Energy storage technology is fundamentally a technology that caters to demand, and its evaluation indicator system encompasses energy indicators, power indicators, scale indicators, lifespan indicators, efficiency indicators, self-discharge indicators, cost indicators, environmental impact indicators, and more. Depending on the application scenario, the types of indicators and their weights required by energy storage technology also vary.
Taking consumer batteries, power batteries, and energy storage batteries as examples, consumer batteries tend to have higher volume and mass energy density, as well as higher charging rates; power batteries require a balance with high cost weight; energy storage batteries can relax energy and power-related indicators to some extent, but they have very high requirements for life and cost-related indicators. Among various energy storage technologies, secondary batteries (electrochemically rechargeable batteries) are a very critical component. They have a wide range of applications and a strong ability to interface with renewable energy power. From the perspective of applicable energy and power ranges, a variety of secondary batteries cover the vast majority of technical needs for energy storage applications, with lithium-ion batteries being the most universal. Overall, energy-related indicators corresponding to “storage capacity” are of the greatest importance at the level of secondary battery technology, and high specific energy (i.e., mass energy density) secondary batteries have the most potential applications, especially in cutting-edge scenarios; rate-related indicators corresponding to “throughput capacity” are also quite important. Of course, the performance of battery materials (sets) is the fundamental determinant of battery performance.
The electrochemical rechargeability of secondary batteries corresponds to the core reversible electrochemical reaction. The oxidizing agent-oxidation product constitutes the active material of the battery’s positive electrode, and the reducing agent-reduction product constitutes the active material of the battery’s negative electrode. According to the basic formula E=U/[1/Qc+1/Qa+minact] (assuming electrode capacity matching, excess electrode material under redundant conditions can be classified as non-active), the higher the specific capacity of the electrode and the greater the potential difference between electrodes, the higher the electrode’s reaction activity, and thus the greater the battery’s specific energy (here not distinguished from energy density). At the same time, electrode materials also need to have a high degree of structural stability (corresponding to the charged and discharged states) and charge reversibility, interface stability (with the electrolyte), chemical stability, thermal stability, a relatively stable voltage platform, and high electronic conductivity and high carrier conductivity, and these properties should ideally be effectively maintained within a wide temperature range. During the electrochemical reaction process, the battery requires the movement of carriers (ions or ion clusters) to provide charge balance for the entire circuit: electrode materials, electrolytes, and electrode-electrolyte interfaces conduct carriers, while electrodes, current collectors, and the external circuit conduct electrons. The bulk conductivity varies with different materials, and so does the interfacial conductivity. The higher the ion diffusion coefficient/electronic conductivity of the battery materials, the higher the rate performance limit of the battery cell. We hope that the secondary batteries that can be widely applied have low costs. This corresponds to the use of elements with high abundance, and the basic material system is cheap, and the battery production process is simple.
Although the basic concept is not complex, lithium batteries are actually a “multi-dimensional composite material” or “multi-functional system” that spans multiple spatial scales, involves various phase types, and continuously changes during use. We conduct classified research on different types of battery materials with a focus, and this focus must also consider the realizability of the overall performance of the battery.
Improving the comprehensive performance indicators of batteries (taking lithium batteries as an example) requires solving a series of problems, addressing the degradation and even failure of battery performance. High energy and high rate capabilities usually amplify the degree and possibility of battery performance degradation and failure. In terms of energy density, a stronger oxidizing positive electrode and a stronger reducing negative electrode correspond to greater capacity or higher potential differences (or both). Large capacity causes greater volume changes in the electrode material when shuttling lithium ions, affecting battery stability; higher potential differences require higher demands on the electrolyte. Fast charging performance of batteries requires a combined effort from the micro and macro levels, a comprehensive trade-off from materials to the system. Temperature is also an important factor affecting performance. Low temperatures are detrimental to battery performance, and high temperatures may pose safety risks. Reasonable temperature control methods are beneficial for the performance and longevity of the battery. The safety of a charged battery is significantly weaker than that of a discharged one. In summary, our high demands on battery performance amplify the degree and possibility of material performance degradation and even failure. For this, various optimization and modification methods for electrode and electrolyte materials have been applied in practice, and the “toolbox” is constantly being enriched and improved.
We once again start with the fundamental formula for energy density/specific energy E=U/[1/Qc+1/Qa+minact]. The potential difference of the battery and energy density are linearly correlated; the contribution of the specific capacities of the cathode and anode to energy density is mathematically symmetrical, but in terms of the performance properties of the electrode materials, there are differences in quality. Generally speaking, phase-change type cathode and anode materials have superior specific capacity-related performance, but their reaction kinetics are poor, and the volume change when absorbing and releasing lithium ions is significant (as previously mentioned); intercalation-type materials are the opposite. The main cathode and anode materials currently used in large-scale applications are intercalation-type materials. Some phase-change type anode materials, represented by silicon, have achieved a small amount of practical application through doping; while phase-change type cathode materials, including chlorides, sulfides, fluorides, iodides, etc., despite continuous scientific research efforts, still have low practical application maturity. The specific capacity of the high-quality and low-cost graphite anode can reach nearly 370mAh/g, not to mention silicon-based anodes; while the specific capacity of the cathode system with a relatively high voltage (with an average of over 3V) is still within 300mAh/g. This also makes the problem of insufficient cathode specific capacity within the entire lithium battery active material system, affecting the overall battery performance, particularly prominent (taking lithium manganate as an example, this article introduces the theoretical specific capacity calculation method for electrode materials:
1.Provide the material chemical formulas for the theoretical lithiated and delithiated states, for lithium manganate these are LiMn2O4 and Mn2O4 respectively;
Lithium cobaltate was the first commercialized layered oxide cathode, possessing a relatively high theoretical specific capacity (274mAh/g, and capacity density), low self-discharge rate, and high cut-off voltage (which is continuously optimized and increased to nearly 4.5V). Its application in the consumer electronics field has persisted to this day. At the same time, cobalt is expensive, and the structural stability of lithium cobaltate at higher delithiated states is generally poor, with thermal stability and high-rate cycle life being a concern. Therefore, researchers have been seeking methods to improve the various shortcomings of lithium cobaltate.
The crystal structures of lithium nickelate and lithium cobaltate are identical, and nickel is cheaper than cobalt. However, the trivalent nickel in lithium nickelate is not stable enough due to the Jahn-Teller effect, and some divalent nickel in lithium nickelate can intermix with lithium ions (which can occur during material synthesis and delithiation, whereas cobalt has a stronger inhibitory effect). During the cycling process, lithium nickelate undergoes a higher degree of irreversible phase transformation, affecting performance and lifespan. Moreover, lithium nickelate has inferior thermal stability compared to lithium cobaltate. A small amount of cobalt and aluminum doping in NCA cathodes, which is an improvement on lithium nickelate: better electrochemical performance and thermal stability, but the structural stability under high pressure may lead to oxygen release. Naturally, researchers have been exploring mixed combinations of central transition metal ions in layered oxide cathodes to enhance the overall performance of the material. The most stable combination is “nickel-manganese balance,” with cobalt further suppressing lithium-nickel mixing. The transition metal ions require atomic-level mixing, equivalent physical positions, and the mainstream synthesis method has become preparing a precursor through liquid-phase mixing, followed by calcination to diffuse lithium ions and control the valence states of the transition metal ions. Thus, the NCM ternary cathode was born. When the nickel content in the ternary cathode is high, the proportion of the highly delithiated state with tetravalent nickel is also high, and the voltage required to achieve this state is relatively low. Therefore, for high-nickel ternary cathodes, the capacity can be fully utilized at a relatively low charging cut-off voltage (for example, the available specific capacity of a typical NCM811 cathode exceeds 200mAh/g at a 4.3V cut-off voltage, and the available specific capacity of the NCM9055 cathode is even higher). It is evident that high-nickel cathodes are beneficial for increasing the energy density of batteries, and they also have lower requirements for the electrolyte’s pressure resistance, which keeps the overall threshold for building the battery material system from being too high. Given the same cut-off voltage, high-nickel cathodes have a higher specific capacity (such as at 4.4V), but there is little practical significance in further increasing the charging cut-off voltage for high-nickel cathodes. For instance, if the charging cut-off voltage is raised to 4.8V, there is basically no effective capacity increase above about 4.4V; the discharge cut-off voltage also effectively starts from 4.4V. Additionally, highly delithiated high-nickel ternary cathodes tend to undergo irreversible phase transformations and cracking, which affects the material’s cycle life. The specific capacity-voltage characteristics of medium-nickel cathode materials (taking NCM424 as an example) differ significantly from those of high-nickel cathodes. When the charging cut-off voltage is not high (such as about 4.3V), the material’s specific capacity is about 160mAh/g; when increased to 4.5V, the specific capacity exceeds 180mAh/g; and when further increased to 4.7V, the specific capacity even exceeds 220mAh/g.
However, the impact of high voltage on the material’s cycle life is also quite significant. This is partly due to the irreversible phase transition, cracking, and even oxygen release in the highly delithiated state of the positive electrode under high voltage, and partly due to the increased side reactions at the interface between the electrode and the electrolyte. If we comprehensively compare positive electrode materials with different nickel contents, we can find that the specific capacity of the NCM9055 positive electrode with a cut-off voltage of 4.4V is similar to that of the NCM111 positive electrode with a cut-off voltage as high as 5V, while the latter corresponds to a higher theoretical energy density of the battery. In other words, the energy densities of batteries with high nickel standard voltage, medium-high nickel higher voltage, and medium nickel high voltage positive electrodes are close. The medium-low nickel extremely high voltage positive electrode is an idealized high energy density battery positive electrode, as it requires extremely high pressure resistance of the electrolyte, and its technical feasibility needs long-term verification. It is likely an effective means to improve the actual charging cut-off voltage of ternary batteries by constructing a pressure-resistant positive electrode surface CEI film with appropriate electrolyte composition and appropriate electrode coating doping. Researchers have confirmed that manganese aggravates lithium-nickel disorder, which has a certain adverse effect on the rate performance of ternary materials; however, the presence of manganese is also beneficial for the stability of lattice oxygen and the layered structure, which in turn is beneficial for the capacity retention of the ternary positive electrode. Therefore, the transition metal ion ratio of the ternary positive electrode needs to be considered comprehensively. The positive electrode particles of ternary materials can be polycrystalline or single-crystalline. Polycrystalline (secondary particles are about a few micrometers to 10 micrometers, corresponding to primary particles of a few hundred nanometers) has a higher specific surface area than single-crystalline (primary particles are on the micron scale), which is beneficial for rate performance. In the process of deepening the understanding of the performance of ternary materials, researchers have also developed a series of modification methods to meet the needs. In terms of bulk doping, researchers have summarized the effects of various elements on the performance of positive electrode materials. For example, doping NCM positive electrodes with aluminum results in NCMA positive electrodes, which have higher aluminum-oxygen bond strength, resulting in smaller lattice deformation of NCMA positive electrodes (-3.6% versus over -4%), higher fracture strength (the fracture strength of NCMA material is as high as 185.7MPa, compared to 125.5MPa for NCA material and 137.2MPa for NCM material), and the material is less prone to cracking and has an improved cycle life.
Researchers recognize that the lithium-rich manganese oxide Li2MnO3, or Li[Li1/3Mn2/3]O2, is equivalent to a compound formed by replacing 1/3 of the octahedral site manganese with lithium in the layered structure of lithium manganese oxide (LiMnO2). It exhibits electrochemical activity at high voltages above 4.5V. Moreover, when part of the octahedral site manganese-lithium pairs (with a stoichiometric ratio of 2:1) are substituted with elements such as nickel, manganese, cobalt, chromium, iron, etc., the resulting so-called lithium-rich manganese-based oxides xLiMO2·(1-x)Li[Li1/3Mn2/3]O2 have an initial reversible specific capacity of over 250mAh/g and a high cut-off voltage close to 5V. This has attracted great interest from materials researchers. The most appealing performance feature of lithium-rich manganese-based cathode materials is their specific capacity of over 250mAh/g within the range of approximately 2.0V-4.8V, but this capacity has not yet been effectively maintained over a larger number of cycles. The first charge curve of lithium-rich manganese-based cathode materials can be observed to have two distinct regions. Within 4.5V, the valence of nickel and cobalt in the conventional layered structure LiMO2 increases, while lithium ions in the conventional layered structure de-intercalate from the cathode and intercalate into the anode, and some lithium ions in the Li2MnO3 layer also de-intercalate into the layered structure (essentially acting as a lithium supplier). The new plateau appearing above 4.5V is attributed by most researchers to the contribution of the Li2MnO3 layer (and its electrochemical activity at high voltages), including the valence change of some manganese, the loss of structural oxygen as oxygen gas and the formation of oxygen vacancies, the loss of electrons by -2 valence oxygen to become -1 valence oxygen to provide charge compensation, lithium ions de-intercalating from the transition metal layer to form Li2O and causing the material structure to rearrange into a conventional layered structure, and the complex charge compensation relationship between lithium, manganese, and oxygen. Experimental research results have also verified the “partial correctness” of the above theoretical generalizations. Some researchers have observed changes in the bond lengths of different chemical bonds in lithium-rich manganese-based cathodes after the first cycle and found that a large amount of oxygen is transferred from its initial position, forming highly contracted transition metal-oxygen octahedra; the distorted lattice during charging does not fully recover during the discharge of the cathode, resulting in voltage hysteresis in the first cycle. Moreover, the distortion of the transition metal-oxygen octahedra will also affect the specific capacity-voltage characteristics of the cathode in subsequent cycles. A prominent feature of the continued cycling of lithium-rich manganese-based cathodes is the continuous decrease in the average discharge voltage. Some research work suggests that the material system will undergo processes such as the continuous de-intercalation of lithium and oxygen, the continuous de-intercalation of transition metals from the layered structure (even into the electrolyte), and the formation of irreversible spinel phases, which affect the average discharge voltage. (If the transition metal center ions continue to de-intercalate, the capacity of the cathode will also be affected.) In addition, changes in the crystal structure can cause stress accumulation. In terms of crystal structure and main components, lithium-rich manganese-based cathodes are very similar to ternary cathodes. Their synthesis methods include solid-phase method, co-precipitation-calcination method, sol-gel-calcination method, hydrothermal method, self-propagating combustion method, etc., but considering the complex structure of lithium-rich manganese-based cathodes and the fact that the internal transition metal atoms are not all in chemically equivalent positions, various liquid-phase mixing methods are appropriate. The correlation between the specific composition and performance of lithium-rich manganese-based cathodes is also very high. For example, some studies have shown that increasing the lithium content (with simultaneous changes in cobalt and nickel content) decreases the average voltage of the cathode; increasing the cobalt content increases the cathode capacity, etc.
If the issues present in the material system are resolved, under conditions where the charging cut-off voltage is as high as about 4.6V, the rich-lithium manganese-based cathode is likely the most promising candidate for the cathode of high-energy density batteries (even if at this point the capacity of high-nickel ternary cathodes remains high, the capacity of medium-high and medium-nickel ternary cathodes significantly increases and the capacity contribution from the high-voltage part is substantial). The impact of capacity and voltage changes during material cycling on the cycle life of the rich-lithium manganese-based cathode is a major issue it faces; the hindrance of lithium transport by rich-lithium manganese oxide and the formation of a relatively thick CEI film on the surface during material cycling also affect rate performance. Based on this, controlling the main elemental components of the rich-lithium manganese-based cathode material, synthesizing appropriate particle morphology and structure, and pairing various modification methods are necessary. Such methods include bulk doping, surface coating, surface treatment, etc., with the aim of suppressing oxygen release, suppressing electrode-electrolyte side reactions, and improving conductivity. Commonly used doping elements include potassium and sodium at the lithium site, cobalt, titanium, zirconium, iron, copper, zinc, tin at the transition metal site; halogens at the oxygen site; boron occupying tetrahedral vacancies, etc. Common coating agents include simple metal oxides such as titanium oxide, zirconium oxide, aluminum oxide, manganese oxide, complex metal oxides, etc.; some phosphates, metal fluorides; some polymers; some other electrode materials, even solid electrolytes, etc. Common surface treatment methods include solution treatment, atmosphere calcination treatment, etc.
3.Olivine cathode, advancing from cost to performance
In the exploration of cathode material systems, researchers have found that materials with XO4 3- ion groups have considerable lithium storage capacity and relatively stable structures. Here, X is represented by phosphorus. After pairing with different types of transition metal center ions, a series of olivine-structured materials belonging to the polyanion category have become an important part of lithium battery cathodes. The theoretical specific capacity of these materials is about 170mAh/g, with different lithium voltage ranges. After considering performance and cost comprehensively, the industry uses iron for the selection of a single transition metal ion, synthesizing the classic cathode material lithium iron phosphate. To this day, lithium iron phosphate is already one of the most widely used cathode materials. In summary, with appropriate means for particle morphology control, particle size control, and carbon coating (also, if the particle size is sufficiently small to be quasi-nano and nano-carbon materials such as carbon nanotubes are doped, the rate performance is improved, but the compaction density is affected), the comprehensive performance of iron lithium can still be optimized. The delithiation and lithiation process of lithium iron manganese phosphate cathode, in addition to containing complete solid solution behavior, also includes a slight change in phase. In the lithium content range of phase change, the delithiation and lithiation capacity of the material will be further affected. The ionic radii of divalent manganese (0.083 nanometers) and divalent iron (0.078 nanometers) are close, and under lithiated conditions, they can infinitely dissolve into each other to form lithium iron manganese phosphate. However, in the delithiated state, the formation of trivalent manganese severely distorts the metal-oxygen octahedron, and this distortion changes the lattice parameters, directly affecting the ability to insert and remove lithium.
The iron-to-manganese ratio is a key parameter for the lithium iron manganese phosphate cathode. Under conditions of high manganese content, the average voltage of the cathode is higher, corresponding to a higher energy density of the battery. However, a large amount of trivalent manganese in the delithiated state can damage the solid solution structure and affect the cathode’s lifespan. Batteries with a balanced iron-manganese state have a slightly lower average voltage, but lithium extraction and insertion are more unobstructed. We can also find that the uniform mixing of iron and manganese at the atomic level is crucial. The enrichment of manganese within micro-regions of the material has a negative impact on lifespan, rate capability, and conductivity. Following the general characteristics of synthesizing inorganic non-metal oxides, laboratories typically use sol-gel, co-precipitation, and other liquid-phase methods for synthesizing lithium iron manganese phosphate. The introduction of doping elements and the combination of solvent systems also affect the morphology, composition distribution, and performance of the final material. The industry has also attempted solid-phase synthesis methods. It can be seen that the lithium iron manganese phosphate particles synthesized by the sol-gel method are slightly more than 1 micron in diameter, with a carbon layer on the surface. Different central transition metal element contents affect the capacity of the cathode material. Under low-rate cycling conditions, when the central transition metal is entirely manganese, the cathode voltage platform is high, but the effective capacity is low. Slightly adding some iron (10%) significantly improves the effective capacity, and the specific capacity-voltage curve of the material still only shows one voltage platform. Continuing to add iron, the effective capacity continues to increase, and two voltage platforms around 4V and 3.5V become apparent. During 50 cycles, no capacity degradation was observed in any of the samples. For lithium iron manganese phosphate cathode samples with an iron-to-manganese ratio of 1:1, research shows that their low-rate capacity performance exceeds 160mAh/g, with two distinct voltage platforms. As the rate increases, the high voltage platform gradually disappears, and the cathode capacity decreases accordingly. At the 1C rate condition, the cathode capacity degrades to about 130mAh/g. Researchers attribute the (relatively) high-performance of the cathode material to the appropriate synthesis process and the iron-to-manganese ratio. Additionally, the grain size and particle size distribution of the iron-manganese-lithium cathode, the point defect situation (such as manganese defects affecting lithium insertion and extraction), and the distribution of modified elements/phases also affect the material performance. Some research indicates that carbon coating, especially carbon nanotube construction of a conductive network coating on iron-manganese-lithium particles, has a significant effect on improving conductivity and rate performance. Other studies show that magnesium, with an ionic radius smaller than manganese and iron, can extend the lithium-oxygen bond in the olivine structure after bulk doping (4% of the total transition metal content), facilitating lithium ion migration.
In summary, if a lithium iron manganese phosphate cathode with a suitable iron-to-manganese ratio, uniformly distributed main components, uniform particle size, regular surface morphology, reasonable doping element distribution, successfully constructed conductive coating layer, and relatively low cost can be obtained, then its core advantage of a higher voltage compared to lithium iron phosphate can be fully realized. To this day, multiple listed companies have made technological arrangements for lithium iron manganese phosphate. Products with medium or medium-high manganese content, as well as doping or standalone use in ternary cathodes, could both become the next “breakthrough” for olivine structure cathodes.
Spinel lithium manganese oxide materials are inexpensive and readily available; their crystal structure corresponds to three-dimensional diffusion channels, and the volume change caused by the insertion and extraction of lithium ions is relatively small, resulting in good rate performance for the material. However, some +3 valence manganese ions (studies have shown that they tend to accumulate on the surface of cathode particles) severely distort the manganese-oxygen octahedron (the so-called J-T effect), causing material cracking and exacerbating electrode-electrolyte side reactions; +3 valence manganese ions also undergo disproportionation and dissolution, with manganese dissolved in the electrolyte eventually depositing on the negative electrode, accelerating electrolyte decomposition, thickening the SEI film on the negative electrode surface, and consuming active lithium in the system; manganese lithium oxide lattices also tend to have/cause some oxygen vacancies, further degrading performance. High-temperature side reactions are exacerbated. All these reasons result in lower battery life for manganese lithium oxide cathodes compared to iron lithium and ternary cathode batteries. Additionally, the capacity and voltage performance of manganese lithium oxide cathodes are not satisfactory. Its specific capacity limit is only about 148mAh/g, lower than the specific capacity limit of 170mAh/g for olivine structure cathodes, and far lower than the specific capacity limit of about 274mAh/g for layered structure cathodes; the average lithium voltage is 4V, with an average specific capacity of only about 115mAh/g. This results in lower system energy density for manganese lithium oxide cathode batteries. The overall balance of these factors means that manganese lithium oxide is only suitable for applications with low requirements for life and energy density, and high sensitivity to cost, such as two-wheeled electric vehicles; the optimistic expectation is for some low-speed electric vehicles, A00-class models to have a slightly larger-scale application. There are various ways to modify the manganese lithium oxide cathode. Under the condition of still using the manganese lithium oxide matrix, various doping and coating methods have been studied, such as a small amount of aluminum doping replacing manganese to improve cycle life, and titanium dioxide nanobelt doping to optimize effective capacity, etc. However, if one wants to significantly improve the overall performance of manganese lithium oxide cathode batteries, the basic idea is also: it is necessary to effectively improve energy density. Since the theoretical specific capacity limit of manganese lithium oxide is limited, it is necessary to build a new material matrix to greatly increase the lithium voltage and simultaneously optimize the bottleneck factors of overall battery performance to meet the usage requirements of electric vehicles. Guided by this idea, research on high-voltage nickel manganese lithium oxide cathode materials has gradually become the “turnaround” key to the spinel structure cathode. After uniformly replacing 25% of the manganese with nickel, making LiMn2O4 become nickel manganese lithium oxide-LiNi0.5Mn1.5O4, researchers obtained a cathode material with a lithium voltage limit of up to 5V and a voltage platform of about 4.7V, which can directly increase the battery’s energy density by about 20% (to a level close to that of ternary battery energy density); in nickel manganese lithium oxide, the valence of manganese is theoretically +4, which makes it less affected by lattice distortion than conventional manganese lithium oxide with much manganese at +3 valence, and the comprehensive performance is improved. However, we must also note that although theoretically nickel is +2 valence and manganese is +4 valence in nickel manganese lithium oxide, in practice (especially during high-temperature synthesis), some oxygen vacancies are still produced and manganese is reduced to +3 valence, which means nickel manganese lithium oxide cannot completely avoid the influence of lattice distortion; furthermore, the higher voltage poses a serious challenge to the existing electrolyte system, with conventional carbonate components facing the risk of decomposition; nickel manganese lithium oxide is also not resistant to hydrogen fluoride (trace amounts of water in the electrolyte will decompose 6F to produce hydrogen fluoride) corrosion, and after being corroded, manganese will still dissolve. This makes the practical application of nickel manganese lithium oxide require effective synthesis and modification methods.
The basic process of the sol-gel method involves preparing a sol containing lithium, nickel, and manganese, drying, and calcining to form a lithium nickel manganese oxide cathode. Its advantages include a high degree of particle crystallization and good dispersibility; however, the disadvantages are higher costs and relatively slow reaction speeds. The size effect of lithium nickel manganese oxide shows that 300-nanometer particles perform better in terms of specific capacity and voltage at different rates than 1-micron particles. Of course, uniformly consistent particles may have a certain negative impact on the compaction density; a larger specific surface area may also trigger more side reactions. Bulk element doping is one of the main methods for modifying lithium nickel manganese oxide, aiming to expand the solid solution region to improve rate performance, enhance structural and thermal stability, etc. Cationic elements such as aluminum, chromium, copper, zirconium, magnesium, cobalt, and titanium, as well as anionic elements such as fluorine, chlorine, phosphorus, and sulfur, have all received relatively positive evaluations. Surface coating is another method to improve the performance of lithium nickel manganese oxide, mainly to inhibit manganese dissolution and side reactions between the electrode and electrolyte, thereby improving the initial efficiency and cycle life. Simple carbon material coatings can improve battery cycle life, and some oxides and organic compounds also have positive effects. In addition, some other surface treatment methods can serve similar purposes. The high voltage of the spinel lithium nickel manganese oxide cathode imposes extremely high demands on the pressure resistance of the electrolyte (and solid electrolyte), which will be discussed in the subsequent electrolyte chapter; other detailed research results on lithium nickel manganese oxide can be found in the research report “Spinel Lithium Nickel Manganese Oxide: Energy Density & Cost Call for Unity.” We estimate the equivalent metal element usage for the cathode of different types of batteries per unit energy as follows in the table. Among them, iron lithium, iron manganese lithium, and nickel manganese oxide save on various expensive transition metals and lithium; the lithium content in the lithium-rich manganese-based cathode is not low, but the amount of expensive transition metal elements is low (estimated based on the material composition of Li1.2Ni0.16Co0.08Mn0.56O2).
The sulfur cathode does not contain lithium (which is another significant difference between sulfur and other cathode materials, besides the phase change/intercalation principle), hence it requires a lithium-containing anode, typically lithium metal. As a high-capacity (1672mAh/g) cathode material, the lithium-sulfur battery formed by pairing sulfur with lithium metal has an extremely high theoretical specific energy density (2.51Wh/g or 2510Wh/kg). For the sulfur cathode, the low lithium-ion diffusion coefficient and electronic conductivity, as well as the high volume change during lithium storage/release, have a significant negative impact on the battery. This can be alleviated by constructing a composite material system (such as compositing with carbon materials and adding binders), sacrificing some specific capacity. During the discharge process of lithium-sulfur batteries, lithium sulfide is not formed all at once but produces various Li2Sx intermediate products. These polysulfides will “shuttle” to the anode side in the electrolyte, directly oxidizing the lithium metal; subsequently, the reduced polysulfides will “shuttle” back to the anode side, reducing the sulfur cathode. Ultimately, this results in battery capacity loss, low coulombic efficiency, and high self-discharge rates. Given this, common methods include adding consumable lithium nitrate to the solvent, regulating the composition, structure, and morphology of the electrodes, and even constructing physical barrier layers in the electrolyte system, which researchers have considered to suppress the shuttle effect. After applying a solid electrolyte, the shuttle effect is fully suppressed, and battery safety is improved. Taking the typical polyethylene oxide-lithium lanthanum zirconium oxide composite solid electrolyte system as an example, the positive electrode capacity can still be maintained above 800mAh/g after 200 cycles, comparable to the aforementioned liquid system. Researchers conclude that, in terms of lithium-sulfur battery research progress, 300Wh/kg is the most reported mass energy density, and the theoretical energy density (considering auxiliary components) is much higher than other mainstream systems; however, the performance of volumetric energy density is quite poor, even falling short of the performance of lithium iron phosphate-graphite anode batteries.
III. Lithium Battery Anode: Striving for Progress at the Pinnacle
The synthesis of lithium graphite intercalation compounds dates back to the 1950s. After being paired with suitable electrolytes, the formation process enables the graphite anode surface to form a SEI (Solid Electrolyte Interphase) film, allowing reversible intercalation/deintercalation of lithium ions. Although the specific capacity of 372mAh/g is not outstanding among anode materials, graphite has balanced overall performance. Therefore, both artificial and natural graphite have become representatives of carbon materials in lithium storage anodes, and even representatives of lithium storage anodes. In addition, some special types of graphite materials, such as certain graphenes and highly oriented pyrolytic graphite, have higher actual lithium storage capacities. The lithium storage potential of the graphite anode is very low, and the formation of the SEI film on its surface stabilizes the entire system during battery formation. The lithium mass transfer of the SEI film includes both interfacial mass transfer and bulk mass transfer. The lithium plating of the graphite anode is also related to temperature conditions. Some research work indicates that when the overall electrode temperature is low, and there are relatively hot spots, the hot spots tend to plate lithium. At present, when the practical product specific capacity is close to the theoretical value, the research and application work on the rate performance of graphite anodes, especially fast charging, is gradually becoming more important. From the perspective of material diffusion barriers, the graphite anode is precisely the anode material suitable for fast charging of EVs. However, the optimization of the graphite anode fast charging performance also requires several conditions. The complete layered structure of graphite requires lithium ions to intercalate from the material end face, and this anisotropy has an adverse effect on the realization of fast charging performance; the specific surface area of graphite particles is small, which limits the fast charging capability, and a large specific surface area has a low initial efficiency. Therefore, researchers need to construct graphite anodes and even composite anodes with appropriate orientations, particle sizes, porosities, surface modification states (such as surface oxidation, amorphous carbon coating), and doping element compositions (such as phosphorus, boron, etc.), to balance specific capacity, initial efficiency, and fast charging capability. Of course, the adaptation of the electrolyte and the reasonable application of conductive agents are also important (discussed below).
Silicon materials have a very high theoretical specific capacity (forming Li22Si5 at high temperatures, corresponding to a capacity of 4200 mAh/g; forming Li15Si4 at room temperature, corresponding to a capacity of 3579 mAh/g; when comparing volumetric energy density, graphite is 837mAh/cm3, while Li15Si4 is 9786mAh/cm3). The delithiation voltage is also lower compared to other anode materials (about 0.5V or less), only slightly higher than graphite. Therefore, silicon-based materials are expected to become the matching anode material for high-energy-density lithium batteries, paired with high-nickel NCM/NCA cathodes (high specific capacity with high specific capacity) to achieve the best results. While demonstrating excellent capacity, silicon-based anode materials also exhibit very significant intrinsic volume changes during lithiation, affecting cycle life. Therefore, alleviating the volume changes of silicon-based anode materials during cycling is a problem that all research work must address. Based on this, silicon-based anodes have given rise to elemental silicon-carbon (and low-dimensional silicon materials) anodes, silicon oxide-carbon anodes, silicon alloys, and other technological approaches. Among these, the first two (collectively referred to as silicon-carbon anodes) are practical subdivision technology route with strong practicalitys. Relying on elemental silicon for anode material construction, the intrinsic volume change of silicon is the first challenge in its practical application. Volume changes cause larger elemental silicon particles to crack and break during multiple cycles, and the physical connection with the conductive agent is also destroyed, affecting the battery cycle life. Researchers have also found that the critical size of the aforementioned elemental silicon particles is about 150nm. In addition to the physical challenge of intrinsic volume changes in silicon, silicon particles also face contact and reaction with the electrolyte, forming a solid electrolyte interface (SEI) film. Unlike the dense, thin, and regular SEI film formed during the conventional cycling of commercial graphite anodes, the SEI film formed by elemental silicon is loose, thick, uneven, and has high impedance, hindering the diffusion of lithium ions. Therefore, the initial efficiency of silicon-based anodes is not high, negatively impacting energy density. Additionally, the SEI film on the surface of elemental silicon will repeatedly peel off, regenerate, and deposit during cycling, consuming active silicon and lithium in the material system, severely degrading battery performance.
Researchers analyzed the elemental distribution of this silicon-carbon composite material (lithiated) and found that the surface nano-pores of the particles are enriched with lithium metal, while the interior is a mixture of lithium, silicon, and carbon. During the lithiation process, Li migrates through the Si-C composite material in the order of C, nano-pores, and Si. The first step is the insertion of Li into the C particles and its diffusion; the second step is the filling of nano-pores formed between the composite particles; the third step is the alloying of lithium with silicon and the beginning of significant volume changes. It is evident that the active component for lithium storage in silicon oxide is still elemental silicon. Li2O produced during the reaction can act as a fast ion channel; lithium silicates such as Li2O and Li4SiO4 can also buffer volume changes during cycling, which is beneficial for achieving better rate performance and a higher cycle life. However, lithium silicates like Li2O and Li4SiO4 are inert phases, which results in a lower theoretical capacity for silicon oxide-carbon anodes compared to elemental silicon-carbon anodes, and a lower first-cycle efficiency compared to elemental silicon-carbon anodes, and even lower than graphite (before the decomposition of Li4SiO4, the theoretical specific capacity of silicon monoxide is 1480mAh/g, with a theoretical first efficiency of 70.9%), ultimately affecting the energy density of the battery. In summary, to enhance the actual specific capacity, rate performance (especially fast charging capability), and cycle life of silicon-based anode materials to meet the growing demands of high-performance power batteries, in addition to controlling the synthesis/modification of silicon/silicon monoxide itself (mechanical grinding or silane decomposition to prepare silicon-based substrates with appropriate particle sizes, followed by different methods of carbon composite and coating, pre-lithiation, and surface structure reinforcement), further adaptation of various components such as electrolytes, conductive agents, and binders is also required. Silicon-based anodes are high-potential but challenging high-performance anode materials. More scientific, engineering, and commercial efforts are on the way.
3.Lithium Metal Anode, the Holy Grail of Thorns
The ultimate goal of all lithium-ion battery negative electrode materials is undoubtedly to have a theoretical specific capacity of up to 3860mAh/g and a lithium metal with a voltage of 0V (considering that lithium is completely introduced from the negative electrode. If lithium is introduced from the positive electrode and there is no lithium in the negative electrode, the specific capacity can even be infinite). Because of this, lithium metal is known as the “holy grail” of negative electrodes. In fact, scientific, engineering, and commercial efforts to use lithium metal as a negative electrode for lithium-ion batteries began long before the successful commercialization of lithium-ion batteries. In 1970, pioneer Michael Stanley Whittingham (later one of the Nobel laureates in Chemistry) invented a lithium battery called titanium disulfide lithium aluminum alloy battery; By 1985, Canadian company Moli Energy had relied on this system to mass produce secondary batteries equipped with lithium metal anodes. Moli Energy’s lithium batteries have a specific energy of over 100Wh/kg and were first applied in the 3C field (handheld computers, cordless phones). Moli Energy was acquired by NEC in the spring of 1990, from the launch of the product in 1985 to the launch of the second generation product in the spring of 1989, and then to the safety incident of the first generation product catching fire and exploding during the same period.
NEC analyzed the performance of lithium metal batteries under low rate charging and discharging conditions: all batteries experienced rapid capacity decay, short circuit failure, and even fire and explosion. After analysis, it was found that the uneven deposition of lithium metal (also believed to be growth) and the formation of lithium dendrites, as well as their fracture/growth piercing through separators, are the “culprits” that cause battery capacity degradation, short circuits, fires, and explosions. In addition to lithium dendrites, the side reactions between lithium and electrolytes, as well as the volume and morphology changes of lithium, also affect the overall performance of batteries. To this day, graphite negative electrode materials have been thoroughly validated by the industry, while lithium metal negative electrodes still attract the attention of a group of researchers: conducting detailed observation of phenomena and mechanism analysis; Finding a suitable electrolyte system to match lithium metal; Modify the electrode itself; Introducing lithium from the positive electrode side allows lithium metal to only exist in the charged state at the negative electrode, creating a ‘negative electrode free battery’ and so on. When lithium metal negative electrode is combined with conventional electrolyte, it is quite difficult to form a uniform, stable and strong protective SEI film, which leads to significant negative effects on the formation and evolution of lithium dendrites, the promotion of side reactions at the lithium negative electrode electrolyte interface, and the volume changes caused by reaction products, amplifying the shortcomings of the battery. Need to study whether to use lithium for the negative electrode and how much lithium to use; It is also necessary to study how to construct the electrolyte system and how to “passivate” lithium metal as much as possible. Researchers have shown that in battery cycling, the side reaction between lithium metal and electrolyte leads to linear capacity decay of the battery; The sudden “drop” in capacity is caused by the polarization caused by the growth of the SEI layer with poor performance during the cycling process. Excessive lithium metal in the negative electrode consumes a large amount of electrolyte, forming a “dry SEI” that is a poorly performing SEI layer; If there is no lithium in the negative electrode (“no negative electrode” battery), dead lithium that does not contribute to capacity will be mixed in the SEI, which also deteriorates the performance of the battery. A 20 micron thick lithium foil surface can grow a “wet SEI” with appropriate thickness and good morphology, which significantly improves the cycling performance of the battery. 600 effective cycles, 350Wh/kg, and 20 micron lithium foil are excellent experimental results for lithium metal negative electrode batteries; Systematically explaining the amount and mechanism of lithium usage is of greater guiding significance. The importance of electrolytes (solvents and additives, lithium salts and additives) for lithium metal negative electrodes is beyond doubt. Electrolyte systems with relatively better performance often use LiFSI as the lithium salt; We often use fluorinated solvents/ether solvents, as well as innovative systems such as sulfonamide solvents. If solid electrolytes are hard, strong, dense, and stable, they can suppress the generation and growth of lithium dendrites during the cycling process of lithium metal batteries. But in fact, lithium dendrites may also “pierce” solid electrolytes (for polymers) or deposit in the gaps inside solid electrolytes (for inorganic substances).
Electrolyte is the “lithium-ion transport river” that enables lithium batteries to form a closed loop. It should consider performance requirements such as ion conductivity (for lithium-ion electrolytes, the ion conductivity is determined by the lithium ion migration rate, lithium ion migration number, and active lithium ion concentration), electronic insulation, good physical contact with the electrode, resistance to positive electrode oxidation, resistance to negative electrode reduction, electrochemical stability, thermal stability, air stability, mechanical stability, and good temperature characteristics corresponding to various indicators, as well as the need for low-cost scaling. Electrolytes have the natural advantage of infiltrating electrodes and have been widely used, with continuous innovation. Due to the potential difference between the positive and negative electrodes that far exceeds the decomposition voltage of water, and without considering expensive ionic liquids, the mainstream technical route for electrolytes is a comprehensive system composed of suitable organic solvents and lithium salts. Its typical lithium-ion conductivity is about 10E-3~10E-2 S/cm. The solvent in the electrolyte is electronically insulated and used to dissolve lithium salts. The basic requirements for electrolyte solvent systems are: having a certain polarity (high dielectric constant) to dissolve lithium salts; Wide electrochemical window (the electrochemical window of the electrolyte is mainly reflected in the electrochemical window of the solvent), resistant to positive electrode oxidation and negative electrode reduction; Low viscosity, easy to infiltrate electrodes and improve low-temperature performance; heat-resisting. So far, there is no single component solvent that can simultaneously meet the above requirements, so the basic idea of constructing a mixed solvent system is very reasonable. The basic consideration for a mixed solvent system is to select solvent components with high dielectric constant and low viscosity. The former corresponds to ethylene carbonate EC and propylene carbonate PC; The latter corresponds to dimethyl carbonate DMC, diethyl carbonate DEC, methyl ethyl carbonate EMC, etc.
The additional functions of solvents, such as improving the solvation properties of lithium ions, synergistically forming and stabilizing solid electrolyte membranes (SEI), assisting flame retardancy, etc., also depend on the action of solvent additives. Solvent additives include conventional chain/cyclic esters (such as vinyl carbonate VC), fluorinated chain/cyclic/amino esters (such as fluorinated vinyl carbonate FEC), sulfate esters (such as vinyl sulfate DTD, vinyl sulfite ES), sulfones, nitriles, phosphorus based additives, silicon-based additives, ethers, heterocyclic compounds, and so on. Researchers are also committed to improving carbonate solvents and developing non carbonate solvent systems to meet the special needs of electrode materials such as lithium metal negative electrodes and high-voltage positive electrodes. The ability to design solvent molecules greatly tests technical strength and even the scientific understanding of the underlying principles of battery electrochemical systems. Fluorinated carbonate solvents have a higher oxidation potential and can be used as a substitute for conventional carbonate solvents to improve the lifespan performance of lithium nickel manganese oxide batteries at room temperature and high temperature. Research has shown that the use of fluorinated ester solvents reduces the decomposition products of both the positive and negative electrodes in the material system, indicating an improvement in the stability of the entire battery system. Fluorinated vinyl carbonate FEC can effectively improve the charge and discharge performance of spinel nickel manganese oxide lithium silicon-based lithium batteries. Researchers have designed a single solvent fluorinated 1,4-dimethoxybutane FDMB and combined it with LiFSI to prepare an electrolyte; Using NCM532 as the positive electrode and lithium metal of different thicknesses as the negative electrode, corresponding soft pack batteries were fabricated; NCM523, 622, and 811 positive electrodes were also used to produce non negative soft pack batteries. On this basis, the battery performance under different conditions (including a highly charged state with a cut-off voltage of 4.4V) was tested, and the relevant mechanisms were analyzed: the coulombic efficiency of the battery under excess lithium conditions reached 99.98%; The initial energy density of 811 positive non negative electrode battery reached 325Wh/kg, and the cycle life of 523 positive non negative electrode battery also exceeded 100 times. Researchers attribute the excellent performance of the battery to the positive effect of extending the solvent carbon chain with CF2 groups. Given the relatively single composition of the electrolyte system used in this research, there is still room for optimizing the material system and improving battery performance in the future. Researchers have designed an electrolyte composed of a single solvent trifluoromethanesulfonyldimethylamine DMTMSA and a standard concentration LiFSI. This system demonstrates the superiority of sulfonyl groups: in terms of their relationship with the positive electrode, there are fewer side reactions, less gas production, less transition metal leaching, and reasonable CEI formation; In terms of the relationship with the negative electrode, it is conducive to stabilizing the lithium metal form. Compared with the conventional LiPF6-EC/EMC/VC electrolyte system, LiFSI-DTMSA exhibits excellent comprehensive performance. Under the condition of a lower cut-off voltage of 3.0V and an upper cut-off voltage of up to 4.7V, the corresponding battery cycled 100 times with a decay of only slightly more than 10%. In addition to the lithium salts mentioned above, lithium salt additives, including phosphates (such as lithium difluorophosphate LiDFP, lithium difluorobisoxalate phosphate LiDFBOP), borates (such as lithium difluorooxalate borate LiBOB, lithium difluorooxalate borate LiDFOB), sulfonyl imide salts (other types besides lithium difluorosulfonyl imide LiFSI and lithium bis trifluoromethanesulfonyl imide LiTFSI), heterocyclic salts, aluminate salts, etc., can optimize the electrolyte liquid phase or electrolyte electrode interface to varying degrees when used properly, and improve the comprehensive performance of the electrolyte. In some cases, the hydrogen fluoride produced by the decomposition of 6F has adverse effects on the performance of the battery, and the high and low temperature performance, as well as the compatibility of some electrodes, are not satisfactory. So the research and application of various new lithium salts also play an important role.
In addition to LiFSI, various other lithium salts (as additives) have also been extensively studied. Research has shown that using 6F alone as a lithium salt, compared to simultaneously using 6F and lithium oxalate borate LiBOB, lithium oxalate borate can passivate the surface of spinel nickel manganese oxide positive electrode and improve the cycle life of the battery. The oxalate borate ions in LiBOB can consume trace amounts of hydrogen fluoride in the system and passivate the surface of the high-voltage positive electrode, forming a stable CEI. During the battery cycling process, a small amount of lithium difluorooxalate borate LiDFOB and lithium tetrafluoroborate are generated, which is beneficial to the overall performance of the battery. A properly matched double salt system may further optimize battery performance. The composite of LiFSI and lithium difluorobis oxalate phosphate (LiDFBOP) for lithium metal battery electrolyte is more effective than LiFSI alone. We have reason to believe that on the basis of existing solvents, lithium salts, and various additives, more new electrolyte systems are poised to emerge. The bulk properties of solvents and solvent additives, the effective dissolution and dissociation of lithium salts in solvent systems, the synergistic solvation characteristics of solvents, solvent additives, and lithium salts, the interface characteristics jointly determined by the three and the electrode, the temperature characteristics corresponding to various properties, and other related mysteries are waiting to be discovered. This form of excavation is not limited to “formula adjustment” and “stir frying”, and molecular design and effective synthesis based on underlying principles will become increasingly important. The optimization progress of electrolyte solvents and lithium salts may lead to a leap in the comprehensive performance of lithium batteries in the next round. Of course, there are considerable barriers to the “formulation design” of electrolyte systems and the “efficient production” of various components. In addition, some ionic liquids have good thermal stability, are not easily volatile, have a wide electrochemical window (some ionic liquids can withstand 5V high voltage), are non flammable, and have high ionic conductivity. Ionic liquid systems themselves are also an important branch of high-performance electrolytes.
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Solid state batteries “encompass a wide range of concepts (roughly distinguished by the proportion of liquid phase inside the battery): quasi solid state batteries, semi-solid state batteries, solid-state batteries, and all solid state batteries. In a relatively strict context, there is no liquid phase present inside the solid-state battery, that is, inorganic or organic polymer solids are used as electrolytes for the battery; In a slightly relaxed context, there is no “flowing liquid component” inside the solid state battery, but it can contain some residual liquid to form jelly like gel; The context continues to be loose, and batteries with solid electrolytes can also be referred to as “solid-state batteries” to a certain extent. Corresponding to the electrolyte, the electrolyte of solid-state batteries is solid electrolyte (also known as solid electrolyte). As an electrolyte, solid electrolytes, like electrolytes, should consider performance requirements such as ion conductivity (for lithium-ion electrolytes, their ion conductivity is determined by lithium ion migration rate, lithium ion migration number, and active lithium ion concentration), electronic insulation, good physical contact with electrodes, resistance to positive electrode oxidation, resistance to negative electrode reduction (which is important for the stability of lithium metal in high-energy density batteries), electrochemical stability, thermal stability, air stability, mechanical stability, good temperature characteristics corresponding to various indicators, as well as the need for low-cost scale promotion. The lithium conduction mechanism of polymer solid electrolytes is significantly different from that of electrolytes. Lithium ions usually migrate in the amorphous region of polymers, including ion migration formed by the local movement of lithium ions accompanying polymer molecular segments, as well as ion migration formed by lithium ions within or between polymer chains, usually in the form of complexation and decomplexion. Therefore, reducing the glass transition temperature of polymers and expanding the amorphous region (plasticization) are the main means to optimize the performance of polymer solid electrolytes. Polymer solid electrolytes generally require doping with lithium salts to obtain lithium ion conductivity. Polymer solid electrolytes include polyethers (typical examples are polyethylene oxide or polyethylene oxide), polydimethylsiloxane, polycarbonate, polyacrylonitrile, polyvinylidene fluoride (and its copolymer polyvinylidene fluoride hexafluoropropylene), polymethyl methacrylate (organic glass), single ion polymers, etc; Some polyionic liquids lithium salts also show gelled state (called ionic gel). Polymer solid electrolytes have lower density and better physical contact properties with electrodes, and are usually easier to process. Polyethylene oxide (PEO) is the most extensively studied polymer solid electrolyte. Its dielectric constant is relatively high (amorphous corresponds to a dielectric constant of 8), which can fully dissolve lithium salts; Good physical contact characteristics with electrodes. However, under room temperature conditions, its crystallization degree is high and its ionic conductivity is low (10E-8~10E-6 S/cm, as a reference electrolyte at the level of 10E-2 S/cm), so various treatment methods such as blending, block grafting, adding plasticizers, adding fillers, etc. are needed for modification. After effective treatment, the ion conductivity of polyethylene oxide based solid electrolyte can reach the level of 10E-4 S/cm at room temperature. Polysiloxane (PS) has better thermal stability, more flexible segments (determined by silicon oxygen silicon bonds), lower glass transition temperature, and stronger resistance to positive electrode oxidation compared to polyethylene oxide. However, its intrinsic polarity is weak, resulting in poor ability to dissolve lithium salts and affecting lithium ion conductivity; The difficulty of large-scale manufacturing is also relatively high. After grafting and block modification of polysiloxane (such as with polyethylene oxide block), its room temperature ionic conductivity can also reach the level of 10E-4 S/cm. Polycarbonate (PC) contains highly polar carbonate groups, with a high dielectric constant and good segment flexibility. Its room temperature conductivity ranges from 10E-5 S/cm to 10E-4 S/cm. However, its resistance to positive electrode oxidation is average, and its chemical stability in contact with lithium metal is also average, making it difficult to manufacture on a large scale. Polyvinylidene fluoride (PVDF) can be used not only as a binder for lithium battery electrode materials, but also as a polymer solid electrolyte matrix. This type of material and its copolymers (PVDF, PVDF-HFP) have high dielectric constant and good chemical stability, but their ionic conductivity is average. In addition, their high crystallinity and hardness require effective plasticization, and their interface stability with lithium metal also needs to be improved.
Other polymer solid electrolyte matrices, such as polyacrylonitrile (PAN) and polymethyl methacrylate (PMMA), are also under research. The main problems are low room temperature ionic conductivity, brittle texture, and poor mechanical properties. The purpose of adding lithium salts to the conventional polymer solid electrolyte matrix is to provide lithium ions, and the actual effect of combining multiple lithium salts has also been studied. Among them, due to the highly delocalized negative charge of the sulfonylimide structure, solid electrolytes based on sulfonylimide lithium salts exhibit strong ion conductivity. Moreover, by further designing the structure of lithium salt anions, introducing specific functional groups, improving lithium ion conductivity, and regulating electrode/electrolyte interface performance, there is room to enhance battery performance. The purpose of filling inorganic fillers (referred to as inert fillers) into conventional polymer solid electrolyte matrices includes reducing crystallinity, forming lithium ion transport channels (at the interface between fillers and matrices), and improving mechanical properties. Common inorganic fillers include silicon dioxide, titanium dioxide, zirconium dioxide, barium titanate, etc. They can be 0-dimensional materials (powders) or 1-dimensional materials (fibers). The uniform and stable dispersion of inorganic fillers in electrolytes, as well as the exploration of appropriate amounts, are important sub fields in the scientific research, engineering, and commercial applications of polymer solid electrolytes. Finally, polymer solid electrolytes also have forward-looking directions such as single ion polymers and polyelectrolyte liquids. The lithium conduction mechanism of inorganic solid electrolytes differs significantly from that of electrolytes. From the perspective of optimizing ion conductivity, it is necessary to find a solid electrolyte matrix with suitable basic element composition and crystal structure, analyze its lithium conduction mechanism (crystalline or amorphous, grain lithium conduction or grain boundary lithium conduction, etc.), and also consider appropriate phase and interface optimization methods. Based on the main anions as a distinguishing criterion, inorganic solid electrolytes can be classified into oxides (crystalline perovskites, sodium superconductors (as a commonly accepted material structure description, some are amorphous), garnets, amorphous LiPON films), sulfides (crystalline lithium superconductors, such as lithium germanium phosphorus sulfur, sulfide silver germanium minerals, as well as some amorphous sulfides), halides, and other categories (some studies also involve nitrides, hydrides, etc.). The lithium conduction mechanism of inorganic solid electrolytes is usually the transition of lithium ions between lattice structures with anions as the framework, and each has its own performance characteristics. Inorganic materials are generally hard, and in addition to bulk ion conductivity, the interface contact between the electrode and electrolyte is also very important. The selection of oxide materials for lithium-ion conduction and electronic insulation is a key focus of solid electrolyte research. Perovskite (ABO3, CaTiO3) structure is one of the classic inorganic crystal structures. In the field of solid electrolytes, lithium lanthanum titanium oxide (LixLayTiO3, LLTO) formed by replacing calcium with lanthanum and lithium has various crystal structures such as cubic, tetragonal, and orthogonal depending on the synthesis conditions and components.
The room-temperature bulk lithium ion conductivity of LLTO can reach the order of 10E-4 to 10E-3 S/cm. However, the synthesis of LLTO is very temperature-sensitive, with high temperatures leading to lithium loss; it has high interfacial resistance; most importantly, the +4 valence titanium is not resistant to reduction, making LLTO unstable against lithium metal, resulting in the formation of products with poor electronic insulation. Additionally, there is research in the industry on anti-perovskite lithium halide oxides. NASICON materials are generally hexagonal phase, with lithium ion conductivity that can reach the order of 10E-4 to 10E-3 S/cm. NASICON solid electrolytes can be crystalline or obtained in a coexistence of amorphous and crystalline ceramics by the method of melting and quenching to alleviate the problem of interfacial resistance. Similarly, due to the presence of +4 valence titanium, LATP has low chemical stability against lithium metal; LAGP has seen some improvement. The typical representative of the garnet structure solid electrolyte (general formula M3N2(SiO4)3, corresponding to the solid electrolyte general formula Li3+xA3B2O12) is lithium lanthanum zirconate (Li7La3Zr2O12, LLZO). LLZO can have both cubic and tetragonal crystal structures. Starting from LLZO, the modification method for garnet solid electrolytes is doping and substitution with elements such as aluminum, tantalum, yttrium, and titanium, with a typical ionic conductivity also in the order of 10E-4 to 10E-3 S/cm. The overall chemical stability of LLZO is fair. Furthermore, the amorphous thin film of lithium phosphorus oxynitride (LiPON) obtained by magnetron sputtering is also an important solid electrolyte. Although the ionic conductivity of this material is not high (in the order of 10E-6 S/cm), its thickness is only 1 micrometer, and it has good thermal stability and a wide electrochemical window, making it suitable as a solid electrolyte for special application solid-state thin-film batteries. Unlike the -2 valence oxygen, the -2 valence sulfur has a larger radius and a greater degree of electron cloud deformation. In sulfide solid electrolytes obtained by substituting sulfur for oxygen in the lattice, the channel size for lithium ion bulk diffusion is larger, and the electrostatic binding of lithium ions is smaller, which makes sulfide solid electrolytes exhibit higher ionic conductivity (simulation calculations suggest that the ionic conductivity of crystalline sulfide solid electrolytes is related to the crystal structure, with body-centered cubic being superior to face-centered cubic); sulfide solid electrolytes are also softer than oxide systems. However, on the other hand, the stability of sulfides is lower than that of oxides, and higher external pressure is needed to maintain the phase of the battery, and they are sensitive to water and oxygen in the air. Additionally, when sulfide solid electrolytes are paired with layered oxide cathodes, a space charge layer is formed at the interface, affecting the ionic conductivity of lithium ions in the vicinity of the interface. LISICON originally refers to the solid solution formed by Li4GeO4 and Zn2GeO4, as well as oxides with the composition Li3+xXxY1-xO4, having a γ-Li phosphate crystal structure. The ionic conductivity of this class of materials at room temperature is relatively low. Researchers later replaced oxygen with sulfur and zinc with phosphorus, obtaining the Li4GeS4-Li3PS4 lithium germanium phosphorus sulfide (LGPS) series of materials. LGPS solid electrolytes and their derivatives with silicon, chlorine, and other substitutions have very high room-temperature ionic conductivity, with Li10GeP2S2 even reaching the order of 10E-2 S/cm, holding an important position in today’s solid electrolyte systems. However, LGPS has poor stability, causing passivation on the cathode and continuous side reactions with lithium metal anodes, limiting its practical application. The thio-argentite structure solid electrolyte Li7PS6 has a lower lithium ion conductivity, but optimizing the composition to Li7P3S11 significantly increases the ionic conductivity to 10E-2 S/cm. On the other hand, after replacing sulfur with halogens to form Li6PS5X (X is a halogen ion, especially chlorine has the best effect, followed by bromine) solid electrolytes, the defect in the material system increases, and the ionic conductivity can be increased to 10E-3 S/cm. After substituting part of the phosphorus with elements such as silicon and germanium (also considering the ionic size, large-radius halogens such as iodine are used in combination), the ionic conductivity will still be improved to some extent, such as Li9.54Si1.74P1.44S11.7Cl0.3 also reaching 10E-2 S/cm. Thio-argentite solid electrolytes are passivated against both the cathode and lithium.
Amorphous sulfide solid electrolytes are sulfide solid electrolytes formed from lithium sulfide and other sulfides, such as Li2S-P2S5, Li2S-SiS2, etc., with diverse compositions. This type of material can be partially doped with sulfides or oxides, or subjected to high-temperature treatment to form a glass ceramic phase, which can increase the room temperature ionic conductivity to the level of 10E-4 to 10E-2 S/cm. Amorphous sulfide solid electrolytes passivate positive electrodes and lithium. The typical components of halide solid electrolytes are LiAX4 or LixAX6, and the electronegativity and radius of halide ions and central transition metals jointly determine the composition and crystal structure. The performance characteristics of halide solid electrolytes are closely related to the type of halide ion, the type of transition metal central element (and the crystal structure of the solid electrolyte). Overall, compared to -2 valent oxygen, -1 valent halide ions such as chlorine, bromine, and iodine have larger radii. The halide solid electrolyte obtained by replacing lattice oxygen with lattice halide ions may have a larger channel size for lithium ion diffusion; The ion polarization ability is weaker, and the electrical binding of lithium ions may be smaller. This makes halide solid electrolytes have the potential for high ionic conductivity. In addition, halide solid electrolytes are softer than oxides. However, it is generally sensitive to humid environments and lithium metal. Halide solid electrolytes typically have strong resistance to positive electrode oxidation. Some research suggests that for solid electrolytes of chlorine and bromine halides, the selection of third main group elements for central transition metal ions results in poor material resistance to reduction; Choosing elements from the third subgroup improves its resistance to reduction. Faced with inorganic solid electrolytes, we have seen a very rich material system and outstanding performance. At the same time, we also believe that further improvement of lithium-ion conductivity and effective contact between electrode electrolytes are still goals that inorganic solid electrolyte researchers need to tirelessly strive to achieve. After analyzing the basic performance characteristics of solid electrolytes separately, we focused on evaluating their performance indicators. In terms of bulk ionic conductivity, some sulfides can be comparable to electrolytes. After considering the dynamic stability, the actual available electrochemical window of solid electrolytes has been expanded to a certain extent (forming nano layers with lithium ion conduction and electronic insulation on the surfaces of the positive and negative electrodes can correspond to dynamic stability); Solid electrolytes with strong valence changing ability corresponding to central ions and solid electrolytes with central ions corresponding to metals that can alloy with lithium are usually not resistant to lithium metal reduction. Most polymer solid electrolytes passivate lithium metal but are unstable at high voltage, while some systems such as polyacrylonitrile are stable at high voltage but unstable to lithium. The thermal stability of solid electrolytes is better than that of electrolytes, especially in oxide systems. The physical contact ability between polymer solid electrolytes and electrodes is good, while the overall contact ability of inorganic solid electrolytes is average. On the positive electrode side, in addition to electrolyte positive electrode composite, commonly used solutions include oxide sintering, sulfide high voltage (which also hinders electrolyte decomposition), etc; On the negative side, solutions include surface modification of solid electrolytes, etc. In addition, lithium dendrites may still form on the negative electrode side during battery cycling, which requires varying degrees of hardness, thickness, density, and current density of the solid electrolyte during cycling. At the same time, considering the anti reduction properties of solid electrolytes, their physical contact ability with lithium, as well as the electrochemical stability during cycling and the generation and development of lithium dendrites, corresponding scientific explorations are still ongoing. Based on the above information, we can compare the comprehensive performance of different types of electrolytes.
Solid electrolytes each have their strengths, but no single solid electrolyte can achieve satisfactory comprehensive performance. Compared to liquid electrolytes, the “traditional weaknesses” of solid electrolytes—lithium ion conductivity and physical contact with electrodes—have not seen fundamental improvements. Of course, lithium batteries are multiphase, multidimensional composite material systems, and in addition to modifying the solid electrolytes themselves, modifying electrode materials to accommodate solid electrolytes is also a critical direction of work, such as surface treatment of lithium metal anodes and surface treatment of ternary cathodes; composites between solid electrolytes, and composites of solid electrolytes with electrolytes, separators, and electrodes, also hold significant importance. Solid electrolytes can be compounded internally, and they can also be compounded with liquid electrolytes. Using polymers as the skeleton network of the electrolyte, lithium salts and plasticizing electrolytes distributed within the polymer matrix as the main lithium-conducting medium result in gel-state electrolytes. The ionic conductivity of such electrolytes can reach 10E-3 S/cm at room temperature, and the assembled batteries can be thinned, increasing volumetric energy density. Lastly, the combination of solid electrolytes with the solid components of traditional liquid lithium-ion batteries has also yielded positive results. Researchers have used NASICON-type solid electrolyte porous Li1.3Al0.3Ti1.7(PO4)3 as a coating layer, compounded with polyethylene separators, and filled/doped LiTFSI in PEO in the pores and outside of the composite separator to obtain a composite solid electrolyte. This solid electrolyte, when combined with a lithium iron phosphate cathode and a lithium metal anode to form a solid-state battery, shows almost no capacity decay after 200 cycles at 0.2C rate at 60°C; and the battery remains very safe after folding, shearing, and other operations. Researchers believe that this “polymer-ceramic-polymer” membrane structure is strong, and a mixed conduction interface is formed at the electrolyte-lithium anode surface, ultimately suppressing the formation of lithium dendrites and ensuring the stability and safety of battery cycling. The processability of solid-state batteries is an important part of their engineering and commercial application. Researchers have analyzed the production processes of typical lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, solid-state batteries, and lithium-air batteries (with overlapping concepts but generality is not lost). From the front-end processes, the process of solid-state batteries is not much different from liquid batteries overall, but the application of lithium metal anodes and the mixing and coating treatment of solid electrolytes require additional process steps. From the middle and back-end segments, solid-state batteries require pressure or sintering, and there is no need for liquidation (hybrid solid-liquid batteries are similar to conventional liquid batteries and still require liquidation). Although the specific technical routes (such as the choice of solid electrolytes and electrode materials, battery production processes, etc.) are still unclear, as challengers, the strategy of solid-state battery industry practitioners is actually clear: to leverage and strengthen some advantages in safety, strive to gain an advantage in energy density, further optimize battery rate, cycle life, and processability, and consolidate the core potential customers in scenarios where solid-state batteries have an advantage (special applications with extreme safety requirements; high-end electric vehicles with long endurance and high safety as selling points). If subsequent performance parameters and cost control capabilities make effective progress, the market space will gradually expand, and it may even become a key technological route for lithium batteries.
1.Pre-lithiation, the desire to fully utilize the active lithium introduced during the battery production process, due to various reasons, cannot all play the role of charge carriers. The irreversible loss of lithium during formation and subsequent cycling processes is one of the important reasons for the degradation of battery performance. Compensating for lithium loss through various means, optimizing the N/P ratio of the battery, and enhancing the actual energy density and lifespan of the battery is a practical choice in situations where it is needed.
Pre-lithiation mainly addresses the lithium consumption of the SEI film on the anode surface, and applying an anode lithium supplement is the most common method for anode lithium supplementation. The first method that comes to mind is adding reducing lithium powder to the anode. Given lithium’s high capacity of up to 3860mAh/g, typically only a small amount needs to be added to achieve the desired lithium supplementation effect. Considering lithium’s extremely high chemical reactivity, researchers usually perform surface stabilization treatment on lithium powder. However, lithium powder can reduce conventional electrolytes, binders, dispersants, and other necessary auxiliary components in the lithium battery production process, so further surface modification is also necessary. Lithium powder also has a considerable level of safety risks, and the standards for its production, transportation, and application with solvents and binders are very strict. Similar to reducing lithium powder, directly contacting the anode with lithium metal (such as lithium foil) can also serve as a pre-lithiation method. At this point, the degree of pre-lithiation is relatively difficult to control, and the technical requirements for lithium foil are also high. Lithium-related alloys (alloys formed by lithium and group IV metals) are also used for lithium supplementation: they have a low lithium voltage, high capacity, and possibly better chemical stability than lithium alone. For example, lithium-silicon alloys coated with artificial SEI can be stable in dry air (but not in humid air). Lithium-related alloys can be added as powders or used as foil materials for contact pre-lithiation. The issues with these lithium supplements are similar to those of metallic lithium powder, and chemical reactivity becomes a barrier to simplifying production processes and safe, large-scale applications. Pre-lithiation through direct contact between the lithium supplement and the anode also requires effective utilization of the lithium source. Unutilized reducing lithium sources may transform into dead lithium, hindering the diffusion and mass transfer of lithium ions in the anode, and can even lead to lithium plating. Some research work indicates that increasing the density of “electronic pathways” at the interface between the anode lithium supplement and the anode, such as replacing mechanical roller pre-lithiation with vacuum thermal evaporation pre-lithiation, can enhance the pre-lithiation effect.
In addition, researchers have also conducted studies on chemical pre-lithiation (treating the anode with a reducing agent containing lithium to transport active lithium to the anode material during redox reactions) and electrochemical pre-lithiation (forming a system with lithium foil, electrolyte, and anode, and applying an external voltage to actively diffuse lithium ions through the electrolyte to the anode to complete the pre-lithiation process). Considering that the consumption of active lithium mainly occurs on the anode side, the method of replenishing lithium on the anode is often referred to as “direct replenishment.” Correspondingly, replenishing lithium on the cathode is also known as “indirect replenishment,” as it requires charging to “push” the excess lithium ions (and electrons) from the cathode replenishment agent or lithium-rich cathode to the anode. There are more options for cathode replenishment agents, including lithium oxides, nitrides, sulfides, metal salts, etc. Unlike anode pre-lithiation, which can use doping mixing or foil contact methods, or even vapor deposition, cathode pre-lithiation often adopts direct doping mixing of replenishment agents into the material system or directly using lithium-rich cathodes, which has a higher compatibility with existing lithium battery manufacturing processes. The specific capacities of different cathode replenishment agents vary; generally, the higher the lithium content, the higher the capacity. Considering both capacity and processability, lithium-rich composite oxides such as lithium-rich nickelate (Li2NiO2) and lithium-rich ironate (Li5FeO4) are commonly used cathode replenishment agents. Overall, the chemical stability of cathode replenishment agents is higher than that of anode replenishment agents (even lithium oxide with strong alkalinity is more stable than metallic lithium), but there are also issues such as gelation during slurry preparation; the pre-lithiation process may produce gas, and if the reaction is incomplete, the battery may experience swelling during subsequent cycling; after the active lithium is removed from the replenishment agent, there are still residual low-capacity materials (which are quite different from lithium metal, lithiated graphite, and lithiated silicon materials), and the impact on the overall battery performance still needs further evaluation; the specific synthesis, modification, and construction of supporting material systems are also underway. Generally speaking, in situations that emphasize safety and process compatibility, and where the demand for replenishment capacity is not high, cathode replenishment is more suitable. When a large capacity replenishment is needed, anode replenishment is more suitable. We can also look at it from another angle: strong reducing replenishment agents pay attention to safety; other replenishment agents pay attention to gas production; all replenishment agents must pay attention to processability, overall battery performance, and overall cost. Relatively speaking, cathode replenishment agents based on various lithium-rich metal salts (and various modified optimization methods) and their applications are more mature; anode replenishment agents based on metallic lithium (and various modified optimization methods) and the use of metallic lithium/neutral lithium compounds and silicon suboxide at high temperatures for anode-side replenishment are in competition (reducing lithium compounds with surface protection applied to the cathode-side replenishment, replenishment electrolytes, etc., are also not lacking in highlights). Our neutral expectation is that for batteries with very high energy density/long life requirements, pre-lithiation technology has the ability to enhance their competitiveness.
As mentioned earlier, in a battery, electrode materials, electrolytes, and the electrode-electrolyte interface conduct carriers, while electrodes, current collectors, and the external circuit conduct electrons. Effective transmission of lithium ions requires high corresponding bulk and interface ionic conductivity, as well as sufficient contact between the electrolyte and the electrode; effective transmission of electrons requires high corresponding bulk and interface electronic conductivity, so applying conductive agents in electrode materials to improve interface electronic transmission characteristics becomes an important measure to optimize battery performance.
Compared to conventional carbon black, carbon nanotubes significantly enhance the electronic conductivity of battery materials. Of course, a certain degree of compounding, especially the combination of carbon nanotubes and high-end carbon black, is of positive significance. When carbon nanotubes with a higher aspect ratio dominate, the electronic conductivity of the battery material is higher. However, this does not mean that the amount and proportion of high aspect ratio carbon nanotubes can be infinitely increased. Both the comprehensive performance and cost of the battery need to be considered. In the foreseeable future, carbon nanotubes will still be representative of high-end conductive agents, and depending on cost control, they will gradually penetrate into the middle and low-end markets.
As previously mentioned, lithium-ion batteries are the best-performing type of battery; other ionic metal batteries such as those based on sodium are also under consideration by researchers due to their unique characteristics. When lithium resources expand slowly, a significant supply gap emerges, and prices skyrocket to record highs; in the future, the certainty of consumption, power, and energy storage sectors makes the long-term demand for lithium resources almost a foregone conclusion, accelerating the research and application process of sodium-ion batteries. As charge carriers, sodium ions and lithium ions have relatively high similarities. Therefore, it is appropriate to start with the construction of the sodium electrochemical material system from the electrolyte. If we compare the bulk ionic conductivity of sodium-ion battery electrolytes, aqueous systems and organic electrolyte solutions perform better than solid electrolytes (in contrast, the ionic conductivity of some solid electrolytes for lithium-ion batteries can rival that of electrolytic solutions). The ionic conductivity of aqueous electrolytes is high, but the electrochemical window is narrow (the theoretical decomposition voltage of water is 1.23V, compared to over 4V for carbonate systems), which significantly affects the energy density of the battery. The organic electrolyte solution is the most suitable type for sodium-ion batteries. The choice of solvent is similar to that of lithium-ion batteries, both using highly mature carbonate-based solvents; lithium salts are replaced with sodium salts. Graphite, as the anode for lithium-ion batteries, has been widely used in industry due to its low voltage against lithium, high specific capacity (compared to other intercalation materials), and also has the ability to store potassium ions, but it is not suitable as the anode material for sodium-ion batteries. During the same period when lithium-ion batteries left the laboratory in the last century, sodium-ion batteries did not become industrialized, and the lack of a suitable anode was an important reason.
The practical capacities of various sodium-based cathode materials range from tens to over 200mAh/g, with sodium voltages in the range of 2 to over 4V. In addition to the specific capacity-voltage characteristics that directly contribute to energy density, the effective diffusion of sodium ions and the stability of various types (composition, valence state, phase structure, aggregation state) are also important. In summary, for the commercialization of sodium-ion batteries in the short to medium term, the most reasonable material system is a hard carbon (dominant) anode, layered oxides (relatively consistent), Prussian blue-Prussian white or polyanionic (the latter two are controversial) cathodes, organic carbonate-sodium salt electrolyte, separator, and aluminum foil current collector. The key production processes and equipment for sodium-ion batteries using organic electrolytes are exactly the same as those for lithium-ion batteries. In contrast, solid-state batteries (regardless of carrier type), lithium-sulfur batteries, lithium-air batteries, etc., to varying degrees, require changes in production processes. Therefore, we can infer that the main production processes and equipment for sodium-ion batteries will not have significant changes compared to the more mature lithium-ion batteries. Sodium-ion batteries may also have certain advantages over lithium batteries in terms of low-temperature performance and safety, the former relying on simple desolvation, and the latter mainly relying on the dual current collector aluminum foil. It can be seen that “conventional” sodium-ion batteries rely on cost advantages and have their own strengths in performance indicators compared to lithium iron phosphate; high-energy sodium-ion batteries may exceed iron lithium in energy density, but their technology maturity is lower, and it is difficult to have a performance advantage, with high uncertainty (considering the less public information on volumetric energy density, the scaling of high-energy density sodium-ion batteries may be even more difficult). It can also be seen that sodium-ion batteries outperform lead-acid batteries in performance (except for extreme safety), and the cost gap can also be effectively narrowed after full development. Therefore, sodium-ion batteries may become a secondary battery with great development potential besides lithium-ion batteries, playing a role in areas such as two-wheel electric vehicles, low-speed electric vehicles, energy storage, and even power.
In addition to +1 valence carriers such as lithium and sodium, the academic community has also studied batteries using magnesium, aluminum, zinc, etc., as carriers. Overall, the difficulty of the cathode and electrolyte for high-valence carriers is very high, and the anode also requires modifications to varying degrees, with a long way to go before practical application. Researchers have discussed the improvement of electrode-electrolyte interface kinetics and suppression of side reactions by chelating magnesium ions in an organic system, achieving a theoretical energy density of 412Wh/kg for magnesium-ion batteries.
Researchers have used vanadium-based oxides, represented by vanadium pentoxide, as cathode materials for zinc storage in aqueous systems, demonstrating high specific capacity (300mAh/g), good rate performance, and a long cycle life (~1000 cycles). 3. All-vanadium redox flow batteries, which use hydrogen ions as the charge carrier, have both distinct advantages and disadvantages. The active materials in all-vanadium redox flow batteries are aqueous solutions of vanadium in different valence states (liquids) stored in tanks. Energy is stored and released by balancing charges through a proton exchange membrane and an external circuit, with the uniformity of the electrode liquid achieved by pumping it to flow (fluid). Since its invention in the 1970s, all-vanadium redox flow batteries have shown excellent cycle life (over ten thousand times) and safety. However, they also have drawbacks such as low energy density, lower energy cycle efficiency, and high initial costs.
VII. Giants Carry the Cauldron, Sailing Far Across the Waves
1.Nobel Prize, The “First Victory” in the Energy Revolution
Looking back at the invention and improvement of lithium-ion batteries, we have witnessed a magnificent “Age of Exploration”. From the perspective of scientific invention, from the proposal of the concept of lithium batteries to the completion of the basic material system, it has taken nearly a century. During this century, a large number of “disruptive” research results have been born. From the perspective of technological practice, many companies have paid a lot of effort for the successful practice of lithium-ion batteries, even including several safety accidents represented by the first application of lithium metal batteries. When we review the history of lithium battery development, what we see is the establishment of the scientific and technological main line over decades or even a century, the pulse of policy rhythm over a few years to a dozen years, and the changes in market temperature over a year or even a smaller period of time; technology, policy, and market promote each other, ultimately profoundly changing people’s lives. Consumer batteries with the highest tolerance for cost took the lead in leaving nickel-cadmium and nickel-metal hydride batteries far behind, making it possible for laptops to “use from New York to Los Angeles”, making it possible for smartphones to control increasingly powerful SOCs, and “charging for five minutes, talking for two hours”. The power battery with high comprehensive requirements for cost and performance took over, and in front of the great, huge, and powerful traditional fuel car industry, it opened a brand new era of electric vehicles. The energy storage battery with higher cost requirements is brewing, and it is competing with the hegemon of energy storage, pumped storage technology, and has a momentum of surpassing. In 2019, the Nobel Prize in Chemistry was awarded to Professor John B. Goodenough of the University of Texas at Austin, Professor M. Stanley Whittingham of Binghamton University in New York, and chemist Akira Yoshino of Asahi Kasei Corporation in Japan, in recognition of their outstanding contributions to the research and development of lithium batteries. The selection committee stated, “This light, rechargeable, and powerful battery is now widely used in various fields, from mobile phones to laptops to electric vehicles. It can also store a large amount of energy from the sun and wind, making a fossil fuel-free society possible.”
2.Forward with Courage, a Shining Future
Looking ahead, there is room for progress in battery technology. In the short to medium term, we will witness the advancement of super high-nickel/higher voltage ternary cathodes, lithium iron manganese phosphate cathodes, higher silicon content anodes, pre-lithiation techniques, electrolytes and conductive agents with optimized compositions and better overall performance, guiding the current lithium-ion battery systems towards a realm of “N specializations and multiple capabilities.” In the medium to long term, spinel nickel manganese oxide cathodes, lithium-rich manganese-based cathodes, lithium metal anodes, high-performance electrolytes, and high-performance solid electrolytes are beckoning us; sodium-ion batteries are expected to become an important niche route with high cost-effectiveness as their selling point; other batteries also have the potential for breakthrough progress. “Next-generation batteries” are bright and illuminated. One day, lithium-ion batteries (and even various new types of batteries) will reach the performance-cost ceiling, becoming a matter of course, just as many great technological achievements have experienced before. Being accustomed to fire, what is taken for granted is warmth. Being accustomed to agriculture, what is taken for granted is satiety. Being accustomed to steel, what is taken for granted is resilience. Being accustomed to industrial products, what is taken for granted is abundance. Being accustomed to ships, trains, and airplanes, what is taken for granted is convenience. Being accustomed to high-performance, low-cost batteries, what is taken for granted is the victory call of the energy revolution, the deep call of the human community’s destiny. All that is taken for granted, converges into creation. That will certainly be remembered by the history of civilization, and even guide the progress of civilization, a shining future.
Reposted from the WeChat public account 3060
https://mp.weixin.qq.com/s/uSoF-CTKD2xaZP3IHFjR4A
