Rechargeable battery technology has continued to advance over the past few decades, and energy density is one of its key performance indicators. Now, Li-ion batteries have surpassed lead-acid and NiMH batteries as a competitive commercial battery, but Li-ion battery performance still needs to be significantly improved to extend the operating time of mobile electronic devices, the range of electric vehicles, etc. In this paper, we provide an overview of post-lithium materials and systems, discussing how each post-lithium technology works, the problems that exist, and evaluating certain battery materials, where we divide the battery system into short-term and long-term technology options.
The general metrics used to evaluate rechargeable batteries include multiplier performance, cost, cycle life, and operating temperature range, but increased energy density has driven advances in battery technology over the past 150+ years, such as lead-acid (1850s), nickel-cadmium (1890s), nickel-metal hydride (1960s), and lithium-ion (1991- ) batteries. Nowadays, the increased power consumption of mobile electronic devices and the extended range of electric vehicles require higher energy density lithium-ion batteries, while the global electric vehicle market is growing more slowly than expected five years ago, reflecting the serious challenges faced by the battery industry. Battery energy density depends mainly on the specific capacity and operating voltage of cathode and anode materials, so battery active materials have been the main research hotspot in the industry in recent years. There is less and less room for technological progress in other components of batteries, including diaphragms, binders, conductive agents, shells, and the various components of electrolyte. Embedded battery materials usually have fewer lithium ion embedding sites and thus limited energy density. In other words, in order to significantly increase the energy density of the battery, it is necessary to go beyond the traditional embedding reaction mechanism and continuously explore new electrochemical reaction systems. Therefore, alloying, conversion reaction-type electrode materials or gas-phase reaction materials are attracting attention because of their potential to surpass the energy density of embedded materials. These new electrochemical systems for energy storage are called post-lithium-ion batteries. The theoretical energy density of post-Li-ion battery system is higher, but the short lifetime is also one of its serious problems, and the main technical challenge is to overcome its poor reversibility. The poor reversibility of post-Li-ion system is mainly due to the unstable phase transition of active materials and uncontrollable side reactions at the electrode/electrolyte interface, so the electrode structure and electrolyte components need to be continuously optimized.
Commercial battery configuration: In large-scale applications, a certain number of cells are assembled into a module, and the design of the module depends largely on the size and shape of the product as well as the internal connection circuit, safety requirements, temperature control, etc. Nowadays, there are three types of commercial batteries: cylindrical, square and soft pack batteries. Among commercial lithium-ion batteries, a typical 18650 cylindrical cell has a volumetric energy density of about 600-650 Wh/L, which is 20% higher than the volumetric energy density of comparable square and soft pack cells, due to the more dense assembly stack in a cylindrical cell. In many practical applications, volumetric energy density is more important because most battery packs have to be designed for a limited volume. Although the volumetric energy density of cylindrical cells is higher, square and soft pack cells are also widely used because of their higher design freedom. Therefore, the volumetric energy density values in the following are based on a soft pack cell with a length, width, and thickness of 300*100*10 mm, in which n negative-diaphragm-positive layers are stacked. The volumetric energy density of a conventional LiCoO2-graphene cell in this soft pack cell is 491 Wh/L, which is equivalent to the volumetric energy density level of many commercially available batteries today. Short-term technology options.
In the pre-lithium era, active electrode materials have been developed to levels that can be partially used in today's products, and continued research on these active materials will continue to increase their content in electrodes.
Silicon anode: Natural and synthetic graphite have long been used as the primary lithium anode active material and are also widely used as a reference for evaluating other emerging anode materials. Silicon anode has been widely studied because of its high theoretical capacity (4200 mAh/g) and suitable working potential (about 0.3 V for lithium potential) has been considered as one of the anode materials with very promising applications. As early as the 1970s, Argonne National Laboratory and General Motors did extensive research on silicon anode with the aim of overcoming the problem of cycle performance degradation due to the huge volume expansion (>300%) during the cyclic lithiation of the battery. Improvements in electrode structure and binder design were used to simultaneously address the problems of pulverization of the active material and shedding from the collector fluid to improve battery cycling performance. Another problem of silicon cathode is the difficulty of forming a stable SEI film on the surface, which prevents effective protection of the electrode surface, and this interface problem can be partially solved by selecting a suitable electrolyte. From the perspective of electrode structure, the synthesis of conductive nano-mesoporous complexes by mesoporous carbon, graphene and carbon nanotubes can effectively alleviate the volume expansion problem of silicon anode and significantly improve the cycling performance. In contrast, SiOx (x ≈ 1) is the first silicon based anode material to be industrially applied because it can be produced on a large scale by gas and liquid phase reaction processes with reliable quality and reasonable price (about $100/kg compared to $10-20/kg for graphene). However, SiOx is usually mixed with graphene, generally at less than 5%, reflecting the less mature technology for silicon anode applications. the main problem with SiOx is the low first coulomb efficiency, which is typically only 50-60% without carbon coating on the surface. The low coulomb efficiency results in the need to load too much cathode material and therefore lowers the overall cell energy density. Designing active silicon structures requires both long cycle life and high first coulomb efficiency, but these two metrics are contradictory. The buffer matrix and mesoporous structure used to mitigate the volume expansion of the silicon cathode to achieve long cycle life is detrimental to the first coulomb efficiency because of excessive irreversible lithium ion consumption and interfacial reactions. In this case, pre-lithiation of silicon cathode containing materials by solution or electrochemical processes is an effective means to solve the first coulomb problem without compromising the cycle life. Polymer binders are also important for improving the cycling performance of silicon cathodes, and traditional PVDFs are being replaced by new binders such as cross-linked polymers, self-healing polymer matrices, carbohydrate-based polymers, and electronically conductive polymers. These new binders can effectively maintain the electrode structure through three-dimensional inter-chain interactions during repeated volume changes of the silicon anode. The basic requirements for new binder research are: first, to maintain the new binder function at a lower binder content, which is now usually less than 5% in industry; second, to develop a hybrid polymer binder capable of forming an effective bond to both silicon and graphene; and finally, to optimize the binder function so that the bonding to silicon is enhanced and the domain limitation to lithium ions is weakened, thus facilitating the improvement of Coulomb efficiency. In addition, supramolecular chemistry through the formation of hydrogen bonds, ionic bonds and π-π interactions is also expected to be used to improve the bonding between the polymer and the active particles.
Based on the soft pack cell dimensions in the figure above, with a high capacity LiNi0.8Co0.1Mn0.1O2 cathode and a graphene:SiOx=1:1 silicon-carbon complex cathode, the energy density is expected to increase by 7.6% from 513 Wh/L (graphene) to 552 Wh/L (silicon-carbon complex), at which point the volume expansion of this complex cathode is approximately 110%. If graphene:pre-lithiated SiOx=1:1 is used as the negative electrode, the corresponding energy density can be increased to 628 Wh/L. This is because the loading of the positive electrode will be reduced after pre-lithiation to make the capacity of the positive and negative electrodes balanced. This energy density value demonstrates that achieving high first Coulomb efficiency and suppressing electrode expansion are essential for obtaining high volumetric energy density cells using the high capacity properties of silicon cathodes.
Layered nickel-rich, lithium-based, and manganese-rich cathode materials: The next generation of emerging cathode materials in the lithium-ion battery field are nickel-rich layered materials, whose origins can be traced back to early research work by Dahn and Thackeray's group. Two other common layered materials that have been around for a long time are LiCoO2 (~145 mAh/g) and LiNi1/3Co1/3Mn1/3O2 (~153 mAh/g). In fact, nickel-rich materials are already being commercialized, and LiNi0.8Co0.15Al0.05O2 is a typical representative. In addition, the introduction of manganese on top of this layered transition metal oxide is beneficial to improve safety and multiplicity performance, so layered high nickel ternary cathode materials LiNixMnyCozO2 (x + y + z = 1) have been developed successively, where x > 0.6. All these layered cathode materials share a common body framework structure in which lithium and transition metals are alternately repeated They are arranged in a cubic dense stacking framework of oxygen atoms. In the case of lithium-based and manganese-rich phase materials, the increase in capacity is due to the presence of Li2MnO3 (in addition to the active LiMO2 lamellar phase), which is activated during lithium removal and oxygen reduction during the first charge. The activation process occurring at voltages greater than 4.7 V is able to provide specific capacities higher than 250 mAh/g for lithium-based and manganese-rich layered cathode materials. Despite their high capacities, both layered materials often face rapid capacity decay during cycling, mainly due to unstable structures and surface state changes. During charging, these two layered materials tend to transform into a more thermodynamically stable spinel phase after the lithium ions are taken off. Crystallographically, this spontaneous transformation is caused by the migration of transition metals into the octahedral sites in the lithium atomic layer where lithium-transition metal mixing occurs, leading to a decrease in the voltage profile and capacity during discharge. This structural instability is also caused by unstable interfaces, where the transition metals from layered nickel-rich, lithium-based, and manganese-rich cathode materials are prone to dissolution, generating irreversible and inactive interfacial compounds. In addition, these nucleophilic cathode materials react interfacially with substances in the electrolyte (e.g. HF, PF5, etc.) to form surface films, which increase the electrode impedance. Then, the formation of spinel phases and structures with concentration gradients on the surface achieved by transition metal doping and surface coating of AlF3, Al2O3, AlPO4, TiO2 and carbon can effectively overcome their failure mechanisms. However, the capacity decay due to thermodynamic phase transition is difficult to progress from the perspective of improving electrolytes, and the actual matching electrolytes for these two types of layered materials is basically based on cost considerations.
Using graphene as the anode active material, LiNi0.8Co0.1Mn0.1O2 or Li1.19Mn0.54Ni0.13Co0.12Ru0.01O2 instead of the conventional lithium cobaltate LiCoO2 as the cathode material, the volumetric energy density of the battery is correspondingly increased from 491 Wh/L to 513 and 524 Wh /L (by 4.4% and 6.7%, respectively). The increase in volumetric energy density is less than expected based on gram capacity calculations due to its lower vibronic density (3.0-3.3 g/cm3) compared to 4.0 g/cm3 for LiCoO2, which also points to a more practical direction for the development of nickel-rich, lithium-based, and manganese-rich cathode materials: increasing the density of the active powder and electrode sheet. . And if paired with a graphene:SiOx=1:1 silicon carbon complex cathode, the corresponding bulk energy density increases to 552 and 661 Wh/L, respectively.
In conclusion, the technical route to achieve the short-term goal of high energy density lithium-ion batteries is to use silicon-carbon composites for the negative electrode and high nickel materials for the positive electrode, which is what the industry is striving to do at this stage.
