Each type of lithium battery has an optimal charging current value under different state and environmental parameters. So, from the perspective of battery structure, what factors affect this optimal charging value?
Microscopic process of charging
Lithium-ion batteries are known as "rocking chair" batteries. Charged ions move between the positive and negative electrodes, transferring charge to power external circuits or be charged by an external power source. Specifically, during charging, an external voltage is applied to the battery's electrodes. Lithium ions are extracted from the positive electrode material and enter the electrolyte, while excess electrons are generated and move through the positive electrode current collector and external circuit to the negative electrode. In the electrolyte, lithium ions move from the positive to the negative electrode, passing through the separator to reach the negative electrode. They then pass through the SEI film on the negative electrode surface and embed into the graphite layered structure of the negative electrode, where they combine with electrons.
Throughout the entire process of ion and electron movement, battery structures that influence charge transfer, whether electrochemical or physical, will affect fast-charging performance.
Fast charging and its requirements for various battery components
To improve the power performance of batteries, efforts need to be made in every aspect of the battery, including the positive electrode, negative electrode, electrolyte, separator, and structural design.
positive electrode
In fact, almost all cathode materials can be used to manufacture fast-charging batteries. The main performance requirements include conductivity (to reduce internal resistance), diffusion (to ensure reaction kinetics), lifespan (no explanation needed), safety (no explanation needed), and appropriate processing performance (the specific surface area should not be too large to reduce side reactions and serve safety).
Of course, the problems to be solved for each specific material may differ, but the common cathode materials can generally meet these requirements through a series of optimizations. However, there are also differences between different materials:
A. Lithium iron phosphate may focus more on solving problems related to conductivity and low temperature. Carbon coating, moderate nano-sizing (note, moderate, not the simple logic of the finer the better), and surface treatment to form ionic conductors are the most typical strategies.
B. Ternary materials already have good electrical conductivity, but their reactivity is too high. Therefore, there is little work on nano-scale production of ternary materials (nano-scale production is not a panacea for improving material performance, especially in the field of batteries, where it can sometimes have many adverse effects). The focus is more on safety and suppressing side reactions (with the electrolyte). After all, one of the biggest weaknesses of ternary materials is safety, and the recent frequent battery safety accidents have placed higher demands on this aspect.
C. Lithium manganese oxide batteries place greater emphasis on lifespan, and there are currently many lithium manganese oxide fast-charging batteries on the market.
negative electrode
When a lithium-ion battery is charged, lithium migrates towards the negative electrode. However, the excessively high potential brought about by the high current of fast charging will cause the negative electrode potential to become even more negative. At this time, the pressure on the negative electrode to rapidly accept lithium will increase, and the tendency to form lithium dendrites will increase. Therefore, during fast charging, the negative electrode must not only meet the kinetic requirements of lithium diffusion, but also solve the safety problems caused by the increased tendency of lithium dendrite formation. So the main technical difficulty of fast charging cells is actually the embedding of lithium ions in the negative electrode.
A. Currently, graphite remains the dominant anode material in the market (accounting for approximately 90% of the market share). The fundamental reason is simple: it's inexpensive, and graphite boasts superior overall processing performance and energy density with relatively fewer drawbacks. However, graphite anodes do have their problems. Their surface is quite sensitive to electrolytes, and lithium intercalation exhibits strong directionality. Therefore, surface treatment of graphite to improve its structural stability and promote lithium-ion diffusion on the substrate is a primary area of focus.
B. Hard carbon and soft carbon materials have also seen considerable development in recent years: hard carbon materials have high lithium intercalation potential and good reaction kinetics due to the presence of micropores in the material; while soft carbon materials have good compatibility with electrolytes, and MCMB materials are also very representative. However, both hard and soft carbon materials generally have low efficiency and high cost (and it is unlikely that they will become as cheap as graphite from an industrial perspective). Therefore, their current usage is far less than that of graphite, and they are used more in some special batteries.
C. What about lithium titanate? In short: the advantages of lithium titanate are high power density and relatively high safety. However, its disadvantages are also obvious: very low energy density and high cost per watt. Therefore, while lithium titanate batteries are a useful technology with advantages in specific situations, they are not very suitable for many applications where cost and driving range are critical.
D. Silicon anode materials are an important development direction, and Panasonic's new 18650 battery has already begun the commercialization process of such materials. However, achieving a balance between pursuing performance through nano-scale technology and the battery industry's general micron-scale requirements for materials remains a challenging task.
diaphragm
For power batteries, high-current operation places higher demands on their safety and lifespan. Membrane coating technology is indispensable, and ceramic-coated separators are rapidly gaining popularity due to their high safety and ability to absorb impurities in the electrolyte, especially for their significant improvement in the safety of ternary lithium batteries.
The main system currently used for ceramic diaphragms is to coat alumina particles onto the surface of traditional diaphragms. A more novel approach is to coat solid electrolyte fibers onto the diaphragm. This results in a diaphragm with lower internal resistance, better mechanical support from the fibers, and a lower tendency to clog the diaphragm pores during service.
The coated diaphragm has good stability and is not prone to shrinkage and deformation leading to short circuits, even at high temperatures. Jiangsu Qingtao Energy Company, with technical support from Academician Nan Cewen's research group at the School of Materials Science and Engineering, Tsinghua University, has done some representative work in this area.
electrolyte
Electrolyte has a significant impact on the performance of fast-charging lithium-ion batteries. To ensure the stability and safety of the battery under high current during fast charging, the electrolyte must meet the following characteristics: A) It cannot decompose; B) It must have high conductivity; C) It must be inert to the positive and negative electrode materials, and cannot react with or dissolve them.
To meet these requirements, additives and functional electrolytes are crucial. For example, the safety of ternary lithium fast-charging batteries is greatly affected by additives, and various high-temperature resistant, flame-retardant, and overcharge-protective additives must be added to improve their safety to some extent. The long-standing problem of high-temperature gas expansion in lithium titanate batteries also needs to be addressed with high-temperature functional electrolytes.
Battery structure design
A typical optimization strategy is stacked vs. wound. The electrodes of a stacked battery are essentially connected in parallel, while those of a wound battery are connected in series. Therefore, the former has much lower internal resistance and is more suitable for power applications.
Alternatively, efforts can be made to address internal resistance and heat dissipation issues by increasing the number of tabs. Furthermore, using high-conductivity electrode materials, employing more conductive agents, and coating thinner electrodes are also strategies to consider.
In summary, factors affecting the movement of internal charges and the rate of insertion into electrode pores all influence the fast charging capability of lithium batteries.
The Future of Fast Charging Technology
Whether fast charging technology for electric vehicles is the future or just a fleeting phenomenon is currently a matter of much debate and no definitive conclusion. As an alternative solution to range anxiety, it should be considered alongside battery energy density and overall vehicle operating costs.
Energy density and fast charging performance are essentially incompatible goals within the same battery; they cannot be simultaneously achieved. Currently, the pursuit of higher battery energy density is the mainstream approach. When energy density is high enough, a vehicle can carry a large enough battery capacity to avoid so-called "range anxiety," reducing the need for faster charging speeds. However, with larger capacities, if the cost per kilowatt-hour of the battery is not low enough, consumers must choose whether to purchase sufficient capacity to alleviate "range anxiety." This perspective gives fast charging its value. Another aspect is the cost of fast charging infrastructure, which is undoubtedly part of the overall cost of promoting electrification in society.
Whether fast charging technology can be widely adopted, which technology—energy density or fast charging technology—develops faster, and which technology can reduce costs more drastically, may play a significant role in its future prospects.
