Factors influencing the fast charging capability of lithium-ion batteries
Each lithium battery has an optimal charging current value under different state parameters and environmental parameters. Then, from the perspective of battery structure, what are the factors that affect this optimal charging value.
The microscopic process of charging
Lithium batteries are known as "rocking chair" batteries, in which charged ions move between positive and negative electrodes to transfer charges to power external circuits or charge from an external power source. In the specific charging process, the external voltage is applied to the two poles of the battery, and the lithium ions are deintercalated from the positive electrode material and enter the electrolyte. At the same time, excess electrons are generated through the positive electrode current collector and move to the negative electrode through the external circuit; lithium ions are in the electrolyte. It moves from the positive electrode to the negative electrode, and passes through the separator to the negative electrode; the SEI film passing through the surface of the negative electrode is embedded in the graphite layered structure of the negative electrode and combines with electrons.
The structure of the battery, whether electrochemical or physical, that affects the charge transfer throughout the ionic and electronic operation will have an impact on the fast charging performance.
Fast charging, requirements for each part of the battery
For batteries, if you want to improve power performance, you need to work hard in all aspects of the battery as a whole, including positive electrodes, negative electrodes, electrolytes, diaphragms, and structural design.
positive electrode
In fact, almost all kinds of cathode materials can be used to make fast-charging batteries. The main performances that need to be guaranteed include conductance (reduce internal resistance), diffusion (guarantee reaction kinetics), life (no need to explain), safety (no need for Explanation), 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 be different, but our common cathode materials can meet these requirements through a series of optimizations, but different materials are also different:
A. Lithium iron phosphate may focus more on solving the problems of electrical conductivity and low temperature. Carbon coating, moderate nano-ization (note that it is moderate, definitely not the simple logic of finer is better), and the formation of ionic conductors on the surface of particles are the most typical strategies.
B. The electrical conductivity of the ternary material itself is relatively good, but its reactivity is too high, so the ternary material is rarely nano-sized (nano-chemical is not an antidote for the improvement of material performance, especially in the field of batteries. There are sometimes a lot of adverse effects), and more attention is paid to safety and inhibition of side reactions (with electrolyte), after all, one of the key points of the current ternary materials is safety, and the recent frequent battery safety accidents are also in this regard. put forward higher requirements.
C. Lithium manganate pays more attention to life. At present, there are many lithium manganate series fast-charging batteries on the market.
negative electrode
When a lithium-ion battery is charged, lithium migrates to the negative electrode. The high potential brought by the high current of fast charging will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode to quickly accept lithium will increase, and the tendency to generate lithium dendrites will increase. Therefore, the negative electrode must not only meet the lithium diffusion requirements during fast charging Therefore, the main technical difficulty of fast charging cells is actually the insertion of lithium ions in the negative electrode.
A. At present, the dominant negative electrode material in the market is still graphite (accounting for about 90% of the market share). There is no other fundamental reason - cheap, and the comprehensive processing performance and energy density of graphite are relatively good, and the shortcomings are relatively few. . Of course, the graphite negative electrode also has problems. Its surface is sensitive to the electrolyte, and the intercalation reaction of lithium has a strong directionality. Therefore, it is mainly necessary to perform graphite surface treatment to improve its structural stability and promote the diffusion of lithium ions on the substrate. direction.
B. Hard carbon and soft carbon materials have also developed a lot in recent years: hard carbon materials have high lithium intercalation potential, and there are micropores in the material, so the reaction kinetics are good; while soft carbon materials have good compatibility with electrolytes, MCMB The material is also very representative, but the efficiency of hard and soft carbon materials is generally low and the cost is high (and it is not very hopeful from an industrial point of view to be as cheap as graphite), so the current consumption is far less than that of graphite, and it is more used in some special on the battery.
C. How about lithium titanate? To put it simply: the advantages of lithium titanate are high power density and safety, and the disadvantages are also obvious, the energy density is very low, and the cost calculated by Wh is very high. Therefore, the viewpoint of lithium titanate battery is a useful technology with advantages in certain occasions, but it is not suitable for many occasions with high requirements on cost and cruising range.
D. Silicon anode material is an important development direction. Panasonic's new 18650 battery has begun the commercial process of such materials. However, how to achieve a balance between the performance pursued by nanotechnology and the general micron-scale requirements of the battery industry for materials is still a challenging task.
diaphragm
For power batteries, high current operation provides higher requirements for their safety and life. Separator coating technology is inescapable. Ceramic-coated separators are rapidly being pushed away because of their high safety and the ability to consume impurities in the electrolyte, especially for the improvement of the safety of ternary batteries.
The main system currently used for ceramic diaphragms is to coat the surface of traditional diaphragms with alumina particles. A relatively novel approach is to coat solid electrolyte fibers on the diaphragm. Such diaphragms have lower internal resistance and better mechanical support for the diaphragm. Excellent, and it has a lower tendency to block the diaphragm pores during service.
The coated diaphragm has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform to cause a short circuit. Jiangsu Qingtao Energy Company, which is technically supported by the research group of Academician Nan Cewen, School of Materials, Tsinghua University, has some representative products in this regard. Work.
Electrolyte
The electrolyte has a great influence on the performance of fast-charging lithium-ion batteries. To ensure the stability and safety of the battery under fast charging and high current, the electrolyte must meet the following characteristics: A) it cannot be decomposed, B) the conductivity must be high, and C) it is inert to the positive and negative materials, and cannot react or dissolve.
If these requirements are to be met, the key is to use additives and functional electrolytes. For example, the safety of ternary fast rechargeable batteries is greatly affected by it, and various additives for high temperature resistance, flame retardant and anti-overcharging must be added to it, in order to improve its safety to a certain extent. The long-standing problem of lithium titanate batteries, high temperature flatulence, also has to be improved by high temperature functional electrolyte.
battery structure design
A typical optimization strategy is stacked VS winding. The electrodes of the stacked battery are equivalent to a parallel relationship, and the winding type is equivalent to a series connection. Therefore, the internal resistance of the former is much smaller, and it is more suitable for power type. occasion.
In addition, you can also work hard on the number of tabs to solve internal resistance and heat dissipation problems. In addition, using high-conductivity electrode materials, using more conductive agents, and coating thinner electrodes are also possible strategies.
In conclusion, the factors that affect the charge movement inside the battery and the rate of intercalated electrode holes will affect the fast charging capability of lithium batteries.
The future of fast charging technology
Whether the fast charging technology of electric vehicles is a historical direction or a flash in the pan, in fact, there are different opinions and no conclusion. As an alternative solution to range anxiety, it is considered on a platform with battery energy density and overall vehicle cost.
Energy density and fast charging performance, in the same battery, can be said to be incompatible in two directions, and cannot have both. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough, a car has enough power to avoid the so-called "mileage anxiety", and the demand for battery rate charging performance will be reduced; at the same time, if the power is large, if the battery cost per kWh is not low enough, then whether it can be used Ding Kemao's purchase of electricity that is "not anxious" requires consumers to make a choice. Thinking about it this way, fast charging has the value of existence. Another angle is the cost of fast charging facilities, which is of course part of the cost of promoting electrification in the whole society.
Whether the fast charging technology can be promoted on a large scale, who develops faster in energy density and fast charging technology, and which of the two technologies reduces costs, may play a decisive role in its future.
