9V rechargeable battery

　　Conditions for fast charging of 9V rechargeable battery

　　Each type of lithium battery has an optimal charging current value under different state parameters and environmental parameters. So, from the perspective of battery structure, what are the factors that affect this optimal charging value?

　　Microscopic process of charging

　　Lithium batteries are called "rocking chair" batteries. Charged ions move between the positive and negative electrodes to achieve charge transfer, power the external circuit 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 lithium ions are deintercalated from the positive electrode material and enter the electrolyte. At the same time, excess electrons are generated and pass through the positive electrode current collector and move to the negative electrode through the external circuit; lithium ions move from the positive electrode to the negative electrode in the electrolyte, pass through the diaphragm to the negative electrode; through the SEI film on the surface of the negative electrode, they are embedded in the negative electrode graphite layered structure and combined with electrons.

　　In the entire process of ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will affect the fast charging performance.

　　Fast charging, requirements for various parts 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, mainly including the positive electrode, negative electrode, electrolyte, diaphragm and structural design.

　　Positive electrode

　　In fact, almost all positive electrode materials can be used to manufacture fast-charging batteries. The main performances that need to be guaranteed include conductivity (reducing internal resistance), diffusion (ensuring reaction kinetics), life (no explanation needed), safety (no explanation needed), and appropriate processing performance (specific surface area should not be too large, reducing side reactions, serving safety). Of course, the problems to be solved for each specific material may be different, but our common positive electrode 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 problems in conductivity and low temperature. Carbon coating, moderate nano-sizing (note that it is moderate, and it is definitely not a simple logic that the finer the better), and surface treatment of particles to form ion conductors are the most typical strategies.

　　B. The conductivity of ternary materials is relatively good, but their reactivity is too high. Therefore, ternary materials are rarely nano-scaled (nano-scale is not a panacea for improving material performance, especially in the field of batteries, there are sometimes many adverse effects). More attention is paid to safety and inhibiting side reactions (with electrolytes). After all, one of the key factors of ternary materials is safety. The recent frequent battery safety accidents have also put forward higher requirements in this regard.

　　C. Lithium manganese oxide is more concerned about lifespan. There are also many lithium manganese oxide fast-charging batteries on the market.

　　Negative electrode

　　When 9V rechargeable battery are charged, lithium migrates to the negative electrode. The excessive potential caused by the high current of fast charging will cause the negative electrode potential to be more negative. At this time, the pressure on the negative electrode to quickly accept lithium will increase, and the tendency to generate 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 generation. Therefore, the main technical difficulty of fast charging cells is actually the embedding of lithium ions in the negative electrode.

　　A. The dominant negative electrode material in the market is still graphite (accounting for about 90% of the market share). The fundamental reason is that it is cheap (you complain about the high price of batteries every day, exclamation mark!), and graphite has excellent comprehensive processing performance and energy density, and relatively few disadvantages. Of course, there are also problems with graphite negative electrodes. Its surface is more sensitive to electrolytes, and the lithium embedding reaction has a strong directionality. Therefore, the main direction of efforts is to treat the graphite surface, improve its structural stability, and promote the diffusion of lithium ions on the base.

　　B. There have been many developments in hard carbon and soft carbon materials in recent years: hard carbon materials have high lithium embedding potentials, and there are micropores in the materials, so the reaction kinetics are good; while soft carbon materials have good compatibility with electrolytes, and MCMB materials are also very representative, but hard and soft carbon materials are generally inefficient and costly (and it is unlikely to be as cheap as graphite from an industrial perspective), so the current usage is far less than that of graphite, and it is more used in some special batteries.

　　C. Some people will ask the author how lithium titanate is. To put it simply: the advantages of lithium titanate are high power density and safety, but the disadvantages are also obvious, the energy density is very low, and the cost is very high when calculated by Wh. Therefore, the author's view on lithium titanate batteries has always been: it is a useful technology with advantages in specific occasions, but it is not very suitable for many occasions with high requirements for cost and mileage.

　　D. Silicon negative electrode materials are an important development direction. Panasonic's new 18650 battery has begun the commercialization process of such materials. However, how to achieve a balance between the pursuit of performance in nano-scale and the general micron-level requirements of the battery industry for materials is still a relatively challenging task.

　　Diaphragm

　　For power batteries, high current operation provides higher requirements for their safety and life. Diaphragm coating technology is unavoidable. Ceramic coating diaphragms are rapidly being promoted because of their high safety and ability to consume impurities in the electrolyte, especially for ternary batteries. The effect of improving the safety is particularly significant. The main system currently used for ceramic diaphragms is to coat alumina particles on the surface of traditional diaphragms. A more novel approach is to coat solid electrolyte fibers on the diaphragms. Such diaphragms have lower internal resistance, better mechanical support for the diaphragms, and lower tendency to block the diaphragm pores during service.

　　Electrolyte

　　The electrolyte has a great influence on the performance of fast-charge 9V rechargeable battery. To ensure the stability and safety of the battery under fast charging and high current, the electrolyte must meet the following characteristics: A) cannot decompose, B) high conductivity, C) inert to positive and negative electrode materials, and cannot react or dissolve. If these requirements are to be met, additives and functional electrolytes are key. For example, the safety of ternary fast-charge batteries is greatly affected by it, and various high-temperature resistant, flame-retardant, and overcharge-resistant additives must be added to protect it in order to improve its safety to a certain extent. The long-standing problem of lithium titanate batteries, high-temperature flatulence, also depends on high-temperature functional electrolytes to improve.

　　Battery structure design

　　A typical optimization strategy is stacking vs winding. The electrodes of stacking batteries are equivalent to parallel connection, while winding batteries are equivalent to series connection. Therefore, the internal resistance of the former is much smaller and more suitable for power applications. In addition, efforts can be made on the number of tabs to solve the internal resistance and heat dissipation problems. In addition, the use of high-conductivity electrode materials, the use of more conductive agents, and the coating of thinner electrodes are also strategies that can be considered.

　　In short, factors that affect the internal charge movement and the rate of embedding into the electrode holes of the battery will affect the fast charging ability of lithium batteries.

　　CATL

　　For the positive electrode, CATL has developed the "super electron network" technology, which makes lithium iron phosphate have excellent electronic conductivity; on the surface of the negative electrode graphite, the "fast ion ring" technology is used for modification. The modified graphite takes into account the characteristics of super fast charging and high energy density. During fast charging, there is no excessive by-product in the negative electrode, which enables it to have 4-5C fast charging capability, achieve 10-15 minutes of fast charging, and can ensure an energy density of more than 70wh/kg at the system level, and achieve a cycle life of 10,000 times (this life is quite high). In terms of thermal management, its thermal management system fully identifies the "healthy charging range" of fixed chemical systems at different temperatures and SOCs, greatly broadening the operating temperature of lithium batteries.

　　Watma

　　Watma is not doing well recently, let's just talk about technology. Wattma uses lithium iron phosphate with a smaller particle size. The common lithium iron phosphate particle size on the market is between 300~600nm, while Wattma only uses lithium iron phosphate with a particle size of 100~300nm, so that lithium ions will have a faster migration speed and can be charged and discharged at a higher rate of current. In systems other than batteries, strengthen the thermal management system and system safety design.

　　Microvast Power

　　In the early days, Microvast Power chose lithium titanate + porous composite carbon with spinel structure that can withstand fast charging and large current as the negative electrode material; in order to avoid the threat of high-power current to battery safety during fast charging, Microvast Power combines non-combustible electrolyte, high-porosity and high-permeability diaphragm technology and STL intelligent thermal control fluid technology to ensure the safety of the battery when fast charging is achieved.

　　In 2017, it released a new generation of high-energy density batteries, using high-capacity and high-power lithium manganese oxide positive electrode materials, with a single energy density of 170wh/kg, achieving 15-minute fast charging, and targeting both life and safety issues.

　　Zhuhai Yinlong

　　Lithium titanate negative electrode is known for its wide operating temperature range and large charge and discharge rate. There is no clear information on the specific technical solution. Talking with the staff at the exhibition, it is said that its fast charging can already achieve 10C and a lifespan of 20,000 times.

　　The future of fast charging technology

　　Is the fast charging technology of electric vehicles a historical direction or a flash in the pan? In fact, there are many different opinions now, and there is no conclusion. As an alternative solution to mileage anxiety, it is considered on the same platform as battery energy density and overall vehicle cost.

　　Energy density and fast charging performance can be said to be two incompatible directions in the same battery, and they cannot be achieved at the same time. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough, a car is loaded with 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 battery cost per kilowatt-hour is not low enough, then whether to buy enough power to "not worry" requires consumers to make a choice. In this way, fast charging has its value. Another perspective is the cost of fast-charging supporting facilities mentioned yesterday, which is of course part of the cost of promoting electrification in the whole society.

　　In conclusion, whether fast-charging technology can be widely promoted, which of the energy density and fast-charging technologies develops faster, and which of the two technologies reduces costs more sharply may play a decisive role in its future prospects.

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