Lithium batteries are called 'rocking chair type' batteries, and charged ions move between the positive and negative electrodes to achieve charge transfer, to supply power to external circuits or to charge from an external power source.
During the specific charging process, the external voltage is applied to the two poles of the battery, lithium ions are deintercalated from the positive electrode material, enter the electrolyte, and excess electrons are generated to pass through the positive current collector, and move to the negative electrode through an external circuit; lithium ions are in the electrolyte Moving from the positive electrode to the negative electrode, passing through the separator to the negative electrode; the SEI film passing through the negative electrode surface is embedded in the negative graphite layered structure and combined with electrons.
During the entire ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will have an impact on the fast charge performance.
Fast charge requirements for various parts of the battery
For the battery, if you want to improve the power performance, you need to work hard in all aspects of the battery, including the positive electrode, negative electrode, electrolyte, diaphragm and structural design.
positive electrode
In fact, almost all kinds of positive electrode materials can be used to make fast-filled batteries. The main properties to be guaranteed include conductance (reduced internal resistance), diffusion (guaranteed reaction kinetics), longevity (no explanation required), and safety (not required). Explain), proper processing performance (specific surface area can not be too large, reduce side reactions, for safety services).
Of course, the problems to be solved for each specific material may vary, but our common cathode materials can be optimized to meet these requirements through a series of optimizations, but different materials are also different:
A, lithium iron phosphate may be more focused on solving the problem of conductance, low temperature. Carbon coating, moderate nanocrystallization (note, moderate, not as fine as possible, simple logic), the formation of ionic conductors on the surface of the particles Is the most typical strategy.
B, the ternary material itself has better conductance, but its reactivity is too high, so the ternary material has little work of nanocrystallization (nanocrystallization is not an antidote to the performance improvement of the metallurgical material, especially in the field of batteries. There are sometimes many reactions in the system. More attention is paid to safety and inhibition (and electrolyte) side effects. After all, the main goal of ternary materials is safety. Recently, battery safety accidents have frequently occurred in this regard. Put forward higher requirements.
C, lithium manganate is more important for life, there are a lot of fast-charge batteries of lithium manganate on the market.
negative electrode
When the lithium ion battery is being charged, lithium migrates to the negative electrode. The excessively high potential caused by the fast charge and high current causes the negative electrode potential to be more negative. At this time, the pressure of the negative electrode rapidly accepting lithium becomes larger, and the tendency to generate lithium dendrites becomes longer. Large, therefore, the negative electrode not only needs to meet the kinetic requirements of lithium diffusion, but also solves the safety problem caused by the increased tendency of lithium dendrite formation. Therefore, the main technical difficulty of the fast charging core is the embedding of lithium ions in the negative electrode. .
A. At present, the dominant anode material in the market is still graphite (about 90% of the market share). The root cause is that it is not cheap - cheap, and the comprehensive processing properties of graphite, the energy density is excellent, and the disadvantages are relatively few. The graphite anode is of course also problematic. The surface is sensitive to the electrolyte. The lithium intercalation reaction has strong directionality. Therefore, it is mainly necessary to work on the graphite surface treatment to improve its structural stability and promote the diffusion of lithium ions on the substrate. The direction.
B, Hard carbon and soft carbon materials have also developed in recent years: Hard carbon materials have high lithium insertion potential, micropores in the materials, and good reaction kinetics; and soft carbon materials have good compatibility with electrolytes, MCMB The materials are also very representative, but the hard and soft carbon materials are generally low in efficiency and high in cost (and Imagine that graphite is as cheap as I hope from an industrial point of view), so the amount is far less than graphite, and more used in some specialties. On the battery.
C, How about lithium titanate? Simply put: The advantages of lithium titanate are high power density, safer, and obvious disadvantages. The energy density is very low. The calculation cost is high according to Wh. Therefore, the viewpoint of lithium titanate battery is one. A useful technique that is advantageous in certain situations, but it is not suitable for many situations where the cost and cruising range are high.
D, silicon anode materials are an important development direction, Panasonic's new 18650 battery has begun a commercial process for such materials. But how to achieve a balance between the pursuit of performance in nanotechnology and the battery industry's general micron-scale requirements for materials, Still a more challenging job.
Diaphragm
For power batteries, high current operation provides a higher level of safety and longevity. Diaphragm coating technology is inseparable. Ceramic coated diaphragms are fast due to their high safety and can consume impurities in the electrolyte. Pushing open, especially for the safety improvement of ternary battery is particularly significant.
The main system currently used in ceramic diaphragms is to coat alumina particles on the surface of conventional diaphragms. A relatively novel approach is to coat solid electrolyte fibers on the membrane. Such membranes have lower internal resistance and the mechanical support effect of the fibers on the membrane is more Excellent, and its tendency to block the diaphragm hole during service is lower.
After the coating, the separator has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform, resulting in short circuit. Jiangsu Qingtao Energy Co., Ltd., technical supporter of the Academic Researcher of Tsinghua University School of Materials, has some representative aspects in this respect. Work, diaphragm as shown below.
Electrolyte
The electrolyte has a great influence on the performance of the fast-charged lithium-ion battery. To ensure the stability and safety of the battery under fast charge and high current, the electrolyte should meet the following characteristics: A) can not be decomposed, B) conductivity should be High, C) is inert to the positive and negative materials and cannot react or dissolve.
If you want to meet these requirements, the key is to use additives and functional electrolytes. For example, the safety of ternary fast-charged batteries is greatly affected by them. Various high-temperature resistant, flame retardant, anti-overcharged types must be added to them. Additive protection can improve the safety of the electrolyte to a certain extent. The problem of the old lithium titanate battery, high temperature flatulence, also depends on the high temperature functional electrolyte.
Battery structure design
A typical optimization strategy is the stacked VS winding type. The electrodes of the laminated 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, which is more suitable for the power type. occasion.
In addition, you can work hard on the number of poles to solve the internal resistance and heat dissipation problems. In addition, the use of high-conductivity electrode materials, the use of more conductive agents, coating thinner electrodes are also considered strategies.
In short, the factors affecting the internal charge movement of the battery and the rate of embedding the electrode cavity will affect the fast charging ability of the lithium battery.
Mainstream manufacturers fast charge technology route overview
Ningde era
For the positive electrode, Ningde era developed the 'super-electronic network' technology, which makes lithium iron phosphate have excellent electronic conductivity; on the surface of the negative graphite, it is modified by 'fast ion ring' technology, and the modified graphite has both super fast charge and high The characteristics of energy density, no excess by-products appear in the negative electrode during fast charging, so that it has 4-5C fast charge capacity, fast charging and charging for 10-15 minutes, and can guarantee the energy density above 70 wh/kg of system level, achieving 10000 Cycle life.
In terms of thermal management, its thermal management system fully recognizes the 'healthy charging range' of fixed chemical systems at different temperatures and SOCs, greatly expanding the operating temperature of lithium batteries.
Waterma
Wattma is not very good recently, let's talk about technology. Waterma uses a smaller particle size of lithium iron phosphate. Currently, the lithium iron phosphate particle size is between 300 and 600 nm, and Waterma only uses Lithium iron phosphate of 100~300nm, so lithium ion will have faster migration speed and can charge and discharge with higher current. On the system other than battery, strengthen the thermal management system and system safety design.
Micro macro power
In the early days, Micro-Power chose a lithium titanate + porous composite carbon with a spinel structure as a negative electrode material; in order to avoid the threat of high power current to battery safety during fast charging, Weihong Power Combined with non-combustion electrolyte, high porosity and high permeability diaphragm technology and STL intelligent thermal control fluid technology, the battery safety is ensured when the battery is fast charged.
In 2017, it released a new generation of high-energy density battery, using high-capacity high-power lithium manganate cathode material, the monomer energy density reached 170wh / kg, achieving 15 minutes fast charge, the target is to balance life and safety issues.
Zhuhai Yinlong
Lithium titanate negative electrode, wide operating temperature range and large charge and discharge rate is known, the specific technical solution, there is no clear information. At the exhibition, talking with the staff, it is said that its fast charge can achieve 10C, life expectancy of 20,000 times.
The future of fast charging technology
The fast charging technology of electric vehicles is the historical direction or the glimpse of the past. In fact, there are many different opinions. As an alternative to solving the mileage anxiety, it is considered on a platform with battery energy density and overall vehicle cost.
Energy density and fast charge performance, in the same battery, can be said to be incompatible in both directions, can not be both. The pursuit of battery energy density, is currently the mainstream. When the energy density is high enough, a car load Large enough to avoid the so-called 'mileage anxiety', the demand for battery rate charging performance will be reduced; at the same time, the power is large, if the cost of battery power is not low enough, then whether you want to buy enough to 'not worry' Electricity, consumers need to make choices, so think, fast charge has the value of existence. Another point is the cost of fast charging facilities, which is of course part of the cost of pushing the whole society.
Whether fast charging technology can be promoted in a large area, energy density and fast charging technology who develops fast, the two technologies who reduce the cost, may play a decisive role in its future.
