Factors affecting fast charging of lithium battery -Lithium - Ion Battery Equipment
Each kind of lithium battery has an optimal charging current value under different state parameters and environmental parameters. From the perspective of battery structure, what are the factors that affect the optimal charging value.
Micro process of charging
Lithium battery is called "rocking chair type" battery. Charged ions move between the positive and negative electrodes to transfer charge, supply power to external circuits or charge from external power sources. During the specific charging process, the external voltage is loaded on the two poles of the battery, lithium ions are disembedded from the positive electrode material and enter the electrolyte, at the same time, excess electrons are generated to move to the negative electrode through the external circuit through the positive collector; Lithium ions move from the positive electrode to the negative electrode in the electrolyte, passing through the diaphragm to the negative electrode; The SEI film on the surface of the negative electrode is embedded into the graphite layer structure of the negative electrode and combines with the electrons.(Lithium - Ion Battery Equipment)
During the operation of ions and electrons, the battery structure that affects charge transfer, whether electrochemical or physical, will affect the fast charging performance.
Fast charging, requirements for all 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, mainly including the positive pole, negative pole, electrolyte, diaphragm and structure design.
positive electrode
In fact, almost all kinds of positive electrode materials can be used to manufacture fast charging batteries. The main properties to be guaranteed include conductivity (reduce internal resistance), diffusion (ensure reaction kinetics), life (no explanation is required), safety (no explanation is required), and proper processing performance (the specific surface area should not be too large, reduce side reactions, and serve safety).
Of course, the problems to be solved for each specific material may be different, but common cathode materials can meet these requirements through a series of optimization, but different materials also have differences:
A. Lithium iron phosphate may focus more on solving the problems of conductivity and low temperature. Carbon coating, moderate nanocrystallization (note that it is moderate, not the simple logic that the finer the better), and the formation of ionic conductor on the particle surface are the most typical strategies.
B. Ternary materials have good electrical conductivity, but their reactivity is too high, so there is little work on nano materials (nano materials are not a panacea for improving material performance, especially in the battery field, there are many reactions sometimes). More attention is paid to safety and inhibition of side reactions (with electrolyte). After all, safety is the lifeblood of ternary materials at present, The recent frequent battery safety accidents also put forward higher requirements in this regard.
C. Lithium manganate is more important for its service life. At present, there are many lithium manganate fast charging batteries on the market.
negative pole
When a lithium ion battery is charged, lithium migrates to the negative electrode. However, the high potential brought by the fast charging large current will lead to the negative electrode potential being 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 the fast charging, the negative electrode should not only meet the dynamic requirements of lithium diffusion, but also solve the security problems caused by the increased tendency to generate lithium dendrites. Therefore, the main technical difficulty of the fast charging core is actually the insertion of lithium ions in the negative electrode.
A. At present, the dominant anode material in the market is still graphite (accounting for about 90% of the market share). The fundamental reason is that it is cheap. Besides, the comprehensive processing performance and energy density of graphite are excellent, with relatively few shortcomings. Of course, there are problems with the graphite anode. Its surface is sensitive to the electrolyte, and the intercalation reaction of lithium has strong directivity. Therefore, the main direction to be worked on is to conduct graphite surface treatment, improve its structural stability, and promote the diffusion of lithium ions on the base.
B. Hard carbon and soft carbon materials also have a lot of development in recent years: hard carbon materials have high lithium intercalation potential, there are micropores in the materials, so the reaction kinetics performance is good; Soft carbon materials have good compatibility with electrolyte, and MCMB materials are also very representative. However, hard and soft carbon materials are generally low in efficiency and high in cost (and it is unlikely to be as cheap as graphite from an industrial point of view). Therefore, the current consumption is far less than graphite and more used in some special batteries.
C. How about lithium titanate? To put it simply: lithium titanate has the advantages of high power density, safety, and obvious disadvantages. Its energy density is very low, and its cost is high when calculated by Wh. Therefore, the view on lithium titanate battery is a useful technology with advantages in specific situations, but it is not applicable to many field mergers that require high cost and 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 pursuit of performance in nano scale and the general requirements of the battery industry for materials in the micron level is still a challenging task.
the diaphragm
For power battery, high current operation provides higher requirements for its safety and life. The membrane coating technology is inextricable. Because of its high safety and the ability to consume impurities in the electrolyte, the ceramic coating membrane is rapidly being pushed away, especially for the improvement of the safety of ternary batteries.
At present, the main system used for ceramic diaphragm is to coat aluminum oxide particles on the surface of traditional diaphragm. A relatively novel method is to coat solid electrolyte fiber on the diaphragm. This kind of diaphragm has lower internal resistance, better mechanical support effect of the fiber on the diaphragm, and has a lower tendency to block the diaphragm hole during service.
The membrane after coating has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform to cause short circuit. Jiangsu Qingtao Energy Co., Ltd., which is supported by the research group of academician Nan Cewen of School of Materials of Tsinghua University, has some representative work in this regard.
electrolyte
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 large current, the electrolyte should meet the following characteristics: A) It cannot be decomposed, B) The conductivity should be high, and C) It is inert to the positive and negative electrode 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 charging battery is greatly affected by it. To improve its safety to a certain extent, it is necessary to add various high-temperature resistant, flame-retardant and overcharge resistant additives. However, the long-standing problem of lithium titanate battery, high temperature gas, must be improved by high temperature functional electrolyte.
Battery structure design
A typical optimization strategy is the laminated VS wound type. The electrodes of the laminated battery are equivalent to a parallel connection, and the wound 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 occasions.
In addition, efforts can be made on the number of lugs to solve the internal resistance and heat dissipation problems. In addition, the use of electrode materials with high conductivity, the use of more conductive agents, and the coating of thinner electrodes can also be considered strategies.
In a word, the factors that affect the charge movement inside the battery and the rate of embedding electrode holes will affect the rapid charging capacity of lithium battery.
The Future of Fast Charging Technology
The rapid charging technology of electric vehicles is a historical direction or a flash in the pan. In fact, there are different opinions and no final conclusion. As an alternative solution to mileage anxiety, it is considered on the same platform as battery energy density and overall vehicle cost.
In the same battery, energy density and fast charging performance can be said to be incompatible in two directions. The pursuit of battery energy density is the mainstream at present. When the energy density is high enough and the load capacity of a car is large enough to avoid the so-called "mileage anxiety", the demand for battery charging performance will be reduced; At the same time, the power is large. If the cost of battery power consumption is not low enough, consumers need to make a choice whether to buy enough "not anxious" power from Keding Kemao. In this way, fast charging has value. Another aspect is the cost of fast charging supporting facilities, which is certainly a part of the cost of the whole society to promote electrification.
Whether the fast charging technology can be popularized in a large area, which of the energy density and fast charging technology develops fast, and which of the two technologies reduces the cost severely, may play a decisive role in its future future.
Micro process of charging
Lithium battery is called "rocking chair type" battery. Charged ions move between the positive and negative electrodes to transfer charge, supply power to external circuits or charge from external power sources. During the specific charging process, the external voltage is loaded on the two poles of the battery, lithium ions are disembedded from the positive electrode material and enter the electrolyte, at the same time, excess electrons are generated to move to the negative electrode through the external circuit through the positive collector; Lithium ions move from the positive electrode to the negative electrode in the electrolyte, passing through the diaphragm to the negative electrode; The SEI film on the surface of the negative electrode is embedded into the graphite layer structure of the negative electrode and combines with the electrons.(Lithium - Ion Battery Equipment)
During the operation of ions and electrons, the battery structure that affects charge transfer, whether electrochemical or physical, will affect the fast charging performance.
Fast charging, requirements for all 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, mainly including the positive pole, negative pole, electrolyte, diaphragm and structure design.
positive electrode
In fact, almost all kinds of positive electrode materials can be used to manufacture fast charging batteries. The main properties to be guaranteed include conductivity (reduce internal resistance), diffusion (ensure reaction kinetics), life (no explanation is required), safety (no explanation is required), and proper processing performance (the specific surface area should not be too large, reduce side reactions, and serve safety).
Of course, the problems to be solved for each specific material may be different, but common cathode materials can meet these requirements through a series of optimization, but different materials also have differences:
A. Lithium iron phosphate may focus more on solving the problems of conductivity and low temperature. Carbon coating, moderate nanocrystallization (note that it is moderate, not the simple logic that the finer the better), and the formation of ionic conductor on the particle surface are the most typical strategies.
B. Ternary materials have good electrical conductivity, but their reactivity is too high, so there is little work on nano materials (nano materials are not a panacea for improving material performance, especially in the battery field, there are many reactions sometimes). More attention is paid to safety and inhibition of side reactions (with electrolyte). After all, safety is the lifeblood of ternary materials at present, The recent frequent battery safety accidents also put forward higher requirements in this regard.
C. Lithium manganate is more important for its service life. At present, there are many lithium manganate fast charging batteries on the market.
negative pole
When a lithium ion battery is charged, lithium migrates to the negative electrode. However, the high potential brought by the fast charging large current will lead to the negative electrode potential being 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 the fast charging, the negative electrode should not only meet the dynamic requirements of lithium diffusion, but also solve the security problems caused by the increased tendency to generate lithium dendrites. Therefore, the main technical difficulty of the fast charging core is actually the insertion of lithium ions in the negative electrode.
A. At present, the dominant anode material in the market is still graphite (accounting for about 90% of the market share). The fundamental reason is that it is cheap. Besides, the comprehensive processing performance and energy density of graphite are excellent, with relatively few shortcomings. Of course, there are problems with the graphite anode. Its surface is sensitive to the electrolyte, and the intercalation reaction of lithium has strong directivity. Therefore, the main direction to be worked on is to conduct graphite surface treatment, improve its structural stability, and promote the diffusion of lithium ions on the base.
B. Hard carbon and soft carbon materials also have a lot of development in recent years: hard carbon materials have high lithium intercalation potential, there are micropores in the materials, so the reaction kinetics performance is good; Soft carbon materials have good compatibility with electrolyte, and MCMB materials are also very representative. However, hard and soft carbon materials are generally low in efficiency and high in cost (and it is unlikely to be as cheap as graphite from an industrial point of view). Therefore, the current consumption is far less than graphite and more used in some special batteries.
C. How about lithium titanate? To put it simply: lithium titanate has the advantages of high power density, safety, and obvious disadvantages. Its energy density is very low, and its cost is high when calculated by Wh. Therefore, the view on lithium titanate battery is a useful technology with advantages in specific situations, but it is not applicable to many field mergers that require high cost and 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 pursuit of performance in nano scale and the general requirements of the battery industry for materials in the micron level is still a challenging task.
the diaphragm
For power battery, high current operation provides higher requirements for its safety and life. The membrane coating technology is inextricable. Because of its high safety and the ability to consume impurities in the electrolyte, the ceramic coating membrane is rapidly being pushed away, especially for the improvement of the safety of ternary batteries.
At present, the main system used for ceramic diaphragm is to coat aluminum oxide particles on the surface of traditional diaphragm. A relatively novel method is to coat solid electrolyte fiber on the diaphragm. This kind of diaphragm has lower internal resistance, better mechanical support effect of the fiber on the diaphragm, and has a lower tendency to block the diaphragm hole during service.
The membrane after coating has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform to cause short circuit. Jiangsu Qingtao Energy Co., Ltd., which is supported by the research group of academician Nan Cewen of School of Materials of Tsinghua University, has some representative work in this regard.
electrolyte
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 large current, the electrolyte should meet the following characteristics: A) It cannot be decomposed, B) The conductivity should be high, and C) It is inert to the positive and negative electrode 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 charging battery is greatly affected by it. To improve its safety to a certain extent, it is necessary to add various high-temperature resistant, flame-retardant and overcharge resistant additives. However, the long-standing problem of lithium titanate battery, high temperature gas, must be improved by high temperature functional electrolyte.
Battery structure design
A typical optimization strategy is the laminated VS wound type. The electrodes of the laminated battery are equivalent to a parallel connection, and the wound 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 occasions.
In addition, efforts can be made on the number of lugs to solve the internal resistance and heat dissipation problems. In addition, the use of electrode materials with high conductivity, the use of more conductive agents, and the coating of thinner electrodes can also be considered strategies.
In a word, the factors that affect the charge movement inside the battery and the rate of embedding electrode holes will affect the rapid charging capacity of lithium battery.
The Future of Fast Charging Technology
The rapid charging technology of electric vehicles is a historical direction or a flash in the pan. In fact, there are different opinions and no final conclusion. As an alternative solution to mileage anxiety, it is considered on the same platform as battery energy density and overall vehicle cost.
In the same battery, energy density and fast charging performance can be said to be incompatible in two directions. The pursuit of battery energy density is the mainstream at present. When the energy density is high enough and the load capacity of a car is large enough to avoid the so-called "mileage anxiety", the demand for battery charging performance will be reduced; At the same time, the power is large. If the cost of battery power consumption is not low enough, consumers need to make a choice whether to buy enough "not anxious" power from Keding Kemao. In this way, fast charging has value. Another aspect is the cost of fast charging supporting facilities, which is certainly a part of the cost of the whole society to promote electrification.
Whether the fast charging technology can be popularized in a large area, which of the energy density and fast charging technology develops fast, and which of the two technologies reduces the cost severely, may play a decisive role in its future future.
