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Lithium batteries are known as "rocking chair type" batteries, in which charged ions move between the positive and negative electrodes to achieve charge transfer, providing power to external circuits or charging from external power sources.

During the specific charging process, an external voltage is applied to the two poles of the battery, causing lithium ions to detach 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 towards the negative electrode through the external circuit; Lithium ions move from the positive electrode to the negative electrode in the electrolyte, pass through the separator, and reach the negative electrode; The SEI film on the negative electrode surface is embedded into the graphite layered structure of the negative electrode and combines with electrons.

The battery structure that affects charge transfer throughout the entire operation of ions and electrons, whether electrochemical or physical, will have an impact on fast charging performance.

Requirements for various parts of the battery for fast charging

For batteries, to improve power performance, efforts need to be made in every aspect of the battery, including positive and negative electrodes, electrolyte, separator, and structural design.

positive electrode

In fact, almost all types of positive electrode materials can be used to manufacture fast charging batteries, and the main performance requirements include conductivity (reducing internal resistance), diffusion (ensuring reaction kinetics), lifespan (no need to explain), safety (no need to explain), and appropriate processing performance (specific surface area should not be too large, reducing side reactions, and serving safety).

Of course, the problems that need to be solved for each specific material may vary, but the commonly used positive electrode materials can meet these requirements through a series of optimizations, but different materials also have differences:

A、 Lithium iron phosphate may focus more on addressing issues related to conductivity and low temperature. Carbon coating, moderate nanomaterialization (note that it is moderate, definitely not the simple logic of finer is better), and surface treatment of particles to form ion conductors are the most typical strategies.

B、 The conductivity of ternary materials is already relatively good, but their reactivity is too high. Therefore, there is little work on nanomaterialization of ternary materials (nanomaterialization is not a panacea for improving material performance, especially in the field of batteries where there are sometimes many reactions). More attention is paid to safety and the suppression of side reactions (with electrolytes). After all, one of the major challenges of ternary materials is safety, and recent frequent battery safety accidents have put forward higher requirements in this regard.

C、 Lithium manganese oxide places greater emphasis on lifespan, and there are currently many lithium manganese oxide based fast charging batteries available on the market.

negative pole

When charging a lithium-ion battery, lithium migrates towards the negative electrode. The high potential caused by fast charging high current can lead to a more negative negative negative electrode potential. At this time, the pressure for the negative electrode to quickly accept lithium will increase, and the tendency to generate lithium dendrites will also increase. Therefore, during fast charging, the negative electrode not only needs to meet the dynamic requirements of lithium diffusion, but also needs to solve the safety problems caused by the increased tendency of lithium dendrite generation. Therefore, the main technical difficulty of fast charging cores is the insertion of lithium ions into 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), and the fundamental reason is that it is cheap, and graphite has excellent processing performance and energy density, with relatively few disadvantages. Of course, graphite negative electrodes also have problems. Their surface is sensitive to electrolytes, and the insertion reaction of lithium has strong directionality. Therefore, surface treatment of graphite to improve its structural stability and promote the diffusion of lithium ions on the substrate is the main direction that needs to be addressed.

B、 In recent years, there have also been many developments in hard carbon and soft carbon materials: hard carbon materials have a high lithium insertion potential, and there are micropores in the material, resulting in good reaction kinetics; Soft carbon materials have good compatibility with electrolytes, and MCMB materials are also representative. However, hard and soft carbon materials generally have low efficiency and high cost (and it is unlikely to be as cheap as graphite from an industrial perspective). Therefore, their current usage is far less than graphite and they are more commonly used in some special batteries.

C、 How about lithium titanate? Simply put, the advantages of lithium titanate are high power density, safety, and obvious disadvantages, such as low energy density and high cost calculated in Wh. Therefore, the view on lithium titanate batteries is a useful technology with advantages in specific situations, but it is not very suitable for many fields that require high cost and range.

D、 Silicon negative electrode materials are an important development direction, and Panasonic's new 18650 battery has begun the commercial process of using such materials. However, achieving a balance between the pursuit of performance in nanotechnology and the general micron level requirements of materials in the battery industry remains a challenging task.

the diaphragm

For power type batteries, high current operation provides higher requirements for their safety and lifespan. The membrane coating technology cannot be bypassed, and ceramic coated membranes are rapidly being pushed forward due to their high safety and ability to consume impurities in the electrolyte, especially for improving the safety of ternary batteries.

The main system currently used for ceramic membranes is to coat alumina particles on the surface of traditional membranes. A relatively novel approach is to coat solid electrolyte fibers on the membrane, which has lower internal resistance, better mechanical support effect of fibers on the membrane, and a lower tendency to block membrane holes during service.

After coating, the diaphragm has good stability and is not prone to shrinkage deformation and short circuit even at high temperatures. Jiangsu Qingtao Energy Company, supported by the research group of Academician Nan Ce Wen from the School of Materials Science and Technology at Tsinghua University, has some representative works in this regard. The diaphragm is shown in the following figure.

electrolyte

The electrolyte has a significant impact 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) non decomposition, B) high conductivity, and C) inert to the positive and negative electrode materials, unable to react or dissolve.

To meet these requirements, additives and functional electrolytes are crucial. For example, the safety of ternary fast charging batteries is greatly affected, and various high-temperature resistant, flame-retardant, and overcharge resistant additives must be added to them for protection in order to improve their safety to a certain extent. The long-standing problem of high temperature swelling in lithium titanate batteries also needs to be improved by high-temperature functional electrolytes.

Battery structure design

A typical optimization strategy is the stacked vs. wound battery. The electrodes of the stacked battery are in parallel, while the wound battery is in series. Therefore, the former has much lower internal resistance and is more suitable for power applications.

Additionally, efforts can be made to increase the number of polar ears to address internal resistance and heat dissipation issues. In addition, using high conductivity electrode materials, using more conductive agents, and coating thinner electrodes are also strategies that can be considered.

In short, factors that affect the internal charge movement and insertion rate into electrode holes in batteries can also affect the fast charging ability of lithium batteries.

Overview of mainstream manufacturers' fast charging technology routes

Ningde Era

For the positive electrode, CATL has developed the "super electronic network" technology, which enables lithium iron phosphate to have excellent electronic conductivity; On the surface of the negative graphite, the "fast ion ring" technology is used for modification. The modified graphite combines the characteristics of super fast charging and high energy density. During fast charging, there are no excessive by-products in the negative electrode, making it capable of 4-5C fast charging, achieving 10-15 minutes of fast charging, and ensuring a system level energy density of over 70wh/kg, achieving a cycle life of 10000 times.

In terms of thermal management, its thermal management system fully identifies the "healthy charging range" of fixed chemical systems at different temperatures and SOC, greatly expanding the operating temperature of lithium batteries.

Waltmal 

Watma hasn't been doing well lately, let's just talk about technology. Waterma uses smaller particle size lithium iron phosphate. Currently, the common particle size of lithium iron phosphate on the market is between 300-600nm, while Waterma only uses 100-300nm lithium iron phosphate. This way, lithium ions will have a faster migration speed and can charge and discharge at a higher rate of current. Strengthen the thermal management system and system safety design on systems other than batteries.

Micro Macro Power

In the early days, Micro Macro Power chose lithium titanate with spinel structure and porous composite carbon as the negative electrode material, which could withstand fast charging and high current; In order to avoid the threat of high-power current to battery safety during fast charging, Micro Macro 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 during fast charging.

In 2017, it released a new generation of high-energy density batteries, using high-capacity and high-power lithium manganese oxide cathode materials, with a single energy density of 170Wh/kg, achieving 15 minute fast charging, with the goal of balancing lifespan and safety issues.

Zhuhai Yinlong

Lithium titanate negative electrode is known for its wide operating temperature range and high charge discharge rate. The specific technical solution is not clearly indicated in the data. At the exhibition, I talked to the staff and it was reported that its fast charging can achieve 10C with a lifespan of 20000 times.

The future of fast charging technology

Whether fast charging technology for electric vehicles is a historical direction or a fleeting phenomenon, there is currently no consensus. As an alternative solution to address range anxiety, it is considered on the same platform as battery energy density and overall vehicle costs.

Energy density and fast charging performance are two incompatible directions in the same battery, and cannot be achieved simultaneously. The pursuit of battery energy density is currently mainstream. When the energy density is high enough and a vehicle is loaded with enough power to avoid the so-called 'range anxiety', the demand for battery rate charging performance will decrease; At the same time, with a large battery capacity, if the cost per kilowatt hour of the battery is not low enough, consumers need to make a choice whether to purchase enough electricity to "not worry". With this in mind, fast charging has value. Another perspective is the cost of supporting facilities for fast charging, which is certainly a part of the overall cost of promoting electrification in society.

Whether fast charging technology can be widely promoted, which one develops faster in terms of energy density and fast charging technology, and which one reduces costs more fiercely, may play a decisive role in its future prospects.

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