What is the key problem with the super fast charging of the power 2s lipo battery?

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This paper comprehensively reviews the factors affecting the fast charging of 2s lipo battery, the main problems and solutions of fast charging from the material level to the system level.

2s lipo battery fast charging background introduction

In recent years, in order to limit the impact of climate change and air pollution, the widespread use of 2s lipo batteries in pure electric vehicles is accelerating. However, compared with traditional fuel vehicles, 2s lipo battery mileage anxiety and long charging time have become the main problems hindering the development of electric vehicles. Therefore, the improvement of fast charging capability has become a common development goal of 2s lipo battery manufacturers and OEMs.

Referring to 2s lipo battery fast charging The following article introduces the charging and discharging principle of 2s lipo battery, and the partners who need it can click to view:
Lipo battery 4s charging and discharging principle, be sure to store it well!
However, studies have shown that low-temperature and high-rate charging will cause accelerated degradation of battery capacity and output power; question. This paper focuses on the review and summary of the existing literature, and analyzes the key technical constraints at each level.

Classification of electric vehicle 2s lipo battery charging

The classification of electric vehicle 2s lipo battery charging includes AC and DC, among which DC charging is faster. Tesla is the first company to use 120kW fast charging; Bosch released a 350kW fast charging plan in 2017, which was implemented in "Taycan" in 2019. Since the current voltage of the 2s lipo batteryPack for vehicles is around 400V, high-power charging of 350kW requires a higher voltage of the 2s lipo batteryPack to avoid problems of excessive current and excessive heat generation. Both the Bosch "Taycan" and Audi's e-tron GT concept car (charging up to 350kW) are equipped with an 800V 2s lipo batteryPack. In December 2018, a joint research group from BMW, Bosch and Siemens achieved fast charging in 450kW CCS mode on two test cars in Germany.

2s lipo battery fast charge limiting factor

Although research on improving the charging power of 2s lipo battery for electric vehicles has made great progress, these fast charging technologies are not suitable for all situations. According to the specific working conditions and charging environment of the electric vehicle, the charging power will gradually decay during the continuous charging process of the 2s lipo battery. In addition, in the fast charging mode, due to safety and other factors, the 2s lipo battery can usually only be charged to 80% of the power; at higher power, the charging rate will gradually decrease to avoid overcharging.

In addition, the charging power is also affected by the 2s lipo battery management system (BMS).(Lipo battery 3s management system and its necessity). The industry is increasingly interested in the field of fast battery charging, and it is necessary to understand the speed-determining steps of different charging methods and their impact on the life of the 2s lipo battery. This paper aims to establish the link between microscopic processes, material properties, 2s lipo battery and pack design and charging strategy optimization from the multiscale and multidisciplinary nature of fast charging.

2s lipo battery fast charging principle

The ideal 2s lipo battery should exhibit long life, high energy density and high power density, so that it can be quickly charged and recharged at any location and at any temperature to meet the long-distance driving requirements of electric vehicles. However, there is a trade-off relationship between these physical properties, and the influence of material and equipment temperature determines the usage threshold of 2s lipo battery.

As the temperature drops, both the charge rate and the maximum voltage should be reduced for safety, making temperature a key limiting factor for fast charging. Among them, as the temperature decreases, the risk of lithium precipitation in the 2s lipo battery will increase significantly. Although many researchers have pointed out that the lithium precipitation of 2s lipo battery often occurs at a temperature lower than 25 ℃, but at high temperature, especially the high charging rate and energy density (partners who do not understand the energy density of 2s lipo battery can refer to this article: 1200mah lipo battry energy density improvement - cell density improvement) is also prone to occur when high. In addition, the relationship between fast charging efficiency and temperature is also very close. The charging efficiency of a 50kW charging pile at 25°C is 93%, but the charging efficiency at -25°C is as low as 39%. This is mainly because the 2s lipo battery BMS has a low temperature. The rated power will be limited.

A common lithium-ion battery is mainly composed of a graphite negative electrode, a lithium metal oxide positive electrode, an electrolyte, a current collector, and a porous separator. When the 2s lipo battery is charged, Li+ is transferred from the positive electrode to the negative electrode through the electrolyte. The main transmission paths are:
1) After solid-state electrodes;
2) through the electrode/electrolyte interface of the positive and negative electrodes;
3) After the electrolyte, including the solvation and desolvation of Li+.
However, improper use conditions of 2s lipo battery often cause a series of side reactions that affect performance and life. In addition, the charge-discharge rate, the internal resistance of the 2s lipo battery and the polarization of the battery will all affect the thermal characteristics of the battery, such as increasing heat generation, reducing charging efficiency and safety, etc.

2s lipo battery fast charging key factors

1) 2s lipo battery negative pole

A large number of studies have shown that the decay of the positive electrode of the 2s lipo battery and the growth of the positive electrode CEI film have no effect on the fast charging speed of the traditional lithium-ion system, so the negative electrode has become the main concern during the charging process.
Under certain circumstances, lithium metal may continue to precipitate into lithium dendrites, and even pierce the separator to cause a short circuit in the 2s lipo battery. Factors affecting Li deposition and deposition structure include the diffusion rate of Li ions in the anode, the electrolyte concentration gradient at the anode interface, the metal salt deposition of the current collector, and the side reactions at the electrode/electrolyte interface.

The research shows that the performance of the negative electrode during the lithium deposition of the 2s lipo battery can be attributed to the influence of the current at the beginning of the lithium deposition on the surface density and internal resistance of the negative electrode. Reducing the negative internal resistance B through the 2s lipo battery design is very important to improve the fast charging capability of the 2s lipo battery.

2) 2s lipo battery fast charging temperature

In addition, the effect of temperature is also very important. Too low or too high temperature will be considered unfavorable to the battery, but the higher temperature of the 2s lipo battery during fast charging will help its own balance, especially for the high specific energy 2s lipo battery.

3) Electrode thickness of 2s lipo battery

The effect of electrode thickness on charging performance also needs attention. Thin electrodes are often considered ideal for lithium ion transport, and as the electrode thickens, it becomes important to ensure sufficient lithium ion concentration at the electrode/electrolyte interface to maintain overpotential stability and reduce the possibility of lithium precipitation. During fast charging of thick-electrode batteries, lithium salts may be deposited at the current collector, resulting in an imbalance in electrode utilization and an increase in the current density of the separator anode.

Well, the above is the whole content of the key issues of the super fast charging of the power 2s lipo battery brought to you by the 2s lipo battery manufacturer CNHL today. Understand, if you want to get more 2s lipo battery information, you can check the following:
Detailed explanation of 6s lipo battery cathode material