With the rapid development of electric vehicles worldwide, the market size of electric vehicles has reached $1 trillion in 2020, and will continue to grow at an annual rate of more than 20% in the future. Therefore, as a major means of transportation, electric vehicles will have higher and higher requirements for the performance of power batteries. The impact of battery attenuation on the performance of power batteries at low temperature cannot be ignored. The main reasons for battery attenuation at low temperature are as follows: first, low temperature affects the battery internal resistance, the heat diffusion area is large, and the battery internal resistance increases. Second, the battery internal and external charge transfer ability is poor, the deformation of the battery will occur local irreversible polarization. Third, the movement of electrolyte molecules is slow at low temperature and it is difficult to diffuse in time when the temperature rises. Therefore, low-temperature battery attenuation is serious, resulting in serious battery performance attenuation.

1. Status quo of low-temperature battery technology

    The technology and material properties of lithium-ion power batteries prepared at low temperature are very demanding. The reason for the serious degradation of lithium-ion power battery performance at low temperature is the increase of internal resistance, which leads to the difficulty of electrolyte diffusion and the shortening of cell cycle life. So in recent years, the research of low temperature power battery technology has made some progress. The performance of traditional high temperature lithium-ion batteries is poor at high temperature, but still unstable at low temperature. Low temperature cell volume, low capacity, low temperature cycle performance is poor; The polarization at low temperature is significantly stronger than that at high temperature. At low temperature, the increase of electrolyte viscosity leads to the decrease of the number of charge and discharge cycles. Cell safety and battery life are reduced at low temperature. Performance degrades at low temperature. In addition, the low temperature battery cycle life is short and the low temperature cell has safety risks, which puts forward new requirements for the safety of power battery. Therefore, the development of stable performance, safe and reliable, long life power battery materials under low temperature environment is the focus of low-temperature lithium-ion battery research. At present, there are mainly the following low-temperature lithium-ion battery materials at home and abroad :(1) lithium metal anode materials: lithium metal is widely used in electric vehicles due to its advantages of high chemical stability, high conductivity and good low-temperature charge-discharge performance; (2) Carbon anode materials are widely used in electric vehicles because of their good heat resistance, low temperature cycle performance, low conductivity and low temperature cycle life at low temperature. (3) Organic electrolyte has good performance at low temperature; (4) Polymer electrolyte: polymer molecular chain is relatively short, high affinity; (5) Inorganic materials: inorganic polymers have good performance parameters (electrical conductivity) and good combining ability between electrolyte activity; (6) less metal oxides; (7) Inorganic substances: inorganic polymers, etc.

2. Influence of low temperature environment on lithium batteries

    The cycle life of lithium battery mainly depends on the discharge process, and the low temperature is the most important factor in the life of lithium battery. In general, at low temperatures, a phase transition will occur on the surface of a battery, resulting in structural damage, along with reduced capacity and cell capacity. Under the condition of high temperature, the formation of gas in the cell will accelerate the thermal diffusion. At low temperature, the gas can not be discharged in time, accelerating the battery liquid phase change; The lower the temperature, the more gas is produced and the slower the liquid phase of the cell becomes. Therefore, at low temperature, the material inside the battery changes more violently and complicated, and the battery material is more likely to produce gas and solid. At the same time, when the temperature is too low, a series of damage reactions such as irreversible chemical bond breaking occur at the interface between the anode material and the electrolytic liquid phase. The self-assembly degree of electrolyte is reduced and the cycle life is shortened. The ability of lithium ion charge transfer to electrolyte decreases; In the process of charge and discharge, a series of chain reactions such as polarization in the process of charge transfer, battery capacity attenuation and internal stress release will be caused, which will affect the cycle life and energy density of lithium-ion batteries. The lower the temperature is at low temperature, the more severe and complex the damage reactions such as REDOX reaction on the battery surface, thermal diffusion and phase transition inside the cell are. Even after the complete damage, the electrolyte self-assembly, the slower the reaction speed, the more serious the battery capacity attenuation, the worse the migration ability of lithium ion charge at high temperature and a series of chain reactions are triggered.

3. The prospect of low temperature for the research progress of lithium battery technology

    At low temperature, the safety, cycle life and temperature stability of the battery cell will be affected. The influence of low temperature on the life of lithium battery cannot be ignored. At present, the development of low-temperature power battery technology using diaphragm, electrolyte, anode and cathode materials has made some progress. In the future, the research and development of low-temperature lithium battery technology should be improved from the following aspects :(1) develop the lithium battery material system with high energy density, long life, low attenuation, small size and low cost at low temperature; (2) Continuously improve the internal resistance control ability of batteries through structural design and material preparation technology; (3) In the development of high capacity and low cost lithium battery system, attention should be paid to the electrolyte additives, lithium ion and positive and negative interface and internal active substances and other key factors; (4) Improve battery cycle performance (charge-discharge ratio energy), battery thermal stability at low temperature, safety of lithium batteries at low temperature, and other battery technology development directions; (5) Developing power battery system solutions with high safety performance, high cost and low cost under low temperature conditions; (6) Develop low-temperature battery related products and promote their application; (7) Developing high-performance low-temperature resistant battery materials and device technology.

    Of course, in addition to the above research directions, there are also many research directions which can further improve the performance of batteries at low temperature, improve the energy density of low-temperature batteries, reduce the attenuation of batteries at low temperature, extend the service life of batteries and other research progress. However, the more important problem is how to realize the commercial use of batteries at low temperature with high performance, high safety, low cost, high mileage, long life and low cost. These problems need to be breakthrough and solved in the current research.

For more information, please pay attention to the public number: Research and development progress of low-temperature power lithium battery technology