Since entering the market, lithium ion batteries have been widely used due to their advantages of long life, large specific capacity and no memory effect. There are some problems in low temperature use of lithium ion batteries, such as low capacity, serious attenuation, poor performance of cycle ratio, obvious phenomenon of lithium evolution and imbalance of lithium removal. However, as the application field continues to expand, the low temperature performance of lithium ion batteries brought more and more obvious constraints.

         It is reported that the discharge capacity of lithium-ion batteries at -20℃ is only about 31.5% of that at room temperature. The operating temperature of traditional lithium ion batteries ranges from -20 ℃ to +55℃. However, in aerospace, military, electric vehicles and other fields, the battery is required to work normally at -40℃. Therefore, it is of great significance to improve the low temperature properties of lithium ion batteries.

         Factors restricting low temperature performance of lithium ion batteries

        At low temperature, the viscosity of electrolyte increases and even partially solidifies, resulting in decreased conductivity of lithium ion battery. The compatibility of electrolyte with cathode and diaphragm becomes worse at low temperature. The negative electrode of lithium ion battery precipitates seriously at low temperature, and the precipitated lithium metal reacts with electrolyte, and the product deposition leads to the increase of solid electrolyte interface (SEI) thickness.
         At low temperature, the charge transfer impedance (Rct) of li-ion battery decreases in the diffusion system of active substance and increases significantly. Discussion on the determinants of low temperature performance of lithium ion batteries

         Expert opinion 1: electrolyte has the greatest impact on the low-temperature performance of lithium-ion batteries, and the composition and physicochemical properties of electrolyte have an important impact on the low-temperature performance of batteries.
      The problems faced by the battery cycle at low temperature are as follows: the electrolyte viscosity will increase and the ionic conduction speed will slow, which will cause the mismatch of electron migration speed in the external circuit.
      Therefore, the battery will be severely polarized and the charging and discharging capacity will decrease sharply. Especially when charged at low temperature, lithium ions are easy to form lithium dendrites on the negative electrode surface, leading to battery failure. The low temperature performance of electrolyte is closely related to the conductivity of electrolyte itself. With high conductivity, electrolyte can transfer ions quickly and exert more capacity at low temperature.
       The more the lithium in the electrolyte dissociates, the more it migrates and the higher its conductivity. The higher the conductivity, the faster the rate of ion conduction, the smaller the polarization and the better the performance of the battery at low temperatures. Therefore, high conductivity is a necessary condition to realize good low temperature performance of lithium ion batteries.
      The conductivity of electrolyte is related to the composition of electrolyte. Reducing the viscosity of solvent is one of the ways to improve the conductivity of electrolyte. Good liquidity of solvent at low temperature is the guarantee of ion transport, and solid electrolyte film formed by electrolyte at negative electrode at low temperature is also the key to affect lithium ion conduction, and RSEI is the main impedance of lithium ion battery at low temperature.

      Expert 2: the main factor limiting lithium ion battery performance at low temperature is the sharp increase of Li+ diffusion impedance at low temperature, but not SEI film.

     Low temperature properties of anode materials for lithium ion batteries

     Low temperature properties of layered anode materials

     Layered structure is the first commercial lithium ion battery anode material, which has not only the incomparable multiplier performance of one-dimensional lithium ion diffusion channel, but also the structural stability of three-dimensional channel. The representative substances include LiCoO2, Li(co1-xnix)O2 and Li(Ni,Co,Mn)O2.

     Xie xiaohua et al. took LiCoO2/MCMB as the research object to test its low-temperature charging-discharge characteristics.

     The results showed that the discharge platform decreased from 3.762V(0℃) to 3.207V(-30 ℃) with the decrease of temperature. Its total battery capacity also decreased sharply from 78.98mA·h(0℃) to 68.55mA·h(-- 30℃).

       2. Low temperature characteristics of spinel structure anode material

        Spinel structure LiMn2O4 anode material, because it does not contain Co element, has the advantage of low cost and no toxicity.

However, the variable valence state of Mn and the jahn-teller effect of Mn3+ lead to structural instability and poor reversibility of this component.

Peng zhengshun et al. pointed out that different preparation methods have a greater impact on the electrochemical properties of LiMn2O4 anode materials. Taking Rct as an example, the Rct of LiMn2O4 synthesized by high-temperature solid phase method is significantly higher than that synthesized by sol-gel method, and this phenomenon is also reflected in the diffusion coefficient of lithium ion. The main reason is that different synthesis methods have great influence on the crystallinity and morphology of the products.

         3. Low temperature characteristics of phosphate anode materials

LiFePO4, together with ternary materials, has become the main body of positive electrode materials for power battery due to its excellent volume stability and safety. The poor performance of lithium iron phosphate at low temperature is mainly due to its material itself as an insulator, low electronic conductivity, poor dispersion of lithium ions, poor conductivity at low temperature, which increases the internal resistance of the battery, greatly affected by polarization, and obstructs battery charge and discharge, so the low temperature performance is not ideal.

          When gu yijie et al. studied the charge-discharge behavior of LiFePO4 at low temperature, they found that its coulombic efficiency decreased from 100% at 55℃ to 96% at 0℃ and 64% at -20 ℃, respectively. The discharge voltage decreases from 3.11v at 55℃ to 2.62v at -20 ℃.

         Xing et al. modified LiFePO4 with nano carbon, and found that after adding nano carbon conductive agent, the electrochemical performance of LiFePO4 was less sensitive to temperature, and the low temperature performance was improved. After modification, the discharge voltage of LiFePO4 decreased from 3.40v at 25℃ to 3.09v at -25 ℃, with a decrease rate of only 9.12%. The cell efficiency was 57.3% at -25 ℃, higher than 53.4% without nano carbon conductive agent.

         Recently there has been a lot of interest in LiMnPO4. The study found that LiMnPO4 has the advantages of high potential (4.1v), no pollution, low price and large specific capacity (170mAh/g). However, due to the lower ionic conductivity of LiMnPO4 than LiFePO4, Fe is often used to partially replace Mn to form LiMn0.8Fe0.2PO4 solid solution in practice.

Low temperature properties of cathode materials for lithium ion batteries

      Compared with anode material, anode material of lithium ion battery deteriorates more seriously at low temperature, mainly for the following three reasons: battery polarization is severe when charged and discharged at low temperature with high multiplier rate, and a large amount of lithium metal deposits on the surface of cathode,
         and the reaction products of lithium metal and electrolyte generally do not have electrical conductivity; From the perspective of thermodynamics, the electrolyte contains a large amount of C -- O, C -- N and other polar groups, which can react with negative electrode materials, and the SEI membrane formed is more susceptible to low temperature. It is difficult to insert lithium into carbon anode at low temperature, and there is asymmetry of charge and discharge.

Study on low temperature electrolyte

    Electrolyte plays the role of Li+ transfer in lithium ion battery, and its ionic conductivity and SEI film formation performance have a significant impact on the low temperature performance of the battery. There are three main indexes to judge the advantages and disadvantages of low-temperature electrolyte: ionic conductivity, electrochemical window and electrode reactivity. And the level of these 3 index, depend largely on its composition material: solvent, electrolyte (lithium salt), additive. Therefore, it is of great significance to study the low temperature performance of the electrolyte for understanding and improving the low temperature performance of the battery. Compared with chain carbonates, the low temperature characteristics of EC - based electrolyte show that the ring carbonates have a tight structure, a large acting force, and a high melting point and viscosity. However, the large polarity brought by the ring structure makes it often have a large dielectric constant. EC solvent has high dielectric constant, high ionic conductivity and excellent film forming performance, which can effectively prevent the co-insertion of solvent molecules and make it indispensable. Therefore, common low-temperature electrolytic liquids are mostly based on EC and mixed with small molecular solvents with low melting point. Lithium is an important component of electrolyte. Lithium salt in electrolyte can not only improve the ionic conductivity of solution, but also reduce the diffusion distance of Li+ in solution. Generally speaking, the higher the concentration of Li+ in solution, the higher the ionic conductivity. But the concentration of lithium ions in the electrolyte is not linearly related to the concentration of lithium salts. This is because the concentration of lithium ion in the solvent depends on the dissociation of lithium in the solvent and the strength of association.

Study on low temperature electrolyte

In addition to the cost of the battery pack, the process factors in the actual operation will also have a great impact on the battery performance. (1) preparation process. Yaqub et al. studied the influence of electrode load and coating thickness on the low temperature performance of LiNi0.6Co0.2Mn0.2O2 /Graphite cell and found that, as far as capacity retention rate is concerned, the smaller the electrode load is, the thinner the coating layer is, and the better its low temperature performance is. (2) charging and discharging state. Petzl et al. studied the influence of low-temperature charge-discharge state on battery cycle life and found that when the discharge depth is large, a large capacity loss will be caused and cycle life will be reduced. (3) other factors. Surface area, pore diameter, electrode density, wettability of electrode and electrolyte, and diaphragm all affect the low temperature performance of lithium ion battery. In addition, the impact of material and process defects on the low temperature performance of the battery can not be ignored.

conclusion

In order to ensure the low temperature performance of lithium ion batteries, the following points need to be done: (1) to form a thin and dense SEI film; (2) ensure that Li+ has a large diffusion coefficient in active substances; (3) electrolyte has high ionic conductivity at low temperature. In addition, the research may also look at another type of lithium ion battery - solid lithium ion battery. Compared with conventional lithium-ion batteries, all-solid lithium-ion batteries, especially all-solid thin-film lithium-ion batteries, are expected to completely solve the problem of capacity attenuation and cycle safety when batteries are used at low temperature.