Since entering the market, lithium-ion batteries have been widely used due to their long life, large specific capacity, and no memory effect. Lithium-ion batteries have low capacity, high attenuation, poor cycle performance, and poor lithium deposition. , deintercalation of lithium imbalance and other issues. With the continuous expansion of the application field, the low temperature performance of lithium-ion batteries is more and more obvious.
According to reports, the discharge capacity of lithium-ion batteries at -20 °C is only about 31.5% at room temperature. The traditional lithium-ion battery operating temperature is between -20~+55 °C. However, in the fields of aerospace, military, electric vehicles, etc. The battery can work normally at -40 ° C. Therefore, improving the low temperature properties of lithium - ion batteries is of great significance.
Factors limiting the low temperature performance of lithium ion batteries
1) In the low temperature environment, the viscosity of the electrolyte increases, or even partially solidifies, resulting in a decrease in the conductivity of the lithium ion battery.
2) The compatibility between the electrolyte and the negative electrode and the separator deteriorates in a low temperature environment.
3) In the low temperature environment, the lithium anode of the lithium ion battery precipitates lithium seriously, and the precipitated metal lithium reacts with the electrolyte, and the product deposition leads to an increase in the thickness of the solid electrolyte interface (SEI).
4) In the low temperature environment, the internal diffusion system of the active material decreases, and the charge transfer impedance (Rct) increases significantly.
Discussion on the decisive factors affecting the low temperature performance of lithium ion batteries
Expert 1: Electrolyte has the greatest influence on the low temperature performance of lithium ion battery, and RSEI is the main impedance of lithium ion battery in low temperature environment.
Expert 2: The main factor limiting the low temperature performance of lithium-ion batteries is the sharp increase in Li at low temperatures. +Diffusion resistance, not SEI film.
Low temperature characteristics of cathode materials for lithium ion batteries
1. Low temperature characteristics of layered structure cathode material
The layered structure, which has the incomparable rate performance of the one-dimensional lithium ion diffusion channel and the structural stability of the three-dimensional channel, is the first commercial lithium ion battery cathode material. Its representative material is LiCoO. 2, Li(Co 1-xNi x)O2And Li(Ni,Co,Mn)O 2Wait.
Xie Xiaohua and others with LiCoO 2/MCMB is the research object, and its low temperature charge and discharge characteristics are tested.
The results show that as the temperature decreases, the discharge platform drops from 3.762V (0°C) to 3.207V (–30°C); the total battery capacity is also reduced from 78.98mA·h (0°C) to 68.55mA·h. (–30°C).
2. Low temperature characteristics of spinel structure cathode material
Spinel structure LiMn 2O4The positive electrode material, because it does not contain Co element, has the advantages of low cost and no toxicity. However, the Mn valence state is variable and Mn 3+The Jahn-Teller effect leads to structural instability and poor reversibility of this component. Peng Zhengshun et al., different preparation methods for LiMn 2O4The electrochemical properties of the positive electrode material have a great influence, taking Rct as an example: LiMn synthesized by high temperature solid phase method 2O4The Rct is obviously higher than that synthesized by the sol-gel method, and this phenomenon is also reflected in the lithium ion diffusion coefficient. The reason is mainly due to the influence of different synthesis methods on the crystallinity and morphology of the product.
3. Low temperature characteristics of phosphate system cathode materials
LiFePO 4Due to excellent volume stability and safety, together with ternary materials, it has become the main material of current power battery cathode materials. Gu Yijie et al. studied LiFePO at low temperature. 4The charge-discharge behavior was found to reduce the coulombic efficiency from 100% at 55°C to 96% at 0°C and 64% at –20°C; the discharge voltage decreased from 3.11V at 55°C to –20°C. 2.62V. Xing et al. utilize nanocarbon to LiFePO 4Modification, found that after adding nano-carbon conductive agent, LiFePO 4The electrochemical performance is less sensitive to temperature and the low temperature performance is improved; LiFePO after modification 4The discharge voltage drops from 3.40V at 25°C to 3.09V at –25°C, a reduction of only 9.12%; and its cell efficiency is 57.3% at –25°C, which is higher than 53.4 without nanocarbon conductive agent. %. Recently, LiMnPO 4Has aroused people's strong interest. Research found that LiMnPO 4Has a high potential (4.1V), no pollution, low price, large specific capacity (170mAh / g) and so on. However, due to LiMnPO 4Than LiFePO 4Lower ionic conductivity, so in practice, it is often used to replace Mn with Fe to form LiMn. 0.8Fe 0.2PO 4Solid solution.
to sum up
Lithium-ion batteries have been well received recently, and there are many concerns about battery safety and cycle life. The research on battery temperature is mainly focused on the capacity attenuation problem when used under high temperature conditions. With the continuous improvement of application standards, correspondingly Lithium-ion batteries are becoming more stringent, and it is imperative to expand their operating temperature range and improve their low-temperature performance.
