Low temperature characteristics of lithium iron phosphate cathode materials

　　LiFePO4, together with ternary materials, has become the main positive electrode material for power batteries due to its excellent volume stability and safety. When studying the charge and discharge behavior of LiFePO4 at low temperatures, Gu Yijie et al. found that its Coulombic efficiency decreased from 100% at 55 ℃ to 96% at 0 ℃ and 64% at -20 ℃, respectively; The discharge voltage decreases from 3.11 V at 55 ℃ to 2.62 V at -20 ℃. Xing et al. modified LiFePO4 using nanocarbon and found that the addition of nanocarbon conductive agents reduced the sensitivity of LiFePO4's electrochemical performance to temperature and improved its low-temperature performance; The discharge voltage of modified LiFePO4 decreased from 3.40 V at 25 ℃ to 3.09 V at -25 ℃, with a decrease of only 9.12%; And its battery efficiency is 57.3% at -25 ℃, higher than 53.4% without nanocarbon conductive agents. Bae et al. analyzed the low-temperature performance of LiFePO4 using numerical simulation methods and pointed out that when the Li+diffusion coefficient is below 0.05 μ When m2/s, it will cause a serious decrease in specific capacity.

　　In recent years, phosphate based cathode materials have made great progress, and in addition to traditional LiFePO4, similar structured Li3V2 (PO4) 3 has also attracted attention. When studying Li3V2 (PO4) 3/C full batteries, Qiao et al. found that under 0.1C charging and discharging conditions, its discharge capacity was 130 mA · h/g at room temperature, but only 80 mA · h/g at -20 ℃; And its rate performance deteriorates more severely at low temperatures. At -20 ℃, the discharge capacity at 5C is only about 7.69% of that at room temperature, while it can hardly discharge at 10C. Rui et al. compared the low-temperature performance of LiFePO4 and Li3V2 (PO4) 3 and found that at -20 ℃, the capacity retention rate of Li3V2 (PO4) 3 was 86.7%, much higher than that of LiFePO4 (31.5%) under the same conditions. This is because the conductivity of LiFePO4 is one order of magnitude smaller than that of Li3V2 (PO4) 3, resulting in a much greater impedance and polarization effect than Li3V2 (PO4) 3; The activation energy of LiFePO4 system is 47.78 kJ/mol, which is much higher than the 6.57 kJ/mol of Li3V2 (PO4) 3, making it more difficult to remove lithium.

　　Recently, LiMnPO4 has aroused strong interest among people. Research has found that LiMnPO4 has advantages such as high potential (4.1 V), no pollution, low price, and large specific capacity (170 mAh/g). However, due to the lower ionic conductivity of LiMnPO4 compared to LiFePO4, Fe is often used to partially replace Mn to form LiMn0.8Fe0.2PO4 solid solutions in practice. The LiMn0.8Fe0.2PO4 solid solution obtained by Yang et al. using co precipitation method has a discharge capacity of 142 mAh/g at 0.1C, 25 ℃, and 72.5 mAh/g at -15 ℃. Martha et al. modified LiMn0.8Fe0.2PO4 (25-60 nm) using carbon coating and achieved good results: the discharge specific capacity can reach 160 mA · h/g at 30 ℃ and 0.2C, and 95 mA · h/g at 10C.

　　With the continuous improvement of application standards, the requirements for lithium-ion batteries have become increasingly strict. Expanding their operating temperature range and improving their low-temperature performance are imperative. From the above research, it can be seen that there is more research on the low-temperature characteristics of LiFePO4 system, while there is relatively less research on the low-temperature characteristics of ternary, Li3V2 (PO4) 3, and high-voltage nickel manganese spinel system cathode materials. Among them, although LiCoO2 was commercialized earlier, its low-temperature performance research is relatively limited due to its gradual withdrawal from the market. Compared to LiFePO4, the low ionic conductivity of LiFePO4 is more sensitive to its constraints at low temperatures, and the modification effect of nanomaterialization and the addition of conductive agents on its low-temperature performance is significant. Compared to the high ionic conductivity of Li3V2 (PO4) 3, V doping may be more beneficial for improving the low-temperature performance of LiFePO4 cathode materials. Compared with high-voltage nickel manganese spinel system and nickel cobalt aluminum ternary system cathode materials, research on their low-temperature performance is relatively limited due to significant high-temperature issues.

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