Low temperature characteristics of lithium iron phosphate cathode material.LR921 battery

　　LiFePO4, along with ternary materials, has become the main cathode material for power batteries due to its excellent volume stability and safety. Gu Yijie et al. found that the Coulombic efficiency of LiFePO4 decreased from 100% at 55 ℃ to 96% at 0 ℃ and 64% at -20 ℃, respectively, when studying its charge discharge behavior at low temperatures; The discharge voltage decreases from 3.11 V at 55 ℃ to 2.62 V at -20 ℃. Xing et al. used nano carbon to modify LiFePO4 and found that adding nano carbon 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 nano carbon conductive agents. Bae et al. used numerical simulation methods to analyze the low-temperature performance of LiFePO4 and pointed out that when the Li+diffusion coefficient is below 0.05 μ At m2/s, it will cause a serious decrease in specific capacity.

　　In recent years, phosphate system cathode materials have made great progress, and in addition to traditional LiFePO4, Li3V2 (PO4) 3 with similar structures has also attracted attention. When studying Li3V2 (PO4) 3/C full cell, Qiao et al. found that under 0.1C charging and discharging conditions, its discharge capacity was 130 mA · h/g at room temperature, while it was only 80 mA · h/g at -20 ℃; And its rate performance deteriorates even more severely at low temperatures. At -20 ℃, the discharge capacity at 5C is only about 7.69% of that at room temperature, while at 10C, it can hardly discharge. 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 the 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 in practice to form LiMn0.8Fe0.2PO4 solid solutions. 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 and 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 can also reach 95 mA · h/g at 10C.

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