18650 battery 3.7v 6000mah.Positive electrode structure

　　The three-dimensional structure of positive electrode materials restricts the diffusion rate of lithium ions, especially at low temperatures. The positive electrode materials of lithium-ion batteries include commercialized lithium iron phosphate, nickel cobalt manganese ternary materials, lithium manganese oxide, lithium cobalt oxide, etc., as well as high-voltage positive electrode materials in the development stage such as lithium nickel manganese oxide, lithium iron manganese phosphate, lithium vanadium phosphate, etc. Different positive electrode materials have different three-dimensional structures. Currently, the main positive electrode materials used in electric vehicle power batteries are lithium iron phosphate, nickel cobalt manganese ternary materials, and lithium manganese oxide. Wu Wendi et al. studied the discharge performance of lithium iron phosphate batteries and nickel cobalt manganese ternary batteries at -20 ℃, and found that the discharge capacity of lithium iron phosphate batteries at -20 ℃ can only reach 67.38% of room temperature capacity, while nickel cobalt manganese ternary batteries can reach 70.1%. Du Xiaoli and others found that the discharge capacity of lithium manganese oxide batteries can reach 83% of their room temperature capacity at -20 ℃.

　　◆ High melting point solvents

　　Due to the presence of high melting point solvents in the mixed solvent of electrolyte, the viscosity of lithium-ion battery electrolyte increases at low temperatures. When the temperature is too low, electrolyte solidification occurs, leading to a decrease in the transmission rate of lithium ions in the electrolyte.

　　◆ Lithium ion diffusion rate

　　The diffusion rate of lithium ions in graphite negative electrodes decreases under low temperature conditions. Xiangyu System studied the effect of graphite negative electrode on the low-temperature discharge performance of lithium-ion batteries and proposed that the increase in charge transfer impedance of lithium-ion batteries in low-temperature environments leads to a decrease in the diffusion rate of lithium ions in the graphite negative electrode, which is an important reason affecting the low-temperature performance of lithium-ion batteries.

　　◆ SEI membrane

　　In low-temperature environments, the SEI film on the negative electrode of lithium-ion batteries thickens, and the impedance of the SEI film increases, leading to a decrease in the conduction rate of lithium ions in the SEI film. Ultimately, the polarization formed during charging and discharging in low-temperature environments reduces the charging and discharging efficiency of lithium-ion batteries.

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