The Dynamic Performance Of Licoo <sub> 2 </ Sub> During Charging And Discharging

A new method to determine the solid diffusion coefficient of Li⁺ in anode materials was introduced, which was called capacity intermittent titration technique (CITT). The solid diffusion coefficient of Li⁺ within LiCoO₂ was determined by CITT. The variation of solid diffusion coefficient of Li⁺ within LiCoO₂ under different voltages and different cycling number was studied. CITT results show that value of Li⁺ solid diffusion coefficient of LiCoO₂ is about 10^-12cm²/s. The value of solid diffusion coefficient of Li⁺ within LiCoO₂ varies not only with voltage, but also with cycling number. Within the charge-discharge cycling with the voltage range from 3.90V to 4.30 V, the solid diffusion coefficient of Li⁺ within LiCoO₂ reaches the minimum at U=3.95V and the maximum at 4.15V, and it decreased with the cycling number increased. It's the results of function with disorder of crystal structure of LiCoO₂ and SEI.The influence of discharge rate and temperature to the performance of cycling of lithium-ion battery was studied. Capacity fade of lithium-ion batteries cycled with different discharge rates were tested at ambient temperature. At 1C discharge rate, the capacity retention was 80% of the initial capacity after 400 cycles. At 0.15C, 0.2C and 0.5C discharge rate, the capacity retention was 91%, 83% and 96%, respectively. It is indicated that the capacity fade was faster at high rate cycling. At 0.5C discharge rate, the capacity retain rate of lithium-ion battery is highest, the capacity fade rate is the lowest and the performance of charge-discharge cycling is the steadiest. These indicate that there is a charge-discharge system that is best for the lithium-ion battery.Capacity fade of lithium-ion batteries cycled with different temperature were tested at 1C charge-discharge rate. Test results show that the battery tested at 57℃shows the worse performance than the one at 25℃. SEM results show that the formation of SEI on the surface of cathode materials cause the increasing of interface impedance between cathode/solute and exacerbating of polarization, thereby result in the decreasing of cycling capacity at high temperature.