Effect of negative/positive capacity ratio on the rate and cycling performances of LiFePO_4/graphite lithium-ion batteries

摘要

The influence of the capacity ratio of the negative to positive electrode (N/P ratio) on the rate and cycling performances of LiFePO4/graphite lithium-ion batteries was investigated using 2032 coin-type full and three-electrode cells. LiFePO4/graphite coin cells were assembled with N/P ratios of 0.87, 1.03 and 1.20, which were adjusted by varying the mass of the graphite negative electrode. Three-electrode cells were also assembled with similar N/P ratios to understand the potential variations of the positive and negative electrodes. The cycling performances of the coin cells were evaluated by repeating the charge-discharge cycle 5000 times (1 C-rate for the 1st–1000th cycle and 2 C-rate for the 1001st–5000th cycle) at cell voltages of 2.5–4.2 V, during which their rate performances were intermittently evaluated (0.1–10 C-rate). Prior to the charge-discharge cycling test, the highest cell specific capacity was obtained from the lowest N/P ratio of 0.87. The coin cell with the highest N/P ratio of 1.20 exhibited the highest capacity retention of 86.5% at 1 or 2 C-rate in the 5000 cycle test. The results obtained using the three-electrode cells showed that the higher N/P ratio elevated the working potential ranges of the positive and negative electrodes. The lower N/P ratio led to deeper Li-ion intercalation for the negative electrode, lowering the potential of both the positive and negative electrodes. The lower N/P ratio was beneficial for achieving full performances from the initial service, but caused larger capacity fading due to the aging by cycling. The higher N/P ratio rather restricted the performances at the initial service, which was useful to prevent the cycling-derived capacity fading. The lowering of rate performance owing to the charge-discharge cycling, particularly at the high current densities of 5 and 10 C-rates, was enhanced by the decrease in the N/P ratio.