The Compatibility Between Li-ion Battery Low Temperatureelectrolyte And LiFePO₄Cathode

LiFeP04cell is one of the most promising energy storage devices. In order to expand its application, it is of importance to investigate effects of electrolyte and LiFePO4cathode parameters on low-temperature performance. Studies of this thesis include:(1) effects of solvent compositions of the electrolyte, and (2) effects of the surface density of the LiFePO4cathode on the low-temperature electrochemical performance of LiFePO4/Li half cells.By probing low temperature performance of LiFePO4/Li cells with different solvent compositions, it is found that the ionic conductivity of the electrolyte is determined by the ratio of the cyclic carbonate (EC, PC) to linear carbonate (DEC, DMC, EMC) solvents. Electrolyte [1M LiPF6/EC+DMC+EMC (1:1:1, v/v)] with33%cyclic carbonate has the highest conductivity. LiFePO4/Li cells using this electrolyte have the highest discharging capacity, the most stable discharge voltage and the lowest polarization at the temperatures from25to-20℃. Results from the cyclic voltammetry and AC impedance tests show that the interface impedance can be reduced by controlling the content of the cyclic carbonate within40%, indicating that compatibility between electrolyte and LiFePO4cathode is improved.By exploring effects of the surface density on LiFePO4/Li half cells’performance, it is shown that the surface density of the pole pieces has little influence to the cell’s room temperature capacity. At0℃, the cells with the surface density between0.317mg/cm2and0.350mg/cm2have low temperature polarization and low temperature capacity.