Effects of current densities on the formation of LiCoO2/graphite lithium ion battery

Abstract

The activation characteristics and the effects of current densities on the formation of a separate LiCoO2 and graphite electrode were investigated and the behavior also was compared with that of the full LiCoO2/graphite batteries using various electrochemical techniques. The results showed that the formation current densities obviously influenced the electrochemical impedance spectrum of Li/graphite, LiCoO2/Li, and LiCoO2/graphite cells. The electrolyte was reduced on the surface of graphite anode between 2.5 and 3.6 V to form a preliminary solid electrolyte interphase (SEI) film of anode during the formation of the LiCoO2/graphite batteries. The electrolyte was oxidized from 3.95 V vs Li+/Li on the surface of LiCoO2 to form a SEI film of cathode. A highly conducting SEI film could be formed gradually on the surface of graphite anode, whereas the SEI film of LiCoO2 cathode had high resistance. The LiCoO2 cathode could be activated completely at the first cycle, while the activation of the graphite anode needed several cycles. The columbic efficiency of the first cycle increased, but that of the second decreased with the increase in the formation current of LiCoO2/graphite batteries. The formation current influenced the cycling performance of batteries, especially the high-temperature cycling performance. Therefore, the batteries should be activated with proper current densities to ensure an excellent formation of SEI film on the anode surface.