Unexpected Relation between Degradation Rate of Lithium Ion Battery and Stage Structure of Graphite Anode

It is very important to understand the degradation behavior of lithium ion battery under various conditions (ex.; state of charge (SOC), temperature) in order to predict the lifetime of the battery. We performed long-term storage tests for lithium ion batteries which consist of layered rock-salt structure cathode and graphite anode on various conditions. During analyzing these test results, we found that the relation between the degradation rate and the SOC is not monotonous. In order to reveal the reason of this peculiar result, we paid attention to the stage structure of graphite. The electrochemical potential of graphite anode is determined by two-phase coexistence relation of stages of Li-graphite intercalation compound. Therefore we can specify the fractions of stages by using differential voltage analysis (dV/dQ). We adopted five stages to characterize the state of anode; stage1, stage2a, stage2b, stage3, and higher stage, and non-lithiated graphite [1]. We estimated the degradation rate of individual stage by solving simultaneous equations of the fraction of each stage and degradation rate. The degradation rate are expected to have negative correlation with the electrochemical potential of stage, however we revealed that the degradation rate of stage2a was much lower than expected rate, as shown in Figure 1. This result suggests that stage2a has high stability against the irreversible reaction accompanied by formation of solid electrolyte interface. Figure 1