Evaluation of the effective reaction zone in a composite cathode for lithium ion batteries

Reaction distribution in the composite electrode of LiCoO2, an acetylene black and an organic binder was evaluated by two-dimensional X-ray absorption spectroscopy (XAS). The valence change of Co ion in LiCoO2 due to the Li de-intercalation during charging was evaluated as the change of the absorption energy of Co K-edge. To demonstrate the reaction distribution due to ionic transportation in the composite cathode, a composite electrode laminated by an aluminum foil and a polyimide film was fabricated in this study. After charging, clear shifts of the absorption energy was observed. The reaction distribution was evaluated by evaluating the peak top energy of Co K-edge as a function of the distance from the edge of the laminated cathode. Two-dimensional mapping of the reaction distribution revealed that the electrochemically active area expands about 700 μm from the edge of the electrode under the charging condition of 0.2 C. This is consistent with a result of electrochemical charging test showing only 42% of the theoretical capacity with the current rate of 0.2 C.