First-Principles Study of H+ Intercalation in Layer-Structured LiCoO2

The electrochemical stability of layer-structured LiCoO2 in a Li+-containing aqueous electrolyte solution is critically dependent on the solution pH. The capacity fades upon cycling in electrolyte solutions below pH 11. We have investigated the detailed atomic-scale mechanism of the failure of LiCoO2 in the presence of H+ using first-principles methods. In layer-structured LiCoO2, lithium ion diffusion paths are two-dimensional channels between the cobalt-oxygen layers. However, in an aqueous electrolyte solution containing a considerable number of H+ ions, H+ will be transported into the cathodes to replace the Li+ ions. Our calculations show that once the H+ ions are intercalated into the LixCoO2 cathode, they may covalently bond to the oxygen ions, thereby decreasing the capacity of the cathodes. We have also found that such hydrogen intercalation increases barriers to the diffusion of lithium ions. Therefore, the channels would be blocked after a sufficient number of H+ ions have intercalated, typically after a few cycles.