Stabilizing Lithia-Based Cathodes through the In Situ Electrochemical Formation of an Inorganic MgF2 Interfacial Coating

Lithia (Li2O)-based cathodes are considered promising alternatives to commercial cathodes because of their high capacity owing to the anionic redox reaction. The capacity of most cathodes is based on the cationic redox reaction induced by heavy transition metals in the structure, whereas that of lithia-based cathodes is based on the anionic redox reaction related to oxygen, which is lighter in weight. However, charged lithia-based cathodes containing Li2O2 and superoxo species are highly reactive to electrolytes, which deteriorates the cathode's electrochemical performance during cycling. Herein, a MgF2 coating is developed through an in situ electrochemical reaction to protect the vulnerable lithia-based cathode. Surface coating can effectively suppress undesirable parasitic reactions at the cathode/electrolyte interface. However, coating the surface of lithia-based cathodes is difficult because it generally requires a high-temperature treatment, causing unwanted side reactions between reactive lithia and the catalyst (or coating material). In contrast, coating via in situ electrochemical reactions can form a stable inorganic layer from dissolved salts in the electrolyte, which is suitable for lithia-based cathodes because no heating is required. Our MgF2 coating efficiently suppressed parasitic reactions between the cathode and electrolyte and increased the available capacity of the lithia-based cathode. Vinylene carbonate (VC) dissolved in the electrolyte also formed an interfacial layer on the cathode that mitigated parasitic reactions and enhanced the electrochemical performance of lithia-based cathode, which was consistent with previous results. However, the VC-based coating was thick and hindered the exchange of electrons and Li ions during cycling, whereas the MgF2 coating derived from a Mg salt was significantly thinner, thus facilitating electron and ion exchange. Therefore, the cell using a Mg salt showed electrochemical performance superior to those using a basic (conventional) electrolyte with and without a VC additive, confirming that the inorganic MgF2 coating effectively improves the electrochemical performance of lithia-based cathodes.