Studies on interfacial reactions of positive electrodes in aqueous lithium-ion batteries

Abstract

Recently, the markets of lithium-ion batteries (LIBs) have been incredibly expanded with the necessity for electric vehicles and energy storage systems (ESSs) . However, as battery explosions have frequently occurred, the safety issue was significantly paid attention. The usage of a non-aqueous electrolyte solution is one of the main reasons for catching fire due to flammability, volatility, and low specific heat capacity. To mitigate this safety issue, an aqueous electrolyte solution has been suggested. However, the electrodes, typically composed of lithium transition metal oxides, are extremely unstable in the aqueous LIBs and provide poor cyclability. In this dissertation, I show electrochemical and chemical reactions of lithium cobalt oxide (LiCoO2, LCO) as the representative positive electrode in LIBs and demonstrate the improved electrode stability through designs of electrode interfaces in aqueous LIBs. In chapter 2, I exhibit the proton insertion, oxygen evolution, and surface structure change of LCO depending the on pH of aqueous electrolyte solutions. Various analyses also reveal structural deformation mechanism of LCO through an irreversible Li+ deintercalation/intercalation process. In chapter 3, I show the role of electrolyte anions correlated with the reactivity of water at the LCO interface using various electrolyte salts. Better cyclability of aqueous LIBs is related to kosmotropic characteristics of anion. Sulfate, as the kosmotropic anion, enhances the hydrogen-bonding strength of water and favorably forms ion pairs with Li+ at the LCO interface, which retards the proton insertion into the LCO and improve a reversibility of the Li+ electrochemistry. In chapter 4, I demonstrate the improved rate capability of LCO by adding a surfactant composed of sulfate ion at the terminal group.