Stabilizing Layered Structure in Aqueous Electrolyte via Dynamic Water Intercalation/Deintercalation

Aqueous lithium-ion batteries (ALIBs) with nonflammable feature attract great attention for large-scale energy storage. However, the layered cathode materials (such as LiCoO₂ ) present serious capacity decay in ALIBs. The degradation mechanism of layered cathode materials in ALIBs is still not clear and an effective strategy to improve cycling stability remains a great challenge. In this work, the authors use LiCoO₂ as a typical example to investigate its structural degradation in aqueous electrolytes. It is found that H⁺ insertion accelerated irreversible layered-to-spinel phase transition is the main reason causing structural degradation and fast capacity fading in LiCoO₂ . Subsequently, Li-excess Li_{1+ t} Co_{1- t} O_{2- t} with intermediate spin Co³⁺ is developed to mitigate H⁺ influence and the adverse phase transition in aqueous electrolyte. It is interesting to discover that reversible water intercalation/deintercalation occurs in the layered structure during charge/discharge, which effectively suppresses the layered-to-spinel phase transition with cycling. Benefiting from the stabilized layered structure, the Li-excess Li_1.08 Co_0.92 O_1.92 shows a significantly improved cycling performance in the neutral aqueous electrolyte with a large specific capacity and excellent rate capability. This work provides a promising structural regulation strategy for the layered cathode materials, enabling their potential application in ALIBs.