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 Li1+tCo1-tO2-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 Li1.08Co0.92O1.92 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.