Critical role of crystal water for a layered cathode material in sodium ion batteries

Layered transition metal oxides are considered promising cathodes for sodium ion batteries (SIBs) due to their superior specific capacities. However, they usually suffer from insufficient cycling and rate performance mainly from the structural instability during repeated cycles. We overcome these longstanding challenges by engaging crystal water in the interlayer space of sodium manganese oxide under the Birnessite framework. The crystal water enhances Na ion diffusion both in the crystal host and at the interface, suppresses fatal Mn²⁺ dissolution, and improves long-term structural stability, leading to excellent performance in rate capability and cycle life. The current study suggests that many hydrated materials can be good candidates for electrode materials of emerging rechargeable batteries that need to deal with the large size or multivalent charge of their carrier ions.