Cu-Doped P2-Na0.7Mn0.9Cu0.1O2 Sodium-Ion Battery Cathode with Enhanced Electrochemical Performance: Insight from Water Sensitivity and Surface Mn(II) Formation Studies

Sodium-ion batteries (SIBs) show great application prospects in large-scale energy storage. P2-type manganese-based layered oxides have received special attention by virtue of their high theoretical capacity, low cost, and environmental friendliness. However, water sensitivity and limited cycling stability hinder their application, especially since the underlying mechanisms for the above two issues are still unclear. In this work, copper substitution is used to suppress the Jahn-Teller effect of Mn³⁺ and affect the corresponding lattice structure. The water sensitivity and charge compensation mechanism were carefully investigated. Results demonstrate that water sensitivity of the electrode is related to the order of Na⁺/vacancy in the Na interlayers since water molecules are more easily inserted into the charged state electrodes, but the tendency for the water uptake does not increase with Na⁺ extraction. Furthermore, Mn²⁺ forms on the surface of electrodes in the initial discharge process, and the redox reaction in the bulk is predominantly between Mn³⁺ and Mn⁴⁺. Cu-substituted in TM layer affects the arrangement of Na⁺/vacancy and suppresses the Mn²⁺ formation on the Na_0.7Mn_0.9Cu_0.1O₂ electrode that results in superior air stability and better storage properties.