Cu²⁺ Dual-Doped Layer-Tunnel Hybrid Na_0.6Mn_1–xCu_xO₂ as a Cathode of Sodium-Ion Battery with Enhanced Structure Stability, Electrochemical Property, and Air Stability

Sodium-ion batteries (SIBs) have been regarded as a promising candidate for large-scale renewable energy storage system. Layered manganese oxide cathode possesses the advantages of high energy density, low cost and natural abundance while suffering from limited cycling life and poor rate capacity. To overcome these weaknesses, layer-tunnel hybrid material was developed and served as the cathode of SIB, which integrated high capacity, superior cycle ability, and rate performance. In the current work, the doping of copper was adopted to suppress the Jahn–Teller effect of Mn³⁺ and to affect relevant structural parameters. Multifunctions of the Cu²⁺ doping were carefully investigated. It was found that the structure component ratio is varied with the Cu²⁺ doping amount. Results demonstrated that Na⁺/vacancy rearrangement and phase transitions were suppressed during cycling without sacrificing the reversible capacity and enhanced electrochemical performances evidenced with 96 mA h g^–1 retained after 250 cycles at 4 C and 85 mA h g^–1 at 8 C. Furthermore, ex situ X-ray diffraction has demonstrated high reversibility of the Na_0.6Mn_0.9Cu_0.1O₂ cathode during Na⁺ extraction/insertion processes and superior air stability that results in better storage properties. This study reveals that the Cu²⁺ doping could be an effective strategy to tune the properties and related performances of Mn-based layer-tunnel hybrid cathode.