Strain buffer of a dual-phase P2/P′2 heterostructure enables cycling stability in Mn-based cathodes

Abstract

Manganese-based layered oxides are regarded as promising cathode materials for sodium-ion batteries, in which the P′2-type NaxMnO2 exhibits high reversible capacity via Mn3+/Mn4+ redox couple. However, the P′2 single-phase material has always been subject to structural degradation driven by Jahn–Teller distortion and anisotropic strain during cycling. Here, by applying different annealing and cooling protocols, we reproducibly obtain a dual-phase P2/P′2 cathode containing in-plane boundary features indicative of local heterostructural coupling. This coupling effect between the two phases redistributes anisotropic deformation, moderates the abrupt high-voltage structural transformation, and retains accessible Na+ transport pathways, thereby improving the reversibility of the overall structural evolution. Consequently, under extended-window testing from 1.5 to 4.4 V, the integrated P2/P′2 Na0.75MnO2 cathode delivers a specific capacity of 214 mAh g–1 at 0.05 C. During the cycling test within 2.0–4.2 V, it retains 94% of the initial capacity after 100 cycles at 1 C. This strain engineering strategy offers a novel solution to the trade-off challenge between capacity and cycling stability in manganese-based layered oxide cathodes.