Dual-enhancements of stability and wettability in O3-Na0.95Ni1/3Fe1/3Mn1/3O2 cathodes by converting surface residual alkali into ultrathin Na2Ti3O7 coatings

Abstract

Na-based layered transition metal oxides with an O3-type structure are considered as promising cathode materials for sodium-ion batteries (SIBs) due to their low cost, wide two-dimensional ion channels and high theoretical specific capacity. However, during storage, exposure to moisture and carbon dioxide in the air results in the formation of increased residual sodium compounds. This degradation exacerbates side reactions with the electrolyte and induces structural collapse, ultimately impairing their electrochemical performance. In this study, two titanium sources (e.g., itanium dioxide nanopowders and tetrabutyl titanate ethanol solution) were applied separately to develop two kinds of Na₂Ti₃O₇ coatings via the reactions between the titanium sources and the residual alkaline species on the O3-Na_0.95Ni_1/3Fe_1/3Mn_1/3O₂ particle surface. Both two Na₂Ti₃O₇ coatings could effectively enhance the electrolyte wettability and Na⁺ conductivity of the coated cathodes, along with reducing the dissolution of transition metals during cathode cycling. In particular, the tetrabutyl titanate-derived Na₂Ti₃O₇ coatings (5–8 nm) on O3-Na_0.95Ni_1/3Fe_1/3Mn_1/3O₂ cathodes realized excellent kinetics (101.9 mA h g⁻¹ at 10C) and optimal cycling stability (85.66% retention over 200 cycles at 1C). These findings demonstrate that the ultrathin Na₂Ti₃O₇ coating strategy effectively enhances layered oxide cathode performance through interface engineering, offering a promising approach for developing air-stable cathodes and emerging as a pivotal technology to advance sodium-ion battery applications.