P2-Na0.67Mn0.7Ni0.2Co0.1O2 stabilized by optimal active facets for sodium-ion batteries.

Abstract

Considering factors such as crustal reserves, atomic mass, redox potential and energy density, sodium-ion batteries (SIBs) are regarded as the most promising alternative to lithium-ion batteries (LIBs). Transition metal-based layered oxides, especially typical Na_xMnO₂, stand out among cathode materials due to their low cost and high energy density. However, Na_xMnO₂ cathodes face several challenges, including Jahn-Teller distortion, manganese dissolution, structural collapse, irreversible phase transition and significant capacity loss. In addition, the significant changes in lattice parameters can lead to the formation of cracks and nanovoids. It has been reported that optimizing the assembly of surface facets can be beneficial to electrochemical properties. In this work, Na_0.67MnO₂ (NMO) with a multilayered structure featuring (distinct {010} active facets and Na_0.67Mn_0.7Ni_0.2Co_0.1O₂ (NMNCO) with a homogeneous polyhedra structure featuring distinct {001} active facets were synthesized. The initial charge capacity of NMNCO reached 110.1 mA h g^-1 at a current density of 10 mA g^-1. After 100 cycles at 100 mA g^-1, it displayed a good cycling retention rate of 82.1 %. The distribution of relaxation times (DRT) reveals the facile transfer of Na⁺ ions in the NMNCO cathode. In addition, the effects of Ni and Co are revealed, and the mechanism underlying the relationship between distinct surface facet and crack formation is studied.