High performance P2 sodium layered oxides: an in-depth study into the effect of rationally selected stoichiometry

Abstract

The search for sodium ion battery cathodes has led to considerable interest in sodium layered oxides due to their attractive properties – e.g. flexibility, versatility, and intrinsically fast Na ion structural diffusion (leading to enhanced rate capability). Based upon our rational approach to material selection (i.e. doping a Mn-rich structure with small quantities of elements carefully selected to enhance performance properties), we present here two high performance materials synthesised via solid-state reaction: P2-Na_2/3Mn_0.9−xNi_xTi_0.05Fe_0.05O₂ (x = 0.10 and 0.20, designated Ni 10% and Ni 20% respectively). Electrochemical characterisation demonstrated that, while both materials compare favourably with the literature, the energy density and cyclability of the Ni 10% material was superior to that of Ni 20%. Detailed examination of the two materials using a range of techniques (including in situ and ex situ X-ray and neutron diffraction, scanning and transmission electron microscopy, solid state NMR, and DFT simulations) provides a good understanding of their relative physiochemical nature and electrochemical behaviour, and demonstrates the power of our stoichiometric selection strategy. In this way this work provides details of both new, high-performance materials, and validates our rational design approach.