High performance P2 sodium layered oxides: an in-depth study into the effect of rationally selected stoichiometry†
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-Na2/3Mn0.9−xNixTi0.05Fe0.05O2 (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.