Currently, there is increasing interest in developing ‘beyond lithium’ battery technologies to augment, or in certain situations replace, lithium ion batteries (LIBs). Room temperature sodium ion batteries (NIBs) offer an attractive combination of low cost and plentiful constituents and a wide range of phases, structures and stoichiometries available for optimisation. Sodium layered oxides are considered to be promising candidates as cathode materials, due to their flexibility and versatility, as well as their intrinsically fast structural diffusion of Na ions which leads to enhanced rate capability. In particular, sodium manganese based layered oxides (generally Na_xMn_1−y−zM_yT_MzO₂, where T_M represents one or more transition metals, and M consists of one or more non-transition metals) are a key family of materials, in part due to the relatively low cost and environmentally friendly nature of the manganese, and consequently are worthy of a detailed investigation. Examination of these systems, particularly in terms of stoichiometry and phase, has shown that significant advances have been made recently, both in terms of understanding the mechanisms behind electrochemical performance, and in terms of refining these to produce improved materials. The goal of this review is to present the current developments in sodium manganese based layered oxides (particularly with respect to electrochemical performance, physical properties and manganese content), to discuss the current state of this field of research and to draw conclusions regarding where future investigations may be most fruitfully directed.

Sodium layered oxide materials show excellent performance as cathodes in sodium ion batteries, leading to considerable interest in routes to improving their properties. A manganese-rich P2/O3-phase Na_2/3Li_0.18Mn_0.8Fe_0.2O₂ material is synthesized, from earth abundant precursors, via a solid state-reaction. Its biphasic nature is confirmed by X-ray diffraction and transmission electron microscopy, and the inclusion of Li by solid state NMR. The pristine electrode delivers a capacity of 125 and 105 mA h g⁻¹ at C/10 and 1C rates, respectively, with a coulombic efficiency of ca. 95 to 99.9% over 100 cycles. In addition, the influence of mechanical post-treatment is explored and shows an increased energy density and capacity retention over 50 cycles when compared to the pristine compound. The strategies outlined in this work not only apply to popular sodium manganese-based layered oxides, but also to the wider family of sodium layered oxide cathode materials in general – providing an additional facile and exploitable route to optimization.