Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries

Sodium-ion batteries (SIBs) are amongst the most attractive alternatives for stationary applications and light electromobility due to potentially substantial cost reductions resulting from the availability, wide distribution, and easily accessible nature its constituents. However, commercialization is hindered - especially by lack of high-performance negative electrodes, little development of advanced electrolytes with suitable electrochemical stability windows (ESW) and electrode-electrolyte interphases (EEI), and the necessity of ongoing optimization of the most promising positive electrodes. Sodium layered oxides (SLOs) are considered one of the best positive electrodes for SIBs, due to relatively facile synthesis, flexibility, versatility, high specific capacity and fast structural Na+ ion diffusion (which potentially enables work at high current densities). Amongst SLOs, sodium manganese-rich layered oxides (SMRLOs) - with general formula NaxMnyTM1-yO2 (y ≥ 0.67; where TM = one or more metal/s) - are the most promising candidates in terms of low-cost, environmental friendliness and cyclability. Advances in research have exploited a wide range of investigative approaches and characterization techniques (e.g. solid-state nuclear magnetic resonance (ssNMR), in situ and ex situ Synchrotron XRD (SXRD), ab initio calculations, etc.) and subsequently established a good understanding of the physicochemical properties of SMRLOs, particularly with respect to their effect on electrochemical performance. The goal of this review is, therefore, to highlight and contextualize the most recent improvements relating to SMRLOs, so as to make available a good understanding of the potential challenges facing commercialization. Conclusions regarding strategies for future SIB commercialization, especially the use of SMRLOs as positive electrodes, are proposed.