(Invited) Exploring Sodium Layered Oxide Cathodes: Designing Tailored Materials

Abstract

Interest in developing energy storage systems to meet a wide array of future challenges has driven research into a range of new battery technologies. Sodium-ion batteries (SIBs) have proven to be particularly popular due to their attractive properties and broad range of potential applications. However, before SIBs take their place in the pantheon of battery technologies, there still remain challenges to overcome - perhaps the most significant of which is the ongoing development of optimal cathode materials.Sodium Mn-rich layered oxides, i.e. NaxMn1-yMyO2 (y ≤ 0.33; M is one or more transition metals, such as Ni, Ti, Fe, etc.), have proven to be a promising family of SIB cathode materials with a low cost, highly tuneable, and environmentally friendly nature synergises well with the main advantages of SIBs.[1] Considerable research in this area, using a variety of conventional and specialised techniques, has led to an increased understanding of the nature and behaviour of these materials, and the key factors affecting their electrochemistry.[1-4]Performance of these materials is frequently governed by their structure, and in this work, we will highlight the importance of taking this into consideration. For example, while Manganese-rich layered oxides are particularly attractive due to their combination of low cost and low toxicity, their performances are often hindered by the effect of Jahn-Teller distortion (resulting from the presence of Mn3+).[5] We will not only discuss this in detail, but also highlight mitigation strategies – such as doping with electrochemically active (e.g. Fe) and inactive (e.g. Mg, Ti) elements, or synergetic P2/O3 combination effect.[5–8] This understanding has enabled the rapid investigation of not only suitable elements (selected with careful attention to electrochemical performance, cost, scarcity, etc.) but also appropriate degrees of doping and substitution.In this presentation, we will highlight some of the most significant Mn-rich materials and their properties, as well as examining and explaining the key parameters and factors which determine their performance. We will explain how this understanding has been used to develop a rational design approach, before summarising the trends in this area and areas of future exploitation.