Sodium layered oxide cathodes: challenges and advances
Abstract: Currently, there is considerable interest in sodium-ion batteries as they represent a potentially low-cost, more plentiful alternative to increasingly scarce and expensive lithium[1–3]. Here we present an overview of sodium cathode materials, and compare some examples of the leading families.
Sodium layered oxides have a diverse range of phases and stoichiometries, making them very versatile systems. Indeed, by careful control of the constituents it is possible to synthesise materials with high voltages, though this mostly requires elements (e.g. nickel and cobalt) with other, less desirable properties (e.g. high toxicity, high cost, scarcity, etc.). Thus, we will examine this important family in detail, relating their stoichiometry to not only their electrochemical performance, but also to these other important parameters.
In this way, it is possible to demonstrate the importance of sodium manganese-based layered oxides - a result of their good combination of attractive properties (e.g. readily available precursors, low cost, relatively environmentally friendly, etc.). Consequently, these materials will be scrutinized with respect to the relevance of the manganese oxidation state to electrochemistry and several key compounds. We will also show how, through transition metal doping/substitution, it is possible to obtain excellent electrochemical performance without sacrificing their positive properties.
These results will then be summarized and discussed, particularly with respect to full-cell configurations. Finally, based upon this understanding, we will draw conclusions about both the current state of the field, and where future research may be most fruitfully directed.