Durable Layered Materials with Abundant Elements for Na Storage Applications

The demand for cost-effective batteries is growing to develop low carbon society. High-energy electrode materials with abundant Mn ions have been developed for Li-ion battery applications,^1-3 which is a suitable system for high-energy density applications at moderate cost. Nevertheless, Li ions, which are the essential element for Li-ion batteries, are also regarded as non-abundant elements, and therefore the development of cost-effective batteries without Li ions is indispensable.
O’3-type NaMnO₂, which is essentially isostructural with LiCoO₂ and LNiO₂, has been extensively studied as a potential positive electrode material for Na-ion battery applications, but practical problem is found in the higher solubility of Mn ions into electrolyte on electrochemical cycles.⁴ In contrast, P’2-type Na_2/3MnO₂ shows excellent reversibility as electrode materials. Higher energy density with good capacity retention is achieved when highly concentrated electrolyte solutions are used.⁵
A practical problem of Na-ion batteries is found in inferior electrode kinetics for hard carbon materials used as negative electrode materials. Although gravimetric energy density is not high enough, Ti-based layered oxides are an attractive candidate for the high-power battery system.^{6, 7} An electrode material with abundant iron ions is also a potential candidate for negative electrode materials of aqueous Na-ion batteries as the extremely low-cost battery system.⁸
From these results, the importance of layered Na insertion materials for advanced and practical Na-ion battery applications is discussed in detail.

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