Sodium and sodium-ion energy storage batteries

► A review of recent advances in the solid state electrochemistry of Na and Na-ion energy storage. ► Na–S, Na–NiCl2 and Na–O2 cells, and intercalation chemistry (oxides, phosphates, hard carbons). ► Comparison of Li+ and Na+ compounds suggests activation energy for Na+-ion hopping can be lower. ► Development of new Na–ion materials (not simply Li analogues) are pivotal for advancement. ► Na-ion batteries can vie with Li-ion batteries – much opportunity and promise for new developments. Owing to almost unmatched volumetric energy density, Li-ion batteries have dominated the portable electronics industry and solid state electrochemical literature for the past 20years. Not only will that continue, but they are also now powering plug-in hybrid electric vehicles and electric vehicles. In light of possible concerns over rising lithium costs in the future, Na and Na-ion batteries have re-emerged as candidates for medium and large-scale stationary energy storage, especially as a result of heightened interest in renewable energy sources that provide intermittent power which needs to be load-levelled. The sodium-ion battery field presents many solid state materials design challenges, and rising to that call in the past couple of years, several reports of new sodium-ion technologies and electrode materials have surfaced. These range from high-temperature air electrodes to new layered oxides, polyanion-based materials, carbons and other insertion materials for sodium-ion batteries, many of which hold promise for future sodium-based energy storage applications. In this article, the challenges of current high-temperature sodium technologies including Na-S and Na-NiCl2 and new molten sodium technology, Na-O2 are summarized. Recent advancements in positive and negative electrode materials suitable for Na-ion and hybrid Na/Li-ion cells are reviewed, along with the prospects for future developments.