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caused by slow oxidation of the electrolyte by the fully charged manganese oxide, corrosion at the current collector interfaces, and/ or mass transfer effects. These phenomena can easily be minimized by lowering the voltage limit upon charge, improving electrolyte compositions and current collector materials, and by judicious use of rest periods between half-cycles. At any rate, these early results demonstrate that the orthorhombic sodium manganese oxide is remarkably stable and undergoes alkali metal intercalation processes readily and reversibly. Conclusions The suitability of the orthorhombic sodium manganese oxide for use as a cathode material in alkali metal secondary batteries has been demonstrated. This material, which has never before been used in a battery, has high specific capacity in both lithium and sodium cells, and discharge characteristics suitable for use with polymer electrolytes. An especially striking feature is the excellent reversibility and stability upon cycling in lithium cells.

Recent developments in rechargeable battery technology have seen a shift from the well-established Li-ion technology to new chemistries to achieve the high energy density required for extended range electric vehicles and other portable applications, as well as low-cost alternatives for stationary storage. These chemistries include Li-air, Li-S, and multivalent ion technologies including Mg , Zn , Ca , and Al . While Mg battery systems have been increasingly investigated in the last few years, Ca technology has only recently been recognized as a viable option. In this first comprehensive review of Ca ion technology, the use of Ca metal anodes, alternative alloy anodes, electrolytes suitable for this system, and cathode material development are discussed. The advantages and disadvantages of Ca ion batteries including prospective achievable energy density, cost reduction due to high natural abundance, low ion mobility, the effect of ion size, and the need for elevated temperature operation are reviewed. The use of density functional theory modeling to predict the properties of Ca-ion battery materials is discussed and the extent to which this approach is successful in directing research into areas of promise is evaluated. To conclude, a summary of recent achievements is presented and areas for future research efforts evaluated.

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