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Na-ion batteries represent an effective energy storage technology with slightly lower energy and power densities but potentially lower material costs than Li-ion batteries. Here, we report a new polyanionic intercalation cathode material of an unusual chemical class: sidorenkite (Na3MnPO4CO3). This carbonophosphate compound shows a high discharge capacity (∼125 mAh/g) and specific energy (374 Wh/kg). In situ X-ray diffraction measurement suggests that sidorenkite undergoes a solid solution type reversible topotactic structural evolution upon electrochemical cycling. Ex situ solid state NMR investigation reveals that more than one Na per formula unit can be deintercalated from the structure, indicating a rarely observed two-electron intercalation reaction in which both Mn2+/Mn3+ and Mn3+/Mn4+ redox couples are electrochemically active.

We present a layered Li-intercalation oxide that operates on the V3+/V4+ redox couple. The stabilization effect of adding a +3 element into the layered LiVO2 has been studied by first principles calculations. We identified Cr substitution into LiVO2 to be promising for stabilizing a layered material based on the V redox couple. Layered LiCrxV1-xO2 (x = 0.1, 0.2, 0.4, 0.5) has been synthesized and tested electrochemically. The improved capacity retention confirms that the structural stability of delithiated LiVO2 is improved by Cr doping.