Sidorenkite (Na₃MnPO₄CO₃): A New Intercalation Cathode Material for Na-Ion Batteries

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.