Structural and Electrochemical Sodium (De)intercalation Properties of Carbon-Coated NASICON-Na3+yV2-yMny(PO4)3 Cathodes for Na-Ion Batteries

Mixed vanadium-/manganese-based NASICON cathodes are attractive for practical sodium-ion battery application due to their low cost and toxicity. Although the previous reports demonstrate remarkable performances of bulk Na3+yV2-yMny(PO4)(3) cathodes, their full utilization is limited by lower electronic conductivities and longer Na-ion diffusion lengths. To overcome this issue, herein, structural and electrochemical Na (de)intercalation properties of carbon-coated nanoscale NASICON-Na3+yV2-yMny(PO4)(3) cathodes are investigated. The Mn-rich carbon-coated cathodes display enhanced cycling stabilities (90% retention after 100 cycles) and rate performances (100 mAh g(-1) at 5C) compared with their bulk counterparts in low-voltage window cycling (3.8-2.75 V) due to efficient carbon coating and particle nanosizing. Upon extending the voltage window to 4.2-2.75 V, the Mn-lean cathodes show better capacity retention (approximate to 100 mAh g(-1) for 50 cycles at 1C) whereas the Mn-rich cathodes undergo structural irreversibility and rapid capacity fading. The in operando X-ray diffraction and ex situ X-ray absorption studies shed insights on the structural (ir)reversibility and redox activities of NASICON cathodes upon cycling in different voltage windows.