Unravelling Li + Intercalation Mechanism and Cathode Electrolyte Interphase of Na 3 V 2 (PO 4 ) 3 and Na 3 (VOPO 4 ) 2 F Cathode as Robust Framework Towards High-Performance Lithium-Ion Batteries.

Although lithium-ion batteries (LIBs) are promising towards high energy density and superior safety energy storage systems (ESS), severe depletion of Li reserve cannot meet the ever-growing demand for LIBs due to the uneven distribution and limited amount of Li resource. Li-free polyanionic cathodes, such as Na 3 V 2 (PO 4 ) 3 (NVP) and Na 3 (VOPO 4 ) 2 F (NVOPF), show intriguing electrochemical performances with prospective future for LIBs due to their appropriate crystallographic sites, robust host structure, and abundant Na resource. In this work, NVP and NVOPF were systematically investigated as cathodes for LIBs using different voltage windows of 2.5-4.3, 2.0-4.3, and 1.5-4.8 V, along with their electrochemical mechanisms, cathode electrolyte interphase properties, and electrode morphologies for comparison. Ex-situ X-ray diffraction, ex-situ X-ray photoelectron spectroscopy, and post-mortem scanning electron microscopy revealed that their mechanisms shifted from a predominant Na + intercalation/deintercalation in the first charging/discharging to a mixed Li + /Na + intercalation/deintercalation at the subsequent cycling. Due to the residual Na + acting as pillar in the structure, NVP and NVPF could serve as robust host framework, providing appropriate crystallographic sites for repeated Li + /Na + intercalation/deintercalation. NVP electrode delivered a higher discharge capacity of 107.6 mAh g -1 with superior capacity retention of 84.3 % after 1000 cycles (2.5-4.3 V, 100 mA g -1 ) than NVOPF electrode (97.3 mAh g -1 , 68.8 %). Electrode polarization and kinetic analysis manifested one energetically similar and two energetically nonequivalent crystallographic Na sites within the R 3 ‾ c and I4/mmm polyanionic structure of NVP and NVOPF. This work comprehensively demonstrates the feasibility and prospect of sodium-based NVP and NVOPF polyanions serving as advanced Li-free cathodes for LIBs, which provides novel insights into seeking Li-free candidates as prospective cathodes for LIBs towards a more sustainable society and a cost-effective battery manufacturing system.