Outstanding specific energy achieved via reversible cycling of V4+/V2+ redox couple in N-doped carbon coated Na3V2(PO4)2F3: An ex-situ XRD, XPS and XAS study

Abstract

Sodium vanadium fluorophosphates [Na3V2(PO4)(3-x)F-3x] are the promising cathode materials due to their high Na-ion intercalation voltage, specific capacity and long cycle life. However, they still lag behind lithium-ion counterparts in specific energy. Extending the working potential window can enhance Na-ions accommodation in Na3V2(PO4)(2)F-3,F- increasing specific energy. Though a few reports have shown improved energy through an extended voltage window, cycle life remains limited. Herein, an in-situ N-doped carbon coated Na3V2(PO4)(2)F-3 (C-NVPF) nanoparticles embedded in mesoporous carbon is synthesized through scalable microwave assisted sol-gel route. The C-NVPF exhibits reversible specific capacity of similar to 126 mAh g(-1) with excellent rate performance up to 10 C-rate. It shows similar to 111 mA h g(-1) with 70 % capacity after 500 cycles at 5.0C. Additional sodium insertion of similar to 1.3 mol at 1.0-4.5 V results in high specific capacity of 210 mAh g(-1). Ex-situ XRD, XAS and EIS study reveal the Na-ion storage mechanism in C-NVPF at different state of charge (SOC). A similar to 3.8 V NVPF//Hard carbon full cell has shown similar to 95 mAh g(-1) and 65 % capacity retention after 100 cycles. The structural and electrochemical studies on additional sodium inserted C-NVPF provides new insights for high-energy SIB development.