Potassium-pillared Na4FeV(PO4)3@C cathode for high-performance sodium-ion batteries

Abstract

Due to the open 3D framework structure and relative high capacity, the NASICON type Na₃V₂(PO₄)₃ has aroused enormous attention as the cathode material for sodium-ion batteries. However, it still suffers from the toxicity and high cost of vanadium elements, coupled with low electronic conductivity. In this study, we partially substituted V with the environmentally friendly and cost-effective transition element Fe and doped K into the Na site to fabricate a series of Na_4-xK_xFeV(PO₄)₃@C composites using a facile sol-gel method. The structural stability, Na⁺ mobility and electronic conductivity can significant improved by replacing Na⁺ with K⁺ and applying a carbon coating. Consequently, the Na_3.9K_0.1FeV(PO₄)₃@C electrode delivers a reversible discharge capacity of 83.85 mAh g⁻¹ at 5C after 3000 cycles, with a capacity retention of 91.7 %. It also exhibits an outstanding rate performance with a specific discharge capacity of 83.88 mAh g⁻¹ even at 20.0C. The kinetic analyses and ex-situ characterizations confirm that a small volume change, the pseudocapacitive- dominated sodium storage behavior and highly reversible redox reaction (Fe²⁺/³⁺ and V³⁺/⁴⁺) occur during the electrochemical reaction process. Finally, the optimized K-doped NFVP cathode also demonstrates great potential in practical utilization through the evaluation of electrochemical performance for full cells.