Bilayered Potassium Vanadate K₀.₅V₂O₅ as Superior Cathode Material for Na-Ion Batteries

Abstract

A bilayered potassium vanadate K0.5V2O5 (KVO) is synthesized by a fast and facile synthesis route and evaluated as positive electrode material for Na-ion batteries. Half the potassium ions can be topotactically extracted from KVO through the first charge, allowing then 1.14 Na+ ions to be reversibly inserted. A good rate capability is also highlighted, with 160 mAh g-1 at C/10, 94 mAh g-1 at C/2, 73 mAh g-1 at 2C and excellent cycling stability with 152 mAh g-1still available after 50 cycles at C/10. Ex-situ X-ray diffraction reveals weak and reversible structural changes consisting in soft breathing of KVO host lattice upon Na extraction-insertion cycles (∆V/V ≈ 3%). A high steadiness of the structure is also evidenced upon cycling, at both the long-range order and atomic scale probed by Raman spectroscopy. This remarkable behavior is ascribed to the large interlayer space of KVO (≈ 9.5 Å) stabilized by pillar K ions, able to accommodating Na ions without critical change in the structure. Kinetics measurements reveal a good Na diffusivity that is little affected upon discharge. This study opens further exploration of potassium vanadates and other bronzes in the field of Na-ion batteries.