Bilayered Potassium Vanadate K_0.5 V₂ O₅ as Superior Cathode Material for Na-Ion Batteries

A bilayered potassium vanadate K_0.5 V₂ O₅ (KVO) is synthesized by a fast and facile synthesis route and evaluated as a positive electrode material for Na-ion batteries. Half the potassium ions can be topotactically extracted from KVO through the first charge, allowing 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^-1 still available after 50 cycles at C/10. Ex situ X-ray diffraction reveals weak and reversible structural changes resulting in soft breathing of the KVO host lattice upon Na extraction-insertion cycles (ΔV/V≈3 %). A high structure stability upon cycling is also achieved, at both the long-range order and atomic scale probed by Raman spectroscopy. This remarkable behavior is ascribed to the large interlayer spacing of KVO (≈9.5 Å) stabilized by pillar K ions, which is able to accommodate Na ions without any critical change to the structure. Kinetics measurements reveal a good Na diffusivity that is hardly affected upon discharge. This study opens an avenue for further exploration of potassium vanadates and other bronzes in the field of Na-ion batteries.