The Sc2WxMo3−xO12 series as electrodes in alkali-ion batteries

Abstract

Herein, the series Sc2WxMo3−xO12 (0 ≤ x ≤ 3) is synthesised and the structure and electrochemical performance in alkali-ion batteries is characterised. The structures remain in the orthorhombic Pnca space group for the whole series with the lattice parameters increasing approximately linearly from {a = 9.6336(2) Å, b = 13.2406(3) Å, c = 9.5413(2) Å} in Sc2Mo3O12 to {a = 9.6735(2) Å, b = 13.3218(3) Å, c = 9.5811(2) Å} in Sc2W3O12. Discharge against Li delivers a high initial discharge capacity of 1200 mA h g−1 for Sc2Mo3O12 with a reversible capacity of about 150 mA h g−1 after 100 cycles. Meanwhile the increase of W content reduces both the initial and overall capacities for all lithium, sodium and potassium half cells. The initial discharge capacity for Sc2W3O12 against lithium is only about 700 mA h g−1 with a reversible capacity of about 100 mA h g−1 after 100 cycles. For all sodium and potassium half cells across the series, the capacities drop dramatically after a few cycles and the reversible capacities are low, below 50 mA h g−1. Structurally, the fully potassium discharged Sc2Mo3O12 partially transforms into a new P[4 with combining macron] space group KMo4O6 phase, while the crystallinity decreases in both fully lithium and sodium discharged Sc2Mo3O12. For Sc2W3O12, only fully potassium discharged Sc2W3O12 shows a decrease in crystallinity, while the fully lithium and sodium discharged Sc2W3O12 appears to become amorphous (or particles are too small to be examined with X-ray diffraction). X-ray absorption spectroscopy demonstrates that the Mo oxidation state changes with different type and amount of alkali-ion discharge. This work illustrates the influence of composition on the electrochemical performance in this family of compounds. © 2021 The Royal Society of Chemistry