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

Abstract

Herein, the series Sc₂W_xMo_3−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 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.