Exceptionally high-energy tunnel-type V1.5Cr0.5O4.5H nanocomposite as a novel cathode for Na-ion batteries

We report a tunnel-type V1.5Cr0.5O4.5H/carbon-nanotube (T-VCr/C) nanocomposite as a new high-energy cathode material for sodium-ion batteries. Structural analyses using Rietveld refinement and bond-valence-energy landscape analysis based on X-ray diffraction reveal the Na+ diffusion paths and possible atomic sizes of Na+ in the V1.5Cr0.5O4.5H structure. Through combined studies using first-principles calculations and various experimental techniques, we confirm that the T-VCr/C nanocomposite delivers a large specific capacity of ~306 mAh g−1, corresponding to 2 mol Na+ de/intercalation at 15 mA g−1, with an average operation voltage of ~2.5 V (vs. Na+/Na) in the voltage range of 1.0–4.0 V based on reversible V3+/V4+ and Cr3+/Cr4+ redox reactions. Even at 900 mA g−1, the T-VCr/C nanocomposite retains a specific capacity of ~214.9 mAh g−1, corresponding to ~70.2% of the capacity measured at 15 mA g−1. Furthermore, over 100 cycles at 300 mA g−1, the T-VCr/C nanocomposite exhibits capacity retention of ~77.1% compared with the initial capacity. Operando/ex-situ X-ray diffraction and X-ray absorption spectroscopy analyses reveal the small structural change of NaxV1.5Cr0.5O4.5H (0 ≤ x ≤ 2) during Na+ de/intercalation based on V4+/V3+ and Cr4+/Cr3+ redox reaction, leading to the outstanding electrochemical performance of the T-VCr/C nanocomposite.