CuVS: A High Rate Capacity and Stable Anode Material for Sodium Ion Batteries.

The ternary compound CuVS exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g at 0.7 A g after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a NaS matrix. The formation of NaS is evidenced by Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or CuS improves the electrical conductivity, leading to superior cycling and rate capability.