CuV2S4: A High Rate Capacity and Stable Anode Material for Sodium Ion Batteries

The ternary compound CuV2S4 exhibits an excellent performance as anode material for sodium ion-batteries with a high reversible capacity of 580 mAh g(-1) at 0.7 A g-1 after 300 cycles. A Coulombic efficiency of approximate to 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 Na2S matrix. The formation of Na2S is evidenced by Na-23 MAS NMR spectra and X-ray powder diffraction. During the charge proCess the nanocrystalline CO 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 Cu2S improves the electrical conductivity, leading to superior cycling and rate capability.