Highly reversible lithium storage in a conversion-type ZnCo₂O₄ anode promoted by NiCl_{2-: X}F_x hydrate

Transition metal oxides are important conversion-type anodes in lithium ion batteries because of their low cost and high specific capacity. However, the poor conversion-reaction reversibility leads to low coulombic efficiency, sluggish reaction kinetics and unsatisfactory long-term cycling stability, thus severely hindering their practical applications. In this research, we propose a facile strategy to achieve highly reversible lithium storage in a ZnCo₂O₄ anode by coating ultrafine NiCl_2-xF_x hydrate on porous ZnCo₂O₄ microspheres. The intimate and well-distributed contact between two components significantly enhances the activity of the reversible conversion reaction. The NiCl_2-xF_x hydrate also promotes the reversible decomposition of organic components in the as-formed solid electrolyte interface, evidenced from several microstructural analyses. Moreover, the ultrafine hydrated NiCl_2-xF_x effectively pins ZnCo₂O₄ to inhibit the detrimental grain coarsening process during cycling. As a result, the hybrid anode shows significantly increased initial coulombic efficiency from 69.1% to 83%, and the round-trip energy efficiency from 50% to 58.7%. It also demonstrates excellent rate capability and superior cycling stability.