One-step electrochemical in-situ Li doping and LiF coating enable ultra-stable cathode for sodium ion batteries

Despite of the higher energy density and inexpensive characteristics, commercialization of layered oxide cathodes for sodium ion batteries (SIBs) is limited due to the lack of structural stability at the high voltage. Herein, the one-step electrochemical in-situ Li doping and LiF coating are successfully achieved to obtain an advanced Na0.79Lix[Li0.13Ni0.20Mn0.67]O2@LiF (NaLi-LNM@LiF) cathode with superlattice structure. The results demonstrate that the Li+ doped into the alkali metal layer by electrochemical cycling act as "pillars" in the form of Li-Li dimers to stabilize the layered structure. The supplementation of Li to the superlattice structure inhibits the dissolution of transition metal ions and lattice mismatch. Furthermore, the in-situ LiF coating restrains side reactions, reduces surface cracks, and greatly improves the cycling stability. The electrochemical in-situ modification strategy significantly enhances the electrochemical performance of the half-cell. The NaLi-LNM@LiF exhibits high reversible specific capacity (170.6 mA h g−1 at 0.05 C), outstanding capacity retention (92.65% after 200 cycles at 0.5 C) and excellent rate performance (80 mA h g−1 at 7 C) in a wide voltage range of 1.5–4.5 V. This novel method of in-situ modification by electrochemical process will provide a guidance for the rational design of cathode materials for SIBs.