Boosting high initial coulombic efficiency of hard carbon by in-situ electrochemical presodiation

Hard carbon (HC) is a promising anode material for sodium ion batteries (SIBs), whereas inferior initial coulombic efficiency (ICE) severely limits its practical application. In the present work, we propose an in situ electrochemical presodiation approach to improve ICE by mixing sodium biphenyl (Na-Bp) dimethoxyethane (DME) solution with DME-based ether electrolyte. A solid electrolyte interface (SEI) could be formed beforehand on the HC electrode and Na+ was absorbed to nanopores and graphene stacks, compensating for the sodium loss and preventing electrolyte decomposition during the initial charge and discharge cycle. By this way, the ICE of half-cells was increased to nearly 100 % and that of full-cells from 45% to 96% with energy density from 132.9 to 230.5 W h kg−1. Our work provides an efficient and facile method for improving ICE, which can potentially promote the practical application of HC-based materials.