Molten-LiCl induced thermochemical prelithiation of SiO x : Regulating the active Si/O ratio for high initial Coulombic efficiency

The low initial Coulombic efficiency (ICE) of SiOx anode caused by the irreversible generation of LiySiOz and Li2O during lithiation process limits its application for high energy-density lithium-ion batteries. Herein, we report a molten-salt-induced thermochemical prelithiation strategy for regulating the electrochemically active Si/O ratio of SiOx and thus enhancing ICE through thermal treatment of pre-synthesized LiNH2-coated SiOx in molten LiCl at 700 °C. Bulk SiOx micro-particle was transformed into pomegranate-like prelithiated micro-cluster composite (M-Li-SiOx) with SiOx core and outer nano-sized agglomerates consisting of Li2Si2O5, SiO2, and Si. Through the analysis of the reaction intermediates, molten-LiCl could initiate reactions and promote mass transfer by the continuous extraction of oxygen component from SiOx particle inner in the form of inert Li2Si2O5 and SiO2 nanotubes to realize the prelithiation. The degree of prelithiation can be regulated by adjusting the coating amount of LiNH2 layer, and the resulted M-Li-SiOx displays a prominent improvement of ICE from 58.73% to 88.2%. The graphite/M-Li-SiOx (8:2) composite electrode delivers a discharge capacity of 497.29 mAh·g−1 with an ICE of 91.79%. By pairing graphite/M-Li-SiOx anode and LiFePO4 cathode in a full-cell, an enhancement of energy density of 37.25% is realized compared with the full-cell containing graphite/SiOx anode. Furthermore, ex-situ X-ray photoelectron spectroscopy (XPS)/Raman/X-ray diffraction (XRD) and related electrochemical measurements reveal the SiOx core and Si of M-Li-SiOx participate in the lithiation, and pre-generated Li2Si2O5 with Li+ diffusivity and pomegranate-like structure reduces the reaction resistance and interface impedance of the solid electrolyte interphase (SEI) film.