The role of solvent during solvothermal synthesis of amorphous SiOx anode materials for advanced Li-ion batteries

Non-stoichiometric SiO_x (0 < x < 1.5) is promising anode active material for next-generation lithium-ion batteries. This work reports the amorphous SiO_x preparation via solvothermal reduction of SiCl₄ by Mg in tetrahydrofuran (THF) or dimethoxyethane (DME) solvents followed by annealing. We provide insights into the synthesis mechanism and solvent role. MgCl₂ is readily dissolved in THF, but not in DME where MgCl₂ remains bound to SiO_x particles. Hydration of MgCl₂ contributes to the SiO_x oxidation degree during washing. Thus, the solvent ability to dissolve and eliminate MgCl₂ by-product during the synthesis affects the сhemical composition and properties of SiO_x. The SiO_x–THF is less oxidized during washing compared with the SiO_x–DME. The electrochemical properties of SiO_x materials depend on their stoichiometry, which may be adjusted by the solvent used in the synthesis. The O/Si ratio increase in SiO_x decreases the reversible capacity and initial Coulombic efficiency but improves the cycling stability.