Reaction mechanism of Li2S-P2S5 system in acetonitrile based on wet chemical synthesis of Li7P3S11 solid electrolyte

[Display omitted] •The formation mechanism of Li7P3S11 electrolyte in acetonitrile is studied.•The reaction in liquid is lone pair electrons on Li2S attacking bridge S on P2S5.•The precipitate and soluble phases are Li3PS4·ACN and Li4P2S7·ACN, respectively.•Low conductivity of liquid-phase synthesized Li7P3S11 is due to residual Li4P2S6.•Soluble Li4P2S7 can be obtained by reacting Li3PS4 with Li2P4S11 or Li4P4S12, etc. All-solid-state lithium batteries have been recognized as the next generation energy storage/conversion devices for many high-power and safe applications. Li7P3S11 glass ceramics, as a promising solid electrolyte, has shown a high application possibility. Although the synthesis of Li7P3S11 by wet-chemical method is more controllable and can be well matched with the existing battery preparation processes, the chemical reaction mechanism of such a liquid-phase reaction process is not fully understood, and also its ionic conductivity is lower than that obtained by solid-state methods. In this paper, we have clarified that the liquid-phase reaction is mainly the process of lone-pair electrons of Li2S to attack the bridged S on P2S5 to obtain compounds with different element ratios, which is explored by recording different reaction times and adjusting different proportions of the phase transitions in acetonitrile (ACN) solvent. The only precipitate phase is found to be Li3PS4·ACN, which can react with soluble phases with higher P content such as Li2P4S11, Li4P4S12 and so on to obtain Li4P2S7 for the formation of Li3PS4·ACN and Li4P2S7·ACN (molar ratio 1:1) in the liquid phase. Finally, Li7P3S11 is formed by the equimolar reaction of above two compounds during the calcination. XPS and Raman results indicate that the low conductivity of liquid-phase synthesized Li7P3S11 solid electrolyte may be induced by the residual Li4P2S6, which is caused by the inadequate contact and coverage of gel-like Li4P2S7 with solid Li3PS4, resulting in the desulfurization reaction of Li4P2S7 during the heat-treatment.