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[Display omitted] • The bifunctional Li 3 Bi-LiBr protective layer is prepared to stabilize Li 7 P 3 S 11 /Li interface via a simple in-situ reaction. • DFT calculations reveal low surface barrier of Li 3 Bi and high interface energy of LiBr. • Li 3 Bi-LiBr modified Li symmetric cells exhibit increased CCD to 0.838 mA/cm2 and cycle stably at 0.5 mA/cm2 at room temperature. • The modification strategy provides possibility for the application of sulfide electrolyte based ASSLMB. All-solid-state lithium metal batteries (ASSLMBs) are well known as the most promising high-energy-density storage systems. However, the unstable interface between sulfide solid electrolyte and lithium (Li) metal anode limits the application of lithium metal in sulfide-based ASSLMBs. Herein, the bifunctional Li 3 Bi-LiBr protective layer is constructed via a simple in-situ reaction between BiBr 3 and lithium metal to solve the instability issue of the Li 7 P 3 S 11 /lithium interface. The merits of the Li 3 Bi-LiBr protective layer reflect in the low surface barriers of Li 3 Bi and the high interface energy of LiBr by density functional theory (DFT) calculations. The combination of both can effectively promote lithium diffusion and avoid lithium aggregation, thus inhibiting the growth of lithium dendrite. Results show that the Li-LBB/Li 7 P 3 S 11 /LBB-Li symmetric battery exhibits dramatically increased critical current density (0.838 mA/cm2) and long cycling lifespan (over 2000 h at 0.1 mA/cm2 and 0.5 mAh/cm2) at room temperature. In addition, a distinct capacity retention enhancement about 19.03% is embodied in the LiNbO 3 @LiCoO 2 /Li 7 P 3 S 11 /LBB-Li full cell. [ABSTRACT FROM AUTHOR]

• The formation mechanism of Li 7 P 3 S 11 electrolyte in acetonitrile is studied. • The reaction in liquid is lone pair electrons on Li 2 S attacking bridge S on P 2 S 5. • The precipitate and soluble phases are Li 3 PS 4 ·ACN and Li 4 P 2 S 7 ·ACN, respectively. • Low conductivity of liquid-phase synthesized Li 7 P 3 S 11 is due to residual Li 4 P 2 S 6. • Soluble Li 4 P 2 S 7 can be obtained by reacting Li 3 PS 4 with Li 2 P 4 S 11 or Li 4 P 4 S 12 , etc. All-solid-state lithium batteries have been recognized as the next generation energy storage/conversion devices for many high-power and safe applications. Li 7 P 3 S 11 glass ceramics, as a promising solid electrolyte, has shown a high application possibility. Although the synthesis of Li 7 P 3 S 11 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 Li 2 S to attack the bridged S on P 2 S 5 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 Li 3 PS 4 ·ACN, which can react with soluble phases with higher P content such as Li 2 P 4 S 11 , Li 4 P 4 S 12 and so on to obtain Li 4 P 2 S 7 for the formation of Li 3 PS 4 ·ACN and Li 4 P 2 S 7 ·ACN (molar ratio 1:1) in the liquid phase. Finally, Li 7 P 3 S 11 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 Li 7 P 3 S 11 solid electrolyte may be induced by the residual Li 4 P 2 S 6 , which is caused by the inadequate contact and coverage of gel-like Li 4 P 2 S 7 with solid Li 3 PS 4 , resulting in the desulfurization reaction of Li 4 P 2 S 7 during the heat-treatment. [ABSTRACT FROM AUTHOR]