IntroductionConventional liquid electrolyte-based lithium-ion batteries have achieved a great commercial success. However, their energy density has theoretical limits, and safety concerns such as electrolyte leakage and thermal runaway remain. Replacing liquid electrolytes with inorganic, nonflammable solid electrolytes can significantly enhance battery safety and potentially enable the application of lithium metal anode in all-solid-state batteries (ASSBs), thereby further improving the energy density. Recent development on solid electrolyte materials has advanced rapidly, with significant improvements in ionic conductivity. Among them, halide electrolytes exhibit a high oxidative stability (i.e., ~4 V) and an excellent compatibility with high-voltage cathodes. However, cations (e.g., Zr⁴⁺, Al³⁺, In³⁺, Y³⁺) in the electrolyte reduce when halide electrolytes come into contact with lithium metal, forming metallic or alloy phases. These electronically conductive phases could cause continuous electrolyte decomposition, ultimately leading to battery failure. To improve interfacial stability, some strategies such as electrolyte doping, introducing interfacial buffer layers, and employing bilayer solid electrolyte structures are implemented to suppress interfacial reactions. However, the selection of buffer layer materials remains limited to a few commonly used compounds, and the degradation mechanisms at the lithium metal/halide electrolyte interface are still unclear in the atomic scale. This study was to employ theoretical calculations to systematically analyze the interfacial reaction between halide electrolytes and lithium metal, including the electronic conductivity of interfacial reaction products and the effects of volume changes.MethodesThe reaction phase equilibria between two contacting solid materials could be constructed by the Pymatgen (Python Materials Genomics) module. Based on enthalpy data retrieved from the Materials Project (MP) database, the interfacial reaction energy was calculated to predict the thermodynamic stability of different materials. This approach is widely used to study the interfacial stability between electrolytes and electrodes in batteries. The fundamental principle is to conceptualize two contacting materials, A and B, as a pseudo-binary system:$$C_{\text{interface}}\text{(}{c}_{\text{A}}\text{,}{c}_{\text{B}}\text{)=}x{c}_{\text{A}}+\left(1-x\right)c_{\text{B}}$$(1)where x is the mole fraction of component A. The total energy of the pseudo-binary interfacial system can be described as the sum of the energies of components A and B, each weighted by their respective mole fractions:$\Delta E_{\text{interface}}\left(c_{\text{A}},c_{\text{B}},x\right)=xE\left(c_{\text{A}}\right)+\left(1-x\right)E\left(c_{\text{B}}\right)$(2)The interfacial reaction energy is described as the reaction energy of the pseudo-binary interfacial system minus the decomposition energies of components A and B:$\Delta E_{\text{D,mutual}}\left(c_{\text{A}},c_{\text{B}},x\right)=\underset{x\in \left[0,1\right]}{\min }\frac{1}{N}\left[E_{\text{eq,interface}}\left(x{c}_{\text{A}}+\left(1-x\right)c_{\text{B}}\right)-x{E}_{\text{D}}\left(c_{\text{A}}\right)-\left(1-x\right)E_{\text{D}}\left(c_{\text{B}}\right)\right]$(3)where E_{eq, interface} is the interfacial reaction energy of the pseudo-binary interfacial system; E_D(c_A) and E_D(c_B) are the decomposition energies of components A and B, respectively; E_D can be obtained by searching the chemical space of all elements of the components and constructing the phase diagram. N is the total number of atoms involved in the reaction. The molar percentage of the electronically conductive phase in the interfacial products is calculated and used as a descriptor for the electronic conductivity of the interphase layer after constructing the reaction phase equilibrium between A and B. The higher the molar percentage is, the stronger the electronic conductivity of the interface layer will be.For interface reaction phase equilibrium: x_AA + x_BB → x_CC + x_DD + x_EE + …. The volume change rate V_c induced by the interfacial reaction can be calculated by$V_{\text{c}}=\frac{\left(x_{\text{C}}{v}_{\text{C}}+x_{\text{D}}{v}_{\text{D}}+x_{\text{E}}{v}_{\text{E}}+\cdots \right)-\left(x_{\text{A}}{v}_{\text{A}}+x_{\text{B}}{v}_{\text{B}}\right)}{x_{\text{A}}{v}_{\text{A}}+x_{\text{B}}{v}_{\text{A}}}\times 100\%$(4)where x is the stoichiometric number of different components in the phase equilibrium; v is the volume of the reactant and product per unit molecule.Results and discussionThe properties of interphases formed between Li-metal and solid electrolyte play a crucial role in the performance of all-solid-state batteries (ASSBs). If the interphase layer exhibits a mixed electronic and ionic conductivity (MIEC), continuous electrolyte decomposition will occur during battery cycling, thus leading to an increased interfacial resistance and an accelerated performance degradation. The complex interfacial reactions generate various interphases, whose volumes may change relative to the reactants (lithium metal and electrolyte). If the interphase layer undergoes volume shrinkage, interfacial separation may occur, exacerbating the inhomogeneous distribution of the interfacial electric field and current. It promotes lithium dendrite nucleation and growth at the interface. The calculation results indicate that the CPP of the interphase layer between halide electrolytes and Li-metal exceeds 9%, leading to the formation of MIEC, which can accelerate electrolyte decomposition during cycling. In addition, the volume shrinkage of interfacial interphase layer (V_c < –17.4%) can also further intensify interfacial separation and dendrite growth. Therefore, optimizing electrolyte composition to reduce interfacial reactivity as well as minimizing CPP and V_c are critical for improving ASSB performance. Based on these interfacial reaction characteristics, selecting coating materials that effectively suppress interfacial reactivity, form a passivating interphase, and exhibit minimal volume changes can significantly enhance the interfacial stability between the Li-metal anode and the electrolyte.ConclusionsHalide electrolytes exhibited an intrinsic thermodynamic instability with Li-metal, originating from the reduction of metal cations in the electrolyte into metallic and alloy phases, with a CPP ranging from 9% to 20%. The presence of these electronically conductive phases rendered the interphases layer a MEIC, leading to continuous reduction of the electrolyte during battery cycling. Also, the interfacial reaction between Li-metal and Li-MF (M = Zr, Nb, Al, Ga, In, Ge, Sn) resulted in interphase layer that underwent volume contraction, with a volume change rate ranging from –36.2% to –17.4%. This could lead to interface separation and the formation of local voids at the interface, which was experimentally confirmed. These phenomena significantly impacted Li-metal deposition and dendrite growth, further deteriorating the performance of Li-metal ASSBs. Based on the interfacial reaction characteristics between Li-metal and halide electrolytes (V_c < –17.4%, CPP > 9%), high-throughput screening was performed to identify materials that could effectively passivate the interface and mitigate reaction-induced volume changes, thereby enhancing the performance of ASSBs. The screening identified binary materials like Li₃N and Li₃P, along with the ternary material Li₉NS₃. These materials were validated to enhance the stability of the Li-metal/electrolyte interface, confirming the scientific validity and effectiveness of the screening criteria.