IntroductionLi₁₀GeP₂S₁₂ solid electrolyte is regarded as a promising candidate for all-solid-state lithium batteries due to its ultra-high ionic conductivity and low grain boundary resistance. However, its instability against moisture and incompatibility with lithium metal impede its application. Elemental substitution or doping is commonly employed to improve the overall properties of solid electrolytes. Based on hard-soft acid-base theory, partially substituting S with O can form a more stable structure, thereby inhibiting hydrolysis and structural damage upon exposure to moist air. However, excessive O doping causes a reduction of ionic conductivity. The introduction of rare-earth elements, such as scandium (Sc) with larger ionic radius, can enlarge the lattice volume and mitigate the adverse effects of O doping. Meanwhile, Sc-containing compounds generated in the interface between solid electrolyte and lithium metal is beneficial to suppress the interfacial side reactions. In this work, Sc and O co-doped Li₁₀GeP₂S₁₂-based electrolytes were synthesized. The moisture stability, interface between the electrolyte and lithium metal and electrochemical performances of all-solid-state lithium batteries were significantly improved.MethodsLi_10+0.5xGe_1–xSc_xP₂S_12–1.5xO_1.5x(x=0, 4%, 8%, 12%, 16%, in mole) solid electrolytes were prepared by ball-milling and subsequent high-temperature sintering. The structure of obtained solid electrolytes was analyzed by a model AXS D8 Advance X-ray diffractometer (Bruker Co., Germany) with Cu K_α radiation in the angular range of 10°–80°. The Raman spectra were recorded by a model inVia-reflex Raman spectrophotometer (Renishaw Co., UK) with an excitation wavelength of 532 nm. The morphology and elemental distribution of the solid electrolyte particles were determined by a model Regulus-8230 scanning electron microscope (SEM, Hitachi Co., Japan) with an energy-dispersive X-ray spectroscope. The ionic conductivity of the solid electrolytes was measured by an electrochemical impedance spectroscope and the electronic conductivity was tested through direct current polarization at 0.5 V by a model 1470E electrochemical workstation (Solartron Co., UK). The air stability of the solid electrolytes was evaluated through the amount of H₂S gas released in a confined environment with approximately 40% relative humidity by a model GX-2009 H₂S gas sensor (Riken Keiki Co., Ltd., Japan).Symmetric batteries were fabricated with two lithium foils attached to both sides of the pelletized solid electrolytes. The critical current density was conducted through galvanostatic cycling at step-increased current densities using a model Land-CT2001A battery test system (Wuhan Rambo Testing Equipment Co., Ltd., China). To assemble LiCoO₂⊥ solid electrolyte/Li all-solid state lithium batteries, composite cathode was prepared by mixing LiCoO₂ and the electrolyte powder in a mass ratio of 70:30. The composite cathode was spread on to one side of the solid electrolyte tablet uniformly and pressed at 360 MPa. The lithium foil was placed on the another side of solid electrolyte to serve as an anode. The charge and discharge measurements of all-solid-state batteries were conducted at room temperature in a voltage range of 3.0–4.2 V using a model Land-CT2001A battery test system.Results and discussionSc and O co-doped Li₁₀GeP₂S₁₂ solid electrolytes are synthesized through ball-milling and subsequent high-temperature sintering at 620 ℃. The XRD patterns and Raman spectra indicate that the optimal doping concentration is 8%, at which the lattice volume is enlarged and no heterogeneous phase is detected. The ionic conductivity of optimized Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12 solid electrolytes sintered at 620 ℃ has a high ionic conductivity of 5.85 mS·cm^–1. In addition, it also exhibits a decreased electronic conductivity from 2.93×10^–8 S·cm^–1 to 1.65×10^–8 S·cm^–1 with a low activation energy of 0.20 eV. After 120-min exposure in a confined environment with a relative humidity of approximately 40%, Li₁₀GeP₂S₁₂ undergoes irreversible hydrolysis and releases 0.59 cm³·g^-1 of H₂S gas. In contrast, Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12 produces only 0.24 cm³·g^–1 of H₂S gas under the same conditions and retains an ionic conductivity of 1.21 mS·cm^–1, which is one order of magnitude greater than that of Li₁₀GeP₂S₁₂. Annealing can restore up to 70.77% of its original ionic conductivity, which is attributed to Sc and O doping that forms a more stable structure, thereby inhibiting the hydrolysis reaction.Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12-based symmetric cell achieves a significant increase in critical current density from 1.0 mA·cm^–2 to 2.4 mA·cm^–2. After undergoing constant-current cycling for 800 h at a current density of 0.1 mA·cm^–2, the polarization voltage of Li\ Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12\Li maintains at ±0.5 V, demonstrating an effective suppression of the side reaction between electrolyte and lithium metal. LiCoO₂\Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12 \Li all-solid-state lithium battery exhibits superior cycling stability and rate performance, with an initial discharge capacity of 128.45 mA·h·g^–1 and a capacity retention of 87.5% after 100 cycles at 0.1 C. Furthermore, the capacity retention remains at 81.7% after 500 cycles at 1 C.ConclusionsSc and O co-doped Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12 solid electrolyte sintered at 620 ℃ had an optimal ionic conductivity of 5.85 mS·cm^–1. The humid air stability was improved after partially substituting S with hard base O. LiCoO₂ | Li all-solid-state lithium battery assembled with Li_10.04Ge_0.92Sc_0.08P₂S_11.88O_0.12 exhibited enhanced long-term cyclic performance and rate capability. This work demonstrated a significant potential of Sc and O co-doping in enhancing both the structural stability and electrochemical performance of Li₁₀GeP₂S₁₂-based solid electrolytes, making them promising candidates for all-solid-state lithium batteries.