Toward Cryogenic Lithium Metal Batteries: An Attempt with Eu Doped LLZO Gel Composite Electrolytes

Garnet-type Li7La3Zr2O12 (LLZO) stands out as one of the most promising ceramic materials for solid-state lithium-ion batteries (LIBs), owing to its high ionic conductivity and electrochemical stability. However, critical challenges such as high interfacial resistance and poor cycling performance, particularly at subzero temperatures, have hindered its widespread application. Herein, we propose a composite solid electrolyte design by dispersing Eu-doped Li7+xLa3Zr2-xEuxO12 as an inorganic filler into a gel polymer matrix based on poly-1,3-dioxolane. This inorganic-polymer hybrid structure significantly enhances the low-temperature performance of LIBs. The incorporation of the gel polymer not only improves the interfacial contact between LLZO and the electrodes but also provides multiple pathways for lithium-ion transport, leading to enhanced ionic conductivity with the value of 5.6 × 10–4 S cm–1 at 20 °C and 9.34 × 10–5 S cm–1 at −20 °C. Electrochemical evaluations demonstrate that the LLZEO/PDL composite electrolyte exhibits remarkable performance at −20 °C: Li|Li symmetric cells achieve stable cycling for over 2000 h at 0.1 mA cm–2, and Li|LiFePO4 full cells deliver a high reversible capacity of 71.8 mAh g–1 at 0.1C, with a capacity retention of 68.8% after 300 cycles.