Dual interface design of Ga-doped Li₇La₃Zr₂O₁₂/polymer composite electrolyte for solid-state lithium batteries

Solid-state lithium-metal batteries (SSLMBs) with a Li₇La₃Zr₂O₁₂-based composite solid electrolyte (CSE) show great potential for overcoming the safety and specific energy concerns of conventional liquid-electrolyte Li-ion batteries. Nevertheless, achieving a satisfactory connection between a solid electrolyte and the cathode and anode materials is a major challenge. A dual interface modification strategy is proposed here to address this problem. CSEs with various fractions of Ga-doped Li₇La₃Zr₂O₁₂ (LGLZO), polyethylene oxide (PEO), and lithium bis(trifluorosulfonyl)imide (LiTFSI) are spin-coated directly onto a lithium iron phosphate (LFP) cathode to improve the cathode/CSE interfacial contact and establish a Li⁺ conducting network within the cathode. The effects of the Ga concentration in LGLZO on CSE conductivity and battery performance are investigated. The LGLZO:PEO:LiTFSI fraction and the number of spin-coated layers are adjusted to optimize battery performance. The advantage of a spin-coated CSE over a freestanding CSE in terms of reducing the migration barrier is demonstrated. In addition, an ionic liquid (IL) interconnection layer is incorporated at the Li/CSE junction to improve wettability. The effects of two IL anions, namely bis(fluorosulfonyl)imide (FSI⁻) and bis(trifluorosulfonyl)imide (TFSI⁻), on interfacial modification are systematically investigated. The optimal ionic conductivity of the CSE is ~1.0 × 10⁻³ S cm⁻¹ at 60 °C. With this SSLMB configuration, the specific LFP capacities are 150 and 141 mAh g⁻¹ at 0.1 and 1 C, respectively. Capacity retention of ~96% after 300 cycles is demonstrated.