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.