Solid-state lithium-metal batteries (SSLMBs) with a composite solid electrolyte (CSE) have great potential for achieving both high energy density and high safety and are thus promising next-generation energy storage devices. The current bottlenecks are a high electrode/electrolyte interface resistance and the limited Li+ conductivity of the solid electrolyte layer. To reduce the interface resistance, a tape casting method is used to directly deposit a CSE layer (∼20 μm) onto a model LiFePO4 cathode. The CSE slurry infiltrates the cathode layer, forming a Li+ conduction network and ensuring intimate contact between the CSE and the cathode. The tape casting parameters, such as the polymer/Li salt ratio, inorganic filler fraction, and casting thickness, for the CSE layer are investigated. To increase Li+ conductivity, Ce substitution is conducted for Li7La3Zr2−xCexO12, x = 0-0.15. The effects of Ce content on the specific capacity, rate capability, and cycling stability of Li//CSE//LiFePO4 cells are systematically studied. Li7La3Zr1.9Ce0.1O12 (i.e., x = 0.1) is found to be the optimal composition; it outperforms Li7La3Zr2O12 and Li6.25Ga0.25La3Zr2O12 in terms of CSE conductivity and SSLMB charge-discharge performance.