<p>To improve the solid–solid interface performance of all solid-state lithium batteries (ASSLBs), a novel sandwich-structured solid electrolyte (SSE, total thickness of 0.7 mm) was investigated. It comprises a central layer of perovskite-type Li_0.37Sr_0.44Zr_0.25Ta_0.75O₃ (LSZT) electrolyte (thickness of 0.5 mm) sandwiched between two layers of composite solid polymer electrolyte (CSPE, each with a thickness of 0.1 mm). The thin CSPE interlayer not only effectively reduces interfacial resistance between LSZT and electrodes, but also suppresses Li-induced reduction degradation of LSZT while ensuring uniform current density distribution across the interface. The SSE demonstrates an ionic conductivity of 8.76 × 10⁻⁵ S·cm⁻¹ at 30°C, increasing to 1.13 × 10⁻³ S·cm⁻¹ at 100°C, with an activation energy of 0.36 eV. In addition, SSE is stable for Li metal and achieves electrochemical stability up to 4.58 V vs. Li⁺/Li. SSE shows outstanding electrode/electrolyte interfacial compatibility and significant suppression of the growth of Li dendrite. Ascribing to these merits, Li ∣ SSE ∣ Li symmetric cell maintained stable operation for 500 h at a current density of 0.3 mA·cm⁻² without short circuit, confirming robust inter-facial compatibility between SSE and Li electrode. The all-solid-state LiFePO₄ ∣ Li battery with SSE has an initial reversible discharge capacity of 109.8 mAh·g⁻¹ and a reversible capacity of 118.1 mAh·g⁻¹ after 50 cycles at a charge/discharge rate of 0.1C (30°C), demonstrating good cycling performance.</p>