As all-solid-state lithium-ion batteries (ASSBs) start to dominate the electronic market, studies regarding to recycling spent batteries remain limited and uncomprehensive. The primary factor causing the failure of batteries is the loss of structural integrity in solid-state electrolytes (SSEs) with limited degradation in the active properties. A promising reprocessing strategy is to repair the fragmented structure. Herein, solid-state composite electrolytes, Li₇La₃Zr₂O₁₂ with polypropylene carbonate (PPC) and lithium perchlorate (LLZO–PPC–LiClO₄), are reprocessed by cold sintering process. The low sintering temperature allows co-sintering ceramics with polymers and salts, thereby enabling densifying composite structures. Reprocessed LLZO–PPC–LiClO₄ show densified microstructures with ionic conductivities above 10⁻⁴ Scm⁻¹ at room temperature and good long-term stability at 0.1 mA h cm⁻² over 2000 hours. The cold sintered full-cell, Li₄Ti₅O₁₂/ reprocessed LLZO–PPC–LiClO₄/LiFePO₄, exhibits impressive electrochemical performance.