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[Display omitted] The huge Li ion transport resistance through the grain boundaries (GBs) among rigid oxide particles forces the adoption of high-temperature sintering (HTS) process over 1000 °C. Nevertheless, the severe side reactions and uncontrollable lithium loss are always companied during the high-cost HTS process, which slows down the pace of oxide solid electrolyte (OSE) for practical application and accelerates the exploration of a new OSE sintering process. Herein, a near-room-temperature (60 °C) cold-sintering process is proposed by filling the GBs with a low-melting-point plastic crystal electrolyte (PCE). Due to the soft property and high-ionic conductivity of PCE, the Li ion transport rate through the GBs is 10 times faster than the bulk phase, endowing the OSE (Li 1.5 Al 0.5 Ge 1.5 (PO 4) 3 chosen as a representative) with a room temperature ionic conductivity of 0.25 mS cm−1. As proof of the concept, the assembled Li symmetrical cells perform a low over-potential of 50 mV with a capacity of 1 mA h cm−2 and full cells delivers a capacity retention of roughly 70% after 820 cycles (1.5 years) at 0.1C. [ABSTRACT FROM AUTHOR]