Polycrystalline lithium-rich oxide (PLRO) is a promising high-capacity cathode for next-generation all-solid-state batteries (ASSBs). However, its full potential is hindered by sluggish Li+ transport at primary interparticle boundaries, mainly due to the limited flowability of inorganic solid-state electrolytes (SEs). Additionally, infiltrating conventional SEs into PLRO can lead to severe interfacial side reactions because of high melting points. Herein, we report a one-step, low-temperature (<200 °C) co-sintering process that simultaneously synthesizes the SE and infiltrates it into the primary interparticle boundaries of PLRO, creating an integrated composite cathode for ASSBs. This process forms a continuous Li+ transport network, enabling deep bulk activation of PLRO. Meanwhile, the co-sintering process modulates the energy bands of the antibonding transition metal 3d-O 2p and nonbonding O 2p at the surface, achieving greater orbital overlap to suppress oxygen release and mitigate interfacial phase transformation. As a result, the PLRO-based ASSBs exhibit an impressive discharge capacity of 271 mAh g-1 at 0.1C, 212 mAh g-1 at 0.5C, and retain 80.0% capacity after 150 cycles. This study highlights the importance of enhancing ion transport to maximize the performance of PLRO-based ASSBs, offering a practical solution for advancing energy storage technologies.