Phase separation enables dual ionic and thermal transport in solid‐state cathodes

Coupled limitations in ionic and thermal transport remain a central challenge for practical solid‐state batteries (SSBs), particularly under fast‐charging conditions and with high‐mass‐loading cathodes. Here we introduce a bicontinuous architecture in composite cathodes, achieved through controlled phase separation between an ionic liquid and a polymer matrix. Phase separation is induced by the solvent‐dependent solubility difference between polymer and the ionic liquid complex, generating a bicontinuous network that offers continuous pathways for accelerated Li + transport and efficient thermal dissipation simultaneously. Density functional theory calculations and finite‐element simulations reveal that this structure concurrently accelerates lithium‐ion transport and facilitates thermal dissipation. This dual enhancement suppresses reaction polarization in thick cathodes, markedly boosting the rate capability of SSBs. A LiNi 0.6 Co 0.2 Mn 0.2 O 2 cathode containing 90 wt% active material with a practical loading of 15 mg cm −2 delivers a specific capacity of 107.7 mAh g −1 at 5 C and room temperature, over two orders of magnitude higher than conventional homogeneous cathodes. The system further sustains >100 mAh g −1 across an exceptionally wide temperature window, from −10°C (0.7 C) to 100°C (30 C). At an even higher loading of 25 mg cm −2 , the cell achieves an areal capacity of 3.2 mAh cm −2 at 1 C (4.5 mA cm −2 ). These findings establish a generalizable design principle for multifunctional cathodes that integrate high energy and power density with robust thermal regulation, advancing the development of next‐generation SSBs.