Fast Li+ transport and superior interfacial chemistry within composite polymer electrolyte enables ultra-long cycling solid-state Li-metal batteries

Polyethylene oxide (PEO)-based composite polymer electrolytes (CPEs) have been considered as the most promising electrolytes for next generation solid-state Li-metal batteries. However, the insufficient ionic conductivity and the poor interfacial compatibility, primarily caused by numerous interfaces between fillers and PEO, has hindered the practical application of PEO-based CPEs at room temperature. Here, we proposed a rational design strategy to construct a novel 3D ion-conducting metal-organic framework (MOF)-based network in PEO-based CPE, where the in-situ growth of densely packed MOFs provide continuous pathways for fast Li+ transport, and the ionic liquid confined in the pores of MOFs significantly improves the interfacial compatibility and Li+ migration kinetics. Accordingly, the resulting PEO-based CPE exhibits a high ionic conductivity of 2.57 × 10−4 S cm−1 and Li+ transference number of 0.59 at room temperature, which enables the stable operation of various state-of-the-art cathodes, including LiFePO4, high-voltage LiNi0.8Co0.1Mn0.1O2, and high-capacity organic cathodes. Remarkably, the Li||LiFePO4 battery demonstrated an unprecedented cycling stability with 86% capacity retention after 2500 cycles at room temperature. Therefore, this work opens new frontiers in engineering CPEs towards ultra-long cycling solid-state Li-metal batteries.