Host–Guest Inversion Engineering Induced Superionic Composite Solid Electrolytes for High-Rate Solid-State Alkali Metal Batteries

Composite solid electrolytes (CSEs) are promising for solid-state Li metal batteries but suffer from inferior room-temperature ionic conductivity due to sluggish ion transport and high cost due to expensive active ceramic fillers. Here, a host-guest inversion engineering strategy is proposed to develop superionic CSEs using cost-effective SiO₂ nanoparticles as passive ceramic hosts and poly(vinylidene fluoride-hexafluoropropylene) (PVH) microspheres as polymer guests, forming an unprecedented "polymer guest-in-ceramic host" (i.e., PVH-in-SiO₂) architecture differing from the traditional "ceramic guest-in-polymer host". The PVH-in-SiO₂ exhibits excellent Li-salt dissociation, achieving high-concentration free Li⁺. Owing to the low diffusion energy barriers and high diffusion coefficient, the free Li⁺ is thermodynamically and kinetically favorable to migrate to and transport at the SiO₂/PVH interfaces. Consequently, the PVH-in-SiO₂ delivers an exceptional ionic conductivity of 1.32 × 10^-3 S cm^-1 at 25 °C (vs. typically 10^-5-10^-4 S cm^-1 using high-cost active ceramics), achieved under an ultralow residual solvent content of 2.9 wt% (vs. 8-15 wt% in other CSEs). Additionally, PVH-in-SiO₂ is electrochemically stable with Li anode and various cathodes. Therefore, the PVH-in-SiO₂ demonstrates excellent high-rate cyclability in LiFePO₄|Li full cells (92.9% capacity-retention at 3C after 300 cycles under 25 °C) and outstanding stability with high-mass-loading LiFePO₄ (9.2 mg cm^-1) and high-voltage NCM622 (147.1 mAh g^-1). Furthermore, we verify the versatility of the host-guest inversion engineering strategy by fabricating Na-ion and K-ion-based PVH-in-SiO₂ CSEs with similarly excellent promotions in ionic conductivity. Our strategy offers a simple, low-cost approach to fabricating superionic CSEs for large-scale application of solid-state Li metal batteries and beyond.