Rapid Li+ transport within a MOF-based composite solid electrolyte enables high-performance FeS2-based quasi-solid-state batteries

Abstract

Pyrite-type iron disulfide (FeS₂), characterized by its abundance, high theoretical specific capacity, non-toxicity, and excellent thermal safety, has attracted extensive attention for secondary energy storage systems. However, liquid-phase electrolyte-based batteries often suffer from drastic volume changes in active materials and polysulfide shuttling during cycling, severely compromising cycling stability. In contrast, employing solid-state electrolytes can suppress these side reactions at the source, providing a promising pathway for the deep utilization of FeS₂. Nevertheless, enhancing the conduction efficiency and long-term stability of Li⁺ in polymer matrices remains a major unresolved challenge. To address this issue, we synthesised a novel lanthanide-based metal–organic framework (La-MOF) material for the first time. Employing solution blending and electrospinning techniques, we uniformly embedded its porous microcrystals within a polyacrylonitrile network. Subsequently, by combining this network with lithium bis(trifluoromethanesulfonyl)imide, we successfully prepared a composite polymer electrolyte. The resulting composite polymer electrolyte (CPE) exhibited an ionic conductivity of 2.75 × 10⁻⁴ S cm⁻¹ at 60 °C, a lithium-ion transference number of 0.84, and an electrochemical stability window extending up to 5.06 V. Theoretical simulations reveal that the La-MOF selectively immobilizes TFSI⁻ anions via multi-site hydrogen bonding and coordination interactions, while facilitating rapid Li⁺ migration through continuous transport pathways. When paired with an FeS₂ cathode, the CPE enables stable cycling with an initial discharge capacity of 828 mAh g⁻¹ at 500 mA g⁻¹ and a retained capacity of 686 mAh g⁻¹ after 400 cycles. This work demonstrates a dual-anchoring mechanism for anion immobilization and cation conduction, providing a feasible strategy toward high-energy quasi solid-state batteries.