Defect-Rich MOFs (MIL-88A) for High-Performance PEO-Based Composite Solid Electrolytes

Metal–organic frameworks (MOFs) with active sites have been proposed as advanced fillers for fabricating PEO-based composite solid electrolytes. However, there is a great need for the design and synthesis of MOFs with more active sites to further increase the ionic conductivity of solid electrolytes. Herein, rich defect sites are constructed via acid etching to scale up the active sites of MOFs (MIL-88A). The etched MIL-88A (EMIL-88A) materials have more pores and exposed unsaturated metal coordination sites, which can facilitate the dissociation of lithium salt through the metal–anion interaction and lead to an outstanding Li⁺ transference number of 0.63. The Fe–O bond formed between metal active sites and PEO can inhibit the crystallization of PEO and provide a fast Li⁺ migration pathway, resulting in a high ion conductivity of 4.2 × 10^–4 S cm^–1 (60 °C). As a result, assembled Li–Li symmetric batteries show good stability for over 300 h at 0.1 mA cm^–2. The assembled LiFePO₄ full batteries deliver a high reversible capacity of 113.2 mAh g^–1 after 250 cycles at 60 °C and 0.5 C. This defect engineering of MOFs offers a promising strategy for PEO-based composite solid electrolytes.