Multiple functional biomolecule-based metal-organic-framework-reinforced polyethylene oxide composite electrolytes for high-performance solid-state lithium batteries

Composite synthesis by adding inorganic fillers to the polymer electrolyte is proposed as a method to obtain solid-state electrolytes (SSE) with high ionic conductivity and long-term stability for high-performance solid-state lithium batteries. Here, we originally introduce biomolecule-based metal-organic frameworks (bio-MOFs) as efficient multiple functional active fillers with naturally abundant functional groups. The proposed bio-MOFs possess plentiful functional Lewis acidic sites composed of open metal nodes and Lewis basic sites consisting of NH₂ functional groups and heterocyclic N atoms to satisfy various requirements, such as high ionic conductivity, wide electrochemical window, high Li⁺ transference number, and long-term stability. Notably, the multiple functional bio-MOF fillers demonstrate good distribution in the PEO matrix owing to strong hydrogen bonding between functional NH₂ groups of bio-MOFs. Moreover, it ensures an enhanced electrochemical window and long-term cycling stability. These improvements are attributed to an increase in the number of free Li⁺ ions due to the strong interaction between the Lewis acidic sites (open metal sites) of the bio-MOF and TFSI⁻ of Li salts and the enhanced amorphous structure of the PEO matrix. Consequently, bio-MOF@polyethylene oxide (PEO):LiTFSI exhibited high ionic conductivity (5.7×10⁻⁵ S·cm⁻¹ at 30 °C and 5.7×10⁻⁴ S·cm⁻¹ at 60 °C), a wide electrochemical window (<4.57 V vs Li/Li⁺), and an excellent Li⁺ transference number (0.63). Moreover, Li/SSE/LiFePO₄(LFP) battery cells utilizing bio-MOF@PEO:LiTFSI exhibited a high specific capacity of 153.9 mAh·g⁻¹ at 0.1 C.