Highly thermostable expanded polytetrafluoroethylene separator with mussel-inspired silica coating for advanced Li-ion batteries

The separator as an important component in Li-ion batteries faces increasing challenges with the next-generation batteries towards higher energy density and longer life. An urgent task is to simultaneously achieve fire resistance, high thermal stability and electrolyte affinity for polymeric separators. Herein, to address all these needs, we report a mussel-inspired silica-coated expanded polytetrafluoroethylene separator fabricated by a multi-step synthesis process of dopamine self-polymerization and subsequent silica in-situ self-assembly. The separator inherits the advantages of expanded polytetrafluoroethylene matrix in terms of significant fire resistance and unique porous structure, especially exhibiting superior thermal dimensional stability up to 300 °C. Additionally, the polydopamine layer acts as a double-sided tape sticking expanded polytetrafluoroethylene matrix and silica layer without any polymeric binders avoiding excessive pore blocking. More significantly, the in-situ grown silica layer combined with the polydopamine layer completely solves the Achilles' heel of polytetrafluoroethylene material to endow the separators with excellent electrolyte affinity. Benefiting from such structural uniqueness, the LiFePO4 half-cell assembled with the separator shows an enhanced discharge capacity of 157.1 mA h g−1 at 0.2C and C-rate capacity of 115.6 mA h g−1 at 5C. Therefore, this study provides inspiration for the expanded polytetrafluoroethylene-based separators facing the next-generation Li-ion batteries.