Engineering of electrospun polyimide separators for electrical double-layer capacitors and lithium-ion cells

Functionalised polyimide (PI) separators, with high electrolyte wettability and good thermo-dimensional stability, in combination with electrolytes provide ionic conductivities > 1 mS for electrical double-layer capacitors (EDLCs) and lithium-ion batteries (LIBs). The mechanical strength of PI separators can be further improved by a factor of 3 via the optimisation of ratio of soft and hard segments, and the introduction of 5% SiO2 nanoparticles (NPs). The electrolyte uptake capability and ionic conductivity of this copolymerized PI separator is also promoted by the addition of SiO2 NPs, favouring the high-rate performances of EDLCs and LIBs. The cyclic voltammetry (CV) curves of an EDLC with this separator show the rectangle shape of a typical capacitor, indicating the low ionic resistance of this copolymerized PI separator. For the usage in the LIBs, on the other hand, a coating of low-density polyethylene (LDPE) onto this PI-SiO2 (PI–SiO2@PE) separator enables the thermal shutdown function of a LIB without sacrificing cell performance. There is no significant change in morphology for this PI-SiO2@PE separator after the 50-cycle full cell test, indicating its promising application potential in LIBs. The study has demonstrated the ability to tailor the multi functionality of the polyimide separator to optimise it properties for specific applications.