Polyvinylidene Fluoride-based Porous Polymer Electrolyte Containing Ionic Bond Modified Comb-like Polystyrene

Polymer electrolyte had become a key material for lithium ion batteries?LIBs?because of its good flexibility,excellent processability and high safety,which can replace liquid electrolytes and seperators.However,the low ionic conductivity of all solid polymer electrolyte?SPEs?and poor interface properties with electrodes prevent the application of SPEs.Porous polymer electrolytes?PPEs?combine the merit of SPEs and liquid electrolytes,which possess high ionic conductivity and good interface properties with electrodes.However,the risk of liquid electrolyte leakage of PPEs is still a big threat for LIBs.In order to solve the problem,polyvinylidene fluoride?PVDF?-based PPEs is modified to reduce the leakage of liquid electrolytes.In this paper,a new type of polystyrene grafted polymer was synthesized and mixed with PVDF matrix.The PPE membrane was fabricated by a phase inversion technique.Then the PPE membrane were immersed in 1 mol L^-1 LiClO₄-EC/PC?v/v = 1:1?electrolyte solution.The PEO derivatives with EO chains were grafted to the polystyrene main chain via ionic bonds,which not only can effectively reduce the degree of crystallinity,promote the EO chain segment motion,also can improve the adsorption and retention of liquid electrolyte.The influence of the molecular weight of polystyrene and the graft rate of PEO derivatives on the ionic conductivity were investigated,respectively.Moreover,we studied the characteristics such as surface morphology,uptake and leakage rate,and ionic conductivity of PPE membranes with different contents of the grafted polymer.The PPE showed the highest ion conductivity when the molecular weight of polystyrene is 1.8×10⁵.And the higher the graft rate of PEO derivatives,the higher ionic conductivity of PPE.The uptake of liquid electrolyte was the highest while the leakage rate decreased remarkably.Finally,the PPE with 30 wt% of the grafted polymer showed the best distribution pore and the highest liquid uptake while the leakage decreased remarkably.The ionic conductivity was one order of magnitude higher than that of pure PVDF.The electrochemical stability window of PPE-3 was 4.8 V?vs Li⁺/Li?which satisfy the requirement of commercial LIBs.The LiFePO₄/PPE-3/Li cell showed excellent cycling stability and rate performance,and the coulombic efficiency remained at 100% after 130 cycles.In order to further reduce the leakage,gelator?MDBS?was added into the PPE-3 to prepare gel porous polymer electrolyte?GPPEs?.When the content of MDBS was 2 wt%?GPPE-2?,the leakage of the membrane decreased remarkably while the liquid uptake was almost the same as GPPE-0?0 wt% MDBS?.And the ionic conductivity of GPPE-2 decreased slightly.