Preparation of PVDF based blend microporous membranes for lithium ion batteries by thermally induced phase separation: I. Effect of PMMA on the membrane formation process and the properties

PVDF/PMMA blend microporous membranes were fabricated via thermally induced phase separation (TIPS) process using a single diluent. Then the blend membranes were soaked in a liquid electrolyte to form polymer electrolytes. Finally, the polymer electrolytes were assembled in coin cells to test electrochemical properties. The effects of PMMA/PVDF weight ratio on the phase diagram of PVDF/PMMA/diluent system, morphology, crystallinity, crystal structure, mechanical properties and porosity of the membranes were discussed. The electrochemical properties of corresponding polymer electrolytes, such as ionic conductivity, cell charge–discharge capacity and discharge performance at different current densities, electrochemical stability windows, were also investigated in this paper. It was found that the cloud point of the system decreased and the membrane morphology changed from cellular structure to network structure with an increase in the amount of PMMA. Both electrolyte uptake of blend membranes and ionic conductivity of corresponding polymer electrolytes also increased with an increase in the amount of PMMA. The maximum ionic conductivity was found to reach 3.38×10−3 S/cm at 25 °C. The graphite/polymer electrolyte/LiFePO4 cells of blend membranes showed higher charge–discharge capacity and better discharge performance at different current densities. Electrochemical stability window was stable up to 4.7 V (vs. Li+/Li).