Improving lithium-ion battery safety through separators with thermal shutdown characteristics induced by thermal expansion microspheres

Battery separators with thermal shutdown capabilities have been developed to improve battery safety. The developed separators incorporate thermally expansive microspheres and have been produced based on poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, with different amounts (0 wt%, 5 wt%, 10 wt% and 15 wt%) of thermally expansive microspheres. The preparation of the separators involves solvent casting with thermally induced phase separation at different temperatures: 20 degrees C and 80 degrees C. The processing conditions allow to tune membranes degree of porosity between 54 % and 5 % depending on the microspheres content. Further, microspheres addition leads to a decrease in the Young modulus from 474 MPa for neat PVDF-HFP membranes to 365 MPa for the membrane with 15 wt% of microspheres. The PVDF-HFP membrane with 15 wt% microsphere content and evaporated at room temperature shows values of 84 %, 0.08 mS cm(-1), 0.37, 2.7, and 145 and for electrolyte uptake, ionic conductivity, lithium transference number, tortuosity, and MacMullin number, respectively. In a half-cell Li/C-LiFePO4, the electrochemical performance of these separators shows reversible cyclability and suitable rate capability with discharge capacity value of 146 mAh.g(-1) at C/8-rate. After 100 cycles, the PVDF-HFP membrane with 15 wt% microsphere content shows a capacity value of 44 mAh.g(-1) at C-rate, with a 5 % capacity fading and a similar to 100 % coulomb efficiency. The thermal shutdown capabilities were verified in charge-discharge tests with applied temperature. The thermal shutdown effect is confirmed at 100 degrees C, temperature at which the microspheres increase in size and collapse the structure of the separator, decreasing battery discharge capacity (C/5-rate) from 120 mAh.g(-1) to 0 mAh.g(-1). The presented approach allows to improve battery safety by properly addressing thermal runaway.