Mechanical Stabilization by Cyclic Compression of Composite Polymer Electrolytes for Lithium-Ion Batteries

Composite polymer electrolytes (CPEs) for lithium-ion batteries provide an effectivebalance of ionic conductivity, mechanical robustness, and safety. Loss of charge capacity, however, is caused by multiple contributing factors, such as dendrite formation, changes in polymer crystallization, and reconfiguration of porous electrodes. In this thesis we specifically examine mechanical changes in the composite electrolyte layer. The hypothesis of this investigation is that the addition of rigid particles to a polymer electrolyte counteracts the fatigue softening effect caused by cyclic dimensional changes, as would be experienced during charge and discharge cycles. We apply cyclic compression to mimic stress cycling that is caused by asymmetric volume changes during charging cycles between anode and cathode. Using a representative composite electrolyte consisting of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles in polyethylene oxide (PEO) with bis(trifluoromethane) sulfonimide (LiTFSI), we experimentally measure stress-strain response, peak stress during cyclic compression, stress relaxation, and surface topography. When tested for 500 cycles at 30% compressive strain, the average normalized peak stress for specimens containing LLZTO was reduced by 12%, compared to a reduction of 25% without LLZTO. These experiments reveal how rigid particles can favorably alter the mechanical response of a composite electrolyte and contribute to more consistent mechanical behavior over the life of lithium-ion batteries