Effect of the soft and hard segment composition on the properties of waterborne polyurethane-based solid polymer electrolyte for lithium ion batteries

A series of waterborne polyurethane (WPU) with various hard segment and soft segment content were synthesized by hexamethylene diisocyanate (HDI), polyethylene glycol (PEG), dimethylol propionic acid (DMPA), and diethylene glycol (DEG). An increase in hard segment content decreased the crystallinity and thermal stability of WPU. Solid polymer electrolytes (SPEs) were prepared by complexing the as-prepared WPU with LiTFSI. The ionic conductivity increased first with the increasing of the hard segment content and then decreased. The compatibility of WPU-based SPEs with lithium electrode was influenced by the hard segment content. The electrochemical stability windows for all the SPEs have reached around 5.0 V (vs. Li+/Li) at 60 °C. A maximum ion conductivity of 5.14 × 10−5 S cm−1 at 25 °C was found for WPU12-20%Li and 1.29 × 10−3 S cm−1 at 60 °C for WPU12-25%Li. All-solid-state LiFePO4/SPE/Li battery based on WPU12-20%Li electrolyte delivered discharge specific capacities of 159 and 162 mAh g−1 under 60 and 80 °C at 0.1 C, respectively. Tuning the appropriate hard and soft segment composition of WPU may ultimately lead to the successful use of WPU-based SPEs for all-solid-state lithium ion batteries.