Highly Ion-Conductive, Elastic, and Adhesive Zwitterionic Polymer Electrolyte for All-Solid-State Lithium Batteries

Currently, lithium-ion batteries (LIBs) are considered to be one of the most popular energy storage systems for electronic devices supported by high energy density, high operating voltage, and favorable cycling performance. However, commercial LIBs with the organic liquid electrolyte and lithium (Li) salts are associated with critical safety issues such as uncontrollable side reactions, toxic liquid electrolyte leakage, flammability of electrolytes, and poor thermal stability. Therefore, replacing the liquid electrolyte with solid electrolytes is quite necessary. Among several solid ion conductors, solid polymer electrolytes (SPEs) can offer excellent flexibility, interfacial compatibility with electrodes, good processibility, low cost, and light weights that can overcome the limitations of ceramic ion conductors. However, current SPEs often encounter limitations such as poor mechanical strength and dimensional thermal stability, inferior electrochemical stability, and low Li+ ion conductivity at room temperature (~10-5 S cm-1 at 25 ℃). Here we present a multifunctional solid polymer electrolyte based on zwitterionic polyurethanes (zPU-SPE) for all-solid-state LIBs (SLBs). Our zPU-SPE exhibits a great potential to overcome current technical limitations of conventional SPE materials in SLB applications (e.g., low Li+ ion conductivity, inferior electrochemical/mechanical stabilities, unsatisfactory suppression of Li dendrite growth). We designed and synthesized a series of zPU [i.e., poly ((diethanolamine ethyl acetate)-co-poly(tetrahydrofuran)-co-(1,6-diisocyanatohexane))]. Our zPU-SPE can host an equal amount of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) without phase separation (up to 90 wt% of LiTFSI loading). The Li-ion conductivity value of zPU exponentially increases with the addition of LiTFSI and reaches 7.4 × 10-4 S/cm at 25° C, almost 14 times higher than that of poly(ethylene oxide) (PEO) SPE with ethylene oxide/Li+ ratio = 16. In addition, its superior adhesion energy (487.5 J/m2 of zPU-SPE vs. 150 J/m2 of commercial 3M Scotch Tape) can minimize interfacial resistance between electrode and SPE, and thus cell resistance of 100-µm-thick zPU SPE is as low as 280 W/cm2 compared to 1230 W/cm2 of the cell with a similar thickness of PEO SPE. Our zPU-SPE also showed an excellent elastic property with a tensile break of 1700% owing to its high density of inter- and intra-molecular hydrogen bonding in polymer matrix. The SLB battery performance of PEO and zPU-SPEs was evaluated using a solid-state Li/SPE/LiFePO4 cell; the assembled SLB cell was cycled at a constant current rate of 1 C at 25 °C. After a discharge capacity of the cell with zPU-SPE stabilized at 100 mAh g-1 after 15 cycles, there was a negligible capacity decrease (only 3% capacity decrease after 500 cycles), delivering a discharge capacity of 97 mAh g-1 with stable Coulombic efficiency (100%) over entire cycles. However, the discharge capacity of Li/PEO/LiFePO4 cells drops rapidly to 3 mAh g-1 after 100 cycles. These results demonstrate that the SLB cell assembled with PCB-PTHFU shows high discharge/charge capacity and excellent capacity retention with stable Coulombic efficiency. The good electrochemical performance of the zPU SPE can be attributed to good compatibility with electrodes, low charge transfer resistance at the interface of electrode/electrolyte, and high Li-ion conductivity, strongly suggesting that zPU SPEs are potential candidates for development of high performance of SLBs. Figure 1