Solid Polymer Electrolyte Based on an Ionically Conducting Unique Organic Polymer Framework for All-Solid-State Lithium Batteries

Solid polymer electrolytes (SPEs) are regarded as a potential candidate for the development of all-solid-state lithium batteries minus the safety issues related to their liquid counterparts. Poly(ethylene oxide) (PEO)-based SPEs with strong capability to dissolve lithium salts have found extensive application in lithium batteries. However, the crystalline nature and propensity of lithium dendrite growth in such SPEs have led to the search for preemptive alternatives like the incorporation of inorganic nanoparticles or porous frameworks as fillers in the polymer matrix. Herein, a unique organic polymer framework (OPF) was synthesized by the Suzuki-Miyaura coupling reaction between 2,5-dibromo-3-(2-(2-(2-ethoxyethoxy)ethoxy)ethyl)thiophene and triazine-phenyl boronic acid. Interestingly, the incorporation of long ethylene glycol side chains in the polymer framework resulted in the amorphous nature of the OPF. Impressively, the percentage of crystallinity of PEO reduced significantly to only 30% in OPFincorporated PEO/lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) composites. Further, the side chains present in a multidirectional manner acted as an interfacial compatibilizer aiding the homogeneous distribution of OPF in the PEO matrix. The SPE showed significantly improved ionic conductivity compared to the pristine samples (9.61 x 10-3 S cm-1 at 55 degrees C) and a lithium-ion transference number of 0.53. The performance of such SPE was further evaluated by assembling Li/LiFePO4 (LFP) coin cells, showing a stable discharge capacity of 130 mAh g-1 at 0.2C after 200 cycles. This work provides an interesting concept for building a unique type of homogeneous OPF/PEO-based composite SPE film that can be utilized as a desirable material in solid electrolytes for lithium battery applications.