Dual anion mediated lithium interface and ion conducting pathway for solvent-free cross-linked polymer electrolytes

Summary:

Due to increase in demand for the batteries with high energy density and flexibility, replacement of conventional carbonate-based liquid electrolytes to solvent-free solid polymer electrolytes (SPEs) is an effective strategy towards safe lithium metal batteries1. Developments in the field of SPEs were mainly focused on improving the intrinsic low ionic conductivity, narrow electrochemical stability window (ESW) and high interfacial resistance to make lithium metal polymer battery (LMPB) into a practical reality2. Beyond these challenges, higher thermal and mechanical stability along with reduced growth of dendrites are considered as the requirements for the application of polymer electrolytes3,4. In this context cross-linked SPEs with dual anion-mediated chemistry can meet all the requirements and thus, considered to be the most suitable candidates for next generation LMPBs.
Instead of employing approaches such as concentrated electrolytes and liquid plasticizers, ionic conductivity of 2*10-4 S cm-1 is achieved at 60°C with dual lithium salts of LiTFSI and LiDFOB along with the cross-linking reaction of allyl-terminated polyethylene glycol oligomers. The LiDFOB as a major lithium-ion source has enhanced the physical as well as the electrochemical properties (ESW > 4.8 V vs. Li|Li+, lithium-ion transference number > 0.4, lithium diffusion coefficient = 2.4*10-12 m2s-1) of the SPE as compared to single-salt electrolyte of LiTFSI. The findings on conducting properties by dual-anions were investigated in detail using MD simulations and an ion transport mechanism is elucidated. Introduction of dual salt approach in cross-linked networks leads to an anion-mediated lithium-ion transfer, which results in enhanced lithium-ion dynamics and interfacial properties. To evaluate the electrochemical performance, cells were assembled using LiFePO4 and LiNi0.8Co0.15Al0.05O2 cathode materials and higher specific capacity, long term cycling and higher coulombic efficiency is achieved with dual salt electrolytes.