Synthesis and Comparative Studies of Solvent-Free Brush Polymer Electrolytes for Lithium Batteries

For next generation lithium batteries, solid polymer electrolytes (SPEs) are essential to meet the challenges of higher safety standards, higher specific energy, and easy processing. Linear polyethylene glycol (PEG) based SPEs are by far the most investigated systems for these requirements. However, the weak mechanical properties, high crystallinity, and consequently moderate ionic conductivity prevent these systems from being used in electrochemical storage devices. We address the question of the influence of the polymer architecture on the above properties by synthesizing bottlebrush copolymers carrying PEG side chains and comparing their electrochemical properties and ionic conductivity with those of the respective linear PEG polymers. For obtaining bottlebrush polymers, first methacrylate (PEGMEMA) and norbornene (Nb-PEGME) macromonomers carrying PEG side chains were synthesized and polymerized using either free radical polymerization or ring-opening metathesis polymerization (ROMP), respectively. We varied the lengths of PEG side chains (1 kg mol–1 and 2 kg mol–1), and the selection of two different backbones results in polymethacrylate Poly(MA)m-graft-PEGME1k,2k and polynorbornene Poly(Nb)m-graft-PEGME2k brushes. All synthesized brush polymers are thermally stable up to 350 °C and show a decreased crystallinity compared to their linear counterparts. Finally, the influence of polymer architecture on ionic conductivity, Li-ion transport number, and electrochemical stability of a series of SPEs obtained thereof by mixing with different amounts of LiTFSI is investigated. The implications of changing a linear polymer system to a brush architecture for potential applications in batteries were examined with respect to thermal properties of the SPEs carrying different O/Li ratios, and we found out that the ionic conductivity scales with the Tg of the system as a generic rule. Electrochemical impedance spectroscopy data facilitated the correlation of the occurring processes in the cell with the distribution of relaxation times (DRT). Ionic conductivities in the range of 10–3–10–4 S cm–1 for the solvent-free SPEs were obtained for a temperature range of 80–25 °C. Furthermore, Li ion transport numbers and electrochemical stability of the brush polymer SPEs are comparable to linear PEG SPEs.