Comparative Study of the Mechanical Reinforcement by Blending, Filling, and Block Copolymerization in Bottlebrush Polymer Electrolytes

Poly(ethylene glycol) (PEG)-based bottlebrush polymer electrolytes exhibit improved room-temperature ionic conductivity and reduced crystallinity compared to those of semicrystalline poly(ethylene oxide) (PEO). However, these graft copolymers suffer from low mechanical stability. Therefore, we synthesized a PEG-based bottlebrush polymer having a polynorbornene backbone using ring-opening metathesis polymerization, and it was mechanically reinforced using three strategies: (a) by blending with a polynorbornene (PNb) homopolymer, (b) filling with TiO2 nanoparticles, or (c) via block copolymerization with a PNb segment. All three systems were converted to solid polymer electrolytes by adding LiTFSI, and their thermal, mechanical, and detailed electrochemical properties in symmetrical Li/SPE/Li cells over a large number of cycles are given. All solid-state lithium metal battery (Li/SPE/LFP) cells were fabricated, and charge/discharge cycles as well as the cycling behavior were comparatively studied. It was found that block copolymerization resulted in the highest storage modulus above 0.1 Hz and overall ionic conductivity (in the whole range of 25 to 80 °C) compared to those of the other two strategies. Furthermore, the highest accessible discharge capacities (159 mA h g–1) and highest capacity retention of 88% after 50 cycles were also achieved with the block copolymer concept.