Fully flexible lithium ion battery based on a flame retardant, solid-state polymer electrolyte membrane

The safety issue of a portable battery has increasingly become paramount important because of intimate contact of wearable devices with the human body. In this study, a flame retardant, thermally stable polymer electrolyte membrane (PEM) has been fabricated based on crosslinkable polyurethane precursor, viz., polyethylene glycol-bis-carbamate dimethacrylate (PEGBCDMA). This PEGBCDMA based PEM is solvent-free, transparent, bendable, and twistable, which makes it an ideal candidate for a fully flexible battery application with safety attributes. Moreover, the above PEM exhibits high ionic conductivity close to the superionic level (8 × 10−4 S cm−1 at room temperature) and electrochemical stability in a large operating window of −0.5 V to 4.5 V. The PEM shows over 80% of specific capacity retention up to 250 cycles tested in the lithium iron phosphate (LiFePO4)/PEM/graphite full cells. Of particular importance is that the present PEGBCDMA based PEM has met most of the application criteria for a flexible solid-state lithium ion battery.