Flexible, nonflammable, highly conductive and high-safety double cross-linked poly(ionic liquid) as quasi-solid electrolyte for high performance lithium-ion batteries

Liquid leakage and lithium dendrite growth are common safety hazards of lithium-ion batteries, and the application of quasi-solid electrolytes are considered to be the most promising material for inhibiting the safety problems. Herein, a new type of bi-functional ionic liquid is synthesized and polymerized into double cross-linked poly(ionic liquid) electrolyte (denoted as PIL-PEI) by dual cross-linking method [ring-opening polymerization network: epoxy groups in 1-vinyl-3-epoxypropylimidazolium bis(trifluoromethanesulfonyl)imide (VEIM-TFSI) cross-linked with amino-groups in poly(ethylene imine) (PEI), radical cross-linking network: VEIM-TFSI cross-linked with poly(ethylene glycol) diacrylate (PEGDA)]. The Double cross-linked network of PIL-PEI reveals satisfactory flexibility, thermal stability and remarkable electrochemical performances (room-temperature ionic conductivity, 1.03 × 10−3 S cm−1 and lithium-ion transference number, 0.47). Furthermore, the compact network of PIL-PEI enables the formation of a strong solid electrolyte interface (SEI) layer on the lithium metal leading to the inhibition of lithium dendrite growth. As results, the batteries electrochemical performances, such as high charge/discharge capacity (165.6 mAh g−1), robust rate behavior (5.0C for 94 mAh g−1) and long-term lifespan (97.8% coulomb efficiency after 200 cycles) are significantly improved after assembled by LiFePO4-based cells equipping PIL-PEI as electrolyte. The soft package cell assembled with PIL-PEI is able to successfully light up the LED light and keep illuminating under varying hazardous situations such as bending and cutting, indicating that the PIL-PEI will have potential application value in highly secure flexible electronic devices.