Recent developments in the ENEA lithium metal battery project

Solvent-free P(EO)20LiTFSI + PYR14TFSI polymer electrolyte films with PYR14+/Li+ mole ratios ranging from 0.96 to 3.22 were prepared by hot-pressing mixtures composed of PEO, LiTFSI and PYR14TFSI of selected stoichiometries. The PYR14TFSI room temperature ionic liquid (RTIL) is homogeneously incorporated into the P(EO)20LiTFSI membrane without phase separation. For a PYR14+/Li+ mole ratio of 3.22, the ionic conductivity was about 2 × 10−4 S/cm at 20 °C, i.e., more than one order of magnitude higher than that of the RTIL-free electrolyte. The electrochemical stability window of the polymer electrolyte containing the RTIL was about 6 V (versus Ag/Ag+). Li/V2O5 cells with the polymer electrolyte (PYR14+/Li+ = 1.92) showed a 60% capacity retention after 80 cycles at 40 °C (the initial capacity was 210 mA h/g). Li/V2O5 cells (PYR14+/Li+ = 1.28) held at 30 °C delivered about 93 mA h/g (at 0.057 mA/cm2), which corresponds to approximately 34% utilization of the active material. These results suggest that the incorporation of the RTILs into PEO-based polymer electrolytes is very promising for the future realization of solid-state lithium metal polymer batteries operating near ambient temperatures.