Polycarbonate-Based Lithium Salt-Containing Electrolytes: New Insights Into Thermal Stability

For investigation of the thermal stability of polycarbonatebased lithium salt-containing electrolytes, polycarbonate-salt mixtures [polyethylene carbonate (PEC) and polypropylene carbonate (PPC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)] were heated to 100 degrees C and the conductivity was monitored with electrochemical impedance spectroscopy for at least 24 h. At a constant high temperature, the observed rise in conductivity can be correlated to degradation of long-chain polymer units to small-chain polymer units as the viscosity decreases with a shorter chain length. In both cases, degradation can be observed. With PEC-LiTFSI, it takes approximate to 9 h until total degradation; with PPC-LiTFSI, the process is slower. Additionally, we repeated the experiments with PEC and other Li salts such as lithium trifluoromethanesulfonate (LiOTf), lithium bis(pentafluoroethanesulfonynimide (LiBETI), and lithium difluoro(oxalato)borate (LiDFOB). These experiments resulted in the degradation being dependent on the electrophilic activation by the lithium salt. With different Li-free salts such as sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), potassium bis(trifluoromethanesulfonyl)imide (KTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), no degradation of the polymer is observable. As a degradation mechanism, we anticipate a depolymerization of PEC at the alpha-carbon of the carbonate group in the polymer chain in the presence of a lithium salt with a weakly coordinating anion.