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

Abstract

For investigation of the thermal stability of polycarbonate-based lithium salt-containing electrolytes, polycarbonate–salt mixtures [polyethylene carbonate (PEC) and polypropylene carbonate (PPC) with lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI)] were heated to 100 °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 ≈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(pentafluoroethanesulfonyl)­imide (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 α-carbon of the carbonate group in the polymer chain in the presence of a lithium salt with a weakly coordinating anion.

Chemistry-Informed Machine Learning for Polymer Electrolyte Discovery

“…Figure f shows an SPE that follows Arrhenius behavior reasonably well, yet the prediction is still quite different than the experimental values. However, the polymer shown in Figure f is known to decompose in the presence of lithium salts, , making experimental measurements of ionic conductivity highly unreliable for this polymer. In this case, the high prediction error signaled problematic experimental data.…”

Ion transport and structural design of lithium-ion conductive solid polymer electrolytes: a perspective

“…Further investigations suggest that low molecular weight components (e.g. PC and EC), originating from the chemical decompositions of PPC and PEC, are responsible for the unexpectedly high ionic conductivities observed for the polycarbonate-based SPEs (figure 8) [118,119].…”

Polypropylene carbonate-based electrolytes as model for a different approach towards improved ion transport properties for novel electrolytes

“…8 c ,10 The conditions for chain degradation of PEC and PPC and the underlying mechanisms were recently studied by Buchheit et al , who concluded that a sterically non-hindered α-carbon in the backbone can readily be attacked by a sufficiently nucleophilic anion if the adjacent carbonate group is polarized by a strong Lewis acid such as Li + . 11 This depolymerization is faster for PEC than for PPC due to the additional methyl group in the latter. Furthermore, the rate of chain degradation increases with LiTFSI concentration and with temperature.…”