Deciphering electrolyte dielectric extended electrochemical stability in lithium-ion capacitors

•Improved cycling stability with increased electrolyte dielectric•High electrolyte dielectric reduces electrode interfacial impedance•PF6− degradation is mitigated with increased electrolyte dielectric•High LiF content on the anode with lower electrolyte dielectric. Solvent properties dominate the characteristics of electrode-electrolyte interfaces and hence influence the performance of lithium-ion capacitors (LiC). In this work, we demonstrate that the electrolyte dielectric influences the electrochemical stability of activated carbon(AC)/hard carbon(HC) LiC. Electrode interfaces from different 1M LiPF6 electrolyte formulations with varying dielectric are probed after stability tests using floating voltage holds (FVH) and galvanostatic charge and discharge (GCD) analysis. The electrode interfacial impedance was inversely proportional to electrolyte dielectric, with a more severe effect on the AC cathode than on the HC anode. Furthermore, surface morphology investigations and species characterisation with SEM and XPS revealed decreased degradation at the electrode surfaces as the electrolyte dielectric was increased. Overall, high electrolyte dielectric improved the electrolyte stability by delaying the PF6− anion degradation and detrimental byproduct formation through decreased ion-pairing and increased PF6− solvation shell resistance to oxidation. These findings are important for developing electrolytes for high-power and long-cycle life LiCs. [Display omitted]