Revealing mechanisms of activated carbon capacity fade in lithium-ion capacitors

The capacity fade mechanism of activated carbon (AC) electrode in Li-ion electrolyte was studied via electro-chemical impedance spectroscopy (EIS) and post-mortem electrode characterizations at different stages of electrochemical cycling. Electrochemical cycling was conducted in half cells incorporating the AC working electrode, Li metal counter electrode, and 1 M LiPF6 in EC:DMC (1:1) electrolyte. Three phases were identified during the ageing process that corresponded with transformation of the passivation layer at the electrode surface and charge transfer impedance derived from the EIS analysis. Surface area and morphology analysis showed that the AC surface was progressively transformed by degradation products that reduced the available surface area and accessibility of electrolyte moieties into the pores. X-ray photoelectron spectroscopy suggested that the degradation products are from the LiPF6 salt decomposition and carbonate solvent decomposition, while Raman analysis demonstrated increased defects in the electrode as cycling progressed. The capacity fade was therefore caused by the synergistic effect of electrolyte degradation and active material transformation.