Electrochemical Kinetic Study of a Polyimide Anode for Lithium-Ion Batteries Using the AC Impedance Technique

Electrochemical impedance spectroscopy (EIS) is selected as the main investigation method to study the underlying mechanisms and electrochemical kinetics of a polyimide anode material for lithium-ion batteries (LIBs) through obtaining the primary parameters of Ohmic impedance (Rs), solid-electrolyte-interphase (SEI) impedance (Rf), and charge-transfer resistance (Rct). A typical polyimide (PNDA) anode material is synthesized using a simple one-step high-temperature polymerization method. The EIS spectrum of PNDA is fitted by two different equivalent circuit models. Charge-transfer activation energies before cycling and after the first cycle are 51.52 and 39.55 kJ mol–1, respectively, implying a large energy barrier for the discharge of PNDA. The Rct and Rf are the key factors influencing the electrochemical performance of PNDA. However, Rs exhibits a slight effect on the total electrochemical kinetics. It shows a decreasing trend with increasing operation temperature, but this positive effect could be almost negligible compared to the kinetics of the total electrochemical processes of lithium embedding. The Rct drops significantly with increasing temperature, reflecting a strong dependence of the charge-transfer process on temperature. However, Rf displays a noticeable increment with temperature, indicating that the SEI on the PNDA is slightly unstable with elevated temperature even below 80 °C. This value is also commonly regarded as the critical temperature for stable SEI on carbon and silicon anodes. The extent of decrease of Rct with increasing temperature is far greater than that of Rf increment, resulting in enhanced total electrochemical kinetics of PNDA with temperature increase, thus enhancing its electrochemical properties at elevated temperature. These parameters of EIS reflect the rate-control step of the electrode-reaction processes and give an evaluation of the electrode-reaction processes. The revealed determining factors of polyimides as anode materials could provide useful insights for the development of high-performance organic electrode materials for LIBs.