Electrochemical Impedance Spectroscopy on ageing Lithium-Ion Batteries

Abstract

The change of Lithium-ion Battery (LiB) model parameters during cyclic ageing is analysed in order to relate it to the capacity of the cell. Therefore, approaches to distinguish between anode and cathode contributions to the full cell impedance spectrum are developed. The results from a new measurement routine, using temperature gradients across a full cell, show that a successful assignment of the charge transfer processes at the cathode and anode is possible. For the analysed cell the cathode charge transfer processes occur at a lower time constant than the related anode processes. With the information of such an assignment, ageing of a 30 mAh pouch cell is analysed with Electrochemical Impedance Spectroscopy (EIS). A Newman-type LiB-model for this cell is extended by a physical ageing model including a Solid Electrolyte Iterphase (SEI), loss of active surface area and loss of active material. The simulation results of this model in combination with the related measurement results are used with a parameter identification algorithm to quantify the thickness of the SEI, the amount of isolated surface area and the loss of active material. The results show that the impedance increase is caused by a decrease of active surface area while the capacity decrease is caused by a loss of active material at the anode and cathode. The SEI thickness has found to increase to 45 nm while the active surface areas of the anode and cathode decrease by 20% and 57% respectively until the End Of Life (EOL) of the cell after 400 cycles. The change of the ageing parameters occurs in correlation to the decrease of the capacity of the analysed cell. Finally, the identifiability of kinetic, transport and ageing parameters, identified using this approach, has been quantified using Fisher-Information-Matrices (FIMs) showing that the impedance spectra are sensitive enough to the ageing parameters to ensure their precise identifiability.