Impedance Characteristics and Diagnoses of Automotive Lithium-Ion Batteries at 7.5% to 93.0% State of Charge

- Tested typical automotive lithium ion batteries (LIB) to obtain the important performance parameters at different values of the state of charge (SOC).
- Based on the obtained charge transfer reaction, diffusion behavior, and status of solid electrolyte interfaces at each state of charge to develop a fractional impedance circuit model for optimizing the operating range of automotive lithium-ion batteries.
- Identified key parameters determining the performance of automotive lithium ion batteries through both measurements and calculations.

Diffusion behaviors and reaction kinetics are factors limiting the rate capacity of lithium ion batteries (LIB), particularly in automotive applications. In order to gain a better understanding of the rate-limiting factors of LIBS, a fractional circuit model is constructed and two feedback loops are added to the impedance diagnosis flowchart in this paper. The fractional impedance model is constructed for a commercially available automotive LIB, featured by low internal resistance, long diffusion lengths, and large interfacial areas. Impedance data collected are then used to quantitatively analyze the charge transfer reaction, the status of the solid electrolyte layer (SEI), and the lithium diffusion behavior at different state of charges (SOCs) of 7.5% ∼ 93.0% based on the constructed model and the calculated characteristics frequencies of the corresponding physiochemical processes. The results indicate that both charge transfer resistance and diffusion resistance increased dramatically with decreased SOC values when SOC≤26.5%. These results suggest that automotive LIBs should not operate at low SOCs of less than 20.0% or depth of discharging higher than 80.0%. Finally, calculations of anode and cathode Warburg impedance percentages offer a simple and quick way to evaluate lithium diffusion abilities through insertion/de-insertion electrodes.