Near-in-situ electrochemical impedance spectroscopy analysis based on lithium iron phosphate electrode

Impedance spectroscopy provides insight into the internal mechanisms of batteries. However, the complexity and data fluctuations in low-frequency parts of impedance spectra often lead to difficulties in analysis, and even neglected in practical applications. To address this issue, we conducted a detailed analysis of lithium iron phosphate (LFP) cells using near-in-situ electrochemical impedance spectroscopy (EIS). The LFP cells exhibited stable charge/discharge platforms, with a narrow reaction voltage range dividing the process into three distinct stages. A near-in-situ EIS test was conducted during charging process, and the changing trend of the impedance spectrum was obtained at different stages, with low-frequency region being particularly prominent. By applying the macrohomogeneous porous electrode theory, we analyzed the impedance features of the LFP porous electrode at low frequencies and established corresponding equivalent circuit models for each stage. Thus, the low-frequency part of the impedance spectrum was accurately fitted and interpreted, and the fitted ionic and electronic resistivities matched well with actual. Our results provide a foundation for further research and development of EIS applications in battery systems, while enhancing the understanding of LFP cell performance.