Combined modeling and impedance experiments for a state-of-charge diagnostics of LiFePO4-based cells

Kurzfassung

Lithium-ion batteries based on lithium iron phosphate (LiFePO4, LFP) positive electrodes are receiving consid-erable attention, being able to supply high power density at high lifetime, reduced material costs and reduced mate-rial toxicity, and exhibiting an improved operational safety. The delithiation of LiFePO4 to FePO4, which can be reversed and cycled, is a heterogeneous two-phase solid-state reaction that can be summarized as LiFePO4 FePO4 + Li+ + e– . Along with these major advantages, LFP also shows some unfavorable properties. The flat profile during discharge makes the assessment of the state-of-charge (SOC) diffi-cult. Furthermore, the cell voltage depends on cycling history, which has been interpreted in terms of tree-ring-type structures in the solid phases upon partial cycling. We present a combined modeling and experimental study of the electrochemical behavior of LFP-based single cells. Goal of the study is to improve understanding of the elec-trochemical behavior and to develop strategies for imped-ance-based and model-based state-of-charge diagnostics. On the experimental side, in-situ electrochemical charac-terization is performed via steady-state charge/discharge measurements and electrochemical impedance spectros-copy. Ex-situ structural characterization is performed via computer tomography for the macro-structure and scanning electron microscopy for the micro-structure. On the theory side, a detailed kinetic model is used for data interpretation. The 1D+1D model represents multi-component transport (mass, charge, heat) within a cell repeat unit (anode, separator, cathode) as well as solid-state diffusion within the active materials. All studies are performed on commercial high-power cells (A123). The combined experimental and simulation results allow a detailed insight into the electrochemical behavior of the cell as a function of operating conditions and state-of-charge. Although the steady-state discharge curves demonstrate the flat discharge characteristics, the im-pedance spectra show a clear variation as function of the state-of-charge. Using a model-based impedance interpretation, strategies for practical SOC diagnostics are being assessed.