No abstract

Lithium iron phosphate is a promising candidate material for Li-Ion batteries. In this study, the rate determining processes are assessed in more detail in order to separate performance limiting factors. Electrochemical impedance spectroscopy (EIS) data of experimental LiFePO 4 /Lithium-cells are deconvoluted by the method of distribution of relaxation times (DRT), what necessitates a pre-processing of the capacitive branch. This results in a separation into cathode and anode polarization processes and in a proposition of a physically motivated equivalent circuit model. We identify three different polarization processes of the LiFePO 4 -cathode (i) solid state diffusion, (ii) charge transfer (cathode/electrolyte) and (iii) contact resistance (cathode/current collector). Our model is then applied to EIS data sets covering varied temperature (0 • to 30 • C) and state of charge (10% to 100%). Activation energy, polarization resistance and frequency range are determined separately for all cathode processes involved. Finally, the tape-casted LiFePO 4 -cathode sheet is modified in porosity, thickness and contact area between cathode/electrolyte and cathode/current collector by a calendering process. Charge transfer resistance and contact resistance respond readily in polarization and relaxation frequency.

Lithium iron phosphate is a promising cathode material for the use in lithium-ion batteries meeting the demands of good stability during cycling and safe operation due to reduced risk of thermal runaway. However, slow solid state diffusion and poor electrical conductivity reduce power capability. For further improvement, all rate determining electrode processes have to be quantified. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the characterization of electrochemical systems. In recent publications (1,2), a physical interpretation for the impedance response of LiFePO 4 /Li-cells was given and an equivalent circuit model was proposed. In this contribution, the proposed equivalent circuit is used to quantify the predominant electrode processes depending on temperature (0°C…30°C), SoC (10% … 100%) and microstructure characteristics (porosity).

Lithium iron phosphate is a promising cathode material for the use in lithium-ion batteries meeting the demands of good stability during cycling and safe operation due to reduced risk of thermal runaway. However, slow solid state diffusion and poor electrical conductivity reduce power capability. For further improvement, the identification of the rate determining processes is necessary. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the characterization of electrochemical systems. In this contribution a deconvolution of the impedance with the distribution of relaxation times (DRT) is used to obtain a better resolution in frequency domain. Therewith, the cathodic and anodic polarization processes are identified and an impedance model for the cell is proposed.

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