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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.

The three-dimensional microstructures of a lab-scale and a high-power LiFePO 4 cathode for lithium-ion cells are analyzed by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. The spatial distributions of (a) carbon black as electronic conductor (b) LiFePO 4 as active material and (c) pore volume are reconstructed by appropriate image processing methods. The global threshold segmentation procedure is replaced by a refined local threshold method, which accounts for gradients in luminosity even within very large imaged volumes. The precise analysis of the high-power cathode demands for reconstructing a very large volume of 18.15 × 17.75 × 27.8 μm 3 , caused by the dual length-scale design of LiFePO 4 , carbon black and pore phase. The microstructure features, (a) electrochemically active surface area and particle size distribution of LiFePO 4 , (b) shape and particle size distribution of carbon black and (c) porosity and tortuosity of the pore phase are compared between lab-scale and high-power cathode.Microstructure characteristics of electrodes influence the dynamic behavior of a lithium-ion cell, and determine the suitability for highenergy or high-power applications. The effects of microstructure characteristics are usually described by effective porous electrode models. 1,2 However, the eligibility of those models is limited: First, the estimation of effective parameters for the model is difficult and second, these models typically assume an idealized microstructure, e.g. spherical particles with one discrete radius. For that reasons it is important to have a method to quantitatively characterize the electrode microstructure. With such a quantitative description also the detailed comparison of different electrodes becomes possible. The reconstruction of an electrode by focused ion beam (FIB) / scanning electron microscopy (SEM) tomography, followed by image processing showed to be a useful tool. 3-6 This technique was applied in lithium-ion cell research for 2D cross-sections of porous electrodes 7,8 and recently for the first 3D reconstruction of active material and pore phase in a LiCoO 2 cathode. 9 Recently, we reported a 3D reconstruction of a LiFePO 4 cathode, which resolved, for the first time, the distribution of all involved phases: carbon black, active material and porosity. 10 The present work consists of two parts. First, the dataset of the labscale LiFePO 4 cathode (sample A) from an earlier work 10 is partially re-analyzed. The classification of the threshold values is improved by the refinement of a segmentation algorithm, which was developed in the previous work. 10 This new algorithm accounts better for gradients (in luminosity or contrast) within large data sets, composed of hundreds of SEM images.In the second part, the reconstruction method is applied in the same way to a LiFePO 4 cathode (sample B) obtained from a disassembled high-power 18650 cell. The high-power LiFePO 4 cathode has a bimodal particle size distribution of submicron LiFePO 4 primary particles ...

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