Three Dimensional Geometric Effects on the Electrochemical Performance of Lithium Ion Betteries
Lithium ion batteries commonly utilize porous materials as electrodes, because the porous structured electrodes have benefits in mass production, low cost, and good performance. However, the heterogeneous geometry of the electrodes causes hardship to solve unpredictable thermal runway, safety, and cell degradation issues. During last decade, advanced 3D imaging techniques, X-ray tomography and FIBSEM, have provided realistic electrode microstructures. The scanned microstructures are employed to fundamentally understand unexpected failure mechanisms. In the present work, graphite anode and LiCoO2 cathode electrodes are employed to investigate geometric characteristics of the inhomogeneous microstructures by ex situ TXM tomography. A pristine and an aged graphite electrodes are generated from the 3D scanning, and their porous geometries are quantitatively analyzed in terms of porosity, tortuosity, specific surface area, and pore size distribution. In general, the aged electrode shows larger porosity, smaller tortuosity, and similar specific surface area. The TXM tomography is utilized to investigate geometric effects on the LIB performance. Geometric properties of different packing density of LiCoO2 electrodes are obtained from the X-ray scanned tomography and electrochemical performance of the electrodes are measured. The geometric and electrochemical analysis reveal that high packing density electrodes have smaller average pore size and narrower pore size distribution, which improves the electrical contact between carbon-binder matrix and LiCoO2 particles. The better contact improves the capacity and rate capability by reducing the possibility of electrically isolated LiCoO2 particles and increasing the electrochemically active area. The results show that increase of packing density results in higher tortuosity, but electrochemically active area is more crucial to cell performance than tortuosity at up to 3.6 g/cm3 packing density and 4 C rate. For in situ TXM tomography, chemical and mechanical stability of Ge0.9Se0.1 electrode are investigated under hard X-ray radiation. Evidence has shown that continuous irradiation has an impact on the microstructure and the electrochemical performance of the electrode. To identify the root cause of the radiation damage, a wire-shaped electrode is soaked in an electrolyte in a quartz capillary and monitored using TXM under hard X-ray illumination. The results show that expansion of the carbonbinder matrix by the accumulated X-ray dose is the key factor of radiation damage. For in situ TXM tomography, intermittent X-ray exposure during image capturing can be used to avoid the morphology change caused by radiation damage on the carbonbinder matrix. To investigate the dynamics of m-sized active materials, a special battery cell is developed to utilize in operando and in situ synchrotron transmission X-ray microscopy (TXM) techniques. Dynamic morphology evolution of Ge and Ge0.9Se0.1 electrodes is tracked by 2D in operando TXM imaging during (de)lithiation and 3D morphology change of the materials are captured by in situ TXM tomography. The observed 2D dynamics of Ge0.9Se 0.1 shows more sudden morphology and optical density change than Ge during the first lithiation. Moreover, the 3D microstructure of Ge0.9 Se0.1 particle preserves its original shape at pristine state after large volume change by the (de)lithiation. The difference between Ge 0.9Se0.1 and Ge is attributed to the super-ionically conductive Li-Se-Ge phase forming in the Ge active material of the Ge0.9Se 0.1 particle, which contributes to fast Li ion pathways in the particle as well as buffering the volume change of Ge.
Fisher, Purdue University.
Mechanical engineering|Energy|Materials science
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