Geometric and electrochemical characteristics of lithium ion batteries

Abstract

The geometric and electrochemical characteristics of different lithium ion batteries (LIBs) are investigated in this study. The core work is to study the impact of the calendering process on NMC cathode electrodes performance. X-ray CT image processing by Python, MATLAB, ImageJ and Avizo is utilized in this study. NMC electrodes with different calendering conditions were fabricated to calculate electrochemical properties of the cells. Charge/discharge of the electrodes under 0.1C, 0.2C, 0.4C, 1C, 2C, 4C and 0.1C (retention test) rates were cycled for three times respectively between 4.2 V and 3.0 V. Electrochemical impedance spectroscopy testing was used to further explain the effects of NMC density on rate capability. Geometric properties of NMC electrodes with different calendering conditions were calculated from the computed tomography data of the electrodes. A synchrotron transmission X-ray microscopy tomography system at the Advanced Photon Source of the Argonne National Laboratory was employed to obtain the tomography data. X-ray CT image processing before the data analysis was introduced. Python based Tomopy and ASTRA toolbox were used to filter the original HDF5 data and reconstruction. ImageJ was used to help remove noise, adjust contrast and cropping. Iso2mesh and image processing tool box were used in MATLAB to generate meshed 3D structure of CT data. Geometric properties of NMC electrodes including porosity, pore size distribution, particle size distribution, specific surface area and tortuosity were calculated from the computed tomography data of the electrodes. The geometric and electrochemical analysis show that calendering can increase the electrochemically active area, which lead to improving of the rate capability. However, more calendering will result in crushing of NMC particles, which can reduce the electrode capacity at relatively high C rates. This study shows that the optimum electrochemical performance of NMC electrode at 94:3:3 weight ratio of NMC:binder:carbon black can be achieved by calendering to 3.0 g/cm3 NMC density. LTAP solid electrolyte and NMC cathode material mix electrode-electrolyte X-ray CT data was studied in last chapter. By using 8 kev X-ray energy, we could distinguish NMC active material, LTAP solid electrolyte and the others three phase. On the basis of NMC electrode image processing method, dilation and multiply threshold method is applied to get three-phase 3D geometry. A comparing of connection area between NMC and LTAP of 700psi and 1300psi electrode was analyzed. Geometric properties like tortuosity, di_usion length and e_ective di_usivity were generated from the CT data.