Study of the Impact of Microstructure on the Effective Electrical Properties of Composite Electrodes for Lithium-Ion Batteries; Acquisition Andsimulations on Real Microstructures

Abstract

Better understanding of how the microstructure of composite electrodes affect their electrochemical performance is still required to achieve Lithium ion batteries of higher energy and power densities [1]. Novel opportunity for progressing in this direction and explore relationships between composite electrode microstructure and electrochemical performance is implementation of X-ray computed tomography (XRCT) and focused ion beam in combination with scanning electron microscopy (FIB-SEM) techniques [2]. Sophisticated characterization techniques such as broadband dielectric spectroscopy (BDS) [3] and/or Pulsed Field Gradient Spin - Echo NMR (PFG-SE NMR) [4] can also be used to measure transport properties and relate them to the composite electrode microstructure. Finally, numerical simulations performed on 3D volumes acquired by XRCT or FIB/SEM is a powerful tool to calculate effective transport properties and finely analyze the impact of the composite electrode microstructure [5].In this work, the microstructure of LiNi0.5Mn0.3Co0.2O2 (hereafter called NMC532)-based positive composite electrodes designed for EV applications were deeply investigated in relationship with their electrical properties.The composite electrode microstructures were acquired numerically in 3D at different scales by X-ray tomography and FIB-SEM tomography. Image analysis tools were developed to extract specific textural parameters (e.g. distribution and percolation of the carbon/binder mixture, type and tortuosity of the porosity, contact area of the active material with these other phases) to assess the electronic and ionic conductivities at different locations and spatial scales. In complement, 3D-resolved simulations with the Fast Fourier Transform (FFT) method were carried out to calculate the effective electronic and ionic conductivities of the composite electrodes and characterize the local current density paths for both type of charges (electrons and ions).From our results, it becomes apparent that it is possible to find a good compromise between the electronic and ionic conductivities that results in improved electrochemical performance by optimizing the microstructure. Acknowledgments Financial funding from the ANR program no. ANR-15-CE05-0001-01 is acknowledged. We also acknowledge the CLYM (Consortium Lyon Saint-Etienne de Microscopie) for the access to the FIB/SEM device used in this study.