Monitoring morphological evolution of L i‐ion cathode secondary particles through In situ FIB/SEM electrochemical experiment

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This last decade, fundamental studies of electrochemical phenomena have been slowed down by a lack of effective in situ (and operando) experimental setup, which is able to clearly identify structural modifications inside and at the surface of electrode materials. The evolution of microstructures, the appearance of cracks and porosities and the transformation of crystalline phases have to be properly investigated in order to get a better insight into the influence of charge/discharge processes in battery materials and the reaction mechanisms implied in electrochemical storage. Improving our understanding of the microstructural changes and crack formation into Li‐ion electrode materials during electrochemical cycling can provide new insight into battery behaviour. In order to monitor microstructural evolution dynamically during electrochemical cycling, we developed a micro‐scale battery set‐up implemented within a FIB/SEM instrument [1]. Secondary particles (figure 1a) are strongly used by industry for positive electrode fabrication. However, so far just a few works were focused on their morphological evolution during electrochemical cycling. Here, single secondary particles of cathode oxide (NCA and NMC) materials with a size of 5–10 µm are attached to the metal pin of the micromanipulator via a conductive carbon bridge formed using GIS (figure 1d/e). The micromanipulator allows moving the particle in the chamber and immersing into an ionic liquid electrolyte (low vapour pressure), which is deposited on a counter electrode, i.e. lithium metal. Electrochemical measurements are carried out using ultra low current instrument (biologic SP200) with two points connection configuration, external probe tip and SEM metal holder are respectively connected. After immersing into electrolyte, the single particle of active materials is cycled in galvanostatic mode with a steady current around 1 nA, which corresponds to a C‐rate of about 1 based on the particle volume and the theoretical capacity [2]. We succeed to carry out in situ experiments inside the FIB/SEM chamber using ionic liquid electrode getting high quality electrochemical measurements in galvatostatic (figure 1f) and impedance modes for single secondary cathode particle. We studied structural modifications of individual particle after each in situ charge/discharge cycle by FIB slicing and SEM imaging. Using specific FIJI plugins and AMIRA software for reconstruction and segmentation steps, we quantified the formation of cracks as a function of cycle number and extracted 2D skeleton and tortuosity (figure 1g/h). Evolution of the discharge capacity was correlated with cracks and porosities appearance inside cathode materials. Impedance measurements suggested an increase of lithium diffusion inside the particle that is relied on the formation of cracks, which induces an enhancement of discharge capacity. On the other hand, the characterization of the 3D structure of these materials is crucial in order to gain a deeper insight into structural configuration and evolution of the discharge capacity. In this purpose, the reconstruction of 3D microstructures by FIB tomography methods was used [3] (figure 1i). The changes of structural parameters such as porosity, grain connectivity and crack propagation that are induced by cycling were extracted from 3D reconstruction and linked with electrochemical properties. Then, 3D structural data was compared to that obtained by 3D Transmission X‐ ray Microscopy (TXM) tomography [4], which is made in APS synchrotron at Argonne NL. Finally, a mechanical strain mode was used to get a better insight into crack formation and evolution into cathode particle.