Multiscale Characterization of Composite Electrode Microstructures for High Density Lithium-ion Batteries Guided by the Specificities of Their Electronic and Ionic Transport Mechanisms

Abstract

The microstructures of Li-ion positive composite electrodes designed for EVs have been characterised at different scales and in particular by FIB/SEM nanotomography. These electrodes are composed of Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 , carbon black (CB), and polyvinylidene fluoride (PVdF). The component proportions in the electrodes and the electrode densities were varied. Specific image analysis tools have been developed to quantify the microstructure parameters that will influence the transport and exchange properties of ionic and electronic charges during battery operation. Different porosities have been highlighted, in particular the micrometric porosity which appears to be the most effective for the ion diffusion in the liquid electrolyte due to its low tortuosity and large intra-connectiviy. Different parallel paths for the transport of electrons in solid phases such as the CB/PVdF percolating network and a hybrid one consisting of CB/PVdF islands distributed on the NMC cluster surface and the NMC grains pertaining to these clusters. This last network can be effective when the CB/PVdF islands allow the electrons to short-circuit the resistive NMC grain boundaries.

3D Quantification of Microstructural Properties of LiNi0.5Mn0.3Co0.2O2 High‐Energy Density Electrodes by X‐Ray Holographic Nano‐Tomography

“…[ 66 ] Thus, the lack of AM/CBD interfacial area, as well as a low TPB p density, can be compensated by the electronic transport through the NMC particles to join the reaction‐sites, as discussed in refs. [ 11,45 ] . In addition, the brutal drop of the rate capability in the high current density region is commonly attributed to the porous electrode effects, that is, through the McMullin number as the main limitation is considered to be in the liquid phase.…”

“…[ 5 ] Commonly, the electrode microstructure is a mixture of different phases: AM, additives such as mixture of conductive carbon (e.g., C65) and binder (e.g., polyvinylidene fluoride (PVdF)), and pores, which are eventually filled with an electrolyte. This microstructure has been reported to play a crucial role in the performance of lithium‐ion battery electrodes, as it affects the effective electronic/ionic transport properties through the morphology of the conducting matrix; [ 6–11 ] the electrochemical kinetics via the interfacial area between phases; [ 12,13 ] as well as the mechanical properties. [ 14 ] Hence, it is of particular importance to understand the interplay between the complex microstructure of a porous electrode and its electrochemical performance.…”

Effective Electronic and Ionic Conductivities of Dense EV-Designed NMC-Based Positive Electrodes using Fourier Based Numerical Simulations on FIB/SEM Volumes

“…Furthermore, in our electrodes we could identify several porosity types [48]: (i) one made of large micrometric cavities within the NMC cluster pack; (ii) one occluded in non-fragmented hollow NMC clusters; (iii) one made of nanometric pores defined by the PVdF/CB mixture morphology and (iv) one due the cracked grain boundaries within fragmented NMC clusters. Quantitatively, the micrometric porosity was defined as pore spaces wider than 60 nm.…”

“…They are fairly dense (porosity is lower than 30 %) and contain a low amount of CB and PVdF additives (total is lower than 6 % by weight). Their microstructures were characterised and digitised in a previous work where the imaging conditions and segmentation process are detailed [48]. Table 1 gives the electrodes mass and volume composition, thickness and density.…”

“…Note that the electrodes contain a CB/PVdF volume fraction significantly higher than the one found in the FIB/SEM volumes (Table 2). This difference is due to the presence of some very bulky CB/PVdF agglomerates resulting from insufficient mixing, or from a lack of stability, in the ink used to manufacture the electrodes [48]. These bulky CB/PVdF agglomerates are few in number and clearly identifiable in the electrode cross-sections and care has been taken to make the FIB/SEM acquisitions away from them.…”