Effect of Nanostructured and Open-Porous Particle Morphology on Electrode Processing and Electrochemical Performance of Li-Ion Batteries

Abstract

A nanostructured, porous NCM cathode material is investigated regarding its behavior during electrode processing and electrochemical performance. The results are related to the densely packed NCM original material from which the nanostructure has been derived. Chemical composition and structural parameters are not affected by the nanostructuring process; changes are limited to the particle morphology in terms of primary particle size, specific surface area, and porosity. Electrodes containing a porous NCM material deliver lower adhesion strength values when adding identical amounts of PVDF binder. Increasing the binder fraction from four to six parts increases also the adhesion strength to an acceptable level without deteriorating the cell capacity. Despite initially high electrode porosities of 65−70%, electrodes with nanostructured NCM are capable of withstanding calendering to 40% porosity without destroying the porous particles. Full-cell tests with 50 mAh pouch cells and graphite anodes reveal substantially improved C-rate capabilities for the nanostructured material in relation to the commercial original NCM. The advantage increases with increasing C-rate and corresponds to shorter diffusion pathways in nanostructured NCM. Remarkably, even at low C-rates (C/20) where diffusion effects are considered secondary, porous NCM lies ahead of the original material. This can be explained by the higher surface area and thereby enlarged interface to the electrolyte, which eases delithiation. Long-term cycling up to 1100 cycles displayed further benefits for the nanostructured active material as one of the most prominent degradation factors, that is, crack formation and particle fragmentation, does not occur throughout the complete cycling procedurein contrast to the bulk particles of original NCM.

Drying of NCM Cathode Electrodes with Porous, Nanostructured Particles Versus Compact Solid Particles: Comparative Study of Binder Migration as a Function of Drying Conditions

“…Calendering leads to a decrease in resistivity for both cathode systems, which is in good agreement with the results of Müller et al [12] For the lowest and the highest drying rate, the resistivity is reduced by 95% in each case of NCM-P. For NCM-C the resistivity decreases by 78% (0.75 g m 2 s À1 ) and 94% (3.5 g m 2 s À1 ). This is because of the improved contacting and percolation due to compaction of the conductive additives between the active material.…”

“…Due to the higher porosity, the resistivity of NCM-P electrodes is significantly higher than that of NCM-C. The measured values of the resistivity in the low drying rate range are in good agreement with the studies of Müller et al, [12] where comparable drying conditions were used. The electrodes of both materials show an increase in resistivity with increasing drying rate.…”

“…Since the solids content and composition are the same for both slurries, the difference in viscosity must be due to the different particle properties of NCM-C and NCM-P. For particle sizes of the same order of magnitude, it can be assumed that particle shape and distribution have less influence. A major difference in the particle morphologies of both particle systems is the larger specific surface area and the internal porosity of the secondary particles of NCM-P. As also discussed by Müller et al, [12] it would be conceivable that during slurry preparation the solvent NMP, including the dissolved PVDF binder, absorbs into the open-pore structure of the particles. A schematic illustration of the processes during slurry preparation is shown in Figure 4.…”

“…Postmortem SEM images show interparticle cracks in the electrode with porous particles that can lead to this increase in contact resistance. [10] Müller et al [12] showed that electrodes containing a porous NCM material have comparatively lower adhesion strength values. The reason for the low adhesion forces within the electrode and to the current collector is that part of the binder has penetrated into the secondary particles and is therefore not available for adhesion.…”

Commercialization‐Driven Electrodes Design for Lithium Batteries: Basic Guidance, Opportunities, and Perspectives

“…160-220 mAh g −1 @ 1 C 2.5-3.5 mAh cm −2 [164][165][166] Commercial NCA ≈20 mg cm −2 ≈2.6 g cm −3 200 mAh g −1 @ 1 C ≈3.6 mAh cm −2 [167] Hierarchical LTO microspheres 3.0-3.5 mg cm −2 1.32 g cm −3…”