Simon Kücher

The porous microstructure of electrodes influences the ionic resistance in lithium-ion cells and is commonly described by the porosity and the tortuosity. Changes in microstructure due to external compression can have a substantial impact on the ionic resistance and therefore the performance of the battery. Properties such as particle size, particle shape, and varying binder composition can affect the microstructure’s impact on ion transport and the compressibility of the electrode. Temperature may also influence mechanical behavior and electrode wettability. Consequently, a combined understanding of mechanical and thermal effects on transport parameters of electrodes is essential for improving electrode design and model parametrization. This study examines the effect of pressure and temperature on the ionic resistance of artificial graphite (AG), lithium-iron-phosphate (LFP), and lithium-nickel-manganese-cobalt-oxide (NMC) electrodes using electrochemical impedance spectroscopy (EIS). The results show that temperature does not significantly affect compressibility. However, AG and NMC exhibit additional temperature-dependent changes in ionic resistance beyond the influence of electrolyte conductivity. This may result from variations in the contact angle between electrode and electrolyte. The MacMullin number increases with higher compression for all observed electrodes, while the tortuosity increases only for the AG and LFP electrodes.

Young’s modulus is a key parameter to describe the compressive behavior of battery electrodes and is therefore frequently employed in mechanical models. In most studies, this property is determined using pristine electrodes. However, during electrochemical cycling, lithium insertion and extraction can alter the mechanical response of the active materials. The literature lacks SOC-dependent compression data of electrodes with different active materials. Especially for LFP electrodes, no SOC dependencies have been reported. This study closes this gap by performing compression tests on graphite, NMC, and LFP electrodes at different states of charge. The results show that the stiffness of graphite and NMC electrodes increases with higher lithium contents, whereas the compressive behavior of LFP remains independent of the lithium content. These findings are consistent with material-level properties reported in the literature. Two hypotheses are proposed to explain this behavior: one hypothesis is related to the contribution of active material particles to electrode deformation and the other hypothesis is related to microstructural changes governed by particle diameter.

Lithium-ion battery separators contribute to the ionic resistance as their microstructure influences the transport of lithium ions through the electrolyte. This effect is commonly characterized by the MacMullin number and tortuosity. Since lithium-ion batteries are usually operated under mechanical pressure, structural changes in the separator due to compression must be considered, as they can significantly affect ionic transport properties. Moreover, different manufacturing processes and material compositions influence the separators’ compressibility and, consequently, ion transport. Temperature may further impact the separator’s mechanical behavior and the electrolyte’s conductivity. Understanding the combined effects of pressure, temperature, and material-specific microstructure is essential for accurate simulation and optimization of lithium-ion battery performance. In this work, pressure is applied onto a symmetrical single-layer pouch cell to determine the ionic resistance through various separators via electrochemical impedance spectroscopy. Our findings demonstrate that the investigated wet-processed separator exhibits a more pronounced temperature-dependent increase in the MacMullin number under pressure than the examined dry-processed separators. Furthermore, the glass fiber separator, commonly used in lab-scale cells, shows approximately twice the relative increase in MacMullin number compared to microporous separators. However, unlike the microporous separators, the ionic resistance of the glass fiber separator decreases under pressure.

Parallel connections of lithium-ion cells in battery systems lead to current distributions between the cells, which impacts fast charging capabilities. This study examines the influence of interconnection resistance, format, electrode design, cell-to-cell variations, and temperature differences on system inhomogeneity and identifies anode potential safety margins that ensure safe charging without lithium plating. To this end, a physico-chemical parameterization of the Molicel INR21700-P45B is presented. An optimized fast-charging profile enables charging from 10%–80% cell capacity in under 10 minutes. The experimental application of the fast-charging profile yielded a result of over 300 equivalent full cycles before reaching 90% state of health. Furthermore, the cell model is scaled to different parallel-connected systems in an extensive simulation study. The interconnection resistance, and analogously the internal-to-interconnection resistance ratio, was found to be the primary factor influencing inhomogeneity in high parallel configurations, whereas cell-to-cell resistance variations are the most significant determinant in low parallel configurations. Variations in cooling were found to be more impactful than initial temperature disparities.

Knowledge about how particle volume changes impact the electrode thickness and microstructural properties like porosity, especially as a function of binder types and contents, is crucial to improving battery design. Single-electrode electrochemical dilatometry provides insights into electrode thickness change during operation. However, commercially available setups have yet to be able to apply sufficient pressure on electrodes to replicate conditions in commercial batteries. This work aims to incorporate a wave spring washer into a commercially available single-electrode electrochemical dilatometer to apply additional force onto the working electrode. Using this setup, the impact of the binder composition on electrode thickness change was investigated using graphite electrodes with different amounts and ratios of styrene-butadiene rubber and carboxymethyl cellulose. It was shown that the electrode volume change is consistently lower than that of the crystallites as obtained by X-ray diffraction in the literature. It is further demonstrated that irreversible thickness change decreases with increasing binder content, indicating that a higher binder content helps prevent particle restructuring during cycling. Lastly, the modified setup exhibits significantly higher reliability than the commercial setup with low pressure, which we relate to electrode bending during lithiation.

In Part I of this work, it was shown that a two-dimensional Doyle-Fuller-Newman (DFN) model can predict inhomogeneous lithium plating during cycling caused by anode overhang. This indicates an increased risk of lithium plating at the cell edge. In Part II, the same model is used to simulate defined cycling conditions of real cells to experimentally validate the proposed model and the edge plating mechanism. The cells used for this purpose are single-layer pouch (SLP) cells instrumented with several spatially distributed gold wire micro-reference electrodes, enabling the measurement of local electrolyte potentials. First, the simulation indicates a significantly inhomogeneous potential distribution during 500-hour potentiostatic storage before the actual study, which is also observed in the local potential measurements of the real cells. Second, the cells are slowly discharged and then fast charged. Consequently, lower local anode potentials are observed near the edge compared to the center, which is consistent with the simulation results. Finally, the predicted and measured lithium plating near the anode edge is cross-validated by visual inspection in post-mortem analyses. The results are particularly relevant for optimizing cell design and operating strategies, as they demonstrate the relevance of considering previous operation during fast charging to avoid inhomogeneous degradation.

Volume change of active materials during the insertion of Li⁺-ions into the electrode’s host structure in lithium-ion batteries (LiBs) has become an increasingly researched topic. Reversible and irreversible thickness changes occur during operation because of active material volume change upon lithiation/delithiation and SEI growth. In LiBs, particularly the volume change of the negative active material can cause mechanical disintegration of the composite coating [1], causing loss of active material. LiBs are usually operated in a compressed state maintained by geometrical boundary conditions to ensure uniform contact between electrode layers. Therefore, the thickness changes can cause increased stack pressure and, subsequently, microstructural changes [2] and thus affect the performance and lifetime of the battery [3]. To better understand the aforementioned phenomena, accurate thickness measurements are required. Measuring the thickness change of a cell stack does not provide accurate information about the individual thickness change of the two electrodes. Therefore, single-electrode electrochemical dilatometry (ECD) can help to better understand the impact of the individual electrodes on cell stack thickness change. Due to the common use of a commercially available setup (EL-Cell, ECD-3-nano, Germany) in literature, most ECD experiments were conducted at a pressure of ~ 16 kPa onto a Ø10 mm working electrode (WE). Only in some publications higher pressures > 0.1 MPa were applied during single electrode measurements [4]. The applied pressure in lab-scale coin cell setups and state-of-the-art LiBs is usually significantly higher (0.1 – 0.5 MPa) than that of single-electrode ECD investigations. Therefore, the question arises whether the pressure has an impact on the measured thickness change. Thus, this work integrates a wave spring washer into a commercially available single-electrode ECD Setup (ECD-3-nano, El-Cell GmbH, Germany) to apply additional pressure onto the WE, without requiring geometrical modifications of the setup. The integration of the wave spring washer increases the applied pressure from 16 kPa to > 100 kPa. Graphite electrodes with different binder types and contents are investigated since distinct measurement artifacts were observed for electrodes with certain binder compositions in preliminary experiments. Electrodes without additional pressure show a first-cycle lithiation thickness increase of up to 30% (see Figure 1), which is much higher than the active material volume change of graphite obtained from XRD data [5] would suggest. For all examined electrodes at lower pressure the first and tenth cycle lithiation thickness change is more than 50 % higher compared to the setup at high pressure. The mechanism causing the discrepancy between measurements at the two pressure levels is identified as the curvature of the electrode edges at low pressures present in commercially available ECDs. Therefore, the setup at increased pressure enables a meaningful investigation of the influence of the binder content on irreversible and reversible thickness change for the electrodes under investigation using more realistic mechanical boundary conditions. Measurements show that irreversible thickness change decreases with increasing binder content. Possibly the higher binder content helps to prevent restructuring of the particles during cycling due to higher mechanical strength of the electrode. Delithiation thickness change is similar among electrodes with normal to high binder contents. Only electrodes with a very low binder content have a significantly lower reversible thickness change. Overall, this study shows that the influence of pressure onto the WE should be considered for future ECD-experiments and setups. This work presents an easy to implement solution to apply additional pressure onto the WE for a common commercially available ECD-setup. References [1] Vetter, J., Novák, P., Wagner, M.R., Veit, C., Möller, K.-C., Besenhard, J.O., Winter, M., Wohlfahrt-Mehrens, M., Vogler, C., and Hammouche, A.: Ageing mechanisms in lithium-ion batteries. In: Journal of Power Sources 147.1-2 (2005), pp. 269–281. doi: 10.1016/j.jpowsour.2005.01.006. [2] Reuter, L., Dickmanns, J.L.S., Kücher, S., Friedrich, S., Morasch, R., Jossen, A., and Gasteiger, H.A.: State-of-Charge Dependent Change in the Microporous Structure of Graphite Electrodes. In: ECS Meeting Abstracts MA2023-02.8 (2023), p. 3392. doi: 10.1149/MA2023-0283392mtgabs. [3] Cannarella, J. and Arnold, C.B.: Stress evolution and capacity fade in constrained lithium-ion pouch cells. In: Journal of Power Sources 245 (2014), pp. 745–751. doi: 10.1016/j.jpowsour.2013.06.165. [4] Hahn, M., Buqa, H., Ruch, P.W., Goers, D., Spahr, M.E., Ufheil, J., Novák, P., and Kötz, R.: A Dilatometric Study of Lithium Intercalation into Powder-Type Graphite Electrodes. In: Electrochemical and Solid-State Letters 11.9 (2008), A151. doi: 10.1149/1.2940573. [5] Schweidler, S., Biasi, L. de, Schiele, A., Hartmann, P., Brezesinski, T., and Janek, J.: Volume Changes of Graphite Anodes Revisited: A Combined Operando X-ray Diffraction and In Situ Pressure Analysis Study. In: The Journal of Physical Chemistry C 122.16 (2018), pp. 8829–8835. doi: 10.1021/acs.jpcc.8b01873. Figure 1