Franz Benjamin Spingler

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.

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

When charging or discharging a lithium-ion-battery (LIB), lithiation or delithiation of the electrodes takes place. Especially in the case of anode active materials, lithiation often leads to a significant volume increase. The latter can cause a rearrangement of the particles. Although the volumetric changes of state-of-the-art cathode materials have been found to be smaller than for anodes, they remain relevant. The combined volumetric changes of anodes and cathodes are an important factor influencing the lifetime of LIBs. An electrochemical dilatometer was used to measure the thickness change of various electrode active materials under minimal constant pressure (≈ 16 kPa): graphite, silicon-graphite (SiG) composite electrodes, and high-voltage spinel lithium-nickel-manganese-oxide (HVS-LNMO). The influencing factors investigated included the initial porosity of the electrodes, the particle shape of graphite, and the silicon content in the case of the silicon-graphite composite electrodes. Regarding all investigated electrodes, the initial electrode porosity is shown to correlate negatively with the irreversible thickness change during the initial cycles. The thickness change in each electrode was constant over the post-formation cycles, regardless of the initial porosity. Spherical particles in graphite resulted in slightly higher thickness changes than flake-type particles. The thickness change of SiGs increased linearly with silicon content.

Lithium plating on the negative electrode of Li-ion batteries remains as a great concern for durability, reliability and safety in operation under low temperatures and fast charging conditions. High-accuracy detection of Li-plating is critically needed for field operations. To detect the lithium plating is to track its multiphysics footprint since lithium plating often is a localized event while the driving force from chemical, electrical, thermal and mechanical origins could vary with time and locality which makes the detection and characterization challenging. Here, we summarize the multiphysical footprints of lithium plating and the corresponding state-of-the-art detection methods. By assessing and comparing these methods, the combination of capacity/voltage differential, R–Q mapping and Arrhenius outlier tracking could be promising and effective for battery diagnosis, prognosis and management. We analyze the origins of quantitative error in sample preparation, overly simplified assumption and dynamic evolution of the plated Li, and recommend the in situ and quantitative chemical analysis method, such as in situ NMR, EPR, X-ray and neutron. In addition, we propose the four conjectures on the capacity plunge, lithium plating, pore clogging, electrolyte drainage and rapid SEI growth, can be aligned and unified to one scenario basically triggered by lithium plating.

Silicon-based electrodes are promising candidates to enable the next generation of Lithium-ion batteries with high energy densities on the cell level beyond 350 Wh kg⁻¹ [1, 2]. Nevertheless, the commercialization of silicon-based electrodes is still hindered due to the large volume expansion during cycling of up to 300% [3]. This volume expansion upon repeated (de-)lithiation of silicon particles worsens the electrode integrity by causing isolation of active material components [4-6]. Continuous electrolyte decomposition at the silicon/electrolyte interface, caused by the repeated volume expansion and contraction, results in a gradual loss of active lithium [7]. While X-ray diffraction (XRD) as well as theoretical methods provide atomic scale information, dilatometry is an appropriate tool for the in-situ investigation of the volumetric work of battery composite electrodes on the macroscopic level. Therefore, the technique itself as well as the obtained data have a high potential for their industrial implementation [8]. In this work, the expansion of a single-side-coated silicon-based anode is investigated with a commercial, high-precision dilatometer, which detects the thickness change perpendicular to the surface of the coating on the electrode level during cycling against a lithium metal counter electrode. The measured samples consist of 95 wt.% active material (0 - 20 wt.% of silicon in combination with 95 - 75 wt.% of graphite, respectively) and 5 wt.% of non-active components. The investigated samples in this work differ in their silicon and graphite content. Moreover, the samples were also investigated electrochemically via conventional coin cells. The comparable electrochemical behavior of the electrodes in both experiment methods is proof of the plausibility of the dilatometry results. Increasing the silicon content led to higher specific capacities of the SiG electrodes. The relative capacity loss and the thickness change increased linearly with the silicon content as well. Based on the characteristic thickness change for the SiGs, we concluded that silicon predominantly lithiated before graphite, and delithiated after graphite. The used non-commercial electrodes were designed and produced within the scope of the project “HighSafeII”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0306B. Literature [1] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, and B. Stiaszny, “Future generations of cathode materials: an automotive industry perspective”, J. Mater. Chem. A, 3 (13), 6709 (2015). [2] O. Gröger, H. A. Gasteiger, and J.-P. Suchsland, “Review—Electromobility: Batteries or Fuel Cells?”, J. Electrochem. Soc., 162 (14), A2605 (2015). [3] D. Ma, Z. Cao, and A. Hu, “Si-Based Anode Materials for Li-Ion Batteries: A Mini Review” Nano-Micro Lett, 6(4), 347 (2014). [4] M. N. Obrovac, L. Christensen, D. B. Le, and J. R. Dahn, “Alloy Design for Lithium-Ion Battery Anodes”, J. Electrochem. Soc., 154 (9), A849 (2007). [5] D. S. M. Iaboni and M. N. Obrovac, “Li₁₅Si₄ Formation in Silicon Thin Film Negative Electrodes”, J. Electrochem. Soc., 163 (2), 255 (2016). [6] M. N. Obrovac and V. L. Chevrier, “Alloy Negative Electrodes for Li-Ion Batteries”, Chem. Rev., 114, 11444 (2014). [7] R. Petibon, V. Chevrier, C. P. Aiken, D. S. Hall, S. Hyatt, R. Shunmugasundaram, and J. R. Dahn, “Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells”, J. Electrochem. Soc., 163 (7), 1146 (2016). [8] Daniel Sauerteig, Svetlozar Ivanov, Holger Reinshagen, Andreas Bund, „Reversible and irreversible dilation of lithium-ion battery electrodes investigated by in-situ dilatometry“, J. Power Sources, 342, 939-946 (2017).

One promising way of compensating for the repeated volume expansion and contraction of silicon as an anode active material in lithium ion batteries (LIBs) is to embed silicon within a graphite matrix. Silicon-graphite (SiG) composites combine the advantageous properties of graphite, i.e., large electrical conductivity and high structural stability, with the advantageous properties of silicon, i.e., high theoretical capacity. Graphite has a much lower volume expansion upon lithiation (≈ 10%) than pure silicon (≈ 300%) and provides a mechanically stable matrix. Herein, we present an investigation into the electrochemical performance and thickness change behavior of porous SiG anode compositions with silicon contents ranging from 0 wt% to 20 wt%. The electrode composites were studied using two methods: in situ dilatometry for the thickness change investigation and conventional coin cells for the assessment of electrochemical performance. The measurements show that the initial thickness change of SiG electrodes increased significantly with the silicon content, but it leveled off during cycling for all compositions. There appears to be a correlation between silicon content and capacity loss, but no clear correlation between thickness change and capacity loss rate was found.

Lithium-ion batteries can experience mechanical loads for a variety of reasons, including the rigidity of the cell casing itself, bracing of cell stacks in a module, which is generally due to limited space in the place of installation, or as a result of accidents or abuse. In all of these cases, and exacerbated by faulty manufacturing or assembly, the mechanical loads may be non-uniform across the cell surface. Here, we present an analysis of the effects of such non-uniform mechanical loads on the current density distribution during charging and show that they can provoke localized lithium plating. Pressure-compression relationships of individual cell components were determined experimentally and implemented into a pseudo-3D axisymmetric electrochemical-mechanical cell model of a 2.1 Ah pouch cell by Kokam, South Korea. The modeling results were successfully validated by comparison to a post-mortem evaluation of pouch cells that were cycled while being locally compressed.

Virtually all types of electrodes used in lithium-ion batteries expand and contract during cycling, which poses an engineering and design challenge. Information provided by X-ray diffraction (XRD) about alterations in the crystal structure of active materials may be insufficient to inform these engineering tasks. This is because it is unclear how these evolutions of the crystal structure translate into the measurable thickness changes at the electrode or cell level. In this study we investigate the thickness changes of electrodes during cycling using a dilatometry setup and compare them to XRD-measured crystal structure changes from scientific literature. Both the reliability of the dilation measurement and the electrochemical performance of the dilatometry setup are thoroughly validated and significantly exceed those of related studies that have been published in recent years. Various laboratory-made graphites as well as LiNi_1/3Co_1/3Mn_1/3O₂ (NMC111), LiNi_0.6Co_0.2Mn_0.2O₂ (NMC622), LiNi_0.8Co_0.1Mn_0.1O₂ (NMC811) and LiNi_0.8Co_0.15Al_0.05O₂ (NCA) electrodes and the positive electrode from a Kokam SLPB356495 pouch cell are investigated. The results show that electrode expansion does not necessarily correlate with the unit cell volume changes of its active materials in any meaningful way and thus only by measuring the expansion of the full electrode can we fully understand and predict its behavior during cycling.

This investigation focuses on significant capacity losses that have been found to occur in commercial lithium-iron phosphate (LFP) / graphite cells after repeated shallow cycling around medium states of charge (SOCs). Many combinations of cycle depth, medium SOC and temperature are examined over up to 10,000 full cycle equivalents. Up to 1000 full cycle equivalents, the capacity loss rate is four times higher when the cells are cycled between 40% and 60% state of charge than it is with full cycles. It is shown that these extraordinary losses occur in three different lithium-iron phosphate / graphite cell models, two 26650-type and one 18650-type, but do not occur in a nickel-manganese-oxide / graphite 18650-type cell. Most of the apparently lost capacity is shown to be recoverable by holding the cells at either 0 % or 100 % state of charge over a period of several days. Differential voltage analysis and visual post-mortem inspection as well as coin cell experiments with harvested electrode material suggest that these capacity losses are caused by strongly non-uniform lithium distributions in the electrodes. Hypothetical mechanisms are presented and discussed that could lead to such non-uniform distributions of lithium forming preferentially in lithium-iron phosphate based cells. The findings presented in this study are believed to be particularly relevant for grid applications like primary control reserve or in other scenarios where the battery mainly serves as a peak power buffer, e. g. in hybrid drivetrains. The graph shows the capacity evolution of LFP / graphite cells cycled with full cycles and with 20 % cycle depth around three different medium SOCs. Capacity loss is largest when cycling between 40 % and 60 % SOC. The photographs show post-mortem views of the graphite anodes of three different LFP / graphite cell models that were cycled using this procedure until reaching ca. 80 % remaining capacity. Gold colored stripes at the top and the bottom of the electrodes indicate a non-uniform distribution of intercalated lithium. Figure 1