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.

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.

Reliable experimental methods for measuring local potentials in lithium-ion battery cells are challenging but vital for a deep understanding of internal processes at the individual electrode level, and to parameterize and validate electrochemical models. Different three-electrode setups and reference electrodes (REs) have been developed in recent years. Some are based on custom laboratory setups or are small, e.g. coin cell sized. This work addresses internal potentials and half-cell impedances in the widely used single-layer pouch (SLP) cell format and proposes a novel multi-reference electrode cell design, enabling spatially resolved measurements. For the first time, it is shown how multiple 25 μm and 50 μm thin gold wire REs, together with a larger LTO-RE, can be used to study occurring inhomogeneities, considering the geometrical anode overhang. Special attention is given to the subtleties of the measurements and their interpretation. Multiple REs allow plausibility checks and confirm stability for both types during a continuous measurement period of more than 7,500 h (>10 months), demonstrating suitability, e.g. for long-term cycling measurements. Results from electrochemical impedance spectroscopy (EIS) and half-cell potential measurements at low currents of C/100 and during fast charging at up to 3C highlight the versatility of the easily reproducible cell design.