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.

The impact of mechanical pressure on electrode stability in full-cells comprising microscale silicon-dominant anodes and NCA cathodes was investigated. We applied different mechanical pressures using spring-compressed T-cells with metallic lithium reference electrodes enabling us to analyze the electrode-specific characteristics. Our investigation covers a wide pressure range from 0.02 MPa (low pressure - LP) to 2.00 MPa (ultra high pressure - UHP) to determine the optimal pressure for cyclic lifetime and energy density. We introduce an experimental methodology considering single-component compression to adjust the cell setup precisely. We characterize the cells using impedance spectroscopy and age them at C/2. In the post-mortem analysis, cross-sections of the aged anodes are measured with scanning electron microscopy. The images are analyzed with regard to electrochemical milling, thickness gain, and porosity decrease by comparing them to the pristine state. The results indicate that cycling at UHP has a detrimental effect on cycle life, being almost two-fold shorter when compared to cycling at normal pressure (NP, 0.20 MPa). Scanning electron microscopy showed a dependency of the thickness and the porosity of the aged silicon anodes on the applied pressure, with coating thickness increasing and porosity decreasing for all pressure settings, and a correlation between thickness and porosity.

Microscale silicon particles have a higher specific capacity but larger volume expansion than graphite particles, leading to particle decoupling and lifetime limitations. This study investigates a wide range of external mechanical pressures from zero (ZP - 0.00 MPa) to high (HP - 0.50 MPa) pressure to determine the optimal pressure for high rate capability, cyclic lifetime, energy density, low temperature rise, and low cell thickness gain. The cells are characterized by rate tests and impedance spectroscopy, and are aged until 70% state of health (SoH). The post-mortem analysis after 70% SoH and thickness measurements over 360 cycles in a compression test bench offer insights into the thickness gain. Electrochemical results reveal an immediate reduction in discharge capacity upon transitioning from normal pressure (NP - 0.20 MPa) to ZP, with NP and HP exhibiting superior performance over aging. The impedance was reduced initially and over aging for higher mechanical pressures, especially the cathode contact resistance, resulting in lower temperature rises during the rate tests. Overall, applying higher pressures reduced the anode and cell thickness gain. Moreover, the porosity decreased with increasing pressure, as determined by mercury intrusion porosimetry and pycnometer measurements. The increase of the anode mass correlates to the total charge throughput, which is pressure-dependent and the highest for NP.

Reducing the capacity utilization of silicon-containing anodes and choosing the optimal full-cell voltage window improve the lifetime significantly. In this study, we investigate how different voltage windows affect the aging modes with a common 50% cycling depth. First, the cyclic stability, the anode potentials, and the polarization increase are analyzed for the different voltage windows using 70 wt% microscale silicon anodes and NCA cathodes with a lithium metal reference electrode to investigate the electrode-specific characteristics. Further, the underlying aging modes are quantified in the post-mortem analysis. Finally, the anode thickness increase is quantified using a dilatometer setup for different anode lithiations. In contrast to the literature, the highest voltage window is most beneficial for the lifetime since high anode delithiation potentials and high surface increases are avoided. The anode potential at the end-of-discharge, the charge-averaged full-cell potentials, and the resistance increase are a function of the state of health (SoH). The common underlying main aging mechanism is the loss of lithium inventory, followed by the loss of anode active material. In contrast, the loss of cathode active materials only plays a minor role.

Silicon is a promising anode material and can already be found in commercially available lithium-ion cells. Reliable modeling and simulations of new active materials for lithium-ion batteries are becoming more and more important, especially regarding cost-efficient cell design. Because literature lacks an electrochemical model for silicon-dominant electrodes, this work aims to close the gap. To this end, a Newman p2D model for a lithium-ion cell with a silicon-dominant anode and a nickel-cobalt-aluminum-oxide cathode is parametrized. The micrometer silicon particles are partially lithiated to 1200 mAh ${\mathrm{g}}_{\mathrm{Si}}^{-1}$. The parametrization is based on values from the electrode manufacturing process, measured values using lab cells, and literature data. Charge and discharge tests at six different C-rates up to 2C serve as validation data, showing a root-mean-squared error of about 21 mV and a deviation in discharge capacity of about 1.3%, both during a 1 C constant current discharge. Overall, a validated parametrization for a silicon-dominant anode is presented, which, to the best of our knowledge, is not yet available in literature. For future work, more in-depth studies should investigate the material parameters for silicon to expand the data available in the literature and facilitate further simulation work.

The degradation mode analysis widely investigates lithium-ion cell aging based on voltage reconstructions. In this work, we adapt and validate this degradation mode analysis method for silicon-dominant anodes with feature-poor potentials. Our motivation is to investigate why reducing the capacity utilization improves lifetime significantly. Firstly, different initial silicon anode and NCA cathode potential parametrizations are investigated. Using T-cells with lithium metal reference electrodes, the fitted anode and cathode potentials could be validated by the measured ones. Secondly, different algorithm parameters for the voltage reconstruction are investigated for reliable results. The resulting root-mean-square-error between the measured and the reconstructed voltages is 3.1 mV at the beginning-of-life and stays below 15 mV over aging until ≈55% state-of-health. The main aging mechanism is the loss of lithium inventory at ≈49%, with a linear correlation to the state-of-health. The loss of cathode active material is minor, with 1.0% to 3.2%, dependent on the voltage window. The loss of anode active material is initially negative due to the gain of amorphous silicon. The degradation modes lead to a significant change in the electrode balancing over aging, reproducing the measured increasing anode and cathode potentials. Finally, coin cell and post-mortem results validated the T-cell results.