• Energy Research

Lithium-ion batteries are the dominating electrochemical energy storage technology for battery electric vehicles. However, additional optimization is needed to meet the requirements of the automotive industry regarding energy density, cost, safety, and fast charging performance. In conventional electrode designs, there is a trade-off between energy density and rate capability. Recently, three-dimensional (3D) structuring techniques, such as laser perforation, were proposed to optimize both properties at the same time and remarkable improvements in fast-charging performance have been demonstrated. In this work, we investigate the effect of structuring techniques on the thermal properties and electrochemical performance of the battery using microstructure-resolved simulations. Particular attention will be paid to the heat evolution and lithium plating during fast charging of the batteries. According to our results, 3D structuring is able to reduce the overall cell resistance by improving the electrolyte transport. This has a positive impact on the fast charging capability of the cell and, moreover, reduces the danger of lithium plating.

Abstract Li-ion batteries are commonly used in portable electronic devices due to their outstanding energy and power density. A remaining issue which hinders the breakthrough e.g. in the automotive sector is the high production cost. For low power applications, such as stationary storage, batteries with electrodes thicker than 300 μm were suggested. High energy densities can be attained with only a few electrode layers which reduces production time and cost. However, mass and charge transport limitations can be severe at already small C-rates due to long transport pathways. In this article we use a detailed 3D micro-structure resolved model to investigate limiting factors for battery performance. The model is parametrized with data from the literature and dedicated experiments and shows good qualitative agreement with experimental discharge curves of thick NMC-graphite Li-ion batteries. The model is used to assess the effect of inhomogeneities in carbon black distribution and gives answers to the possible occurrence of lithium plating during battery charge. Based on our simulations we can predict optimal operation strategies and improved design concepts for future Li-ion batteries employing thick electrodes.

It is well known that the spatial distribution of the carbon‐binder domain (CBD) offers a large potential to further optimize lithium‐ion batteries. However, it is challenging to reconstruct the CBD from tomographic image data obtained by synchrotron tomography. Herein, several approaches are considered to segment 3D image data of two different cathodes into three phases, namely, active material, CBD, and pores. More precisely, it is focused on global thresholding, a local closing approach based on energy‐dispersive X‐ray spectroscopy data, ak‐means clustering method, and a procedure based on a neural network that has been trained by correlative microscopy, i.e., based on data gained by synchrotron tomography and focused ion beam scanning electron microscopy data representing the same electrode. The impact of the considered segmentation approaches on morphological characteristics as well as on the resulting performance by spatially resolved transport simulations is quantified. Furthermore, experimentally determined electrochemical properties are used to identify an appropriate range for the effective transport parameter of the CBD. The developed methodology is applied to two differently manufactured cathodes, namely, an ultrathick unstructured cathode and a two‐layer cathode with varying CBD content in both layers. This comparison elucidates the impact of a specific structuring concept on the 3D microstructure of cathodes.

Most cathode materials for Li-ion batteries exhibit a low electronic conductivity. Therefore, a considerable amount of conductive additives is added during electrode production. A mixed phase of carbon and binder provides a 3D network for electron transport and at the same time improves the mechanical stability of the electrodes. However, this so-called carbon binder domain (CBD) hinders the transport of lithium ions through the electrolyte and reduces the specific energy of the cells. Therefore, the CBD content is an important design parameter for optimal battery performance. In the present study, stochastic 3D microstructure modeling, microstructure characterization, conductivity simulations as well as microstructure-resolved electrochemical simulations are performed to identify the influence of the CBD content and its spatial distribution on electrode performance. The electrochemical simulations on virtual, but realistic, electrode microstructures with different active material content and particle size distributions provide insights to limiting transport mechanisms and optimal electrode configurations. Furthermore, we use the results of both the microstructure characterization and electrochemical simulations to deduce extensions of homogenized cell models providing improved predictions of cell performance at low CBD contents relevant for high energy density batteries.

Abstract Fast charging is one of the main challenges in Lithium-ion battery applications. Especially at low temperatures and high C-rates, capacity loss due to lithium plating is identified as the main aging effect. Electrochemical models are able to predict the lithium plating onset conditions, as they provide information about the local potentials and lithium concentrations within the individual electrodes. Due to the narrow potential window of graphite, a precise determination of the sensitive parameters is needed for an accurate prediction of the plating onset. Experimental parameterization is needed as each cell has a specific geometry and the transport parameters are material and geometry-dependent. Literature values are scattered and often do not provide information on the electrode geometry. In this study, a non-isothermal electrochemical 3D model was experimentally parameterized and used to investigate the lithium plating onset at low temperatures. The whole set of geometrical, transport and kinetic model parameters were determined at different temperatures and states of charge and the results were validated against the individual potentials of a multi-layer pouch cell. Good predictions of lithium plating onset were obtained. The study shows that the model can be used to develop fast-charging strategies for commercial lithium-ion batteries at low temperatures.

Abstract A transient 2D physical continuum-level model for analyzing polymer electrolyte membrane fuel cell (PEMFC) performance is developed and implemented into the new numerical framework NEOPARD-X. The model incorporates non-isothermal, compositional multiphase flow in both electrodes coupled to transport of water, protons and dissolved gaseous species in the polymer electrolyte membrane (PEM). Ionic and electrical charge transport is considered and a detailed model for the oxygen reduction reaction (ORR) combined with models for platinum oxide formation and oxygen transport in the ionomer thin-films of the catalyst layers (CLs) is applied. The model is validated by performance curves and impedance spectroscopic experiments, performed under various operating conditions, with a single set of parameters and used to study water management in co- and counter-flow operation. Based on electrochemical impedance spectra (EIS) simulations, the physical processes which govern the PEMFC performance are analyzed in detail. It is concluded that the contribution of diffusion through the porous electrodes to the overall cell impedance is minor, but concentration gradients along the channel have a strong impact. Inductive phenomena at low frequencies are identified from physics-based modeling. Induction is caused by humidity dependent ionomer properties and platinum oxide formation on the catalyst surface.

Silicon (Si) anodes attract a lot of research attention for their potential to enable high‐energy density lithium‐ion batteries (LIBs). Many studies focus on nanostructured Si anodes to counteract deterioration. Herein, LIBs are modeled with Si nanowire anodes in combination with an ionic liquid (IL) electrolyte. On the anode side, elastic deformations to reflect the large volumetric changes of Si are allowed. With physics‐based continuum modeling, insight into usually hardly accessible quantities like the stress distribution in the active material can be provided. For the IL electrolyte, the thermodynamically consistent transport theory includes convection as relevant transport mechanism. The volume‐averaged 1d+1d framework is presented and parameter studies are performed to investigate the influence of the Si anode morphology on the cell performance. The findings highlight the importance of incorporating the volumetric expansion of Si in physics‐based simulations. Even for nanostructured anodes — which are said to be beneficial concerning the stresses — the expansion influences the achievable capacity of the cell. Accounting for enough pore space is important for efficient active material usage.