Virtual design and development of novel electrode concepts for Lithium-Ion batteries

Li-Ion batteries are commonly used in portable electronic devices due to their outstanding energy and power density. However, in order to reach the requirements of the automotive industry for next-generation electric vehicles regarding safety, life-time, energy density, and rate capability further developments are inevitable. Additionally, a reduction of material and production costs is needed to further increase the market penetration of electric vehicles. Currently, several novel cell chemistries1 based on abundant materials such as magnesium2 and sodium3 are under investigation, however, at the moment Li-Ion batteries are the only commercial product coming close to the requirements specified above. The capacity of state-of-the-art Li-Ion batteries is intrinsically limited by its intercalation chemistry 4. During charge and discharge of the battery lithium ions are shuttling between the positive and negative electrode where they are inserted in the host structure of the active materials. In order to improve the energy density the share of inactive materials such as separators, current collectors, housing, etc. needs to be reduced. This can be done by either making these components thinner, which bears some safety risks, or by increasing the areal capacity and/or thickness of the electrode layers. The latter approach has the advantage that less electrode layers are needed to assemble a battery of the same capacity which also reduces production time and cost. However, increasing the active material loading can cause transport limitations5 of the shuttling lithium ions which limits the rate capability of the cell and, thus, is problematic for the targets of the automotive industry regarding fast charging. Moreover, new cell designs and production methods6,7 might need to be developed. A possible route towards high capacity electrodes for Li-Ion batteries is the development of new structuring techniques by e.g. laser perforation. Computer simulations can be a very useful and versatile tool to find optimal cell designs or electrode structures. Our virtual design approach is based on 3D micro-structure resolved simulations in the in-house software package BEST (Battery and Electrochemistry Simulation Tool)8. The governing conservation equations for mass, charge, and energy were derived in an approach based on non-equilibrium thermodynamics9 and allow tracking important quantities like the local concentration of lithium in the electrolyte and active material or the temperature distribution in the cell. Based on this information limiting processes for the global battery performance or life-time can be detected and different electrode designs can be evaluated. An important basis for predictive simulations is a sound parameterization of the model. Electrochemical parameters and transport parameters can be determined in independent model experiments and a brief overview of the methodology will be given10. The second corner stone of the simulations are the electrode structures themselves. Virtual electrode structures are obtained by tomographic methods like Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) and x-ray Computed Tomography (CT) or virtual stochastic 3D geometry generators11. The latter are parametrized with tomography data and allow exploring a large parameter space of realistic electrode structures. This methodology gives the opportunity to correlate material and structural properties with the performance of the battery and therefore, provides an important design tool for the processing of improved electrode geometries. In the presentation a short introduction to the fundamentals and working principles of Li-Ion batteries will be given. A focus will be set on current limitations and future targets. In the second part I will introduce the virtual design approach developed in our group and demonstrate the capabilities of the approach with the help of a few selected design studies of novel electrode processing concepts

Analysis of limiting factors for Li-Ion battery performance and life-time: Micro-structure resolved simulations with BEST

In our presentation we will show examples of micro-structure resolved simulations in our battery simulation tool BEST (Battery and Electrochemistry Simulation Tool [1]). BEST was originally developed for the simulation of Li-Ion batteries in the former group of one of the authors (AL) at the Fraunhofer ITWM Kaiserslautern. The governing equations are derived in a rigorous approach from fundamental non-equilibrium thermodynamics and are implemented based on the CoRheos framework for complex and granular flow. DLR/HIU and Fraunhofer ITWM are collaborating to develop models for additional physical and chemical processes as well as new battery chemistries and to implement them in the software package. BEST is under constant active development and gives insights to fundamental physical processes as well as the latest developments for state-of-the-art battery materials. Li-Ion batteries are commonly used in portable electronic devices due to their outstanding energy and power density. Remaining issues which hinder a breakthrough e.g. for stationary storage applications or electric vehicles are high production costs as well as safety risks. Recently, new battery concepts with thicker electrodes (>300 µm) or solid electrolytes were suggested to resolve these issues [2]. In both cases mass and charge transport limitations can be severe at already small currents due to long transport pathways, small transport coefficients, or inhomogeneous material properties. This could be a trigger for degradation effects, such as Li plating at the graphite anode, and reduces the lifetime of the battery. A thorough understanding of relevant processes within the electrodes is urgently needed to avoid these problems. The electrode micro-structures of our simulations are either taken from tomography data [3] or geometries generated in GeoDict. Our detailed 3D studies allow important insights on cell operation and reveal detrimental structural properties for battery performance. For instance, we are able to quantify the effect of an inhomogeneous distribution of conductive additive in the case of thick electrodes or imperfect impregnation of the electrodes with the solid electrolyte. Moreover, we investigate the occurrence of degradation processes, such as Li plating during battery charge. Our approach allows analyzing limiting processes and critical operation conditions and predicts possible optimization and operation strategies to improve the performance and life-time of Li-Ion batteries

First steps towards a model of Mg-S batteries

The use of alkaline/alkaline earth metal as anode material, such as Li, Na, Mg, or Zn offers many benefits compared to conventional intercalation chemistry based battery technologies. Lithium-sulfur batteries are one of most investigated systems of that type. In recent years magnesium-sulfur batteries are highly discussed as a next-generation energy storage system. Magnesium can be directly used as anode material due to its dendrite-free deposition and thus increases the safety as well as energy density of such a cell. Two electrons are stored per Mg atom which compensates the rather low discharge potential of magnesium-sulfur cells of 1.7 V and provides a high capacity of 3832 mAh/cm3 and 2230 mAh/g [1]. The system offers a theoretical energy density of over 3200 Wh/l, which is beyond that of lithium-sulfur batteries and is therefore very promising for automotive and stationary applications [2]. Furthermore magnesium and sulfur are both naturally abundant, low in price and non-toxic. However, magnesium-sulfur batteries are in a very early stage of research and development. The first proof of concept was reported by Kim et al. in 2011. Critical issues of this first cell were a short lifetime of only two cycles and an initial discharge potential of only 1 V indicating side reactions [3]. A new electrolyte recently developed by Zhao-Karger et al. pushed the system to the next level and a cell with a lifetime of more than 50 cycles and a discharge potential near the theoretical value was demonstrated. However, analogous to lithium-sulfur batteries magnesium-sulfur batteries show polarization effects during charging, low cyclic stability, initial capacity fading, and a polysulfide shuttle. The reaction mechanism is proposed analogous to those of lithium-sulfur batteries but is still not investigated in detail. To the best of our knowledge there are no continuum models of magnesium-sulfur batteries in the literature. Therefore, we present the first step towards a mechanistic model of magnesium-sulfur cells. Our aim is to provide more insight in the operation of Mg-S batteries and to support the experimental progress. We use a continuum model based on porous electrode and dilute solutions theory. The transport of dissolved species is described by the Nernst-Planck equation where transport occurs via diffusion and migration. Our kinetic model includes a reduced reaction mechanism from our previous work on Li-S cells which was able to reproduce the key experimental results [4]. In this work we additionally take into account adsorption and desolvation effects [5] which are expected to be prominent for Mg cations. In close collaboration with experimentalists we aim at guiding new developments of the Mg-S system

Modeling of structured electrodes in lithium-sulfur batteries

Due to their high theoretical capacity lithium-sulfur batteries (Li/S) are envisioned as next-generation storage technology for electric vehicles [1]. However, several challenges obviate a successful commercialization of the battery. Most of them are related to the high solubility of intermediate polysulfide species and the resulting so-called ‘shuttle effect’. The transport of polysulfides between positive and negative electrode causes a decay of capacity and low coulombic efficiency upon cycling. Experimental studies [2] demonstrate that nano-structuring of carbon/sulphur composite electrodes significantly improves the cyclability of the battery. Micro-porous particles [3], hollow carbon spheres (HCS) or carbon nano tubes (CNTs) are used to encapsulate the sulphur and to prevent transport of polysulfides to the anode. In our contribution we present results of a detailed 1D single particle continuum model describing reaction and transport inside a representative spherical particle [4], [5]. On the surface we assume that only lithium ions are able to enter and leave the particle. This relatively simple model gives some interesting insights on the behavior of the particle during battery operation. Most prominently, we identified an additional overpotential during discharge resulting from the transport of Li ions against a concentration gradient into the particle. In a following step we couple the single particle model to a macroscopic model of a full battery cell. Results of the 1+1D full cell simulations are parameterized and validated based on experimental data [3]. Systematic parameter studies reveal the influence of novel electrode geometries on battery performance and are able to guide future improvements in electrode design

Homogenized lattice Boltzmann model for simulating multi-phase flows in heterogeneous porous media

A homogenization approach for the simulation of multi-phase flows in heterogeneous porous media is presented. It is based on the lattice Boltzmann method and combines the grayscale with the multi-component Shan–Chen method. Thus, it mimics fluid–fluid and solid–fluid interactions also within pores that are smaller than the numerical discretization. The model is successfully tested for a broad variety of single- and two-phase flow problems. Additionally, its application to multi-scale and multi-phase flow problems in porous media is demonstrated using the electrolyte filling process of realistic 3D lithium-ion battery electrode microstructures as an example. The approach presented here shows advantages over comparable methods from literature. The interfacial tension and wetting conditions are independent and not affected by the homogenization. Moreover, all physical properties studied here are continuous even across interfaces of porous media. The method is consistent with the original multi-component Shan–Chen method (MCSC). It is as stable as the MCSC, easy to implement, and can be applied to many research fields, especially where multi-phase fluid flow occurs in heterogeneous and multi-scale porous media

Influence of Electrode Structuring Techniques on the Performance of All-Solid-State Batteries

All-solid-state batteries (ASSBs) offer a promising route to safer batteries with superior energy density compared to conventional Li-ion batteries (LIBs). However, the design of the composite cathode and optimization of the underlying microstructure is one of the aspects requiring intensive research. Achieving both high energy and power density remains challenging due to limitations in ionic conductivity and active material loading. Using structure-resolved simulations, we investigate the potential of perforated and layered electrode designs to enhance ASSB performance. Design strategies showing significant performance increase in LIBs are evaluated regarding their application to ASSBs. Composite cathodes with solid electrolyte channels in the structure do not significantly increase cell performance compared to unstructured electrodes. However, the design with a two-layer cathode proves promising. The layered structure effectively balances improved ionic transport due to increased solid electrolyte fraction at the separator side and substantial active material loading through increased active material fraction at the current collector side of the cathode. Our research highlights key challenges in ASSB development and provides a clear direction for future studies in the field.Comment: 46 pages, 15 figure

A novel continuum model and simulation study of metal (Mg & Li) -sulfur battery with sulfurized polyacrylonitrile (SPAN) cathodes

Magnesium-sulfur (Mg-S) batteries are considered promising contenders for the next generation of batteries due to a set of advantages: high gravimetric energy density, high abundancy of both Mg and S, absence of scarce elements such as nickel or cobalt, and a reduced tendency to dendrite formation [1]. Unfortunately, similar to Lithium-Sulfur batteries, Mg-S batteries show a low coulombic efficiency and fast self-discharge due to the polysulfide shuttle. Several mitigation strategies to reduce the polysulfide shuttle effect, have been developed for Li-S batteries and some of these concepts have been also transferred to Mg-S batteries [2]. One of the promising approaches is to covalently bind the sulfur to a polymer backbone. Long cycle life and high specific capacities have been demonstrated for sulfurated poly(acrylonitrile) (SPAN) cathodes in lithium-based batteries and, more recently, the proof-of-concept was also shown for Mg-SPAN batteries [3,4]. We present a novel continuum model for both Li & Mg-SPAN batteries, which includes redox reactions of sulfur covalently bound to the polymeric backbone of SPAN, species transport, and electrochemical reactions of the polysulfides in solution. The model requires a number of parameters which however can beextracted from structural and electrochemical characterization. Using our model we performed simulation studies screening the influence of different parameters on the cell performance. We demonstrate that the morphology of the electrode is essential for Mg-SPAN battery performance. In our contribution we aim on providing more insights into the degradation mechanisms and limiting factors for battery performance, which are able to guide new developments for Mg-SPAN batterie

Systematic Workflow for Efficient Identification of Local Representative Elementary Volumes Demonstrated with Lithium-Ion Battery Cathode Microstructures

The concept of a representative elementary volume (REV) is key for connecting results of pore-scale simulations with continuum properties of microstructures. Current approaches define REVs only based on their size as the smallest volume in a heterogeneous material independent of its location and under certain aspects representing the same material at the continuum scale. However, the determination of such REVs is computationally expensive and time-consuming, as many costly simulations are often needed. Therefore, presented here is an efficient, systematic, and predictive workflow for the identification of REVs. The main differences from former studies are: (1) An REV is reinterpreted as one specificsub-volume of minimal size at a certain location that reproduces the relevant continuum properties of the full microstructure. It is therefore called a local REV (lREV) here. (2) Besides comparably cheap geometrical and statistical analyses, no further simulations are needed. The minimum size of the sub-volume is estimated using the simple statistical properties of the full microstructure. Then, the location of the REV is identified solely by evaluating the structural properties of all possible candidates in a very fast, efficient, and systematic manner using a penalty function. The feasibility and correct functioning of the workflow were successfully tested and validated by simulating diffusive transport, advection, and electrochemical properties for an lREV. It is shown that the lREVs identified using this workflow can be significantly smaller than typical REVs. This can lead to significant speed-ups for any pore-scale simulations. The workflow can be applied to any type of heterogeneous material, even though it is showcased here using a lithium-ion battery cathode