Influence of Conductive Additives and Binder on the Impedance of Lithium-Ion Battery Electrodes: Effect of Morphology

Most cathode materials for lithium ion batteries exhibit a low electronic conductivity. Hence, a significant amount of conductive graphitic additives are introduced during electrode production. The mechanical stability and electronic connection of the electrode is enhanced by a mixed phase formed by the carbon and binder materials. However, this mixed phase, the carbon binder domain CBD , hinders the transport of lithium ions through the electrolyte pore network. Thus, reducing the performance at higher currents. In this work we combine microstructure resolved simulations with impedance measurements on symmetrical cells to identify the influence of the CBD distribution. Microstructures of NMC622 electrodes are obtained through synchrotron X ray tomography. Resolving the CBD using tomography techniques is challenging. Therefore, three different CBD distributions are incorporated via a structure generator. We present results of microstructure resolved impedance spectroscopy and lithiation simulations, which reproduce the experimental results of impedance spectroscopy and galvanostatic lithiation measurements, thus, providing a link between the spatial CBD distribution, electrode impedance, and half cell performance. The results demonstrate the significance of the CBD distribution and enable predictive simulations for battery design. The accumulation of CBD at contact points between particles is identified as the most likely configuration in the electrodes under consideratio

Theoretical study of the reaction of LiBH4 with MgH2 in presence of carbon substrate

In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH 4 and MgH 2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH 4 , MgH 2 and carbon layers we did some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB 2 lattice

Suppression of inherent ferromagnetism in Pr-doped CeO2 nanocrystals

Ce1-xPrxO2-delta (0 LT = x LT = 0.4) nanocrystals were synthesized by self-propagating method and thoroughly characterized using X-ray diffraction, Raman and X-ray photoelectron spectroscopy and magnetic measurements. Undoped CeO2 nanocrystals exhibited intrinsic ferromagnetism at room temperature. Despite the increased concentration of oxygen vacancies in doped samples, our results showed that ferromagnetic ordering rapidly degrades with Pr doping. The suppression of ferromagnetism can be explained in terms of the different dopant valence state, the different nature of the vacancies formed in Pr-doped samples and their ability/disability to establish the ferromagnetic ordering

Microstructure-Resolved Impedance Simulations for the Characterization of Li-Ion Battery Electrodes

The production of Li-Ion battery electrodes is a highly interconnected process and many parameters determine the functionality of the final battery cell. Therefore, characterization techniques are very important to monitor the quality of the electrodes and to analyze deviations in electrode performance. The impedance of the porous electrode is a characteristic performance indicator, relatively easy to measure, and the corresponding spectra provide a comprehensive overview of characteristic timescales of different processes. For a detailed analysis impedance spectra are commonly evaluated integrally with the help of equivalent circuit models. However, often the performance of the electrode is affected by local structural inhomogeneities due to compression in the calendering process or an unfavorable binder and/or carbon black distribution. For instance, it was found that harsh drying conditions cause binder migration to the electrode surface and consequently reduce the rate capability1. In this contribution we interpret impedance spectra of Li-ion battery positive electrodes with the help of 3D microstructure-resolved simulations2. This allows us to study in detail the effect of local structural inhomogeneities on the electrode impedance and, thus, performance. NMC electrodes with different thickness and density were prepared and characterized electrochemically by galvanostatic cycling and electrochemical impedance spectroscopy. Impedance spectra were recorded on symmetrical cells3 which are especially advantageous for the characterization of electrode transport properties. Reconstructions of the electrodes were created with the help of synchrotron tomography and a 3D stochastic structure generator4. The resulting microstructures are then input to microstructure-resolved electrochemical simulations. With the help of our simulations we are able to extract the contribution of the carbon black and binder network to the overall pore transport resistance by comparing our simulations to the experimental data. Additionally, we use different models for the spatial distribution of binder and carbon black to mimic different drying conditions and investigate the effect on the electrode impedance and cell performance

Microstructure-Resolved Impedance Simulations for the Characterization of Li-Ion Battery Electrodes

Li-Ion batteries are commonly used in portable electronic devices and state-of-the-art electric vehicles due to their outstanding energy and power density. At high current densities, e.g. during fast charging, the transport of Li-ions in the electrolyte is decisive for the performance of the battery cell and optimized electrode designs are required to reduce mass transport limitations. In this respect the impedance of the porous electrode is a characteristic performance indicator and is commonly evaluated integrally with the help of equivalent circuit models. However, often the performance of the electrode is affected by local structural inhomogeneities due to compression in the calendering process or an unfavorable binder and/or carbon black distribution. For instance, it was found that harsh drying conditions cause binder migration to the electrode surface and consequently reduce the rate capability1. In this contribution we present simulated impedance spectra of Li-ion battery positive electrodes based on 3D microstructure-resolved simulations2 which allows us to study in detail the effect of local structural inhomogeneities on the electrode impedance and, thus, performance. NMC electrodes with different thickness and density were prepared and characterized electrochemically by galvanostatic cycling and electrochemical impedance spectroscopy. Impedance spectra were recorded on symmetrical cells3 which are especially advantageous for the characterization of electrode transport properties. Reconstructions of the electrodes were created with the help of synchrotron tomography and a 3D stochastic structure generator. The resulting microstructures are then input to microstructure-resolved electrochemical simulations. Impedance spectra of the symmetrical cells and half-cells with Li counter electrode were simulated with a potential step and current relaxation technique4. With the help of our simulations we are able to extract the contribution of the carbon black and binder network to the overall pore transport resistance by comparing our simulations to the experimental data. Additionally, we use different models for the spatial distribution of binder and carbon black to mimic different drying conditions and investigate the effect on the electrode impedance and cell performance

Influence of conductive additive and binder domain distribution and its structural properties on macroscopic impedances

The conductive additive and binder domain (CBD) is an essential component of Lithium-ion battery electrodes. It enhances the electrical connectivity and mechanical stability within an electrode matrix. Migration of the binder during electrode drying leads to an inhomogeneous distribution of the CBD, impeding transport of lithium ions in the electrolyte, and diminishing the electronic pathways between solid particles[1]. The effect of this migration on the electrochemical performance of NMC622 electrodes is quantitatively investigated via microstructure-resolved 3D simulations and compared with experimental results. The virtual electrode microstructures are based on tomographic data. The valuable information derived from combining microstructure-resolved models[2] with electrochemical impedance spectroscopy (EIS) simulations on symmetric cells is used to characterize the lithium ion transport in the electrode pore space, including the contributions of the CBD. Additionally, half-cell discharge simulations are conducted to quantify the effect on performance. In the above simulations, the CBD is treated as a homogenized phase with effective transport parameters, not resolving its internal nano-structure. A key aspect for predictively determining the physical and chemical processes occurring are the intrinsic properties of the CBD. To develop a more predictive model, we need to characterize and understand the properties of the CBD on the nano-scale. In the present contribution, high-resolution 3D FIB-SEM data is used to obtain further geometric information on the porous networks within the CBD, shedding light on its effective ionic conductivity. This information is then fed back to the model, allowing us to account for the tortuosity on the nano-scale in the CBD domain

Effect of a Heterogeneous Distribution of the Conductive Additives and Binder Domain on the Impedances of Lithium-Ion Battery Electrodes

The conductive additive and binder domain (CBD) is an essential component of Lithium-ion battery electrodes. It enhances the electrical connectivity and mechanical stability within an electrode matrix. Migration of the binder during electrode drying leads to an inhomogeneous distribution of the CBD, impeding transport of Lithium ions into the electrodes, and diminishing the electronic pathways between solid particles. Therefore, we investigate the effect of binder migration on the electrochemical performance of NMC622 electrodes via microstructure-resolved 3D simulations, and compare them with experimental results. The virtual electrode microstructures are based on tomographic data. The valuable information derived from combining microstructure-resolved models with electrochemical impedance spectroscopy (EIS) simulations on symmetric cells is used to characterize the Lithium-ion transport in the electrode pore space, including the contributions of the CBD. Additionally, half-cell discharge simulations are also conducted. Through our work, we demonstrate the significance of the CBD distribution and enable predictive simulations for future battery design