Geometric and Electrochemical Characteristics of NMC Electrodes with Different Calendering Conditions

poster abstractThe energy and power capabilities of Li ion batteries (LIBs) have been considered critical factors to determine the commercial values of the LIB powered applications. Many efforts have been done to improve the energy density and rate capability of LIBs. In addition to intrinsic material properties of anode and cathode active materials, the structure of electrode at micro and nano scales also plays a critical role in determining the energy density and rate capability of a LIB [1-3]. Calendering is a process in battery manufacturing to lower the porosity of the electrode and increase electrical contact. Increased calendering can increase the packing density of active materials in LIB electrodes, thereby increasing the volumetric energy density. The specific energy density is also increased by calendering via decreasing the percentage of inactive materials, such as current collector and separator. However, higher fraction of active materials in LIB electrodes can change electrodes’ structural properties significantly, such as porosity, specific surface area, pore size distribution and tortuosity [4]. To this end, there are few reports on the geometric characteristics and their impact on the electrochemical performance of LIB electrodes with different calendering conditions due to the inhomogeneity, complexity, and three-dimensional (3D) nature of the electrode’s microstructure [5-6]. Recently, porous electrode microstructures have been reconstructed by advanced tomography techniques such as X-ray nano-computed tomography (nano-CT) and focused ion beam scanning electron microscope (FIB-SEM)[7-8]. The reconstructed microstructures have been employed to investigate the geometric characteristics and spatial inhomogeneity of porous electrodes. In this study, we investigated real 3D Li[Ni1/3Mn1/3Co1/3]O2 (NMC) electrode microstructures under different calendering conditions and the effect of calendering on the performance of LIBs[4]. To investigate geometric characteristics of porous microstructures, cathode electrodes were fabricated from a 94:3:3 (weight %) mixture of NMC, PVDF, and super-P carbon black. To change the calendering condition, initial thickness of the electrodes was set 50μm, 80um, 90um, 100um. Then all electrodes were pressed down to 50 μm by using a rolling press machine. A synchrotron X-ray nano-CT at the Advanced Photon Source of Argonne National Lab was employed to obtain morphological data of the electrodes, with voxel size of 58.2 × 58.2 × 58.2 nm3. The morphology data sets were quantitatively analyzed to characterize their geometric properties. The geometric analysis showed that high packing density can result in smaller pore size and more uniform pore size distribution. The specific surface area and tortuosity of different electrodes will be reported. The charge/discharge experiments were also conducted for these electrodes. The geometric properties and cell testing results will be analyzed and reported

Modeling and simulation of heat of mixing in lithium ion batteries

poster abstractHeat generation is a major safety concern in the design and development of lithium ion batteries (LIBs) for large scale applications, such as electric vehicles. The total heat generation in LIBs includes entropic heat, enthalpy, reaction heat, and heat of mixing (1-3). The heat of mixing will be released during relaxation of Li ion concentration gradient. For instance, after the drivers turn off their vehicles, the generation of entropy, enthalpy and reaction heat in LIBs will stop, but the heat of mixing is still being generated. Thomas and Newman derived methods to compute heat of mixing in LIB cells and investigated the heat of mixing on a Li|LiPF6 in ethylene carbonate:dimethyl carbonate|LiAl0.2Mn1.8O4-δF0.2 cell (4). The objective of this study is to investigate the influence of heat of mixing on the LIBs with different materials, porosities, particle sizes, and charge/discharge rate and to understand whether it is necessary to consider heat of mixing during the design and development of LIBs. In this study, a mathematical model was built to simulate heat generation of LIBs using COMSOL Multiphysics. The LIB model was based on Newman’s model. LiCoO2 was applied as the cathode materials, and LiC6 was applied as the anode material. The results of heat of mixing were compared with the other heat sources to investigate the weight of heat of mixing in the total heat generation. Table 1 shows the heat of mixing, irreversible heat, and reversible heat in anode and cathode electrodes at 5 min during a 2 C discharge process. As shown in Table 1, the heat of mixing in cathode is smaller than the heat of mixing in anode, mainly due to the lower Li ion diffusivity and larger particle size of LiC6. The heat of mixing is not as much as the irreversible heat and reversible heat, but it cannot be neglected for this operating condition. The heat of mixing in different LIB cells and under different operating conditions will be reported. The mathematical model: Mathematical model equations: = ( − ) + + Σ Δ + Σ Σ ( − ) = [ 1 2 ∙ ( − ,∞)] =

Geometric characteristics of 3D reconstructed anode electrodes of lithium ion batteries

The realistic 3D microstructure of lithium ion battery electrodes plays a key role in studying the effects of inhomogeneous microstructures on the performance of LIBs. However, the complexity of realistic microstructures implements significant computational cost on numerical simulation of large size samples. In this work, we used tomographic data obtained for a commercial lithium ion battery graphite electrode to evaluate the geometric characteristics of the reconstructed electrode microstructure. Based on the analysis of geometric properties, such as porosity, specific surface area, tortuosity, and pore size distribution, a representative volume element that retains the geometric characteristics of the electrode material was obtained for further numerical studies. In this work, X-ray micro-CT with 0.56 μm resolution was employed to capture the inhomogeneous porous microstructures of lithium ion battery anode electrodes. The Sigmoid transform function was employed to convert the initial raw tomographic images to binary images. Moreover, geometric characteristics of an anode electrode after 2400 1 C charge/discharge cycles were compared with those of a new anode electrode to investigate morphological change of the electrode. In general, the cycled electrode shows larger porosity, smaller tortuosity, and similar specific surface area compared to the new electrode

Simulation of Heat Generation in a Reconstructed LiCoO2 Cathode during Galvanostatic Discharge

A three dimensional numerical framework with finite volume method was employed to simulate heat generation of a semi lithium ion battery (LIB) cell during isothermal galvanostatic discharge processes. The microstructure of the LIB cathode electrode was experimentally determined using X-ray nano computed tomography technology. Heat generation in the semi LIB cell during galvanostatic discharge processes from different mechanisms, such as electronic resistive heat, ionic resistive heat, contact resistive heat, reaction heat, entropic heat and heat of mixing, was investigated. The spatial distribution of heat generation rates from different mechanisms was also studied. The simulation results demonstrate that the magnitude of heat generation rates spans a wide range in the electrode due to structural inhomogeneity. The simulation results of heat generation from the three dimensional model and the porous-electrode theory model were compared in this study. It is found that the typical Bruggeman coefficient, 1.5, underestimated ionic resistance in the electrolyte and overestimated electronic resistance in the cathode particles. In general, the three dimensional model predicted more heat generation than the porous-electrode theory model at large discharge rates due to the wider distribution of physical and electrochemical properties

Polarization Analysis Based on Realistic Lithium Ion Battery Electrode Microstructure Using Numerical Simulation

poster abstractThe performance of lithium ion battery (LIB) is limited by the inner polarization and it is important to understand the factors that affect the polarization. This study focuses on the polarization analysis based on realistic 3D electrode microstructures. A c++ software was developed to rebuild and mesh the microstructure of cathode and anode electrodes through Nano-CT and Micro-CT scanned images respectively. As a result, the LIB model was composed of electrolyte, cathode and anode active materials and current collectors. By employing 3D finite volume method (FVM), another c++ code was developed to simulate the discharge and charge processes by solving coupled model equations. The simulation revealed the distribution of physical and electrochemical variables such as concentration, voltage, current density, reaction rate, et al. In order to explore the correlation of local effects and electrode structural heterogeneity, the cathode electrode were divided equally into 8 sub-divisions, of which the porosity, tortuosity, specific surface area were calculated. We computed the polarizations in the sub-divisions due to different sub-processes, i.e., the activation of electrochemical reactions and charge transport of species. As shown in Fig. 1, the tortuosity is very irregular because of unevenly distributed cathode particle size and packing pattern with low porosity. There are no exact and direct relations among porosity, tortuosity and specific surface area. Fig. 2 shows that the polarizations are related to the porosity in sub-divisions. The knowledge from the study will help to figure out the mechanism of polarization and power loss in LIB, which could be useful to improve LIB design and manufacturing. Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850. Fig. 1 Porosity and tortuosity in sub-divisions of a cathode electrode Fig. 2 Intercalation reaction polarization and ionic conduction polarization of sub-divisions at 120 sec during a 5 C charging proces

3D Simulation of diffusion induced stress in realistic LiCoO2 electrode particles of lithium ion battery generated by nano-CT

Diffusion induces stresses in the electrode during charge and discharge processes of lithium ion batteries, which can cause deformation and even fracture, further result in the fade of capacity and duration. The 3D model coupling diffusion and induced stress is applied to the reconstructed LiCoO2 electrode particles determined by X-ray nanocomputed tomography technology, of which the nonuniform electrochemical intercalation reaction takes place on the surface. A code is developed to simulate the fully coupled diffusion and induced stress in the LiCoO2 electrode particles at different discharge rates. The simulations demonstrate the variable distribution such as concentration, reaction rate, hydrostatic stress, Von-Mises stress, and so on. The influence of the geometric characteristics of LiCoO2 electrode particle and material properties on the variables is revealed. The investigation can help to improve lithium ion battery design and manufacture through understanding the relationship between electrode morphology and mechanical endurance

Hard X-ray-induced damage on carbon–binder matrix for in situ synchrotron transmission X-ray microscopy tomography of Li-ion batteries

The electrode of Li-ion batteries is required to be chemically and mechanically stable in the electrolyte environment for in situ monitoring by transmission X-ray microscopy (TXM). Evidence has shown that continuous irradiation has an impact on the microstructure and the electrochemical performance of the electrode. To identify the root cause of the radiation damage, a wire-shaped electrode is soaked in an electrolyte in a quartz capillary and monitored using TXM under hard X-ray illumination. The results show that expansion of the carbon–binder matrix by the accumulated X-ray dose is the key factor of radiation damage. For in situ TXM tomography, intermittent X-ray exposure during image capturing can be used to avoid the morphology change caused by radiation damage on the carbon–binder matrix

Microstructure evolution of high capacity anode electrode by in-situ and in-operando X-ray nano-CT

poster abstractAlloy-typed materials have been studied as an anode active material to develop high energy density lithium ion batteries (LIBs). Especially, lithium alloys based on the group IV elements (Si, Ge, and Sn) are potential candidates for the anode material because of their high theoretical capacities and low operating voltages. Lithiation and delitiation of the anode alloys accompany large volume change that causes fractures, pulverizations, and delamination of the electrodes. The mechanical degradation reduces the reversible capacity and shortens the cycle life of the alloy anode LIBs. Particle fracture has been alleviated by nano-structuring the alloy-type anode materials due to the facile strain accommodation and the short diffusion path for electron and lithium ion transport in these nanostructured electrodes. However, nano-structured particles have low tap density and lead to lower energy density anodes, making scale up difficult. The surface area of the material increases with decreasing particle size, which leads to large irreversible capacity loss due to the formation of the solid electrolyte interphase (SEI). Currently, a fundamental understanding of the impact of a high capacity electrode’s microstructure change on LIB performance is still lacking due to the inhomogeneity, complexity, and 3D nature of the electrode’s microstructure. In this study, a novel approach is proposed to gain greater understanding of the microstructure change of the alloy anode electrodes and its impact on the electrochemical performance. A special LIB cell was designed to monitor the microstructure change of high capacity anode electrodes with the synchrotron X-ray nano-CT technique at the Advanced Phothon Source of Argonne National Lab. The cell is composed of a quartz capillary housing and a wire-typed electrode to maximize X-ray penetration for the nano-CT scan. The structural evolution of the alloy electrodes is monitored to investigate crack propagations and pulverizations under in-operando 2D x-ray CT scan. Moreover, in-situ 3D x-ray CT scan enables to study the anisotropic volumetric changes at different voltage states. This simultaneous structural and electrochemical investigation of the alloy electrodes is an essential study to understand the fundamental degradation mechanism of high capacity lithium alloy anode

Three-Dimensional Reconstruction and Analysis of All-Solid Li-Ion Battery Electrode Using Synchrotron Transmission X-ray Microscopy Tomography

A synchrotron transmission X-ray microscopy tomography system with a spatial resolution of 58.2 nm at the Advanced Photon Source was employed to obtain three-dimensional morphological data of all-solid Li-ion battery electrodes. The three-phase electrode was fabricated from a 47:47:6 (wt %) mixture of Li(Ni1/3Mn1/3Co1/3)O2 as active material, Li1.3Ti1.7Al0.3(PO4)3 as Li-ion conductor, and Super-P carbon as electron conductor. The geometric analysis show that particle-based all-solid Li-ion battery has serious contact interface problem which significantly impact the Li-ion transport and intercalation reaction in the electrode, leading to low capacity, poor rate capability and cycle life

Geometric Characteristics of Lithium Ion Battery Electrodes with Different Packing Densities

poster abstractThe microstructure of electrodes plays a critical role in determining the performance of lithium ion batteries (LIBs), because the microstructure can affect the transport and electrochemical processes within electrodes (1-3). Increasing the volume fraction of active materials in the electrode will increase the energy density. However, the electrodes’ structural properties could also be changed significantly and the critical physical and electrochemical processes in LIBs will be affected. Therefore, the performance of a LIB can be optimized for a specific operating condition by designing electrode microstructures. For instance, Hellweg suggested a spatially varying porous electrode model to improve lithium ion transport in electrolyte phase at high charge/discharge rates (4). He showed that the power density of the graded porosity electrode was higher than a homogeneous porosity electrode without energy loss. In this study, we investigate the realistic geometric characteristics of electrode microstructures under different packing densities and the effect of packing density on the performance of LIBs. Moreover, a spatially varying porous electrode will be studied to increase the electrode energy density without losing rate capability. To investigate geometric characteristics of porous microstructures, cathode electrodes were fabricated from a 94:3:3 (weight %) mixture of LiCoO2 (average particle radius = 5 μm), PVDF, and super-P carbon black. To change the packing density, initial thickness of the electrodes was set in a range of 40 ~ 80 μm. Then all electrodes were pressed down to 40 μm by using a rolling press machine. A synchrotron X-ray nano-computed tomography instrument (nano-CT) at the Advanced Phothon Source of Argonne National Lab was employed to obtain morphological data of the electrodes, with a spatial resolution of 60 nm. The morphology data sets were quantitatively analyzed to characterize their geometric properties. Fig. 1 shows the porosity (ε), specific surface area (As, μm-1), tortuosity (τ), and pore size distribution of 4 different electrode microstructures. The pore size distribution of the un-pressed electrode (ε =0.56, black color) demonstrates nonuniformly dispersed active material. The highest packing density electrode (ε =0.36, red color) shows the highest tortuosity. The charge/discharge experiments were also conducted for these 4 different electrodes. The geometric properties and cell testing results will be analyzed and reported. Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850. Fig. 1 Geometric characteristics (porosity ε, specific surface area As, tortuosity τ, pore size distribution) of xray generated porous electrode microstructure with different packing densities