Browsing by Author "Bo, Yan"
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Item Geometric Characteristics of Lithium Ion Battery Electrodes with Different Packing Densities(Office of the Vice Chancellor for Research, 2015-04-17) Lim, Cheolwoong; Lee, Wen Chao; Bo, Yan; Song, Zhibin; De Andrade, Vincent; De Carlo, Francesco; Kim, Youngsik; Zhu, LikunThe 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.Item Microstructure evolution of high capacity anode electrode by in-situ and in-operando X-ray nano-CT(Office of the Vice Chancellor for Research, 2016-04-08) Lim, Cheolwoong; Kang, Hixiao; Bo, Yan; Zhu, LikunAlloy-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.Item Modeling and simulation of heat of mixing in lithium ion batteries(Office of the Vice Chancellor for Research, 2015-04-17) Song, Zhibin; Bo, Yan; Lim, Cheolwoong; Zhu, LikunHeat 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 ∙ ( − ,∞)] = −