Simulation of Galvanostatic Discharge of the Li x C 6 /Liquid Electrolyte/Li y (Ni a Co b Mn c )O 2 Cell
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Simulation of Galvanostatic Discharge of the LixC6/Liquid Electrolyte/Liy(NiaCobMnc)O2 Cell Rajeswari Chandrasekaran1, Yeonkyeong Seong2, Chulheung Bae1, Joosik Jung2, Kwangsoo Kim2, Kyeongbeom Cheong2, Theodore Miller1 1 Research and Advanced Engineering, Ford Motor Company, Dearborn, MI, United States. 2 Samsung SDI, Yong-in, Kyung-gi, Korea, Republic of. ABSTRACT An isothermal, physics-based model was developed in COMSOL multiphysics software to simulate the galvanostatic discharge performance of LixC6/Liquid Electrolyte/ Liy(NiaCobMnc)O2 dual lithium-ion insertion cell at 298 K. Modeling results are compared with experimental data to provide further insight into design and optimization of these cells for advanced electric vehicles. INTRODUCTION As new materials are investigated for lithium-ion cells for advanced electric vehicles, fundamental understanding of the thermodynamic, kinetic and transport properties of these materials and other design parameters (such as tortuosity) are paramount. Accurate phenomenological cell model is important for optimizing the design (from the standpoint of performance, life, safety, etc.), effective scale-up and prognostics and diagnostics of degradation during cycle and storage [1]. Modeling enables detailed spatial-temporal analyses of responses of interest at a fraction of cost and time as compared to experiments. Therefore, an isothermal, physics-based model is used to simulate the performance of lithium-ion cells during galvanostatic discharge at various current densities. Results are validated with experimental data and the influences of various parameters are identified. THEORY & MODEL PARAMETERS The detailed theory and equations used in the isothermal, physics-based, dual lithium-ion insertion cell sandwich model are available in the references [2], [3] and are not repeated here. Graphite (LixC6) and Liy(NiaCobMnc)O2 are the negative and positive electrode active materials respectively and a porous separator is used in between them. The porous composite electrodes consist of active material, binder and conductive carbon (if needed). The pores are filled with electrolyte (LiPF6 in EC: EMC: DMC in the ratio 3:4:3, with additives). The simulations at all galvanostatic discharge rates are carried out at 298 K, to the cut-off potential of 2.8 V (as in experiments) using COMSOL multiphysics finite element software (version 4.3). The thermodynamic open-circuit potentials (OCP) vs. composition of the electrodes from GITT (Galvanostatic Intermittent Titration Technique) measurements are provided in Figure 1. The bulk ionic conductivity of the electrolyte measured in the presence of additives is provided in Figure 2. The solid phase diffusivities of lithium in the electrodes as a function of the lithium intercalation coefficient estimated from experiments are provided in Figure 3. Table 1 gives the remaining parameters used in this report. The transference number, the diffusion coefficient of the electrolyte, kinetic reaction rate constants and the anodic and cathodic transfer coe
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