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the evolution of the phases’ active surface areas. Finally, a possible mode of degrada- tion – the formation of Li2S at the negative electrode – was integrated into the model. The chapter continued with remarks on the steps taken to successfully implement the model as an extension to the existing modeling framework Denis. Finally, the strategy for model calibration and a consideration of the relevant errors were presented. The presentation of the simulation results in chapter 5 started off with the results of the global model in section 5.1. Here, transport in the liquid electrolyte and cycling protocols were studied, illustrating the advantages of suppressing sulfur mobility in the electrolyte and CCCV charging to fully recover the capacity after discharging. Also the effect of a slow “refresh” cycle was illuminated. The more detailed multi-step model was first presented with literature parametrization in section 5.2. As shown by discharge profiles, impedance spectra, and the evolution of solid and dissolved species, this model generally produces plausible results, including the well-known two distinct voltage plateaus during discharge and the formation/dissolution of solid S8 and Li2S. The results confirm the findings previously published in Ref. [196]. Similar to that publication, however, the end of discharge could not be explained conclusively. Therefore, in a second step, all parameters in the multi-step model were rigorously calibrated to match the experimental data presented in chapter 3. The calibration as well as a partial validation were described in sections 5.3 and 5.4. A broad selection of data was considered for the calibration, ranging from literature values to the ex- perimentally known composition of the cell, and its electrochemical performance in the form of discharge profiles and impedance spectra. Several important parameters were determined by fitting this data. For each fit, a parameter variation was presented, illustrating the effect of the parameter. After correct parametrization, simulation re- sults reproduced experiments to a reasonably good degree and could be used to better understand and explain the empirical findings. More simulations were presented in section 5.5, including the evolution of the grand total and individual concentrations of dissolved species, as well as a study of the formation of a passivating Li2S film on the electrochemically active carbon in the positive electrode. For the calibrated model, the resistance of this film actually triggers the end of discharge for a wide range of oper- ating conditions. Finally, the effect of adding the polysulfide shuttle and the related deposition of Li2S at the negative electrode to the model was studied in section 5.6. For the highly porous cell simulated, the slightly reduced Coulombic efficiency and the deposition of several ppm Li2S at the lithium electrode caused only minor effects in terms of capacity and voltage profiles. 139PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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