Reactive Molecular Dynamics Simulations of Lithium Secondary Batteries - Interfaces and Electrodes
Open Access
- Author:
- Islam, Md Mahbubul
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 01, 2016
- Committee Members:
- Adrianus C Van Duin, Dissertation Advisor/Co-Advisor
Adrianus C Van Duin, Committee Chair/Co-Chair
Donghai Wang, Committee Member
Hosam Kadry Fathy, Committee Member
Lasse Jensen, Outside Member - Keywords:
- eReaxFF
ReaxFF
Reactive molecular dynamics
Solid eletrolyte interphase
Lithium-sulfur battery
Lithium-ion battery - Abstract:
- Over the last two decades, lithium-based batteries have revolutionized the energy storage technologies. Li-ion batteries have found widespread use in portable electronics and electric vehicle applications. However, a detailed understanding of the battery chemistry, especially the formation of a solid electrolyte interphase (SEI)—a thin passivation layer which is generated during the first charge cycle due to the reduction of electrolytes—is still elusive. The mass scale commercialization of electric vehicles requires the storage capacity beyond the conventional Li-ion batteries, which spurred research interests towards Li-S technologies. Li-S batteries are attractive for their very high capacity and energy density, but their commercial application has been thwarted due to several critical limitations stemming from electrolyte dissociation chemistry and electrode material properties. To investigate the current issues associated with the Li-ion and Li-S batteries and to find possible countermeasures, we used both a newly developed computational tool eReaxFF and the standard ReaxFF reactive molecular dynamics simulations in the following research areas: 1) We developed a computational method, eReaxFF, for simulating explicit electrons within the framework of the standard ReaxFF reactive force field method. We treat electrons explicitly in a pseudoclassical manner that enables simulation several orders of magnitude faster than quantum chemistry (QC) methods, while retaining the ReaxFF transferability. We describe in this thesis the fundamental concepts of the eReaxFF method, and the integration of the Atom-condensed Kohn-Sham DFT approximated to second order (ACKS2) charge calculation scheme into the eReaxFF. We trained our force field to capture electron affinities (EA) of various species. As a proof-of-principle, we performed a set of molecular dynamics (MD) simulations with an explicit electron model for representative hydrocarbon radicals. We establish a good qualitative agreement of EAs of various species with experimental data, and MD simulations with eReaxFF agree well with the corresponding Ehrenfest dynamics simulations. The standard ReaxFF parameters available in literature are transferrable to the eReaxFF method. The computationally economic eReaxFF method will be a useful tool for studying large-scale chemical and physical systems with explicit electrons as an alternative to computationally demanding QC methods. 2) A detailed understanding of the mechanism of the formation of SEI is crucial for designing high capacity and longer lifecycle lithium-ion batteries. The anode side SEI is primarily comprised of the reductive dissociation products of the electrolyte molecules. Any accurate computational method to study the reductive decomposition mechanism of electrolyte molecules is required to possess an explicit electronic degree of freedom. In this study, we employed our newly developed eReaxFF method to investigate the major reduction reaction pathways of SEI formation with ethylene carbonate (EC) based electrolytes. In the eReaxFF method, a pseudo-classical treatment of electrons provides the capability to simulate explicit electrons in a complex reactive environment. Our eReaxFF predicted simulation results of the EC decomposition reactions are in good agreement with the quantum chemistry data available in literature. Our MD simulations capture the mechanism of the reduction of the EC molecule due to the electron transfer from lithium, ring opening of the EC to generate EC-/Li+ radicals, and subsequent radical termination reactions. Our results indicate that the eReaxFF method is a useful tool for large-scale simulations to describe redox reactions occurring at electrode-electrolyte interfaces where quantum chemistry based methods are not viable due to their high computational requirement. 3) Li-S batteries still suffer several formidable performance degradation issues that impede their commercial applications. The lithium negative electrode yields high anodic capacity, but it causes dendrite formation and raises safety concerns. Furthermore, the high reactivity of lithium is accountable for electrolyte decomposition. To investigate these issues and possible countermeasures, we used ReaxFF reactive molecular dynamics simulations to elucidate anode-electrolyte interfacial chemistry and utilized an ex-situ anode surface treatment with Teflon coating. In this study, we employed Li/SWCNT (single-wall carbon nanotube) composite anode instead of lithium metal and tetra (ethylene glycol) dimethyl ether (TEGDME) as electrolyte. We find that at a lithium rich environment of the anode-electrolyte interface, electrolyte dissociates and generates ethylene gas as a major reaction product, while utilization of Teflon layer suppresses the lithium reactivity and reduces electrolyte decomposition. Lithium discharge from the negative electrode is an exothermic event that creates local hot spots at the interfacial region and expedites electrolyte dissociation reaction kinetics. Usage of Teflon dampens initial heat flow and effectively reduces lithium reactivity with the electrolyte. 4) Sulfur cathodes of Li-S batteries undergo a noticeable volume variation upon cycling, which induces stress. In spite of intensive investigation of the electrochemical behavior of the lithiated sulfur compounds, their mechanical properties are not very well understood. In order to fill this gap, we developed a ReaxFF interatomic potential to describe Li-S interactions and performed MD simulations to study the structural, mechanical, and kinetic behavior of the amorphous lithiated sulfur (a-LixS) compounds. We examined the effect of lithiation on material properties such as ultimate strength, yield strength, and Young’s modulus. Our results suggest that with increasing lithium content, the strength of lithiated sulfur compounds improves, although this increment is not linear with the lithiation. The dependence of the mechanical properties and failure behavior on the loading rate of the amorphous lithiated sulfur compositions was also studied. The diffusion coefficients of both lithium and sulfur were computed for the a-LixS system at various stages of Li-loading. A Grand canonical Monte Carlo (GCMC) scheme was used to calculate the open circuit voltage (OCV) profile during cell discharge. The calculated OCV is consistent with prior experimental results. Our ReaxFF potentials also reproduced experimentally observed volume expansion of a-LixS phases upon lithiation. The Li-S binary phase diagram was constructed using genetic algorithm based tools. These simulation results provide insight into the behavior of sulfur-based cathode materials that are needed for developing high-performance lithium-sulfur batteries.