Reaxff And Ereaxff Reactive Force Field Development And Applications To Energy Storage Interfaces

The depletion of fossil fuels necessitates alternate and clean energy sources. Lithium-ion batteries and solid oxide electrocatalysis devices are some of the most popular candidates. However, further improvements of these energy storage devices are essential in order to meet the ever-increasing global energy demand. Improvement of the performance of these high energy chemical systems is directly linked to the understanding and improving the complex physical and chemical phenomena and exchanges that take place at their different interfaces. Surfaces or interfaces, structures created between dissimilar media, such as liquids and solids, and interphases, structures arising in between these dissimilar media, present great challenges for their study and understanding since these are the regions where myriad events such as electron transfer, ion transfer and migration, reactions, and solvation/desolvation processes take place and significantly alter their landscape. In order to investigate the physical and chemical interactions at the interfaces of energy storage devices such as Li-ion batteries and solid oxide electrocatalysis devices, we used ReaxFF and eReaxFF reactive molecular dynamics simulations in the following research areas: 1) In the electrode/electrolyte interface of a typical lithium-ion battery a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from anode to the electrolyte reduces the Li+-ion and makes them more reactive resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended existing ReaxFF reactive force field parameter sets to organic electrolyte species such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate and LiPF6 salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to existing ReaxFF training data and subsequently, we trained the ReaxFF parameters with the aim to find the optimal reproduction of the DFT data. In order to discern the characteristics of Li neutral and cation, we have introduced a second Li parameter set to describe Li+-ion. ReaxFF is trained for Li-neutral and Li+-cation to have similar solvation energies but unlike the neutral Li, Li+ will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li+-ions are reduced to Li-atoms, which can be simulated using a Monte-Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li+-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of EC molecules solvating the Li-atom. 2) Graphene, a 2D material arranged in an sp2-bonded hexagonal network, is one of the most promising materials for lithium-ion battery anodes due to its superior electronic conductivity, high surface area for lithium intercalation, fast ionic diffusivity and enhanced specific capacity. A detailed atomistic modeling of electronic conduction and non-zero voltage simulations of graphitic materials require the inclusion of an explicit electronic degree of freedom. To enable large length and time scale simulations of electron conduction in graphitic anodes, we developed an eReaxFF force field describing graphitic materials with an explicit electron concept. The newly developed force field, verified against quantum chemistry-based data describing, amongst others, electron affinities and equation of states, reasonably reproduces the behavior of electron conductivity in pristine and imperfect graphitic materials at different applied temperatures and voltages. Our eReaxFF description is capable of simulating leakage of excess electrons from graphene which are captured by exposed lithium ions; a common behavior at the anode/electrolyte interface of a lithium-ion battery. Finally, the initiation of Li-metal-plating observed at the graphene surface reveals the eReaxFF force field's potential for the future development of Li-graphene interactions with explicit electrons. 3) Electrocatalysis results in the change of the rate of an electrochemical reaction occurring on an electrode surface by varying the electrical potential. Electrocatalysis can be used in hydrogen generation and the generated hydrogen can be stored for future use in fuel cells for clean electricity. The use of solid oxide in electrocatalysis specially in hydrogen evolution reaction is promising. To enable large length and time scale atomistic simulations of solid oxide electrocatalysis for hydrogen generation, we developed an eReaxFF force field for barium zirconate doped with 20 mol% of yttrium (BZY20). All parameters for the eReaxFF were optimized to reproduce quantum mechanical (QM) calculations on relevant condensed phase and cluster systems describing oxygen vacancies, vacancy migrations, water adsorption, water splitting and hydrogen generation on the surfaces of the BZY20 solid oxide. Using the developed force field, we performed zero-voltage molecular dynamics simulations to observe water adsorption and the eventual hydrogen production. Based on our simulation results, we conclude that this force field sets a stage for the introduction of explicit electron concept in order to simulate electron conductivity and non-zero voltage effects on hydrogen generation. Overall, the work described in this dissertation demonstrate how atomistic-scale simulations can enhance our understanding of processes at interfaces in energy storage materials.