Development And Application Of A Reaxff Reactive Force Field For Solid Electrolyte Interphase Study In Silicon Based Li Ion Batteries

The amount of energy required to satisfy the demands of the human population may not be able to be supplied solely by fossil fuels as the world population continues to rise and the supply of fossil fuels starts to decline. Lithium-ion batteries (LIBs), since being suggested as commercialized energy storage systems by Sony company in the 1990s, have been considered an important device in alternative energy solutions owing to their high energy density, wide working voltage, long cycling life, and low self-discharge rate. 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, which means structures built between dissimilar media such as liquids and solids, and interphases, which means structures formed within these dissimilar media, present significant 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 occur and significantly alter their configuration. A detailed understanding of 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. It is well known that the SEI has a strong influence on the battery performance characteristics, such as irreversible capacity, safety, and cycle life, when the SEI is a thin layer between the liquid electrolytes and anode surfaces formed by the electrochemical reductive decomposition reaction of the electrolyte during the initial few cycles. A stable SEI with full surface coverage over the electrode is important for achieving optimal electrochemical performances of Li-ion batteries. Understanding the SEI is quite challenging due to its complicated and amorphous structure. In order to investigate the physical and chemical interactions at the interfaces of energy storage devices such as Li-ion batteries a, we used ReaxFF reactive molecular dynamics simulations in the following two research areas: 1. In the last decade silicon has attracted significant attention as a potential next generation anode material for Li-ion batteries (LIBs) due to its high theoretical specific capacity (3579 mAh/g (Li15Si4)) compared to that of the commonly used graphite (372 mAh/g). However, despite the apparent attractiveness of Si in view of its application in LIBs, it is known to suffer from severe degradation problems which lead to performance losses of Si-based anodes, and the electrochemical outcome of the degradation is well documented in the literature: rapid capacity fading of the anode accompanied by the increased internal resistance of the cell. Full utilization of silicon's potential as an anode material is thus prevented by incomplete understanding of the degradation mechanisms and the resulting inability to implement effective mitigation tactics. To study the Si anode degradation at the anode/electrolyte interface, we have developed a ReaxFF reactive force field simulation protocol. In this protocol, a delithiation algorithm is employed. This novel systematic delithiation algorithm helps to capture the effect of different delithiation rates, which plays an important role in the irreversible structural change of delithiated Si. Besides, the fundamental of degradation was investigated by analyzing the relationship between the depth of discharge and corresponding volume and structural changes at different rates. 2. The SEI (solid-electrolyte interphase) is important for protecting silicon anodes in batteries from losing both silicon and electrolyte through side reactions. A major issue with this technology is SEI breakdown caused by cracking in silicon particles. A strategy is presented for creating a self-sealing SEI that automatically covers and protects the cracked surface of silicon microparticle anodes by bonding an ion pair to the silicon surface. The cations in the bond prevent silicon-electrolyte reactions while the anions migrate to the cracked surface and decompose more easily than the electrolyte. The SEI formed in this way has a double layer structure with a high concentration of lithium fluoride in the inner layer. To study the electrode electrolyte reactions at the anode/electrolyte interface, we have developed ReaxFF reactive force field parameter sets to organic electrolyte species such as ethylene carbonate, N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI), LiPF6 salt and lithium-Silicon oxygen electrode. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and bonding energies of Li-electrolyte structures were generated and added to ReaxFF training data and subsequently, we trained the ReaxFF parameters with the aim to find the optimal reproduction of the DFT data. This force field is capable of distinguishing Li interaction with electrolyte in presence of self-sealing layer. Moreover, these findings provide a new way to design a stable SEI at the highly dynamic electrode-electrolyte interface. In addition to this battery work, we also studied halogen interaction with platinum surfaces, a system that has received considerable attention in catalysis and semiconductor mannufacturing. These halogen gases are applied as platinum mobilizers in both deposition and corrosion or etching processes since PtCl4 adsorption from an electrolyte and subsequent reduction to metallic Pt clusters is an electrochemical pathway for obtaining highly dispersed, catalytically active Pt surfaces. A novel ReaxFF reactive force field has been developed to understand the size and shape-dependent properties of platinum nanoparticles for the design of nanoparticle-based applications. The ReaxFF force field parameters are fitted against a quantum mechanical (QM) training set containing the adsorption energy of Cl and dissociative HCl on Pt (100) and Pt (111), the energy-volume relations of PtCl2 crystals, and Cl diffusion on Pt (100) and Pt (111). ReaxFF accurately reproduces the QM training set for structures and energetics of small clusters and PtClx crystals. The predictive capacity of the force field was manifested in molecular dynamics simulations of the Cl2 and HCl molecules interactions on the (100) and)111(surfaces of c-Pt crystalline solid slabs. The etching ratio between HCl and Cl2 are compared to experimental results, and satisfactory results are obtained, indicating that this ReaxFF protocol provides a useful tool for studying the atomistic-scale details of the etching process.