Reaxff Reactive Force Field Modeling Of High Capacity Electrodes In Lithium Ion Batteries And Two Dimensional Materials

1. Development and application of ReaxFF for Energy Storage Materials Energy storage as a key aspect of assimilating renewable energy sources in power grids made the development of high capacity batteries an important technical challenge. The proliferation of portable electronics, hybrid electric vehicles (HEVs), and large scale energy storage has drawn significant attention toward the development of new generation-Lithium ion batteries (LIBs). Currently, most of commercial LIBs use graphitic anodes due to its long cycle life, high conductivity, abundance, and relatively low cost. However, the low specific charge capacity "(375 mAh" "g" ^"-1" ")" of graphite caused researchers to dedicate extensive attempts to find better alternative anode materials. Among all potential anodes, Silicon (Si), which can host a large amount of Lithium (Li) - each silicon atom can host up to 4.4 lithium atoms - is the most promising candidate. Nevertheless, associated with its large charge capacity of" 4200 mAh" "g" ^"-1" , insertion of lithium into silicon causes a large volumetric expansion of up to" 300%" , resulting in mechanical degradation and fracture. On the other side of the battery, the cathode, which mainly controls the voltage of LIB, should be tailored so that it can react with Li in a reversible manner. Sulfur (S) has been identified as one of most promising cathode materials for its high theoretical capacity, five times higher than that of the LiCoO2/graphite system. However, since the capacity of sulfur and its intermediate lithium polysulfide products rapidly decay due to the dissolution of intermediate polysulfides into the electrolytes, the commercialization of Li/S has been hindered. In this thesis study, systematic Molecular Dynamics (MD) simulations have been performed using ReaxFF reactive potentials to study the atomistic mechanisms governing the chemo-mechanical degradation in high-energy density anode and cathode materials. Our modeling results have shed light on the electrochemical insertion process of Li into the new high-capacity electrodes and have provided fundamental guidance to the rational designs of the next generation high capacity electrode materials with enhanced capacity retention and durability. 2. Application of ReaxFF for tow-dimensional materialsWhile the first part of this thesis covers battery applications, the second part of this thesis is devoted to the application and development of ReaxFF to model two-dimensional (2D) materials. The discovery of graphene in 2004 was the moment of the birth of an emerging research realm of 2D materials. Due to its exceptional electronic, optoelectronic and chemo-mechanical properties, graphene became ushers in the field of nano-transistors, photovoltaics, light emitting devices, optical sensors and topological field effect transistors. However, the ever increasing demands of semiconductor industry to utilize novel materials with a wider band-gap and superior structural, thermal and electrical properties has stimulated extensive scientific efforts to develop and synthesis of new generation of graphene-liked 2D-materials. Of the various proposed materials, transition metal dichalcogenides (TMDCs), such as MoS2 and WS2 have been recognized as promising candidates. It is well established that mechanical strain and geometry changes could dramatically affect the band structure and in turn electronic properties of 2D materials and in turn affect the performance and workability of the nano-transistors made of these materials. In this study, we present the development of a new ReaxFF reactive potential which can accurately describe the thermodynamic and structural properties of MoS2 sheets. Extensive Density Functional Theory (DFT) simulations are carried out to produce the required data-set for optimization of the new empirical potential parameters. This force field is able to accurately predict the mechanical properties and elastic constants of single layer MoS2. The newly developed potential is also successfully applied to estimate the formation energies of various types of point-defects (5 different vacancy types) along with the vacancy migration barriers and transition of 2H (semiconducting) 1T (metallic) phases. The energetics of ripplocations, a recently observed defect in van der Waals layers, is examined and the interplay between these defects and sulfur (S) vacancies is studied. As strain-engineering of MoS2 sheets is as an effective way to manipulate the electronic and optical properties of this material, the new ReaxFF description can provide a comprehensive insight about the morphological changes under various conditions of loading and defects to further tuning the band-gap properties in these 2D-structures. Recent experimental advances however confirm the possibility of further tuning the electronic properties of MoS2 through the fabrication of single-layer heterostructures consisting of semiconducting (2H) and metallic (1T) MoS2 phases. However, despite of technological and scientific interests, there exist limited information concerning the mechanical properties of these systems. Consequently, this investigation aims to provide a general vision regarding the mechanical properties of all-MoS2 single-layer structures at room temperature. This goal was achieved by performing extensive classical molecular dynamics simulations using a recently developed RexFF forcefield. We first studied the chirality dependent mechanical properties of defect-free 2H and 1T phases. Our atomistic modeling results for pristine 2H MoS2 were found to agree fairly well with the experimental tests. We finally discussed the mechanical response of 2H/1T single layer heterostructures. Our atomistic results suggest all-MoS2 heterostructures as suitable candidates to reach a strong and flexible material with tunable electronic properties.