• EC-LiPF6 system
  • Lithiated sulfur cathode
  • Anodic discharge of Li-S battery

 

Electrolyte chemistry in Li-S batteries

Lithium-sulfur batteries are amongst the most appealing choices for the
next generation large-scale energy storage applications. However, these
batteries still suffer several formidable performance degradation issues
that impede its commercialization. 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 lithium rich environment at 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

Mechanical properties of lihitated sulfur cathode

Sulfur is a very promising cathode material for rechargeable energy storage devices. However, sulfur cathodes undergo a noticeable volume variation upon cycling, which induces mechanical 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 molecular dynamics (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 lithiation. 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 profile during cell discharge. The Li–S binary phase diagram was constructed using genetic algorithm based tools. Overall, these simulation results provide insight into the behavior of sulfur based cathode materials that are needed for developing Li-S batteries.

Interaction of Hydrogen with Iron and Iron Carbide Interfaces

Hydrogen embrittlement (HE) is a well-known material phenomenon that causes significant loss in the mechanical strength of structural iron and often leads to catastrophic failures.  In order to provide a detailed atomistic description of HE we have used a reactive bond order potential to describe adequately the diffusion of hydrogen as well as its chemical interaction with other hydrogen atoms, defects, and the host metal. The currently published ReaxFF force field for Fe/C/H systems was originally developed to describe Fischer-Tropsch (FT) catalysis and had been especially trained for surface formation energies, binding energies of small hydrocarbon radicals on different surfaces of iron and the barrier heights of surface reactions.  We merged this force field with the latest ReaxFF carbon parameters and used the same training data set to refit the Fe/C interaction parameters. The present work is focused on evaluating the applicability of this reactive force field to describe material characteristics, and to study the role of defects and impurities in the bulk and at the precipitator interfaces. We study the interactions of hydrogen with pure and defective α-iron (ferrite), Fe3C (cementite), and ferrite-cementite interfaces with a vacancy cluster. We also investigate the growth of nano voids in α-iron using a grand canonical Monte Carlo (GCMC) scheme.  The calculated hydrogen diffusion coefficients for both ferrite and cementite phases predict a decrease in the work of separation with increasing hydrogen concentration at a ferrite-cementite interface, suggesting a hydrogen-induced decohesion behavior. Hydrogen accumulation at the interface was observed during molecular dynamics (MD) simulations, which is consistent with experimental findings. These results demonstrate the ability of the ReaxFF potential to elucidate various aspects of hydrogen embrittlement in α-iron and hydrogen interaction at a more complex metal/metal carbide interface.

Explicit electron in ReaxFF (eReaxFF)

The treatment of explicit charge and polarization is essential for force-field descriptions applied to systems such as rechargeable battery interfaces and ferro/piezo-electric materials. Essentially, such descriptions require a classical treatment for an explicit electron. Recently, potentials including some form of explicit electron description have been introduced, such as the electron force-field (eFF and the LEWIS force-field. Nonetheless, these methods have not yet been demonstrated to accurately simulate complex materials and intricate chemistries. To extend the ReaxFF description to include chemistry dependent on electron diffusion, we have introduced explicit electron-like and hole-like particles that carry negative (-1) and positive (+1) charges, respectively. The electron and hole particles interact with atomic centers through a single Gaussian function. We implemented charge-valence coupling, which allows the electron or hole particle to modify the number of valence electrons in a host atom, thus ensuring the appropriate change in valence when calculating the degree of over or under coordination in the host atom. Interfacial chemistry at the electrode-electrolyte interfaces are crucial for Li-based rechargeable battery performance. In typical Lithium ion batteries (LIBs) with the most prevalent carbonate type electrolytes, anode-electrolyte interface undergoes reduction reaction, while oxidation occurs at the cathode-electrolyte interface. Decomposition of the electrolytes during battery cycling creates a thin passivation layer, known as solid electrolyte interphase (SEI). This interfacial chemistry involves explicit electron flow and redox reactions. In this study, we employed the newly developed electron version of ReaxFF (eReaxFF) for training our force field to capture electron affinities of various species. We are studying electron dynamics on different molecules and validating our findings against Ehrenfest dynamics results.

 

Mechanical properties of Stanene

Stanene, a graphene like two dimensional honeycomb structure of tin has attractive features in electronics application. In this study, we performed molecular dynamics simulations using modified embedded atom method potential to investigate mechanical properties of stanene. We studied the effect of temperature and strain rate on mechanical properties of α-stanene for both uniaxial and biaxial loading conditions. Our study suggests that with the increasing temperature, both the fracture strength and strain of the stanene decrease. Uniaxial loading in zigzag direction shows higher fracture strength and strain compared to the armchair direction, while no noticeable variation in the mechanical properties is observed for biaxial loading. We also found at a higher loading rate, material exhibits higher fracture strength and strain. These results will aid further investigation of stanene as a potential nano-electronics substitute.