The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method. INTRODUCTIONAtomistic-scale computational techniques provide a powerful means for exploring, developing and optimizing promising properties of novel materials. Simulation methods based on quantum mechanics (QM) have grown in popularity over recent decades due to the development of user-friendly software packages making QM level calculations widely accessible. Such availability has proved particularly relevant to material design, where QM frequently serves as a theoretical guide and screening tool. Unfortunately, the computational cost inherent to QM level calculations severely limits simulation scales. This limitation often excludes QM methods from considering the dynamic evolution of a system, thus hampering our theoretical understanding of key factors affecting the overall behaviour of a material. To alleviate this issue, QM structure and energy data are used to train empirical force fields that require significantly fewer computational resources, thereby enabling simulations to better describe dynamic processes. Such empirical methods, including reactive force-field (ReaxFF), 1 trade accuracy for lower computational expense, making it possible to reach simulation scales that are orders of magnitude beyond what is tractable for QM.Atomistic force-field methods utilise empirically determined interatomic potentials to calculate system energy as a function of atomic positions. Classical approximations are well suited for nonreactive interactions, such as angle-strain represented by harmonic potentials, dispersion represented by van der Waals potentials and Coulombic interactions represented by various polarisation schemes. However, such descriptions are inadequate for modelling changes in atom connectivity (i.e., for modelling chemical reactions as bonds break and form). This motivates the

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 lithium-sulfur batteries.

We present a computational tool, 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 delineate here 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 the 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.

A detailed understanding of the mechanism of the formation of the solid electrolyte interphase (SEI) is crucial for designing high-capacity and longer-lifecycle lithium-ion batteries. The anode-side SEI primarily consists of the reductive dissociation products of the electrolyte molecules. Any accurate computational method for studying the reductive decomposition mechanism of electrolyte molecules is required to include 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, electrons are treated explicitly in a pseudoclassical manner. The method has the ability to simulate explicit electrons in a complex reactive environment. Our eReaxFF-predicted results for the EC decomposition reactions are in good agreement with the quantum chemistry data available in the literature. Our molecular dynamics (MD) simulations capture the mechanism of the reduction of the EC molecule due to electron transfer from lithium, ring opening of 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 because of their high computational requirements.

Room temperature sodium−sulfur (Na−S) batteries, because of their high theoretical energy density and low cost, are considered as a promising candidate for next-generation energy storage devices. However, the practical utilization of the Na−S batteries is greatly hindered by various deleterious factors such as dissolution of sodium polysulfides (Na 2 S n ) into the electrolyte commonly termed as "shuttle effect," sluggish decomposition of solid Na 2 S, and poor electronic conductivity of sulfur. To overcome the challenges, we introduced single-layer vanadium disulfide (VS 2 ) as an anchoring material (AM) to immobilize higher-order polysulfides from the dissolution and also to accelerate the otherwise sluggish kinetics of insoluble short-chain polysulfides. We employ density functional theory (DFT) calculations to elucidate the Na 2 S n interactions at the VS 2 interfaces. We show that the adsorption strengths of various Na 2 S n species on the VS 2 basal plane are adequate (1.21−4.3 eV) to suppress the shuttle effect, and the structure of Na 2 S n are maintained without any decomposition, which is necessary to mitigate capacity fading. The calculated projected density of states (PDOS) reveals that the metallic character of the pristine VS 2 is retained even after Na 2 S n adsorption. The calculated Gibbs free energy of each elementary sulfur reduction reaction indicates a significant decrement in the free energy barrier due to the catalytic activity of the VS 2 surface. Furthermore, VS 2 is found to be an excellent catalyst to significantly reduce the oxidative decomposition barrier of Na 2 S, which facilitates accelerated electrode kinetics and higher utilization of sulfur. Overall, VS 2 with strong adsorption behavior, enhanced electronic conductivity, and improved oxidative decomposition kinetics of polysulfides can be considered as an effective AM to prevent the shuttle effect and to improve the performance of Na−S batteries.