Insight into Sodium Storage Behaviors in Hard Carbon by ReaxFF Molecular Dynamics Simulation

Abstract

The uncertainty in the Na-storage behaviors has seriously prevented the optimization of hard carbon electrodes, while molecular simulations can provide some unique research perspectives. In this work, a large-scale molecular model with over 65,000 carbon atoms was constructed for commercial hard carbon by combining experimental characterization and theoretical modeling, which well reproduces some key structural features. Based on such an experimentally determined hard carbon model, reactive molecular dynamics simulations were performed to obtain more detailed insights into the nature of Na storage at the atomic scale. It is found that the computational findings are basically in agreement with the electrochemical results. The final structure–capacity–potential relationship not only verifies the controversial “card-house” theory but also unveils a new mechanism involving small sodium clusters on defective sites in the high-potential sloping region. On this basis, it yielded a clear picture of the Na-storage behaviors in hard carbon, which consists of sodium adsorption, intercalation, and pore filling. Besides, several design strategies were proposed to maximize the capacity of hard carbon. Such fundamental research based on ReaxFF simulation can open new studying routes for elucidation of the Na-storage mechanism and may pave the way for designing high-performance hard-carbon-based anodes.

Review: Insights on Hard Carbon Materials for Sodium‐Ion Batteries (SIBs): Synthesis – Properties – Performance Relationships

“…[130] Hard carbon materials have a non-graphitizable and disordered structure, which includes various structural defects, amorphous regions, pseudo-graphitic domains (short turbostratic stacked graphene layers), and nanopores (Figure 8b). The HC structure is very complex, and several 2D [23,156] and 3D [157][158][159][160] models have been reported in the literature to describe it, as nicely detailed in the review of Passerini et al [130] Figures 8c and d show a representation of these HC structures, as proposed by the authors of the present work. Stevens and Dahn's [23] model is the most accepted one, although improvements are still required.…”

New Template Synthesis of Anomalously Large Capacity Hard Carbon for Na‐ and K‐Ion Batteries

“…[6] Although the detailed mechanism of Na insertion into HC is still under debate, recent studies, including various characterizations, and theoretical calculations of Na-inserted HC, have suggested the following sequence of three steps by sweeping from higher to lower potentials: 1) Na adsorption at various defect sites, 2) Na intercalation into the interlayers of pseudo-graphitic domains, and 3) the formation of quasi-metallic Na clusters within internal pores. In the final step, Na cluster formation in nanopores by applying a lower potential has been established by multiple groups through computational studies, [18][19][20] and experimental characterizations, including Xray scattering methods, [3,[21][22][23][24] solid state Na-NMR for chemical state analysis, [24][25][26] and other analyses, such as spectroscopy [27] and dilatometry. [7] Based on these findings, the most intuitive approach for enhancing HC capacity is to design a pore structure optimized for Na storage.…”

Molecular Structure Evaluation and Image-Guided Atomistic Representation of Hard Carbon Electrodes

“…g −1 and 6.67 m 2 g −1 , respectively. Previous studies 38,39 demonstrated that increases of measurable porosity are very strongly associated with lower reversible capacities. Therefore, it is crucial to control pore distribution of subsequent molecular models based on experimental data.…”