Hard Carbon Ethylene Carbonate Interfaces – Atomic Scale Insight into Surface Termination, Defects, Functional Groups, and Roughness on Electrolyte Breakdown, Metal Adsorption, Intercalation and Nucleation
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Hard Carbon Ethylene Carbonate Interfaces – Atomic Scale Insight into Surface Termination, Defects, Functional Groups, and Roughness on Electrolyte Breakdown, Metal Adsorption, Intercalation and Nucleation
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<jats:p> In the search for post lithium ion batteries (LIBs), sodium ion batteries (NIBs) are gaining traction, but fundamental understanding of the atomic scale interactions at the anode electrolyte interfaces in NIBs remains poorly understood. In moving from LIBs, the LIB anode material graphite was found to be unsuitable for NIBs. Instead, hard carbon anode materials have arisen as one of the most promising electrode materials for sodium ion batteries (NIBs), which are also suitable for use with carbonate based electrolyte solvents. Hard carbons are complex amorphous carbon structures with randomly orientated, defective, and curved graphene nanosheets, and turbostratically stacked graphitic layers. This complex structure leads to a plethora of carbon structural motifs being present in the anode, leading to a large number of potential morphologies having to be taken into account when investigating the anode electrolyte interface.</jats:p>
<jats:p>In this study, we use ab initio molecular dynamics to investigate the effect of surface roughness, disorder, defects, and termination on the electrolyte anode interactions in the EC (electrolyte solvent) | hard carbon (anode) system. These simulations show that EC intercalation and breakdown is directly influenced by the carbon surface termination and surface roughness. Furthermore, defects were found to be more probable to form in terms of defect formation energies at strained and curved carbon morphologies. These defects were also found to have a direct impact on the sodium binding to the anode. The effect of all these different structural motifs on sodium adsorption, intercalation, and diffusion were simulated to form an understanding of the anode electrolyte interfacial reactions. One consequence of electrolyte molecule breakdown, but could also be an effect of anode synthesis method, is the presence of functional groups at the carbon surface. From density functional theory simulations of functional groups O, OH, NH<jats:sub>2</jats:sub>, and COOH at different carbon motifs (planar basal plane, H-terminated edges, and curved morphologies) we could show that O-functional groups from an energetical perspective are highly probable to form on these surfaces, and can act as favourable sodium storage sites. The hydroxyl group on the other hand were found to form inorganic NaOH compounds, which could contribute to irreversible capacity.</jats:p>
<jats:p>Acknowledgment</jats:p>
<jats:p>The financial support from EPSRC (Engineering and Physical Sciences Council) under the grant number EP/M027066/1, and EP/R021554/2, is acknowledged. </jats:p>