This paper reveals the existence of a critical separation distance (dc) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than dc, the lattice behaves mechanically as if there is a single defect. For a distance less than dc, the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation (θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of “interacting monovacancies (iMVs)” are either higher or smaller than that of the lattice containing a pair of “non-interacting monovacancies (nMVs),” suggesting the existence of a critical orientation angle θc. For θ<θc, the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ≥θc, the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius.
Core-shell and core-gradient hybrid cathode materials for lithium-ion batteries display enhanced rate capability over their homogeneous counterparts. The apparent enhancement of transport is shown to arise from advective flow of Li+ from the higher free-energy core towards the lower free-energy shell compositions. First-principles analysis of a planar model of these hybrid structures concludes that the inbuilt free-energy gradient enhances the Li+ de-intercalation process by reducing the average overpotential during extreme fast-charging. Analysis of representative LiNi0.8Co0.1Mn0.1O2||LiNi0.4Co0.2Mn0.4O2 core/shell reveals: (i) an optimal components ratio exists that maximizes storage capacity during fast-charging and (ii) components should be selected with appreciably large chemical potential difference between the core and shell to further exploit the free-energy gradient effects provided volume ratios are optimized against the potential gradient. In the case of NCM811||NCM424 studied herein, a balanced (ca. 40/60 vol.%) structure appears optimal. This finding indicates that the shell must not necessarily be confined to a thin chemically-protective coating; higher relative volumes of the lower free-energy shell may provide performance benefits at high-rates. The presented insights will serve towards optimizing and developing high capacity, more rate capable core-shell particles for extreme fast charging batteries. Figure 1
Core–shell and core-gradient hybrid cathode materials for lithium-ion batteries display enhanced rate capability over their homogeneous counterparts. The apparent enhancement of transport is explained herein as resulting from advective flow of Li+ from the higher free-energy core towards the lower free-energy shell compositions. First-principles analysis of a planar model of these hybrid structures concludes that the inbuilt free-energy gradient enhances the Li+ de-intercalation process by reducing the average overpotential during extreme fast-charging. Analysis of representative LiNi0.8Co0.1Mn0.1O2∣∣LiNi0.4Co0.2Mn0.4O2 core/shell reveals: (i) an optimal components ratio exists that maximizes storage capacity during fast-charging and (ii) components should be selected with appreciably large chemical potential difference between the core and shell to further exploit the free-energy gradient effects provided volume ratios are optimized against the potential gradient. In the case of NCM811∣∣NCM424 studied herein, a balanced (ca. 40/60 vol.%) structure appears optimal. This finding indicates that the shell must not necessarily be confined to a thin chemically-protective coating; higher relative volumes of the lower free-energy shell may provide performance benefits at high-rates. The presented insights will serve towards optimizing and developing high capacity, more rate capable core–shell particles for extreme fast charging batteries.
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