Curvature drastically changes diffusion properties of Li and Na on graphene
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esearch Letters
Curvature drastically changes diffusion properties of Li and Na on graphene Yang Wei Koh, Bioinformatics Institute, 30 Biopolis Street, #07-10 Matrix, Singapore 138671, Singapore Sergei Manzhos, Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Block EA #07-08, 117576 Singapore, Singapore Address all correspondence to Sergei Manzhos at [email protected] (Received 16 April 2013; accepted 18 June 2013)
Abstract We present a comparative ab initio study of surface diffusion of Li and Na on planar and curved graphene. The barrier for diffusion is ~0.1 eV lower for Na than for Li, and is changed significantly by curvature. The maximum change is similar for Li for Na, of the order of ±0.1 eV on the convex and concave sides. The difference in barrier for metal atoms adsorbed on the concave and convex sides can reach 0.2 eV. This modulation of the diffusion barrier by curvature is therefore expected to affect significantly the rate capability of graphene-based anodes.
Introduction Metal-ion batteries are today the technology of choice for mobile applications, with Li ion batteries providing the highest energy densities among commercial batteries.[1] Even higher specific capacity and higher rate batteries are required to achieve, respectively, all-electric vehicles and economical large-scale storage required for integrating clean yet intermittent sources of electricity such as solar or wind.[2] Specifically, for large-scale storage, sodium-ion batteries are a promising technology, as Na is cheaper and more abundant than Li.[3] Diffusion properties of metal ions in electrode materials are a key determinant of achievable cycling rates. Graphene has emerged as a viable anode material for electrochemical batteries, providing a high electric conductivity and decent Li diffusivity, with the diffusion barrier of about 0.3 eV.[4–6] Graphene can be used as a storage medium for ions[7] and as a conducting binder for graphene–nanoparticle composites providing high connectivity between particles of high-specific capacity materials even as they undergo significant expansion and contraction on cycling.[8,9] Surface diffusion of metal atoms/ions on graphene, together with insertion into and bulk diffusion in the nanoparticles, determines the overall rate capability of the anode. In these applications, graphene is significantly curved.[4,8,9] Graphene is intrinsically curved,[10] and graphene used as an anode has crumpled paper morphology with kinks on the scale of a few nm.[11,12] Curvature is known to result in changes in the electronic properties of graphene. For instance, significant effects on graphene– H2 interactions and on the phonon spectrum have been reported.[13,14] It is expected to affect graphene–Li, Na interactions, but to what extent is unknown. Li storage in a related system, carbon nanotubes (CNT), has been extensively studied, and
the effect of curvature on adsorption and diffusion was identified.[15,16] To the best of our knowledge, there are no calculations of the
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