Bo SONG Shanghai Institute of Applied Physics, CAS Dec., 2012 Intercalation and diffusion of lithium ions in a CNT bundle by ab initio molecular dynamics simulations 北大新材料论坛

Outline  Motivations  Molecular model and methods  MD simulations and mechanism underlying  Conclusion and outlook

Energy is the key challenge for human! Motivations

 Energy!!! 1.Energy conversion? 2.Energy storage? Motivations

 A high charge-and-discharge rate  A high capacity  A high cyclability Motivations Way to store energy with Century

Motivations Nanotechnology, in the past two decades, provides a novel approach to improve energy storage device Energy Environ. Sci 2, 589 (2009)

Motivations Li battery with nanotube-based anode is believed to be one of the most promising electrochemical energy storage systems Li-ions in nanotubes Nano Lett. 9, 3844 (2009)

Motivations  Providing the visual of lithium intercalation and diffusion in the battery.  Understanding mechanism under the intercalation and diffusion.  Greatly promote the development and application of the Li battery.

Intercalation and diffusion of lithium ions in a CNT bundle by ab initio molecular dynamics simulations Energy Environ. Sci. 4, 1379 (2011)

Model and methods  Purple: lithium  Black: carbon  White: hydrogen Midpoint A among 3 CNTs Midpoint B between 2 CNTs Molecular model by AIMD (10, 0) zigzag, Å x 7.97 Å

MD simulations  Conducting 10,000-fs simulation for each initial conformation MD results A typical animation Li atom would lose 0.8 electrons

the interior channel of the CNT the interstitial channel between CNTs  All Li ions moving into the interior channel of the CNT or into the interstitial channel between CNTs

8 Li ions  Axial and radial distributions of the 8 Li ions 2.0 Å 6.7 Å

16 Li ions  Results of 16 Li ions in CNTs bundle Uniformly, 2.0 Å 6.7 Å ???

 Snapshot of Li ions in CNTs bundle 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle B

 Snapshot of Li ions in CNTs bundle 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle B

 Distribution of Li ions in x-y plane 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle

 Adsorption energy of one lithium atom with the nanotubes Mechanism underlying  Putting one Li atom in the interior or interstitial channels.  Moving it along the Z dirction. r Z E ad = E(Li + CNTs) – E(Li) – E(CNTs)  E ad = E(Li + CNTs) – E(Li) – E(CNTs)

 Adsorption energy of a lithium atom with the nanotubes - ■ -: r = 0.0 Å - ● -: r = 1.0 Å - ▲ -: r = 2.0 Å Z r Interior channel of the nanotube

▲ ●  Adsorption energy of a lithium atom with the nanotubes - ∆ - : A position - ○ -: B position - ■ -: r = 0.0 Å - ● -: r = 1.0 Å - ▲ -: r = 2.0 Å Interstitial channel between the nanotubes 32 1 A B

16 Li ions in CNTs bundle Third channel  Snapshot of Li ions in CNTs bundle 8 Li ions in CNTs bundle Two channels

▲ ●  Adsorption energy of a lithium atom with the nanotubes - ∆ - : A position - ○ -: B position - ■ -: r = 0.0 Å - ● -: r = 1.0 Å - ▲ -: r = 2.0 Å Interstitial channel between the nanotubes 32 1 A B A little large???

Cation-pi interaction  Physics underlying: Cation-pi interaction Li + Cation-induced re-arrangement of pi-electrons E ad = eV Cation-pi interaction

 Cation-pi interaction for the interior channel of the nanotube - ■ -: r = 0.0 Å - ● -: r = 1.0 Å - ▲ -: r = 2.0 Å Z r Li + E ad = eV

- ○ -: B position 32 1 B  Cation  2pi for the interstitial channel between NTs ● Li +

∆- : A position A  Cation  3pi for the interstitial channel between NTs ▲ Li +

16 Li ions in CNTs bundle Third channel  Snapshot of Li ions in CNTs bundle by Cation  n pi 8 Li ions in CNTs bundle Two channels

 Cation  n pi interaction for the interstitial channel between NTs ▲ Li + ● ???

 Cation  3 pi interaction V.S. Cation  2pi interaction J. Am. Chem. Soc. 134, (2012) Cation  n pi interaction

 Li ions intercalation and diffusion in a CNT bundle.  Interior and interstitial channels opened for Li ions.  For CNT with a small diameter, the adsorption energy at the site among three nanotubes is much higher than that in the interior channels.  Li ions located among three neighboring NTs would be very difficult to be removed from a bundle of nanotubes.  Irreversible storage capacity in a NT-based Li battery.  Keeping the nanotubes apart with an appropriate distance would hinder or promote the formation of irreversible intercalation and storage capacity. Conclusion Control the irreversible intercalation

Acknowledgement  Prof. David Tomanek  Profs. Haiping Fang, Jijun Zhao  My students

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