1. bosong@sinap.ac.cn 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 bosong@sinap.ac.cn 北大新材料论坛

2. bosong@sinap.ac.cn Outline  Motivations  Molecular model and methods  MD simulations and mechanism underlying  Conclusion and outlook

3. bosong@sinap.ac.cn Energy is the key challenge for human! Motivations

4. bosong@sinap.ac.cn  Energy!!! 1.Energy conversion? 2.Energy storage? Motivations

5. bosong@sinap.ac.cn  A high charge-and-discharge rate  A high capacity  A high cyclability Motivations Way to store energy with Century

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

7. bosong@sinap.ac.cn 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)

8. bosong@sinap.ac.cn 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.

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

10. bosong@sinap.ac.cn 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, 15.61 Å x 7.97 Å

11. bosong@sinap.ac.cn MD simulations  Conducting 10,000-fs simulation for each initial conformation MD results A typical animation Li atom would lose 0.8 electrons

12. bosong@sinap.ac.cn 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

13. bosong@sinap.ac.cn 8 Li ions  Axial and radial distributions of the 8 Li ions 2.0 Å 6.7 Å

14. bosong@sinap.ac.cn 16 Li ions  Results of 16 Li ions in CNTs bundle Uniformly, 2.0 Å 6.7 Å ???

15. bosong@sinap.ac.cnL  Snapshot of Li ions in CNTs bundle 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle B

16. bosong@sinap.ac.cn  Snapshot of Li ions in CNTs bundle 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle B

17. bosong@sinap.ac.cn  Distribution of Li ions in x-y plane 16 Li ions in CNTs bundle 8 Li ions in CNTs bundle

18. bosong@sinap.ac.cn  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)

19. bosong@sinap.ac.cn  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

20. bosong@sinap.ac.cn ▲ ●  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

21. bosong@sinap.ac.cn 16 Li ions in CNTs bundle Third channel  Snapshot of Li ions in CNTs bundle 8 Li ions in CNTs bundle Two channels

22. bosong@sinap.ac.cn ▲ ●  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???

23. bosong@sinap.ac.cn Cation-pi interaction  Physics underlying: Cation-pi interaction Li + Cation-induced re-arrangement of pi-electrons E ad = -1.67 eV Cation-pi interaction

24. bosong@sinap.ac.cn  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 = -1.67 eV

25. bosong@sinap.ac.cn - ○ -: B position 32 1 B  Cation  2pi for the interstitial channel between NTs ● Li +

26. bosong@sinap.ac.cn 32 1 -∆- : A position A  Cation  3pi for the interstitial channel between NTs ▲ Li +

27. bosong@sinap.ac.cn 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

28. bosong@sinap.ac.cn 32 1  Cation  n pi interaction for the interstitial channel between NTs ▲ Li + ● ???

29. bosong@sinap.ac.cn  Cation  3 pi interaction V.S. Cation  2pi interaction J. Am. Chem. Soc. 134, 12104 (2012) Cation  n pi interaction

30. bosong@sinap.ac.cn  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

31. bosong@sinap.ac.cn Acknowledgement  Prof. David Tomanek  Profs. Haiping Fang, Jijun Zhao  My students

32. bosong@sinap.ac.cn Foundation

33. thank you for your attention!