Silicon Nanowire Composite Anodes for Lithium-Ion Batteries

Wanli Xu and John C. FlakeCain Department of Chemical Engineering

Louisiana State University, Baton Rouge

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Comparison of the different battery technologies in terms of volumetric and gravimetric energy density.

Why Lithium-Ion Batteries ? Light-weight High energy storage capability High operating voltage

Lithium-Ion Rechargeable Batteries

Anode: xLi+ + 6C + xe- ↔ LixC6

Cathode: LiCoO2 ↔ xe- + Li(1-x)CoO2

Tarascon, J. M.; Armand, M., Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, (6861), 359-367.

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Graphite LiCoO2

Yoshio, M.; Tsumura, T.; Dimov, N., Electrochemical behaviors of silicon based anode material. Journal of Power Sources 2005, 146, (1-2), 10-14.

Target Anode Capacity: 1000-2000 mAhg-1

How to increase total capacity ?

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Silicon as Anode Material

Chan, C. K.; Hailin, P.; Gao, L.; McIlwrath, K.; Xiao Feng, Z.; Huggins, R. A.; Yi, C., High-performance lithium battery anodes using silicon nanowires. Nature Nanotechnology 2008, 3, (1), 31-35.

4200 mA·hg-1 for Li4.4Si phase (graphite 372 mA·hg-1 for LiC6 )

~300% silicon volumetric change causing anode pulverization and large capacity fade as cell cycles

VLS-grown silicon nanowire (SiNW) can avoid pulverization.

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Nanowire Anodes• Nanowires can accommodate the volume changes

on cycling.• Only need ~15% Nanowires to double the cell

capacity.• Can we develop a composite anode with graphite

and nanowires?– Conductive– Resist Mechanical Failure– Cost Effective

• We propose a new nanowire composite anode made from electroless etching of Silicon (no vacuum processing).

Ag+e- →Ag Si + 6F- → SiF6

2- + 4e-

o Facile fabricationo Low cost o Mass production

Cross-section and top view SEM imagines of SiNWs on substrate

STEP 1: SiNW Fabrication via Electroless Etching

7

Anode Preparation:• Separation• Centrifugation• Mixing• Drying

A)SiNWs as fabricated by electroless etching.B) Graphite anode as fabricated .C) Silicon nanowire composite anode as fabricated.

A

B C

STEP 2: SiNW Composite Anode Preparation

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• 3 electrodes• Reference/Counter electrode: Li foil • Electrolyte: 1 M LiPF6 in EC/DMC • ~C/10 from 0.01 to 1.5 V vs. Li/Li+

• Anode capacity is the combination of both graphite and silicon components.

• The loss of capacity is mainly attributed to the silicon loss.

charge/discharge capacity of SiNW composite anode for the 1st 2nd and 5th cycles

STEP 3: Anode Assembly and Testing

Li Li

LiPF6 in EC/DMC

A

V

Working(SiNW composite)

Reference Counter

SEM image of SiNW composite anode

SiNW composite anode surface as prepared SiNW composite anode after 1 st charging cycle

• SiNW swells in diameter• No pulverization or cracking • An SEI layer covers the anode surface after first cycle

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Charge/discharge capacities versus cycle number

~2x

SiNW composite anode capacity vs. graphite

811mAhg-1

512 mAhg-1

310 mAhg-1

Conclusions Silicon nanowire composite anodes for lithium ion

battery are presented.

Nanowire structure can accommodate large volume change of silicon during the charge/discharge cycles.

2× capacity increase with only 15% silicon nanowire addition.

Facile and low-cost preparation of anodes for lithium ion battery.

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Joel Nino BugayongSri Sai VeguntaPurnima Narayanan

SPONSORS:

Acknowledgements

Electrochemical behavior of the SiNW composite anode

• Anode capacity is the combination of both graphite and silicon components.

• The loss of capacity is mainly attributed to the silicon loss.

charge/discharge capacity of SiNW composite anode for the 1st 2nd and 5th cycles

Differential charge/discharge capacity of SiNW composite anode for the 1st 2nd and 5th cycles

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A)Top-view of SiNWs as fabricated by electroless etching. B) Top-view of SiNWs after 11 cycles at both 25µA/cm2 charging and discharging rate. C) Tiled view of SiNWs as fabricated by electroless etching. D) Tilted view of SiNWs after 100 cycles at 12C scanning rate.

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No binder or other special treatment was applied to prepare the sample. Charging and discharging rates are 25µAcm-2.

Free-standing Silicon Nanowire Anode