Silicon based Nano-Scoops for High-Power Lithium-Ion Battery Anodes

Si

Al

C

10%

94%280%

C

AlSi

Before Lithiation

After Lithiation

10%

94%280%

C

AlSi

10%

94%280%

10%

94%280%

C

AlSi

Before Lithiation

After Lithiation

Rahul Mukherjee(Graduate Student)

Department of Mechanical,Aerospace and Nuclear Engineering

Rensselaer Polytechnic Institute110 8th Street, Troy,

New York, USA.

Research Objective• Need for a power source

that can provide high power density and high energy density

• Research aim: To design electrode nano-architectures that can retain mechanical integrity over hunderds of cycles while providing a high capacity even when cycled at very high C-rates (> 20C)

Source: M. Winter, R. J. Brodd, Chem. Rev. 2004, 104, 4245.

Li-ion Cell Structure

Cathode: It is the source of Lithium.Ex: LiCoO2, LiFePO4

Anode: It is a host material for insertion of Li+.Ex: Graphite, porous carbon

Electrolyte: It offers a medium for the transport of Li+.Ex: solution of lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, in an organic solvent such as alkene carbonates

Source: R. Teki, M. K. Datta, R. Krishnan, T. C. Parker, T. –M. Lu, P. N. Kumta, N. Koratkar, Small, 2009, 5, 2236.

Is there room for improvement ?• Charge / discharge capacity: It is a measure of the total charge per unit

weight stored or recovered from the electrode material. The standard units for specific capacity are (mAh/g). The specific capacity is also a measure of the “Energy Density” of the battery.

• C-rate: A rate of nC corresponds to a full discharge in 1/n hours. This parameter monitors rate of charge/discharge as well as magnitude of current. It captures the “Power Density” of the battery.

Alternative Anode Materials: Quest for Higher Energy Density !

Alloy Capacity (mAh/g) Volumetric change (%)

Li22Si5 4200 400

Li3As 840 201

Li3Sb 564 147

LiAl 993 94

LiC6 372 10

Silicon has been proposed as the anode instead of carbon.Higher the Li capacity, larger the accompanying volumetric change

Source: A. Patil, V. Patil, D. Shin, J. Choi, D. Paik, S-J. Yoon, Mater. Res. Bull. 2008, 43, 1913.

Prior Art: Silicon Films as AnodeAdvantage: High theoretical charge capacity of 4200 mAh/g (10 times larger than carbon) Disadvantage: 400 % volume expansion leading to pulverization and delamination of the electrode films.

(a) Specific capacity plotted as a function of cycle number. (b) Stress-induced cracking of the film after a few cycles. (c) Delamination and peeling of the film from the collector electrode after extended cycling [2]

Source: J. P. Maranchi, A. F. Hepps, A. G. Evans, N. T. Nuhfer, P. N. Kumta, J. Electrochem. Soc. 2006, 153, A1246.

Nano-Silicon Reports

(a) Scanning electron micrographs of the porous Si particles indicating a pore wall size of ~40 nm. (b) Capacity vs. cycle number.

Source: H. Kim, B. Han, J. Choo, J. Cho, Angew. Chem. 2008, 47, 10151.

(a) Concept schematic of Si nanowire electrode (b) Scanning electron micrograph of Si nanowires that comprise the device anode. (c) Capacity vs. cycle number. Source: C. K. Chan, H. Peng, G. Lin, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui, Nat. Nanotechnol. 2008, 3, 31.

0.05C

Our Concept: Strain graded carbon-aluminum-silicon nano-scoop anode architecture

• C nanorods – oblique angle flux (85° from normal)

• Al and Si scoops – normal flux incidence

• Tested at 40C (51.2 A/g), 60C (76.8 A/g), 100C (128 A/g)

10%

94%280%

C

AlSi

Before Lithiation

After Lithiation

10%

94%280%

C

AlSi

10%

94%280%

10%

94%280%

C

AlSi

Before Lithiation

After Lithiation

200 nm

Si

Al

C100 nm

Si wafer 200 nm

Si

Al

C100 nm

Si wafer 200 nm

Si

Al

C100 nm

200 nm200 nm

Si

Al

C100 nm

Si wafer

Coin cell testing

C-Al-Si Nanorods: Oblique Angle Deposition (OAD) C-Al-Si Nanorods: Oblique Angle Deposition (OAD)

+Ar

Sputter target (Pt)

Pt atoms

Substrate

Oblique angle θ

will reach the target point

Obliquely incident particles

hθ

will be captured by the tall

surface feature

Incident atoms preferentially land on taller islands due to the shadowing effect:physical self-assembly!

will reach the target point

Obliquely incident particles

hθ

will be captured by the tall

surface feature

Incident atoms preferentially land on taller islands due to the shadowing effect:physical self-assembly!

Growth mechanism:

+Ar

Sputter target (Pt)

Pt atoms

Substrate

Oblique angle θ

+Ar

Sputter target (Pt)

Pt atoms

Substrate

Oblique angle θ

will reach the target point

Obliquely incident particles

hθ

will be captured by the tall

surface feature

Incident atoms preferentially land on taller islands due to the shadowing effect:physical self-assembly!

will reach the target point

Obliquely incident particles

hθ

will be captured by the tall

surface feature

Incident atoms preferentially land on taller islands due to the shadowing effect:physical self-assembly!

Growth mechanism:

Sputter Target (C, Al, Si)

Results: Specific Capacity vs. Cycle Index

C-rate: 40C

Evidence of Li Insertion into C-Al-Si Anode Unlithiated

Lithiated: 1C

Lithiated: 40C

Direct Physical Evidence ofLi+ insertion intoThe Si Scoops

Effect of Strain Gradation

CAlSiSi

Cr Vs.(0%)

(280%)

(10%)

(94%)(280%)

CAlSiSi

C Vs.

(280%)

(10%)(10%)

(94%)(280%)

Results: Ragone Plot

Advantages:

Energy density of 100 Wh/kg at C-rate of 40C

Power density as high as 250 kW/kg

Stability in performance

Journal Publication: Nano Letters

5th Highest Downloaded Paper on Nano Letters Web-site in Jan/Feb 2011

Current Work : Mass Scalable Geometry

ISSUES

POOR DIFFUSIVITY OF LITHIUM IN MICRON-

LONG SILICON SPRINGS

POOR CONDUCTIVITY OF SILICON SPRINGS

HINDERING ELECTRON TRANSFER

Research Objective :Mass Scalable Silicon geometry for Li-ion batteries

Approach :Deposition of micron long Silicon springs

(a) (b)

(a) Cross-Section SEM Image of Silicon arm-based spiral. (b) Cross-Section SEM image of Si-Al helical spiral