Managed by UT-Battellefor the Department of Energy

Sheng Dai, Xiao-Guang Sun,

Bingkun Guo

Oak Ridge National Laboratory

One Bethel Valley Road,

Oak Ridge, TN

May 12, 2011

Hard Carbon Materials for High-Capacity Li-ion Battery Anodes

Project ID: ES104This presentation does not contain any proprietary,

confidential, or otherwise restricted information

Overview

• Start date: - June, 2010

• Budget:-Funding received in FY 10: $120K

-Funding for FY 11: $300K

Barriers

• Barriers addressed • Low energy density• Poor cycle

performance• Cost

Outline

• Background– Why hard Carbon?– Why is hard carbon more suitable

for vehicle application? – Why mesoporous carbon?

• Progress report– Objective– Challenges and Approaches– Milestones– Progress towards milestones

• Future work

• Summary

Li, Dai*, et. al, J. Am. Chem. Soc. 2004, 126, 12782; Li, Dai*, et. al, Chem. Mater. 2005, 17, 1717 Liang, Dai*, J. Am. Chem. Soc. 2006, 128, 5316; Wang, Dai*, et. al., Langmuir 2008, 24, 7500 Wang, Dai*, et. al., Chem. Mater. 2008, 20, 4800; Lee, Dai*, et. al., J. Am. Chem. Soc. 2009, 131, 4596Wang, Dai*, Angew. Chem. Int. Ed. 2010, 49, 6664; Wang, Dai*, et. al., Chem. Mater. 2010, 22, 2178

Why hard carbon?

Disadvantages of Graphite• Limited Capacity: 372 mAh/g• Susceptibility to exfoliation by

electrolytes• Poor rate performance• Difficulty to form composites

and molecularly doped anodes

0 5 10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

10C

MC-550 Graphite

2CC/10

Capa

city (

mAh/g

)

Cycle number

C/10

1C

5C

Advantages of Hard Carbon• High Capacity• No intercalation-induced

exfoliation• Fast rate capability• Easy to form composites and

molecularly doped anodes

Why is hard carbon more suitable for Vehicle applications?

• The I/V curve of hard carbon based battery can be used as a gaugefor power management

• The I/V curve of graphite based battery can’t be used as a gaugefor power management unless sophisticated power management system is in place!

Endo et al, SCIENCE, 264, 6556 (1994). Endo et al, Carbon, 38, 183 (2000).

0 20 40 60 80 100 1200

300

600

900

1200

1500

1800

0.2C

0.1C

50C30C10C20C

5C2C

1C0.5C

0.2C

MC-550 C-550

Ca

paci

ty (m

Ah/g

)

Cycle number

0.1C

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500 MC550 C550

Amou

nt A

dsor

bed

(cm

3 g-

1 )

Relative Pressure (P/P0)

Why mesoporous hard carbon?

MC-550: 576 (m2/g) 10.1 nmC-550: 569 (m2/g) <1.2 nm

1. Li can be stored in the nanopores of the carbon via interfacial andsurface charging!

2. The mesoporous structure provides fast Li transport channel and alsoreduces the solid-state diffusion length for Li and thus render high ratecapability.

3. The mesoporous structure can buffer well against the local volumechange during the Li insertion/extraction reactions and thus enhancethe structural stability.

Objectives

To develop low-cost hard carbon materials with capacity higher than 372 mAh g-1 and offering good cycle performance for uses in lithium ion batteries targeted on EV applications.

Challenges and Approaches

1. Improve cycle performance by surface coating or doping

2. Improve initial coulombic efficiency (CE) by surface coatingwith single ion conductors

3. Improve volumetric capacityby forming carbon compositewith metal oxide

• Capacity fade with cycling

• Low initial coulombicefficiency

• Low density (low volumetric capacity)

Challenges Approaches

Milestones

• Finish investigation on the effect of carbonization temperature on physical and electrochemical properties of carbon materials. (03/30/2010)

• Improve long cycle performance of the carbon half cells. (9/30/2011)

• Optimize carbon || Li half cells and reduce the initial irreversible capacity. (09/30/2011)

• Identify the structural and surface changes of the carbon electrode after cycling. (09/30/2011)

Effect of Carbonization Temperature on cell performance

• Increasing temp. results in H, O reduction

• Surface area remains almost same

• Li stores in defects (Nanopores, Cavities)• Unstable defects result in the reduction

of the capacity with cycling.• Low initial capacity and coulombic efficiency are partially related to the lowcut off voltage of 2.0V; use 3.0V insteadwill increase capacity 200-300mAh g-1

and coulombic efficiency 15%-20%. 0 5 10 15 20 25 30 35 40

200

400

600

800

1000

1200

1400

1600

1800

550 oC 650 oC 750 oC 850 oC

Capa

city (

mAh

/g)

Cycle number

0.05 mA

0.10 mA

0.15 mA

0.20 mA

15wt% PVdF0.05 - 2.0V

T / oC C % H % N % O % BET (m2/g)

Pore Size(nm)

1st intercalation capacity (mAh g-1)

1st de-intercalation capacity (mAh g-1)

Coulombicefficiency (%)

550 82.90 2.55 0.98 10.08 517.6 9.0 1674.5 685.3 40.9

650 88.05 2.13 0.53 5.86 517.4 5.9 1212.5 526.2 43.4

750 88.17 1.38 0.52 3.09 505.1 7.5 1164.1 481.5 41.3

850 91.30 1.29 <0.5 3.64 530.1 6.2 1072.6 418.1 39.0

Approach 1: Improve Cycle Performanceby surface coating

0 20 40 60 800

200400600800

100012001400160018002000

MC-550-ppy-1 MC-550-ppy-2 MC-550

10C5C

2C1C

C/2C/5

Capa

city

(mAh

/g)

Cycle number

C/10

BET (m2/g)

Pore Size (nm)

MC-550 497 10.1MC-550-PPy-1 292.2 8.2MC-550-Ppy-2 232.5 7.5

• The surface coating of Polypyrrole (ppy) reduces unstable defects of mesoporous carbon and improves its cycle performance.

• The less contribution from surface charging due to the reduced surfacearea after Ppy coating is the cause of capacity reduction.

•Enhance electronic conductivity

0 10 20 30 40 50 600

100

200

300

400

500

MC-550 MC-550-0.1CNT MC-550-0.2CNT

50C30C

20C

10C

2C

Capa

city

(mAh

/g)

Cycle number

5C

• The distribution of CNT in the carbon matrix enhances the electronicconductivity of mesoporous carbon.

• The mesoporous structure of the composite provides fast Li+ transportchannels.

MC-550: 497 m2/g; 10.1 nm

MC-550-0.1CNT;499 m2/g;10.9 nm

MC-550-0.2CNT;484 m2/g;11.4 nm

Approach 1: Improve Cycle Performance by surface coating (con’t)

•Enhance electronic conductivity

0 50 100 150 200 250 300 350 400 450 500 5500

50

100

150

200

250

300

350

10% CNT 20% CNT

Cap

acity

/ m

Ah

g-1

Cycle number

5C

• Under the current rate of 5C, the cell with 20wt%CNT reaches stable cycling quickly! • Cell with 20wt% CNT has a lower charge transfer resistance that facilitates faster charge/discharge.

0 10 20 30 40 500

10

20

30

40

50

C-550-0.1CNT After cycle

-Z'

(Ω)

Z(Ω)

0 10 20 30 40 500

10

20

30

40

50

C-550-0.2CNT After cycle

Z (Ω)- Z

' (Ω)

Approach 1: Improve Cycle Performance by surface coating (con’t)

Approach 2: Improve coulombic efficiency by surface coating with single ion conductors

• The improvement in the initial coulombic efficiency by surface coating with single ion conductors is very limited.• Surface coating with single ion conductors indeed improved the cycling

stability.

0 200 400 600 800 1000 1200 1400 1600 18000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

E / V

Capacity / mAh g-1

C550; CE = 0.574 C550-PALi; CE = 0.597 C550-PSSALi-MALi; CE = 0.604

C-550: 497 m2/g; 10.1 nmC550-PALi: 210 m2/g; 9.3 nmC550-PSSA-co-MALi: 224 m2/g; 8.9 nm

0 5 10 15 20 25 30 35 40 45 500

200

400

600

800

1000

1200

1400

1600

1800

C/20

CC/2

C/5

C/10

C550 C550-PALi C550-PSSA-co-MALi

Cap

acity

(mA

h g-1

)

Cycle number

C/20

• Use Single ion conductors as artificial SEI layer on carbon surface toimprove initial coulombic effciency

• Surface coating with single ion conductors reduces both SEI andcharge transfer resistance, either before cycling and after cycling.• PSSA-MALi is more effective than PALi in reducing the charge transfer resistance.

Approach 2: Improve coulombicefficiency by surface coating with single

ion conductors (con’t)

0 20 40 60 80 100 1200

20

40

60

80

100

120

C550 C550-PALi C550-PSSALi-MALi

-Z"

/ Ω

Z' / Ω

Before Cycling

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

After Cycling

C550 C550-PALi C550-PSSALi-MALi

-Z"

/ ΩZ' / Ω

0 5 10 15 200

200400600800

100012001400160018002000

MC-550-0.2LT; CE = 0.527 MC-550-0.1LT; CE = 0.496 MC-550; CE = 0.504

Capa

city (

mAh

/g)

Cycle number

C/10

BET (m2/g)

Pore Size (nm)

MC-550 497 10.1MC-550-0.1LT 344 9.4MC-550-0.2LT 329 8.9

• Coating is a suitable method to reduce unstable defects of mesoporous carbons and improve their cycle performance.

• Needs new approaches, new coating technique or combination of both to improve initial coulombic efficiency.

Approach 2: Improve coulombicefficiency by surface coating with single

ion conductors (con’t)

Approach 3: Improve volumetric capacity by forming carbon composite with metal oxide

10 20 30 40 50 60 70 800

50

100

150

200

250

300

350

400

Inte

nsity

(a.u

.)

2θ (degree)

Cr2O3: JCPDS #06-0504

0 100 200 300 400 500 60040

50

60

70

80

90

100

Wei

ght (

%)

Temperature (0C)

Carbon: 43 wt% Cr2O3: 57 wt%

• Form composite with metal oxide that have comparable capacity within the same discharge voltage range as mesoporous carbon but having poorreversibility.• Mesoporus carbon matrix provide electron conductivity and confinement effect to improve the overall performance.

Densities:Cr2O3 : 5.22 g/cm3; C550: 1.90 g/cm3; Composite: 3.0g/cm3

The density is increased by 58%!

0 5 10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

1600

1800

100mA/g

MC550-Cr2O3

Commercial Cr2O3

Capa

city

(mAh

/g)

Cycle number

50mA/g

200mA/g500mA/g

0 200 400 600 800 1000 1200

0.0

0.5

1.0

1.5

2.0

2.5

3.0 1st cycle 2nd cycle 3rd cycle

Volta

ge (V

)

Capacity (mAh/g)

3-0.01V

The cycling stability was greatly improved as a result of composite.

MC550: 497 m2/g; 10.1 nmMC550-Cr2O3: 505 m2/g; 4.5nm

Conversion (displacement)6Li + Cr2O3 ↔ 3Li2O + 2Cr Cr Li2O

CrCr

Li2OLi2O

Li2O

Approach 3: Improve volumetric capacity by forming carbon composite with metal

oxide (con’t)

Fulvio, Dai*, et. al., Adv. Funct. Mater., 2011, in press

Future Work

• Continue to seek suitable candidates and new methods in modifying the surface of mesoporous carbons to increase the initial coulombic efficiency and improve the cycle performance.

• Seek other high density or high capacity active materials to increase the volumetric capacity density of mesoporous carbons.

• Focus on characterization and diagnostic studies using XPS, XRD, EIS, SEM/TEM, Neutron etc.

Summary

• Hard carbons with high capacity (1000mAh g-1) have been made and their cycle stability has been improved by surface coating with polypyrrole and doping with carbon nanotube.

• Different single ion conductors have been used to improve the initial coulombic efficiency, however, only limited success is achieved.

-- Further improvement in initial coulombic efficiency is pendingupon characterization of the modified electrodes after first cycle to understand morphology, composition and their effectson coulombic efficiency.

-- Needs new approaches, new coating technique or combination of both to improve initial coulombic efficiency.

• The volumetric density of mesoporous carbon has been increased by forming composite with Cr2O3.