Managed by UT-Battelle for 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: ES104 This presentation does not contain any proprietary, confidential, or otherwise restricted information
20
Embed
Hard Carbon Materials for High-Capacity Li-ion Battery … · Outline • Background – Why hard Carbon? – Why is hard carbon more suitable for vehicle application? – Why mesoporous
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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
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
• 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
• 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
• 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.
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.