V. Focused Fundamental Research - EERE - U.S. Department of ...

V.C.14 Hard Carbon Materials for High-Capacity Li-ion Battery Anodes
(ORNL)
Sheng Dai
Oak Ridge National Laboratory (ORNL)
1 Bethel Valley Rd.
P.O. Box 2008, MS-6201
Oak Ridge, TN 37831-6053
Phone: (865) 576-7307; Fax: (865)-576-5235
E-mail: dais@ornl.gov
Collaborators: Xiao-Guang Sun, Miaofang Chi, ORNL
Start Date: June 2010
End Date: September 2012
Objectives
The objective of this project is to develop low-cost
hard carbon materials with a high capacity (>372 mAh g -1 )
and reliable performance for applications in lithium ion
batteries.
Technical Barriers
Challenges for application of lithium ion batteries in
electric vehicles include low energy density, poor cycle
life, large irreversible capacity loss, poor rate capability,
and calendar life.
Accomplishments
· Evaluated the carbonization temperature effect on the
carbon half cell performance.
· Ealuated the effect of binder amount and pore size on
the carbon half cell performance.
· Evaluated the surface coating and bulk doping effect
on the electron conductivity of the carbon and the
accompanying carbon half cell performance,
especially the rate capability and long cycle stability.
· Evaluated the surface coating of single ion conductor
on improving the initial coulombic efficiency and cell
cycling stability.
· Finished the comparison of cell performance of
mesoporous carbon with commercial carbons
Introduction
Thousands of carbonaceous materials are
commercially available, and lithium can be inserted
reversibly within most of these. The structure of carbons
depends strongly on the type of organic precursors used to
make them. Carbonaceous materials have traditionally
been divided into two groups: soft and hard. The soft
carbons can be graphitized completely upon heating to
above 3000°C, whereas the hard carbons are very difficult
to graphitize. The capacity of graphite (soft carbon) is
limited to 372 mAh g -1 , which is associated with its
maximum LiC 6 stage. On the other hand, disordered hard
carbons with different degrees of graphitization have been
reported to exhibit stable capacities exceeding 500mAh/g.
The reversible capacities of many hard carbons for lithium
depend on both pyrolysis temperature and precursor type.
In addition, the mechanism of lithium insertion in
carbonaceous materials also depends on the carbon type.
Approach
Hard carbon materials with tailored surface areas and
nanoscopic architectures have been synthesized in our
group via a self-assembly method [1,2]. The unique ORNL
methodology for synthesis of these hard carbon materials
has several advantages: (1) tunable pore sizes (2–18 nm),
(2) adjustable surface areas, (3) controlled variation of
carbon/hydrogen ratios and chemical compositions through
carbonization temperature and precursors, (4) tunable
interfacial structures, and (5) controllable morphologies.
The unique characteristics of our hard carbon materials
permit us to systematically correlate the synthesis
conditions with their reversible lithium intercalation
capacity and provide a rare opportunity to investigate the
structure-function relationship associated with hard
carbons for energy storage. The lithium intercalation
mechanism will also be studied in detail to guide the future
synthesis of carbon materials with high, stable capacities
against cycling for applications in lithium ion batteries.
Results
The mesoporous carbons obtained at different
temperatures under nitrogen atmosphere all had a similar
BET surface area of around 500m 2 /g with pore sizes in the
range of 6-9 nm. However, the intercalation capacity of the
mesoporous carbon obtained at 550°C (MC550) showed
much higher capacities than those obtained at 650, 750 and
850°C under the same testing conditions. So we have
focused on improving the cell performance of MC550. To
test the rate capability of MC550, we used the theoretical
capacity of graphite, 372 mAh g -1 , to calculate the needed
current. During the optimization of the electrode, we found
Energy Storage R&D 590 FY 2011 Annual Progress Report