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

V.B.12 Developing Materials for Lithium-Sulfur Batteries (ORNL)
Liang – ORNL
Capacity (mAh g -1 )
1400
1200
1000
800
600
400
200
Charge, pore volume 2.35
Discharge, pore volume 2.35
Charge, pore volume 1.12
Discharge, pore volume 1.12
Pore volume 2.35
0
5 10 15
0
20
Cycle Number
Figure V - 65: Cycling performance of S/C composites with 50% sulfur
loading. Carbon hosts have pore volumes of 2.35 and 1.12 cm 3 /g. Surface
areas are ~ 800 m 2 /g for both materials. Capacity is normalized by the sulfur
alone.
Electrolyte Additives. To overcome the corrosion
problem associated with the LiBr additive, a number of
alternative compounds have been investigated. Organic
alternatives include benzoquinone,
Tetracyanoquinodimethane (TCNQ), and 2,3-Dichloro­
5,6-dicyano-1,4-benzoquinone (DDQ). These compounds
have electrochemical redox potentials between 2.5 and 3.9
V. However, the chemical compatibility of benzoquinone
and TCNQ with bare lithium anode is a problem. DDQ has
better chemical stability with lithium metal. It was found
that using DDQ as the additive [5% in tetraglyme] reduces
the capacity of S by half. The possible reason for this
unexpected capacity loss could be the polymerization of
DDQ at high concentration. The resulting polymer coats
the carbon electrode surface and compromises the
electrochemical performance of the cathode. When the
concentration of DDQ was reduced to 1%, the
electrochemical performance was improved comparing
with the 5% DDQ in tetraglyme. Inorganic alternatives to
LiBr were also investigated. Promising results were
obtained when a phosphorous sulfide additive was used in
the cell. As shown in Figure V - 66a, the charge/discharge
curves have plateaus close to the ideal curves. The cell has
a good retention of capacity at 0.1 C rate. The cut-off cell
voltage has been set to 1.9 V because the additive can be
reduced at the cathode when the discharge voltage is below
1.9 V. The stability of this phosphorous sulfide additive is
still under investigation. The mechanism of how the
additive works will be studied in future experiments.
Protection of Lithium Anode. To protect lithium
from reacting with the sulfur species, a novel anode made
of metal lithium with a pre-formed solid electrolyte
interphase (SEI) on its surface was designed and
implemented in the Li-S batteries. The tenet of the design
is to have an ionic conductor covering the surface of
lithium metal, thereby impeding surface reaction of lithium
with polysulfide.
Lithium thiophosphate salts were chosen to form the
100 SEI. Lithium thiophosphates are fast Li ion conductors.
Their ionic conductivity ranges from 10 -6 to 10 -3 S/cm
80
depending on the composition. It was found that Li 2 S x ­
60
P 2 S 5 can react with lithium metal to form a solid
electrolyte of lithium thiophosphate at the surface of
40 lithium metal. This solid electrolyte conducts lithium ions
while it blocks the access of polysulfide to the lithium.
20 Because the polysulfide shuttle phenomenon is more
Coulombic Efficiency (%)
severe at low cycling rates than at high rates, the cell was
intentionally cycled at a low rate of 0.1 C to demonstrate
the protection of lithium metal through this solid
electrolyte coating. Shown in Figure V - 67a is the voltage
profile of the first cycle. The discharge curve has two
plateaus at 2.4 and 2.0 V, corresponding to the reduction of
long chain polysulfides (sulfur atoms greater than 4) and
short chain polysulfides (sulfur atoms less than 4),
respectively. The ratio of the first plateau to the second
plateau is exactly 1:3. The same ratio of the high to low
plateaus was found in the charging curve. The constant
ratio of 1:3 of these plateaus at both charging and
discharge cycles proves that the polysulfide shuttle has
been completely blocked by this solid electrolyte coating
on the surface of lithium metal. The coulombic efficiency
shown in Figure V - 67b is above 98% for all cycles. The
energy efficiency is about 90% at 0.1 C. Capacity decay
during cycling was still present. It is known that the
capacity decay is caused by the precipitation of Li 2 S in the
liquid electrolyte.
Sample Transfer Stage. A new sample transfer stage
has been developed for taking samples from a glove box to
a scanning electron microscope (SEM) without exposure to
air and moisture. The transfer stage operates by the
difference of pressure inside and outside the SEM
chamber: when a sample is loaded in a glove box and
exposed to a slight vacuum, the atmospheric pressure
pushes the lid against the o-ring seal. The sample is
protected by the transfer stage as it is moved from the
glove box to the SEM. After the sample stage is loaded
inside the SEM chamber, the higher vacuum causes the lid
to pop open, thus exposing the sample for imaging. This
sample stage enables the analysis of lithium anodes after
battery cycling.
Conclusions and Future Directions
Because of the poor electronic conductivity of sulfur,
the high surface area carbon is the enabler of achieving the
maximum utilization of sulfur under electrochemical
cycling. The porosity is important for the retention of
capacity. With the same sulfur loading, excess pore
volume accommodates liquid electrolyte and facilitates the
mass transport of lithium ions. Physical adsorption by
porous carbon is insufficient to completely eliminate the
polysulfide shuttle.
Energy Storage R &D 526 FY 2011 Annual Progress Report