Lithium-Sulfur Rechargeable Batteries: Characteristics
Lithium-Sulfur Rechargeable Batteries: Characteristics, State
of Development, and Applicability to Powering Portable Electronics
Frank B. Tudron, James R. Akridge, and Vincent J. Puglisi
Sion Power Corporation
Tucson, AZ 85747
Abstract
Lithium-sulfur technology developed at Sion
Power forms the basis for a practical, high
energy, light-weight, 2 V rechargeable battery
system. The specific energy of this system
exceeds that of state-of-the-art lithium ion by a
factor of greater than 2-to-1, while the energy
density stands at an equivalent level. Unit cell
safety is improved beyond historical lithium
metal rechargeables, due to the absence of
dendrite formation in the sulfur chemistry. The
combination of high energy and light weight
make this an attractive technology for payloadcritical applications.
Introduction
Lithium-sulfur (Li-S) rechargeable battery
technology is capable of a specific energy of
400Wh/kg and an energy density of 425
Wh/liter. Further evolution of the technology
has the potential to achieve a 25%
improvement beyond these values. The
specific energy exceeds currently used lithium
ion rechargeables by a factor of over 2 to 1;
the energy density is equivalent to lithium
ion's. That is, Li-S provides the same runtime
for a portable computer in less than half the
weight or twice the runtime in the same weight
while having a volume comparable to lithium
ion. This performance level is obtained using
sulfur’s and lithium’s high specific energies
and novel electrode designs. Whereas the
lithium ion active material couple yields about
500 Wh/kg ideally, the Li-S active material
couple is more capable, yielding 2500 Wh/kg.
Also, the ability to use plastic substrates for
the sulfur cathode and vacuum deposition of
lithium for the lithium anode provides weight
savings not realized in other rechargeable
technologies.
Prototype Li-S cells, operating at 250 to 300
Wh/kg and similar Wh/L values, demonstrate
rate capability surpassing 3C and a
temperature range of –60oC to +60oC. When
charged and discharged at –10oC to –20oC at
the 3C rate, the cell provides about 80% of the
amp-hours and more than 75% of the watthours delivered at room temperature. At
temperatures above 45oC, the cell provides
more capacity at a higher voltage yielding
about a 10% Wh improvement relative to the
room temperature value. A Ragone plot
comparison shows that the Li-S cell will deliver
higher specific energy than other rechargeable
technologies such as LiIon, NiMH and NiCd at
any discharge power. Self-discharge is < 15%
per month.
Safety is improved relative to lithium ion in
that, unlike lithium ion, lithium dendrites do not
form in Li-S cell. This is a result of the
electrolyte/liquid cathode system employed.
End of life capacity and voltage are
determined by sulfur electrode fatigue and not
by lithium electrode failure. Abuse testing at
milestone design points during technology
development
has
shown
acceptable
characteristics. The technology will meet
safety standards. Guidelines are provided for
prospective users to assess how this enabling
technology will impact their applications in the
future.
Background and Status
Sion Power’s Li-S rechargeable battery
development aimed to exploit the attributes of
the chemistry and design possibilities to
satisfy the power requirement of payloadsensitive
applications.
The
chemistry’s
attributes are unique, exceeding all existing
commercialized rechargeable chemistries in
virtually all performance categories. In
particular, its high specific energy (Wh/kg)
directly addresses the prospective user’s need
for additional runtime for a given battery
weight or equal runtime for less battery weight.
For example, with this battery chemistry, the
goal of achieving 8-hr runtime with an internal
computer battery having a reasonable weight
is possible. This will come with no energy
density decrease, that is, the battery volume to
provide a given runtime will not increase. This
is possible due to the extremely high energy
provided by the materials used in the Li-S cell
when compared with the materials used in the
lithium ion cell as illustrated in Figure 1. As
shown, the theoretical energy provided by Li-S
exceeds that provided by lithium ion by a
significant margin both in specific energy and
energy density. And as will be shown, the
advantage in specific energy has already been
realized in early prototypes. Figure 2 illustrates
the evolution in specific energy that has taken
place together with improvement in cycle life.
Discharge
Charge
Cathode
S8
Li 2 S 8
Li 2 S 6
Li 2 S 4
Li 2 S 3
Li 2 S 4
Li 2 S 3
Li 2 S 2
Li 2 S
Polysulfides
Diffusion
through
Separator
Porous
Separator
Shuttle
S8
Li 2 S 8
Li 2 S 6
Li
+
Polysulfides reduction on the Anode surface
S p e c ific
Li o
V o lu m e tric
Anode
Lithium plating-stripping
V o lu m e tric
2500
W H /K g
2660
W H/L
S p e c ific
Figure 3: Chemical processes in the Li-S
rechargeable cell
1810
W H/L
580
W H/K g
L i-S
L i-io n
Figure 1: Theoretical Energy Density
Comparison
Li-S Energy Density & Cycle Life Evolution
2400
Discharge Capacity (mAH)
2000
Alpha 6.6
2004 Est.
300 WH/Kg
ENERGY DENSITY
INCREASE
1600
Alpha 6.5
2004 Current
250 WH/kg
1200
Alpha 6.6
2004 Current
270 WH/Kg
CYCLE LIFE INCREASE
800
Alpha 6.0
2000
400
Alpha 6.0
2001
Alpha 6.1
2002
Alpha 6.1
2003
and plated out on charge. The sulfur chemistry
is more complex in that a series of sulfur
polymers are formed. Higher polymer states
exemplified by Li2S8 are present at high states
of charge, the charged form of the battery.
Lower polymer states, exemplified by Li2S, are
present at low states of charge, the
discharged form of the battery. Fundamental
understanding of the sulfur electrochemistry
has driven significant practical advances in
specific energy and energy density. Sion
Power has improved the sulfur utilization
dramatically from about 46 to over 90 %. The
result is that a gram of sulfur that was
providing about 800 mAh now provides about
1500 mAh with no accompanying increase in
weight or volume of the cell. This improvement
is depicted in Figure 4.
Improvement in Utilization of Sulfur
0
0
50
100
150
200
250
300
350
400
450
Discharge profiles at 350 mA
Cycle Life (Cyles)
Figure 2: Multi-generation Evolution of Li-S
Performance
Figure 3 illustrates the electrochemical
working of the cell. At the negative electrode,
lithium is dissolved into solution on discharge
2.3
Voltage
It should be noted that with dramatic increases
in specific energy, there is typically a
corresponding decrease in cycle life, followed
by a period of improvement and so on.
Prototype cells in the range of 300 to 350
Wh/kg are now undergoing test. Energy
density has been tracking the specific energy
very closely to date, but is projected to
surpass that value as geometric design factors
assume a larger role relative to chemistry
factors in any improvements obtained.
2.4
2.2
Theory
2.1
2.0
Old
Chemistry
1.9
New
Chemistry
1
New
Chemistry
2
1.8
0
500
1000
Specific capacity mAh/g
1500
Figure 4: Advances in sulfur usage-path to
the present.
.
Charge Retention with Stand Time
CHARGE PROTOCOL: Constant Current/Voltage w/ Taper
2 min after Charge
2.3
65 oC
0.3
2.2
0.2
2.0
0.1
1.8
Voltage (V)
25 oC
2.4
Voltage
0.4
45 oC
Current, A
2.6
2.5
2.1
8 days after Charge
1.9
1.7
0.0
0.2
0.4
0.6
0.8
1.0
1.2
DISCHARGE CAPACITY (Ah)
0
0
100
200
300
Time, min
400
500
Figure 5: Voltage-limited taper charge
protocol
Ease of charge and charge termination is of
paramount importance in devising an
application suitable battery system. Figure 5
shows an industry-accepted charge protocol
using constant current charging terminated on
reaching a pre-set voltage The initial charging
can be followed by a tapering current,
maintaining the pre-set voltage. This method
is simple, inexpensive to implement and well
understood by both battery and device
manufacturers. Studies have shown that the
lithium sulfur electrochemistry adheres to this
accepted standard charge protocol.
As can be seen, the charge voltage increases
during the charge until it ultimately exhibits a
steep rise. In the test being illustrated, at the
pre-set voltage, in this case 2.5 V, the current
is tapered to lower values forcing the voltage
to remain at 2.5. Further, this repeats itself in
the same manner at all temperatures tested:
23, 45, and 65 oC.
After charging, the user typically does not
immediately use the device: some extended
and variable time may transpire before use.
Tests are run on charged cells to characterize
the self-discharge or charge retention over
stand time. Figure 6 illustrates the voltage and
ampere-hours delivered by the lithium sulfur
cell following a charged stand time of over 8
days.
Figure 6: Charge retention of Li-S cells
As can be seen, the high voltage plateau is
slightly depressed but the low voltage plateau is
not affected. In addition the discharge capacity
(Ah) is about 96% of that delivered with no
stand. This performance is comparable with
that of LiIon. The 3C rate discharge of the
lithium sulfur cell at various temperatures is
shown in Figure 7. An approximate 10%
improvement in watt-hours is obtained when
discharge is conducted at 65 Co versus 25 Co.
This is the result of an increase in delivered
ampere-hours plus an increase in the average
voltage.
2.6
2.4
2.2
+65 oC
Voltage
1.6
2.0
1.8
1.6
1.4
- 20 oC
- 40 oC
- 10 oC
+25 oC
o
- 30 C
- 60 oC
- 50 oC
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Ah
Figure 7: Temperature dependence of Li-S 3C
rate discharge
Other characteristics of this technology include
high discharge rate capability extending
beyond 6 times the rated capacity and the
ability to obtain useful energy at temperatures
ranging from a low of minus 60 Co to a high of
plus 60 Co. The latter comes with the additional
benefit of being able to perform the charge as
well as the discharge at the test temperature.
These attractive features come together in
designs currently exhibiting 300 to 350 Wh/kg
and similar Wh/L values. Cycle life for these
high specific energy and energy density
designs is still low approaching 100 cycles but
as shown in Figure 2 (vide supra), historically
this is typical. That is, as the energy within the
cell is dramatically increased, cycle life
becomes less until refinement of the design
drives that characteristic back to acceptable
values.
Application Testing
To study the operation of the lithium- sulfur
battery, a Hewlett Packard TC1000 pen tablet
was acquired for test. A smart battery pack
was designed in the fall of 2003 using
prototype lithium-sulfur cells with 250 Wh/kg
specific energy (Figure 8).
WinHEC: HP Pen Tablet & Prototype Li-S Battery
provides 50 Wh (10.5 V, 4.8 Ah capacity). The
LiIon original equipment battery with the
computer delivers about 40 WH and is
characterized by a rated voltage of 11.1 volts
and a capacity of 3.6 AH. It consists of 6 cells
arranged in 2 parallel strings of 3 cells.
Table 1 compares the prototype lithium sulfur
battery with the target commercial product. A
100 Wh battery is projected for production
using cells having a specific energy of 350
WH/kg.
Lithium Sulfur Battery Specification
Item
Voltage (V)
Capacity (AH)
WH/ battery
Cells (#)
Cell WH/kg
Cell WH/liter
Battery WH/kg
Battery WH/liter
Prototype (1)
Production (2)
10.5
4.8
50
20
250
265
10.5
8.8
100
20
350
400
265 (3)
320 (4)
Note 1: Prototype developed for WinHEC
Note 2: Initial production battery for pen tablet applications
Note 3: Projected based on current battery designs w/ cell contribution at 75 %
Note 4: Projected based on current battery designs w/ cell contribution at 80%
Table 1: Comparison of the Li-S proto-type
smart battery with its commercial counterpart
Summary
10.5 V, 50 WH Battery utilizing 250 WH/kg prototype cells,
discreet electronic components & machined plastics!
Figure 8: Li-S smart battery pack
configuration used for application testing
Discreet electronic, components provided
control, monitoring and safety circuits. Fuel
gauging was not included but will be
incorporated in a follow-on project. The plastics
were machined parts. Although functionally
sophisticated, the battery design was not
optimized for either weight or volume nor was it
possible to insert the battery into pen tablet
battery compartment since the form factor was
different. The battery consisted of 20 cells (4
parallel strings of 5 series-connected cells) and
safety and control electronics. The battery
Lithium-sulfur technology provides specific
energy and energy density values far superior
to any recharge-able battery technology in
commercial production or on the research
horizon. Charge retention with storage and
charge termination protocols are con-sistent
with the LiIon technology. Characteristics such
rate capability and consistently high capacity
delivery in the temperature range 23 oC to 60
o
C are far superior to those of LiIon and thus
can be device and application enabling.
Areas for additional advancement are:
increasing specific energy and energy density;
increasing cycle life; continuous improvement in
safety of cells and battery packs; and finally,
exploiting lithium sulfur’s unique chemistry for
battery fuel gauging.
Acknowledgement
The authors would like to thank Dr. Yuriy
Mikhaylik
for
his
important
technical
contributions.
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