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Sulfur Electrode for Sodium Sulfur Cells
김고운, 박윤철, 조남웅, 정기영*
Goun Kim, Yoon-Cheol Park, Namung Cho, Keeyoung Jung
포항산업과학연구원 ES소재연구그룹
(Energy Storage Materials Research Group, Research Institute of Industrial Science and Technology, RIST)
INTRODUCTION
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BACKGROUND
 Principles of cell operation
Fig 2. EMF of the sodium sulfur cell at 350℃
discharging
Discharge: 2Na+ + xS2- = Na2Sx(ℓ)
Charge: Na2Sx(ℓ) = 2Na+ + Sx2Overall: 2Na(ℓ) + xS(ℓ) = Na2Sx
EMF = 2.076~1.74V @350℃
Fig 1. Na2S/S Phase Diagram
EXPERIMENTAL
The sulfur electrode is typically made of a graphite felt filled with
molten sulfur. The graphite felt is used because of its good corrosion
resistance against highly corrosive environments (molten sulfur and
sodium poly-sulfides) at its service temperature of 350oC, and high
electronic conductivity. The sulfur electrodes have been fabricated an
d subject to cell tests via the following procedure.
 Manufacturing process for sulfur electrode
RESULTS
1. Changes in cell voltage
Tubular cell
2.4
Planar cell
Theoritical OCV
50th DC-C
100th DC-C
150th DC-C
200th DC-C
Charge
2.2
Theoretical OCV
2.0
2.4
Operating Voltage (V)
discharging
When a sodium sulfur cell is subject to charge/discharge cycles, the
charge and discharge curves as a function of the depth of discharge
(DoD) follows the open circuit voltage (OCV) curves but with parallel
shifts due to the iR drop. The EMFs at different DoDs are determined
by the composition of the sodium polysulfides. For example, the cell
is initially discharged in a two phase regime which consists of an
immiscible mixture b-sulfur and sodium penta-sulfide (Na2S5.1). The
EMF in two phase region is invariant until all the b-sulfur is converted
to Na2S5.1. Thereafter, Na2S5.1 is decomposed to lower polysulfides of
Na2Sx (3<x<5.1), the free energy of formation of which is lower than
that of is Na2S5.1. Therefore, the EMF of the cell is linearly decreasing.
Operating Voltage (V)
ESS
 One of the most important features in determining the long term electrochemical performance of sodium
sulfur (NaS) batteries is the resistance increase over accumulated cycles. The increase in cell resistance results
from degradation of the sulfur electrode, metal current collector, and the increased impurity concentration in
the electrode. To ensure the cell performance over its lifetime (>15 years), understanding the electrochemical
behavior of sulfur electrodes is of critical.
 In this study, in order to investigate the geometry of cathode felts, two identical cathode felts, but with
different geometries, i.e. planar and tubular, were fabricated. The two sulfur electrodes were installed in
planar and tubular cells, respectively, and their electrochemical behaviors were compared.
1.8
Charge
2.2
Theoretical OCV
2.0
1.8
Discharge
graphite fiber
1.6
10
20
30
40
50
Discharge
60
70
1.6
80
10
20
30
Depth of Discharge (%)
400×400㎛
1000㎛
200th discharge
2000㎛
glass fiber
Fig 5. 3D image of sulfur felt
 Effective factors for sulfur electrode
1. operating temperature
2. electrode thickness
3. charge regime and recharge time
(current density/charge voltage)
4. electronic conductivity of carbon matrix
5. geometry of electrode
(a) tubular type
Planar
-0.005
Planar
-0.010
20
40
60
80
-0.010
80
60
40
20
Depth of Discharge (%)
(b) planar type
2.1
10 mA/cm2
50 mA/cm2
2.0
Solid electrolyte
(β/β”-Alumina)
S
100 mA/cm2
1.9
100mA/cm2 DC_start
100mA/cm2 DC_end
50mA/cm2 DC_start
50mA/cm2 DC_end
10mA/cm2 DC_start
10mA/cm2 DC_end
1.8
1.7
60~130mm
1.6
Planar NaS cell
S
Theoretical OCV
0.000
Fig 3. Sulfur electrodes
Sulfur Electrode
80
4. Effect of current density on discharge characteristics for tubular cell
Anode(Na)
500mm
Tubular
0.005
Depth of Discharge (%)
Insulating Ring
Tubular NaS cell
0.000
-0.005
 Specification of test cells
S
Tubular
Theoretical OCV
0.005
∂V / ∂(DoD)
Fig 4. Fabrication process for sulfur felt by needle punching
70
Theoritical OCV
Tubular cell
Planar cell
0.010
∂V / ∂(DoD)
after needle punching
60
200th charge
Theoritical OCV
Tubular cell
Planar cell
0.010
before needle punching
50
3. Derivations of cell voltages w.r.t. depth of discharge(DoD)
needle
needle punching
40
Depth of Discharge (%)
Cell voltage (V)
glass fiber sheet
Theoritical OCV
50th DC-C
100th DC-C
200th DC-C
Tubular cell
Specification
Planar cell
800cm2
Active area
13~100cm2
10mm
Thickness of cathode
10mm
T5-4
Cathode felt
T5-4
350oC
Operating temperature
350oC
150W
Power
2.2W
1,000Wh
Energy capacity
9~75Wh
100mA/cm2
Current density during discharge
100mA/cm2
80mA/cm2
Current density during charge
80mA/cm2
200 cycles
Number of cycle
200 cycles
10
20
30
40
50
60
70
80
90
Depth of Discharge (%)
• Current interruption tests were conducted with a 120W/550Wh
mid-sized tubular NaS cell.
• Significant EMF drop was observed at high current density (10, 50,
and 100mA/cm2) from the current interruption test.
• Insufficient flux of sulfur and Na2Sx caused formation of lower
Polysulfide which has lower EMFs.
SUMMARY
1. Discharge-charge characteristics of tubular and planar sodium sulfur cells with the same cathode materials, but with different geometry,
were compared.
2. Both cells were tested over 200 cycles with 100mA/cm2 of current density and 80mA/cm2 of current density during discharge and charge,
respectively.
3. Cell test results showed different shape of discharge curves between tubular and planar cell.
4. The planar cell shows more reliable performance.
5. Surface area density of felts changes along the cell thickness direction of cathode in tubular cell.
6. Differences of surface density and viscosity have an effect on the flow of sulfur and Na2Sx.
The presented work has been financially supported by POSCO and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the authority of the Ministry of Trade, Industry, and Energy of
the Republic of Korea (contract Nos. 2012T100100643).