A Comparison of Nonwoven Separators for Supercapacitors

1 EXECUTIVE SUMMARY

Six separators were tested from two suppliers, Dreamweaver

International and each at 40, 30 and 25 microns, in supercapacitors

assembled of commercial electrodes in a pouch cell. Separator

properties, scanning electron micrographs, and capacitor

performance were all measured for each material. The following

conclusions were reached:

SEMS: Scanning electron micrographs (SEMs) revealed that the

competitor separators are composed primarily of fibrillated cellulose

microfibers with diameters in the range from 1-4 microns. DWI

separators have similar composition, with a much higher population

of fibers with diameters in the range of 0.2 – 0.4 microns.

Separator Properties: The separators from the different companies

had, on average, similar basis weights, thickness, and porosity. The

pore size and bubble point for DWI separators was slightly higher, but

with the following advantages:

Gurley: a much lower (63%) Gurley air resistance.

Strength: 21% higher tensile strength

Modulus: 129% higher modulus

Moisture: 25% lower moisture content

Capacitor Performance: All of the materials

showed a similar 24 hour self-discharge. On

average, the DWI materials showed 9% higher

capacitance, and 27% lower ESR. In the most

dramatic comparison, at 30 microns, the DWI

Titanium 30 had 13% higher capacitance and

61% lower ESR, as shown in the graph.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Capacity ESR

Cap

acit

y (F

/g)

or

ESR

(O

hm

)

Supercapacitor Comparison

Leading Competitor 30micron Dreamweaver Titanium 30

Competitor

DWI Titanium

40

Summary: DWI Titanium separators

used a higher population of nano-sized

fibers to provide higher electrical

conductivity and capacitance in a

separator that is also stronger, with

lower moisture content.

2 ABSTRACT

Two separator companies have designed nonwoven separators specifically for electrolytic double layer

capacitors (EDLC), including supercapacitors, ultracapacitors, and EDLC capacitor – battery hybrids. The

leading competitor uses cellulose fibrillated to 1-4 micron fibers in a uniform nonwoven web, and is the

leader in the industry. Dreamweaver International (DWI) uses cellulose fibrillated to approximately 250

nanometer diameter fibers, combined with microfibers of approximately 5 microns diameter, also in a

uniform nonwoven web. In this paper, three separators are compared from each company. They are

measured for thickness, moisture content, porosity, mean flow pore size, bubble point, tensile strength,

and tensile modulus and Gurley air resistance. Images were taken under a scanning electron microscope.

In addition, EDLCs were made using production electrodes and measured for self-discharge, capacitance

and internal resistance. These measurements are compared to commercial capacitors from two

manufacturers. While both companies make materials that are very suitable for use in a wide variety of

EDLCs, on average the DWI materials showed 9% higher capacitance, and 27% lower ESR. They are also

21% stronger on average, and have 25% lower moisture content.

3 INTRODUCTION

Electric double layer capacitors (EDLCs) are energy storage devices capable of providing very high power,

up to 100 times that of even high rate lithium ion batteries. This allows applications that otherwise could

not be done, such as high power signal conditioning in the electric grid, regenerative braking in busses

and other large transport, and energy recovery in construction where heavy lifting is involved. In addition,

EDLCs are finding application in portable electronics, helping to extend battery life, improve burst

communications, and provide rapid charging.

Because the power can be so high, especially in high

energy applications of EDLCs such as regenerative

braking for large vehicles, the ohmic losses due to

internal resistance of the EDLC can cause considerable

heating and loss of efficiency. One driver of the energy

loss is the separator. If a separator could be provided

with significantly lower internal resistance, it could

improve the performance of the EDLC, lowering ohmic

losses, reducing operating temperature and increasing

energy efficiency.

4 EXPERIMENTAL

In this study, commercially available separators from two manufacturers are compared. The first

manufacturer is Dreamweaver International (DWI) and the second is a leading competitor. The tests listed

in Table 1 were performed on each separator. All of the testing was done by outside test labs, which are

also listed along with the test procedure where there is a standard available. Other procedures are

described below.

Table 1: All of the testing was done at outside labs, using standard test procedures where available, as listed below.

Test Procedure Test Laboratory Standard or Equipment

Scanning electron microscope Clemson University Electron Microscopy Laboratory

Hitachi Analytical Variable Pressure Scanning Electron Microscope SU6600

Basis weight Herty Advanced Materials Development Center

TAPPI T220

Thickness Herty AMDC TAPPI T220

Moisture content Herty AMDC TAPPI T220

Tensile strength Herty AMDC TAPPI T220

Tensile modulus Herty AMDC TAPPI T843

Gurley Herty AMDC TAPPI T460

Mean flow pore size Porous Materials Inc ASTM F316

Bubble point Porous Materials Inc ASTM F316

Self-discharge Polystor Maccor 4000, method below

Capacitance Polystor Maccor 4000, method below

Internal resistance Polystor Maccor 4000, method below

4.1 SCANNING ELECTRON MICROSCOPE Images were taken at 500x, 1000x, 2000x, 5000x and 10,000x magnification using a Hitachi Analytical Variable

Pressure Scanning Electron Microscope SU6600. Eight sets were taken with each material, for a total of 40 images.

4.2 SEPARATOR PROPERTIES Separator properties were measured at the Herty Advanced Materials Development Center, and at Porous

Materials, Inc. according to the test methods listed above.

4.3 CAPACITANCE TESTS PolyStor built cells using the separator materials specified below and production EDLC electrodes. The cells were ~

5cm x 5cm square single cells, but used double sided electrodes (most production electrodes are double sided). The

separator materials and electrodes were dried under vacuum at 120C overnight prior to cell assembly. All cells were

dried at 80C under vacuum overnight after assembly, but prior to electrolyte fill. The cells used acetonitrile with 1M

TEATFB salt blended by PolyStor using high purity materials. The cells were slightly overfilled with electrolyte and

included extra volume for a gas pocket. They were clamped using plastic plates with binder clips. The weight

included the weight of the double-sided electrode and current collector. Because only one side of the electrode is

used in the test, the capacitance values are low, approximately half of what would be achieved in a production cell.

The capacitors were charged to 2.70V at ~10 mA/F rate and held for 10 minutes at 2.70V. Capacity was measured in

Whr/g by immediate discharge from 2.70 to 0.10 V at a discharge current of ~10 mA/F. To measure D.C. ESR

(equivalent series resistance), the capacitor was charged to 2.70V, held at 2.70V for 10 minutes and then

immediately discharged at ~100 mA/F. The voltage drop at the constant current (Id) was used to calculate the DC

ESR:

The reported ESR is normalized for a 1 cm2 square area of capacitor.

The 24 hour self-discharge was measured by charging the capacitors to 2.70V and holding for 10 minutes before

removing the charge voltage. The voltage was then recorded continuously for 24 hours. The capacitors were then

charged to 2.70V, held for 10 minutes at 2.70V, and capacitance and D.C. ESR were measured using the methods

described above.

5 RESULTS

5.1 SCANNING ELECTRON MICROSCOPE PICTURES Representative high magnification scanning electron microscope images of the various materials are

shown below.

5.1.1 Competitor’s 40 Micron

5.1.2 Competitor’s 30 Micron

5.1.3 Competitor’s 25 Micron

5.1.4 DWI Titanium 40

5.1.5 DWI Titanium 30

5.1.6 DWI Titanium 25

5.1.7 SEM Image Discussion

All of the materials were made primarily from fibrillated cellulose, made into a nonwoven sheet that looks

and feels like paper. In addition, all of the materials appeared uniform and homogeneous, especially at

low magnification (images not shown). At high magnification, it can be seen that the competitor’s

materials are constituted primarily of fibers that range in diameter from 1-4 microns, with many fibers in

the range of 1-2 microns, and very few fibers less than 1 micron in diameter. The DWI separators, in

contrast, have a much higher population of fibers below 1 micron, and a large population of fibers in the

200 – 500 nanometer diameter range.

5.2 SEPARATOR PROPERTIES Table 2: Separator properties for each of the six separators tested, along with the average for each supplier.

Units Competitor 40 Micron

Competitor 30 Micron

Competitor 25 Micron

Average Competitor

DWI Titanium

40

DWI Titanium 30

DWI Titanium 25

Average DWI

Basis Weight

g/m2 28 20 12 20 21 18 18 19

Thickness (7.3 psi)

microns 43 32 30 35 39 33 28 33

Thickness (12.6 psi)

microns 41 31 28 33 37 32 27 32

Thickness (25 psi)

microns 38 27 22 29 35 30 25 30

Porosity % 55% 57% 72% 61% 60% 61% 54% 58%

Pore Size microns 0.44 0.6 0.6 0.55 0.96 1.4 1.0 1.1

Bubble Point

microns 1.5 1.7 3.4 2.2 2.9 4.3 3.2 3.4

Gurley seconds 171 80 104 118 37 39 55 44

MD Strength

kg/cm2 250 99 174.5 210 185 240 212

MD Modulus

kg/cm2 9400 6200 7800 17500 16000 20000 17833

Moisture Content

% 8% 7% 8% 8% 6% 5% 6% 6%

Separator properties are reported in Table 2, above, along with the average for each supplier. For

Competitor 30 micron, there was not enough material available for tensile tests. There are several

significant results:

Compressibility: All of the materials are compressible, changing thickness by 3-8 microns over the

pressure range tested. The competitor materials were slightly more compressible, compressing

an additional 6 microns compared to 3 for the DWI materials.

Porosity: For 30 and 40 micron materials, the DWI porosity was higher, at ~60% compared to

~56%. However, at 25 microns, the competitor material had significantly higher porosity.

Pore Size: Both the bubble point and pore size were higher for the DWI materials, with the bubble

points for the DWI materials all very similar to the competitor 25 micron.

Gurley: The DWI materials had much lower Gurley air resistance than the competitor materials.

Normally, this would correspond to lower internal resistance as well. See capacitance testing

below.

MD Strength & Modulus: All of the materials except the competitor 25 micron had MD strength

near 200 kg/cm2. The competitor 25 micron was significantly lighter weight and higher porosity

than the rest of the field, which resulted in a lower tensile strength and modulus.

Moisture Content: The DWI materials has 25% lower moisture content, likely due to the inclusion

of PVA microfibers rather than solely cellulosic materials.

5.3 SUPERCAPACITOR TESTING Table 3: Performance in supercapacitors for each of the six separators tested, along with the average for each supplier.

Units Competitor 40 Micron

Competitor 30 Micron

Competitor 25 Micron

Average Competitor

DWI Titanium 40

DWI Titanium 30

DWI Titanium 25

Average DWI

24 hr Self Discharge

% 53% 57% 56% 56% 52% 56% 52% 53%

Capacity Ah/g 0.024 0.023 0.027 0.025 0.025 0.026 0.029 0.027

Capacity F/g 30.4 29.4 34.5 31.4 32.4 33.2 36.8 34.1

ESR Ohm 9.5 17.6 8.6 11.9 11.2 6.8 8.1 8.7

The results of the testing of supercapacitors is shown above in Table 3. There are several results:

Self-discharge: Very little difference was seen between the materials in 24 hour self-discharge of

the cells. That the self-discharge is higher than productions cells is likely due to cell construction.

Capacitance Trends: On average the capacitance was higher for thinner materials, with the only

exception being the competitor 30 micron separator.

Capacitance Comparison: On average, the capacitors made with DWI separators had higher

capacitance, a total of 9% across the three separator types by each manufacturer, and higher at

each thickness.

ESR: As a whole, the ESR was 27% lower for the DWI separators than the competitor materials.

Most notably was the difference at 30 microns, with the competitor 30 micron material having

more than 150% higher ESR than DWI Titanium 30.

Lowest ESR: The lowest ESR by far was the DWI Titanium 30, with an ESR of 6.8 Ohm. The closest

competitor’s material was the 25 micron, with an ESR of 8.6 Ohm-cm.

5.4 COMMERCIAL SUPERCAPACITOR TESTS Two commercial one farad supercapacitors were also tested under the same protocol as above, and then

disassembled to determine electrode weight and area. Both commercial parts show lower ESR and lower

capacitance per gram, indicating that a higher surface area, lower electrode thickness strategy was taken

in order to reduce ESR to the lowest possible. The Maxwell part shows much lower ESR than that Nichicon

part. This compares to the parts made with the competitor’s separators and DWI separators, where the

capacitance is near the practical limit for carbon electrodes (these were double sided electrodes, but only

a single side was measured, which would effectively double the capacitance if a bulk supercapacitors were

made using both sides. The practical maximum is around 70 F/g.) From this comparison with commercial

supercapacitors, the following conclusions can be supported:

The capacity compares well, considering a high energy design.

The ESR compares as would be expected given the relative capacitances.

The self-discharge is lower for the commercial supercapacitors, which likely has to do with cell

design and formation processes, neither of which were optimized for the DWI and competitor

cells.

Units Maxwell (BCAP0001P270

T(0))

Nichicon (1F, 2.7V UM(M)

1205 PET)

Average Commercial

Average Competitor

Average DWI

24 hr Self Discharge % 29% 32% 31% 56% 53%

Capacity Ah/g 0.006 0.005 0.006 0.025 0.027

Capacity F/g 7.6 6.6 7.1 31.4 34.1

ESR Ohm 1.65 6.81 4.2 11.9 8.7

6 DISCUSSION AND

CONCLUSIONS

An interesting comparison can be made

between the competitor’s 30 micron material

and the DWI 30 micron material (Titanium 30).

In those cells, the DWI separator provided 13%

higher capacitance, 61% lower ESR, and

equivalent self-discharge. At 40 and at 25

microns, the products from the two

companies’ performance is more similar.

SEMS: Scanning electron micrographs (SEMs)

revealed that the competitor’s separators are

composed primarily of fibrillated cellulose

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Capacity ESR

Cap

acit

y (F

/g)

or

ESR

(O

hm

)

Supercapacitor Comparison

Leading Competitor 30micron Dreamweaver Titanium 30

microfibers with diameters in the range from 1-4 microns. DWI separators have similar composition, with

a much higher population of fibers with diameters in the range of 0.2 – 0.4 microns

Separator Properties: The separators from the different companies had, on average, similar basis weights,

thickness, and porosity. The pore size and bubble point for DWI separators was slightly higher, but with

the following advantages:

Gurley: a much lower (63%) Gurley air resistance.

Strength: 21% higher tensile strength

Modulus: 129% higher modulus

Moisture: 25% lower moisture content

Capacitor Performance: All of the materials showed a similar 24 hour self-discharge. On average, the

DWI materials showed 9% higher capacitance, and 27% lower ESR. In the most dramatic comparison, at

30 microns, the DWI Titanium 30 had 13% higher capacitance and 61% lower ESR, as shown in the graph

below.

In conclusion, DWI Titanium separators used a higher population of nano-sized fibers to provide higher

electrical conductivity and capacitance in a separator that is also stronger, with lower moisture content.

7 REFERENCES

7.1 TEST LABORATORIES Herty Advanced Materials Development Center: www.herty.com. Contact Martha Simmons,

msimmons@herty.com, (912) 963-2641.

Clemson University Electron Microscopy Facility: http://www.clemson.edu/centers-

institutes/cuadvancedmaterialscenter/electron-microscope/. Contact George Wetzel,

gwetzel@g.clemson.edu.

Porous Materials Incorporated: www.pmiapp.com. Contact Dr. Krishna Gupta, gupta@pmiapp.com, (607)

257-5544.

Polystor: www.polystor.com. Contact Dr. James Kaschmitter, james.Kasch@gmail.com, (925) 570-7251.

7.2 AUTHOR INFORMATION Dr. Brian Morin is co-founder, President & COO of Dreamweaver International. He may be contacted at

Brian.Morin@Dreamweaverintl.com, and at 864-968-3321.