A Continuum of Separator Options for Next Generation Lead‐Acid BatteriesR. Waterhouse, C. La, L. Keith, K. Hanawalt, D. Merritt, D. Walker, J. Moore, C. Rogers, J. Kim, J. Frenzel, M. Warren, J. Norris, D. Trueba, and R.W. Pekala

September 12 , 2014

Background

ENTEK focused on providing new low resistance separator to EFB/ECL market (start‐stop) without compromising mechanical properties

Explored key characteristics that influence electrical resistance % porosity Wettability  Tortuosity

Improvements achieved through modifications to formulation and process conditions

Worked with customers and verified improved battery performance

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Separator ER

Separator resistance is a function of electrolyte resistivity (acid) plus the design, pore structure, and composition of the separator.

Resistance of electrolyte within a porous structure (Ω): R = pLτ2 / P A 

Where  p = resistivity of the electrolyte,  f ([C], T)L = thickness of the separator (design)τ = tortuosity of the pore path (structure)P = porosity filled with acid (structure and composition)A = cross‐sectional area through which ions flow

New product attributes Higher porosity Lower tortuosity Sustained wettability through interface modification(s)

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Electrical resistance ‐‐‐ PALICO measurement

Electrical resistance of a PE separator is traditionally measured using the Palico Low Resistance Measuring System: RSES: the areal resistance of the Separator‐Electrolyte System RPalico: The resistance value in milliohms measured by the Palico instrument 

times the area of the aperture (32.26 cm²) RE: The areal resistance of the electrolyte that would occupy the same 

volume as the SES 4

V’1 V’2

I

V1 V2

I

R’ = (V’ 1-V’2)/I = R1 + RSES + R2With Separator

R = (V 1-V2)/I = R1 + RE +R2Without Separator

RPalico = (R’ – R) = (RSES – RE)

R1 RSES R2 R1 RE R2

Separator ER Model

5

V2

R SES

The resistivity of the electrolyte‐filled separator can be calculated for various separators from the ER measurement data and the geometry of the rib profile.

Separator schematic with major and minor ribs.The space filled by the acid‐containing separator and the space filled with acid comprise the separator‐electrolyte system (SES).

Rmajor

Relyte,bw

Rbw

Relyte,minor

V1

Rminor

Pore Structure

The pore structure of PE/silica separator is characterized by its heterogeneity: Hydrophilic silica aggregates of different sizes Hydrophobic polyethylene fibrils Differences in structure between surface (polymer‐rich) and bulk 

(silica‐rich) Not all pore volume is filled with electrolyte or stays filled with 

electrolyte Total pore volume ≠ acid accessible pore volume

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Constricted Pore

BlindPore

Variety of pores

SEM ‐‐‐ Bulk Structure

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UHMWPE

Silica Aggregates

SEM: Surface Structure ‐ Rib Side

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SEM: Surface Structure ‐ Flat Side

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Tortuosity – Electrical Resistance Measurement

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1.00

1.50

2.00

2.0 2.2 2.4 2.6 2.8 3.0

Tort

uosi

ty

SiO2/PE Ratio

Tortuosity vs. SiO2/PE Ratio - 0.25mm BW: Determined from ER Measurement

SiO2/PE Water Porosity Tortuosity2.3 57 1.372.75 60 1.312.9 60 1.28

Tortuosity – Diffusional Resistance Measurement

Separator samples were boiled in DI water for 10 minutes, and equilibrated at room temperature prior to measurement: Conductivity of the solution in the diffusate compartment was 

measured with time for 1 hour

Tortuosity was determined from the measured diffusional resistance.

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Conductivity Probe

Separator

KCl Feed Compartment: C0 = 1.0M

D = 1.9 X 10‐9 m2/sec

Diffusate Compartment

Reference: C. Labbez, et al., Desalination, 141 (2001) p. 291

Cf(t) Cd(t)

Tortuosity Measurement – Diffusional Resistance

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Diffusion through a membrane separating two compartments:

Slope of the left‐hand‐side of eq. (2) vs. time can be used to calculate the diffusional resistance

C0: initial concentration in the feed compartmentCf(t): concentration of electrolyte in the feed compartment at time tCd(t): concentration of electrolyte in the diffusate compartment at time tA: Separator area exposed to the solutionsV: volume of solution in one compartmentRd: Diffusional resistance of separator

εDτtR

2

d

t: separator thicknessτ : tortuosityD: Diffusivity of soluteε : porosity

ln[(C0-2Cd(t))/C0] vs. Time

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0 500 1000 1500 2000 2500 3000 3500 4000Time (sec.)

ln[(C

0-2C

d(t))

/C0]

Slope = -(2*A)/(V*Rd)

Diffusional resistance is related to tortuosity by:

(1)

ln (2)

Tortuosity Comparison

Difference between the two methods is 12‐15%: Electrolyte: H2SO4 vs. KCl Concentration: ~ 5M for H2SO4 vs. 1M for KCl

Increasing SiO2/PE ratio from 2.3 to 2.9 only results in a small increase in water porosity and a small decrease in tortuosity.

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1.00

1.50

2.00

2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

Tort

uosi

ty

SiO2/PE Ratio

Tortuosity vs. SiO2/PE Ratio - 0.25mm BW: Determined from Diffusional and Electrical Resistance Measurement

Diffusional Resistance Electrical Resistance

How does the new Low Resistance Separator differ from a traditional PE/SiO2 Separator?

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New 2.6

New 2.3

New 2.1

New 1.8

Low ER separator ‐ Tortuosity

Change in % porosity and pore size distribution leads to lower tortuosity.

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1.00

1.50

2.00

1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

Tort

uosi

ty

SiO2/PE Ratio

Tortuosity vs. SiO2/PE Ratio: Determined from Diffusional and Electrical Resistance Measurement

Determined from Diffusional Resistance

Determined from Electrical Resistance

Standard Separator

LER SeparatorProduct SiO2/PE

Water Porosity (%)

Tortuosity - Diffusion

Standard Separator 2.3 57 1.63Standard Separator 2.75 60 1.49Standard Separator 2.9 60 1.51

LER Separator 1.8 60 1.30LER Separator 2.6 64 1.35

Electrical Resistance

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New

New

New

Initial product Launch ‐ 162‐0.8‐0.25 GE_LR

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‐ 34%

+ 22%+ 21%

+ 13%

Battery Performance Results

Lower internal resistance:  Rbatt decreased by 0.07‐0.16 mΩ

Better Cold Cranking Performance EN U10: increased by 100 to 300mv. EN U30: increased by 60 to 140mv.

Excellent cycle life 50% DOD cycles 17.5% DOD cycles

Low water consumption

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Advanced Development Activities

MOTIVATION

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1. Low Resistance Separator2. Melt blown polymer fiber mats3. High porosity silica‐filled sheets4. Crosslinked polymer gels

Porosity Range

55%PE/SiO2 

Separators

93%AGM

How can we create separators with 60‐90% porosity ?

Melt‐blown Fiber Technology

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Air In

Polymer Melt In

Attenuated Fiber

Air

Spinning Nozzles

Spinning Nozzles

Air Gap

Bottom View

Sectional View

Air Gap

Orifice

Polymer Melt In

Bottom View Sectional View

1. AGM Substitute2. Extraction‐Free SLI separators

AGM – BGO Grade

SEM Comparison

Fiber size comparable to AGM can be achieved. Polymer fiber mats exhibit sustained wettability

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Polymer Melt Blown Fiber Mat

Porosity and Wicking Behavior

Wicking height was determined after 2 minutes in H2SO4, 1.28 SG.

Wicking height varied between 62mm to 72mm: Comparable to the H&V AGM sample –

65mm 23

50

55

60

65

70

75

80

85

90

95

100

1 2 3 4 5

Percen

t Porosity

Lane Number

Run #1 Run #3 Run #4 Run #9

Porosity between 86% and 89% was achieved.0

10

20

30

40

50

60

70

80

1 2 3 4 5Wicking

 Height (mm)

Lane Number

Run #1 Run #3 Run #4 Run #9

Compressibility Behavior

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Melt blown fiber mat and AGM samples had the same initial thickness –about 1.7mm measured at 10kPa, dry.

Melt blown polymer fiber mat demonstrates minimal shrinkage upon wetting, and more compression resistance under stress.

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

2.00

0 10 20 30 40 50 60

Fibe

r She

et T

hick

ness

(mm

)

Compressive Stress (KPa)

Melt Blown Fiber Mat: Sheet Thickness vs. Compressive Stress

MBFM, Dry State MBFM, Wet StateAGM, Dry State AGM, Wet State

AGM

Melt Blown Polymer Fiber Mat

Calendered Sheet

The fiber mats were calendered using one groove roll and one flat roll: Longitudinal rib profile was introduced on the calendered fiber mats Precusor fiber mats were densified more aggressively

75%‐80% densification Final backweb thickness ~ 0.4mm

Preferred thickness is 0.15‐0.25mm

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Rib profile

Electrical Resistivity

Samples were soaked in H2SO4, 1.28 SG, at room temperature for 20 minutes prior to ER measurement.

Electrical resistance depends on the extent of calendering and the final porosity. The instantaneous wettability in H2SO4 allows the calendered polymer fiber mats to 

achieve soaked electrical resistivity that is better than the boiled electrical resistivity of a standard PE/SiO2 separator.

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0

1000

2000

3000

4000

5000

6000

MBFM140407.4

MBFM140421.1

MBFM140421.3

MBFM140421.4

MBFM140421.9

0.25mmStandard PE

Separator

Elec

tric

al R

esis

tivity

(mΩ

-cm

) Boiled ER Value

Mechanical Properties – BW Puncture

When normalized for backweb thickness, the calendered polymer fiber mats exhibit puncture strength comparable to a PE/SiO2separator

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0

10

20

30

40

50

60

0

5

10

15

20

25

MBFM140407.4

MBFM140421.1

MBFM140421.3

MBFM140421.4

MBFM140421.9

0.25mmStandard PE

Separator

Nor

mal

ized

Pun

ctur

e St

reng

th (N

/mm

)

Punc

ture

Str

engt

h (N

)Puncture Strength Normalized Puncture Strength

SEM: High OA Silica‐filled Sheets

FG silica‐filled sheet at SiO2/PE=5 has similar porosity with larger amount of PE fibrils that enhance mechanical properties

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TX; SiO2/PE = 10FG; SiO2/PE =5

0

0.5

1

1.5

2

2.5

3

3.5

4

0.0010.010.1110

Log diffe

rential intrusion

 (mL/g)

Pore size diameter (μm)

FG/PE=5

AB/PE=5

TX/PE=10

UK 0.15 mm 2.6 control

Hg Porosimetry ‐ Pore Size Distribution

Highly filled silica sheets show larger pore size distribution and higher tortuosity compared with standard separator 29

Crosslinked Polymer Gel Formation in Sulfuric Acid

30

O

NH2

acrylamideHN

HN

O

H2N

O O

H2N

O

HN

C OC O

H2NO

NH

O

NH

N,N'-methylene-bis-acrylamide

C O

H2N

C O

HN

(NH4)2S2O8+

Lead

10% PA gel in H2SO4

SEM: Polyacrylamide Gel (Supercritical Drying)

31

Cycle Performance

Test conditionCharge rate=1CCut-off voltage in charge= 2.45 VCut-off voltage in discharge= 1.3 V

Cell performance is similar to controls with AGM separator

320

0.02

0.04

0.06

0.08

0.1

0.12

0 10 20 30 40 50 60

Resistance (Ω)

Cycle

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50 60

Discharge capacity (A

h)

Cycle

PAM009 (10% polymer in prismatic cell )

Prismatic cell assembly

Summary

Value creation through product and manufacturing innovation is the foundation of ENTEK

ENTEK is working on next generation separators for deep cycle, start‐stop, and high power battery applications

ENTEK would like to work with battery manufacturers on new growth opportunities that could change the way that separators are integrated into Pb‐acid cell designs

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