Richard S. Baldwin

Glenn Research Center, Cleveland, Ohio

Monica Guzik

Rose-Hulman Institute of Technology, Terre Haute, Indiana

Michael Skierski

Michigan State University, East Lansing, Michigan

Properties and Performance Attributes of Novel

Co-extruded Polyolefi n Battery Separator Materials

Part 1: Mechanical Properties

NASA/TM—2011-216979/PART1

March 2011
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Richard S. Baldwin

Glenn Research Center, Cleveland, Ohio

Monica Guzik

Rose-Hulman Institute of Technology, Terre Haute, Indiana

Michael Skierski

Michigan State University, East Lansing, Michigan

Properties and Performance Attributes of Novel

Co-extruded Polyolefi n Battery Separator Materials

Part 1: Mechanical Properties

NASA/TM—2011-216979/PART1

March 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors would like to express their appreciation for project support from the NASA Exploration Technology Development 

Program (ETDP). Also, support from two NASA Glenn University Programs student internship opportunities—the NASA 

Undergraduate Student Research Program (USRP) and the NASA Lewis’ Educational and Research Collaborative

Internship Program (LERCIP)—was appreciated.

Available from

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either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-216979/PART1 1 

Properties and Performance Attributes of Novel Co-extruded 

Polyolefin Battery Separator Materials 

Part 1: Mechanical Properties

Richard S. Baldwin 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Monica Guzik 

Rose-Hulman Institute of Technology 

Terre Haute, Indiana 47803 

Michael Skierski 

Michigan State University 

East Lansing, Michigan 48824 

Abstract 

As NASA prepares for its next era of manned spaceflight missions, advanced energy storage 

technologies are being developed and evaluated to address future mission needs and technical 

requirements and to provide new mission-enabling technologies. Cell-level components for advanced 

lithium-ion batteries possessing higher energy, more reliable performance and enhanced, inherent safety 

characteristics are actively under development within the NASA infrastructure. A key component for safe 

and reliable cell performance is the cell separator, which separates the two energetic electrodes and 

functions to prevent the occurrence of an internal short-circuit while enabling ionic transport. Recently, a 

new generation of co-extruded separator films has been developed by ExxonMobil Chemical and 

introduced into their battery separator product portfolio. Several grades of this new separator material 

have been evaluated with respect to dynamic mechanical properties and safety-related performance 

attributes. This paper presents the results of these evaluations in comparison to a current state-of-thepractice separator material. The results are discussed with respect to potential opportunities to enhance the 

inherent safety characteristics and reliability of future, advanced lithium-ion cell chemistries. 

Introduction 

As NASA embarks on a renewed human presence in space, human-rated electrical energy storage and 

power generation technologies that demonstrate reliable performance in a variety of unique mission 

environments will be required. To address the future performance and safety requirements for the energy 

storage technologies that will enhance and enable future aerospace missions, advanced rechargeable 

lithium-ion battery technology development has been pursued within the NASA Exploration Technology 

Development Program’s (ETDP) Energy Storage Project, with an emphasis on addressing performance 

technology gaps between state-of-the-art capabilities and critical future mission requirements. 

Recently, lithium-ion batteries have been qualified to provide electrical power needs for life support 

hardware associated with manned extravehicular activities in space. Currently, both cell and cell 

component development efforts are focused on improving the overall safety, as well as the 

electrochemical attributes, of such battery systems in order to optimize their performance for manned 

space applications. A key cell component of a lithium-ion battery which significantly impacts both of 

these performance features is the battery separator. 
NASA/TM—2011-216979/PART1 2 

Battery Separator Properties and Characteristics for 

Rechargeable Lithium-based Cells 

The function and reliability of the separator is critical for the optimal performance and safety of a 

lithium-ion cell. In nonaqueous, liquid-electrolyte cells that exemplify a lithium-ion cell chemistry, the 

separator is typically a non-electrically-conducting porous electrolyte-filled media or membrane, which is 

sandwiched between and in contact with the two active, solid electrodes. Its roles are to prevent direct 

electronic contact between the two electrodes, which would result in a short-circuit, and to allow the flow 

of ionic species within the cell. Certain separator materials can also function as internal cell safety 

devices. The separator affects the internal cell resistance, stability, cycle-life, operating temperature range 

and cell kinetic parameters such as discharge and charge rates. By its ability to regulate the electrolyte 

distribution between the active electrodes, as a result of its morphology and permeability characteristics, 

the separator limits ionic diffusion and recombination rates, and, thus, impacts cell capacity, power and 

available energy. With respect to the inherent safety of a lithium-ion cell, the separator may afford passive 

protection and tolerance to cell abuse conditions that may arise, and it can enhance the overall cell safety 

by providing another level of internal redundancy (i.e., complement cell designs that incorporate safety 

devices such as a positive temperature coefficient (PTC) resistor and/or a current interrupt device (CID)). 

The structural and physiochemical properties of the separator material strongly influence the overall 

cell performance, although the separator does not “actively” participate in the battery operation. The mere 

presence of the separator component within a cell adversely affects both electrical and mass-related 

performance, as it adds electronic resistance and occupies limited space. For electrochemical cell 

chemistries in which the liquid electrolyte solution serves only as an ionic conductor, the separator should 

be as thin as possible and porous, but yet possess the required physical strength to maintain the 

mechanical and electrical separation between the solid electrodes. These characteristics are essential in 

order to achieve high energy and power densities for applications requiring such performance attributes. 

Thus, the practical design of a separator is an art, as there is a compromise between the degree of porosity 

of the material and its mechanical strength. For battery safety, a battery separator for utilization with 

lithium-based cell chemistries should have the inherent ability to shut the battery down if overheating 

occurs. Several comprehensive reviews of battery separators in general (Refs. 1 and 2) have appeared in 

the literature, as well as some with a primary emphasis on lithium-based cell technologies (Refs. 3 to 5). 

Recently, a NASA applications-focused summary review was prepared which elucidated primary 

separator properties and characteristics for rechargeable lithium-based cells, how the separator properties 

influence cell performance and safety and how degradation of the separator component may result in 

overall cell failure (Ref. 6). 

In summary, the overall mechanical strength of the separator is characterized in terms of several 

factors, each possessing desired requirements and performance metrics. Tensile strength and related 

properties are strongly dependent upon the material manufacturing process. Uniaxially-oriented separator 

films are generally preferred for material dimensional stability, as they exhibit a minimal tendency to 

shrink at elevated temperatures or when exposed to electrolyte. High puncture strength is important to 

prevent the penetration of particulate electrode material through the separator, especially during cell 

assembly, which could lead to an electrical short circuit. High mechanical strength is important, 

especially in a uniaxially-oriented separator film’s machine direction, as the material is subjected to 

considerable tension in this direction during the winding operation of a spiral-wound battery fabrication 

procedure, and contraction of the width during the operation would be undesirable. Biaxially-oriented 

films are more uniformly strong in both the machine and transverse directions. 

State-of-the-Art Microporous Polymeric Membranes 

Presently, most commercially-available lithium-ion spiral-wound cylindrical and prismatic cells and 

flat-plate cells containing a liquid organic electrolyte employ a microporous polymeric membrane 

separator due to advantages of performance, safety, and cost. These separators are based on polyolefin 
NASA/TM—2011-216979/PART1 3 

materials, such as polyethylene (PE), polypropylene (PP) and blends of such. Such polyolefin-based 

separators, which afford both excellent chemical stability and mechanical properties, are manufactured by 

either a wet or a dry process, both of which employ orientation steps to introduce porosity and increase 

tensile strength. The wet process, or phase inversion process, involves mixing a polyolefin resin with a 

hydrocarbon liquid or low molecular weight oil, melting the mixture, extruding the melt into a sheet, 

orientating the sheet and extracting the liquid phase. An advantage of the wet process is that separator 

films can be created which exhibit uniform microporous fibril structures and uniform physical properties, 

which can provide stable battery performance and reliable safety. Polyolefin battery separators made by 

the wet process are available from several sources, such as a widely used polyethylene-based material 

from ExxonMobil Chemical and its Japanese affiliate Tonen Chemical. 

For practical lithium-ion cell performance most microporous separators have a thickness well below 

50 μm and average pore diameters below 1 μm. For high energy density applications, separator 

thicknesses of 25 μm are desirable in order to achieve a lower internal resistance and higher rate 

capability, however, the safety concern with respect to mechanical integrity and penetration must be 

considered. The tortuous and interconnected porous structure of a wet-process made material is more 

suitable for a cell with a long cycle life requirement. This is because the resulting microstructure from the 

wet-process aids in suppressing the growth of dendritic lithium metal on the anode surface during fast or 

low-temperature charging (Ref. 4). Desired values of key mechanical properties of separators for 

conventional lithium-ion cell chemistries are shown in Table 1. 

TABLE 1.—KEY SAFETY-RELATED MECHANICAL PROPERTIES OF 

LITHIUM-ION CELL SEPARATOR MATERIALS (REF. 2). 

Co-extruded ExxonMobil Battery Separator Grades 

In the 2006-07 timeframe, the introduction of novel co-extruded battery separator grades by 

ExxonMobil Chemical created new opportunities in lithium-ion battery technology innovation and 

performance improvements. Based upon wet, bi-orientation process technology, the co-extruded 

polyolefin grades afford a new flexibility and versatility to the company’s commercial battery separator 

product portfolio, which had a heritage of mono-layer grades based upon polymer membrane technology 

attributed to their affiliate, Tonen Chemical. Tonen, which is based in Japan, had supplied the 

microporous film for the first lithium-ion battery introduced in 1991 by Sony. 

The ExxonMobil co-extruded multi-layer separator materials, which are based on tailored, heatresistant polymer formulations, feature significant improvements in both porosity, which leads to lower 

internal cell resistance, and thermal properties compared to their predecessors (Ref. 7). Improved thermal 

meltdown properties can extend the mechanical integrity of the separator film for a longer time during a 

thermal event, giving the cell more time to dissipate the generated heat, thus, decreasing the probability of 

a more catastrophic event. 

The co-extrusion process for polymeric film formation is based upon bringing selected thermoplastic 

polymers simultaneously together in an extrusion die, and they subsequently exit from the die in a planar 

form as a multi-layer structure. Each layer is formed by a separate polymer feed stream into the extrusion 

die, and the discrete layers are bonded together at their interface by a commingling of the polymers. 

Generally, the feed streams are of dissimilar polymers with respect to their chemical nature (e.g., 

polyethylene (PE) and polypropylene (PP)) and/or polymers of similar chemical nature but having 

Cell Safety-related Separator Characteristics 

• Low dimensional shrinkage (5 percent) 

• High puncture strength (300 g/mil) 

• Tensile strength (1000 kg/cm2) 

• Elevated-temperature melt integrity (150 C) 
NASA/TM—2011-216979/PART1 4 

dissimilar properties (e.g., molecular weight, rheology, density, additives, etc.). Additives to the polymers 

can modify or enhance the performance or properties of an individual layer or the overall separator, and 

such modifications may include: 

(1) Lowering the melting temperature of the polymer. 

(2) Improving the melt integrity of the film (e.g., adding inorganic fillers, fine fibers, high melting 

point organic materials, etc.) 

(3) Improving the separator strength (e.g., adding elastomers, fine fibers, etc.) 

(4) Improving the separator surface wettability (e.g., adding wetting agents) 

(5) Improving the polymer processing (e.g., adding waxes, fluoropolymers, etc.) 

(6) Improving the separator flame retardant nature (e.g., adding phosphate esters, etc.)

After extrusion of a multi-layered, nonporous precursor separator, the precursor is processed (e.g., by a 

wet process) to form the micropores of desired porosity and tortuosity. 

The shutdown property of a polymeric separator material can internally provide a margin of safety 

against an external short-circuit, accidental overcharge or an abuse condition resulting in an elevated cell 

temperature. A shutdown separator closes off its micropores when an internal temperature threshold (e.g., 

130 C) is exceeded, essentially shutting down the activity (e.g., the flow of ions) of the battery. In a 

scenario where the internal temperature is rising rapidly, however, the separator film can melt, thereby 

eliminating the physical barrier. Per company literature, the ExxonMobil Chemical/Tonen-developed film 

can retain the original shutdown temperature of the polyolefin material and, additionally, retain its 

mechanical integrity for an additional 40 C. By not melting until a higher temperature is reached, the 

new separator film provides more time for the malfunctioning cell to dissipate its heat, reducing the 

potential for transferring the heat to its neighbors and setting off a larger thermal event (Ref. 9).Enhanced 

thermal mechanical properties of the co-extruded grades are illustrated in Figure 1, which is 

representative strain versus temperature data from ExxonMobil’s product literature. As illustrated in the 

figure, a larger temperature differential (i.e., safety enhancement) between the shutdown and meltdown 

temperatures is evident for the co-extruded material, although the two materials have similar visual 

appearances as the temperature is increased. 
NASA/TM—2011-216979/PART1 5 

Experimental 

Samples of the co-extruded polyolefin battery separator films under investigation in this report were 

obtained from the ExxonMobil Chemical Company, Macedon, New York. Experimental laboratory 

assessment results for the key mechanical properties for four grades of co-extruded films, which are 

identified by ExxonMobil as V25EKD, V25CGD, V20EHD, and V20CFD, are presented in this report. 

The numerical designation in the identification label signifies the nominal film thickness in microns (μm). 

For comparisons of the material properties and performance attributes of the new co-extruded grades to a 

state-of-the-practice separator, a microporous polyethylene Tonen SETELA separator, grade E16MMS 

was employed. This mono-layer grade, which has been produced in Japan since the early 1990’s, 

presently serves as the standard separator component in some lithium-ion cell designs. All separator 

samples were inspected for uniform appearance and thickness, and were stored in a low relative humidity 

(2 percent) dryroom environment. 

The procedure (Ref. 10) for determining the puncture strength of a separator sample utilized a blunt 

tip on the end of a digital force gauge which recorded the force at which the separator was punctured 

while slowly moving the tip into the material using a hand-wheel motored test stand. Specifically, an 

Imada DS2 force gauge (20 N or 2 kilograms-of-force (kgf) capacity) was employed and operated in 

conjunction with accompanying penetrating pins and a vertical hand wheel test stand. 

For the characterization of key separator sample mechanical and rheological properties, including 

sample mechanical integrity at elevated temperatures for an assessment of safety performance, dynamic 

mechanical analysis (DMA) (Ref. 11) techniques were employed, utilizing a TA Instruments Q800 

Dynamic Mechanical Analyzer, which is shown in Figure 2, located in a low-humidity dryroom. The 

Q800 DMA instrument can operate over a wide temperature range (–150 to 600 C) and provides 

multiple modes of deformation including dual/single cantilever and 3-point bending, tension, 

compression, and shear. The sample clamps are individually calibrated for data accuracy and the elegant 

but simple design facilitates sample mounting. 

In conjunction with pertinent, property-specific analytical methodologies, the DMA instrumental 

method is used to study a material’s response to stress, temperature and frequency, among other variables. 

A sinusoidal, constant (step) or fixed-rate force is applied to a separator sample to cause a mechanical 

deformation and to measure the sample’s response as a function of temperature or time. Specific 

experimental procedures for the measurement and analysis of a separator sample’s (1) thermal shrinkage, 

(2) tensile strength, and (3) melt integrity characteristics can be found in Reference 10. The tabulated 

results presented in this report for these characteristics represent average values for at least five 

experimental sample trials. 
NASA/TM—2011-216979/PART1 6 

Results and Discussion 

Lithium-ion Cell Performance and Safety—Separator Effects 

Although classified as an inactive cell component, the separator plays a critical role in obtaining 

practical and optimal cell-level performance and the inherent safety of the cell. In addition to initially 

possessing optimal properties for meeting the requirements for a specific application, the material 

properties must be uniform and stable over the desired application/mission life. With respect to cell and 

battery-level safety, the internal cell separator can complement safety control circuits possessing 

redundant safety features (e.g., CID, PTC, vent, etc.). 

In order to achieve good performance, the separator should possess a uniform pore structure and 

exhibit desired dimensional stability (e.g., low shrinkage) and a high ionic conductivity when imbibed 

with electrolyte. The separator property of high ionic conductivity, which is a function of multiple 

material properties, has a pronounced effect upon such cell properties as capacity, rate performance, fast 

charge capability, overall cell resistance and cycle life. For safety concerns, the advantages of utilizing a 

thin separator must be weighed against the reduction in mechanical integrity, and the cell design should 

reflect such. External mechanical abuse (e.g., “nail” penetration or crush) or an internal short-circuit could 

result in an abnormal increase in cell temperature, which, as discussed below, could seriously 

compromise safety. If an undesirable cell overcharge condition occurs, dendritic lithium metal could form 

on the negative electrode surface, which could penetrate the separator and result in an internal shortcircuit and an undesirable increase in internal cell temperature. 

Thermal Properties and Separator Shutdown Performance 

If a lithium-ion cell is accidentally overcharged or abused, heat can be generated that could seriously 

compromise cell and battery-level safety, which is especially critical for human-rated applications. Above 

a threshold temperature, a “self-heating” condition could occur due to exothermic reactions occurring 

internally within the cell (Ref. 12). Such reactions may include reactions between lithium and electrolyte 

and the thermal decomposition of internal cell components. If the internal heat generation is allowed to 

continue, a catastrophic “thermal runaway” condition could occur, which would be a serious safety 

concern for a manned application. 

For battery safety with lithium-based cell chemistries, especially for manned applications, it is 

desirable for a state-of-the-art battery separator to possess an inherent ability to shut the battery down 

(i.e., terminate the flow of electric current) if overheating occurs. Near the melting temperature of the 

specific polymeric material, the pores collapse, and this event is accompanied by a significant increase in 

both the separator and overall cell impedances, resulting in a moderation of electrochemical activity and 

current flow. Optimization of the shutdown response and efficiency may be achievable by tailoring 

material properties and processing methods. 

The separator material should also possess high-temperature melt integrity, and exhibit mechanical 

robustness above the shutdown temperature, as characterized by a high rupture temperature. After a 

separator shutdown occurrence, the cell temperature is likely to continue to increase. The separator must 

maintain mechanical integrity and high impedance at elevated temperatures in order to prevent the 

electrodes from making physical contact and creating the safety hazard of an internal short-circuit. 

Ideally, the mechanical integrity of the separator should be maintained for a long enough period of time 

for heat to be sufficiently dissipated and a thermal cool-down to occur. Dynamic mechanical analysis 

(DMA) provides a measure of a material’s melt integrity. 
NASA/TM—2011-216979/PART1 7 

Mechanical Properties of ExxonMobil Polyolefin Battery Separator Materials 

Puncture Strength 

Puncture strength is the weight/force that must be applied to a needle or probe to force it completely 

through a separator. It is used to indicate the tendency of a cell separator to be resist penetration and 

internal short-circuiting that may be caused by the rough surface of an electrode during the battery 

assembly or by electrode surface changes that can occur during charge-discharge cycles. If particulate 

material from the electrodes penetrates the separator, an electrical short will occur, creating a safety 

hazard. The puncture strength should be at least 300 grams-of-force per mil of thickness (g/mil) for 

lithium-ion cells (Ref. 2). 

Experimentally-measured relative values of puncture strength for the four co-extruded separator films 

and for the “baseline” Tonen, E16MMS, mono-layer grade material is presented in Figure 3, along with 

ExxonMobil literature values. 

Figure 3 compares the average film experimental puncture strength values for at least five samples of 

each film grade, and the results are not normalized with respect to film thickness, so as to afford direct 

comparisons of the different films. In each material grade designation, the numerical portion refers to the 

material thickness in microns. The experimentally-measured numerical values are not absolute material 

specifications, as the measured values are only relative to the specific test conditions and methodology 

employed in our laboratory, and this is reflected by the differences shown between the experimental and 

the company-projected values in the figure. Comparing the Tonen E16MMS grade with the thicker coextruded grades, the V20EHD and V25EKD grades show an enhanced safety feature with respect to 

puncture strength, which is coupled with a higher porosity (i.e., a higher power capability) for all four of 

the co-extruded grades. 
NASA/TM—2011-216979/PART1 8 

Dynamic Mechanical Properties—Tensile Strength 

The separator is typically wound with the electrodes under tension during cell fabrication. The 

winding machine combines the two electrodes and separator strip on a mandrel and winds the 

combination in a jellyroll fashion into a cell core. Cylindrical cells are wound on a round mandrel while 

prismatic cells utilize a flat paddle. The separator must not elongate significantly under the tension or 

change width. A tensile strength specification is sometimes given, but the key parameter is Young’s 

modulus in the machine direction (MD). As Young’s modulus is difficult to measure, the “2 percent 

offset yield” is a viable measure; less than “2 percent offset” at 1000 psi is acceptable for many winding 

machines (Ref. 2). This “offset yield strength” represents an approximation of the elastic limit of the 

material, and is expressed in terms of percent strain. It is the stress that corresponds to the point of 

intersection of a stress-strain graph and a line parallel to the straight line portion of the graph. Offset 

refers to the distance between the origin of the stress-strain diagram, and the point of intersection of the 

parallel line and the zero stress axis. 

A separator should be strong enough to withstand mechanical handling during cell winding and 

assembly. It should be dimensionally stable and should not neck down (i.e., contract in width) during 

winding. A decrease in width will allow the electrodes to touch each other and create a short. Thus, the 

tensile property of the separator should be very strong in the machine direction (i.e., winding direction). 

The strength in the transverse direction is not as critical as that in the machine direction during the process 

of making spiral-wound batteries (Ref. 13). Flat-plate cells often utilize a wound electrode-separator 

sandwich, also, which is heated and pressed flat. For weaved or stacked cell components, which are held 

together be pressure, a tension-applying winding operation would not necessarily be employed in the cell 

fabrication. 

While the separator is under tension and the percent strain is increased on the sample, the film 

experiences two types of deformations (1) elastic deformation, which is the distance that the film can be 

stretched and still return to its initial length, and (2) plastic deformation, in which the film is continually 

being stretched and becoming permanently deformed. The instrumental DMA analysis performed on the 

separator samples provides the following multiple indicators of separator strength while under tension: 

(1) Yield strength—Occurs at the transition from elastic to plastic deformation 

(2) Ultimate strength—Maximum stress that a material can undergo while under tension 

(3) Breaking strength—Stress on the material at the point of rupture 

Representative graphical tensile strength data for a single experimental trial for the ExxonMobil separator 

films are shown in Figures 4 and 5 for the machine and transverse directions, respectively. Average 

results from the multiple trials are shown in Tables 2 and 3. 

TABLE 2.—AVERAGE EXPERIMENTAL MACHINE DIRECTION TENSILE 

STRENGTH RESULTS FOR EXXONMOBIL SEPARATOR FILMS 

Material Displacement at 

break, 

μm 

Strain at 

break, 

percent 

Ultimate 

stress, 

MPa 

Ultimate 

strain, 

percent 

Stress at 

yield, 

MPa 

Young’s 

modulus, 

MPa 

V20CFD 16532 222.6 32.91 205.0 18.3 153.2 

V25CGD 19490 251.6 49.93 231.1 24.3 178.0 

V20EHD 19030 240.2 50.96 205.04 26.14 210.3 

V25EKD 17128 226.6 46.11 172.70 22.44 138.0 

E16MMS 18190 235.3 57.28 216.2 16.30 156.6 
NASA/TM—2011-216979/PART1 9 

TABLE 3.—AVERAGE EXPERIMENTAL TRANSVERSE DIRECTION TENSILE 

STRENGTH RESULTS FOR EXXONMOBIL SEPARATOR FILMS

Material Displacement at 

break, 

μm 

Strain at 

break, 

percent 

Ultimate 

stress, 

MPa 

Ultimate 

strain, 

percent 

Stress at 

yield, 

MPa 

Young’s 

modulus, 

MPa 

V20CFD 14152 171.5 54.48 130.5 36.26 305.4 

V25CGD 18071 227.9 45.65 162.5 26.04 106.9 

V20EHD 15538 191.5 69.60 166.1 45.75 176.5 

V25EKD 15738 189.2 46.12 145.3 39.72 137.7 

E16MMS 10570 127.2 67.21 92.63 42.31 301.8 

 

Examining Figures 4 and 5, as the percent of strain applied to the film is increased, the amount of 

stress applied is also increased. When comparing the observed data for the machine direction to the 

transverse direction, the machine direction tends to require a higher percent strain in order for the 

separator to rupture. This is because the separator experiences a lower stress when transitioning from 

elastic to plastic deformation, therefore more strain can be applied before film rupture occurs. The point at 

which the graph turns from a relatively steep initial slope to a more gradual slope is where the 

transformation between elastic and plastic deformation occurs. In general, the tensile properties in both 

directions are of similar magnitude for all of these film grades, as they were fabricated by a wet-process, 

as described earlier, and do not exhibit anisotropy. Differences in tensile properties between the various 

grades can be attributed to differences in material composition and film processing. It can also be noted 

that as Young’s modulus increases for the co-extruded grade materials in both the machine and transverse 

directions, there is a tendency for the ultimate stress to increase as well. In general, the percent strain at 

the breaking point for the co-extruded film grades was comparable to that for the E16MMS film in the 

machine direction, whereas significantly higher percent strain values were observed for the co-extruded 

films in the transverse direction. 
NASA/TM—2011-216979/PART1 10 

Dynamic Mechanical Properties—Thermal Shrinkage 

Lithium-ion batteries can be poisoned by water, and therefore materials going into a cell are typically 

dried at 80 C under vacuum. Under these conditions the separator must not shrink significantly and 

certainly must not wrinkle. The requirement of 5 percent shrinkage after 60 min at 90 C (in vacuum), is 

a reasonable generalization for comparing the shrinkage characteristics of separator films that may 

undergo drying protocols or be subjected to elevated temperatures. 

The procedure to evaluate thermal shrinkage utilizing the DMA employed a very low constant 

0.001N applied force that simulated holding the separator sample at a constant temperature with minimal 

applied tension. The temperature was rapidly ramped to 90 C and then held isothermal for 1 hr. The 

displacement of the sample serves as an indicator of the shrinkage in the particular direction the sample 

was tested in. Representative graphical thermal shrinkage data for a single experimental trial for the 

ExxonMobil separator films are shown in Figures 6 and 7 for the machine and transverse directions, 

respectively. Average results from the multiple trials are shown in Tables 4 and 5. 

 
NASA/TM—2011-216979/PART1 11 

TABLE 4.—AVERAGE EXPERIMENTAL MACHINE DIRECTION THERMAL 

SHRINKAGE RESULTS FOR EXXONMOBIL SEPARATOR FILMS

Material Average displacement, 

mm 

Average shrinkage, 

percent 

V20CFD 0.0947 0.8201 

V25CGD 0.1574 1.3566 

V20EHD 0.0679 0.5181 

V25EKD 0.0805 0.5867 

E16MMS 0.3237 2.7761 

TABLE 5.—AVERAGE EXPERIMENTAL TRANSVERSE DIRECTION THERMAL 

SHRINKAGE RESULTS FOR EXXONMOBIL SEPARATOR FILMS 

Material Average displacement, 

mm 

Average shrinkage, 

percent 

V20CFD 0.0544 0.4254 

V25CGD 0.0468 0.3945 

V20EHD 0.0029 0.0244 

V25EKD 0.0714 0.5432 

E16MMS 0.1975 1.6986 

Comparing the thermal shrinkage results in both machine and transverse directions, the average 

displacement and average shrinkage values for all of the film grades were higher in the machine direction. 

Even though these separator grades tend to shrink to a greater degree in the machine direction, none of the 

samples approached the maximum separator requirement limit of 5 percent shrinkage. As represented in 

Figure 6, the co-extruded grades exhibit significantly less thermal shrinkage than the E16MMS grade in 

the machine direction. Most of the co-extruded grade separators had relatively low thermal shrinkage 

results; all of them had less than 2 percent shrinkage in the machine direction and less than 1 percent in 

the transverse direction. 
NASA/TM—2011-216979/PART1 12 

Dynamic Mechanical Properties—Melt Integrity 

Melt integrity is a critical property that affects the mechanical and electrical properties of a battery. 

Separators with good mechanical integrity over a wide temperature range can provide a greater margin of 

safety for lithium-ion cells by preventing the electrodes from contacting one another. The temperature at 

which the separator loses physical integrity (viscosity becomes low enough to allow the electrodes to 

contact) can be identified by measuring the elongation of a separator film under load (~5 g/cm) as a 

function of temperature. 

Utilizing thermal mechanical analysis (TMA) with a DMA, the separator film is held under a constant 

load/stress with a temperature ramp, and the degree of elongation or displacement is measured. The 

temperature at which the elongation increases dramatically is when the separator loses its mechanical 

integrity and, thus, its safety factor. 

Typically, polyolefin separators show some shrinkage initially, and then start to elongate and, 

ultimately, break (rupture). This test utilizes a small separator sample (about 10 mm length (MD) and 

about 3 mm width (TD)), which is held in the film tension clamp. The sample is held under a small, 

constant 0.02 N force while the temperature is ramped at 5 C per minute past the melting point of the 

material until the tension ruptures the film. Three parameters are reported from the procedure: 

(1) Shrinkage onset temperature 

(2) Deformation temperature 

(3) Rupture temperature 

The locations of these temperature parameters on a typical melt integrity analysis curve plotting film 

displacement as a function of temperature are illustrated in Figure 8, in which curves for both a 

monolayer Tonen polyethylene material and a multi-layer (i.e., PP/PE/PP) Celgard separator material are 

shown for comparison. For enhanced internal cell safety, separators with a rupture temperature greater 

than 150 C are desirable for lithium-ion cells. 

In Figure 9, representative melt integrity data for a single experimental trial for the ExxonMobil 

separator films are presented graphically. Average results from multiple trials are summarized in Table 6. 

In these analyses, a chosen deformation temperature is representative of the transition from sample 

shrinkage to elongation. Melt integrity data was only compiled for displacement in the machine direction 

(MD); duplicate trials with the Tonen E16MMS film samples verified identical graphical results in both 

the MD and TD directions. 
NASA/TM—2011-216979/PART1 13 

TABLE 6.—AVERAGE EXPERIMENTAL MELT INTEGRITY TEMPERATURE PARAMETERS 

FOR EXXONMOBIL SEPARATOR FILMS 

Material Shrinkage onset temperature,

C 

Deformation temperature,

C 

Rupture temperature, 

C 

V20CFD 125.38 150.15 178.38 

V25CGD 129.34 153.8 181.4 

V20EHD 157.3 167.7 191.1 

V25EKD 142.68 162.92 183.63 

E16MMS 121.35 160.3 173.06 

As illustrated in Figure 9, the graphical representation shows a general trend for the co-extruded 

grades—for a grade with a higher observed deformation temperature, a higher rupture temperature is also 

observed. The V25EKD and the V20EHD materials exhibiting the highest rupture temperatures of the 

four co-extruded grades. As the films are being heated, between their shrinkage onset temperatures and 

their defined deformation temperatures the polyolefin fibril compositions are undergoing melting, which 

is accompanied by a decrease in the film porosity. If this occurred in a lithium-ion cell, the separator 

would be undergoing the “shutdown” phenomena, accompanied by a significant increase in separator and 

overall cell impedances, as described earlier. Following shutdown, the melt integrity data shows that the 

co-extruded grades, with their higher rupture temperatures relative to the E16MMS grade, would afford a 

higher degree of separator mechanical integrity and electrode isolation for a longer period of time as the 

temperature continued to rise to the point of actual rupture (i.e., total separator meltdown). Also, the 

lower slope of the displacement versus temperature data exhibited for the co-extruded grades after their 

deformation temperatures are reached reflects a more gradual increase in strain upon and meltdown of the 

separator compared to the mono-layer grade material. 
NASA/TM—2011-216979/PART1 14 

Conclusions 

The laboratory assessment results for the puncture strength and dynamic mechanical properties for 

four grades of novel ExxonMobil co-extruded polyolefin battery separators suggest that these materials 

may afford an enhanced level of internal safety-related performance and reliability for lithium-ion cell 

chemistries relative to some current state-of-the-practice cell separator component materials. The 

improved meltdown properties of these materials extend the integrity of the electrode-separating barrier 

for a longer time during a thermal event, allowing the cell additional time to dissipate heat and avoiding 

the occurrence of a more catastrophic event. Improved permeability attributes of these materials may also 

afford enhanced cell-level energy and power capabilities. 

As ExxonMobil and other companies advance their separator material development and 

manufacturing capabilities to address forthcoming electric vehicle market opportunities, it is of interest to 

NASA to leverage such technology advancements in order to address its advanced energy storage and 

battery technology needs for future aerospace missions, especially when enhanced safety and high 

reliability are highly desired for manned missions. For advanced high-energy lithium-ion cell designs, 

such as those addressed by the NASA ETDP Energy Storage Project, enhanced cell-level safety 

performance, which could be afforded by a slightly thicker and higher temperature-tolerant separator 

component as compared to “baseline” materials, could overshadow a rather minimal projected negative 

impact on cell specific energy due to an increased component mass (i.e., separator and its imbibed 

electrolyte) and volume. Depending upon mission power and rate requirements, the enhanced 

permeability characteristics of the safety-enhanced ExxonMobil co-extruded materials could offset cell 

specific energy differences even further. 

References 

1. W. Boehnstedt, in Handbook of Battery Materials, Besenhard, J.O., Ed., VCH Wiley, Amsterdam and 

New York, 1999. 

2. P. Arora and Z. Zhang, Chem. Rev., 104, 4419 (2004). 

3. R. Spotnitz, in Handbook of Battery Materials, Besenhard, J.O., Ed., VCH Wiley, Amsterdam and 

New York, 1999. 

4. S. Zhang, J. Power Sources, 164, 351 (2007). 

5. Z. Zhang and P. Ramadass, in Lithium-Ion Batteries: Science and Technologies, Yoshio, 

M., et al., Eds, Springer, New York, 2009. 

6. R.S. Baldwin, NASA/TM—2009-215590 (2009). 

7. T. Ishihara, et al., in EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 

Proceedings, Stavanger, Norway, 2009. 

8. R. Call, et al., U.S. Patent Application 20080118827, 2006. 

9. www.greencarcongress.com/2007/05/tonen_liion_bat.html 

10. R.S. Baldwin, et al., NASA/TM—2010-216099 (2010). 

11. K.P. Menard, Dynamic Mechanical Analysis: a Practical Introduction, CRC Press, 2008. 

12. J. Dahn, et al., Solid State Ionics, 69, 265 (1994). 

13. D. Ihm, et al., J Power Sources, 109, 388 (2002). 
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Properties and Performance Attributes of Novel Co-extruded Polyolefin Battery Separator 

Materials 

Part 1: Mechanical Properties

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WBS 038957.04.07.03.01.03 

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John H. Glenn Research Center at Lewis Field 

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14. ABSTRACT

As NASA prepares for its next era of manned spaceflight missions, advanced energy storage technologies are being developed and evaluated 

to address future mission needs and technical requirements and to provide new mission-enabling technologies. Cell-level components for 

advanced lithium-ion batteries possessing higher energy, more reliable performance and enhanced, inherent safety characteristics are 

actively under development within the NASA infrastructure. A key component for safe and reliable cell performance is the cell separator, 

which separates the two energetic electrodes and functions to prevent the occurrence of an internal short-circuit while enabling ionic 

transport. Recently, a new generation of co-extruded separator films has been developed by ExxonMobil Chemical and introduced into their 

battery separator product portfolio. Several grades of this new separator material have been evaluated with respect to dynamic mechanical 

properties and safety-related performance attributes. This paper presents the results of these evaluations in comparison to a current state-ofthe-practice separator material. The results are discussed with respect to potential opportunities to enhance the inherent safety characteristics 

and reliability of future, advanced lithium-ion cell chemistries.

15. SUBJECT TERMS

Lithium batteries; Storage batteries; Electric batteries; Electrochemical cells; Energy storage; Space missions; Spacecraft power 

supplies; Electric power supplies 

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