battery separator.pdf

Battery SeparatorsPankaj Arora* and Zhengming (John) Zhang

Celgard, LLC, 13800 South Lakes Dr., Charlotte, North Carolina 28273

Received March 30, 2004

Contents1. Introduction and Scope 44192. Battery and Separator Market 44203. Separator and Batteries 44214. Separator Requirements 44225. Separator Types 4422

5.1. Microporous Separators 44225.2. Nonwovens 44225.3. Ion Exchange Membranes 44235.4. Supported Liquid Membranes 44235.5. Polymer Electrolyte 44235.6. Solid Ion Conductors 4423

6. Separator for Nonaqueous Batteries 44236.1. Lithium Ion 4424

6.1.1. Separator Development 44246.1.2. Separator Requirements 44276.1.3. Separator Properties/Characterization 44296.1.4. Effect of Separator on Cell Performance

and Safety4436

6.2. Lithium Polymer 44406.3. Lithium-Ion Gel Polymer 44416.4. Lithium Primary Systems 4443

6.4.1. Separator Requirements 44436.4.2. Chemistries 4444

7. Separator for Aqueous Batteries 44457.1. Leclanche (Zinc Carbon) 44467.2. Alkaline Zinc MnO2 44467.3. Lead-Acid Batteries 4447

7.3.1. Flooded Electrolyte Lead Acid 44477.3.2. Valve Regulated Lead Acid (VRLA) 4449

7.4. Nickel Systems 44507.4.1. NickelCadmium 44507.4.2. NickelMetal Hydride 44517.4.3. NickelHydrogen 4452

7.5. Zinc Systems 44527.5.1. SilverZinc 44527.5.2. NickelZinc 44547.5.3. ZincAir 44557.5.4. ZincBromine 4456

7.6. Redox Flow Batteries 44568. Mathematical Modeling of Batteries/Separators 44579. Summary 4458

10. Future Directions 445811. Acknowledgments 445912. References 4459

1. Introduction and ScopeMany advances have been made in battery tech-

nology in recent years, both through continuedimprovement of specific electrochemical systems andthrough the development and introduction of new

* Corresponding author. E-mail: [email protected]. Tele-phone: 704 587 8478. Fax: 704 588 7393

Pankaj Arora is a Senior Research Engineer at Celgard LLC in Charlotte,NC. He specializes in the design and modeling of electrochemical powersources and is currently working in the Battery Applications Laboratoryof Celgard, where he helps guide separator development work for lithiumbatteries. He has a B.Tech. degree in Electrochemical Engineering fromthe Central Electrochemical Research Institute in Karaikudi, India, and aPh.D. degree in Chemical Engineering from the University of SouthCarolina, Columbia, SC. Pankaj can be reached by email at [email protected].

Zhengming (John) Zhang is Vice President of New Technology at CelgardLLC in Charlotte, NC. He has been working in Solid State Ionics, Batteries,and Battery Separators since 1984. He has published more than 50 papersand patents and has co-edited a book on battery. He has been a invitedspeaker for many professional conferences, invited lecturer for UnitedNations Development Program, and is a Visiting Professor at XiamenUniversity, Fujian, China. He has a B.S. in Mechanical Engineering fromShanghai University, Shanghai, China, an M.S. in Electrochemistry fromShandong University, Jinan City, China, and a Ph.D. in Materials Chemistryfrom the University of California at Santa Barbara, Santa Barbara, CA.John can be reached by email at [email protected].

4419Chem. Rev. 2004, 104, 44194462

10.1021/cr020738u CCC: $48.50 2004 American Chemical SocietyPublished on Web 10/13/2004

battery chemistries. Nevertheless, there is still no oneideal battery that gives optimum performanceunder all operating conditions. Similarly, there is noone separator that can be considered ideal for allbattery chemistries and geometries.

A separator is a porous membrane placed betweenelectrodes of opposite polarity, permeable to ionic flowbut preventing electric contact of the electrodes. Avariety of separators have been used in batteries overthe years. Starting with cedar shingles and sausagecasing, separators have been manufactured fromcellulosic papers and cellophane to nonwoven fabrics,foams, ion exchange membranes, and microporousflat sheet membranes made from polymeric materi-als. As batteries have become more sophisticated,separator function has also become more demandingand complex.

Separators play a key role in all batteries. Theirmain function is to keep the positive and negativeelectrodes apart to prevent electrical short circuitsand at the same time allow rapid transport of ioniccharge carriers that are needed to complete thecircuit during the passage of current in an electro-chemical cell.1,2 They should be very good electronicinsulators and have the capability of conducting ionsby either intrinsic ionic conductor or by soakingelectrolyte. They should minimize any processes thatadversely affect the electrochemical energy efficiencyof the batteries.

Very little work (relative to research of electrodematerials and electrolytes) is directed toward char-acterizing and developing new separators. Similarly,not much attention has been given to separators inpublications reviewing batteries.1-10 A number ofreviews on the on cell fabrication, their performance,and application in real life have appeared in recentyears, but none have discussed separators in detail.Recently a few reviews have been published in bothEnglish and Japanese which discuss different typesof separators for various batteries.11-20 A detailedreview of lead-acid and lithium-ion (li-ion) batteryseparators was published by Boehnstedt13 and Spot-nitz,14 respectively, in the Handbook of BatteryMaterials. Earlier Kinoshita et al. had done a surveyof different types of membranes/separators used indifferent electrochemical systems, including batter-ies.11

The majority of the separators currently used inbatteries were typically developed as spin-offs ofexisting technologies. They were usually not devel-oped specifically for those batteries and thus are notcompletely optimized for systems in which they areused. One positive result of adapting existing tech-nologies is that they are produced in high volume atrelatively low cost. The availability of low costseparators is an important consideration in thecommercialization of batteries, because the batteryindustry traditionally operates with thin profit mar-gins and relatively small research budgets.

The purpose of this paper is to describe the varioustypes of separators based on their applications inbatteries and their chemical, mechanical and elec-trochemical properties, with particular emphasis onseparators for lithium-ion batteries. The separator

requirements, properties, and characterization tech-niques are described with respect to lithium-ionbatteries. The separators used in other batteries areonly discussed briefly. Despite the widespread useof separators, a great need still exists for improvingthe performance, increasing the life, and reducing thecost of separators. In the following sections, anattempt is made to discuss key issues in variousseparators with the hope of bringing into focuspresent and future directions of research and devel-opment in separator technologies.

2. Battery and Separator MarketThe battery industry has seen enormous growth

over the past few years in portable, rechargeablebattery packs. The majority of this surge can beattributed to the widespread use of cell phones,personal digital assistants (PDAs), laptop computers,and other wireless electronics. Batteries remainedthe mainstream source of power for systems rangingfrom mobile phones and PDAs to electric and hybridelectric vehicles. The world market for batteries wasapproximately $41 billion in 2000, which included$16.2 billion primary and $24.9 billion secondarycells.21

A recent study from Freedonia has predicted ag-gregate U.S. demand for primary and secondarybatteries to climb 5.5% annually through 2007 to $14billion. The growth will be driven by strong demandfor battery-powered electronic devices like digitalcameras and 3G wireless phones, and increasingproduction of electrical and electronic equipment. Thesecondary battery demand is expected to outpace theprimary battery market gains through 2007 benefit-ing from strong growth in the use of high-drainportable electronic devices. The lead-acid batterieswill account for over half of all rechargeable demandin 2007, although lithium-ion and NiMH batterieswill record the strongest growth. Alkaline batteriescould remain the dominant type, accounting for morethan two thirds of all primary battery sales in 2007.22

The rechargeable battery (NiCd, NiMH, andlithium-ion) market for 2003 for portable electronicswas around $5.24 billion, around 20% more then2002. The lithium-ion battery market was around$3.8 billion (73%). They are now used in more than90% of cellphones, camcorders, and portable comput-ers, worldwide, and have also been adopted in powertools recently.23

The tremendous progress in lithium-ion cells isclearly visible with as much as a 2-fold increase inthe volumetric and gravimetric energy density forboth 18650 and prismatic cells between 1994 and2002. In past few years the lithium-ion productionhas expanded to South Korea (Samsung, LG, etc.)and China (BYD, B&K, Lishen, etc.) from Japan.Several Japanese (Sanyo, Sony, MBI, NEC, etc.) andKorean (LG Chemical) manufacturers have alsomoved their manufacturing plants to China.23 Japan,which controlled 94% of the global rechargeablebattery market in 2000, has seen its market sharedrop to about 65% of the global market.23-25 Thecontinued growth in lithium-ion battery market hasled to a strong demand for battery separators. All the

4420 Chemical Reviews, 2004, Vol. 104, No. 10 Arora and Zhang

major separator manufacturers (Celgard, Asahi, andTonen) have either increased their capacity in 2003or are planning to increase it in 2004.26-28

There is not too much information available on thebattery separator market in the literature. It isestimated that about 30% of the rechargeable lithiumbattery market or $1.5 billion is the size of thebattery materials or components market. Batteryseparators for lithium batteries are about a $330million market within the total battery componentsmarket.29,30 Recently, the Freedonia Group has re-ported that the U.S. demand for battery separatorswill increase to $410 million in 2007 from $237million in 1977 and $300 million in 2002, respec-tively.31,32

3. Separator and Batteries

Batteries are built in many different shapes andconfigurationssbutton, flat, prismatic (rectangular),and cylindrical (AA, AAA, C, D, 18650, etc.). The cellcomponents (including separators) are designed to

accommodate a particular cell shape and design. Theseparators are either stacked between the electrodesor wound together with electrodes to form jellyrollsas shown in Figure 1. Stacked cells are generally heldtogether by pressure from the cell container. Thelithium-ion gel polymer stacked cells are preparedby bonding/laminating layers of electrodes and sepa-rators together. The separator properties should notchange significantly during the bonding process. Insome cases, the separators are coated to help inbonding process, thus reducing the interfacial resis-tance.33-35

In the conventional way of making spirally woundcells, two layers of separators are wound along withthe positive and negative electrodes, resulting in apositive/separator/negative/separator configuration.They are wound as tightly as possible to ensure goodinterfacial contact. This requires the separators tobe strong to avoid any contact between the electrodesthrough the separator. The separator also must notyield and reduce in width, or else the electrodes maycontact each other. Once wound, the jellyroll is

Figure 1. Typical battery configurations: (a) button cell; (b) stack lead-acid; (c) spiral wound cylindrical lithium-ion; (d)spiral wound prismatic lithium-ion.

Battery Separators Chemical Reviews, 2004, Vol. 104, No. 10 4421

inserted into a can, and filled with electrolyte. Theseparator must be wetted quickly by the electrolyteto reduce the electrolyte filling time. A header is thencrimped into the cell to cover the can from top. Insome prismatic cells, the jellyroll is pressed at hightemperatures and pressures and then inserted intothin prismatic (rectangular) cans. A typical 18650lithium-ion cell uses around 0.07-0.09 m2 of separa-tor, which is approximately 4-5% of the total cellweight.36

4. Separator RequirementsA number of factors must be considered in selecting

the best separator for a particular battery andapplication. The characteristics of each availableseparator must be weighed against the requirementsand one selected that best fulfills these needs. A widevariety of properties are required of separators usedin batteries. The considerations that are importantand influence the selection of the separator includethe following:

Electronic insulatorMinimal electrolyte (ionic) resistanceMechanical and dimensional stabilitySufficient physical strength to allow easy handlingChemical resistance to degradation by electrolyte,

impurities, and electrode reactants and productsEffective in preventing migration of particles or

colloidal or soluble species between the two electrodesReadily wetted by electrolyteUniform in thickness and other propertiesThe order of importance of the various criteria

varies, depending on the battery applications. Theabove list presents a broad spectrum of requirementsfor separators in batteries. In many applications, acompromise in requirements for the separator mustgenerally be made to optimize performance, safety,cost, etc. For example, batteries that are character-ized by small internal resistance and consume littlepower require separators that are highly porous andthin, but the need for adequate physical strength mayrequire that they be thick.

In addition to the above general requirements eachbattery type has other requirements essential forgood performance and safety. For example, separa-tors in sealed nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries should be highlypermeable to gas molecules for overcharge protection,the separator in lithium-ion cells should have ashutdown feature for good safety, separators foralkaline batteries should be flexible enough to bewrapped around the electrodes, and the separator foran SLI (starting, lighting and ignition) battery couldalso serve as a mechanical-shock cushion.

5. Separator TypesSeparators for batteries can be divided into differ-

ent types, depending on their physical and chemicalcharacteristics. They can be molded, woven, non-woven, microporous, bonded, papers, or laminates.In recent years, there has been a trend to developsolid and gelled electrolytes that combine the elec-trolyte and separator into a single component.

In most batteries, the separators are either madeof nonwoven fabrics or microporous polymeric films.Batteries that operate near ambient temperaturesusually use separators fabricated from organic ma-terials such as cellulosic papers, polymers, and otherfabrics, as well as inorganic materials such asasbestos, glass wool, and SiO2. In alkaline batteries,the separators used are either regenerated celluloseor microporous polymer films. The lithium batterieswith organic electrolytes mostly use microporousfilms.

For the sake of discussion, we have divided theseparators into six typessmicroporous films, non-wovens, ion exchange membranes, supported liquidmembranes, solid polymer electrolytes, and solid ionconductors. A brief description of each type of separa-tor and their application in batteries are discussedbelow.

5.1. Microporous SeparatorsThey are fabricated from a variety of inorganic,

organic, and naturally occurring materials and gen-erally contain pores that are greater than 50-100 in diameter. Materials such as nonwoven fibers (e.g.nylon, cotton, polyesters, glass), polymer films (e.g.polyethylene (PE), polypropylene (PP), poly(tetrafluo-roethylene) (PTFE), poly(vinyl chloride) (PVC)), andnaturally occurring substances (e.g. rubber, asbestos,wood) have been used for microporous separators inbatteries that operate at ambient and low tempera-tures (

The materials used in nonwoven fabrics include asingle polyolefin, or a combination of polyolefins, suchas polyethylene (PE), polypropylene (PP), polyamide(PA), poly(tetrafluoroethylene) (PTFE), polyvinylidinefluoride (PVdF), and poly(vinyl chloride) (PVC).Nonwoven fabrics have not, however, been able tocompete with microporous films in lithium-ion cells.This is most probably because of the inadequate porestructure and difficulty in making thin (

6.1. Lithium IonThe past decade has seen significant advances in

the ambient temperature lithium battery technology.Lithium-ion batteries are the preferred power sourcefor most portable electronics because of their higherenergy density, longer cycle life, and higher opera-tional voltage as compared to NiCd and NiMHsystems. In 2002, 66% of the total rechargeablebattery market for mobile IT and communicationdevices used lithium-based batteries and the restused nickel-based batteries.38,39

A typical lithium-ion cell consists of a positiveelectrode composed of a thin layer of powdered metaloxide (e.g., LiCoO2) mounted on aluminum foil anda negative electrode formed from a thin layer ofpowdered graphite, or certain other carbons, mountedon a copper foil. The two electrodes are separated bya porous plastic film soaked typically in LiPF6 dis-solved in a mixture of organic solvents such asethylene carbonate (EC), ethyl methyl carbonate(EMC), or diethyl carbonate (DEC). In the charge/discharge process, lithium ions are inserted or ex-tracted from the interstitial space between atomiclayers within the active materials.

Sonys introduction of the rechargeable lithium-ionbattery in the early 1990s precipitated a need for newseparators that provided not only good mechanicaland electrical properties but also added safety througha thermal shutdown mechanism. Although a varietyof separators (e.g., cellulose, nonwoven fabric, etc.)have been used in different type of batteries, variousstudies on separators for lithium-ion batteries havebeen pursued in past few years as separators forlithium-ion batteries require different characteristicsthan separators used in conventional batteries.

A novel microporous separator using polyolefinshas been developed and used extensively in lithium-ion batteries since it is difficult for conventionalseparator materials to satisfy the characteristicsrequired in lithium-ion batteries. In lithium-ionbatteries two layers of separators are sandwichedbetween positive and negative electrodes and thenspirally wound together in cylindrical and prismaticconfigurations. The pores of the separator are filledwith ionically conductive liquid electrolyte.

Microporous polyolefin membranes (see Figure 2)in current use are thin (2.0 A h) cylindrical cells, and 9 m separators inlithium-ion gel polymer cells.

Nonwoven materials have also been developed forlithium-ion cells but have not been widely accepted,in part due to the difficulty in fabricating thinmaterials with good uniformity and high strength.14Nonwoven separators have been used in button cellsand bobbin cells when thicker separators and lowdischarge rates are acceptable.

6.1.1. Separator Development

The process for making lithium-ion battery separa-tors can be broadly divided into dry45,46 and wet47processes. Both processes usually employ one or moreorientation steps to impart porosity and/or increasetensile strength. The dry process involves melting apolyolefin resin, extruding it into a film, thermally

Figure 2. Polyolefin separators used in lithium-ion batteries.


annealing it to increase the size and amount oflamella crystallites, and precisely stretching it toform tightly ordered micropores.48-52 In this process,a row lamellar crystal structure is generated in thepolymer in the initial extrusion step. This nonporousstructure is highly oriented as a result of extrusionand annealing conditions. The films are then stretchedto form micropores. This microporous structure iscontinuous throughout the bulk interior of the mem-brane.53

Polypropylene and polyethylene microporous filmsobtained by this method are available from Cel-gard48,50,54,55 and Ube.56 The dry process is technologi-cally convenient because no solvents are required.However, only a uniaxial stretching method has beensuccessful to date, and as a result, the pores areslitlike in shape and the mechanical properties offilms are anisotropic. The tensile strength in thelateral direction is relatively low.

Wet process (phase inversion process)57,58 involvesmixing of hydrocarbon liquid or some other low-molecular weight substance generally with a poly-olefin resin, heating and melting the mixture, ex-truding the melt into a sheet, orientating the sheeteither in the machine direction (MD) or biaxially, andthen extracting the liquid with a volatile solvent.45,59Separators made by the wet process are availablefrom Asahi Kasei,60 Tonen,61-63 and Mitsui Chemi-cals64 and more recently from Polypore/Membranaand Entek.65 The structure and properties of themembranes can be changed by controlling the com-position of the solutions and the evaporation orsubtractions of solvents in the gelation and solidifica-tion processes. The separators made by wet processuse ultrahigh-molecular-weight polyethylene (UH-MWPE). The use of UHMWPE gives good mechanicalproperties as well as some degree of melt integrity.

Ihm et al. have given a nice overview of the wetprocess by preparing a separator with polymer blendsof high-density polyethylene (HDPE) and ultrahighmolecular weight polyethylene (UHMWPE).58 Theyshowed that the mechanical strength and drawingcharacteristics are influenced by the content and themolecular weight of the UHMWPE contained in a

polyolefin blending solution. The manufacturing pro-cess of typical microporous film by dry and wetprocess is compared in Table 3.

A simplified flowchart for separator manufacturingprocess is shown in Figure 3.66 The virgin polymeris prepared and mixed with processing aids (e.g.,antioxidants, plasticizer, etc.) and then extruded. Theextruded polymer then goes through different steps,which vary from process to process. For the dryprocess, it can involve film annealing and stretching,while for the wet process, it can involve solventextraction and stretching. The finished film is thenslit into required widths and packed into boxes andshipped to the battery manufacturers. With theadvent of thinner separators, the film handlingduring manufacturing steps has become very impor-tant for the final quality of the film. Each step of theseparator manufacturing process has online detectionsystems to monitor the quality of the separators.

Uniaxially oriented films generally have highstrength in only one direction, whereas biaxiallyoriented films are relatively strong in both machinedirection (MD) and transverse direction (TD). Al-though intuitively one might expect biaxially orientedfilms to be preferred over uniaxially oriented films,

Table 2. Major Manufacturers of Lithium-Ion Battery Separators along with Their Typical Products

manufacturer structure composition process trade name

Asahi Kasai single layer PE wet HiPoreCelgard LLC single layer PP, PE dry Celgard

multilayer PP/PE/PP dry CelgardPVdF coated PVdF, PP, PE, PP/PE/PP dry Celgard

Entek Membranes single layer PE wet TeklonMitsui Chemical single layer PE wetNitto Denko single layer PE wetDSM single layer PE wet SolupurTonen single layer PE wet SetelaUbe Industries multi layer PP/PE/PP dry U-Pore

Table 3. Manufacturing Process of Typical Microporous Film

process mechanism raw material properties typical membranes manufacturers

dry process drawing polymer simple process PP, PE, PP/PE/PP Celgard, Ubeanisotropic film

wet process phase separation polymer + solvent isotropic film PE Asahi, Tonenpolymer + solvent + filler large pore size PE Asahi

high porosity

Figure 3. Generalized process for lithium-ion separatormanufacturing.66 Each step of the separator manufacturingprocess has online detection systems to monitor the qualityof the separator.


in practice biaxial orientation provides no perfor-mance advantage. In fact, biaxial orientation tendsto introduce TD shrinkage. This shrinkage, at el-evated temperatures, can allow electrodes to contacteach other. The separator must have sufficientstrength in the machine direction so that it does notdecrease in width or break under the stress ofwinding. The strength in the transverse direction isnot as important as that in the machine directionduring the process of making spirally wound batter-ies. The minimum generally practical requirementfor the mechanical strength of the 25-m separatoris 1000 kg/mm2.58

The typical properties of some commercial mi-croporous membranes are summarized in Table 4.Celgard 2730 and Celgard 2400 are single layer PEand PP separators, respectively, while Celgard 2320and 2325 are trilayer separators of 20 and 25 mthickness. Asahi and Tonen separators are singlelayer PE separators made by the wet process. Basicproperties, such as thickness, gurley, porosity, melttemperature, and ionic resistivity are reported inTable 4. These properties are defined in section 6.1.3.

Efforts have been made to find a new route for dryprocess using biaxial stretching techniques for pre-paring polypropylene microporous films, which mayhave submicrometer pore sizes and narrow pore sizedistributions and high permeability to gases andliquids combined with good mechanical properties.The biaxially stretched polypropylene microporousfilms (Micpor) were made by using nonporous polypro-pylene films of high -crystal content.67 The porosityof these films can be as high as 30-40%, with anaverage pore size of approximately 0.05 m. Thepores on the surface were almost circular in shapecompared to slitlike pores observed in uniaxialstretched samples and exhibited high permeabilityto fluids with good mechanical properties and almostcircular pore shape with narrow pore size distribu-tion.68-70

The PP/PE bilayers40 and PP/PE/PP trilayer sepa-rators were developed by Celgard. Multilayer separa-tors offer advantages by combining the lower meltingtemperature of PE with the high-temperature strengthof PP. Nitto Denko has also patented a single-layerseparator made from a blend of PE/PP by the drystretch process.71 According to the patent, the sepa-rator has microporous regions of PE and PP. Onheating in an oven, the impedance of the separatorincreases near the melting point of PE and theimpedance remains high until beyond the meltingpoint of PP. However, battery performance data havenot been presented.

Microporous polyethylene separator material com-posed of a combination of randomly oriented thickand thin fibrils of ultrahigh molecular weight poly-ethylene (UHMWPE), Solupur, manufactured byDSM Solutech, is also an interesting separator mate-rial for lithium-ion batteries. Solupur is fabricatedin standard grades with base weights ranging from7 to 16 g/m2 and mean pore size ranging from 0.1 to2.0 m and a porosity of 80-90%.72 Ooms et al.carried out a study on a series of DSM Solupurmaterials with different permeability. Rate capabilityand cycling tests of these materials were comparedwith commercial available separators in CR2320 typecoin cells. Solupur materials showed low tortuosity,high strength and puncture resistance, excellentwettability, and good high rate capability and low-temperature performance because of its high porosityand UHMWPE structure.73

Recently Nitto Denko has developed a batteryseparator made by a wet process that had highpuncture strength and high heat rupture resistance.74They used a polyolefin resin with a high molecularweight rubber as its main component materials andcross-linked through oxidation in air. The meltrupture temperature, as measured by thermomech-nical analysis was over 200 C in this material. Theyalso tried cross-linking ultrahigh molecular weightpolyethylene with electron-beam and ultraviolet ir-radiation, but this had the side effect of causingdeterioration in the polyolefin including rupture ofthe main chains and therefore resulted in reducedstrength.

ENTEK Membranes LLC has developed Teklonsa highly porous, ultrahigh molecular weight polyeth-ylene separator for lithium-ion batteries. At thewriting of this publication, the separator is availablein small quantities. Pekala et al. characterized Cel-gard, Setela, and Teklon separators in terms of theirphysical, mechanical, and electrical properties.75

Celgards separators are by far the best-character-ized battery separators in the literature as they havebeen widely used in numerous battery systems.Bierenbam et al.45 has described the process, physicaland chemical properties, and end-use applications.Fleming and Taskier76 described the use of Celgardmicroporous membranes as battery separators. Hoff-man et al.77 presented a comparison of PP and PECelgard microporous materials. Callahan discusseda number of novel uses of Celgard membranes.Callahan and co-workers98 also characterized Celgardmembranes by SEM image analysis, mercury poro-simetry, air permeability, and electrical resistivity,and later they characterized the puncture strength

Table 4. Typical Properties of Some Commercial Microporous Membranes

separator/properties Celgard 2730 Celgard 2400 Celgard 2320 Celgard 2325 Asahi Hipore Tonen Setela

structure single layer single layer trilayer trilayer single layer single layercomposition PE PP PP/PE/PP PP/PE/PP PE PEthickness (um) 20 25 20 25 25 25gurley (s) 22 24 20 23 21 26ionic resistivitya ( cm2) 2.23 2.55 1.36 1.85 2.66 2.56porosity (%) 43 40 42 42 40 41melt temp (C) 135 165 135/165 135/165 138 137

a In 1 M LiPF6 EC:EMC (30:70 by volume).


and temperature/impedance data for Celgard mem-branes.40 Spotnitz et al. reported short-circuit behav-ior in simulated, spirally wound cells, as well asimpedance/temperature behavior and thermomechan-ical properties.108 Yu78 found that a trilayer structureof PP/PE/PP Celgard microporous membranes pro-vided exceptional puncture strength.

Nonwoven materials such as cellulosic fibers havenever been successfully used in lithium batteries.This lack of interest is related to the hygroscopicnature of cellulosic papers and films, their tendencyto degrade in contact with lithium metal, and theirsusceptibility to pinhole formation at thickness of lessthan 100 m. For future applications, such as electricvehicles and load leveling systems at electric powerplants, cellulosic separators may find a place becauseof their stability at higher temperatures when com-pared to polyolefins. They may be laminated withpolyolefin separators to provide high-temperaturemelt integrity.

Asahi Chemical Industry carried out an explor-atory investigation to determine the requirements forcellulose based separators for lithium-ion batteries.79In an attempt to obtain an acceptable balance oflithium-ion conductivity, mechanical strength, andresistance to pinhole formation, they fabricated acomposite separator (39-85 m) that consists offibrilliform cellulosic fibers (diameter 0.5-5.0 m)embedded in a microporous cellulosic (pore diam-eter: 10-200 nm) film. The fibers can reduce thepossibility of separator meltdown under exposure toheat generated by overcharging or internal short-circuiting. The resistance of these films was equal toor lower than the conventional polyolefin-based mi-croporous separators. The long-term cycling perfor-mance was also very comparable.

Pasquier et al.80 used paper based separators in flatpouch type lithium-ion batteries and compared theperformance with cells made with Celgard typepolyolefin based separators. The paper separatorshad good wetting properties and good mechanicalproperties but did not provide the shutdown effectessential for large lithium-ion batteries. Their resis-tance was similar to polyolefin separators, and whenall water traces were removed from paper, theircycling performance was similar to that of Celgardseparators. The paper-based separators can be usedin small flat pouch type cells where high strengthand shutdown behavior is not required. For largerspherically wound cells, which require strong separa-tors with a shutdown feature, one can never usepaper-based separators.

Recently Degussa announced that they have de-veloped Separion separators for lithium batteries bycombining the characteristics of flexible polymericseparators with the advantages of chemical andthermally resistant and hydrophilic ceramic materi-als. Separion is produced in a continuous coatingprocess. Ceramic materials, e.g., alumina, silica, and/or zirconia are slip coated and hardened onto asupport.81,82 According to Degussa, Separion separa-tors have an excellent high temperature stability,superior chemical resistance, and good wettability,

especially at low temperatures. They tested theperformance and safety behavior of Separion separa-tor in 18650 cells and found the performance to becomparable to that of polyolefin-based separators.83

The potential use of polymeric ion-exchange mem-branes in the next generation single-ion secondarylithium polymer batteries was shown by Sachan etal.84,85 Conductivities exceeding 10-4 S/cm with trans-ference numbers of unity were achieved for Nafionconverted to the Li+ salt form.

To obtain a thin (less than 15 m) separator forlithium batteries, Optodot has taken a differentapproach of high-speed coating of a metal oxide sol-gel coating on a smooth surface followed by a delami-nation step to provide the free-standing separator.Using this approach, separator with thicknesses from6 to 11 m was made on large-scale productioncoating equipment.86 They found that the sol-gelseparators with a thickness in the middle of thisrange of 8-9 m have the preferred combination ofthinness and strength. The metal oxide sol-gelcoating is water-based with no organic solventspresent. The coating formulations include a polymerand a surfactant. The polymer provides improvedcoating rheology, mechanical strength, and otherproperties. The surfactant provides improved wettingproperties on the substrate. The films prepared werearound 11 m thick, with 45% porosity, and werecompletely wettable in nonaqueous electrolyte andhad a melt temperature greater than 180 C. Thesefilms are relatively thin and should help in increasingthe capacity but may not be strong enough for tightlywound cells. Moreover, the shutdown temperature ofthe separator seems to be very high and thus notsuitable for lithium-ion batteries.

Gineste et al. carried out the grafting of hydrophilicmonomers onto PP or PE separators to improve thewettability of separators used in secondary lithiumbatteries with a lower content of wetting agents.87,88They used a PP film (Celgard 2505) of 50 mthickness after irradiating in air by electron beamswith a dose ranging from 0.5 to 4 Mrad. The irradi-ated film was grafted by a monofunctional monomer(acrylic acid, AA), in the presence of difunctionalcross-linking agent (diethylene glycol dimethacrylate,DEGDM). The separators start loosing mechanicalproperties, when the grafting ratio is higher than50%.

6.1.2. Separator Requirements

In lithium-based cells, the essential function ofbattery separator is to prevent electronic contact,while enabling ionic transport between the positiveand negative electrodes. It should be usable on high-speed winding machines and possess good shutdownproperties. The most commonly used separators forprimary lithium batteries are microporous polypro-pylene membranes. Microporous polyethylene andlaminates of polypropylene and polyethylene arewidely used in lithium-ion batteries.89 These materi-als are chemically and electrochemically stable insecondary lithium batteries.


The general requirements90 for lithium-ion batteryseparators are given below.

6.1.2.1. Thickness. The lithium-ion cells used inconsumer applications use thin microporous separa-tors (

on a table parallel with a straight meter stick. Theskew should be less than 0.2 mm/m of separator.

6.1.2.14. Shutdown. Lithium-ion batteries sepa-rators provide some margin of protection againstshort circuit and overcharge in lithium-ion cells. Theseparators exhibit a large increase in impedance attemperature about 130 C that effectively stops ionictransport between the electrodes.91,92 The greater themechanical integrity of the separator above 130 C,the greater the margin of safety the separator canprovide. If the separator loses mechanical integrity,then the electrodes can come into direct contact, reactchemically, and result in thermal runaway. Theshutdown behavior of a separator can be character-ized by heating the separator (saturated with elec-trolyte) to high temperatures and simultaneouslymonitoring the electrical resistance of the separa-tor.92,108

6.1.2.15. High-Temperature Stability. A separa-tor might provide an extra margin of safety if it canprevent the electrodes from contacting one anotherat high temperatures. Separators with good mechan-ical integrity at high temperatures can provide agreater margin of safety for lithium-ion cells. Ther-mal mechanical analysis (TMA) can be used tocharacterize the high-temperature stability of sepa-rators. Utilizing TMA, the separator is held underconstant load and the degree of elongation vs. tem-perature is measured; at the temperature where theseparator loses mechanical integrity, the elongationincreases dramatically.

6.1.2.16. Electrode Interface. The separatorshould form a good interface with the electrodes toprovide sufficient electrolyte flow.

In addition to the above properties, the separatormust be essentially free of any type of defects(pinholes, gels, wrinkles, contaminants, etc.). All ofthe above properties have to be optimized before amembrane qualifies as a separator for a lithium-ion

battery. The general requirements for lithium-ionbattery separators are also summarized in Table 5.

6.1.3. Separator Properties/Characterization

Separators are characterized by structural andfunctional properties; the former describes what theyare and the latter how they perform. The structuralproperties include chemical (molecular) and microc-rystalline nature, thickness, pore size, pore sizedistribution, porosity, and various chemical andphysical properties such as chemical stability, andelectrolyte uptake. The functional properties of inter-est are electrical resistivity, permeability, and trans-port number. It is useful to characterize separatormaterials in terms of their structural and functionalproperties and to establish a correlation of theseproperties with their performance in batteries. Avariety of techniques are used to evaluate separators.Some of these techniques are discussed in thissection.

6.1.3.1. Gurley. Separator permeability is usuallycharacterized by air permeability. The gurley numberexpresses the time required for a specific amount ofair to pass through a specific area of separator undera specific pressure. The standard test method isdescribed in ASTM-D726 (B).

The gurley number is used to characterize separa-tors because the measurement is accurate and easyto make, and deviations from specific values are agood indication of problems. Air permeability (gurley)is proportional to electrical resistance (ER), for agiven separator morphology.98 Gurley can be used inplace of ER measurements once the relationshipbetween gurley and ER is established. A lower gurleyvalue means higher porosity, lower tortuosity, andaccordingly lower ER.

6.1.3.2. Electrical Resistance. The measurementof separator resistance is very important to the artof battery manufacture because of the influence the

Table 5. General Requirements for Lithium-ion Battery Separator90

parameter goal

thicknessa,b (m)

separator has on electrical performance. Electricalresistance is a more comprehensive measure ofpermeability then the gurley number, in that themeasurement is carried out in the actual electrolytesolution. The ionic resistivity of the porous membraneis essentially the resistivity of the electrolyte that isembedded in the pores of the separator. Typically, amicroporous separator, immersed in an electrolytehas an electrical resistivity about 6-7 times that ofa comparable volume of electrolyte, which it dis-places. It is a function of the membranes porosityand tortuosity, the resistivity of the electrolyte, thethickness of the membrane, and the extent to whichthe electrolyte wets the pores of the membrane.93 Theelectrical resistance of the separator is the trueperformance indicator of the cell. It describes apredictable voltage loss within the cell during dis-charge and allows one to estimate rate limitations.

Classical techniques for measuring electrical re-sistivity of microporous separators have been de-scribed by Falk and Salkind5 and by Robinson andWalker.94 The resistivity of an electrolyte is moreaccurately determined by ac methods since dc canpolarize the electrodes and cause electrolysis of thesolution. Modern ac impedance measuring systemsallow rapid measurements of cell resistance over awide range or frequencies from which resistance canbe calculated free of capacitance effects. Comparedto the dc techniques, the equipment required and thetheory necessary to interpret the ac techniques aremore complex; however, ac measurements yieldinformation about long-range migration of ions andpolarization phenomena occurring within the cell. Inan ac measurement, a sinusoidal voltage is appliedto a cell, and the sinusoidal current passing throughthe cell as a result of this perturbation is determined.A four-electrode cell is usually used for resistivitymeasurements. The outer two electrodes serve toapply a sinusoidal potential, and the resulting cur-rent passing through the inner two electrodes ismeasured. This technique is employed to avoid thecomplications arising from a nonuniform potentialfield near the outer two electrodes. An excellentreview of experimental techniques for measuringelectrical resistivity in aqueous solution is avail-able.95,96

The separator resistance is usually characterizedby cutting small pieces of separators from the fin-ished material and then placing them between twoblocking electrodes. The separators are completelysaturated with the electrolyte. The resistance () ofthe separator is measured at a certain frequency byac impedance techniques. The frequency is chosen sothat the separator impedance is equal to the separa-tor resistance. To reduce the measurement error, itis best to do multiple measurements by adding extralayers. The average resistance of single layer isdetermined from multiple measurements. The spe-cific resistivity, Fs ( cm), of the separator saturatedwith electrolyte is given by

where Rs is the measured resistance of separator in

, A is the electrode area in cm2, and l is thethickness of membrane in cm. Similarly, the specificresistivity of the electrolyte, Fe ( cm), is given by

where Re is the measured resistance of electrolyte in. The ratio of the resistivity of a separator mem-brane to that of the electrolyte is called the Mac-Mullin number, Nm, which can be used to predict theinfluence of the separator on battery performance.97

where is the tortuosity and is the porosity of theseparator. The MacMullin number describes therelative contribution of a separator to cell resistance.It is almost independent of electrolyte used and alsofactors out the thickness of the material. It assumesthat the separator wets completely in the electrolyteused for the test. From eqs 1 and 3, the electricalresistance of a microporous membrane is given by thefollowing5,114

It has been shown for Celgard membranes that themembrane resistance can be related to the gurleynumber by98

where Rm is the membrane resistance (), A is themembrane area (cm2), Fe is the specific electrolyteresistance ( cm), tgur is the gurley number (10 cm3air, 2.3 mmHg), d is the pore size, and 5 18 10-3is a scaling factor.

The usual procedure for characterizing batteryseparators is to cut several test samples from thefinished material. Thus, only a small portion of theseparator is actually examined. Ionov et al. hasproposed an alternative technique to measure theresistance of a separator over a large separatorarea.99 In this technique, the separator material ispassed through an electrolyte bath between electricalresistance measuring transducers. The set of trans-ducers installed in the bath transverse to the movingsheet of separator material examines the wholesurface of the material. If the production processensures good uniformity in the physicochemicalproperties of the separator material over the wholesurface, the transducer outputs will clearly be closeto one another. A nonuniform separator will causesignificant deviations from the average value atvarious sections of the material. In this case, thesections having lower or higher resistance comparedwith the average value should be regarded as flawed.

6.1.3.3. Porosity. The porosity is important forhigh permeability and also for providing a reservoirof electrolyte in the cell. Higher and uniform porosityis desirable for unhindered ionic current flow. Non-

Fs )RsA

l(1)

Fe )ReA

l(2)

Nm )FsFe

) 2

(3)

Rm ) Fe(2lA) (4)

RmA )Fe

5 18 10-3tgurd (5)


uniform porosity leads to nonuniform current densityand can further lead to reduced activity of theelectrodes. Cell failure can result because duringdischarge some areas of the electrodes work harderthen other.

Porosity of a separator is defined as the ratio ofvoid volume to apparent geometric volume. It isusually calculated (eq 6) from the skeletal density,basis weight, and dimensions of the material and somay not reflect the accessible porosity of the material.

The standard test method is described in ASTMD-2873. The actual or accessible porosity can also bedetermined by the weight of liquid (e.g., hexadecane)absorbed in the pores of the separator. In thismethod, the separator weight is measured before andafter dipping in hexadecane solvent, and the porosityis calculated (eq 7) by assuming that volume occupiedby hexadecane is equal to the porous volume of theseparator.

6.1.3.4. Tortuosity. Tortuosity is the ratio of meaneffective capillary length to separator thickness. Thetortuosity factor, of a separator can be expressedby

where ls is the ion path through the separator and dis the thickness of the separating layer.

Tortuosity is a long-range property of a porousmedium, which qualitatively describes the averagepore conductivity of the solid. It is usual to define by electrical conductivity measurements. With knowl-edge of the specific resistance of the electrolyte andfrom a measurement of the sample membrane resis-tance, thickness, area, and porosity, the membranetortuosity can be calculated from eq 3.

This parameter is widely used to describe the ionictransport by providing information on the effect ofpore blockage. A tortuosity factor ) 1, therefore,describes an ideal porous body with cylindrical andparallel pores, whereas values of > 1 refer to morehindered systems. Higher tortuosity is good fordendrite resistance but can lead to higher separatorresistance.

6.1.3.5. Pore Size and Pore Size Distribution.For any battery applications, the separator shouldhave uniform pore distribution to avoid performancelosses arising from nonuniform current densities. Thesubmicrometer pore dimensions are critical for pre-venting internal shorts between the anode and thecathode of the lithium-ion cell, particularly sincethese separators tend to be as thin as 25 m or less.

This feature will be increasingly important as batterymanufacturers continue to increase the cell capacitywith thinner separators. The pore structure is usuallyinfluenced by polymer composition, and stretchingconditions, such as drawing temperature, drawingspeed, and draw ratio. In the wet process, theseparators produced by the process of drawing afterextraction (as claimed by Asahi Chemical and MitsuiChemical) are found to have much larger pore size(0.24-0.34 m) and wider pore size distribution thanthose produced by the process of extraction (0.1-0.13m) after drawing (as claimed by Tonen).58

The testing of battery separators and control oftheir pore characteristics are important requirementsfor proper functioning of batteries. Mercury porosim-etry has been historically used to characterize theseparators in terms of percentage porosity, mean poresize and pore size distribution.100 In this method, thesize and volume of pores in a material are measuredby determining the quantity of mercury, which canbe forced into the pores at increasing pressure.Mercury does not wet most materials, and a forcemust be applied to overcome the surface tensionforces opposing entry into the pores.

The hydrophobic (e.g. polyolefins) separators arealso characterized with Aquapore (non-mercury po-rosimetry) technique, where water is used in placeof mercury. This is a very useful technique forcharacterizing polyolefin-based separators used inlithium batteries.101 Porosimetry gives pore volume,surface area, mean pore diameter, and pore sizedistribution. In a typical experiment, the sample isplaced in the instrument and evacuated. As thepressure increase, the quantity of water forced intothe pores increases in proportion to the differentialpore volume, the size of the pores corresponding tothe instantaneous pressure. Thus, increasing thepressure on a membrane having a given pore sizedistribution results in a unique volume vs pressureor pore diameter curve. The pressure required forintrusion of water in to a pore of diameter, D, is givenby following equation

where D is the diameter of the pore assuming thepore to be cylindrical, p is the differential pressure, is the surface tension of the nonwetting liquid,water, and is the contact angle of water. The poresgenerally are not of spherical shape of a constantdiameter. They usually vary in their form and size.Thus, statements of any pore diameter are alwaysto be viewed with the above in mind.

Another technique, capillary flow porometry hasbeen developed by Porous Materials Inc.102 to char-acterize battery separators.103,104 The instrument canmeasure a number of characteristics of batteryseparators such as size of the pore at its mostconstricted part, the largest pore size, pore sizedistribution, permeability, and envelope surfacearea.109

Scanning electron microscopy (SEM) is also usedto examine separator morphology. SEM pictures ofsome commercial membranes are shown in Figures

porosity (%) )

[1 - (sample weight/sample volume)polymer density ] 100 (6)

porosity (%) )volume occupied by hexadecane

(volume of polymer +volume occupied by hexadecane)

100 (7)

)lsd

(8)

D ) 4 cos p

(9)


4-6. The surface SEM of Celgard 2400, 2500, and2730 are shown in Figure 4. It is clear from theimages that the pores are uniformly distributed. BothCelgard 2400 and 2500 are single layer PP separa-tors, but the pore size of Celgard 2500 is substantiallylarger than Celgard 2400. Thus, it has lower resis-tance and is more suited for high rate applications.

Figure 5 shows the surface SEM and cross-sectionSEM of Celgard 2325. The surface SEM only showsthe PP pores while the PE pores are visible in thecross-section. It is clear from the image that all threelayers are of equal thickness. The SEM of separatorsmade by wet process are shown in Figure 6. The porestructure of all of these membranes is very similar.

Figure 4. Scanning electron micrographs of the surface of single layer Celgard separators used in lithium batteries: (a)2400 (PP), (b) 2500 (PP), and (c) 2730 (PE).

Figure 5. Scanning electron micrographs of Celgard 2325 (PP/PE/PP) separator used in lithium-ion batteries: (a) surfaceSEM and (b) cross-section SEM.


Asahi-1 (Figure 6b) separator has significantly largerpores compared to the other membranes.

Image analysis has been used to characterize thepore structure of synthetic membrane materials.105The Celgard films have also been characterized byscanning tunneling microscopy, atomic force micros-copy, and field emission scanning electron micros-copy.53,106 The pore size of the Celgard membranescan also be calculated from eq 5, once the MacMullinnumber and gurley values are known.

6.1.3.6. Puncture Strength. A separator is re-quired to have sufficient physical strength to endurethe rigors of cell assembly and day-to-day charge-discharge cycling. Physical strength is required towithstand basic handling, cell blocking/assembly,physical shock, punctures, abrasion, and compres-sion.

The puncture strength (PS) is the weight that mustbe applied to a needle to force it completely througha separator.45,107 It has been used to indicate thetendency of separators to allow short-circuits in a cellthat may occur due to holes generated in the separa-tor by the rough surface of an electrode during thebattery assembly and charge-discharge cycle. ThePS requirement for lithium-ion batteries is higherthem lithium-foil batteries, because the separatormust contend with two rough surfaces. Commercially

available puncture strength machines made for tex-tiles tend to give meaningless results when testingbattery separator membranes. More reproducibleresults can be obtained with a load frame (such asan Instron Machine). The mix penetration strengthis a better measure of mechanical strength for batteryseparators as it measures the force required to createa short through the separator when electrode mix ispushed through it.

The strength of the separator depends greatly onthe materials used and the manufacturing method.The wet-biaxial method simultaneously stretches inthe MD and TD directions and thus achieves amaterial that has tensile modulus and rupturestrength in both directions. Both high polymer en-tanglement and stretching help increase the physicalstrength of the separator.

6.1.3.7. Mix Penetration Strength. The forcerequired to create a short through a separator dueto mix (electrode material) penetration defines mixpenetration strength. In this test force (with a 1/2 in.diameter ball) is applied on the positive electrode/separator/negative electrode sandwich, and the forceat which the mix penetrates through the separatorand creates an electronic short is called mix penetra-tion force. Mix penetration strength is used toindicate the tendency of separators to allow short-

Figure 6. Scanning electron micrographs of separators made by wet process and used in lithium-ion batteries: (a) Setela(Tonen), (b) Hipore-1 (Asahi), (c) Hipore-2 (Asahi), and (d) Teklon (Entek).


circuits during battery assembly. The mix penetra-tion resistance test is more closely related to particlepenetration resistance compared to puncture resis-tance.49

6.1.3.8. Tensile Strength. The tensile strengthmeasurements (e.g., Youngs modulus, percent offsetstrength, elongation at break, and stress at break)can be made by utilizing widely known standardprocedures. These tests are carried out in both MDand TD directions. The tensile properties are depend-ent on the manufacturing process. The uniaxiallyoriented films have high strength in only one direc-tion, whereas biaxially oriented films are moreuniformly strong in both MD and TD directions.ASTM test method D88-00, Standard test methodfor tensile properties of thin plastic sheeting, is anappropriate test.

The separator should be strong enough to with-stand mechanical handling during cell winding andassembly. It should be dimensionally stable andshould not neck down during winding. The decreasein width will allow the electrodes to touch each otherand create a short. Thus, the tensile property of theseparator should be very strong in MD directioncompared to TD direction.

6.1.3.9. Shrinkage. Shrinkage test is carried outon both MD and TD directions. In this test, thedimensions of separators are measured and thenstored at 90 C for a fixed time. The shrinkage is thencalculated from the change in dimensions as shownin eq 10.

where Li is the initial length and Lf is the final lengthof separator after high temperature storage. Theuniaxially stretched separators tend to shrink in theMD direction only, while the biaxially stretchedseparators shrink in both MD and TD directions. Theshrinkage of separators can also be compared bycarrying out the thermal mechanical analysis (TMA)test at a constant load and rate.

6.1.3.10. Shutdown. Separator shutdown is auseful and essential mechanism for limiting temper-

ature and preventing venting in short-circuited cells.108It usually takes place close to the melting tempera-ture of the polymer when the pores collapse turningthe porous ionically conductive polymer film into anonporous insulating layer between the electrodes.At this temperature there is a significant increasein cell impedance and passage of current through thecell is restricted. This prevents further electrochemi-cal activity in the cell, thereby shutting the cell downbefore an explosion can occur.

The ability of the PE based separator to shutdownthe battery is determined by its molecular weight,percent crystallinity (density) and process history.Material properties and processing methods mightneed to be tailored so that the shutdown response isspontaneous and complete. The optimization needsto be done without affecting the mechanical proper-ties of the material in the temperature range ofinterest. This is easier to do with the trilayer separa-tors manufactured by Celgard since one material isutilized for the shutdown response and another forthe mechanical properties. Polyethylene containingseparators, in particular trilayer laminates of polypro-pylene, polyethylene, and polypropylene, appear tohave the most attractive properties for preventingthermal runaway in lithium-ion cells.109,110 The shut-down temperature of 130 C is usually enough tocontrol the cell heating and avoid thermal runawayin lithium-ion cells. A lower temperature shutdownwill be desirable if it does not affect the separatormechanical properties or high-temperature cell per-formance in any adverse way.

The shutdown property of separators is measuredby measuring the impedance of a separator while thetemperature is linearly increased.92,108 Figure 7 showsthe actual measurement for Celgard 2325 membrane.The heating rate was around 60 C/min, and theimpedance was measured at 1 kHz. The rise inimpedance corresponds to a collapse in pore structuredue to melting of the separator. A 1000-fold increasein impedance is necessary for the separator to stopthermal runaway in the battery. The drop in imped-ance corresponds to opening of the separator due tocoalescence of the polymer and/or to penetration ofthe separator by the electrodes; this phenomenon is

Figure 7. Internal impedance (at 1 kHz) of Celgard 2325 (PP/PE/PP) separator as a function of temperature. Heatingrate: 60 C/min.

shrinkage (%) )Li - Lf

Li 100 (10)


referred to as a loss in melt integrity. This test isfairly reliable in indicating the temperature at whichthe impedance rises but shows some variability incharacterizing the subsequent drop in impedance.

In Figure 7, the shutdown behavior of a multilayer(PP/PE/PP) separator (Celgard 2325) is shown. Theimpedance rise occurred near the melting point ofpolyethylene (130 C) and remained high until suchtime as the melting point of polypropylene (165 C)is attained. The shutdown temperature of the sepa-rator is governed by the melting point of the separa-tor material. At the melting point the pores in theseparator collapse to the form a relatively nonporousfilm between the anode and the cathode. This wasconfirmed by DSC as shown in Figure 8. The DSCscan in Figure 8 gives a peak melting temperatureof 135 C for Celgard 2730, 168 C for Celgard 2400,and 135/165 C for Celgard 2325. The shutdownbehavior of thinner separators (

about 5 mm width), which is held in mini-instron typegrips. The sample is held with a constant 2 g loadwhile the temperature is ramped at 5 C/min pastthe melting point until the tension ruptures the film.Three parameters are reported from TMA testsshrinkage onset temperature, melt temperature, andmelt rupture temperature. It has proved to be a morereproducible measure of melt integrity of the separa-tor.108

Figure 9 shows the TMA data for two differentCelgard membranes. The shrinkage onset tempera-ture, deformation temperature, and rupture temper-ature are summarized in Table 6. The single layerPP membrane (Celgard 2400) showed a higher soft-ening temperature (121 C), a deformation temper-ature around 160 C, and a very high rupturetemperature around 180 C. The multilayer polypro-pylene/polyethylene/polypropylene separator (Cel-gard 2325) combined the low-temperature shutdownproperty of polyethylene with the high-temperaturemelt integrity of polypropylene, resulting in a sepa-rator with softening (105 C) and melt temperature(135 C) very similar to that of PE and rupturetemperature (190 C) very similar to that of PP.

Separators with melt integrity greater than 150 Care desirable for lithium-ion cells. The trilayer sepa-rators with polypropylene on the outside help inmaintaining the melt integrity of the separators athigher temperatures compared to single layer PEseparators. This is especially important for biggerlithium-ion cells being developed for hybrid andelectric vehicles.

6.1.3.12. Wettability and Wetting Speed. Twophysical properties of separators, which are impor-tant to the operating characteristics of a battery, areelectrolyte absorption and electrolyte retention. Anygood separator should be able to absorb a significantamount of electrolyte and also retain the absorbedelectrolyte when the cell is in operation. These aremore important in sealed cells where no free elec-trolyte is present. A maximum amount of electrolytein the separator is desirable to achieve minimum cellinternal resistance.

The separator wettability can limit the perfor-mance of batteries by increasing the separator andcell resistance. Separator wetting speed can be cor-related with electrolyte filling time in real cells. Thewetting speed is determined by the type of polymer(surface energy), pore size, porosity, and tortuosityof the separators. There is no generally accepted testfor separator wettability. However, simply placing adrop of electrolyte on the separator and observingwhether the droplet quickly wicks into the separatoris a good indication of wettability. The contact angleis also a good measure of wettability.

The uptake of electrolyte by many hydrophobicpolymer separators can be enhanced either by wet-ting agents or ionic-functional groups (e.g. ion-exchange membranes).

6.1.4. Effect of Separator on Cell Performance and Safety

Although the material of a battery separator isinert and does not influence electrical energy storageor output, its physical properties greatly influence theperformance and safety of the battery. This is espe-cially true for lithium-ion cells, and thus the batterymanufacturers have started paying more attentionto separators while designing the cells. The cells aredesigned in such a way that separators do not limitthe performance, but if the separator properties are

Figure 9. TMA of Celgard 2400 (PP) and 2325 (PP/PE/PP). A constant load (2 g) is applied while the temperature isramped at 5 C/min.

Table 6. TMA Data for Typical Celgard Separators

Celgard 2400 Celgard 2325

shrinkage onset temp (C) 121 106deformation temp (C) 156 135, 154rupture temp (C) 183 192


not uniform, or if there are other issues, it can affectthe performance and safety of cells. This section willfocus on the effect of the separator properties on cellperformance and safety. Table 7 shows differenttypes of safety and performance tests for lithium-ionbatteries and the corresponding important separatorproperty and how it affects performance and/orsafety.

To achieve good performance of lithium-ion cells,the separators should have low resistance, low shrink-age and uniform pore structure. The separator withhigh resistance will perform poorly during high ratedischarge and will also increase the cell chargingtime. Low shrinkage is a very important character-istic for separators, especially for higher capacitycells. These cells are used in high-speed laptopcomputers, which can experience higher tempera-tures (70-75 C) under certain conditions.117 Thiscan lead to shrinkage of separators and ultimatelyhigher cell resistance and poor long-term cycling. Theshrinkage in TD direction can lead to safety issuesbecause of an internal short between the electrodes.Larger pores can lead to shorts during cell manufac-turing or can fail during hipot testing. Larger poreswill allow more soft shorts and higher self-discharge,especially during high-temperature storage. Verysmall pore size can lead to higher resistance andpoor cycle life during high-temperature cycling andstorage. Thus, the pore size of the separator shouldbe optimized to achieve good strength and perfor-mance.

One of the ways to increase cell capacity is bydecreasing the thickness of separators. The newerhigh capacity cells (>2.0 A h) generally use 20 and16 m separators as compared to 25 m separatorsused in cells with 1.6-1.8 A h capacity. The thinnerseparators offer lower resistance and help in increas-ing the capacity. However, they can hold less elec-

trolyte and their mechanical strength is not as highas thicker separators. Thus, appropriate changesshould be made in cell design to keep the cell safe.The handling and manufacturing of thinner separa-tors is also a challenge for the separator manufactur-ers. They are required to maintain the same electricaland mechanical properties and to have better qualityfor thinner separators. The separator manufacturershave installed better controls and quality standardsand have started offering 16-m separators. A lot ofbattery experts are of the opinion that the 16-m isthe thinnest they can use and still maintain thestringent performance and safety requirements oflithium-ion cells.

The separators inside the lithium-ion batteriesexperience extreme oxidizing environment on theside facing the positive electrode and extreme re-ducing environment on the side facing the negativeelectrode. The separators should be stable in theseconditions during long-term cycling especially athigh temperatures. Separators with poor oxidationresistance can lead to poor high-temperature stor-age performance and poor long-term cycling be-havior. The oxidation resistance properties of trilayer(PP/PE/PP) separators with PP as the outside layerand PE as inner layer is superior compared topolyethylene separators. This is because of the betteroxidation resistance properties of polypropylene incontact with the positive electrode in a lithium-ioncell.

The products formed by the decomposition of theelectrolyte can also block the pores of the separator,leading to increase in cell resistance. The separatorswith lower resistance also helps in better low tem-perature performance. At very low temperatures, theresistance of the electrolytes is very high and thussmaller contribution from separator helps in keepingthe cell resistance lower.

Table 7. Safety and Performance Tests for Lithium-Ion Batteries and the Corresponding Important SeparatorProperty and Its Effect on the Cell Performance and/or Safety

cell property separator property comments

cell capacity thickness cell capacity can be increased by making the separator thinnercell internal resistance resistance separator resistance is a function of thickness, pore size, porosity,

and tortuosityhigh rate performance resistance separator resistance is a function of thickness, pore size, porosity,

and tortuosityfast charging resistance low separator resistance will aid in overall faster charging by

allowing higher and/or longer constant current charginghigh-temp storage oxidation resistance oxidation of separators can lead to poor storage performance and

reduce performance lifehigh-temp cycling oxidation resistance oxidation of separators can lead to poor cycling performanceself-discharge weak areas, pinholes soft shorts during cell formation and testing can lead to

internal current leakagelong-term cycling resistance, shrinkage, pore size high resistance, high shrinkage and very small pore size can

lead to poor cycling performanceovercharge shutdown behavior; separator should completely shutdown and then maintain its

high-temp melt integrity melt integrity at high tempexternal short circuit shutdown behavior separator shutdown stops the cells from overheatinghotbox high-temp melt integrity separator should be able to keep the two electrodes apart at

high tempnail crush shutdown (to stop delayed failure) in the case of internal shorts, the separator may be the only

safety device to stop the cell from overheatingbar crush shutdown (to stop delayed failure) in the case of internal shorts, the separator may be the only

safety device to stop the cell from overheating


Zeng et al.118 has shown that small amounts ofactive lithium metal could be added to a lithium-ionbattery via the separator by using vacuum depositiontechniques. The lithium films (4-8 m) were depos-ited onto a microporous PP film and showed that thelithium electrochemically reacted with either elec-trode and thus the intrinsic irrereversible capacityof the negative electrode was compensated for usingvolumetrically efficient lithium metal. This may bea novel idea to allow higher capacity designs but islikely impractical and uneconomical due to issuesinvolved with lithium plating on polymer films andhandling the resulting films.

The lithium-ion cells have demonstrated power losswhen aged and/or cycled at high temperatures. Norinet al.119 demonstrated that the separator is at leastpartly responsible for the power loss due to theintrinsic increase in its ionic resistance. They showedthat impedance increased significantly upon cyclingand/or aging of lithium-ion cells at elevated temper-atures and that separators accounts for 15% of thetotal cell impedance rise. They later reported that theloss in ionic conductivity of the separator was due toblocking of the separator pores with the productsformed due to electrolyte decomposition, which wassignificantly accelerated at elevated temperatures.120

The U.S. Department of Transportation (DOT)classifies all lithium-ion batteries as hazardous ma-terials for shipping in the same category as lithiummetal primary batteries.121 It grants exceptions basedon the cell capacity and ability of the cells to passspecified tests. There are several groups that regu-late, or provide testing, to verify safe operation oflithium-ion cells under abuse conditions. In addition,the UL Laboratories,122,123 the International Electro-technic Commission,124 and the United Nations125have developed standardized safety testing proce-dures. These tests are designed to ensure that cellsare safe to ship and are resistant to typical abuseconditions such as internal shorting, overcharge,overdischarge, vibration, shock, and temperaturevariations that may be encountered in normal trans-portation environments.

Underwriters Laboratories (UL) requires that con-sumer batteries pass a number of safety tests (UL1642126 and UL-2054127). There are similar recom-mendations from UN for transport of dangerousgoods,128 and from the International ElectrotechnicalCommission (IEC) and Japan Battery Association.129An abnormal increase in cell temperature can occurfrom internal heating caused by either electricalabusesovercharge or short circuitsor mechanicalabusesnail penetration or crush. A higher cell tem-perature could also be a result of external heating.For this reason, battery packs containing lithium-ion cells are designed with safety control circuits thathave redundant safety features (PTC, CID, vent,thermal fuse, etc.). Shutdown separators are one ofthe safety devices inside the cell and act as a lastline of defense. The separator shutdown is irrevers-ible, which is fine for polyethylene-based separators,which melt around 130 C.

The impedance of the separator increases by 2-3orders of magnitude due to an increase in cell

temperature, which results from cell abuse (e.g.,short circuit, overcharge). The separator should notonly shutdown around 130 C, but it should alsomaintain its mechanical integrity at higher temper-atures, preferably at temperatures. If the separatordoes not shutdown properly then the cell will con-tinue to heat during an overcharge test and can leadto thermal runaway. The high-temperature meltintegrity of separators is also a very importantproperty to keep the cell safe during extendedovercharge or during extended exposure to highertemperatures.

Figure 10 shows a typical short-circuit curve foran 18650 lithium-ion cell with shutdown separator,LiCoO2 positive electrode, and MCMB carbon nega-tive electrode. The cell does not have other safetydevices (e.g., CID, PTC), which usually work beforeseparator shutdown. As soon as the cell is short-circuited externally through a very small shuntresistor, the cell starts heating because of the largecurrent drained through the cell. The shutdown ofthe separator, which occurs around 130 C, stops thecell from heating further. The current decrease iscaused by increase of battery internal resistance dueto separator shutdown. The separator shutdownhelps in avoiding the thermal runaway of the cell.

Cells can be overcharged when the cell voltage isincorrectly detected by the charging control systemor when the charger breaks down. When this hap-pens, the lithium ions remaining in the cathode areremoved and more lithium ions are inserted into theanode then under standard charging conditions. Ifthe lithium insertion ability of the carbon anode issmall, lithium metal in the form of dendrites may bedeposited on the carbon and this causes a drasticreduction in thermal stability. At higher chargingrates, the heat output increases greatly because thejoule heat output is proportional to I2R. Severalexothermic reactions (e.g., reaction between lithiumand electrolyte, thermal decomposition of anode andcathode, thermal decomposition of electrolyte, etc.)occur inside the cell as its temperature increases.Separator shutdown happens when the cell temper-ature reaches the melting point of polyethylene asshown in Figure 11. The CID and PTC of the 18650cells was removed to test the performance of separa-tor alone. The current decrease is caused by increaseof battery internal resistance due to separator shut-down. Once the pores of the separator have closeddue to softening, the battery cannot continue to becharged or discharged, and thus thermal runawayis prevented. During continued overcharge, the sepa-rator should maintain its shutdown feature andshould not allow the cell to heat again. It should alsomaintain its melt integrity and should not allow thetwo electrodes to touch each other.

The separator should also not allow any dendriteto penetrate through the separator to avoid internalshorts. During an internal short, the separator is theonly safety device that can stop the thermal runaway,if the failure is not instant. If the heating rate is toohigh, then instant failure will occur which cannot bestopped by separator shutdown. If the heating rate


is not too high then the separator shutdown can helpin controlling the heating rate and stop thermalrunaway.

Generally in a nail penetration test, an instanta-neous internal short would result the moment thenail is tucked into the battery. Enormous heat isproduced from current flow (double layer dischargeand electrochemical reactions) in the circuit by themetal nail and electrodes. Contact area varies ac-cording to depth of penetration. The shallower thedepth, the smaller the contact area and thereforethe greater the local current density and heat pro-

duction. Thermal runaway is likely to take place aslocal heat generation induces electrolyte and elec-trode materials to decompose. On the other hand, ifthe battery is fully penetrated, the increased contactarea would lower the current density, and conse-quently all tests would pass the nail penetration test.Internal short-circuit tests are more difficult to passthen the external short-circuit tests described earlier,because contact area between metal nail contact issmaller than the contact area between current col-lectors, where the current density would therefore belarger.

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positivetemperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

Figure 11. Typical overcharge behavior of a 18650 lithium-ion cell with shutdown separator. The PTC (positive temperaturecoefficient) and CID (current interrupt device) were removed from the cell header.


Figure 12 shows the typical nail penetration be-havior of a 18650 lithium-ion cell with shutdownseparator, LiCoO2 positive electrode, and MCMBcarbon negative electrode. Clearly, there was a volt-age drop from 4.2 to 0.0 V, instantaneously, as thenail penetrates through (when an internal shortcircuit occurs), and the temperature rose. When theheating rate is low, the cell stops heating when thetemperature is close to separator shutdown temper-ature as shown in Figure 12a. If the heating rate isvery high, then the cell continues to heat and failsthe nail penetration test as shown in Figure 12b. Inthis case, the separator shutdown is not fast enoughto stop cell from thermal runaway. Thus, the separa-tor only helps in avoiding delayed failures in the caseof internal short circuit as simulated by nail and barcrush tests. Separators with high-temperature meltintegrity and good shutdown feature (to avoid de-layed failures) are needed to pass the internal short-circuit test. Thinner separators (

dium for ionic transport but also they function as theseparator which insulates the cathode from theanode. Consequently, the polymer electrolyte musthave sufficient mechanical integrity to withstandelectrode stack pressure and stresses caused bydimensional changes, which the rechargeable elec-trodes undergo during charge/discharge cycling.

Lithium polymer electrolytes formed by dissolvinga lithium salt LiX (where X is preferably a large softanion) in poly(ethylene oxide) PEO can find usefulapplication as separators in lithium rechargeablepolymer batteries.138-140 Thin films must be used dueto the relatively high ionic resistivity of these poly-mers. For example, the lithium-ion conductivity ofPEO-Li salt complexes at 100 C is still only about1/100th the conductivity of a typical aqueous solution.

A polymer electrolyte with acceptable conductivity,mechanical properties and electrochemical stabilityhas yet to be developed and commercialized on alarge scale. The main issues which are still to beresolved for a completely successful operation of thesematerials are the reactivity of their interface withthe lithium metal electrode and the decay of theirconductivity at temperatures below 70 C. Croce etal. found an effective approach for reaching both ofthese goals by dispersing low particle size ceramicpowders in the polymer electrolyte bulk.141,142 Theyclaimed that this new nanocomposite polymer elec-trolytes had a very stable lithium electrode interfaceand an enhanced ionic conductivity at low tempera-ture, combined with good mechanical properties. Fanet al.143 has also developed a new type of compositeelectrolyte by dispersing fumed silica into low tomoderate molecular weight PEO.

The gel type polymer electrolyte prepared bydispersing ceramic powders (e.g., Al2O3) into a matrixformed by a lithium salt solution contained in a poly-(acrylonitrile) (PAN) network was reported by Ap-petecchi et al.144 These new types of composite gelelectrolytes had high ionic conductivity, wide elec-trochemical stability, and particularly, high chemicalintegrity even at temperatures above ambient. Kimet al.145 used a blend of PVdF-HFP and PAN as amatrix polymer to attain high ionic conductivity andgood mechanical strength. The PAN can give me-chanical integrity and structural rigidity to a porousmembrane without inorganic fillers. The high ionicconductivity was due to the high volume of pores anda high affinity of the membrane for electrolytesolution.146

6.3. Lithium-Ion Gel PolymerThe solid polymer electrolyte approach provides

enhanced safety, but the poor ambient temperatureconductivity excludes their use for battery applica-tions, which require good ambient temperature per-formance. In contrast, the liquid lithium-ion tech-nology provides better performance over a widertemperature range, but electrolyte leakage remainsa constant risk. Midway between the solid polymerelectrolyte and the liquid electrolyte is the hybridpolymer electrolyte concept leading to the so-calledgel polymer lithium-ion batteries. Gel electrolyte isa two-component system, viz., a polymer matrix

swollen with a liquid electrolyte. The gel polymerelectrolyte approach to the lithium-ion technologycombines the positive attributes of both the liquid(high ionic conductivity) and solid polymer electro-lytes (elimination of leakage problems).

Gel polymer lithium-ion batteries replace the con-ventional liquid electrolytes with an advanced poly-mer electrolyte membrane. These cells can be packedin lightweight plastic packages as they do not haveany free electrolytes and they can be fabricated inany desired shape and size. They are now increas-ingly becoming an alternative to liquid-electrolytelithium-ion batteries, and several battery manufac-turers, such as Sanyo, Sony, and Panasonic havestarted commercial production.147,148 Song et al.149have recently reviewed the present state of gel-typepolymer electrolyte technology for lithium-ion bat-teries. They focused on four plasticized systems,which have received particular attention from apractical viewpoint, i.e., poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN),150 poly(methyl methacry-late) (PMMA),151,152 and poly(vinylidene fluoride)(PVdF) based electrolytes.153-157

One particular version of the lithium-ion gel poly-mer cells, also known as plastic lithium-ion cell(PLION), was developed by Bellcore (now TelcordiaTechnologies).158-160 In this case, Gozdz et al. devel-oped a microporous plasticized PVdF-HFP basedpolymer electrolyte which served both as separatorand electrolyte. In PLION cells, the anode andcathode are laminated onto either side of the gellablemembrane. Good adhesion between the electrodesand the membranes is possible because all threesheets contain significant amounts of a PVdF copoly-mer that can be melted and bonded during thelamination step.

The PVdF-HFP separators used in PLION cellswere around 3 mil thick, and had poor mechanicalproperties. It has been reported that the major sourceof rate limitation in PLION cells was the separatorthickness.161 The rate capability of these cells can besignificantly improved by decreasing the separatorthickness to that typically used in liquid electrolytesystem. Moreover, in the absence of shutdown func-tion, the separator does not contribute to cell safetyin any way. Park et al. reported that the HFP contentin separators did not have any significant impact oncell performance.162 The Bellcore process has provento be an elegant laboratory process but is difficult toimplement in large-scale production.

To overcome the poor mechanical properties ofpolymer and gel polymer type electrolytes, mi-croporous membranes impregnated with gel polymerelectrolytes, such as PVdF, PVdF-HFP, and othergelling agents, have been developed as an electrolytematerial for lithium batteries.163-173 Gel coated and/or gel-filled separators have some characteristics thatmay be harder to achieve in the separator-free gelelectrolytes. For example, they can offer much betterprotection against internal shorts when compared togel electrolytes and can therefore help in reducingthe overall thickness of the electrolyte layer. Inaddition the ability of some separators to shutdown


at a particular temperature allows safe deactivationof the cell under overcharge conditions.

The shutdown behavior of PVdF coated Celgardtrilayer membranes is shown in Figure 13. Theshutdown is defined by the sharp increase in resis-tance around 130 C. The PVdF coating should beporous and should not block the pores to maintainsimilar ionic conductivity. The scanning electronmicrographs of the PVdF coated membrane is shownin Figure 14. The cross section SEM of Celgard 3300provides visual evidence that the coating is porousand is not blocking the pores of the top PP layer.

Abraham et al.174 were the first ones to proposesaturating commercially available microporous poly-olefin separators (e.g., Celgard) with a solution oflithium salt in a photopolymerizable monomer anda nonvolatile electrolyte solvent. The resulting bat-teries exhibited a low discharge rate capability dueto the significant occlusion of the pores with thepolymer binder and the low ionic conductivity of thisplasticized electrolyte system. Dasgupta and Ja-cobs163,175 patented several variants of the process forthe fabrication of bonded-electrode lithium-ion bat-teries, in which a microporous separator and elec-trode were coated with a liquid electrolyte solution,such as ethylene-propylenediene (EPDM) copolymer,and then bonded under elevated temperature andpressure conditions. This method required that thewhole cell assembling process be carried out underscrupulously anhydrous conditions, which made itvery difficult and expensive.

More recent methods proposed by Motorola176,177and Mitsubishi Electric178 researchers differ in imple-mentation details, but they share a common featurein that a separate adhesive layer (PVdF) is appliedto the separator and used to bond the electrode andthe separator films, using in the first case the hot,liquid electrolyte as an in situ PVdF plasticizer.Recently, Sony179,180 researchers described the use ofa thin, liquid electrolyte-plasticized polyacrylonitrile

layer directly applied either to the electrode or theseparator surfaces as an effective ion-conductiveadhesive. Sanyo181,182 investigators, on the otherhand, used thermally polyerizable additives to gel,or solidify, liquid electrolyte solutions in a wound,packaged battery.

The ceramic fillers (e.g. Al2O3, SiO2, TiO2) cangreatly influence the characteristics and propertiesof polymer electrolyte by enhancing the mechanicalstability and the conductivity.141,183-186 -LiAlO2,Al2O3, and MgO were used as fillers by Prosini etal.187 in a PVdF-HFP polymer matrix to form self-standing, intrinsically porous separators for lithium-ion batteries. The MgO based separators showed thebest anode and cathode compatibilities.

Liu et al.188 has successfully prepared a PVdF-HFP/PE composite gel electrolyte by cast method.They showed that when the PE content was over 23wt %, the electrical impedance of the composite gelelectrolyte increased rapidly by several orders ofmagnitude, around the melting point of PE. The SEMpictures showed that the PE particles were fused andformed into a continuous film at or near the PEmelting point, which cuts off the ion diffusion. Thisshutdown feature of the composite gel electrolyte canhelp in preventing the cell runaway under abusiveusage. Similarly, poly(ethylene oxide) (PEO) coatedseparators were prepared by Kim et al.189 by coatingPEO onto a microporous PE separators. The ionicconductivity of PEO coated membranes was higherthen the base film. Kim et al. prepared the polymerelectrolytes by coating poly(ethylene oxide) (PEO) a

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