A separator is a permeable membrane placed between a battery's anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell.[1]
Separators are critical components in liquid electrolyte batteries. A separator generally consists of a polymeric membrane forming a microporous layer. It must be chemically and electrochemically stable with regard to the electrolyte and electrode materials and mechanically strong enough to withstand the high tension during battery construction. They are important to batteries because their structure and properties considerably affect the battery performance, including the batteries energy and power densities, cycle life, and safety.[2]
History
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Unlike many forms of technology, polymer separators were not developed specifically for batteries. They were instead spin-offs of existing technologies, which is why most are not optimized for the systems they are used in. Even though this may seem unfavorable, most polymer separators can be mass-produced at a low cost, because they are based on existing forms of technologies.[3] Yoshino and co-workers at Asahi Kasei first developed them for a prototype of secondary lithium-ion batteries (LIBs) in 1983.
Schematic of a lithium ion batteryInitially, lithium cobalt oxide was used as the cathode and polyacetylene as the anode. Later in 1985, it was found that using lithium cobalt oxide as the cathode and graphite as the anode produced an excellent secondary battery with enhanced stability, employing the frontier electron theory of Kenichi Fukui.[4] This enabled the development of portable devices, such as cell phones and laptops. However, before lithium ion batteries could be mass-produced, safety concerns needed to be addressed such as overheating and over potential. One key to ensuring safety was the separator between the cathode and anode. Yoshino developed a microporous polyethylene membrane separator with a “fuse” function.[5] In the case of abnormal heat generation within the battery cell, the separator provides a shutdown mechanism. The micropores close by melting and the ionic flow terminates. In 2004, a novel electroactive polymer separator with the function of overcharge protection was first proposed by Denton and coauthors.[6] This kind of separator reversibly switches between insulating and conducting states. Changes in charge potential drive the switch. More recently, separators primarily provide charge transport and electrode separation.
Materials
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Materials include nonwoven fibers (cotton, nylon, polyesters, glass), polymer films (polyethylene, polypropylene, poly (tetrafluoroethylene), polyvinyl chloride), ceramic[7] and naturally occurring substances (rubber, asbestos, wood). Some separators employ polymeric materials with pores of less than 20 Å, generally too small for batteries. Both dry and wet processes are used for fabrication.[8][9]
Nonwovens consist of a manufactured sheet, web or mat of directionally or randomly oriented fibers.
Supported liquid membranes consist of a solid and liquid phase contained within a microporous separator.
Some polymer electrolytes form complexes with alkali metal salts, which produce ionic conductors that serve as solid electrolytes.
Solid ion conductors, can serve as both separator and the electrolyte.[10]
Separators can use a single or multiple layers/sheets of material.
Production
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Polymer separators generally are made from microporous polymer membranes. Such membranes are typically fabricated from a variety of inorganic, organic and naturally occurring materials. Pore sizes are typically larger than 50-100 Å.
Dry and wet processes are the most common separation production methods for polymeric membranes. The extrusion and stretching portions of these processes induce porosity and can serve as a means of mechanical strengthening.[11]
Membranes synthesized by dry processes are more suitable for higher power density, given their open and uniform pore structure, while those made by wet processes are offer more charge/discharge cycles because of their tortuous and interconnected pore structure. This helps to suppress the conversion of charge carriers into crystals on anodes during fast or low temperature charging.[12]
Dry process
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The dry process involves extruding, annealing and stretching steps. The final porosity depends on the morphology of the precursor film and the specifics of each step. The extruding step is generally carried out at a temperature higher than the melting point of the polymer resin. This is because the resins are melted to shape them into a uniaxially-oriented tubular film, called a precursor film. The structure and orientation of the precursor film depends on the processing conditions and the resin's characteristics. In the annealing process, the precursor is annealed at a temperature slightly lower than the polymer's melting point. The purpose of this step is to improve the crystalline structure. During stretching, the annealed film is deformed along the machine direction by a cold stretch followed by a hot stretch followed by relaxation. The cold stretch creates the pore structure by stretching the film at a lower temperature with a faster strain rate. The hot stretch increases pore sizes using a higher temperature and a slower strain rate. The relaxation step reduces internal stress within the film.[13][14]
The dry process is only suitable for polymers with high crystallinity. These include but are not limited to: semi-crystalline polyolefins, polyoxymethylene, and isotactic poly (4-methyl-1-pentene). One can also use blends of immiscible polymers, in which at least one polymer has a crystalline structure, such as polyethylene-polypropylene, polystyrene-polypropylene, and poly (ethylene terephthalate) - polypropylene blends.[9][15]
Dry Microstructure
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After processing, separators formed from the dry process possess a porous microstructure. While specific processing parameters (such as temperature and rolling speed) influence the final microstructure, generally, these separators have elongated, slit-like pores and thin fibrils that run parallel to the machine direction. These fibrils connect larger regions of semi-crystalline polymer, which run perpendicular to the machine direction.[11]
Wet process
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The wet process consists of mixing, heating, extruding, stretching and additive removal steps. The polymer resins are first mixed with, paraffin oil, antioxidant and other additives. The mixture is heated to produce a homogenous solution. The heated solution is pushed through a sheet die to make a gel-like film. The additives are then removed with a volatile solvent to form the microporous result.[16] This microporous result can then be stretched uniaxially (along the machine direction) or biaxially (along both the machine and transverse directions, providing further pore definition.[11]
The wet process is suitable for both crystalline and amorphous polymers. Wet process separators often use ultrahigh-molecular-weight polyethylene. The use of these polymers enables the batteries with favorable mechanical properties, while shutting it down when it becomes too hot.[17]
Wet Microstructure
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When subjected to biaxial stretching, separators formed from the wet process have rounded pores. These pores are dispersed throughout an interconnected polymer matrix.[11]
Choice of polymer
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The chemical structure of polypropylene The chemical structure of polyethyleneSpecific types of polymers are ideal for the different types of synthesis. Most polymers currently used in battery separators are polyolefin based materials with semi-crystalline structure. Among them, polyethylene, polypropylene, PVC, and their blends such as polyethylene-polypropylene are widely used. Recently, graft polymers have been studied in an attempt to improve battery performance, including micro-porous poly(methyl methacrylate)-grafted[16] and siloxane grafted polyethylene separators, which show favorable surface morphology and electrochemical properties compared to conventional polyethylene separators. In addition, polyvinylidene fluoride (PVDF) nanofiber webs can be synthesized as a separator to improve both ion conductivity and dimensional stability.[3] Another type of polymer separator, polytriphenylamine (PTPAn)-modified separator, is an electroactive separator with reversible overcharge protection.[6]
Placement
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Side view of a batteryThe separator is always placed between the anode and the cathode. The pores of the separator are filled with the electrolyte and packaged for use.[18]
Essential properties
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There are multiple factors that contribute to the overall mechanical profile of a separator.
Tensile strength
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Puncture strength
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Defects
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Many structural defects can form in polymer separators due to temperature changes. These structural defects can result in a thicker separators. Furthermore, there can be intrinsic defects in the polymers themselves, such as polyethylene often begins to deteriorate during the stages of polymerization, transportation, and storage.[30] Additionally, defects such as tears or holes can form during the synthesis of polymer separators. There are also other sources of defects can come from doping the polymer separator.[2]
Use in Li-ion Batteries
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Polymer separators, similar to battery separators in general, act as a separator of the anode and cathode in the Li-ion battery while also enabling the movement of ions through the cell. Additionally, many of the polymer separators, typically multilayer polymer separators, can act as “shutdown separators”, which are able to shut down the battery if it becomes too hot during the cycling process. These multilayered polymer separators are generally composed of one or more polyethylene layers which serve to shut down the battery and at least one polypropylene layer which acts as a form of mechanical support for the separator.[6][31]
Separators are also subjected to numerous stresses during battery assembly and battery usage. Common stresses include tensile stresses from dry/wet processes and compressive stresses from the volumetric expansion of electrodes and required forces to ensure sufficient contact between components. Dendritic lithium growths are another common source of stress. These stresses are often applied concurrently, creating a complex stress field that separators must withstand. Additionally, standard battery operation leads to the cyclic application of these stresses. These cyclic conditions can mechanically fatigue separators, which reduces strength, leading to eventual device failure.[32]
Other types of battery separators
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In addition to polymer separators, there are several other types of separators. There are nonwovens, which consist of a manufactured sheet, web, or mat of directionally or randomly oriented fibers. Supported liquid membranes, which consist of a solid and liquid phase contained within a microporous separator. Additionally there are also polymer electrolytes which can form complexes with different types of alkali metal salts, which results in the production of ionic conductors which serve as solid electrolytes. Another type of separator, a solid ion conductor, can serve as both a separator and the electrolyte in a battery.[10]
Plasma technology was used to modify a polyethylene membrane for enhanced adhesion, wettability and printability. These are usually performed by modifying the membrane on only its outermost several molecular levels. This allows the surface to behave differently without modifying the properties of the remainder. The surface was modified with acrylonitrile via a plasma coating technique. The resulting acrylonitrile-coated membrane was named PiAn-PE. The surface characterization demonstrated that PiAN-PE's enhanced adhesion resulted from the increased polar component of surface energy.[33]
The sealed rechargeable nickel-metal hydride battery offers significant performance and environmental friendliness above alkaline rechargeable batteries. Ni/MH, like the lithium-ion battery, provides high energy and power density with long cycle lives. This technology's greatest problem is its inherent high corrosion rate in aqueous solutions. The most commonly used separators are porous insulator films of polyolefin, nylon or cellophane. Acrylic compounds can be radiation-grafted onto these separators to make their properties more wettable and permeable. Zhijiang Cai and co-workers developed a solid polymer membrane gel separator. This was a polymerization product of one or more monomers selected from the group of water-soluble ethylenically unsaturated amides and acid. The polymer-based gel also includes a water swellable polymer, which acts as a reinforcing element. Ionic species are added to the solution and remain embedded in the gel after polymerization.
Ni/MH batteries of bipolar design (bipolar batteries) are being developed because they offer some advantages for applications as storage systems for electric vehicles. This solid polymer membrane gel separator could be useful for such applications in bipolar design. In other words, this design can help to avoid short-circuits occurring in liquid-electrolyte systems.[34]
Inorganic polymer separators have also been of interest as use in lithium-ion batteries. Inorganic particulate film/poly(methyl methacrylate) (PMMA)/inorganic particulate film trilayer separators are prepared by dip-coating inorganic particle layers on both sides of PMMA thin films. This inorganic trilayer membrane is believed to be an inexpensive, novel separator for application in lithium-ion batteries from increased dimensional and thermal stability.[35]
References
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Pioneering work of the lithium battery began in 1912 under G.N. Lewis, but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the 1980s but failed because of instabilities in the metallic lithium used as anode material. (The metal-lithium battery uses lithium as anode; Li-ion uses graphite as anode and active materials in the cathode.)
Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest specific energy per weight. Rechargeable batteries with lithium metal on the anode could provide extraordinarily high energy densities; however, it was discovered in the mid-1980s that cycling produced unwanted dendrites on the anode. These growth particles penetrate the separator and cause an electrical short. The cell temperature would rise quickly and approach the melting point of lithium, causing thermal runaway, also known as “venting with flame.” A large number of rechargeable metallic lithium batteries sent to Japan were recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a man’s face.
The inherent instability of lithium metal, especially during charging, shifted research to a non-metallic solution using lithium ions. In 1991, Sony commercialized the first Li ion, and today this chemistry has become the most promising and fastest growing battery on the market. Although lower in specific energy than lithium-metal, Li ion is safe, provided the voltage and currents limits are being respected. (See BU-304a: Safety Concerns with Li-ion)
Credit for inventing the lithium-cobalt-oxide battery should go to John B. Goodenough (1922). It is said that during the developments, a graduate student employed by Nippon Telephone & Telegraph (NTT) worked with Goodenough in the USA. Shortly after the breakthrough, the student traveled back to Japan, taking the discovery with him. Then in 1991, Sony announced an international patent on a lithium-cobalt-oxide cathode. Years of litigation ensued, but Sony was able to keep the patent and Goodenough received nothing for his efforts. In recognition of the contributions made in Li-ion developments, the U.S. National Academy of Engineering awarded Goodenough and other contributors the Charles Stark Draper Prize in 2014. In 2015, Israel awarded Goodenough a $1 million prize, which he will donate to the Texas Materials Institute to assist in materials research.
The key to the superior specific energy is the high cell voltage of 3.60V. Improvements in the active materials and electrolytes have the potential to further boost the energy density. Load characteristics are good and the flat discharge curve offers effective utilization of the stored energy in a desirable and flat voltage spectrum of 3.70–2.80V/cell.
In 1994, the cost to manufacture Li-ion in the 18650 cylindrical cell was over US$10 and the capacity was 1,100mAh. In 2001, the price dropped to below $3 while the capacity rose to 1,900mAh. Today, high energy-dense 18650 cells deliver over 3,000mAh and the costs are dropping. Cost reduction, increased specific energy and the absence of toxic material paved the road to make Li-ion the universally accepted battery for portable applications, heavy industries, electric powertrains and satellites. The 18650 measures 18mm in diameter and 65mm in length. (See BU-301: A look at Old and New Battery Packaging)
Li-ion is a low-maintenance battery, an advantage that most other chemistries cannot claim. The battery has no memory and does not need exercising (deliberate full discharge) to keep it in good shape. Self-discharge is less than half that of nickel-based systems and this helps the fuel gauge applications. The nominal cell voltage of 3.60V can directly power mobile phones, tablets and digital cameras, offering simplifications and cost reductions over multi-cell designs. The drawbacks are the need for protection circuits to prevent abuse, as well as high price.
Lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor. (The anode of a discharging battery is negative and the cathode positive (see BU-104b: Battery Building Blocks). The cathode is metal oxide and the anode consists of porous carbon. During discharge, the ions flow from the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions flow from the cathode to the anode. Figure 1 illustrates the process.
Figure 1: Ion flow in lithium-ion battery.Li ion batteries come in many varieties but all have one thing in common – the “lithium-ion” catchword. Although strikingly similar at first glance, these batteries vary in performance and the choice of active materials gives them unique personalities. (See BU-205: Types of Li-ion-ion)
Sony’s original lithium-ion battery used coke as the anode (coal product). Since 1997, most Li ion manufacturers, including Sony, shifted to graphite to attain a flatter discharge curve. Graphite is a form of carbon that has long-term cycle stability and is used in lead pencils. It is the most common carbon material, followed by hard and soft carbons. Nanotube carbons have not yet found commercial use in Li-ion as they tend to entangle and affect performance. A future material that promises to enhance the performance of Li-ion is graphene.
Figure 2 illustrates the voltage discharge curve of a modern Li-ion with graphite anode and the early coke version.
Several additives have been tried, including silicon-based alloys, to enhance the performance of the graphite anode. It takes six carbon (graphite) atoms to bind to a single lithium ion; a single silicon atom can bind to four lithium ions. This means that the silicon anode could theoretically store over 10 times the energy of graphite, but expansion of the anode during charge is a problem. Pure silicone anodes are therefore not practical and only 3–5 percent of silicon is typically added to the anode of a silicon-based to achieve good cycle life.
Using nano-structured lithium-titanate as an anode additive shows promising cycle life, good load capabilities, excellent low-temperature performance and superior safety, but the specific energy is low and the cost is high.
Experimenting with cathode and anode material allows manufacturers to strengthen intrinsic qualities, but one enhancement may compromise another. The so-called “Energy Cell” optimizes the specific energy (capacity) to achieve long runtimes but at lower specific power; the “Power Cell” offers exceptional specific power but at lower capacity. The “Hybrid Cell” is a compromise and offers a little bit of both. (More on BU-501: Basics About Discharging)
Manufacturers can attain a high specific energy and low cost relatively easily by adding nickel in lieu of the more expensive cobalt, but this makes the cell less stable. While a start-up company may focus on high specific energy and low price to gain quick market acceptance, safety and durability cannot be compromised. Reputable manufacturers place high integrity on safety and longevity. Table 3 summarizes the advantages and limitations of Li-ion.
Most Li-ion batteries share a similar design consisting of a metal oxide positive electrode (cathode) that is coated onto an aluminum current collector, a negative electrode (anode) made from carbon/graphite coated on a copper current collector, a separator and electrolyte made of lithium salt in an organic solvent. Table 3 summarizes the advantages and limitations of Li-ion.