A separator is an ion-permeable barrier placed between the anode and the cathode of a battery. The separator, as the name implies, mechanically separates the two electrodes from each other, and is an electronic insulator at a given voltage to prevent electrical short circuiting of the battery. The separator, with an added electrolyte, enables the transport of ionic charge carriers (the positively and negatively charged ions) through the internal circuit of the galvanic cell; this stream of ions closes the circuit during the passage of current. Given the numerous types of separators available for battery development and the important role they play in a galvanic cell, the proper choice of the separator for new battery development is critical.
In the first part of this 2-part blog series, we will briefly review different types of separators used in batteries. The second part will elaborate on separator properties, and how these features render the separators suitable for certain battery applications.
What are the Common Types of Battery Separators Used?
Separators utilized in galvanic cells can be divided into six different types, based on their physical and chemical characteristics. These include:
Supported liquid membranes
Solid ion conductors
You may have noticed that types 5 and 6 above combine the features of electrolytes and separators. These have been addressed in a previous blog post on the choice of a proper electrolyte for a given custom battery application.
1. Microporous separators
Microporous separators can be manufactured from various inorganic materials (e.g., glass, silica), organic materials (non-woven nylon, cotton, or polyester fibers, or porous forms of polymer films, e.g., polyvinylchloride, polypropylene, and cellulose acetate), or naturally occurring raw materials, such as rubber, asbestos, and wood. Their pore size exceeds 5-10 nanometers in diameter. Such separators allow the electrolyte to get impregnated into their pores. Microporous separators are typically saturated with the liquid electrolyte, and act as an electrolyte reservoir for the cell. Ultrathin cells, for example, may operate exclusively with the electrolyte solution stored in the separator and the porous electrode materials. Microporous polyolefin separators are widely used in lithium ion batteries, where the pores of the separator are filled with the ionically conductive liquid electrolyte.
The choice of a microporous separator depends on battery chemistry, as the separator should withstand oxidizing and reducing conditions at high positive and negative voltages, respectively, and should be chemically inert in contact with the electrolyte. There is also a trade-off between separator thickness and porosity, i.e., the ability to store sufficient electrolyte. For lithium-ion batteries the separator could have a built-in safety mechanism, which shuts down the cell, when overheating.
2. Nonwoven separators
Nonwoven materials are defined as porous fabrics, composed of a random array of fibers having the specific function to mechanically separate solid phases and components. They are textiles, in the form of sheets, web, or matt, manufactured from directionally or randomly oriented fibers in a dry-lay or wet-lay manufacturing process. Materials used in nonwoven fabrics can be glass fibers or glass fibers impregnated with a phenolic resin, PVA fibers and binders, single polyolefin and combination of polyolefins (including polyethylene, polypropylene, polyamide, PTFE, PVC, and others). They can be composites made of polymer fibers (e.g., polyacrylonitrile) and ceramics containing polyolefin nonwoven.
The pore size of nonwoven can range from nanometers to micrometers. Typically, nonwoven separators are resistant to both acids and alkali, they are highly porous and consistent. These features render them a preferred separator protection layer for stationary and traction batteries. Lightweight, wet laid nonwovens, consisting of cellulose and poly(vinyl alcohol) or polyester spunbond products are popular in primary alkaline cells and in Ni-MH batteries of various sizes.
3. Ion-exchange membranes (IEM)
Ion-exchange membranes can be divided into cation-exchange membranes and anion-exchange membranes. They are typically made of polymeric materials containing small pores, with diameters < 2 nanometers. Grafted ion-exchange groups are present on the polymer chains of these membranes. These groups control the ion transport properties across the membrane, and allow discriminating between permeating ions based on their charge.
For example, if the separator exhibits cation-exchange properties, it will preferentially transfer protons or alkali metal ions across the separator, while anions and larger size cations are rejected so they are not being moved from one side of the membrane to the other. In other words, the ion-exchange membrane differentiates between cations and anions, a property termed as permselectivity. IUPAC defines permselectivity as the preferential permeation of certain ionic species through ion-exchange membranes. The greater the permselectivity, the more discriminating the membrane transport is. Nevertheless, greater permselectivity (i.e., ion discriminating power) translates into greater ionic resistance of the separator, which is not desirable, as it involves more operating voltage drop in the battery.
Ion exchange membranes (IEM) represent key materials for vanadium redox flow batteries, which are suitable for large scale energy storage. The role of the IEM is to prevent cross mixing of the positive and negative electrolytes, while still allowing the transport of ions to complete the circuit during the passage of current. Ion exchange membranes enable vanadium redox flow battery (VRB), iron/chromium, bromine/polysulfide, or hybrid redox-flow battery (e.g., zinc/bromine) applications.
4. Supported liquid membranes
Supported liquid membranes, also referred to as Ionogels, are a relatively new class of materials, which are made of an ionically conducting liquid immobilized inside a polymer matrix. The liquid phase is retained in the pores of the solid matrix by capillary forces. The liquid stored in the micropores must be chemically stable, insoluble in the electrolyte, thermally stable and non-flammable, combined with high ionic conductivity and wide electrochemical stability window.
As microporous substrates one can use polypropylene, polysulfone, PTFE, and cellulose acetate. The supported liquid can be a gel, normally a two-component system made from a solvent and a gelling agent. This approach allows the design of unique materials with the high ionic conductivity of a liquid or gel electrolyte combined with the easy handling properties of a solid electrolyte. To date, membrane supported gel polymer electrolytes have found success in lithium polymer batteries.
5. Polymer electrolytes
Polymer electrolytes form complexes with alkali metals, to yield ionic conductors that serve as solid electrolytes. Polymers that can be made from these complexes include polyethylene oxide (PEO), polypropylene oxide (PPO), and polyvinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-HFP). Their ionic conductivity is low at room temperature because of the ohmic resistance of the electrolyte/electrode interface. Nevertheless, because of their rigid structure, polymer electrolytes act as both the electrolyte and separator. Solid polymer electrolytes (SPEs) serve in the cell as both separator and electrolyte.
Mainly used in fuel cells and electrolysis cells, polymer electrolytes also have applications in commercial lithium cells or batteries based on other chemistries. They convey enhanced safety over conventional lithium ion batteries. They have a good ability to absorb and retain the lithium ion containing electrolyte. Advanced gel polymer electrolytes have been developed for state-of-the-art lithium-ion polymer (Li-Po) batteries. They enable Li-Po batteries with a wide variety of shapes, sizes, and dimensions.
6. Solid Ion Conductors
Solid ion conductors (also referred to as solid electrolytes or ionic conductors) are solids that conduct electricity by passage of ions. Usually, only one type of ion (either cation or anion) is primarily mobile. Solids with room-temperature ionic conductivity greater than 10-2-10-3 S/m are called fast ion or superionic conductors. The use of solid ion conductors simplifies battery construction, given that they act as both the separator and the electrolyte. When an electrical potential gradient (voltage) or a chemical gradient (concentration difference) is present, the solid ion conductor allows for the migration of selected ions through their molecular structures.
Primarily used in solid oxide fuel cells, solid-state (ceramic) electrolytes have applications in lithium ion batteries; enabling all-solid-state cells that are safe in operation, do not exhibit leakage, poor chemical stability, or flammability. Furthermore, solid-state Li batteries can operate at high voltage, thus, producing high power density. Special applications with solid ion conductors include rechargeable sodium ion batteries. The limitations of solid ion conductors include lower rate capability and fragility of thin solid separators.
In this article, we have summarized the various types of separator choices. In our next article, we will describe the application requirements to consider for selecting the best separator. You can also check out the battery fundamentals or the battery resources sections of our blog for useful information. As always feel free to contact us for battery development questions. Subscribe to our blog for updates related to batteries.