Lithium microporous separator materials

Table l. Commercially available microporous membrane materials used as separators in lithium-ion batteries. 

They are fabricated from a variety of inorganic, organic, and naturally occurring materials and generally contain pores that are greater than 50—100 A 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)), and naturally occurring substances (e.g. rubber, asbestos, wood) have been used for microporous separators in batteries that operate at ambient and low temperatures (<100 °C). The microporous polyolefins (PP, PE, or laminates of PP and PE) are widely used in lithium based nonaqueous batteries (section 6.1), and filled polyethylene separators in lead-acid batteries (section 7.3), respectively. 

A novel microporous separator using polyolefins has been developed and used extensively in lithium-ion batteries since it is difficult for conventional separator materials to satisfy the characteristics required in lithium-ion batteries. In lithium-ion batteries two layers of separators are sandwiched between positive and negative electrodes and then spirally wound together in cylindrical and prismatic configurations. The pores of the separator are filled with ionically conductive liquid electrolyte. 

Solid electrolytes for lithium-ion batteries are expected to offer several advantages over traditional, nonaqueous liquid electrolytes. A solid electrolyte would give a longer shelf life, along with an enhancement in specific energy density. A solid electrolyte may also eliminate the need for a distinct separator material, such as the polypropylene or polyethylene microporous separators commonly used in contemporary liquid electrolyte-based batteries. Solid electrolytes are also desirable over liquid electrolytes in certain specialty applications where bulk lithium-ion batteries as weU as thin-film lithium-ion batteries are needed for primary and backup power supplies for systems, devices, and individual integrated circuit chips. 

Fignre 27.3 shows a typical spectroelectrochemical cell for in sitn XRD on battery electrode materials. The interior of the cell has a construction similar to a coin cell. It consists of a thin Al203-coated LiCo02 cathode on an aluminum foil current collector, a lithium foil anode, a microporous polypropylene separator, and a nonaqueous electrolyte (IMLiPFg in a 1 1 ethylene carbonate/dimethylcarbonate solvent). The cell had Mylar windows, an aluminum housing, and was hermetically sealed in a glove box. 

In most batteries, the separators are either made of nonwoven fabrics or microporous polymeric films. Batteries that operate near ambient temperatures usually use separators fabricated from organic materials such as cellulosic papers, polymers, and other fabrics, as well as inorganic materials such as asbestos, glass wool, and Si02. In alkaline batteries, the separators used are either regenerated cellulose or microporous polymer films. The lithium batteries with organic electrolytes mostly use microporous films. 

In lithium-based cells, the essential function of battery separator is to prevent electronic contact, while enabling ionic transport between the positive and negative electrodes. It should be usable on highspeed winding machines and possess good shutdown properties. The most commonly used separators for primary lithium batteries are microporous polypropylene membranes. Microporous polyethylene and laminates of polypropylene and polyethylene are widely used in lithium-ion batteries. These materials are chemically and electrochemically stable in secondary lithium batteries. 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

The materials used in nonwoven fabrics include a single polyolefin, or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), polytetrafluoroethylene (PTFE), polyvinylidine fluoride (PVdF), and polyvinyl chloride (PVC). Nonwoven fabrics have not been able to compete with microporous films in lithium-ion cells. This is primarily because of the inadequate pore-size structure and difficulty in making thin (<25 pm) nonwoven fabrics with acceptable physical properties. However, nonwoven separators have been used in button cells and bobbin cells when thicker separators and low discharge rates are acceptable. 

Microporous polymer membranes are the most commonly used separators in lithium-ion batteries. Majority of the microporous polymer membrane separators are based on semicrystalline polyolefin materials, such as polyethylene (PE), polypropylene (PP), PE-PP blends and high-density polyethylene (HDPE)-ultrahigh molecular polyethylene (UHMWPE). 

Lithium batteries using solid cathodes typically use a thin lithium-metal foil or disk as the anode a transition metal oxide, metal sulfide, or a fluoride as the cathode and an organic electrolyte. The cathode material is either coated onto a foil substrate (often aluminum) or embedded into an expanded metal or perforated metal substrate. The anode and cathode are separated by a microporous plastic membrane separator, usually polyethylene and/or polypropylene. 

The electrochemically active electrode materials in Li-ion batteries are a lithium metal oxide for the positive electrode and lithiated carbon for the negative electrode. These materials are adhered to a metal foil current collector with a binder, typically polyvinylidene fluoride (PVDF) or the copolymer polyvinylidene fluoride-hexafluroropropylene (PVDF-HFP), and a conductive diluent, typically a high-surface-area carbon black or graphite. The positive and negative electrodes are electrically isolated by a microporous polyethylene or polypropylene separator film in products that employ a liquid electrolyte, a layer of gel-polymer electrolyte in gel-polymer batteries, or a layer of solid electrolyte in solid-state batteries. 

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