Separator (electricity)
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.
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.
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.
However, before lithium ion batteries could be mass-produced, safety concerns needed to be addressed such as overheating and over potential.
[5] In the case of abnormal heat generation within the battery cell, the separator provides a shutdown mechanism.
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).
[8][9] Nonwovens consist of a manufactured sheet, web or mat of directionally or randomly oriented fibers.
The extrusion and stretching portions of these processes induce porosity and can serve as a means of mechanical strengthening.
The extruding step is generally carried out at a temperature higher than the melting point of the polymer resin.
The hot stretch increases pore sizes using a higher temperature and a slower strain rate.
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] The wet process consists of mixing, heating, extruding, stretching and additive removal steps.
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.
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.
These cyclic conditions can mechanically fatigue separators, which reduces strength, leading to eventual device failure.
There are nonwovens, which consist of a manufactured sheet, web, or mat of directionally or randomly oriented fibers.
Ni/MH, like the lithium-ion battery, provides high energy and power density with long cycle lives.
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.
This inorganic trilayer membrane is believed to be an inexpensive, novel separator for application in lithium-ion batteries from increased dimensional and thermal stability.
