Gel electrolyte separators

Abraham etol. [43] suggested the use of polyacrylamide gels supported in microporous membranes. This approach could provide the packaging advantages 

Since the realization in the early 1980s that poly(ethylene oxide) could serve as a Hthium-ion conductor in Hthium batteries, there has been continued interest in polymer electrolyte batteries. Conceptually, the electrolyte layer could be made very thin (5 rm) and so provide higher energy density. Fauteux et al. [47] reviewed the state of polymer electrolyte technology in 1995. To summarize here briefly, a polymer electrolyte with acceptable conductivity ( 10 Scm ), transport number ( 0.9), mechanical properties, and electrochemical stability to high voltage positives has yet to be developed. Ten years later the situation has not changed. 

Avestor spent hundreds of millions of dollars to develop a lithium metal polymer battery, but the company went out of business. The battery used a low-voltage LiV30g positive to stabilize the polymer electrolyte, which consisted of a PE oxide polyether copolymer and hthium perfluorosulfonimide (Li(CF3S0CNS02CF3)). The battery was heated above 40 °C in order to achieve a conductivity of 10 S cm . The battery proved ineffective for use in electric vehicles, but showed some promise in telecommunications applications where cycling was limited. However, even the telecommunications apphcation did not work out. 

Interest in polymer electrolytes has been renewed by Seeo. According to news accounts, Seeo has developed a high-conductivity polymer. According to a patent apphcation [48], the polymer is a block copolymer consisting of polystyrene-block-poly(ethylene oxide) (SEO) diblock copolymer and requires elevated temperature to achieve a conductivity of 10 S cm .  

Recently, Nitto Denko has patented a single-layer separator made from a PE/PP blend by the dry stretch process [24], According to the patent, the separator has microporous regions of PE and PP. On heating in an oven, the impedance of the separator increases near the melting point of PE and the impedance remains high until beyond the melting point of PP. However, battery performance data have not been presented. 

The liquid electrolytes used in lithium batteries can be gelled by addition of a polymer [25] or fumed silica [26], or by cross linking of a dissolved monomer [271. Depending on the mechanical properties, gelled electrolytes can be used as separators, or supported by a conventional [27] 

Gozdz et al. (of Bellcore) [25] recognized that poly (vinylidene difluoride) hexafluoropropylene (PVDF HFP) copolymers could form gels with organic solvents and developed an entire battery based on this concept. Typically, the gel separator is 50 pm thick and comprises 60wt. % polymer. In the Bellcore process the separator is laminated to the electrodes under pressure at elevated temperature. The use of the PVDF HFP gelling agent increases the resistivity of the electrolyte by about five times which limits the rate capability of such batteries. 

Dasgupta and Jacobs [29] patented a concept of using a gel layer in combination with a microporous membrane. The gel layer acts as an adhesive bridge between separator and electrodes, just as in the flat pack Zn/MnC cell [30], The microporous membrane (for example, Celgard membrane) provides excellent mechanical 

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Battery makers sometimes view separators with disdain the separator is needed but does not actively contribute to battery operation. Consequently, very little work (relative to that on electrode materials and electrolytes) is directed towards characterizing separators. In fact, development efforts are under way to displace microporous membranes as battery separators and instead to use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by elimi-... 

Doyle et al. [40] used a mathematical model to examine the effect of separator thickness for the PVDF.HFP gel electrolyte system and found that decreasing separator thickness below 52 pm caused only a minor decrease in ohmic drop across the cell. The voltage drops in the electrodes were much more significant. They state that their model predictions were confirmed experimentally. 

Lithium secondary batteries can be classified into three types, a liquid type battery using liquid electrolytes, a gel type battery using gel electrolytes mixed with polymer and liquid, and a solid type battery using polymer electrolytes. The types of separators used in different types of secondary lithium batteries are shown in Table 1. The liquid lithium-ion cell uses microporous polyolefin separators while the gel polymer lithium-ion cells either use a PVdF separator (e.g. PLION cells) or PVdF coated microporous polyolefin separators. The PLION cells use PVdF loaded with silica and plasticizer as separator. The microporous structure is formed by removing the plasticizer and then filling with liquid electrolyte. They are also characterized as plasticized electrolyte. In solid polymer lithium-ion cells, the solid electrolyte acts as both electrolyte and separator. 

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... 

Recent 7Li-NMR studies on gel electrolytes based on PAN/EC/PC/LiC104 confirms that the onset of the narrowing of linewidth of the 7Li-NMR signal is closely correlated with the Tg of the material [Fig. 22] as is found for PEO-LiX electrolytes [266]. Further from the similarity of the 7Li-NMR linewidths ( 300 Hz at 300 K) observed in gel electrolytes with that of PEO-LiX electrolytes in comparison with liquid electrolytes containing EC-PC and LiX (15 Hz at 295K), it is concluded that the interpretation of the high conductivity of gel electrolytes in terms of microscopic packets of liquid electrolyte separated by regions of inert PAN matrix may be erroneous [266]. 

Typically, the solid-state electrolyte can function as both a substrate and separator in the fabrication of a flexible supercapacitor by two steps. For instance, a freestanding PVA/ H3PO4 film was obtained by first casting the gel electrolyte on a glass slide and then peeled off from the glass. It had been further sandwiched between two composite electrodes that were prepared from conducting polymers and nanoparticles to produce the flexible supercapacitor (Fig. 9.3G) (Liu et al., 2010,2013b). 

A typical charge-discharge cycle is shown in Figure 7.14, and confirms the feasibility of the PAN-based gel electrolytes as separators in lithium-ion batteries by showing that the cell can indeed be cycled with a good capacity delivery. 

Due to its many advantageous properties (low cost, fast color change, good contrast, stability, etc.), PANI is also a favorite material for use in electrochromic display devices. Pictures of a PANI-based flexible device are shown in Fig. 7.11. The display pattern, which consists of 25 pixels and the connections that allow each pixel to be driven separately, was fabricated by depositing gold onto a plastic sheet. Another plastic sheet covers the display. The electrochemical switching is executed using a counterelectrode, which also serves as a reference electrode, and an acidic gel electrolyte is placed between the two sealed plastic sheets. 

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