Porous gel polymer electrolyte

Abstract The chapter begins by discussing the characters and composition of polymer electrolytes for electrochromic devices. It then describes the four types of the polymer electrolytes dry solid polymer electrolyte, gel polymer electrolyte, porous gel polymer electrolyte and composite solid polymer electrolyte, their preparation procedures and properties especially ion conductivity of the samples. Finally, new types of polymer electrolytes including proton-conducting, alkaline, single ionic polymer electrolytes and electrolytes with ionic liquids are also introduced. 

Generally, polymer electrolytes used for ECDs can be classified into the following four types according to their physical configuration and chemical composition dry solid polymer electrolyte (DSPE), gel polymer electrolyte (GPE), porous gel polymer electrolyte (PGPE) and composite solid polymer electrolyte (CSPE). 

Porous gel polymer electrolyte (PGPE) and composite solid polymer electrolyte (CSPE)... 

Poly( vinylidene fluoride)-hexafluoropropylene(P(VdF-HFP) )-based porous gel polymer electrolyte (PGPF)... 

A reversible lithium-air system was first implemented on a laboratory scale in 1996. In this cell, the gel-polymer electrolyte was pressed between lithium foil on the one side and an air electrode on the other. (Later, usual liquid electrolyte in a porous, for example, glass fabric, separator was often used in lithium-air batteries). The whole cell was sealed into a plastic container ( coffee bag ) and small holes were made in the container wall adjacent to the air electrode to supply air under discharge and remove oxygen under charging. The air electrode was made of a mixture of particles of polymer electrolyte and carbon black with the catalyst supported on its surface (cobalt phthalocyanine). 

Senthilkumar, S. T., R. K. Selvan, J. S. Melo, and C. Sanjeeviraja. 2013. High performance solid-state electric double layer capacitor from redox mediated gel polymer electrolyte and renewable tamarind fruit shell derived porous carbon. ACS Applied Materials Interfaces 5 10541-10550. 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, polytetrafluoroethylene, and cellulose acetate, have been used for porous substrates for supported liquid membranes. The PVdF-coated polyolefin-based microporous membranes used in gel polymer lithium-ion battery fall into this category. Gel polymer electrolytes/membranes are only discussed briefly. 

Kim, D.W. and Sun, Y.K. (2001) Electrochemical characterization of gel polymer electrolytes prepared with porous membranes. J. Power Sources. 

While the electrolyte layer used commercially is usually electrolyte solvent/Li salt with a porous separator (e.g., Celgard ), polymer addition to the electrolyte layer is becoming more prevalent due to safety considerations. With polymer addition, two types of systems are generally considered, namely, solid polymer electrolyte and a gel polymer electrolyte. Poly(ethylene oxide) is the preferred material for the solid polymer electrolyte which is comprised of polymer and Li salt with limited to no organic electrolyte solvent. The gel polymer electrolyte is comprised of polymer, Li salt and a large amount of organic electrolyte solvent. Poly(vinylidene fluoride) and copolymers, PMMA, and PAN are often noted as key candidates for gel polymer electrolyte layers. Not surprising, polymer blends have been proposed for both... 

Lee KM, Hsu CY, Chiu WH, Tsui MC, Tung YL, Tsai SY, Ho KC (2009) Dye-sensitized solar cells with a micro-porous Ti02 electrode and gel polymer electrolytes prepared by in situ cross-link reaction. Sol Energy Mater Sol Cells 93(ll) 2003-2007... 

In gel polymer electrolytes based on PVDF or P(VDF-HFP) plasticized with organic electrolytes such as carbonates, there are two ion-conducting phases. One is the solution filling the pores, which has weak interactions with the pol)uner and is a free solution. The other is in the amorphous domain of the pol)uner. The interaction of the electrolyte solution with the amorphous phase increases the ratio of the amorphous phase by the incorporation of the carbonate this increases the ionic conductivity but decreases the mechanical strength of the porous membrane. The ionic conductivity of the gel polymer electrolyte increases with the amoimt of electrolyte solution, and this increase is larger for samples with lower porosity. This indicates that the ion conduction is mainly along the polymer and less along the pores. The membrane is thermally and mechanically stable up to 100°C, both before and after electrolyte uptake [15]. 

Some soluble polymers such as PVDF can also be used to cover the surface of PE, forming a gel polymer electrolyte. This membrane has a good porous structure. To improve the binding with the electrode, poly(vinly acetate) (PVAc) is injected into PVDF and PE nonwoven fabric by dipping. The... 

The matrices of polymers such as poly(vinyl pyrrolidone) (PVP), polysul-fone, poly(trimethylene carbonate) (PTMC), triethylene glycol diacetate-butyl propenoate copolymer [28], and cellulose [29] are different from the mentioned polymers in Sections from 11.1 to 11.5. For example, when porous polysulfone is used as the polymer carrier, the ionic conductivity (3.93 x 10 S/cm at room temperature) and mechanical performance are greatly improved after adding plasticizers. When organic electrolyte is added to PTMC, the uptake ability is greatly improved because its structure is similar to that of the organic electrolyte. Methylcellulose (MC) is prepared easily as a porous polymer membrane, as illustrated in Figure 11.34. It can absorb liquid electrolyte to become a gel polymer electrolyte whose ionic conductivity is 0.2 mS/cm and lithium-ion transference number is 0.29. These results can compare with the commercial separator [29]. 

CHENG, c., WAN, c., WANG, Y., Preparation of porous, chemicaUy cross-hnked, PVdF-based gel polymer electrolytes for rechargeable hthium batteries, J. Power Sources, 2004,134(2), 202-10. 

We have already mentioned the interest of the separator in the case of gel-polymer electrolytes. This element is critical in the case of liquid electrolytes. The functirm of this membrane is to separate to prevent physical contact of the positive and negative electrodes. It must permit free ion flow, so that it must be porous. On the other hand, it should be an electrical insulator to prevent any electronic flow that... 

Both the anode and the cathode are composed of a coating of the electrochemically active material onto a current collector (copper or aluminum). Another key component of the battery is the separator that physically separates the two electrodes and prevents contact between them. In the case of a liquid technology battery, a polyolefin separator is typically used and a liquid electrolyte is used to transport the Li ions from one side of the porous separator to the other. In the case of a polymer Li ion battery, a polymer, such as PVDF, is used to form a porous structure, which is then swollen with a Li" " conducting liquid electro-lyte. " This results in a gel-type electrolyte, which plays the dual role of electrolyte and separator, with no free liquid present. 

There are several different modes of conducting electrophoresis in capillary columns. We have just discussed at length the basic one, CZE, which is conceptually the simplest. It is conducted with a liquid buffer of uniform composition of electrolyte concentration and pH level. There are more complex versions, in which the buffer liquid is enmeshed in a porous gel of hydrophilic polymer, or the concentration or the pH of the separation buffer is not constant along the length of the column, leading to variations in the migration velocities of individual ions as the separation proceeds. Several of these will be discussed subsequently. 

The liquid membrane type of separator relies on the concepts developed for liquid membranes for separation. The Hquid is retained in the pores of a solid porous matrix. Unlike liquid membranes for separations, the liquid in separators must be insoluble in the electrolyte but must provide ionic conductivity. The polyviny-lidine fluoride (PVDF) coated polyolefin-based membrane used in gel-polymer lithium-ion batteries is an example of the Hquid membrane type of separator. 

However, electrospun PAN fibrous membranes had undesirable effects, for example, liquid extraction from the gel, which resulted in decreased ionic conductivity of the polymer electrolytes upon long-term storage. Also, the porous PAN membranes were very brittle for the reason that the interaction of adjacent cyanide groups increases the resistance of interior rotation of the main chain and thus decreases the flexibility of the polymer chain [32]. Thus, the separator combination of thermally stable nonwoven frame with the electrospun PAN fibrous membranes is required in high-power battery. 

A gel polymer membrane based on P(MMA-AN-VAc) has been prepared by emulsion polymerization and phase inversion, and exhibits low crystallinity and Tg. Its ionic conductivity at room temperature is 3.48 x 10 g/cm, and its electrochemically stable voltage is above 5.0 V (vs. LP/Li). By further adding fumed silica, the semicrystalline state is changed into an amorphous porous structure. When 10 wt% fumed silica is added, the porosity of the polymer increases with an even distribution of pores. This intercoimected porous structure can improve the electrolyte retention ability and increase the ionic conductivity of the gel polymer from 3.48 x 10 g/cm to 5.13 x 10 3 S/cm. At the same time, the thermal and electrochemical stability of the membrane and the cycling performance of the assembled battery are improved. 

During the preparation of a gel copolymer P(VDF-HFP) electrolyte membrane by the phase inversion method, using different solvents such as NMP and NAf-dimethylformamide) (DMF) and nonsolvents such as dibutyl phthalate (DBF) and di-(2-ethylhexyl phthalate) (DEHP) will result in different pore sizes and different porosities. As a result, they are called porous polymer electrolytes, hi fact, they are gel electrolytes. The micromorphology of e membrane is related to the preparation conditions, but the solvents and nonsolvents used do not affect the ionic conductivity much, under certain conditions. After adding a certain amount of plasticizer, the pores will be filled by the plasticizer, and the ionic conductivity is 4.07 x 10 S/cm, with an electrochemical window of 4.5 V. 

Motorola Company developed a multiphase gel electrolyte by physical modification. One phase is a polar polymer such as PVDF or PAN, which can absorb solvent and offer channels for ion conduction. The other phase is a nonpolar polymer such as PP or PTFE (15-25 wt%), which absorbs little or no solvent, provides mechanical support, and reduces the volume change during the gelling process. It can also form a multilayer polymer electrolyte similar to that formed with the porous PP membrane (see Section 12.5). 

A compromise is to add some gelled electrolyte. Commercial cells use a porous polyethylene or polypropylene separator filled with a polymer and gel filling with a liquid electrolyte. They offer improved safety with more resistant to overcharge and less chance for electrolyte leakage. 

---