PAN-based gel polymer electrolytes

PAN-based gel polymer electrolytes are among the earliest and most investigated gel polymer electrolytes. Their ionic conductivities at room temperature are relatively high. 

The interactions between the PAN matrix and LP ions and the ion conduction mechanisms in PAN-based gel polymer electrolytes are somewhat complicated. Besides the movement of ions in the plasticizer, ion movement in the polymer matrix does also occur. In addition, ion movements also occur in the solvation phase between the polymer and the organic solvent. At high plasticizer concentrations, the main contribution to ion conduction is the movement of ions in the plasticizer. 

Alternative routes to obtain lithium-ion plastic batteries have considered the use of PAN-based gel-type polymer electrolytes as separators. These electrolyte membranes, although macroscopically solid, contain in their structure the active liquid electrolyte (Figure 7.7). Therefore, they have a configuration which in principle allows a single lamination process for the fabrication of the lithium-ion battery, i.e., a process that avoids intermediate liquid extraction-soaking activation steps. 

Gel electrolytes consisting of poly(acrylonitrile) (PAN) as the polymer matrix have high mechanical strength even when they contain large amounts of liquid components. This property mainly comes from the -CN functionality in the chemical structure of the matrix. The PAN-based gels also show wide potential window and good compatibility with electrode materials. 

Akashi, H., Shibuya, M., Orui, K., Shibamoto, G., and Sekai, K. (2002) Practical performances of Li-ion polymer batteries with LiNio.sCoo.2O2. MCMB, and PAN-based gel electrolyte. J. Power Sources, 112, 577-582. 

This discussion shows that the gel electrolyte must match the use of the battery, requiring optimization of the composition of the gel polymer electrolyte, the supporting salt and its concentration, and the solvent. PAN gel electrolytes made using different solvents, lithium salts, and composition will display different behaviors with respect to the ionic conductivity, lithium-ion transference number, electrochemical window, cyclic voltam-metric behavior, and compatibility with electrodes. Table 11.1 lists the ionic conductivity at room temperature of some gel electrolytes based on PAN. Because the PAN chain contains highly polar -CN groups, which exhibit poor compatibility with lithium metal electrodes, the passivation of the interface between the gel electrolyte and lithium metal electrode is crucial. At the same time, PAN has a high crystallization tendency. At elevated temperatures, the liquid electrolyte and plasticizer will separate therefore, the polymer is modified, mainly by copolymerization and cross-linking. 

Gel polymer lithium-ion batteries replace the conventional liquid electrolytes with an advanced polymer electrolyte membrane. These cells can be packed in lightweight plastic packages as they do not have any free electrolytes and they can be fabricated in any desired shape and size. They are now increasingly becoming an alternative to liquid-electrolyte lithium-ion batteries, and several battery manufacturers. such as Sanyo. Sony, and Panasonic have started commercial production.Song et al. have recently reviewed the present state of gel-type polymer electrolyte technology for lithium-ion batteries. They focused on four plasticized systems, which have received particular attention from a practical viewpoint, i.e.. poly(ethylene oxide) (PEO). poly (acrylonitrile) (PAN). ° poly (methyl methacrylate) (PMMA). - and poly(vinylidene fluoride) (PVdF) based electrolytes. ... 

One may then conclude that, the gel-type electrolytes, and the PAN-based ones in particular, have electrochemical properties that in principle make them suitable for application in versatile, high-energy lithium batteries. In practice, their use may be limited by the reactivity towards the lithium electrodes induced by the high content of the liquid component. Indeed, severe passivation phenomenon occurs when the lithium metal electrode is kept in contact with the gel electrolytes [60, 69]. This confirms the general rule that if from one side the wet-like configuration is essential to confer high conductivity to a given polymer electrolyte, from the other it unavoidably affects its interfacial stability with the lithium metal electrode. 

Once the compatibility of the gel-type electrolyte with both anode and cathode materials is ascertained, one can proceed with the combination of the two for the fabrication of polymer-based lithium-ion battery prototypes. A few examples of these prototypes have been reported at the laboratory level scale. One is provided by a battery of the type C/ LiC104-EC-PC-PAN/LiCryMn2.y04. 

A gel electrolyte is a polymer gel that confines a liquid electrolyte. Gel electrolytes have better ionic conductivity than polymer electrolytes. Wang et al. have studied poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) gel electrolyte with plasticizers such as EC-DMC, and PC-EC-DEC for Li-S batteries at room temperature. The ionic conductivity is improved significantly to about 1.2 X 10 Scm, which is much higher than that of PEO-based polymer electrolyte. High discharge capacity and utilization of sulfur are obtained with PAN-S [70, 71] and C-S [46, 69] composite electrodes. However, much lower utilization of sulfur is obtained when PVDF [134] or PVDF-HFP [119] are combined with TEGDME as a plasticizer. When TEGDME combines with EC as the plasticizer for microporous PVDE-HEP gel electrolytes, a better performance is observed in Hthium-sulfur batteries [31]. 

Up to now, many types of polymer gel electrolytes have been used in quasi-solid-state DSSCs, including polyacrylonitrile (PAN), poly(ethylene oxide) (PEO), polymethylmethacrylate (PMMA), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). PVDF-HFP exhibits high ionic conductivity and stability at room temperature however, the complex preparation technology and the poor mechanical strength of these gel polymer-based DSSCs represent a bottleneck to their introduction to the market. To overcome this problem, the electrospinning of such polymers has been performed with the aim of integrating the resulting fibrous, easy-to-obtain, and low-cost materials as electrolytes [23]. 

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