PAN-based polymer electrolytes

Scrosati and coworkers [165] have fabricated an all solid lithium battery by combining a PAN based polymer electrolyte (containing EC and PC) with a lithium metal anode and a poly pyrrole (pPy) film cathode. Although the Coulom-bic efficiency was found to be high, near 90%, the battery has a poor shelf life. 

An assessment of the relative diffusion rates of ionic and molecular species in the PAN-based electrolyte may be made from the diffusion coefficients calculated for ferrocene from cyclic voltammograms. Some data are presented in Table 3.8. The ratio of diffusion coefficients of ferrocene in the PAN-based polymer electrolyte and PC/LiC104 liquid electrolyte at room temperature is the same as that obtained for the conductivity of LiC104 in these electrolytic media. It may be noted here that the ferro-cene/ferrocenium couple has been shown [36] to be useful for the overcharge protection of secondary Li batteries. 

FIGURE 38.50 Cycling data for a Li/PAN-based polymer electrolyte/oxygen battery at room temperature in an atmosphere of oxygen. The cathode contained 20 w/o catalyzed Chevron carbon black and 80 w/o polymer electrolyte. The battery was discharged at 0.1 mA/cm and charged at 0.05 mA/cm. ... 

Figure 22.6 The intermittent discharge curve and the open-circuit voltages of a lithium/PAN-based polymer electrolyte-oxygen cell at a current density of 0.1 mAcm at room temperature in an oxygen atmosphere. (Reproduced from Abraham and Jiang [13].)...

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. 

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]. 

The majority of electrochemical cells to have been constructed are based on PEO, PAN, or PVdF [101]. Recently, the Yuasa Corporation have commercialized solid polymer electrolyte batteries, primarily for use in devices such as smart cards, ID cards, etc. To date, the batteries which have been manufactured and marketed are primary lithium batteries based on a plasticized polymer electrolyte, but a similar secondary battery is expected [120]. 

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. ... 

PAN-based lithium-salt electrolytes are obtained by plasticizing the electrolytes with propylene carbonate, ethylene carbonate, dimethylformamide, dimetltylsulfoxide, etc. These compotrrrds are considered solvents in other applications bnt in these electrolytes they are used to lower the glass transition temperature, dissolve salt, and make the polymer amorphous. Some of these functions are typical of plasticizers. An improved version of the electrolyte is based on ternary mixtrues of plasticizers consisting ethylene, propylene andbntylene carbonates. This mixture improves low temperature conductivity. 

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. 

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. 

The based electrolytes are presented in Figure 3.9. Even at — 10°C, conductivities of 0.4x10" and 1.1 x 10 Scm" were obtained, respectively, for the electrolytes containing LiCFsSOs and LiC104. The conductivity at each temperature showed dependence on the nature of the polymer network, the relative amounts of... 

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. 

Xie D, Jiang YD, Pan W, Li D, Wu ZM, Li YR (2002) Fabrication and characterization of polyanUine-based gas sensor by ultra-thin film technology. Sens Actuators B 81 158-164 Yasuda A, Doi K, Yamaga N, Fujioka T, Kusanagi S (1992) Mechanism of the sensitivity of the planar CO sensor and its dependency on humidity. J Electrochem Soc 139 3224-3229 Zawodzinski TA, Springer TE, Uribe F, Gottesfeld S (1993) Characterization of polymer electrolytes for fuel cell applications. Solid State Ionics 60 199-211... 

Electrospun polymers exhibit low levels of molecular defects, thus optimizing strength and creating order that leads to higher conductivity of 700 to 900 mS.cm 1 after carbonization at temperatures between 700 and 800°C [69]. The amorphous character of the CNFs allows effective functionalization and activation of fhe lowly graphitic form. Lowly graphitic PAN-based CNFs synfhesized by Kim et al. with steam activation (1100 m. g i) at 750°C showed 120 F.g i at 1 A.g i in KOH electrolyte [70]. They also showed that polyamic acid (PAA) fibers sfeam activated at 750°C (1400 m. g i), produced 160 F.g at 1 A.g i in KOH electrolyte [71]. 

Raman spectroscopy is sensitive to both the chemical and the stmctural variations of a material, liquid or solid/ As an in situ technique, Raman spectroscopy has been used to characterize the crystalline structural variation of graphite anodes and Li vPj and LiMn O cathodes in lithium ion batteries during lithium ion insertion and extraction. In the authors laboratory, Raman spectroscopy was used to extensively study the strong interactions between the components of polyacrylonitrile (PAN)-based electrolytes, the competition between the polymer and the solvent on association with the Li ions, the ion transport mechanisms of both salt-in-polymer and polymer-in-salt electrolytes. Based on the Raman spectroscopic study, Li ion insertion and extraction mechanisms in low-temperature pyrolytic carbon anode have also been proposed. " In many cases, Raman spectroscopy is used as compensation to the IR spectroscopy to give a complete understanding to the structure of a substance though there are as many cases that Raman spectroscopy is used independently. 

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