Lithium-solid polymer electrolyte cells

A possible solution to this problem is to use an electrolyte, such as a solid polymer electrolyte, which is less reactive with lithium metal [3]. Another simple solution is the lithium-ion cell. 

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]. 

State-of-the-art thin film Li" cells comprise carbon-based anodes (non-graphitic or graphite), solid polymer electrolytes (such as those formed by solvent-free membranes, for example, polyethylene oxide, PEO, and a lithium salt like LiPFe or LiCFsSOs), and metal oxide based cathodes, in particular mixed or doped oxides... 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

Organic polydisulfides were combined with lithium metal in solid-polymer electrolytes [102-105] cf. No. 17 in Table 10 and Section 2. The cells were cycled at 80 °C. 

Fig. 1. Configuration for a solid polymer electrolyte rechargeable lithium cell where the total thickness is 100 Jm.

Electrodes and cell components must be thin to minimise the internal resistance of the batteries the total cell can be less than 0.2 mm thick. Figure 12.11 shows the construction of a multi-layer film, rechargeable lithium polymer battery, using a solid polymer electrolyte. A thin lithium metal foil acts as an anode. The electrolyte is polyethylene oxide containing a lithium salt, and the cathode is a composite of the electrolyte and a... 

Macklin, W. J., and Neat, R. J. (1992). Performance of titanium dioxide-based cathodes in a lithium polymer electrolyte cell. Solid State Ionics, 53, pp. 694-700. 

Polymer electrolytes are used in lithium ion rechargeable batteries. Pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEG), or polyphosphazenes. Chlorinated PVC blended with a terpoly-mer comprising vinylidene chloride/acrylonitrile/methyl methacrylate can make a good polymer electrolyte. Rechargeable lithium ion cells use solid polymer electrolytes. Plasticized polymer electrolytes are safer than liquid electrolytes because of a reduced amount of volatiles and flammables. The polymer membrane can condnct lithinm ions. The polymer membrane acts as both the separator and electrolyte [7],... 

Many solid electrolytes are known today and it can be expected that their importance will further increase, especially for electrochemical devices. For example, beta-alumina solid electrolyte (BASE) is a fast ion conductor, which is used as a membrane in electrochemical cells. It can contain small ions like sodium, which show a high mobility. More classical examples are electrolytes based on lithium or silver iodide where the small cations are very mobile [13]. Note that solid polymer electrolytes are also a rapidly growing field [14]. 

The next entry is for Nafion, a proton-conducting fluorosulfonic acid ionomer material which in membrane form is widely used in PEM fuel-cell technology. The conductivity value quoted is for a fully hydrated membrane at an ambient temperature. Note that the conductivity is less than that of a comparable aqueous acid solution, for example 0.5 M sulfuric acid, but by a factor of only 3-4. Heavily hydrated Nafion membranes contain a lot of water, and consequently they behave a lot like aqueous acid solutions. The next three entries are for various gel and solid-polymer electrolytes containing lithium salts. All these material are membranes some contain some potentially volatile solvents, while others do not. Conductivities for these materials are low relative to true liquid solvents but they are still well within the range of usable values for electrochemical experiments. The semi-solid character of these materials, combined with their near-zero volatility (for solid-polymer electrolytes which do not contain volatile solvents), makes them suitable for use under high-vacuum conditions which makes them potentially useful for fabrication of electrochemical devices which are targeted for use in vacuum or under conditions which could otherwise result in solvent loss by evaporation. 

A variety of materials have been investigated for the positive electrode (the cathode during discharge), such as intercalation solid compounds, soluble inorganic cathodes, and polymeric materials. Liquid aprotic organic and inorganic electrolytes are used in many cells. Solid polymer electrolytes are also popular as they may provide a safer design because of their lower reactivity with lithium. These materials are identified in Fig. 34.1. 

The ionic conductivity of most solid polymer electrolytes is significantly lower than that of the liquid electrolytes. Cells must he designed with thin electrodes and cell components to minimize the internal cell resistance. The total thickness of a cell assembly is as low as 200 fim or thinner. An alternative is to operate at higher temperatures where the conductivity is higher. While this may he acceptahle for electric-vehicle and stand-by batteries, it will not be acceptable for many portable consumer applications. Newer polymer electrolytes are being developed using plasticizers or gel-type polymers. These methods increase the conductivity of the polymers, but since they contain organic solvents, they will be more reactive with the lithium anode. 

The polymer Li-ion cells described here may be more accurately described as employing a gel electrolyte, as the electrolyte contains a monomeric, volatile liquid component absorbed into a polymeric host, in contrast to technologies which do not employ a volatile, liquid component, such as solid polymer electrolyte batteries. Because of the poor conductivity of currently available solid polymer electrolytes (solid polymer lithium batteries developed to date operate at 40°C to 80°C to accommodate the low conductivity of the electrolyte) (see Sec. 34.4.2), current polymer Li-ion batteries incorporate less viscous, liquid components to improve the conductivity of the electrolyte, enabling their use at ambient temperatures. 

Irradiated PVDF and poly(VDF-co-TrFE) copolymer possess ferroelectric properties that allow the use of such fluorinated polymer in the domain of captors, sensors, and detectors [47,194]. Another interesting property of crosslinked poly(VDF-co-HFP) copolymer is their insolubihty in organic solvent [195]. Cured fluorinated polymers can be processed as membranes for many electrochemical applications such as fuel cell and batteries [196]. For example, a poly(VDF-co-HFP) copolymer has been crossUnked with various systems such as polyols [197], by irradiation with electron beam or y-rays [197] or with aliphatic amines [198] in order to elaborate a solid polymer electrolyte for non aqueous lithium battery [197,198]. This electrolyte is particularly interesting for its ionic conductivity, its adhesion with an electro-conductive substrate and also remarkably enhanced heat resistance. 

Since the type of electrolyte material dictates operating principles and characteristics of a fuel cell, a fuel cell is generally named after the type of electrolyte used. For example, an alkaline fuel cell (AFC) uses an alkaline solution such as potassium hydroxide (KOH) in water, an acid fuel cell such as phosphoric acid fuel cell (PAFC) uses phosphoric acid as electrolyte, a solid polymer electrolyte membrane fuel cell (PEMFC) or proton exchange membrane fuel cell uses proton-conducting solid polymer electrolyte membrane, a molten carbonate fuel cell (MCFC) uses molten lithium or potassium carbonate as electrolyte, and a solid oxide ion-conducting fuel cell (SOFC) uses ceramic electrolyte membrane. 

The most simple and efficient approach is based on gelation which is a simple method that allow a good compromise between the retention of the IL and its fluidity inside the polymeric network. These so called ion gels are simpler than solid polymer electrolytes and exhibit improved conductivities. In fact ion gels hold both the processability and mechanical properties of polymers, but with added physico-chemical properties and were primary developed as replacements for current solid-state polyelectrolytes in energy devices, such as dye-sensitized solar cells, supercapacitors, lithium ion batteries, and fuel cells. (Fernicola et al., 2006 Galinski et al., 2006 Le Bideau et al., 2011 Lu et al., 2002 Mazille et al., 2005 Stephan, 2006)... 

Ogumi, Z. Uchimoto, Y. Takehara, Z. Kanamoii, Y. (1989). Preparation of Ultra-Thin Solid-State Lithium Batteries Utilizing a Plasma-Polymerized Solid Polymer Electrolyte. /, Chem. Soc., Chem. Commun., Vol. 21, pp. 1673-1674 Ohnishi, R. Katayama, M. Takanabe, K Kubota, J. Domen, K (2010). Niobium-Based Catalysts Prepared by Reactive Radio-Frequency Magnetron Spnitteiing and Arc Plasma Methods as Non-Noble Metal Cathode Catalysts for Polymer Electrolyte Fuel Cells. Electrochim. Acta, Vol. 55, pp. 5393-5400 Papadopoulos, N.D. Karayiarmi, H.S. Tsakiridis, P.E. Perraki, M. Hiistoforou, E. (2010). 

Harwell UK are developing a solid state cell based on four very thin layers (50-60 pm each) comprising a lithium foil-polymer electrolyte-composite cathode-nickel foil current collector (lOym). 

Polyphosphazenes have emerged as a class of promising solid polymer electrolyte materials for energy applications due to their inherently high stability and a wide range of synthetic variability. Herein, a summary is presented on the synthesis of polyphosphazenes, membrane fabrication and characterization, and applications for lithium batteries, fuel cells, and dye-sensitized solar cells (DSSCs). 

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