Mechanical properties, lithium polymer batteries

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... 

Poly (ethylene oxide) (PEO) - LiX complexes appear to be the most suitable electrolytes for lithium polymer batteries, however, the local relaxation and segmental motion of the polymer chains remain a problem area (Armand et al., 1997). Therefore, the PEO-based electrolytes show an appreciable ionic conductivity only above 100°C (Gorecki et al., 1986). This is, of course, a drawback for applications in the consumer electronic market. On the other hand, the gel polymer electrolytes although offer high ionic conductivity and appreciable lithiiun transport properties it suffers from poor mechanical strength and interfacial properties (Croce et al., 1998 Gray et al., 1986 Kelly et al., 1985 Weston et al., 1982). Recent studies reveal that the nanocomposite polymer electrolytes alone can offer safe and reliable lithium batteries (Appetecchi... 

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)... 

The polymer component in these batteries fulfills the function of a medium for ionic transport and a separator. The polymers are polyethers, PEO, or PPO. However, the lithium salts, dissolved in these polymers, have 100-fold lower conductivity than that of a lithium salt dissolved in water. The low conductivity below 70 °C, the reactivity of the interface with the lithium metal electrode, and the issues related to mechanical properties and electrochemical stability need to be resolved before the lithium polymer battery has acceptable performance. The use of inorganic composite membranes, described in a subsequent section, has been shown to result in improved ionic conductivity. 

Lithium-Ion Cel Polymer Battery The Hthium-ion gel polymer batteries offer better performance than that of solid polymer electrolyte batteries. The gel electrolyte is a polymer matrix swollen with a liquid electrolyte, and the batteries that employ gel electrolyte are known as gel polymer batteries. The detailed information on this type of battery can be found in other chapters of this book and in a review [33]. Most of the gel electrolytes have been made employing PEO, poly(acrylonitrile) (PAN) [34], poly(methyl methacrylate) (PMMA) [35, 36], and PVdF [37, 38]. The poor mechanical properties of polymer and gel polymer electrolytes have led to an alternative approach where microporous membranes are impregnated with gel polymer electrolytes [39-42]. The process builds upon the work of Abraham et al. who saturated commercial polyolefin separators with a solution of lithium salt in a photopolymerizable monomer and a nonvolatile... 

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]... 

Kureha KF polymer is a PVdF product developed by Kureha Corporation, Tokyo Japan. KF polymer has a high chemical resistance and desirable mechanical properties. One of its most remarkable characteristics is that the irregular bonding of its molecular chain is lower than that of any of the other PVdFs, an attribute that leads to a perfectly crystallized polymer. These valuable properties allow lithium-ion secondary batteries to combine long-term, stable performance with a minimum amount of swelling by the organic electrolyte. 

The ceramic fillers (e.g., AI2O3, SiOa, TiOa) can greatly influence the characteristics and properties of polymer electrolyte by enhancing the mechanical stability and the conductivity [135, 175-178]. Prosini et al. [179] in a PVdF-HFP polymer matrix used y-LiAlOa, AI2O3, and MgO as fillers to form self-standing, intrinsically porous separators for lithium-ion batteries. The MgO-based separators showed the best anode and cathode compatibilities. 

Chen ZH, Christensen L, Dahn JR (2004) Mechanical and electrical properties of poly (vinylidene fluoride-tetrafiuoroethylene-propylene)/super-S carbon black swelled in liquid solvent as an electrode binder for lithium-ion batteries. J Appl Polym Sci 91 2958-2965... 

Zhonghai and co-workers [154] studied the mechanical properties of carbon fibre reinforced films of this polymer as a function of the degree of crosslinking with triethylenetetramine. In general, an improvement in mechanical properties resulted in little or no effect on the electrical properties of this polymer, which is a strong candidate for the fabrication of lithium battery electrodes. 

In China, there are a number of researchers who commit themselves to the preparation of polymer electrolyte with high ion conductivity. Wang et al. (2002) reported a PMMA-based electrolyte crosslinked by ethylene glycol dimethacrylate (EGD) for a lithium ion battery. As the network structure was introduced, the mechanical property of the GPE was improved. The maximum ion conductivity was 2 x 10 S/cm containing 25% MMA, 2% EGD and 73% plasticizer at room temperature. Additionally, the lithium cell fabricated with this gel polymer electrolyte exhibited excellent electrochemical properties. 

No information is given on the mechanical properties. Angell and coworkers have pointed out that it is not necessary to have a room temperature ionic liquid [26]. Indeed solid electrolytes have many well-known attractive properties for lithium-ion batteries. Several lithium salts have given conductivities comparable with PEO - lithium salt mixtures, which are heavily weighted with polymer. Since these values are not really adequate for most lithium ion battery applications, however, they are not covered further here. 

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