# A Review on Nanocomposite Materials for
## 6 Binder
### Separator  
Separator is another inactive component in Li-ion batteries, but it plays a critical role in the operation and safety of such batteries. Separator physically separates the anode and cathode to prevent the electrical short circuit, while allowing Li-ion diffusion due to its porous structure in between the anode and cathode during cycling of a cell. Therefore, an ideal separator should be a good electronic insulator, mechanically robust, chemically inert under the operating conditions of the battery, and should have good wettability in non-aqueous liquid electrolytes [18]. The most common types of Li-ion battery separators are microporous and non-woven separators, former being the dominant in the Li-ion battery market [161]. The microporous separators are mainly made of polyolefins such as polyethylene (PE) and polypropylene (PP). The scanning electron microscope (SEM) images of a multilayered composite separator manufactured by Celgard(tm) are given in Figure 16. The multilayer structure (PP/PE/PP) is instrumental in preventing the thermal runaway of the Li-ion battery via thermal shut-down mechanism [161].  
The thermal stability, wettability and mechanical properties of the separators can be improved by surface coating, grafting and blending techniques [18]. The surface coating of the microporous separators with high temperature polymers such as polyimide, inorganic nanoparticles such as Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), or the combination of polymers and inorganic nanoparticles have shown to reduce the thermal shrinkage, and improved the mechanical properties and wettability of the separators [162; 163; 164; 165]. It is important to note that surface coating creates an additional layer on the separator, which may limit the Li-ion diffusion rate hence the performance (i.e., cycle life and rate capability) of the cell. Although the surface coating is effective in improving the aforementioned properties of the separator, uniformity, thickness and adhesion of the coated layer are of concern in practical applications [18]. To circumvent these issues associated with the surface coating technique, surface grafting can be utilized. Unlike coding, grafting results in the permanent attachment of the desired functional groups via covalent bonding. The surface grafting can be achieved by UV-irradiation, plasma treatment and high energy radiation methods [18]. The siloxane grafted PE separator prepared by electron beam irradiation showed enhanced ionic conductivity and wettability as well as improved cycle life at high voltage operating conditions [166]. Similar results were obtained for glycolyl methacrylate grafted PE separator [167].  
Traditionally, non-woven separators are not considered to be competitive with the microporous separators due to their large pore structure and difficulty in manufacturing them thin (<25 \(\upmu\)m) with the desired mechanical properties [18; 161]. One of the promising non-woven (polyethylene terephthalate) separators with ceramic coating (i.e., Al\({}_{2}\)O\({}_{3}\)) on both sides (i.e., Degussa-Separion(tm) is shown in  
Figure 16: SEM images of Celgard 2325 (PP/PE/PP) separator used in lithium-ion batteries: (**a**) surface SEM and (**b**) cross-section SEM images. Adapted with permission from [161], © American Chemical Society, 2004.  
Figure 17. This commercially available separator have good wettability, superior chemical resistance and very low shrinkage at high temperatures [18].