# A Review on Nanocomposite Materials for
## 6 Binder  
Binder is one of the inactive components in a Li-ion battery electrode, but its role is critical for proper operation of the Li-ion batteries. Binder serves as a polymeric matrix that connects active materials to each other and to the current collector in a composite Li-ion battery electrode, and accommodates volume changes due to de-/lithiation of active materials during battery operation. An ideal binder should have: (a) high binding strength, (b) favorable mechanical properties to accommodate volume changes, (c) high electronic conductivity, (d) optimal porous structure for high ionic conductivity and (e) chemical inertness [16; 150]. Currently, polyvinylidene fluoride (PVDF) is the most common binder despite its shortcomings. PVDF is not conductive and does not store Li hence considered as an inactive material. Several different electroactive polymers  
Figure 14: The schematic of the pomegranate-inspired Si anode design in **(a)** 3D and **(b)** 2D before and after cycling (in the lithiated state); **(c)** The variation of the specific SEI area and the number of primary nanoparticles in one pomegranate particle versus its diameter. Adapted with permission from [149], © Nature Publishing Group, 2014.  
such as polyaniline and polypyrrole were proposed to replace PVDF. These multifunctional binders improved the rate capability and cycle life of the LiFePO\({}_{4}\) which has an intrinsically poor electronic conductivity [151; 152; 153; 154]. The polyaniline binder also improved the irreversible capacity loss during the first cycle in a graphite anode [155]. As discussed earlier, the alloy anodes show significant volume change upon de-/lithiation; therefore, an ideal binder should be highly flexible and mechanically robust to keep active materials together during cycling. Different composite elastomeric binders were developed for Si alloy anodes [156; 157; 158]. The PVDF-tetrafluoroethylene-propylene (PVDF-TFE-P) binder can be extended to 100% strain, whereas PVDF can only be extended to 10% strain. The PVDF-TFE-P binder enhanced the cycle life of a-Si\({}_{0.64}\)Sn\({}_{0.36}\) alloy anode reasonably [157]. Another promising elastomeric binder is styrene-butadiene-rubber (SBR) and sodium-carboxyl-methylcellulose (SCMC) composite. As compared to PVDF, SBR + SCMC binder showed smaller moduli, a larger maximum elongation, and a stronger adhesion strength hence better cycle life in Si alloy anodes [158]. Crosslinking is an effective strategy to improve the mechanical properties of polymeric binders. By crosslinking poly(acrylic acid) with SCMC, [PERSON] et al. obtained a capacity of 2000 mAh/g for Si anode after 100 cycles [159]. Recently, a new type of binder, hyperbranched \(\beta\)-cyclodextrintrim polymer and a dendritic gallic acid cross-linker with six adamantane units, for Si anodes based on \"dynamic crosslinking\" method was developed as shown in Figure 15 [160]. The dynamic crosslinking refers to supramolecular host-guest interactions. The distinctive feature of this polymer is that it can restore the broken connections between polymer chains during cycling; therefore, it can be considered as a self-healing binder. The Si anode based on this novel composite binder retained 90% of its initial capacity after 150 cycles (\(\sim\)1550 mAh/g).  
Figure 15: (**a**) Schematic and structures of hyperbranched R-, \(\beta\)-, and \(\gamma\)-cyclodextrintrim polymer (CDp); (**b**) Dynamic cross-linking mechanism of \(\beta\)-CDp and 6 adamantane (AD) in an electrode matrix along with schematic and structures of guest molecules incorporating AD moiety. Adapted with permission from [160], © American Chemical Society, 2015.