# A Review on Nanocomposite Materials for
## 5 Anode Materials
### Alloy Anodes  
To meet the battery system targets set by the U.S. DOE given in Table 2, it is necessary to improve the gravimetric and volumetric energy density of the batteries significantly which can only be achieved by developing novel materials with enhanced Li storage properties. To achieve this goal, one of the alternatives is to replace graphite, either completely or partially, with alloy anode materials. The most promising and well-studied alloy anodes are presented in Figure 13.  
Figure 12: Schematic of the SEI layer on (**a**) pristine and (**b**) carbon coated graphite. Adapted with permission from [138], © Elsevier, 2001. One should note that, currently Li metal anode is not considered to be a viable candidate due to dendrite formation and the accompanying safety concerns [142]. Among different alloy anodes given in Figure 13, Si and Sn have attracted the most attention due their superior gravimetric and volumetric capacity. However, these alloy anodes experience large volume change (>300%) upon Li de-/ intercalation which causes particle cracking and pulverization upon cycling which leads to poor cycle life. Additionally, this large volume change during cycling prevents the formation of a stable SEI layer on the alloy anodes, which continuously consumes Li ions, and results in low Coulombic efficiency. One simple but effective strategy to suppress the rapid deterioration of cycle life in Si anodes is to prepare a composite anode with low Si (i.e., Si < 33 wt %) and high binder (i.e., 33-56 wt %) content [143]. However, high inactive mass in this composite cathode severely limits the specific capacity (i.e., \(\sim\)250 mAh/g after 400 cycles). Another approach is to partially replace graphite with Si. The graphite-Si composite anodes with carbon coating showed better performance than Si alone, but quick degradation of cycle life is still a concern in these type of materials [144; 145]. The carbon and metal coating techniques were also applied to improve the cycle life and Coulombic efficiency of Si anodes [146; 147]. These coatings reduce the electrolyte decomposition on Si anodes by creating a buffer zone between the electrolyte and Si, but due to significant volume change of Si anode, the carbon coating loses its structural integrity by cycling, and fresh Si surfaces are exposed to electrolyte eventually. In another study, nanocrystalline Si particles of 20-80 nm that were encapsulated in a carbon aerogel showed a stable reversible capacity of 1450 mAh/g for 50 cycles [147]. The carbon aerogel works as a cushion during the expansion-contraction of the nanocrystalline Si particles, and maintains the three dimensional electrical network; this way, Si particles stay in physical contact with the carbon matrix at all times. More recent studies on Si anodes concentrated on minimizing the Si surface area exposed to electrolyte by encapsulating single Si nanoparticle in a carbon shell (i.e., yolk-shell design) which has enough void space to accommodate significant volume changes during de-/lithiation of Si [148]. This strategy minimizes the carbon shell deformation upon cycling, hence helps with obtaining a stable SEI layer over the carbon shell. This Si-carbon composite showed a reversible capacity of 1500 mAh/g at 1C rate after the initial formation cycles, and kept 74% of its initial capacity after 1000 cycles. In a similar  
Figure 13: Theoretical gravimetric and volumetric capacities of C (LiC\({}_{6}\)), Si (Li\({}_{3.75}\)Si), Sn (Li\({}_{4.4}\)Sn), Al (LiAl), P (Li\({}_{3}\)P) and Sb (Li\({}_{3}\)Sb). Adapted with permission from [141], © Royal Society of Chemistry, 2011.  
approach, inspired by a pomegranate fruit, carbon coated Si nanoparticles are further encapsulated in a thick secondary carbon layer as shown in Figure 14[149]. As a result, the SEI layer mostly forms on this secondary carbon layer (i.e., lower surface area) instead of the individual Si nanoparticles (i.e., higher surface area), Figure 14c.