## 6 Functional Properties and Applications of Nanocomposites
### Lithium-ion batteries
#### 6.5.3 Nanocomposites as electrodes and supercapacitors  
It was already shown in Table 2 that nanocomposites based on inorganic nanoparticles and carbon became of particular interest as anode materials in lithium-ion-batteries. Nanoscaled materials are expected to enhance the performance significantly as the reduced dimensions increase the rate of lithium insertion and removal due to the extremely short diffusion lengths [227]. Additionally, electron transport within the particles is enhanced, and the high surface areas permit high contact areas with the electrolytes. Up to now, graphite is the standard material for anodes in commercial lithium-ion-batteries with a theoretical capacity of 372 \(\mathrm{mAh/g}\) and a more or less stable cycling stability. Nanomaterials based on tin dioxide (SnO\({}_{2}\)) possess very promising potential as anode material because they exhibit in principle much higher specific capacities (790 \(\mathrm{mAh/g}\)). Unfortunately, bulk and pristine SnO\({}_{2}\) anode material shows very poor long-term cycle stability due to internal stresses caused by the large volume change (\(>\)200%) during the alloying process from Li and Sn forming Li\({}_{4.4}\)Sn. This alloying process results in cracks and loss of electrical contact at the anode. Many research efforts therefore focus on nanocomposite materials based on SnO\({}_{2}\) or TiO\({}_{2}\), combined with carbon, graphite, or graphene, acting as:  
* a barrier to suppress the aggregation of active particles
* a buffering matrix to relax the volume expansion during the lithiation/delithiation process
* an improvement of the conductance of the electronic material. The following explanations focus on anodes, fabricated from nanocomposite powders using additionally carbon black for conductivity and a binder. [PERSON] _et al._ showed [228] that the cycling stability can be improved significantly compared to pristine SnO\({}_{2}\) when using 7 to 10 \(\mathrm{nm}\) SnO\({}_{2}\) nanoparticles dispersed in graphite. They also could enhance the specific capacity of these nanocomposites compared to graphite. Due to further particle size reduction of SnO\({}_{2}\) in the range of 2 to 3 \(\mathrm{nm}\) by applying a microwave-assisted synthesis, the cycling stability could further be enhanced [229]. Nevertheless, after a certain amount of cycles the capacity decreased below the one of graphite. [PERSON] _et al._[56] developed TiO\({}_{2}\)/C core/shell nanocomposites using commercial TiO\({}_{2}\) nanoparticles. Conductive additive and additional binder is needed with this setup. Their material also showed enhanced cycling stability compared to the pure metal oxide. Also nanocomposites using PPy show an enhanced cycling stability compared to SnO\({}_{2}\). This was shown by [PERSON] _et al._[230]. Recently, [PERSON] _et al._[59] could realize anodes of SnO\({}_{2}\) at C core/shell spheres as active material, retaining their cycle stability after 18 cycles, and a specific capacity still higher than carbon. The authors attributed this stability to the presence of the carbon shell, enhancing the conductivity of SnO\({}_{2}\) cores and suppressing the aggregation of active particles during cycling. A very promising development is metallic Sn nanoparticles encapsulated in an elastic hollow carbon sphere [65]. With this type of material, even after the 100 th cycle 66.2 % of the theoretical specific capacity was retained, with a capacity significantly higher than the one of graphite. Carbon encapsulation of SnO\({}_{2}\)[58] also results in higher reversible specific capacity, and a capacity of over 400 mAh/g after 30 cycles. [PERSON] _et al._[231] report about the development of SnO\({}_{2}\)/C nanocomposite anodes with superior cycling capacity. They could realize an anode material containing 75 wt % of SnO\({}_{2}\) nanoparticles with sizes around 3-4 nm, highly dispersed in a carbon precursor matrix derived from glucose. This material was characterized by a stable relative charge capacity of 610 mAh/g even after 200 cycles.