## 6 Functional Properties and Applications of Nanocomposites
### Lithium-ion batteries
#### 6.5.1 Polymer-nanocomposite electrolytes applying passive ceramic nanofillers  
In the early seventies of the last century the first solid solvent-free polymer electrolyte was presented in a short note by [PERSON] and coworkers [196]. To achieve an acceptable ionic conductivity suitable alkali salts must be soluble in the polymer. The polar molecular structure of polyethyleneoxide (PEO) enables the formation of stable polymer-salt-complexes PEO-LiX. Pure PEO possesses, under ambient conditions, a poor ionic conductivity of 10\({}^{-8}\) S/cm, which can be improved by the addition of lithium halides by a factor of 100 [193,194]. The semicrystalline PEO showed, depending on the average molecular weight, a TG around -64 \({}^{\circ}\)C and a melting temperature around 65 \({}^{\circ}\)C [193]. In the intermediate range the presence of semicrystalline domains hindered the lithium-ion mobility resulting in a poor ionic conductivity. The addition of organic plasticizers like highly polar solvents (ethylenecarbonate, propylenecarbonate, dimethylcarbonate, and others) with a relative permittivity around 50-70 for better salt solvation or ceramic particles suppressed the crystallization enabling an enhanced amount of amorphous domains [197]. These early research efforts showed that the application of submicron sized ceramics instead of micron-sized fillers forming polymer-nanoparticle composites is favorable enabling an ionic conductivity around 10\({}^{-5}\) S/cm near room temperature [198,199]. The latter authors found, that the addition of Al\({}_{2}\)O\({}_{3}\) yielded an increase of the ionic conductivity only below the PEOs melting temperature. Passing the phase transition the filler causes a stiffening of the polymer host, and the reduced polymer chain mobility lowered the ionic conductivity [199]. [PERSON] and coworkers used the effective medium theory for a description of the temperature, composition, grain size and solid filler load of the composite electrolyte conductivity [200]. The model predicted the influence of the grain size and the filler concentration on the conductivity. The conductivity increase could be addressed to the formation of a flexible amorphous layer at the interface between the polymer matrix and the fine particles [200]. A direct comparison of nanosized (13 nm) and micron-sized (\(<\)10 um) Al\({}_{2}\)O\({}_{3}\) as passive fillers, dispersed in PEO-LiBF\({}_{4}\), was investigated by [PERSON] _et al._[201]. As observed earlier, an optimized nanofiller content (around 10 wt %) for a pronounced ambient temperature ionic conductivity increase was found. The addition of nanosized TiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) enabled a ionic conductivity of 10\({}^{-5}\) S/cm at 30 \({}^{\circ}\)C and of 10\({}^{-4}\) S/cm at 50 \({}^{\circ}\)C [202]. The ionic conductivity was at all investigated temperatures (below and beyond the melting temperature around 60 \({}^{\circ}\)C) higher than the electrolyte without ceramic filler. The large surface area of the applied nanofillers did not affect the ionic conductivity remarkably in a negative way and yielded an improved mechanical stability of the electrolyte applied as a polymer membrane usable as separator [202]. [PERSON] and coworkers researched the impact of nanosized TiO\({}_{2}\) (P25, Degussa, today Evonik Industries) on two different amorphous polyethylene glycol (PEG)-LiX mixtures (X: LiClO\({}_{4}\), Li[(CF\({}_{3}\)SO\({}_{2}\))\({}_{2}\)N] prior and after polymerization to the final polymer [203]. In case of high conducting salt concentrations the addition of the P25 increased the ionic conductivity after polymerization. The authors explained the increase by the influence of the nanofiller on the ion-aggregation reducing the ion-pair formation. A comprehensive investigation of the influence of 30 micro- and nanofillers (Al\({}_{2}\)O\({}_{3}\), AlN, SiO\({}_{2}\), TiO\({}_{2}\), ZrO\({}_{2}\), and others) on the ionic conductivity, TG and melting behavior of PEO-LiClO\({}_{4}\) complexes was published by [PERSON] _et al._[204].