| Several metals are known to react with lithium to form useful anode materials exhibiting high capacity for Li-ion battery application. However, the cycle life of these materials is generally poor since the large volume changes (≅300∼600%) caused by the reaction of lithium with these metals inevitably leads to decrepitation, cracking and crumbling of the electrode. In order to overcome this problem, researchers have invoked the concept of ‘active-inactive’ composites, which can help to reduce or prevent mechanical damage caused by the ensuing volume expansion of the anode during electrochemical cycling. In this study, silicon and tin have been used as active components because of their high theoretical capacity of 4000 mAh/g, and 990 mAh/g, respectively. In order to generate the active-inactive nanocomposite, carbon and non-oxide ceramics have been studied as the primary inactive components.; Tin metal is typically malleable at room temperature and mechanical milling of Sn to form nanosized particles is extremely difficult. On the other hand, Si exhibiting high mechanical brittleness (Knoop hardness: 820 kg/mm 2) can be easily pulverized into nanosized particles by mechanical milling. However, the generation of nanosized Si by chemical routes poses significant challenges. As a result, high-energy mechanical milling has been utilized to generate nanocomposites of Si/TiN, Si/TiB2 and Si/SiC. The electrodes prepared by HEMM do not exhibit any change in the phases formed as well as the microstructure, indicating the structural stability of the nanocomposites. However the nanocomposites obtained from HEMM generally exhibit a loss in capacity after extended milling. Detailed systematic impedance analysis, electronic conductivity measurement and high-resolution electron microscopy studies have shown that the loss in capacity is caused by embedding of the extremely fine Si particles (10∼20 nm) within the inactive matrices. This results in high charge-transfer resistances on the order of ∼600 ohm thereby preventing the Li-ions from alloying and de-alloying with Si resulting in an inactive material.; Tin-based nanocomposites on the other hand, have been synthesized by pyrolyzing the precursors generated by the infiltration of organotin compounds such as tetraethyl tin into mechanically milled PS-resin. The Sn/C electrodes exhibit a promising initial discharge capacity of ∼480mAh/g that lowers to a value of 460 mAh/g after 30 cycles. (Abstract shortened by UMI.)... |
