Mesoporous Si / Preparation And Electrochemical Properties Of Zro <sub> 2 </ Sub> Nano-composite Films

There is an intense interest in developing new high capacity electrode material for lithium ion batteries to catch up with the development of HEV and portable devices. Silicon is the one of most attractive anode material of because of its largest theoretical insertion capacity of all known host materials. The lithium-ion insertion and desertion in silicon and silicon based compound have been investigated in this thesis. In order to improve the electrochemical performance of silicon based materials, mesoporous structured Si/ZrO₂ nanocomposite thin film electrodes were prepared by sol-gel method, and their electrochemical performances were investigated.It has been commonly accepted that the serious volume expansion (about 300%) and pulverization caused by lithium-ion insertion and desertion, which leads to a loss of electrical conductivity, are the main reasons for the poor cyclability of silicon. The electrochemical properties and structure transformation during cycle process of nanoscale silicon powder with the sizes of 30 nm-60 nm was researched. It has been found that the crystal structure was destroyed by inserting and deserting of lithium ions during the cycling process. Compared with microscale silicon, the nanoscale silicon had an improved cyclability, but its first irreversible discharge capacity was 35%, and the volume expansion had not been alleviated. A mesoporous Si/ZrO₂ nanocomposite film was fabricated by surface sol-gel process. The Si nanoparticles as active material were dispersed homogeneously into the mesoporous ZrO₂ film. The mesoporous Si/ZrO₂ exhibited good electrochemical performance with a reversible capacity of 1690 mAh/g and a high coulomb efficiency of 84.5%. This implied that the irreversible capacity fading was obviously suppressed during cycling.According to our research, mesoporous ZrO₂ has no lithium insertion ability, it is the inertia phase and only play the role of structure-sustain. In the charge-discharge process of Si/ZrO₂ thin film, active Si formed Li-Si alloy with lithium-ion and the then the Li-Si alloy decomposed. This mesoporous structure can endure the volume variation of Si, improve the transferring of ions within the phases and reduce the diffuse resistance of the anode, Moreover, the electrolyte can easily filter in the mesoporous nanostructure, thus improve the contact between Si powder and electrolyte. Therefore, the mesoporous Si/ZrO₂ nanocomposite thin film electrode has such excellent electrochemical performance.It was found that the mol ratio of nanoscale Si powder and mesoporous Z1O2 has important effect on the electrochemical performance of Si/ZrO2 thin film electrode. The initial discharge capacity of the electrode increased with the increase content of pure silicon, but on the contrary, the first discharge coulomb efficiency of the electrode decreased. The more the mol ratio was, the cycle stability became worse. The most serious decrease occurred when the content of silicon was 58%.The mosoporous Si/ZrCb thin film electrodes with different mesoporous diameters were obtained by using different surfactants. The size of mesoporous has also a direct effect on the electrochemical performance of Si/ZrC>2 thin film electrode. With the increase of mesoporous diameter, the first discharge coulomb efficiency of the electrode became better, and the battery showed a higher reversible discharge capacity.