Preparation And Properties Of Li 2 Ti ₂ 12 Composites

In order to improve the electronic conductivity of spinel Li4Ti5O12(LTO) and suppress the interface reaction between the LTO surface and the electrolyte, enhancing the electrochemical performance of the LTO, a layer of carbon was successfully in-situ coated on the surface of spinel LTO by rheological phase method with phenolic resin as carbon source. The effect of the synthesis condition of phenolic resin (the molar ratio of formaldehyde and resorcinol, and the reaction time), carbon content and the kinds and content of the added transition metal elements during the synthesis process of phenolic resin on the electrochemical performances of LTO/C composites were investigated. The structure and morphology of the prepared samples were characterized by the XRD, SEM and TEM. EIS, CV and galvanostatic charge-discharge test were employed to investigate electrochemical performance of LTO/C composites.Carbon coating could obviously improve the electrochemical performance of the LTO. Three kinds of LTO/C composites were synthesized with glucose, resorcinol and phenolic resin as carbon sources, respectively. The results indicated that the LTO/C composites with phenolic resin as carbon source showed the best electrochemical performance, which should be due to the higher graphitization of coated carbon layer. In addition, as results the optimized LTO/C composite with the molar ratio of formaldehyde and resorcinol of2.5:1, reaction time of6h, exhibited the best rate capability. When the carbon content in the LTO/C composite is4.22wt%, its initial discharge specific capacity is126.9mAh/g at5.0C and112.5mAh/g at10.0C, respectively, and after100cycles at5.0C, it could still remain at124.7mAh/g.The addition of during the synthesis process of phenolic resin could improve the degree of graphitization of the carbon in the LTO/C composites, and the added transition metal element could enter into the lattice of LTO during the sintering process, forming the Li4Ti5-XMXO12(M represented Fe, Co, Ni and Zr). The addition of Fe, Co and Ni could improve the graphitization of carbon in the samples, however, the Fe, Co and Ni could enter into the lattice of LTO and decrease its conductivity, resulting in the lower discharge capacity and worse electrochemical performance. The addition of Zr not only improve the degree of graphitization of the carbon but also expand the migration channel of Li+, which was beneficial for the diffusion of Li+. Therefore, the Zr-doped LTO/C composite (LTO/ZC) exhibited higher discharge capacity and better electrochemical performance. The LTO/ZC composites with0.15Zr (the x value in the Li4Ti5-XZrXO12) content displayed the best rate capability of132.2mAh/g at5.0C and119.7mAh/g at10.0C, respectively. It could still remain at127.9mAh/g after100cycles at5.0C. In addition, there was almost no SEI film formed on the surface of LTO/ZC electrode, whereas a significant SEI film existing on the surface of pristine LTO electrode after galvanostatic charge-discharge tests at the cut-off voltages of1V～3V. This indicated that surface carbon coating layer could cover the catalytic active sites of LTO and separate it from the electrolyte, suppressing the interface reaction between the LTO surface and the electrolyte.