Investigation Of Carbon-coated Li_1-xTi₂O₄（0＜x＜0.5） And Nano-silicon/Carbon/Grahite Composites Anode Materials

As new types of anode materials for lithium- ion batteries, spinel-type LiTi2O4 has very good cyclic performance and high electrical conductivity; Silicon-based material has very high specific capacity. However, both of them have their own disadvantages,respectively. The synthesis of LiTi2O4 is difficult and the electrochemical reaction is complex. Si undergoes large volume changes during charge/discharge process, resulting in poor cyclic performance. In this thesis, magnesiothermic reduction has been used to fabricate nanosized carbon-coated TiO and carbon-coated Si as precursors, respectively, Then, carbon-coated Li_1-xTi₂O₄（0<x<0.5） and nano-silicon/carbon/grahite composites were synthesized by solid-state reaction process. XRD,XPS,SEM,TEM,charge/discharge test,cyclic voltammetry and AC impedance test have been employed to investigate their microstructure and electrochemical performance.The Li_1-xTi₂O₄ anode materials exhibit a reversible two-stage lithiation process with a voltage plateau at ca.1.5V and a voltage slope in a range of ca.0.7V to 0.45V. With increase of the Ti³⁺ proportion,Li1-xTi2O4 shows lower capacity at the 1.5 V voltage plateau but higher capacity at the voltage slope. The voltage slope of ca.0.7V to 0.45 V may be ascribed to Ti³⁺/Ti2+ redox couple. Li_1-xTi₂O₄ anode materials with different Ti³⁺ proportion display excellent cyclic stability and reversible capacity. The first-cycle charge capacities can reach ca. 200mAh/g. The reversible capacity retentions are around 95% after 100 cycles. The rate capability is improved with increase of Ti³⁺ proportion in the materials.The carbon coating of nanosized Si and the addition of graphite can improve the cyclic stability and the first-cycle coulomb efficiency（>90%） of the nano-silicon/carbon/grahite composites materials simultaneously. The nano-silicon/carbon/grahite composite with mass ratio of nano Si/C :graphite equal to 1 :2（Si:16wt.%） exhibits the best cyclic stability and rate capability. The first-cycle reversible capacity is 662.4mAh/g at current density of 0.1 mA/cm2,the first-cycle coulomb efficiency is 90.9% and the reversible capacity retention is as high as 90.8% after 100 cycles,which make the nano-silicon/carbon/grahite composites promising anode materials for high energy-density lithium- ion batteries.