Study On The Preparation And Surface Modification Of Nano-Li₄Ti₅O₁₂ Electrode And Its Lithiation/deliatiation Behavior

Cubic spinel Li4Ti5O12 shows increasing promise as a suitable anode material for high-power lithium ion batteries(LIB) due to its excellent structural and thermal stability, stable cycling performance, improved stability, low cost and extremely small expansion and contraction during charge/discharge cycles. Unfortunately, the low intrinsic electronic conductivity, severe gassing behavior during storage and charge/discharge cycles and small theoretical capacity greatly hinders its practical application. Herein, we report the facile synthsis of a high performance Li4Ti5O12/C composite with lump and nanosheets morphology, and discussed the influence of surface modification with nitrogen-doped carbon and metal oxide on the suppression of gassing behavior in Li4Ti5O12-based LIBs. Further more, the enhancement of specific capacity by extending the voltage range of Li4Ti5O12 anode was investigated.First, we report a novel Li4Ti5O12/C composite with lump morphology and super rate performance, which is prepared by a hydrothermal process followed by low temperature heat treatment. In this hydrothermal preparation possess, a tertiary ammonium surfactant, cetyltrimethyl ammonium bromide(C16H33(CH3)3NBr, abbreviated as CTAB) was introduced, which not only functions as a surfactant to restrain the growth of Li4Ti5O12 particles, but also acts as a carbon source.Then, sheets of Li4Ti5O12 with high crystallinity are coated with nitrogen-doped carbon(NC-LTO) using a controlled process, comprising hydrothermal reaction followed by chemical vapor deposition(CVD). Acetonitrile(CH3CN) vapor is used as carbon and nitrogen source to obtain a thin coating layer of nitrogen-doped carbon. The layer enables the NC-LTO material to maintain its sheet structure during the high-temperature CVD process and to achieve high crystallinity. Doping with nitrogen introduces defects into the carbon coating layer, and this increased degree of disorder allows fast transportation of lithium ions in the layer. As a consequence, the rate and cycling performance are greatly enhanced. Further study indicates that surface modification with nitrogen-doped carbon could enhance the structural stability of Li4Ti5O12 electrode and suppress the gassing of Li4Ti5O12-based battery. The metal oxide modification also can suppress the interfacial reaction between Li4Ti5O12 and electrolyte solution.The extending of working voltage range of Li4Ti5O12 is an effective method to enhance the specific capacity and offers the opportunity to increase the energy density of full Li4Ti5O12-based battery. Herein, we report the exploration on the enhancement of specific capacity of Li4Ti5O12 at various rates enabled by extending the working voltage range. The Li4Ti5O12 discharged to 0V exhibited markedly enhanced capacity at low current rates(<5C), while possessing a rapid capacity fading at high current rates(>10C). The plateau at about 1.5V over the entire rates was shortened and the charge/discharge potential gap was remarkably increased with the widened discharge voltage range(i.e., from 2.5~1.0V to 2.5~0V). The decreased Li-ion diffusion caused by an extra Li ion intercalation together with increased internal impedance induced by high resistive solid electrolyte interface(SEI) layer significantly enhanced the polarization, which in turn hindered the reaction from Li4Ti5O12 to Li7Ti5O12 and further to Li9Ti5O12.In summary, the research results of this thesis is singnificant to design the high energy density Li4Ti5O12, solve the gas behavior and promote the development of high density, long life Li4Ti5O12-based battery.