The Preparation And Electrochemical Performance Of Lithium Titanate Anode Materials Based On Metallic Titanium

During the Li-insertion/delithiation process,Li₄Ti₅O₁₂is the cathode of a lithium-ion battery with small rate of unit cell volume change and a high lithium insertion voltage of 1.55 V?vs.Li⁺/Li?.Furthermore,as the electrode during the cycle is relatively stable,irreversible loss of Li⁺in the electrolyte is reduced,the cycle performance and safety of the battery is improved,therefore we have seen rapid development in the field of lithium ion power batteries.However,the lower electronic conductivity and theoretical capacity(175 mAh g^-1)lead to poor power performance and energy density of the lithium titanate battery,which limited its commercial application.In this dissertation,three reaction mediums?solid phase,molten salt phase and liquid phase reaction media?were used to address the above problems.Reductive titanium metal powder,cathodic current and surface-modified lithium titanate materials were used.In a molten salt medium,the titanium metal powder was used as a titanium source to prepare lithium titanate electrode material through in-situ self-growth.In a liquid phase medium,it is prepared by slow hydrolysis.Both presented good electrochemical performance.Titanium dioxide and lithium carbonate were used as the starting materials.In the solid-phase synthesis process,different qualities of coarse-grain titanium powders were added as reducing agents to prepare lithium titanate materials with different amount of self-doped Ti³⁺.The XPS convolution analysis shows that Ti³⁺content gradually increases with the increase of reducing agent dosage,which improved lithium titanate performance and maintained good stability.The LTO--15%sample received 200charge-discharge cycles at 10 C rate and discharge specific capacity of 127.5 mAh g^-1with a capacity retention rate of 96.9%.Based on the starting materials of titanium dioxide and lithium carbonate,ultrafine metal titanium powder was added as a doping agent during the solid phase synthesis process.By means of high temperature thermal reduction reaction,Ti³⁺self-doping and bulk metal doping modification was separately performed on the surface of prepared lithium titanate and inside bulk phase.At the same time,the size of as-formed particles were reduced to develop a synergistic enhancement effect and strengthen the electrochemical performance of lithium titanate.At a rate of 0.5 C,the discharge specific capacity of the LTO-Ti sample was 178 mAh g^-1.At a rate of 5 C,its capacity retention rate was 91.2%after 200 charge-discharge cycles,and the Coulomb efficiency was close to 100%.Lithium titanate was successfully prepared via in-situ self-growth by using metal titanium powder as an initial titanium source.XRD/XPS/TEM results show that Ti³⁺self-doped and bulk-phase doped Ti powder on the surface of lithium titanate sample have smaller particle size,which indicates enhanced electrochemical performance.At the rate of 15 C,the discharge capacity was 110 mAh g^-1,and there was only 2.2%capacity loss after 200 charge-discharge cycles at rate of 2 C.By taking titanium dioxide and lithium carbonate as the starting materials,the lithium titanate was prepared through the molten salt method.Then,based on the reduction ability of cathode current,a relatively novel molten salt electro-decorative interface treatment technology was used to modify the interface of lithium titanate again,which significantly changed grain shape,reduced grain size and manifested enhanced electrochemical performance.The discharge specific capacity of the LTO-ME sample was 78 mAh g^-1at a rate of 20 C,and the capacity retention rate was 92.3%after 900charge-discharge cycles at a rate of 5 C.Finally,ultrafine metal powders were adopted as the titanium source to prepare intermediates?C1?with slow hydrolysis ability through the liquid phase synthesis method.During the subsequent hydrolysis,a standard spherical lithium titanate precursor was prepared by using the slow hydrolysis process,in contrast to the rapid hydrolysis process of tetrabutyl titanate as commercial organic titanium source.Due to its slow hydrolysis characteristics,C1 promotes the formation of spherical hydrolysates with the most stable surface potential during the prolonged hydrolysis process.Besides,after subsequent high temperature heat treatment,Li₄Ti₅O₁₂-TiO₂bicrystal phase material was generated.The introduction of TiO₂ phase prominently reinforced the specific capacity of the composites.Due to the synergetic enhancement effect of incorporated metal titanium phase and self-doped Ti³⁺on electronic conductivity,along with the spherical nanostructure of the final material,the transmission distance of Li⁺ions in electrode active material was shortened so as to endow it with excellent electrochemical performance and a tap density of 1.2-1.3 g/cm³.Moreover,its capacity and performance were reinforced.The initial discharge specific capacity was 465 mAh g^-1at a rate of 0.1 C,225.4 mAh g^-1at a rate of 1 C and was105.8 mAh g^-1at a rate of 20 C.