Study On Preparation And Modification Of TiO₂ As Anode Material For Sodium Ion Batteries

V Lithium-ion batteries have many advantages, such as high energy density, long cycle life, no memory effect and environmental friendly. It has gained recognition and affirmation in the field of portable electronic products and large-scale energy storage. However, limited lithium resources impeded its development and application. The sodium ion batteries has more attraction than lithium ion batteries when applied in large-scale energy storage and smart grid because of the more abundant and cheaper of sodium. The research of cathode material of sodium ion battery has made a certain progress, while little for anode materials, mainly including carbon, alloy and transition metal oxides. TiO₂ is regarded as a kind of new anode material for alternating carbon material. The change of lattice parameters is very small after embedding of Na+. So it is a material with excellent cycle stability, extremely good resistance to overcharge performance, thermal stability and security. But it has drawbacks itself as anode material for sodium ion battery, the poor electronic conductivity, which greatly influence its ratio performance. In this paper, the sol-gel method and hydrothermal synthesis method were used for synthesizing anatase TiO₂ nanoparticles. Its electrochemical performance and conductivity were improved by doping with niobium and boron.There are many differences about sodium reservoir performance between anatase and rutile TiO₂. The performance of anatase TiO₂ is better than rutile TiO₂. It can be found that different experimental conditions can affect the formation of TiO₂ crystal when prepared by sol gel method. It indicates that the proportion of oxidant and reductant, calcination temperature are two major factors of affecting its formation of crystal. When the proportion of oxidant and reductant is more than 1:1 and calcination temperature is over 500 ℃, rutile TiO₂ will appear. The amount will increase with the increasing proportion of oxidant and reductant and the rising of calcining temperature. The materials obtained at the condition of 1:1 and 500℃ are anatase TiO₂, morphology is uniformly, and show excellent electrochemical performance. The capacity of sample at 0.1 C, 0.2 C, 0.5 C, 1.0 C and 2.0 C is 150.3 m Ah/g, 136.1 m Ah/g, 111.4 m Ah/g, 90.8 m Ah/g and 78.4 m Ah/g, respectively. The capacity is 149.1 m Ah/g when the ratio back to 0.1 C.Doping is an effective method to improve the conductivity of materials. Niobium element was successfully prepared via sol-gel method. XRD, TEM and XPS test show that the niobium was doped into material interior and the lattice parameters were increased. Niobium doping can widen the ion channels obtained by the results of calculating of diffusion coefficient, which can improve the ionic conductivity. The discharge capacity of 2 % Nb-TiO₂ at 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C is 177.5 m Ah/g, 165.2 m Ah/g, 150.7 m Ah/g, 129.2 m Ah/g, 112.2 m Ah/g and 100.6 m Ah/g, respectively. However, the discharge capacity of pristine TiO₂ only has 150.4 m Ah/g, 136.6 m Ah/g, 112.5 m Ah/g, 92.6 m Ah/g, 78.5 m Ah/g and 54.6 m Ah/g.Although nano crystal TiO₂ can be obtained via sol-gel method, aggregation phenomenon will appear. While the materials obtained via hydrothermal method can be more uniformly and the size will smaller. In order to further improve the performance of TiO₂ for sodium ion battery, smaller nanoparticles and boron element doping were synthesized by hydrothermal method. Boron element were successfully doped into the interior of material obtained the results of XRD, HRTEM and XPS. Boron element can restrain the growth of the crystal grain, which leads to larger specific surface area. In addition, the calculating of diffusion coefficient shows that the boron doping can increase the diffusion coefficient, which makes it easier to sodium ion for transporting. The capability of materials can improved after doping. Its charge capacity can reach 225 m Ah/g for the first time at 0.1 C, when the ratio increase to 2 C, capacity fell to about 147 m Ah/g. However, the charge capacity of undoped sample is only 164.8 m Ah/g at 0.1 C and decrease to 116 m Ah/g at 2 C.This research explores the mechanism of storing sodium for TiO₂ by all kinds of test analysis. The XRD results of different states during charge and discharge show that the XRD diffraction peak of TiO₂ become weaker gradually during the process of sodiation, and will not increase after desodiation. There are no obvious changes of XRD diffraction peak of both niobium and doping boron compared with pristine TiO₂. But the niobium element doping can broaden its ion channel and increase its ionic conductivity, boron element can restrain the growth of the cirystal grain and increase its specific surface area, which are conducive to ion for embedding in the process of transmission.