In order to confirm that the metal oxide/TiO2 secondary heterostructure is an excellent electrode material for lithium ion batteries, we took the Co3O4/TiO2 secondary heterostructure as an example to investigate its electrochemical performance as an anode material for lithium ion batteries.
Figure 1(a) is the cyclic voltammetry curve of Co3O4/TiO2 electrode material in the voltage range of 0.01~3V. Obviously, the redox peak is located at 1.7/2.1V, which is the characteristic of the transformation of anatase TiO2 to orthorhombic LiχTiO2 peak, and its transformation mechanism is (TiO2+xLi++xe–LiχTiO2)[27-29]. Besides, the reduction peaks at 0.98 V and 0.66 V are caused by the reduction of Co3O4 to Co and amorphous Li2O and the formation of solid electrolyte membranes (ie, SEI membranes). Usually, the oxidation of Co to Co3O4 and the decomposition of amorphous Li2O will show an oxidation peak at 2.1V, but only one oxidation peak appears in the cyclic voltammetry curve, which may be caused by the overlapping of the two oxidation peaks. Next, we investigated the lithium storage properties of the Co3O4/TiO2 electrode material.
Figure 1(b) shows the charge-discharge curve of the electrode material at a current density of 200 mA/g, and the charge-discharge plateau of the charge-discharge curve is consistent with the cyclic voltammetry curve. The first discharge and charge capacities are 632.5 mA•h/g and 499.7 mA•h/g, respectively, and the Coulombic efficiency is 79%. When cycled for 480 cycles, the electrode material showed a weak decay trend, and its specific capacity was still 602.8 mA·h/g [Fig. 1(c)]. To show the superiority of the Co3O4/TiO2 electrode material as an electrode for Li-ion batteries, we also tested the electrochemical performance of the single components (TiO2 fibers and Co3O4 nanomaterials), respectively.
In contrast, the initial discharge and charge capacities of TiO2 fibers are very low, only 275.8 mA•h/g and 159.8 mA•h/g when used as the anode material for lithium-ion batteries. While Co3O4 nanomaterials have high initial charge-discharge capacity, they exhibit very poor cycling performance (Figure 2). By comparing the above results, the composite material inherits the advantages of the two components, namely the cycle stability of TiO2 and the high capacity of Co3O4, which fully exerts the synergistic effect of the two components and realizes the optimization of performance. Next, we continued to investigate the rate performance of the Co3O4/TiO2 electrode material. It can be seen from Figure 1(d) that the composite material also exhibits excellent rate performance compared with TiO2 nanofibers.
For example, at current densities of 400 mA/g and 1000 mA/g, the composites have specific capacities of 444.6 mA•h/g and 397.5 mA•h/g, while those of pure titanium dioxide are only 125.8 mA•h/g and 87.4mA•h/g, it is not difficult to find by comparing the values, the capacity of the composite material is more than 3 times that of pure titanium dioxide. And the composite can still recover the initial capacity even after 70 high-capacity cycles. These results demonstrate the excellent rate capability of the Co3O4/TiO2 electrode material.
In summary, the Co3O4/TiO2 secondary heterostructure has extremely excellent performance as a negative electrode material for lithium ion batteries. This excellent performance is not caused by the simple compounding of the two compounds, but the result of integrating the two to form a special secondary structure through rational design. First, it is easy to understand that the Co3O4/TiO2 composite exhibits a higher capacity than TiO2 because of the introduction of a high-capacity Co3O4 component. The composite secondary material not only retains the high capacity of Co3O4, but also inherits the good cycle stability of TiO2. Second, the peculiar structure of the composite also contributes to the improvement of its electrochemical performance.
For example, the voids between the Co3O4 sheet-like structures increase the surface area, ensure the efficient utilization of electrode materials, and play a positive role in increasing the capacity. Moreover, the ultra-long secondary structure can effectively avoid the self-aggregation of the electrode material, buffer the volume expansion of the electrode material, maintain the effective contact between the electrode material and the electrolyte, and play a positive role in improving the cycle stability. . In addition to the enhanced cycling stability, the Co3O4/TiO2 secondary heterostructure also exhibits excellent rate capability. This is because the addition of oxides enhances the electrical conductivity of the composite. To confirm the effect of conductivity, we performed electrochemical impedance measurements on 3 materials. The charge transfer resistances of the three materials are TiO2(730.0V)>Co3O4/TiO2(170.9V)>Co3O4(137.2V). Obviously, the addition of oxides enhances the impedance of the composites, thereby improving the rate capability of the composites.
We designed a simple and effective strategy that combines electrospinning technology and hydrothermal method to induce metal oxides (including Co3O4, Fe2O3, Fe3O4, and CuO) to grow on the surface of TiO2 fiber membranes, thereby obtaining metal oxide/ TiO2 secondary heterostructure. The Co3O4/TiO2 secondary heterostructure exhibits high capacity, enhanced cycling stability and rate capability when used as an anode material for Li-ion batteries. This enhanced performance is attributed to the efficient integration of Co3O4 and TiO2, which exerts their synergistic effect. We also believe that such exotic secondary heterostructures can also be widely used in photocatalytic water splitting and sensing.