In order to confirm that the Fe3O4/TiO2 secondary heterostructure has excellent electrochemical performance as a negative electrode material for lithium ion batteries, and further confirm that the composite of the two types of metal oxides can effectively exert the synergistic effect of the two, we investigated its use as a lithium ion battery. Electrochemical performance of battery anode materials. Figure 1(a) is the cyclic voltammetry curve of Fe3O4/TiO2 electrode material in the voltage range of 0.01~3V. Obviously, the redox peak at 1.7V/2.1V is the characteristic of the transformation of anatase TiO2 to orthorhombic LixTiO2 peak, and its transformation mechanism is TiO2+xLi++xe–LixTiO2[22~24]. Besides, the sharp reduction peak at 0.7 V can be attributed to the reduction of Fe3O4 to Fe and the formation of amorphous Li2O and the formation of solid electrolyte film (ie, SEI film). The broad oxidation peak at 1.7V is caused by the oxidation of Fe to Fe3+[14,15]. For comparison, we also tested the cyclic voltammetry curves of TiO2 and Fe3O4 electrode materials in the voltage range of 0.01~3V (Fig. 2). Clearly, the cyclic voltammetry curves of the composites are consistent with the respective cyclic voltammetry curves of the single components, which also explains the formation of Fe3O4/TiO2 secondary heterostructures. In addition, for the TiO2 electrode material, after 3 cycles, the cyclic voltammetry curve remains basically unchanged, indicating that the TiO2 electrode material has very good cycle stability
sex. For the Fe3O4 electrode material, the first circle exhibits a large irreversible capacity, illustrating its poor cycling stability. For the electrode material with Fe3O4/TiO2 secondary heterostructure, its first irreversible capacity is obviously between the two, which preliminarily shows that the composite is beneficial to improve the cycle stability of metal oxides.
Figure 1(b) shows the charge-discharge curve of Fe3O4/TiO2 secondary heterostructure electrode material at a current density of 100 mA/g. The charge-discharge plateau of the charge-discharge curve is consistent with the cyclic voltammetry curve, which can be clearly observed. to the discharge platform at 1.75V and 0.7V and the charge platform at 1.7V and 2.1V. The first discharge and charge capacities are 783.6 mA•h/g and 494.5 mA•h/g, respectively, and the Coulombic efficiency is 63.1%. When cycled for 200 cycles, the electrode material showed a weak decay trend, and its specific capacity was still 454.5 mA•h/g [Fig. 1(c)]. To show the superiority of the Fe3O4/TiO2 electrode material as an electrode for Li-ion batteries, we also tested the electrochemical performance of the single components (TiO2 fibers and Fe3O4 nanomaterials), respectively. In contrast, the initial charge and discharge capacities of TiO2 fibers as anode materials for Li-ion batteries are very low, only 225.1 mA•h/g and 377.5 mA•h/g [Fig. 3(a)]. While Fe3O4 nanomaterials have high initial charge-discharge capacities of 768.5 mA•h/g and 1063.4 mA•h/g, respectively [Fig. 3(b)], they show very poor cycling performance after 70 cycles. Afterwards, its specific capacity is only 157.7 mA•h/g [Fig. 3(c)]. By comparing the above results, the composite material is obviously
Inheriting the advantages of the two components (i.e., the cycle stability of TiO2 and the high capacity of Fe3O4), the synergistic effect of the two is fully exerted, and the performance is optimized. Moreover, the Fe3O4/TiO2 secondary heterostructure exhibits excellent cycling performance even at high current density, for example, when the current density is 1A/g, the specific capacity remains at 187.8mA·h/g after 400 cycles ; When the current density is 2A/g, the specific capacity remains at 134.3 mA•h/g after 400 cycles; when the current density is 3A/g, the specific capacity remains at 112.2mA•h/g after 400 cycles [Fig. 3(d)]. In contrast, at such a high current density, the specific capacity of TiO2 fibers is only 92 mA·h/g [Fig. 3(e)]; while the specific capacity of Fe3O4 nanomaterials is only 128 mA·h/g after 50 cycles of cycling g [Fig. 3(f)].
Next, we continued to investigate the rate capability of Fe3O4/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 0.2A/g, 0.4A/g, 0.6A/g, 2A/g, 4A/g and 6A/g, the FesO,/TiO2 electrode material exhibited 481.4mA·h/g, The specific capacities of 335.4mA•h/g, 297.8mA•h/g, 199.1mA•h/g, 99.5mA•h/g and 65mA•h/g are more than 2 times that of pure titanium dioxide. In contrast, Fe3O4 nanomaterials exhibit very poor rate performance even at very small currents (Fig. 4). More importantly, the Fe3O4/TiO2 electrode material can still recover the initial specific capacity even after 120 high-capacity cycles. These results demonstrate the excellent rate capability of Fe3O4/TiO2 electrode material.
According to the Joule effect, the battery will generate a lot of heat during the charging and discharging process, which is bound to increase the ambient temperature of the battery operation. In addition, due to the different ambient temperature in different regions, the battery may be used in a relatively low temperature environment (such as northern China). Therefore, it is necessary to investigate the electrochemical properties of electrode materials at different temperatures. In view of this, we investigated the rate capability of Fe3O4/TiO2 secondary heterostructures at different temperatures, as shown in Fig. 5. When the ambient temperature is 50°C, the Fe3O4/TiO2 secondary heterostructure exhibits enhanced rate capability, corresponding to a specific capacity of 605mA·h/g at current densities of 200mA/g, 2000mA/g and 6000mA/g, respectively. g, 269.1mA•h/g and 70mA•h/g; and when the ambient temperature is 25℃, the specific capacities corresponding to the same current density are 481.4mA•h/g, 199.1mA•h/g and 65mA•h, respectively /g; when the ambient temperature drops to 0 °C, the specific capacities corresponding to the same current density are 398.2 mA•h/g, 96.1 mA•h/g and 26.6 mA•h/g, respectively. Obviously, the capacity of the electrode material increases under high temperature conditions, which is mainly caused by the high temperature intensifying the movement of ions in the electrolyte and reducing the impedance of the battery. However, under high temperature conditions, the cycle stability of the electrode material is weakened, which may be caused by the decomposition of the electrolyte and the destruction of the interface between the electrode and the electrolyte due to the high temperature [25~27]. While at low temperature, the capacity of the electrode material is reduced, but the cycling stability is increased, which is mainly due to the weakened movement rate of ions in the low-temperature electrolyte, but a stable solid-state electrolyte interfacial film is formed. Although the electrode material has low capacity at low temperature, it still exhibits a reversible capacity of over 320 mA·h/g at a current density of 200 mA/g. This indicates that the Fe3O4/TiO2 secondary heterostructure can be applied in a wide temperature range with excellent electrochemical performance.
In summary, the Fe3O4/TiO2 secondary heterostructure has extremely excellent performance as a lithium-ion battery anode material, and this excellent performance is the result of the synergistic effect of integrating the two types of electrode materials. First, it is understandable that the composite exhibits a higher capacity than TiO2 due to the introduction of high-capacity components. Moreover, the introduced high-capacity component enhances the electrical conductivity of the composite material system, thereby improving the rate capability of the composite material. In this chapter, we also tested the electrochemical impedance of three materials. The test results are shown in Figure 6. The charge transfer resistances of the three materials are TiO2>Fe3O4/TiO2>Fe3O4. Clearly, the addition of oxides enhances the impedance of the composites, thereby improving the rate capability of the composites, which is consistent with the results in Chapter 6.
To further prove that the excellent performance exhibited by the composites is caused by the synergistic effect after the formation of the secondary heterostructure, we tested the cycling performance of different types of electrode materials. As shown in Figure 7, we used the single-component TiO2 nanofibers and Fe3O4/TiO2 secondary heterostructures as a reference, and tested the TiO2 nanofibers and Fe3O4 nanoparticles according to their performance in the Fe3O4/TiO2 secondary heterostructures. The cycling stability of the physical mixing ratio was investigated, and the cycling stability of the secondary heterostructures with a small amount of Fe3O4 particles coated with TiO2 was compared. The results show that the secondary heterostructure of TiO2 coated with a small amount of Fe3O4 particles maintains good cycling stability, but exhibits lower capacity due to fewer high-capacity components. While the physical mixing of TiO2 nanofibers and Fe3O4 nanoparticles in proportion shows high initial capacity, but poor cycling stability. Obviously, this excellent performance is not caused by the simple mixing of the two compounds, but the result of integrating the two to form a special secondary structure through rational design.
As mentioned above, the excellent properties of composite materials are not caused by the simple mixing of the two compounds, but the result of the combination of the two to form a special structural synergistic effect. This synergistic effect imparts structural integrity to the electrode material. The composite one-dimensional secondary structure can alleviate the volume expansion of Fe3O4 caused by the deintercalation of lithium ions during the charging and discharging process, effectively inhibit the pulverization of the electrode, and improve the cycling stability of the electrode, as shown in Figure 8(a). shown. However, simple physical mixing cannot alleviate the volume expansion, so that the volume expansion and pulverization of Fe3O4 occur, so the cycle stability is greatly reduced, as shown in Fig. 8(b). In addition, the reduction of Fe3O4 to Fe in the composite electrode material can increase the reversibility of the reaction between TiO2 and lithium ions. To further confirm our proposed mechanism, we tested the SEM images of Fe3O4/TiO2 secondary heterostructures and their physical mixing at a current density of 100 mA/g and 20 cycles, as shown in Figure 9. It can be seen from Figure 9 that the morphology of the Fe3O4/TiO2 secondary heterostructure remains basically unchanged after 20 cycles, while in the physically mixed electrode materials, pulverized particles appear, which may be the fragmentation of Fe3O4 after cycling. caused. This experimental result fully confirms our proposed mechanism, so the synergistic effect plays a crucial role in improving the electrochemical performance of such electrode materials.
