Next, we investigate the electrochemical performance of porous Fe2O3 nanotubes as anode materials for Li-ion batteries. Cyclic voltammetry is a direct and effective method to investigate the electrochemical properties and electrochemical history of materials, so we first used cyclic voltammetry to characterize the samples with a scanning speed of 0.1mV/s and a voltage range of 0~3.0V. As shown in Fig. 1(a), in the cyclic voltammetry curve of the first circle, three reduction peaks appeared at the voltages of 1.55V, 0.89V and 0.55V. Among them, the reduction peaks located at 1.55V and 0.89V can be attributed to the conversion of Fe2O3 to α-LixFe2O3 and Li2Fe2O3 [3l-33]. The peak at 0.55 V is attributed to the conversion from Fe(III) to Fe(0) and the decomposition of the electrolyte. The corresponding oxidation peak at 1.85V is attributed to the conversion from Fe(0) to Fe(II) and Fe(III), which reversibly generates Fe2O3 and the subsequent cyclic voltammetry curves show obvious differences. This is due to irreversible phase transformation during delithiation/intercalation.
First, the reduction peaks at 1.55 and 0.89 V disappeared in the second cycle of cyclic voltammetry, suggesting that the transformation of Fe2O3 into α-LixFe2O3 and Li2Fe2O3 is irreversible. Second, the peak intensity of the cyclic voltammetry curve was significantly reduced, implying that the capacity of the electrode material decreased during cycling. However, the subsequent cyclic voltammetry curves were almost overlapping, suggesting that the electrode material has high reversibility and cycling stability during subsequent cycling.
Figure 1(b) shows the charge-discharge curves of porous Fe2O3 nanotubes at a current density of 100 mA/g. This curve not only exhibits redox peaks consistent with the cyclic voltammetry curve, but also exhibits high initial charge-discharge capacities (1041.1 mA•h/g and 1407.9 mA•h/g). Not only that, the porous Fe2O3 nanotubes can still exhibit a high specific capacity (987.7 mA h/g) even after 250 cycles at a current density of 200 mA/g, thus exhibiting good cycling performance [Fig. 1(c). )]. Although the initial coulombic efficiency of the electrode material was low, only 73.9%, the coulombic efficiency increased to 95% after 5 cycles, and increased to 98% after 50 cycles, which also showed that the electrode material had good reversible performance. Interestingly, the charge-discharge capacity of the electrode material showed a decreasing trend in the initial 50 cycles, dropping to 512.6 mA·h/g and 524.3 mA·h/g, but then the charge-discharge capacity showed a rising trend. The trend, after 250 cycles, reached 995mA•h/g and 987.7mA•h/g. This is similar to the reported transition metal oxide electrode materials [34~36]. The decrease in capacity is mainly caused by the aggregation and pulverization of Fe2O3 particles due to the de/intercalation of lithium ions during the initial charge-discharge process. As the Fe2O3 particles become smaller and smaller, the dissolution and pulverization are effectively suppressed, thereby enhancing the reversibility of the electrode material, resulting in a stable enhancement trend in the capacity of the porous Fe2O3 nanotubes.
Considering the practical application performance, it is necessary to investigate the rate capability of porous Fe2O3 nanotubes. Figure 1(d) shows the rate performance of this electrode material at different current densities. At current densities of 0.4A/g, 0.6A/g and 1A/g, the specific capacities of the electrode material were 868.4mA•h/g, 554.4mA•h/g and 358.8mA•h/g, respectively. And after 60 cycles, the specific capacity of the electrode material can still recover to 881.9 mA•h/g. Therefore, the electrode material has outstanding rate capability. Obviously, these outstanding electrochemical properties are closely related to the special structure of electrode materials. Porous Fe2O3 nanotubes not only provide efficient one-dimensional electron transport channels and short lithium ion diffusion paths, but also improve the contact between the active material and the electrolyte, effectively slow down the expansion of the electrode material during cycling, control the capacity fading, and improve the Cycling stability, and can alleviate the expansion and pulverization of the electrode material caused by the lithium ion deintercalation process, thereby greatly improving the cycle stability performance.