Fig. 3a shows the CV curves for the first four cycles of Ge/rGO/CNTs anode at a sweep speed of 0.1 mV/s and a potential window of 0.01–3 V. During the initial embedding process of Li
+, the CV curve of anode has an irreversible reduction peak at 0–0.5 V, which is attributed to the formation of solid electrolyte interphase (SEI) film [
33]. In addition, the second cycle has a wide cathodic peak at 0.29 V, which is the reversible alloying reaction of Ge-Li to generate various amorphous Li
xGe phases. The oxidation peaks observed at approximately 0.41 V and 0.57 V can be attributed to the delithiation process of Li
xGe alloys, indicating a gradual removal of lithium ions. The cycle curves of the third and fourth cycles coincide well, which ensures the cycle stability of the anode. However, a small peak at 1.12 V is also detected in the CV curves, which is ascribed to the partial surface oxidation of Ge to GeOx because nanosized Ge crystal is more susceptible to the oxidation. The lithium storage capacity of Ge/rGO/CNTs nanocomposites was systematically studied by constant current charge-discharge test at a potential window of 0.01–3 V and a current density of 0.1 A/g (
Fig. 3b). It can be seen that the discharge and charging platform of the anode corresponds to the position of redox peak in
Fig. 3a. The Ge/rGO/CNTs anode has high initial discharge specific capacity (1630 mAh/g) and charge specific capacity (1185 mAh/g), and the initial Coulomb efficiency (ICE) is 73%. The low Coulomb efficiency is because the SEI layer is generated due to the large area of direct contact between the anode and electrolyte during the first charge/discharge process, and the consumption of Li
+ leads to the generation of irreversible capacity and the loss of Li
+ [
34]. The CE value of the second cycle rose to 95.93% and showed an increasing trend with the cycle proceeding.
Fig. 3c and Fig. S5a (Supporting information) show the cycle performance of the seven materials for 100 cycles at 0.2 A/g. It can be seen that the discharge specific capacities of the anodes are maintained at 972.4 mAh/g (Ge/rGO/CNTs), 688.7 mAh/g (Ge/rGO), 601.7 mAh/g (Ge/CNTs), 59.4 mAh/g (Ge), 759.5 mAh/g (Ge/rGO/CNTs-5), 911.4 mAh/g (Ge/rGO/CNTs-20) and 824.6 mAh/g (Ge/rGO/CNTs-40). Ge/rGO/CNTs has better cycling performance.
Fig. 3d and Fig. S5b (Supporting information) display the rate performance of seven samples at various current densities. Obviously, Ge/rGO/CNTs anode has the best rate performance. The steady-state reversible capacities of Ge/rGO/CNTs anode at 0.1, 0.2, 0.5, 1 and 2 A/g are 829.5, 708.2, 675.3, 560 and 363 mAh/g, respectively. When the current density returns to 0.1 A/g, the reversible capacity of Ge/rGO/CNTs anode still reaches 894.9 mAh/g, because the 3D carbon network constructed from GO and CNTs can effectively alleviate the large volume changes of Ge/rGO/CNTs anode and promote the diffusion/ transport of Li
+ and electrons, thus improving the lithium storage performance. At a higher current density of 0.5 A/g, the long-cycle performance is also studied (
Fig. 3e and Fig. S5c in Supporting information). The discharge capacity of Ge/rGO/CNTs anode after 300 cycles is 614.5 mAh/g, compared with the discharge capacity of the second cycle (983.4 mAh/g), the capacity retention rate is 62.5%. It indicates that Ge/rGO/CNTs anode with an appropriate ratio of rGO and CNTs has good long-term cycling stability, because less CNTs added will easily make rGO nanosheets to agglomerate and reduce the performance, and more CNTs added will enlarge the space of rGO layers, resulting in larger volume expansion [
35]. Therefore, the combination of an appropriate amount of CNTs and GO, which can effectively alleviate the volume expansion of Ge particles, and improve the electrical conductivity and electrochemical activity. The prepared Ge/rGO/CNTs, Ge/rGO, Ge/CNTs, Ge/rGO/CNTs-5, Ge/rGO/CNTs-20 and Ge/rGO/CNTs-40 anodes show better electrochemical performance than Ge anode, proving the feasibility of synthetic materials.