## 3 Results and Discussion
### Electrochemical Properties of the NFs  
PCGA measurements were performed to get cathodic and anodic profiles in quasi-reversible conditions. In this technique, the potential (ranging from 3.0 to 0.01 V vs. Na\({}^{+}\)/Na) is decreased (cathodic branch) or increased (anodic branch) by 4 mV and, for each step, the charge is collected until the current becomes less than 20 mA g\({}^{-1}\). Considering the theoretical specific capacity of hematite (1005 mAh g\({}^{-1}\)), the current threshold corresponds to C/50, which should allow to collect all the available charge. The corresponding profiles are reported in Figure 6a. The first cycle Coulomb efficiency was 55%, probably due to both the large amount of carbon at the electrode and the conversion reactions already discussed for similar phases [36]. After the first cycle, the profiles showed the typical pseudocapacitive behavior of these composites, with three different regions of different slopes. After the initial cycle, the specific capacities were around 300 mAh g\({}^{-1}\). Galvanostatic cycling was performed to evaluate the rate capability and the capacity retention of the electrodes (Figure 6b). The GCPLs at low currents (C/20) confirmed the capacities observed in PCGA, with an average value of 320 mAh g\({}^{-1}\) in the anodic process. The large uncertainty of the low current capacities (C/20 and C/10) can be attributed to the intrinsic multi-phase nature of the doped oxide, because the different phases have probably different kinetics affecting the extension of reaction with the sodium ions cycle by cycle. At higher rates, the data precision increases, because the slower phases became less active and do not substantially contribute to the value. The capacity decreased with the C rate, and 270, 240, 205, 175, and 140 mAh g\({}^{-1}\) were obtained at C/10, C/5, 1C, and 2C, respectively. On the contrary, the Coulomb efficiencies increased with the current (from 94% at C/20 to 99.2% at 2C), pointing out the presence of parasitic cathodic processes  
Figure 6: Electrochemical properties of the Fe\({}_{2}\)O\({}_{3}\):Ge NFs. (**a**) Potential profiles versus the specific capacity and (**b**) anodic and cathodic specific capacities versus number of cycles at different rates and corresponding Coulombic efficiency. with lower kinetic compared to the charge accumulation reaction. After the 60 th cycle (2C), the current was set back to C/20 to evaluate the capacity retention after 70 cycles, which was calculated as 97% with a good and stable recovering of the initial value. Thus, the electrodes showed good specific capacity as anode for sodium ion batteries, with excellent rate capability (140 mAh g\({}^{-1}\) at 2C which corresponds to 2 A g\({}^{-1}\)) thanks to the synergistic combination of the electronic transport properties of the complex material, the nanostructured morphologies and the pseudo-capacitive nature of the charge storage mechanism. These performances exceeded those of pure hematite and were in agreement with those obtained from metalloid-doped hematites [36] (Figure S9). Since Fe\({}_{2}\)O\({}_{3}\):Ge NFs and Fe\({}_{2}\)O\({}_{3}\):Si NFs were prepared by using the same dopant-iron mass ratio (1:9), the larger relative amount of silicon incorporated in the Fe\({}_{2}\)O\({}_{3}\) lattice may partly account for the higher specific capacity of Fe\({}_{2}\)O\({}_{3}\):Si NFs at lower rates. In any case, the different percentages of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) (and amorphous) phases present in the Fe\({}_{2}\)O\({}_{3}\):Si NFs and Fe\({}_{2}\)O\({}_{3}\):Ge NFs made it hard to make a direct comparison considering only the atomic chemical physical properties. To test the cyclability of the materials at constant current, 180 cycles at C/10 (70 mA g\({}^{-1}\)) have been performed (Figure S10). The initial capacity values are in agreement with the rate test of Figure 6b, with an average value of 230 \(\pm\) 10 mAh g\({}^{-1}\) (cycles 1-5).