## 4 Conclusions  
The thorough characterization of the investigated electrospun Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\):Ge NFs via SEM, TEM, MRS, XPS, X-ray and neutron diffraction and XAS allow concluding that:  
* Reference Fe\({}_{2}\)O\({}_{3}\) NFs, exhibiting grainy structure, consist exclusively of hematite.
* Fe\({}_{2}\)O\({}_{3}\):Ge NFs have a more complex architecture and composition. They are formed by finer and elongated nanostructures developing mainly along the fiber axis; although \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) are the dominant phases, an amorphous component is also present.
* Germanium, mostly dispersed as an ionized impurity (with 4+ oxidation state), occupies the tetrahedral sites of the maghemite lattice and probably the defective hematite surface sites.
* The synergy between nanostructured morphology and electronic transport properties of Fe\({}_{2}\)O\({}_{3}\):Ge NFs, together with the pseudo-capacitive nature of the charge storage mechanism, are responsible for their excellent rate capability as anode for SIBs (140 mAh g\({}^{-1}\) at 2C).  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/2076-3417/11/4/1483/s1](https://www.mdpi.com/2076-3417/11/4/1483/s1)]([https://www.mdpi.com/2076-3417/11/4/1483/s1](https://www.mdpi.com/2076-3417/11/4/1483/s1)), Figure S1: Sketch of the synthesis route followed to produce the electrospun iron oxide NFs, Figure S2: Photo of the as- calcined Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\):Ge NFs., Figure S3: Elemental composition of the NFs, Figure S4: XRD patterns of the investigated NFs, Figure S5: Micro-Raman spectra, as measured at different random locations in the sample, Figure S6: Deconvolution of the O 1s core level profiles of Fe\({}_{2}\)O\({}_{3}\) NFs and Fe\({}_{2}\)O\({}_{3}\):Ge NFs, Figure S7: Linear combination fit of the spectrum of Fe\({}_{2}\)O\({}_{3}\)Ge NFs acquired at the Fe K-edge, Figure S8: EXAFS signal and Fourier transform for the spectrum of Fe\({}_{2}\)O\({}_{3}\):Ge NFs at the Ge K-edge, Figure S9: Results of the rate capability test on Fe\({}_{2}\)O\({}_{3}\):Ge NFs, compared with previously investigated Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\):Si NFs, Figure S10. Results of the cyclability test on Fe\({}_{2}\)O\({}_{3}\):Ge NFs. Table S1: Surface composition of the NFs, Table S2: Binding energies and fractional area of the components of O 1s core level in the NFs, Table S3: parameters of the EXAFS fit.  
**Author Contributions:** Conceptualization, C.F., R.R. and S.S.; methodology, B.P., C.F., G.B., C.R., M.F., P.G., S.P., C.T. and L.S.; validation and formal analysis, C.R., R.R. and L.S.; data curation C.F. and S.S.; writing--original draft preparation C.F., R.R. and S.S.; writing--review and editing, C.F., R.R. and S.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was partially funded by the Italian Ministry of University and Research (MUR) through the project \"Towards sustainable, high-performing, all-solid-state sodium-ion\" (PRIN 2017 MCEEY4).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data and processing aids, required to reproduce the findings of this work, cannot be shared at this time as they form the basis for an ongoing study.  
**Acknowledgments:** Financial support from the Italian Ministry of University and Research (MUR) through PRIN project 2017 MCEEY4, TRUST (towards sustainable, high-performing, all-solid-state sodium-ion batteries) is thankfully acknowledged. The Authors gratefully thank for beam time provision both ELETTRA synchrotron radiation facility (exp. 20195497) and ILL ([[https://doi.org/10.5291/ILLDATA.EASY-641](https://doi.org/10.5291/ILLDATA.EASY-641)]([https://doi.org/10.5291/ILLDATA.EASY-641](https://doi.org/10.5291/ILLDATA.EASY-641))).  
**Conflicts of Interest:** The authors declare no conflict of interest.