Analysis of the Franklinite-Gahnite (ZnFe2-xAlxO4) Solid Solution Series: Insights into the Use of Doped Ferrites as Active Material for Zinc-ion Batteries

The cubic spinel franklinite (zinc ferrite, ZnFe2O4) is a well-studied material with a comparatively small band gap of 1.9 eV and widely studied magnetic properties [1,2]. This makes it a compelling candidate for a number of different applications ranging from photocatalysis to data storage. Recently, there have also been attempts to use zinc ferrite as an active material at the negative electrode of lithium-ion batteries (LIBs) due to its high theoretical volumetric capacity of 1142 mA h cm-3, but these have failed due to the poor chemical stability of the material during repeated lithium (de)insertion [1-3]. In contrast, first attempts to use doped zinc ferrite as an active material at the positive electrode of zinc-ion batteries (ZIB) showed promising results [4]. Compared to the standard tetragonal spinel material ZnMn2O4, the cubic titanium-doped ferrite in this study showed higher potentials vs. Zn/Zn2+. However, the problems of transition metal leaching in contact with aqueous electrolytes were similar to those of manganite.In the present study on the solid solution series of franklinite-gahnite, the iron is partially or completely replaced by Al3+. Since iron leaching occurs especially when Fe3+ in the structure is reduced to Fe2+ upon Zn2+ insertion, a sufficient amount of non-redox-active Al3+ could stabilize the structure in contact with aqueous electrolytes, but will strongly affect the ionic conductivity and possibly the cyclability. According to LIB studies, there might be a sweet spot in the range below x = 1.0 for ZnFe2-xAlxO4 [5]. In addition, increasing the aluminum content will result in lower electron conductivity due to an increased band gap [6], which is not necessarily desirable for an active material, but can be compensated by techniques such as carbon coating. It is also expected that the magnetic and optical properties will change significantly with increasing Al content. Homogeneous powders with the composition ZnFe2-xAlxO4 (x = 0.0 to 2.0) were synthesized by a Pechini type synthesis method. Calcined powders were then used to produce electrode sheets for application in ZnFe2-xAlxO4 ||Zn cells, while sintered ceramic pellets were used for materials level investigations. Using a variety of analytical techniques (XRD, impedance spectroscopy, CV, reflectance measurements, etc.) we succeeded in obtaining a holistic picture of the entire solid solution series, which will be discussed in terms of a defect model and potential applications in ZIB and beyond.REFERENCES[1] W. Schiessl, W. Potzel, H. Karzel, M. Steiner, G.M. Kalvius, Phys. Rev. B 53 (1996) 9143[2] M. Bohra, V. Alman, R. Arras, Nanomater 1 (2021) (5) 1286[3] M.M. Thackeray, Adv Energy Mater 11 (2021) (2) 2001117[4] S. Krämer, J. Hopster, A. Windmüller, M. Grünebaum, R.-A. Eichel, T. Jüstel, M. Winter, K. Neuhaus, Energy Adv. 3 (2024) 2175[5] I. Quinzeni, V. Berbenni, D. Capsoni, M. Bini, J. Solid State Electrochem. 22 (2018) 2013-2024[6] S. Gul, M.A. Yousuf, A. Anwar, M.F. Warsi, P.O. Agboola, I. Shakir, M. Shahid, Ceram. Internat. 46 (2020) 14195-14205