Apart from the ubiquitous lithium-ion batteries, the focus of research has recently increasingly fallen on energy storage alternatives that promise high energy density with cheaper and more available starting materials, like e.g. zinc ion batteries (ZIB) or zinc metal batteries (ZMB). So far, mainly Mn- or V-based oxides or Prussian blue analogues have been investigated as promising cathode materials for ZIB/ZMB.
In this study, the suitability of the spinel material ZnFe2O4 for use as active material at the positive electrode in ZIB/ZMB is investigated. ZnFe2O4 is already a well-investigated material with possible applications ranging from magnetic data storage to photoelectrochemical water splitting to active materials in lithium ion batteries. Other than the closely related ZnMn2O4 there are no reports on ZnFe2O4 being applied in ZIB/ZMB, even though there are theoretical works that suggest it as promising candidate which should show a high specific capacity and good zinc ion conductivity besides the already confirmed good electronic conductivity.
In addition to pure ZnFe2O4, part of the Fe3+ in our study was substituted with Ti4+ to achieve stabilization of Zn vacancies in the material and to increase ionic conductivity. Ceramic pellets with the composition ZnFe2-xTixO4 and x = 0 to 0.25 were prepared via a Pechini synthesis route and investigated regarding their electrochemical characteristics. Subsequently, cathode sheets from both pure and Ti-doped materials were prepared and measured in a cell setup using 0.5 M zinc triflate in acetonitrile as electrolyte in an “anode-free” approach with tin foil as negative electrode. Water-based electrolytes were tested as well but showed strong problems with Fe leaching from the spinel and with gas formation.
The electronic conductivity of ZnFe2O4 is reduced by Ti addition, hence Ti is not a typical n-type dopant, although in total, the electronic conductivity of the Ti-doped materials is still on a very high level (roughly 0.01 to 0.1 mS cm-1 at 20 °C). The amount of Ti dopant is limited, as already at x=0.13, small traces of secondary phases were detected in XRD. In our study, pure and Ti-doped zinc ferrite was found to work as active material in ZIBs, but samples with x  0.13 did not show a stable cycling behavior. The effect of Ti-addition on the magnetic characteristics is probably comparably small, as all synthesized pellets could be picked up with a magnet.
In summary, it has been successfully demonstrated that pure and Ti-doped ZnFe2O4 can be applied as active material at the positive electrode in ZIBs. As was to be expected, however, the measured specific capacities are very low, as the material has not yet been optimized for use as an active material, but the capacities are in a comparable dimension to those of unoptimized ZnMn2O4.
Acknowledgements: The authors thank Prof. Thomas Jüstel for help and discussion. Patent on Ti-doped ZnFe2O4 as active material for the positive electrode is pending.