Ca- and Al-doped ZnFe2O4nanoparticles as possible anode materials

Spinel ferrites are an amazing class of materials that can find application in different fields, from sensors and lithium-ion batteries
to the intriguing biomedical field. For the use as anode in lithium-ion batteries, ZnFe2O4 is rather competitive due to low price,
abundance, environmental benignity, working voltage of ~1.5 V, and, most importantly, a high theoretical specific capacity
(~1072 mA h g−1). For its practical application, however, some issues must be overcome, in particular its fast capacity fading and
poor rate capability resulting from an inherent low electronic conductivity. Possible strategies are represented by ferrite carbon
coating/embedding, peculiar synthesis routes, and doping. In this frame, we synthesized Ca- and Al-doped ZnFe2O4 nanoparticles
by using microwave-assisted combustion synthesis, followed by a classical carbon coating (determined as about 5 wt% by
thermogravimetry). A good solubility of Ca and Al up to 25 atom% on both Zn and Fe sites was obtained. Cyclic voltammetries
evidenced redox reactions involving Zn and Fe ions, but also the Al intervention could be supposed. Galvanostatic charge–
discharge cycles proved that particularly Al ions were useful to improve the anode structural stability at high C rate (up to 3C),
thanks to the stronger Al–O bonds with respect to Fe–O ones. A further improvement of capacities comes from the use of sodium
alginate as binder to substitute polyvinylidene fluoride in the anode preparation.