Investigation and optimization of the iron(II) sulfide starting material employed as an anode in alkaline batteries

Abstract

NATURAL SCIENCE RESEARCH PROJECT (not Master Thesis) In this work it has been shown that Nickel-Iron(II) sulfide (Ni-FeS) batteries, prepared with ball milled FeS as the active anode material, demonstrate faster activation (about 6 charge-discharge cycles), lower self- discharge (8% in 24hr) and higher discharge capacity (343 mAh/g) than virgin FeS batteries. However, their lifetime is reduced to ~200 charge-discharge cycles compared to ~300 for virgin FeS batteries. Therefore, the effect of one-hour heat treatment (300°C-900°C) on virgin FeS as well as ball-milled FeS was studied. The hypothesis was that heating would lead to more crystalline and/or stable FeS and therefore a prolonged lifetime. Characterization of samples was performed by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Mössbauer spectrometry. Battery performance was tested by galvanostatic charge-discharge measurements with discharge capacity, activation time and self-discharge as important parameters. The results have shown that commercial, as received FeS of 99.9% purity is actually a mixture of FeS, iron deficient Fe(1-x)S, metallic iron and free sulfur. Furthermore, the effect of heat treatment on virgin FeS is negative in terms of lifetime. On ball milled FeS the heat treatment has shown to retain a higher discharge capacity for the full 200 cycle lifetime, together with a small overall lifetime improvement. In this case, heating should be performed after ball milling. Whilst heating had an effect there was no clear trend with heating temperature. The differences in battery performance between ball milled and virgin FeS will be discussed on a mechanistic level with particle size deemed to be the most important factor. The effects of heating are more complex and not yet clear, but the first steps towards a clearer insight are given.