Energy storage for portable applications requires batteries capable of high capacity and cyclability. Conversion batteries operate by a different mechanism, giving rise to theoretical capacities several times larger than intercalation materials. Iron disulfide (FeS2) is a promising cathode material for conversion batteries due to its high theoretical capacity (726 mA.h g-1) and low materials cost. Electrochemical performance of FeS2 batteries will be discussed in the context of a local structure investigation at various states of charge. The local atomic structure of the active material is probed with pair distribution functions (PDFs) from total X-ray scattering. This type of analysis provides a valuable quantitative insight into the local chemical environment of Li, Fe, and S. A comparison of the pristine cathode before cycling and at various states of charge in the first two cycles of discharge show the formation of ternary Li—Fe—S and substoichiometric Fe—S phases as well as the formation of nanoparticle domains from the initially micron-sized particles of FeS2. Decreased coherence length and increased atomic disorder of the intermediate phases correlate with the electrochemical performance of the battery and capacity fade. The evolution of local structure of FeS2 with cycling may be tied with other transition metal sulfides to inform what chemistries are most promising for more reversible and efficient conversion materials. Such fundamental understanding is essential for improving conversion battery technology as a viable alternative to intercalation for next generation energy storage.

Unraveling the mechanism of transition metal sulfide conversion electrodes with local structure methods

251st National Meeting (2016)

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