Sodium- and potassium-ion battery systems have garnered recent attention due to their potential for lower cost when compared to lithium-ion systems. However, electrode materials often undergo more substantial volumetric changes in Na- and K-ion systems, which may influence phase transformation pathways and cycle life. For the continued development of these batteries, it is critical to understand reaction mechanisms in individual electrode particles when using different ions; this knowledge will guide the engineering of particles with optimal size, shape, and structure. In this study, nanoscale transformation pathways in different sulfide materials (Cu2S and FeS2) are investigated in real time with in situ transmission electron microscopy (TEM) during electrochemical reaction with Li, Na, and K. Cu2S nanocrystals exhibit significantly different morphological changes during reaction with Na compared to Li, with a more uniform distribution of Cu after discharge in the Na case. Na-Cu2S half cells exhibit excellent cycle life (negligible capacity decay over 400 cycles), which is likely related to these different reaction pathways. During reduction of FeS2, the different alkali metal ions induce a conversion reaction that proceeds by a two-phase mechanism in all cases. However, the mechanical integrity of individual reacted FeS2 nanocrystals was found to differ with each of the alkali metal ions. Notably, despite lithiation causing much smaller volume expansion than the reaction with Na or K, fracture only occurred during reaction with lithium. This unexpected behavior is due to differences in the evolution of the reaction front shape during reaction with different alkali ions, as well as the different mechanical properties of the reacted phases. These results indicate that unforeseen nanoscale reaction pathways may allow for the effective use of large-volume-change conversion and alloying electrode materials for rechargeable Na- and K-ion based batteries, which would be a significant step towards energy-dense battery systems beyond Li-ion.

In situ TEM investigation of the electrochemical reaction pathways of sulfide materials with different alkali ions

255th National Meeting (2018)

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