Impact of Electrolyte Additives on Interphase Chemistry and Performance in Potassium-Ion Batteries

Potassium-ion batteries are an emerging energy storage technology with promising aspects including high access and good distribution of potassium sources, earth-abundant electrode materials, and voltages comparable to lithium-ion batteries.[1] At the forefront for electrode materials are graphite, as the negative electrode, and Prussian blue analogues, for the positive electrode. While these materials offer good theoretical capacities and redox potentials, their high performance is often achieved in relatively expensive electrolytes based on potassium bis(fluorosulfonyl) amide (KFSA) and potassium bis(trifluoromethanesulfonyl) amide (KTFSA). As an alternative, the costs of the electrolyte could be minimized through low concentration additives in an inexpensive electrolyte, such as potassium hexafluorophosphate (KPF₆) in a carbonate ester solvent.[2] In this work, we explored fluorosulfonyl-type additives that have a similar structure to the electrolyte KFSA, among others, at low concentration (below 10 weight %). We focused on the impact of these additives on graphite and manganese Prussian blue (K₂Mn[Fe(CN)₆]) performance within KPF₆ electrolytes. We found the use of even 1 weight percent of KFSA or dimethyl sulfamoyl fluoride additives could improve the initial charge/discharge coulombic efficiency and subsequent capacity access. We further explored the additive impact on rate performance and charge transfer properties. Lastly, we evaluated the resulting interphase chemistry through surface techniques (e.g. XPS) and other analyses to understand how the additives react at the electrode surfaces. Alike to the additives applied in lithium-ion batteries,[3] we found evidence that small differences in the additive structure can strongly impact the resulting degradation reactions and their subsequent impact on cell performance.<br/><br/>References<br/>[1] T. Hosaka, <i>et al</i>. <i>Chem. Rev</i>. <b>120</b>, 6358 (2020).<br/>[2] T. Hosaka, <i>et al</i>., <i>ACS Appl. Mater. Interfaces</i>, <b>12</b>, 34873 (2020).<br/>[3] C. Forestier, <i>et al</i>., <i>J. Power Sources</i>, <b>330</b>, 186 (2016).