Impact of the Salt Anion on K Metal Reactivity in EC/DEC Studied Using GC and XPS Analysis

Abstract

To develop K-ion batteries, the potassium metal reactivity in a half-cells must be understood. Here, it is shown first that the K metal leads to the migration of the electrode degradation species to the working electrode surface so that half-cells’ solid electrolyte interphase (SEI) studies cannot be trusted. Then, the K metal reactivity was studied by combining gas chromatography (GC)–mass spectrometry, GC/Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis after storage in ethylene carbonate/diethylene carbonate (EC/DEC) wo/w 0.8 M KPF6 or KFSI. A comparison with Li stored in EC/DEC wo/w 0.8 M LiPF6 was also performed. Overall, full electrolyte degradation pathways were obtained. The results showed a similar alkali reactivity when stored in EC/DEC with the formation of a CH3CH2OCO2M-rich SEI. For a MPF6-based electrolyte, the reactivity was driven by the PF6 – anion (i) forming mostly LiF (Li metal) or (ii) catalyzing the solvent degradation into (CH2CH2OCOOK)2 and CH3CH2OCOOK as main SEI products with additional C2H6 release (K metal). This highlights the higher reactivity of the K system. With KFSI, the reactivity was driven by the FSI– anion degradation, leading to an inorganic-rich SEI. These results thus explain the better electrochemical performance often reported in half-cells with KFSI compared to that with KPF6. Finally, the understanding of these chemically driven electrolyte degradation mechanisms should help researchers to design robust carbonate-based electrolyte formulations for KIBs.

Recent Advances in Electrolytes for Potassium‐Ion Batteries

“…On the other hand, KFSI efficiently inhibits the growth of potassium metal dendrites and suppresses side reactions. [119][120][121] Nevertheless, typical KFSIbased and KTFSI-based electrolytes cause severe corrosion on the Al current collector >4.0 V. Furthermore, if ether solvents are used, the electrolytes become more prone to decomposition at relatively low voltages due to the higher HOMO energy levels of ether molecules, which prevents the electrolytes from being applied to the production of high-voltage cathodes and full cells. Both problems mentioned above, fortunately, can be avoided by concentrated electrolytes, which will be discussed in detail in the electrolyte optimization section below.…”

Binder Chemistry Dependent Electrolyte Reduction in Potassium‐Ion Batteries: A Successive, Two‐Step Reduction Way

“…The deconvoluted peak at 532.3 eV is attributed to the ROCO 2 K and RCO 2 K, and K x PF y O z is identified at 535.5 eV, (it also may attribute to the Auger of Na in CMC‐ and SA‐derived SEI, especially in inner SEI layer). The deconvoluted peak area ratio of ‐O‐CO 2 ‐(≈531.2 eV, assigned to K 2 CO 3 ) and ‐OCO 2 K (≈533.4 eV, RO‐CO 2 K, assigned to PEDC), ($${A_{{{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3}}}{\rm{/}}{A_{ - {\rm{OC}}{{\rm{O}}_2}K}}$$) exceeds the balance value 1 (except the first point probably because of the surface sensitive feature of XPS) and increases gradually upon Ar + ion bombardment [ 7,21 ] (Figure 2e,f). This corroborates that i) more K 2 CO 3 are formed with respect to PEDC on CMC‐based electrodes in step I, and ii) the nearer the graphite surface, the more K 2 CO 3 are produced.…”

“…) exceeds the balance value 1 (except the first point probably because of the surface sensitive feature of XPS) and increases gradually upon Ar + ion bombardment [7,21] (Figure 2e,f). This corroborates that i) more K 2 CO 3 are formed with respect to PEDC on CMC-based electrodes in step I, and ii) the nearer the graphite surface, the more K 2 CO 3 are produced.…”

“…[ 6 ] And the organic compounds such as (CH 2 CH 2 OCO 2 K) 2 and CH 2 CH 2 OCO 2 K are the main SEI products, resulting from the high reactivity of the K system. [ 7 ] Nevertheless, given the complexity of the battery system, especially exacerbated by the diverse set of electrode materials, electrolyte compositions, and operating parameters, [ 8 ] a clear understanding of the key electrolyte decomposition process and the growth mechanism of SEI remains elusive.…”

Optimized electrode formulation for enhanced performance of graphite in K-ion batteries

“…So far, most of the studies focused on the electrolyte formulation (solvents and salt(s) ratios) to form a stable/passivating solid electrolyte interphase (SEI) without hindering the power performance [10][11][12][13][14][15]. However, half-cells results should be taken with caution as the highly reactive K metal was proved to significantly affect electrochemical performance (K metal polarization) [3,7,8,32] as well as the SEI formation (cross talking mechanism) [16,17,33]. Considering the electrode materials and formulation, i.e.…”