Self-supported carbon nanofibers as negative electrodes for K-ion batteries: Performance and mechanism

Abstract

Self-standing carbon nanofibers (CNF) were electrospun and tested in K-ion batteries (KIB). The comparison of the electrochemical performance of KIB using potassium bis(fluorosulfonyl)imide (KFSI) and potassium hexafluorophosphate (KPF 6) carbonate-based electrolytes revealed that, despite the coulombic efficiency is more readily stabilized with KFSI than with KPF 6 , the long-term cycling is quite the same, with a specific capacity of 200 mAh.g-1 for the CNF electrode. Post-mortem X-ray photoelectron spectroscopy analysis shows a more stable solid electrolyte interphase (SEI) for KIB employing KFSI. Finally, the K + ion storage mechanism was investigated by combining cyclic voltammetry and operando Raman spectroscopy, highlighting a combination of adsorption and intercalation processes. However the rate capability is better with the KPF 6 salt due to SEI layers formed at both CNF and K metal electrode, highlighting that full cell may lead to even superior results.

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

“…Moreover, Figures and S1 show that this phenomenon also occurs for graphite and KVPO 4 F half-cells, so that such interaction (also called cross-talk) between electrodes is thus driven by the K metal; (ii) the Coulombic efficiency (CE) of K metal plating/stripping is about 50% for conventional 0.8 M KPF 6 EC/DEC compared to 33% for 0.8 M KFSI and 99% for 0.8 or 5 M KFSI DME . This explains why KFSI DME electrolytes and especially highly concentrated ones show superior performance in half-cells. ,− In other words, the CE measured in half-cells is partially driven by the K metal. Note that highly concentrated KFSI DME electrolytes enable the use of KFSI up to 5 V ( vs K/K + ), although the high salt amount may be a cost issue for the application.…”

“…This result highlights the much higher reactivity of the K metal and/or the formation of a more stable/passivating SEI with the KFSI salt compared to KPF 6 in EC/DEC, in agreement with previous studies. 14 XPS Analysis. Figure 4 shows the SEI composition as obtained from XPS quantification for Li and K metal electrodes after 3 weeks of storage in EC/DEC w/wo MPF 6 or KFSI salts.…”

“…(iii) K metal polarization is nearly 0.1 V higher for 0.8 M KFSI than for 0.8 M KPF 6 , which explains the lower rate capability generally observed with KFSI EC/DEC compared to that with KPF 6 EC/DEC, especially for negative electrodes. 14 Note that special care during K metal preparation can decrease its polarization and improve its stability over time. 22,23 In any case, electrode capacities obtained in half-cells are thus partially driven by the K metal.…”

“…Indeed, it is well known that the quality of the solid electrolyte interphase (SEI) formed , and possible gaseous products strongly impact the cycling . For instance, the use of KPF 6 (used in more than 75% of half-cell studies) is a default choice as it leads mostly to an organic SEI with poor stability. ,− Indeed, inorganic SEI should be preferred because compounds such as Li 2 CO 3 and LiF bring stability to the SEI, as previously shown for Li-ion and Na-ion batteries. , Contrary to KPF 6 , inorganic-rich and stable SEIs were often reported for KFSI-based electrolytes with enhanced electrochemical performance. ,− Moreover, among the solvents used, it has been shown that the use of a linear alkyl carbonate without a co-solvent is not appropriate and that the addition of ethylene carbonate seems beneficial for electrolyte stabilization . However, conventional 0.5–1.0 M KFSI in ethylene carbonate/diethylene carbonate (EC/DEC) implies corrosion of the Al current collector starting at different potentials above 3.5 V as a function of the salt concentration. , …”

Solid‐State Post Li Metal Ion Batteries: A Sustainable Forthcoming Reality?

“…The resulting data analysis reveals that the electrochemical sodiation mechanism is mainly based on the reduction of Ti 4+ to Ti 3+ , going along with the irreversible amorphisation of the pristine anatase structure. More classical experimental operando protocols have been reported [ 191 ] to investigate the performance and mechanism of carbon nanofibers as negative electrodes in K‐ion batteries using operando Raman spectroscopy. Similar to the case of graphite electrodes, [ 192 ] the K‐ion incorporation into amorphous carbon nanofibers occurs though a complex combination of absorption and intercalation processes, like in the electrochemical sodiation of hard carbons.…”