Nature of the Cathode–Electrolyte Interface in Highly Concentrated Electrolytes Used in Graphite Dual-Ion Batteries

Abstract

Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li + /Li 0 and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode–electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF 6 , LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g –1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF 6 -based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g –1 . The poor performance in LiPF 6 -containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in Li x PF y , PO x F y , Li x PO y F z , and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF 6 -based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.

The Role of Electrolyte Composition in Enabling Li Metal‐Iron Fluoride Full‐Cell Batteries

“…Full survey and region spectra of the cathodes and anodes after two cycles are provided in Figures S19 and S20 (Supporting Information), respectively, with the most relevant region spectra shown in Figure 5 . Beginning with the F 1s region of the cycled cathodes (Figure 5a ), we observe peaks corresponding to LiF (≈684.1 eV), [ 28 ] fluorosulfonyl (687.4 eV), [ 29 ] and —CF 3 species (688.2 eV). [ 28 ] The lack of Fe 2p peaks in the survey spectra of the cathodes (Figure S19c , Supporting Information) indicates these F species are likely in the surface CEI rather than the bulk of the electrode.…”

“…Turning next to chemical characterization, we study the surface chemistry of the electrodes after cycling using XPS. Full survey and region spectra of the cathodes and anodes after two cycles are provided in Figures S19 and S20 5a), we observe peaks corresponding to LiF (≈684.1 eV), [28] fluorosulfonyl (687.4 eV), [29] and -CF 3 species (688.2 eV). [28] The lack of Fe 2p peaks in the survey spectra of the cathodes (Figure S19c, Supporting Information) indicates these F species are likely in the surface CEI rather than the bulk of the electrode.…”

Ternary Ionogel Electrolytes Enable Quasi‐Solid‐State Potassium Dual‐Ion Intercalation Batteries

“…The maximum anodic current decreased from 0.1 μA cm À2 for the first cycle to %0.02 μA cm À2 for the third cycle. The areal current was substantially lower as compared with the values reported for ionic liquid-based electrolytes (e.g., 1 mA cm À2 for 1 M LiFSI in Pyr 14 FSI), [26] which indicated that the gel electrolyte suppressed parasitic reactions. This finding is particularly encouraging, as most of the state-of-the-art electrolytes available for DIBs (and in particular FSI-based alternatives) tend to cause extensive damage to the Al current collector during long-term cycling.…”

“…Thus, mostly highly concentrated electrolytes (HCEs) are used in DIBs along with thick and very porous separators. [6] The majority of HCEs contain unusually high amount of salt (3-5 mol L À1 ) dissolved in carbonate solvents which are prone to degradation beyond 4.5 V. [26] Under such circumstances, cell damage is inevitable as the salt and solvent molecules undergo oxidative degradation resulting in poor coulombic efficiency and graphite exfoliation in the long run. [6,12] Replacing liquid electrolytes with gel polymer electrolytes (GPEs) can potentially mitigate parasitic reactions involving free solvent molecules.…”

Relieving hydrogen evolution and anodic corrosion of aqueous aluminum batteries with hybrid electrolytes

“…As shown in Fig. S19,† there are obvious peaks in the F 1s spectrum that corresponding to intercalated (688.4 eV) and absorbed (686.7 eV) F species, 37 respectively. Even at high scan rates, the redox peaks are still clearly observed without obvious polarization, indicating the fast-electrochemical kinetic of KVO.…”