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.

Concentrated electrolytes have the potential to increase the stability for batteries with lithium metal anodes. In this study, liquid electrolytes were created by mixing ethylene carbonate (EC), a solid at room temperature, with a high concentration of LiTFSI salt. The binary LiTFSI–EC highly concentrated electrolytes have the benefit of extremely low volatility as compared to conventional organic electrolytes and also allow for cycling vs Li metal anodes. Using a LiTFSI–EC electrolyte with molar ratio 1:6, the Coulombic efficiency for Li plating/stripping on Cu is 97% at a current density of 1 mA cm–2 with a 2 mAh cm–2 capacity, pointing to a practically useful performance. In a full cell setup using a commercial LiFePO4 (LFP) cathode, the efficiency is maintained, proving compatibility. In comparison to other carbonate-based electrolytes, there is less accumulation of decomposition products on the surface of a cycled Li film, which in part explains the improved cycle life. In all, this electrolyte system shows promise in terms of electrochemical stability and may allow for safe Li metal batteries due to the inherent physical stability.

Sustainable battery materials and chemistries are required to complement the growing demand for renewable energy from solar farms, wind mills, and hydroelectric power stations which are characterized by either demand fluctuations or periodic supply interruptions. [1] To keep a balance between supply and demand at all times, the intermittent nature of renewables should be leveled using stationary batteries deploying abundant, inexpensive, and nontoxic materials. [2,3] Current stationary batteries rely heavily on expensive transition metals (e.g., nickelcadmium or nickel-metal hydride batteries, and vanadium redox flow batteries), toxic elements (e.g., nickelcadmium and lead-acid batteries), or require high temperature to operate (e.g., sodium-sulfur batteries operating at 300À350 C). [3,4] In the interest of avoiding toxic and expensive minerals, there is a pressing need for sustainable battery materials that can provide comparable performance and cycle life. In this regard, the dual-ion battery (DIB) concept has emerged as a promising chemistry for future energy storage applications. In contrast to the "rocking chair" model in lithium-ion batteries, the energy storage mechanism in an archetype of a DIB is underpinned by the simultaneous intercalation of cations and anions from the electrolyte into, respectively, negative and positive electrodes containing graphite. [5,6] In a lithium DIB, anion intercalation and extraction require an operating voltage window ranging from 3 to 5.2 V versus Li þ /Li with the extent of reversibility ultimately depending on the type of graphite, electrolyte-salt concentration, type of electrolyte solvent, type of anion in the electrolyte, working temperature, and amount of electrolyte in the cell. [7,8] Reported gravimetric capacities for half-cells generally vary between 80 and 140 mAh g À1 with discharge voltages averaging 4.5 V. [6,9] On the basis of these metrics, lithium metal DIBs can be optimized to deliver cell-level energy density and specific energy above 200 Wh L À1 and 100 Wh kg À1 , respectively, better than lead-acid batteries (50-80 Wh L À1 or 20-55 Wh kg À1 ) and comparable with Ni-metal hydride batteries (150-220 Wh L À1 or 50-70 Wh kg À1 ) or Na-S batteries (150-300 Wh L À1 or 80-150 Wh kg À1 ). [6,10] Using graphite as the anion-hosting electrode, a wide selection of materials can be utilized in the negative electrode, which allows for increasingly diverse electrodeelectrolyte combinations in the design of DIBs. In addition, the fact that the negative and positive electrodes host different ions during cell operation relaxes the requirements on the type of electrolytes used. Thus, a stationary battery based on a DIB technology presents significant strategic advantages such as the likelihood of replacing transition metal-containing cathodes with graphite or organic materials and a broad choice of electrolytes. [11][12][13] These benefits are, in turn, anticipated to facilitate the transition to beyond Li-ion technologies harnessing more abundant Na þ -, K þ -, Ca 2þ...

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