The electrode potential-dependent formation of oxygen species on copper in noncomplexing aqueous media, encompassing oxide phase films and adsorbed oxygen/hydroxide, are explored at different pH values by means of surface-enhanced Raman spectroscopy (SERS). This technique provides a monolayer-sensitive in- situ vibrational probe, which can follow potential-dependent surface speciation on voltammetric or longer time scales. In alkaline NaClO4 electrolytes (pH 13), the cyclic voltammetric peaks associated with copper oxide phase-film formation and removal are correlated quantitatively with simultaneously acquired SER spectral sequences. The latter indicate the sequential formation of Cu2O and then mixed Cu2O/Cu(OH)2 layers, diagnosed by the appearance of metal−oxygen lattice vibrations at 625/525 and 460 cm-1, respectively. The potential-dependent speciation is in concordance with the Pourbaix diagram, certifying the “bulk-phase” nature of the films. The Raman band intensity−film thickness correlation (the latter deduced from the voltammetric Coulombic charges) indicate that the vibrational spectral responses are limited to the first 15−20 monolayers, consistent with earlier SERS observations and theoretical predictions. Weaker bands at ca. 800 and 460 cm-1 are discernible at more negative potentials, suggestive of hydroxide adsorption. Similar, although thinner, oxide films were deduced to form in neutral 0.1 M NaClO4. In the additional presence of chloride under these conditions, a potential-sensitive competition between the formation of a CuCl and a more passivating Cu2O phase film was evident from SERS. While oxide phase films are absent on copper in 0.1 M H2SO4 and 0.1 M HClO4, an adsorbed oxygen species was nonetheless detected from a broad SERS band at ca. 625 cm-1. This feature, which was deduced to involve oxygen rather than hydroxyl from an absence of a frequency shift upon H/D solvent isotopic substitution, is evident throughout most of the “polarizable potential” region on copper in acid, ca. −0.7 to −0.1 V vs SCE. The likely nature and reasons for its remarkable prevalence on copper in acidic media are discussed with reference to the recent literature.

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A high performance Li-S battery with novel fluoroether-based electrolyte was reported. The fluorinated electrolyte prevents the polysulfide shuttling effect and improves the Coulombic efficiency and capacity retention of the Li-S battery. Reversible redox reaction of the sulfur electrode in the presence of fluoroether TTE was systematically investigated. Electrochemical tests and post-test analysis using HPLC, XPS, and SEM/EDS were performed to examine the electrode and the electrolyte after cycling. The results demonstrate that TTE as a cosolvent mitigates polysulfide dissolution and promotes conversion kinetics from polysulfides to Li2S/Li2S2. Furthermore, TTE participates in a redox reaction on both electrodes, forming a solid electrolyte interphase (SEI) which further prevents parasitic reactions and thus improves the utilization of the active material.

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The fluorinated electrolyte containing a fluoroether 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was investigated as a new electrolyte for lithium-sulfur (Li-S) batteries. The low solubility of lithium polysulfides (LiPS) in the fluorinated electrolyte reduced the parasitic reactions with Li anode and mitigated the self-discharge by limiting their diffusion from the cathode to the anode. The use of fluorinated ether as a co-solvent and LiNO 3 as an additive in the electrolyte shows synergetic effect in suppressing the self-discharge of Li-S battery due to the formation of the solid electrolyte interphase (SEI) on both sulfur cathode and the lithium anode. The Li-S cell with the fluorinated electrolyte showed prolonged shelf life at fully charged state. Lithium-sulfur (Li-S) batteries are considered to be promising candidates which can satisfy the demand for high energy density batteries in electronic and transportation devices due to their high theoretical capacity, intrinsic overcharge protection, low cost and nontoxicity. 1,2 Sulfur, one of the most abundant elements in the earth's crust, has a theoretical capacity value of 1675 mAh/g and is the cheapest solid state cathode material for energy storage devices.Despite the considerable advantages, there are several major issues that impede the practical applications of the Li-S battery.3 Sulfur undergoes a series of structural and morphological changes during charge and discharge, which results in the formation of soluble Li 2 S x (4 < x < 8) and insoluble Li 2 S 2 and Li 2 S. The formation of soluble lithium polysulfides (LiPS) and their chemical reaction with the electrolyte and lithium anode leads to low coulombic efficiency and rapid capacity fading. [4][5][6][7] In addition, Li-S battery suffers from severe self-discharge, which is the biggest hurdle for the commercialization of this battery. 8 A secondary battery will lose charge capacity when stored for a period of time at a certain temperature. This behavior is known as self-discharge and depends on the battery chemistry, electrode composition, choice of current collector, electrolyte formulation, and the storage temperature. For Li-S batteries, the self-discharge is a well-known issue due to the severe corrosion of lithium metal anode in the presence of the LiPS in the electrolyte. 5,6,8 Many attempts have been made to overcome the poor cycle life and low sulfur utilization of Li-S batteries.7-14 However, there are only a few publications focused on solving the self-discharge issue of the Li-S battery. Kazazi et al. have reported that the corrosion of the aluminum current collector and the shuttle mechanism play a significant role in the self-discharge of Li-S cells; therefore, LiNO 3 is a suitable electrolyte additive candidate to prevent self-discharge due to its effect on shuttle prevention.8 Mikhaylik and Akridge reported that self-discharge mainly attributed from the high plateau polysulfides. Electrolytes with higher salt concentration also showed lower rates of Li corrosion wit...

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