This comparative work studies the self-enforcing heterogeneity of lithium deposition and dissolution as the cause for dendrite formation on the lithium metal anode in various liquid organic solvent based electrolytes. In addition, the ongoing lithium corrosion, its rate and thus the passivating quality of the SEI are investigated in self-discharge measurements. The behavior of the lithium anode is characterized in two carbonate-based standard electrolytes, 1 M LiPF6 in EC/DEC (3 : 7) and 1 M LiPF6 in EC/DMC (1 : 1), and in two alternative electrolytes 1 M LiPF6 in TEGDME and 1 M LiTFSI in DMSO, which have been proposed in the literature as promising electrolytes for lithium metal batteries, more specifically for lithium/air batteries. As a result, electrolyte decomposition, SEI and dendrite formation at the lithium electrode as well as their mutual influences are understood in the development of overpotentials, surface resistances and lithium electrode surface morphologies in subsequent lithium deposition and dissolution processes. A general model of different stages of these processes could be elaborated.
Rechargeable alkaline zinc-air batteries promise high energy density and safety but suffer from the sluggish 4 electron (e−)/oxygen (O2) chemistry that requires participation of water and from the electrochemical irreversibility originating from parasitic reactions caused by caustic electrolytes and atmospheric carbon dioxide. Here, we report a zinc-O2/zinc peroxide (ZnO2) chemistry that proceeds through a 2e−/O2 process in nonalkaline aqueous electrolytes, which enables highly reversible redox reactions in zinc-air batteries. This ZnO2 chemistry was made possible by a water-poor and zinc ion (Zn2+)–rich inner Helmholtz layer on the air cathode caused by the hydrophobic trifluoromethanesulfonate anions. The nonalkaline zinc-air battery thus constructed not only tolerates stable operations in ambient air but also exhibits substantially better reversibility than its alkaline counterpart.
Dual-graphite cells have been proposed as electrochemical energy storage systems using graphite as both, the anode and cathode, whereas the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode during charge. On discharge, cations and anions are released back into the electrolyte. In this contribution, we present highly promising results for "dual-ion cells" based on intercalation of bis(trifluoromethanesulfonyl)imide anions into a graphite cathode from an ionic liquid-based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr 14 TFSI). As the compatibility of this ionic liquid with graphitic anodes is relatively poor, metallic lithium and lithium titanate (Li 4 Ti 5 O 12 ) are used as anode. As both cations and anions participate in the charge/discharge reaction and other anode materials than graphite are possible, we propose the name "dual-ion cells" for these systems. The cell performance was studied in terms of cut-off voltage, temperature, cycling stability, self-discharge and rate performance. Depending on the cut-off voltage and temperature, coulombic efficiencies of more than 99 % and specific discharge capacities exceeding 100 mAh g −1 (based on graphite cathode weight) were achieved. Furthermore, this system provides an excellent cycling stability and capacity retention above 99 % after 500 cycles, outperforming reported organic solvent-based dual-graphite or dual-ion cells. Graphite is a redox-amphoteric intercalation host and therefore cations and anions can be electrochemically intercalated at different potentials yielding so-called donor-type or acceptor-type graphite intercalation compounds (GICs).1,2 Currently, the predominantly used donor-type GIC is LiC x . The LiC x /C x redox couple is the major active compound for state-of-the-art negative electrodes in lithium ion batteries. [3][4][5][6][7] Compared to the limited number of cationic intercalation guests, there is a broad spectrum of different anions capable to form acceptor-type GICs. Examples are hexa-or tetrafluoride guest species, e. g. PF 6− , AsF In 1938, Rüdorff and Hofmann developed the first ion transfer or rocking chair cell based on the shuttling of HSO 4 − anions between two graphite electrodes ( Figure 1a).14 This cell can be considered as the ancestor of the well-known lithium-ion cell, where lithium cations are transferred between two insertion electrodes during the charge/discharge process (Figure 1b). 3 In the 1990s, a rechargeable electrochemical energy storage system, using graphite as positive and negative electrode material in combination with a non-aqueous electrolyte has been introduced by and Carlin et al. 27,28 Carlin et al. investigated the reductive and oxidative intercalation of different cations and anions from ionic liquid-based electrolytes (without any additional lithium salt), such as 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI + PF 6 − ). This system, a so-called dual-graphite cell, was bas...
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