The Influence of Water and Protons on Li2O2Crystal Growth in Aprotic Li-O2Cells

Schwenke, Konrad; Metzger, Michael; Restle, Tassilo M. F.; Piana, Michele; Gasteiger, Hubert A.

doi:10.1149/2.0201504jes

Cited by 225 publications

(313 citation statements)

References 57 publications

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“…Based on the resolution of the SEM micrographs, a less than 10 nm thick deposit appears to have formed with no sign of larger toroidal-shaped particles, in accordance with previously reported data at comparable current densities (0.1 mA/cm 2 geometric surface area ∼ 0.46 mA/m 2 BET ). 23 Upon charge, the morphology of the pristine electrode is retained, thus supporting the notion that the discharge/charge chemistry solely involves formation/dissolution of nanometer-thin surface deposits without any major structural changes imposed on the carbonaceous electrode. Figure 1d shows the X-ray diffractogram of the (1) discharged and (2) pristine C65 electrodes along with (3-8) reference powders.…”

Section: Resultssupporting

confidence: 53%

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

Guéguen

Novák

Berg

2016

J. Electrochem. Soc.

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The surface chemistry of the commonly employed positive electrode substrate Super C65 carbon was investigated during the 1 st cycle of a Li-O 2 battery with a typical ether electrolyte (0.2 M LiTFSI in Diglyme) by performing in situ online electrochemical mass spectrometry (OEMS), ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). During discharge, a nanometer-thin (< 5 nm) layer of Li 2 O 2 forms homogeneously throughout the electrode before it is passivated at a final specific charge of ∼365 mAh/g. Higher discharge charge rates lead to thinner and more densely packed layers of Li 2 O 2 . Electrolyte decomposition also occurs in parallel, as evidenced by the continuous increase of LiF and sulfur containing species of the degraded LiTFSI salt. On charge, O 2 evolves initially according to a 2e − /O 2 rate, but as the cell over-potentials increase, O 2 evolution rate approaches 4e − /O 2 , thus demonstrating a significant extent of parasitic side-reactions. Unlike the discharge, the charge leads to inhomogeneous electrode reactions causing a continuous removal of Li 2 O 2 with no indication of LiO 2 or Li 2 O intermediates. The decomposition side-products formed during discharge are also removed and the spectra of the pristine electrode are nearly retained at a full charge. Our results show that the analysis of Li-O 2 battery chemistry requires broad approach based on the combination of various electrode surface sensitive techniques, such as XPS and OEMS, in order to provide further fundamental understanding of the mechanisms behind the complex electrochemistry involving the Li and O 2 as well as several other predominant degradation products of the electrode and electrolyte. Non-aqueous electrolyte based Li-O 2 batteries have been a subject of intensive research during the past years, mainly because of their high theoretical specific charge (∼1100 mAh/g Li2O2 ), which provide promising prospects for their implementation in future energy storage systems.1 Several challenges do however remain, of which the most notable are the low round-trip efficiency and poor cycling performance, both partly related to the electrode/electrolyte instability.2,3 On discharge of the Li-O 2 cell, gaseous oxygen dissolves in the electrolyte and is reduced at the positive porous electrode where it combines with Li + (from the negative Li metal electrode) to form solid Li 2 O 2 deposits according to forward reaction of 2Li + O 2 Li 2 O 2 . Upon charge, the reaction is reversed to release the initial reactants Li + and O 2 gas. Despite extensive research on several classes of electrodes and electrolytes (including carbons, gold, carbides, oxides substrates combined with carbonate, sulfoxide, [4][5][6] nitrile, 4,5,7 amide, ionic liquid 8,9 or ether 10-13 based electrolytes, to mention a few), no composition has so far displayed sufficient chemical stability toward the reduced oxygen species to sustain a satisfactory number of discharge/charge cycles.1 The cell failure has been...

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Section: Resultssupporting

confidence: 53%

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

Guéguen

Novák

Berg

2016

J. Electrochem. Soc.

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“…While these spherical discharge products are consistent with previous results for Li 2 O 2 formed on Li-O 2 battery cathodes, we cannot rule out the formation of other side-products. This increase in particle size is also possibly reflected in a downshifting of the solution driven peak position for ORR caused by more solution mediated Li 2 O 2 growth as described by Aetukuri et al 45 Additionally, the density of discharge products on the cathodes containing PEO and PTFE (which showed almost exclusively solution driven processes in Figures 4e-4f) are much higher than that seen for PVP (which showed a larger contribution from surface processes). In contrast, the PVDF cathode showed a different discharge product morphology.…”

Section: Resultsmentioning

confidence: 68%

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O₂Batteries

Geaney

O’Dwyer

2015

J. Electrochem. Soc.

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In this report we examine the influence of electrode binder and electrolyte solvent on the electrochemical response of carbon based Li-O 2 battery cathodes. Much higher discharge capacities were noted for cathodes discharged in DMSO compared to TEGDME. The increased capacities were related to the large spherical discharge products formed in DMSO. Characteristic toroids which have been noted in TEGDME electrolytes previously were not observed due to the low water content of the electrolyte. Linear voltage sweeps were used to investigate ORR in both of the solvents for each of the binder systems (PVDF, PVP, PEO and PTFE) and related to the Li 2 O 2 formed on the cathode surfaces. Galvanostatic tests were also conducted in air as a comparison with the pure O 2 environment typically used for Li-O 2 battery testing. Interestingly, tests for the two electrolytes showed opposite trends in terms of discharge capacity values with capacities increased in TEGDME (compared to those seen in O 2 ) and decreased in DMSO. The report highlights the key roles of electrolyte and cathode composition in determining the stability of Li-O 2 batteries and highlights the importance of identifying more stable electrolyte/cathode pairings. Li-O 2 batteries are an exciting class of energy storage devices with exceptional theoretical capacities which could facilitate longrange electrical vehicles if fully optimized systems are realized. [1][2][3][4][5] Energy storage in Li-O 2 batteries proceeds via different mechanisms to those associated with conventional Li-ion batteries, necessitating detailed studies into the fundamental processes associated with discharge and charge.6-8 It has been shown in a number of studies that the energy storage mechanism for Li-O 2 batteries involves the reversible formation/decomposition of Li 2 O 2 upon discharge and charge respectively.9-13 While the O 2 required to form Li 2 O 2 during discharge can theoretically be provided from ambient air, the majority of systems investigated to date have used pure O 2 to avoid unwanted side-reactions due to the ingress of atmospheric CO 2 and H 2 O.14,15 The formation of parasitic by-products in Li-O 2 batteries (which have been found to form extensively on charging due to cathode and electrolyte instabilities) is a major hurdle to their widespread implementation. 16,17 The nature of Li 2 O 2 (i.e morphology, crystallinity, size and location on the cathode) formed during discharge, and its impact on capacity, cycle life and charging behavior has attracted recent research interest.18-20 A number of reports have presented the formation of Li 2 O 2 toroids on cathode surfaces during discharge in a variety of cathode/electrolyte systems. 7,8,12,19,[21][22][23][24][25] Adams et al. showed that the formation of these toroids was related to the applied current for a given system (TEGDME/LiTFSI electrolyte and Super P carbon cathode). 26Their results suggested that low applied currents tend to favor the formation of large crystalline Li 2 O 2 toroids on the cathode surface with ...

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“…The quantity of Li 2 O 2 formed was determined by ultraviolet-visible spectrometry (Thermo Evolution 200) using an ultraviolet-visible titration method reported previously 20,52 . The unwashed discharged electrode and separators were added to a vial containing a known amount of water; Li 2 O 2 reacts with water to produce H 2 O 2 in solution.…”

Section: Methodsmentioning

confidence: 99%

“…In the absence of DBBQ, reduction of O 2 to Li 2 O 2 proceeds via the LiO 2 intermediate 15,16,[18][19][20][21] , and it is the need to reach the potential for formation of LiO 2 that pins the O 2 reduction at a potential (discharge plateau in a Li-O 2 cell) significantly negative of the standard potential for Li 2 O 2 formation, 2.96 V (Fig. 1).…”

Section: The Mechanism Of O 2 Reduction In the Presence Of Dbbqmentioning

confidence: 99%

“…In contrast, if Li 2 O 2 can be induced to grow in the electrolyte solution then high discharge capacities at relatively high rates and avoiding early cell death is possible 15 . It is clearly important to operate a Li-O 2 battery in which Li 2 O 2 grows in solution.A number of groups have elucidated the mechanism of O 2 reduction to Li 2 O 2 on discharge 15,16,[18][19][20][21] . The reduction proceeds through the following general steps: 22,[27][28][29][30] .…”

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confidence: 99%

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Erratum: Promoting solution phase discharge in Li–O2 batteries containing weakly solvating electrolyte solutions

Gao¹,

Chen²,

Johnson³

et al. 2016

Nature Mater

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On discharge, the Li-O 2 battery can form a Li 2 O 2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li 2 O 2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the T he high theoretical specific energy of the rechargeable Li-O 2 battery has generated intense interest in the possibility of a practical device that could deliver energy storage significantly in excess of today's lithium-ion batteries 1-9 . However, major challenges hinder the development of such a technology 1-6,10-14 . Typically a Li-O 2 battery is composed of a lithium metal anode separated by an aprotic electrolyte solution from a porous O 2 cathode. The reaction at the cathode involves, on discharge, the reduction of O 2 to form Li 2 O 2 , with oxidation of the latter on charge. Growth of Li 2 O 2 on the cathode surface leads to low capacities, poor rates and early cell death [15][16][17] . In contrast, if Li 2 O 2 can be induced to grow in the electrolyte solution then high discharge capacities at relatively high rates and avoiding early cell death is possible 15 . It is clearly important to operate a Li-O 2 battery in which Li 2 O 2 grows in solution.A number of groups have elucidated the mechanism of O 2 reduction to Li 2 O 2 on discharge 15,16,[18][19][20][21] . The reduction proceeds through the following general steps: 22,[27][28][29][30] . High donor number salts have been shown to increase the capacity fourfold and reduce the discharge overpotential by ∼30-50 mV over low donor number salts 22 . Viologens 27,28 , phthalocyanines 29 and quinones 30 have been investigated as possible soluble reduction catalysts. Although the studies of such catalysts are important, in most cases there is little or no direct evidence demonstrating that they promote formation of Li 2 O 2 in solution and not on the electrode surface because they rely on electrochemical measurements alone. Yet past work on Li-O 2 batteries has shown how essential it is to provide more than electrochemical evidence in this field 31 . In some cases, soluble catalysts show an increase in discharge voltage (lower overpotential) as small as, for example, 40 mV (refs 28,29), which is very unlikely to be sufficient to shut off the direct reduction of O 2 to Li 2 O 2 , essential to stop detrimental Li 2 O 2 film formation. Also, none of the previous studies in low donor number solvents exhibited a significant increase in capacity on discharge at a relatively high rate, which is important for a successful Li-O 2 battery.Here we demonstrate that addition of DBBQ (2,5-di-tert-butyl-1,4-benzoquinone) to a weakly solvating (low donor number) electrolyte solution, LiTFSI in ether 22 , promotes O 2 reduction to Li 2 O 2 in solution while halving the discharge overpotential (increasing the discharge potential), suppressing the growth of a Li 2...

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The Influence of Water and Protons on Li₂O₂Crystal Growth in Aprotic Li-O₂Cells

Cited by 225 publications

References 57 publications

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O₂Batteries

Erratum: Promoting solution phase discharge in Li–O2 batteries containing weakly solvating electrolyte solutions

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The Influence of Water and Protons on Li2O2Crystal Growth in Aprotic Li-O2Cells

Cited by 225 publications

References 57 publications

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2Batteries during the 1st Cycle

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2Batteries during the 1st Cycle

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O2Batteries

Erratum: Promoting solution phase discharge in Li–O2 batteries containing weakly solvating electrolyte solutions

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The Influence of Water and Protons on Li₂O₂Crystal Growth in Aprotic Li-O₂Cells

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O₂Batteries