The Effect of Water on the Discharge Capacity of a Non-Catalyzed Carbon Cathode for Li-O2 Batteries

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367

This study provides an analysis of the technological barriers for all-electric vehicles, either based on batteries (BEVs) or on H 2 -powered proton exchange membrane (PEM) fuel cells (FCEVs). After an initial comparison of the two technologies, we examine the likely limits for lithium ion batteries for BEV applications, and compare the projected cell-and system-level energy densities with those which could be expected from lithium-air and lithium-sulfur batteries. Subsequently, we will review the current development status of H 2 PEM fuel cells, with particular attention to their viability with regards to the required amount of platinum and the resulting cost and availability constraints. It is widely accepted that global warming is caused by CO 2 emissions and that they must be substantially reduced in order to prevent climate change. Since ≈23% of the world-wide CO 2 emissions are due to transportation, ≈75% of which are contributed by the road sector (numbers from 2012 1 ), a reduction of CO 2 emissions from vehicles is imperative to combat global warming. Towards this goal, many countries have passed legislature to lower passenger vehicle emissions over the long term like, e.g., the European Union mandate for 95 g CO2 /km fleet average emissions by 2020.2 The analysis by Eberle et al. 3 in Figure 1 suggests that this rather ambitious goal can only be met by means of extended range electric vehicles or all-electric vehicles in combination with the integration of renewable energy (e.g., wind and solar). Without increased integration of renewable energy sources and basing the calculations on the current European electricity generation mix, the only vehicle concept which could meet the 95 g CO2 /km target are pure battery electric vehicles (BEV100 in Fig. 1). However, for electricity produced entirely by renewable energy sources, the 95 g CO2 /km target could also be met by extended range electric vehicles with 40 miles all-electric range (E-REV40 in Fig. 1), if 50% of driving is powered by the battery (i.e., the average driving range would have to be below 80 miles), or by fuel cell electric vehicles (FECVs), with hydrogen produced by water electrolysis. While these propulsion concepts look promising, their contribution to CO 2 emission savings in the transportation sector would only be meaningful if their market penetration were substantial. In the absence of government regulations, the latter largely hinges on consumer acceptance, which in turn strongly depends on cost. In addition, in the case of BEVs, recent studies clearly showed that BEV driving range (closely followed by cost) are the predominant variables determining consumer acceptance. 4 In the following we will thus focus on the two vehicle types, which would be capable to meet and exceed the CO 2 emission targets of 95 g CO2 /km on the long-term, viz., pure BEVs and hydrogen powered FCEVs. For both vehicle types, but particularly for the latter, meaningful CO 2 emission reductions require the predominant use of renewable energy, which in turn necess...

Section: Lithium-oxygen Battery -Energy Density Projections and Challmentioning

confidence: 99%

Review—Electromobility: Batteries or Fuel Cells?

Gröger

Gasteiger

Suchsland³

2015

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367

“…This is comparable to a doubling of capacity in our previous study using Vulcan electrodes. 6 The minor difference might be related to the lower area specific discharge current for the Vulcan electrodes (880 μAm −2 external ), which, based on Figure 1, would still be in the high current density regime, but owing to the substantially lower discharge rate, a slightly higher capacity is not unexpected. A considerably larger effect was obtained when increasing the water content in the catholyte up to 1%, which resulted in a ≈25-fold increase of the capacity of a cell discharged at the same current.…”

Section: Resultsmentioning

confidence: 94%

“…Thus, at low rates, Li 2 O 2 precipitates more slowly, leading to the formation of big toroidal crystals composed from the initially formed disks in a secondary nucleation, as described by Mitchell et al 9 Effect of water on the Li 2 O 2 yield.-Although the crystals observed by SEM are similar to Li 2 O 2 crystals reported by others without intentional addition of water, it is important to verify the chemical nature of the main discharge product, especially as LiOH has been hypothesized to be one of the major products in cells discharged in presence of water. 6,22 XRD is usually the method of choice to detect the formation of crystalline Li 2 O 2 ; e.g., XRD was employed to prove that Li 2 O 2 is the main discharge product in ether-based electrolytes in early 2011. 40 Figure 3 shows that without addition of water, no crystalline discharge product can be observed in our experiment due to the very small amount of discharge product on the low-surface-area carbon fiber electrode.…”

Section: Resultsmentioning

confidence: 99%

“…2 In our earlier work, we found a direct correlation between the measured external surface area of a variety of carbon-based cathodes and their first discharge capacity, suggesting that a ≈2 monolayer thick Li 2 O 2 film is sufficient to block the discharge reaction. 5 On the other hand, we observed discharge capacities corresponding to more than 2 monolayers only when using electrolytes which contain species reactive towards the oxygen reduction reaction intermediate superoxide O 2 •− , such as propylene carbonate 6 or non-purified tetraglyme. 7 In contrast, Linda Nazar's group reported recently a substantial capacity increase when discharging at extremely low rates.…”

mentioning

confidence: 78%

See 1 more Smart Citation

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

Schwenke

Metzger

Restle

et al. 2015

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225

285

Aprotic Li-O 2 cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe low discharge capacities and formation of passivating layers of Li 2 O 2 on the electronically conducting cathode support, while other groups report large capacities and toroidal Li 2 O 2 crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O 2 cells, having a large effect on discharge capacity and Li 2 O 2 morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li 2 O 2 is still the main discharge product in Li-O 2 cells containing water, and moreover the Li 2 O 2 yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components.

“…Given that the DMSO absorbs more water than the TEGDME electrolyte, corrosion of the Li anode is expected given the high reactivity of Li anodes with H 2 O. 47 The Li foil in the case of the TEGDME electrolyte was not corroded and was still shiny after the test. Reactions occurring at the Li anode surface have not been widely studied/ reported (as a function of electrolyte type, water content, cathode composition etc.)…”

Section: Resultsmentioning

confidence: 99%

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

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 ...