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The water catalysis at oxygen cathodes of lithium-oxygen cells.

Li F, Wu S, Li D, Zhang T, He P, Yamada A, Zhou H - Nat Commun (2015)

Bottom Line: However, even in the state-of-the-art lithium-oxygen cells the charge potentials are as high as 3.5 V that are higher by 0.70 V than the discharge potentials.This can significantly reduce the charge overpotential to 0.21 V, and results in a small discharge/charge potential gap of 0.32 V and superior cycling stability of 200 cycles.The overall reaction scheme will alleviate side reactions involving carbon and electrolytes, and shed light on the construction of practical, rechargeable lithium-oxygen cells.

View Article: PubMed Central - PubMed

Affiliation: 1] Energy Interface Technology Group, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan [2] Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

ABSTRACT
Lithium-oxygen cells have attracted extensive interests due to their high theoretical energy densities. The main challenges are the low round-trip efficiency and cycling instability over long time. However, even in the state-of-the-art lithium-oxygen cells the charge potentials are as high as 3.5 V that are higher by 0.70 V than the discharge potentials. Here we report a reaction mechanism at an oxygen cathode, ruthenium and manganese dioxide nanoparticles supported on carbon black Super P by applying a trace amount of water in electrolytes to catalyse the cathode reactions of lithium-oxygen cells during discharge and charge. This can significantly reduce the charge overpotential to 0.21 V, and results in a small discharge/charge potential gap of 0.32 V and superior cycling stability of 200 cycles. The overall reaction scheme will alleviate side reactions involving carbon and electrolytes, and shed light on the construction of practical, rechargeable lithium-oxygen cells.

No MeSH data available.


Proposed reaction mechanism, LSV and gas analysis.(a) (i) is a spontaneous process; (ii) is promoted over MnO2 nanoparticles in Ru/MnO2/SP; and oxidation of LiOH in (iii) occurs at low charge overpotentials over Ru nanoparticles. (b) Linear scanning voltammetry (LSV) curves of the Ru/SP electrodes with and without LiOH under Ar atmosphere. (c) Gas chromatography (GC) analysis on the gas evolved in a charging process.
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f5: Proposed reaction mechanism, LSV and gas analysis.(a) (i) is a spontaneous process; (ii) is promoted over MnO2 nanoparticles in Ru/MnO2/SP; and oxidation of LiOH in (iii) occurs at low charge overpotentials over Ru nanoparticles. (b) Linear scanning voltammetry (LSV) curves of the Ru/SP electrodes with and without LiOH under Ar atmosphere. (c) Gas chromatography (GC) analysis on the gas evolved in a charging process.

Mentions: Based on the above results, a mechanism for the discharging and charging process of the cell with Ru/MnO2/SP and the electrolyte containing a trace amount of H2O can be proposed and schematically described in Fig. 5a. On discharging, O2 accepts electrons via the external circuit and is reduced to generate the primary discharge product Li2O2 (refs 40, 41, 42). At the same time, the Li2O2 reacts with H2O from the electrolyte and is converted to LiOH via Steps (i and ii). Although Step (i) is an equilibrium26, it can be largely promoted to move forward by the conversion of one of the products H2O2 to H2O over MnO2 via Step (ii)36. The two Steps (i and ii) occur sequentially, and quickly transform Li2O2 to LiOH as long as H2O remains in the electrolyte. This has been confirmed by the presence of substantial LiOH in discharged cathodes as revealed by both of the XRD patterns and the IR spectra in Fig. 2. This is consistent with the observations of Aetukuri et al.25, and Schwenke et al.26, where the H2O in electrolytes is possibly consumed by the employed Li anode or saturated by the product LiOH, and the lack of a promoter like MnO2 for Step (ii) made the equilibrium reaction in Step (i) to move backward and result in the major discharge product Li2O2 as detected.


The water catalysis at oxygen cathodes of lithium-oxygen cells.

Li F, Wu S, Li D, Zhang T, He P, Yamada A, Zhou H - Nat Commun (2015)

Proposed reaction mechanism, LSV and gas analysis.(a) (i) is a spontaneous process; (ii) is promoted over MnO2 nanoparticles in Ru/MnO2/SP; and oxidation of LiOH in (iii) occurs at low charge overpotentials over Ru nanoparticles. (b) Linear scanning voltammetry (LSV) curves of the Ru/SP electrodes with and without LiOH under Ar atmosphere. (c) Gas chromatography (GC) analysis on the gas evolved in a charging process.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525193&req=5

f5: Proposed reaction mechanism, LSV and gas analysis.(a) (i) is a spontaneous process; (ii) is promoted over MnO2 nanoparticles in Ru/MnO2/SP; and oxidation of LiOH in (iii) occurs at low charge overpotentials over Ru nanoparticles. (b) Linear scanning voltammetry (LSV) curves of the Ru/SP electrodes with and without LiOH under Ar atmosphere. (c) Gas chromatography (GC) analysis on the gas evolved in a charging process.
Mentions: Based on the above results, a mechanism for the discharging and charging process of the cell with Ru/MnO2/SP and the electrolyte containing a trace amount of H2O can be proposed and schematically described in Fig. 5a. On discharging, O2 accepts electrons via the external circuit and is reduced to generate the primary discharge product Li2O2 (refs 40, 41, 42). At the same time, the Li2O2 reacts with H2O from the electrolyte and is converted to LiOH via Steps (i and ii). Although Step (i) is an equilibrium26, it can be largely promoted to move forward by the conversion of one of the products H2O2 to H2O over MnO2 via Step (ii)36. The two Steps (i and ii) occur sequentially, and quickly transform Li2O2 to LiOH as long as H2O remains in the electrolyte. This has been confirmed by the presence of substantial LiOH in discharged cathodes as revealed by both of the XRD patterns and the IR spectra in Fig. 2. This is consistent with the observations of Aetukuri et al.25, and Schwenke et al.26, where the H2O in electrolytes is possibly consumed by the employed Li anode or saturated by the product LiOH, and the lack of a promoter like MnO2 for Step (ii) made the equilibrium reaction in Step (i) to move backward and result in the major discharge product Li2O2 as detected.

Bottom Line: However, even in the state-of-the-art lithium-oxygen cells the charge potentials are as high as 3.5 V that are higher by 0.70 V than the discharge potentials.This can significantly reduce the charge overpotential to 0.21 V, and results in a small discharge/charge potential gap of 0.32 V and superior cycling stability of 200 cycles.The overall reaction scheme will alleviate side reactions involving carbon and electrolytes, and shed light on the construction of practical, rechargeable lithium-oxygen cells.

View Article: PubMed Central - PubMed

Affiliation: 1] Energy Interface Technology Group, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan [2] Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

ABSTRACT
Lithium-oxygen cells have attracted extensive interests due to their high theoretical energy densities. The main challenges are the low round-trip efficiency and cycling instability over long time. However, even in the state-of-the-art lithium-oxygen cells the charge potentials are as high as 3.5 V that are higher by 0.70 V than the discharge potentials. Here we report a reaction mechanism at an oxygen cathode, ruthenium and manganese dioxide nanoparticles supported on carbon black Super P by applying a trace amount of water in electrolytes to catalyse the cathode reactions of lithium-oxygen cells during discharge and charge. This can significantly reduce the charge overpotential to 0.21 V, and results in a small discharge/charge potential gap of 0.32 V and superior cycling stability of 200 cycles. The overall reaction scheme will alleviate side reactions involving carbon and electrolytes, and shed light on the construction of practical, rechargeable lithium-oxygen cells.

No MeSH data available.