Identifying Electrochemical Processes in the Lithium

Identifying Electrochemical Processes in the Lithium-Oxygen
Battery by Solid State NMR Spectroscopy
Michal Leskes(1), Amy J. Moore(1), Gillian R. Goward(2) and Clare P. Grey(1)
(1) Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
(2) Department of Chemistry, McMaster University, 1280 Main St. W. Hamilton, Ontario, Canada , L8S 4M1
The Lithium-Oxygen battery
Library of possible electrochemical products
The lithium-oxygen battery is, in principle, a promising candidate for use as an energy
storage system. Theoretically, it can store 3,505Whkg-1 (approaching an order of
magnitude more than a conventional lithium ion battery) based on the reaction (in a nonaqueous electrolyte) of Li and O2 to form lithium peroxide (Li2O2) and including the
weight of the reactants1.
Detecting the 17O spectral signature of various lithiumoxygen compounds at high magnetic fields allows us to
identify them when they are formed in the battery .
Fitting the second-order
quadrupole line shape we can
determine the NMR
parameters and simulate
the spectra at the same
conditions (magnetic field and
magic angle spinning
frequency). The various
species are clearly
distinguishable by their 17O
spectra.
eCharge
O2
electrolyte
Discharge
Electrolyte
decomposition
products
composite
carbon cathode
In practice the development of the battery is still at initial stages with operating cells
falling short of their promising potential2. Among the challenges to be addressed are the
identification of stable electrolyte systems, inert and porous cathode materials and
efficient catalytic species. These can only be achieved with a careful analysis of the
electrochemical products formed during the operation of the cell. Here we employ a
multi-nuclear solid state NMR spectroscopy which enables us to monitor the evolution of
these products during electrochemical cycling and gain insight into processes affecting
capacity fading.
Characterization by Solid state NMR
We have recently demonstrated how solid state NMR (ssNMR) spectroscopy, in
particular of the 17O nucleus, is a powerful tool in the investigation of the lithium-air
battery as it allows a clear distinction between the main products formed in the cell –
lithium peroxide and lithium carbonate3.
The advantages of solid state NMR are:
Li2O2
Allows a clear distinction between
the main discharge products.
Detects products formed in the bulk
of the cathode as well as on the
surface.
Detects both crystalline and amorphous
materials.
carboxylic
acids
amino
acids
gas
phase
organic
mol.
17O
17O
NMR of cycled cathodes
enrichment of the products
Cycling the battery with 17O enriched oxygen atmosphere
results in isotope enrichment of the products which can
be identified and monitored during the cycle.
77%
discharge
Li+
Li2O2
6%
14%
3%
vacuum
line
84%
6%
2%
partial charge
lithium metal
anode
Li(s) + O2(g)
17O
8%
51%
22%
8%
inorganic
mol.
Taking into account the
relaxation of the different
species in order to get more
quantitative analysis
(I=5/2) quadrupole coupling constant, Cq, is a sensitive probe to its chemical
environment and can be used to uniquely identify the peroxide species.
Li-O2
cell
the cell is evacuated
to ~0.8bar and cooled
to~5C. Then filled
back to ~1bar with
enriched gas
18%
17O
enriched O2 gas
1H
ssNMR of cycled cathodes
*
*
*pvdf
1
P.G. Bruce, S. A. Freunberger, L. J. Hardwick, J-M Tarascon, Nature Materials 2012, 11, 19.
G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, The Journal of Physical Chemistry Letters 2010, 1, 2193. L. J. Hardwick, P.G. Bruce, Current
Opinion in Solid State and Materials Science 2012, 16, 178.
3M. Leskes, N.E. Drewett, L.J. Hardwick, P. G. Bruce, G. R. Goward, C. P. Grey, Angewandte Chemie International Edition 2012, 24, 2880.
 
2
Cell design and electrochemistry
13C
=  (1 − 
)
ssNMR of cycled isotope enriched cathodes
Top case (with holes)
The cathode reacts with lithium
peroxide upon charging forming
lithium carbonate. From the
second cycle carbonate
accumulates on the surface.
Stainless steel mesh
cathode (40% C)
separator +electrolyte
lithium
current collector
spring
coin cell
Li213CO3
Bottom case
O2 out
O2 in
Battery stopped at various stages,
cathode extracted, washed, dried
and packed in the NMR rotor
-
13C
cathode
1H
spectra are used to monitor the evolution of lithium
hydroxide and formate.
1H-6Li 2D correlations
aid in filtering the pvdf signal and
identifying a fragment of the DME formed at initial discharge.
Indicating proximity in
space between the
cathode and the
carbonate
D1Ah/g
LiOH
1M LiTFSI, DME, 70mA/g
+
1H
LiOCH2CH2OLi
cycling
HCO2Li
13C-13C
2D correlation (RFDR mixing, 20ms
at 10kHz MAS)
discharge
charge
1H
17O
6Li
Conclusions
• Lithium peroxide is the main discharge product in the initial cycle in DME accompanied by non-negligible electrolyte decomposition forming lithium
hydroxide, carbonate and formate.
• Upon charge significant amounts of lithium peroxide decompose below 4.5V.
• While the hydroxide decomposes upon charging, formate accumulates on the cathode surface.
• The carbon cathode, though inert during the first discharge, is unstable in the presence of peroxide at higher voltages forming a layer of carbonate that
blocks the surface.
• Limiting the capacity to 1000mAh/g results in similar distribution of products with a slight decrease in the charge potential, possibly due to a thinner
insulating layer of products.
• We have demonstrated that a multinuclear solid state NMR approach is a powerful method for directly detecting product formation and decomposition
within the cathode, a necessary step in the evaluation of new electrolytes, catalysts and cathode materials for the development of a viable lithium-air
battery.
Acknowledgments
The UK 850 MHz Solid-State NMR Facility used in this research was funded by EPSRC and BBSRC, as well as by the University of Warwick with partial support through Birmingham Science City
Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). We also thank the National Ultrahigh-Field NMR Facility for Solids
(Ottawa, Canada) for access to the 900 MHz NMR spectrometer. We are grateful for Prof. Dominic Wright for his help and advice on synthesis and Johnson Matthey for discussions and partial funding of AJM. ML is
an awardee of the Weizmann Institute of Science—National Postdoctoral Award Program for Advancing Women in Science and thanks the EU FP7 Marie Curie actions for a intra-European fellowship.