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Na Solid-State NMR Investigations of the Electrochemical Reaction Products of Sodium-Oxygen Cells
1
1
1
2
Zoë E. M. Reeve , Kristopher J. Harris , Christopher J. Franko , Victor Terskikh and Gillian R. Goward
1
1. Department of Chemistry and Chemical Biology, McMaster Unversity, Hamilton, ON, Canada L8S 4M1
2. National Ultrahigh-Field NMR Facility for Solid, 1200 Montreal Road, Ottawa, ON, K1A 0R6
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For the first time solid-state Na NMR is demonstrated to be a diagnostic tool for evaluating reaction products formed
within the sodium–oxygen (Na-O2) battery. The Na-O2 battery is an inexpensive, high energy density storage device;
[1]
comprised of an air electrode, a non-aqueous electrolyte and a Na metal anode. At the cathode during discharge,
molecular oxygen is reduced to the superoxide radical, which further reacts producing either one or both of the desirable
[2-4]
products; sodium superoxide (NaO2) and sodium peroxide (Na2O2).
In addition to the
anticipated electrochemistry, the superoxide radical also attacks the electrolyte resulting
in electrolyte decomposition species, where sodium carbonate (Na2CO3) is the main
NaO2
[1]
undesirable product. Currently the underlying battery chemistry is still unclear but that
can be revealed through the careful characterization of electrochemically-cycled
electrodes.
[5-6]
Na2O2
17
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Based on CASTEP
calculations both O and Na are predicted to be useful NMR
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probes. The focus has been to evaluate the diagnostic power of Na NMR as Na is
100% naturally abundance. Thus experiments can be preformed directly on cycled
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cathodes. This contrasts O NMR where isotropic labeling is necessary due to the
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extremely low natural abundance of O (0.04%).
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Na2CO3
Experimentally NaO2, Na2O2 and Na2CO3 are readily distinguishable in the Na NMR
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spectrum, Figure 1. Each species has a unique Na NMR signature. In a mixed sample
(the potential reaction products are ground together) the presence of NaO2 is confirmed
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by a short T1 relaxation time and a temperature dependent Na chemical shift because
NaO2 is a paramagnetic material. The diamagnetic Na2O2 and Na2CO3 spectra are
strongly influenced by the
quadrupole interaction. In a
mixed sample, the Na2O2
Na B
quadrupole lineshape is Na A
separated from the Na2CO3
lineshape by triple quantum
Na2O2
magic
angle
spinning
[7-8]
(3QMAS) ,
shown
in
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Figure 1 - Na 20 kHz MAS Figure 2. This demonstrates
spectra of the potential that we can identify Na2O2
in
a
multi-component
reaction products
system.
Na A
Na C
Na B
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The Na NMR investigation has been extended to probe
electrochemically-cycled materials. Through a combination of
1D and 2D NMR techniques we can determine which
reaction product is electrochemically formed in real Na-O2
cells.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Na2CO3
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Figure 2 – 2D Na-3QMAS spectrum of a Na2O2 and
Na2CO3 mixtures where the MAS speed is 20 kHz
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