Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 48 (2015)

99Ru Mössbauer spectroscopy of Na-ion butteries of Na2RuO3 (I)Y. Kobayashi,＊1,＊2 M. Okubo,＊3 H. Haba,＊1 and A. Yamada＊3

Sodium-ion batteries have attracted considerable
attention recently, because of their potential forlarge-scale applications: higher power and less expensive
batteries compared to Li-ion batteries can be realized byutilizing abundant and polarizable sodium ion as amobile charge carrier. One of the main obstacles torealizing high performance sodium-ion batteries is thelow specific capacity of the electrode materials. Thus, it
is particularly important to develop novel Na-ion cathodematerials with a high capacity as well as a high operating
potential.
Our group has recently reported the electrochemical
properties of Na excess transition metal oxide
Na[Na1/3Ru2/3]O2, where the cation arrangement in the[Na1/3Ru2/3] layer is controlled between the ordered anddisordered states by synthetic conditions. Importantly,
the ordered and disordered Na[Na1/3Ru2/3]O2 exhibit
significant differences in the electrochemical properties1).
In this work, we conducted Ru Mössbauer spectroscopy
for the ordered and disordered Na[Na1/3Ru2/3]O2 toclarify the valence states of Ru ions during
desodiation/sodiation.
The source nuclide, 99Rh (T1/2=15.0 d) for 99Ru
Mössbauer spectroscopy was produced via the 99Ru (p,n)
99
Rh reaction. The 96%-enriched 99Ru metal powder wasirradiated by protons with E = 12 MeV and I = 10 µA at
the AVF Cyclotron. The 99Ru metal powder was packed
into an Al holder, which was tightly fixed to theirradiation equipment for cooling by He gas flow andwater flow during the irradiation. After irradiation forabout 24 hours, the target was used as a 99Rh Mössbauer
source without being subjected to annealing or chemicaltreatment2).
99
Ru absorption Mössbauer spectra of Na[Na1/3Ru2/3]O2
samples were obtained using a conventional Mössbauer
spectrometer. Owing to the relatively high energy of theMössbauer -ray (89.8 keV), both the source and theabsorbers were maintained at liquid-helium temperature
in a cryostat.
The obtained spectra are shown in Fig. 1 and 2. The
Mössbauer spectrum for the disordered Na[Na1/3Ru2/3]O2(Fig. 1 (a)) shows a broad singlet with isomer shift () ofെ0.30 mm/s and linewidth () of 0.95 mm/s. The  value
is typical of RuIV, although the large linewidth suggests
structural disorder around Ru. The Mössbauer spectrumfor the ordered Na[Na1/3Ru2/3]O2 (Fig. 1 (b)) shows asinglet with  = െ0.27 mm/s and  = 0.63 mm/s,
indicating RuIV. Thus, regardless of the synthetic＊1＊2＊3RIKEN Nishina CenterUniversity of Electro-Commun.
University of Tokyo

conditions, the valence states of Ru in both compounds
are the same. However, the linewidth for the ordered
compound is much smaller than that of the disorderedone, most likely due to the ordered honeycomb
arrangement of Na and Ru in the [Na1/3Ru2/3] layer.
On charging the ordered compound, the Mössbauer
spectrum, as shown in Fig.2, cannot determine theelectronic structure accurately, in part due to the small
amounts of the obtained sample. However, a preliminary
result indicates oxidation of Ru from tetravalent topentavalent, from the obtained  value of +0.21 mm/s
with a small quadrupole splitting of 0.60 mm/s. Further
experiments with a larger amount of samples are now inprogress to reveal the reaction mechanism ofNa[Na1/3Ru2/3]O2.

(b)

(a)

Fig. 1. 99Ru Mössbauer spectra of (a) disorderedand (b) ordered Na[Na1/3Ru2/3]O2 at 4.2 K.

Fig. 2. 99Ru Mössbauer spectra at 4.2 K of theordered Na[Na1/3Ru2/3]O2 after electric charge.
References

1) M. Tamaru, et al., Electrochem. Commun. 33, 23 (2013).
2) Y. Kobayashi et al., J. Phys. 217, 012023 (2010).

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