Ⅲ-3. Radiochemistry & Nuclear Chemistry

9 9 RuRIKEN Accel. Prog. Rep. 50 (2017)

Mössbauer spectroscopy of Na-ion batteries of Na 2 RuO 3 (III)K. Takahashi,1 Y. Kobayashi,1,2 H. Haba,2 and H. Ueno2

Sodium-ion batteries are attractive energy storage
media owing to its abundance of sodium, compared withlithium-ion batteries. There have been many
investigations of the development of Na+-containing
electrodes as new cathodes.1,2) We recently reported theelectrochemical properties of Na2RuO3 as the cathodematerial.3) Na2RuO3 has a layered structure in which thefirst layer is composed of Na while the second layer
contains Na and Ru in the ratios 1:3 and 2:3, respectively.
It was known that layered Na2RuO3 has two phases inthe [Na1/3Ru2/3]O2 layers, namely ordered and disorderedarrangements, depending on the sintering time. Theordered Na2RuO3 phase has honeycomb-type cation
ordering in the [Na1/3Ru2/3]O2 layers. On the other hand,the disordered Na2RuO3 phase has a random distributionof Na and Ru in [Na1/3Ru2/3]O2 layers. The twopolymorphs
exhibitsignificantdifferences
inelectrochemical properties.
In this study, the disordered Na2RuO3 phase and theNa-deficient oxides that imitate discharged samples ofNa2RuO3, which were obtained after a short sintering
time of 12 h were characterized by XRD and 99Ru
Mössbauer spectroscopy to understand the change ofoxidation state and coordination environment of the Ru
atoms caused by the deficiency of Na+ ions.
Disordered Na2RuO3 and non-stoichiometric Na-Ru
oxides (atomic ratios of Na/Ru = 1.5, 1.0, 0.5, and 0.2)
were synthesized by conventional solid-state reaction.2)
RuO2 and NaHCO3 were mixed with different atomic
ratios of Na/Ru thoroughly and sintered at 850ºC for 12
h in an Ar atmosphere. The source nuclide, 99Rh
(T1/2=15.0 d) of 99Ru Mössbauer spectroscopy wasproduced by the 99Ru(p,n)99Rh reaction at the AVF
Cyclotron. After p-irradiation for 24 h, the target wasused as the Mössbauer source with no chemicaltreatment. 99Ru Mössbauer spectra were obtained by aconventional arrangement, but both the source and theabsorbers were maintained at 4.2 K in a liquid He
cryostat during the measurements.4,5)
The XRD pattern of disordered Na2RuO3 showed atrigonal structure with a = 0.313 nm and c = 1.605 nm.
The parameters were consistent with those reported inRef. 3. The 99Ru Mössbauer spectrum of disordered
Na2RuO3 obtained at 5.0 K is shown in Fig. 1 (a). The
spectrum is similar to that of ordered Na2RuO3, reported
in Ref. 4, and is reasonably fitted by a doublet with anisomer shift (δ) of –0.34(1) mm/s and a quadrupolesplitting (ΔEQ) of 0.30(5) mm/s. The linewidth of 0.44(5)mm/s was slightly broadened compared to that ordered
Na2RuO3 owing to the shorter sintering time.
The XRD of non-stoichiometric Na-Ru oxides sintered1
2

Dep. of Eng. Sci., University of Electro-Commun.
RIKEN Nishina Center- 255 -Fig. 1. 99Ru Mössbauer spectra of (a) disordered
Na2RuO3 and non-stoichiometric Na-Ru oxides(Na/Ru = (b) 1.5, (c) 1.0, (d) 0.5, and (e) 0.2) obtainedby sintering at 850 ºC for 12 h in an Ar atmosphere.

with Na/Ru = 1.5 indicated that the sample crystallized
in two phases of Na2RuO3 (85%) and NaRu2O4 (15%).
NaRu2O4 is in the orthorhombic space group Pnm andconsists of double chains of edge sharing RuO6octahedra.6) The Mössbauer spectrum of Na-Ru oxides
with Na/Ru = 1.5 is shown in Fig. 1 (b). It was difficult
to analyze the two separated components, Na2RuO3 andNaRu2O4. However, assuming that the spectrum consistsof one component, the Mössbauer spectrum could have δ= 0.42(1) mm/s and ΔEQ = 0.49(1) mm/s. The value ofδ indicated that the oxidation state of Ru ions wasreduced from Ru4+ to Ru3.5+ on average. The detailed
discussion between the oxidation states of the Ru atoms
and the Na/Ru ratios of non-stoichiometric Na-Ru oxides
is under consideration.
References
1) K. M. Mogare et al., Z. Anorg. Allg. Chem. 630, 547 (2004).
2) M. Tamaru et al., Electrochem. Commun. 33, 23 (2013).
3) B. M. de Boisse et al., Nature Commun. doi 10.1038/ncomms
11397 (2016).
4) K. Takahashi et al., RIKEN Accel. Prog. Rep. 48, 242 (2015) .
5) Y. Kobayashi et al., J. Phys. 217, 012023 (2010).
6) K. A. Regan et al., J. Solid Stat. Chem. 179, 195 (2006).