Ⅲ-3. Radiochemistry & Nuclear Chemistry99

RIKEN Accel. Prog. Rep. 53

Ru and 57 Fe Mössbauer spectroscopic studies of Na2 Ru1−x Fex O3 ofsodium-ion battery electrodeK. Hamano,∗1,∗2 Y. Kobayashi,∗1,∗2 H. Haba,∗2 and H. Ueno∗2∗1∗2Dep. of Eng. Sci., University of Electro-Commun.
RIKEN Nishina Center- 167 -1.01

Relative transmission (a. u.)

10.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
-4-3-2-101234Velocity mm/Fig. 1. 57 Fe Mössbauer spectrum of disordered
Na2 Ru0.99 Fe0.01 O3 measured at 15.0 K.
1.001

Relative transmission (a. u.)

Sodium-ion batteries have attracted attention as thenext generation of batteries after Li-ion batteries forlarge-scale applications owing to the abundance andlow cost of sodium.1–3) The use of Na-excess metal oxides, Na2 MO3 (M: transition metals), as cathode materials may realize large capacities through an additional
oxygen redox reaction. Among Na-excess metal oxides,
Na2 RuO3 is expected to be the leading cathode material. In this study, we demonstrate the oxidation states
and coordination environments of Ru ions in Na2 RuO3
and Na2 Ru1−x Fex O3 (x = 0.01 and 0.05) after repeating the charge/discharge processes by means of 99 Ru
and 57 Fe Mössbauer spectroscopy, XRD, and electrochemical measurements. Mössbauer spectroscopy is a
powerful tool to understand the oxidation states ofRu ions. However, the measurements of 99 Ru Mössbauer spectra can only be performed at low temperatures because the Mössbauer transition energy is relatively high (Eγ = 89.8 keV).4,5) A 57 Fe-doped sampleof Na2 Ru1−x Fex O3 was prepared to estimate the temperature dependence of the oxidation states of Ru sites
by 57 Fe Mössbauer spectroscopy.
Na2 RuO3 has a layered structure in which the ﬁrst
layer is composed of Na+ and the second layer contains Na+ and Ru4+ in the ratios 1:3 and 2:3, respectively. It is known that Na2 RuO3 has two phases,
namely, ordered and disordered arrangements, whichcorrespond to sintering times of 48 h and 12 h, respectively. Ordered Na2 RuO3 has honeycomb-type cation
ordering in the [Na1/3 Ru2/3 ]O2 layers, whereas disordered Na2 RuO3 has a random distribution of Na+ andRu4+ in the [Na1/3 Ru2/3 ]O2 layers. The two polymorphs exhibit signiﬁcant diﬀerences in electrochemical properties.3)
A sample of disordered Na2 Ru0.99 Fe0.01 O3 was prepared through a solid-state reaction. A stoichiometricmixture of RuO2 , NaHCO3 , and Fe2 O3 was pressedand sintered at 850◦ C for 12 h in an Ar atmosphere.
The Na2 Ru0.99 Fe0.01 O3 sample was conﬁrmed to bein a single phase by XRD. For 99 Ru Mössbauer spectroscopy, the source nuclide 99 Rh (T1/2 = 15.0 d) wasproduced through the 99 Ru(p, n)99 Rh reaction at theAVF Cyclotron. 99 Ru Mössbauer spectra were obtained by a conventional arrangement, but both thesource and absorbers were maintained at 4.2 K in aliquid He cryostat during the measurements.
The 57 Fe Mössbauer spectrum of disordered
Na2 Ru0.99 Fe0.01 O3 shows a doublet peak, as shown inFig. 1. The doublet was analyzed with the distribu-1

0.999

0.998

0.997

0.996

0.995
-4-3-2-101234Velocity mm/Fig. 2. 99 Ru Mössbauer spectrum of disordered
Na2 Ru0.99 Fe0.01 O3 measured at 5.0 K.

tion of quadrupole splitting (∆EQ ) because Fe ionswere randomly present at on Na and Ru sites. The
mean values of the obtained distributions of the isomer
shift (δ) and ∆EQ were 0.48(1) mm/s and 0.64 mm/s,
respectively. Fe ions were in a trivalent state in the[Na1/3 Ru2/3 ]O2 layers. The 99 Ru Mössbauer spectrum shows a broadened and asymmetric single line, asshown in Fig. 2. The isomer shift was −0.30(1) mm/s,
and the Ru ion was in a typical Ru4+ state. It is speculated that the line broadening is caused by an increasein the oxidation state of a part of the Ru ions from
Ru4+ to Ru5+ and a signiﬁcant distortion of the octahedron of RuO6 with the introduction of a Fe3+ ion.
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. 7, 11397 (2016).
4) K. Takahashi et al., RIKEN Accel. Prog. Rep. 48, 242(2015).
5) Y. Kobayashi et al., J. Phys. 217, 012023 (2010).