III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

99

Ru and 57 Fe Mössbauer spectroscopic studies of Na2 Ru1−x FexO3of sodium-ion battery electrode (2)K. Hamano,∗1,∗2 Y. Kobayashi,∗1,∗2 H. Haba,∗2 and H. Ueno∗2

Sodium-ion batteries have the potential to be thenext-generation batteries to replace lithium-ion batteries owing to the abundance of its raw materials.
Na2 RuO3 is a kind of Na-excess layered oxide andis promising as a cathode material for Na-ion batteries.1) In this study, we demonstrate the oxidation
states and the coordination environments of Ru ionsin Na2 RuO3 after charging and discharging processes
via Mössbauer spectroscopy, X-ray diﬀraction (XRD),and electrochemical analysis.2)
A sample of Na2 Ru0.99 Fe0.01 O3 was prepared via
a solid-state reaction. Stoichiometric mixtures ofRuO2 , NaHCO3 , and Fe2 O3 were pressed and sintered at 850◦ C for 48 h in an Ar atmosphere. The
sample was conﬁrmed to possess a single phase byXRD.2) In the electrochemical treatment, a mixtureof Na2 Ru0.99 Fe0.01 O3 , acetylene black, and polyvinylidene diﬂuoride as a binder was applied on an Al foil.
The Al foil with Na2 Ru0.99 Fe0.01 O3 and a carbon sheet
were used as the cathode and the anode, respectively.
As the electrolyte, 1 M NaBF4 solution was used.1)
These materials were packed in a battery cell and subsequently charged by applying with a DC current of
4 V for 0.5 h. In the XRD pattern after charging, thecharacteristic peaks of triclinic ilmenite-type NaRuO3were assigned, in addition to those of honeycomb-type
Na2 RuO3 .
For performing 99 Ru Mössbauer spectroscopy, thesource nuclide 99 Rh (T1/2 = 16.1 d) was produced by
the nuclear reaction of 99 Ru(p, n)99 Rh at the AVF Cyclotron. 99 Ru Mössbauer spectra were measured at
5 K in a conventional liquid He cryostat.3) The 99 Ru
Mössbauer spectra of Na2 Ru0.99 Fe0.01 O3 obtained at
5 K before and after charging are shown in Figs. 1(a)and (b), respectively. The spectrum before charging
was ﬁtted by a broadened single line, indicating Ru4+
with the isomer shift δ = −0.27(1) mm/s.2,3) The
spread of the linewidth was considered to be caused by
the doping of Fe atoms. Concurrently, the spectrum
measured after charging had a poor quality owing to
the small amount of sample packed into the battery
cell. However, this spectrum was analyzed to consistof two components having hyperﬁne magnetic ﬁelds(Hhf ) with quadrupole splitting (∆EQ ). The red component (δ = −0.27(4) mm/s, ∆EQ = 0.51(9) mm/s,
Hhf = 4.7(2) T) was consistent with the oxidation
state of Ru4+ before charging. The blue component (δ = −0.01(4) mm/s, ∆EQ = 0.07(7) mm/s,
Hhf = 17.9(9) T) presented a larger isomer shift and a∗1∗2Fig. 1. 99 Ru Mössbauer spectra of Na2 Ru0.99 Fe0.01 O3(a) before charging and (b) after charging.

larger internal magnetic ﬁeld. The value of δ indicatedthat the oxidation state of Ru after charging changed
from 4+ to 5+. It is probable that the oxidation number of Ru ions increased as Na+ ions were removed
by charging, and that Hhf was generated as the structure changed to a three-dimensional ilmenite structurefrom a two-dimensional layer structure with a Na+ deﬁciency. This speculation is supported by the observation of magnetic splitting in the 57 Fe Mössbauer spectrum at low temperatures. The ratio of the area intensity of the blue component was approximately 66%,
which is almost consistent with the Coulomb numberof the one-electron oxidation reaction under the electrochemical conditions of 4 V for 0.5 h.
References1) B. M. de Boisse et al., Nat. Commun. 7, 11397 (2016).
2) K. Hamano et al., RIKEN Accel. Prog. Rep. 53, 167(2019).
3) Y. Kobayashi et al., Inorg. Chem. 31, 4570 (1992).

Dep. of Eng. Sci., University of Electro-Commun
RIKEN Nishina Center- 154 -1