PD Research Report for the 2015 year
Name（Research group） Wang Heng
（Yoshikawa Research Group, Graduate School of Science and Technology）
Research Theme

Development of electrode fabrication method for new active material

Research PeriodApril 1, 2015 ~ March 31, 2016

Research Results（about 2,500 characters in Japanese, about 65 lines times 90 characters in English）
Recently, polyoxometalates and metal–organic frameworks (MOFs) have been found to havepotential as the cathode materials for rechargeable batteries. Polyoxometalates are promisingmultifunctional materials which have numerous applications in catalysis, photoluminescence, as singlemolecule magnets, and as proton-conductive materials. One of the most important features of POMs isthe ability to configure or tailor their redox properties, which allows the development of new functional
systems.
We have reported the rechargeable molecular cluster batteries, which were formed by a lithium
anode and a cathode active material, polyoxometalate, [PMo 12 ]3–. In operando Mo K-edge X-ray
absorption fine structure (XAFS) measurements on these cathodes revealed that [PMo 12 ]3– functioned as
an “electron sponge”, cycling reversibly by 24 electrons between [PMo 12 ]3– and [PMo 12 ]27– during
charging/discharging process. However, the energy density and power density of these cathode materials
were limited by the slow rate of charging/discharging: capacities in excess of 200 Ah/kg could beobtained after ten cycles, but only at a low current of 1 mA. Vanadium based polyoxometalates haveexhibited high capacities and rapid charging/discharging than the polyoxometalates [PMo 12 ]3–, but theirreaction mechanism remained unclear. Herein, in order to investigate the reaction mechanism andfundamental role of Li 7 [V 15 O 36 (CO 3 )] molecule in the batteries, I performed in situ XAFS analysis .
The molecular structure of Li 7 [V 15 O 36 (CO 3 )] is shown in the inset of Figure 1. The spherical anion{V 15 O 36 } of Li 7 [V 15 O 36 (CO 3 )] has a “hollow sphere” structure formed by linkage of 15 tetragonal VO 5pyramids. The 15 vanadium atoms are arranged on the surface of a sphere from the center of the cluster.
{V 15 O 36 } formally contains eight V4+ centers and seven V5+ centers. Li 7 [V 15 O 36 (CO 3 )] shows
reversible redox behavior over a wide voltage window 1.9-4.0 V. XAFS analysis is a powerful techniquefor observing the oxidation state of the absorber atoms and the local structures around them. I carried out
in situ vanadium K-edge XAFS analyses of the charging/discharging processes of the Li 7 [V 15 O 36 (CO 3 )] battery
to reveal the microscopic mechanisms of their battery
performance.

Figurecharging/discharging

1showscyclesofthefirsttwoaninsitu

Li 7 [V 15 O 36 (CO 3 )] battery obtained at a constant currentof 50mA/g in the voltage range of 1.9-4.0 V. A
reversible capacity of around 250 Ah/kg was obtained.
This value is similar to that of coin cell battery, whichmeans this electrochemical cell is suitable for in situ
XAFS study.

Figure 1. Charging/discharging curves of
Li7[V15O36(CO3)]
batteries.
(inset:molecular structure of Li7[V15O36(CO3)]).

By using a linear relationship between thevanadium oxidation state and the X-ray absorption edgeenergy for the reference materials, the averaged valenceof the vanadium ions in Li 7 [V 15 O 36 (CO 3 )] was obtained
from the normalized vanadium K-edge X-ray absorption
near edge structure spectra (XANES). The results areshown in Figure 2, where the values of averaged valence
are plotted as a function of the battery voltage. The initialvalue of averaged valence before the first charging is ca.

Figure 2. Averaged vanadium valence of4.7,

annealing

Li7[V15O36(CO3)] in the Li7[V15O36(CO3)]

Li 7 [V 15 O 36 (CO 3 )] sample. This value increase to 4.96

batteries as a function of the cell voltage.

whichagrees

withtheneutral

after the first charging. In the first discharging, vanadium
averaged valence exhibits a decrease from the initial value to 4.06. The change in averaged valence by
0.9 during the discharging processes agrees with the observed battery capacity of ca. 250 Ah/kg.
Extended X-ray absorption fine structure (EXAFS) analyses were performed in order to obtainthe structural change of the Li 7 [V 15 O 36 (CO 3 )] molecule. Figure 3 shows the Fourier transform spectra ofthe k3-weighted data for the samples denoted by 1C, 1D, 2C,and 2D. After the first charging, sample 1C exhibits threemain peaks at 1.2, 1.6 and 2.7 Å. The peaks at 1.2 Å and1.6 are assigned to short and long V-O distances, whilst theother one is attributed to V-V distance. After the firstdischarge, 1D exhibits a different curve, the peaks at 1.2,
1.6 and 2.7 Å shift to shorter distance. This suggests thatthe distances of V-O and V-V decrease after discharging.
After the second charging, sample 2C exhibits the initialthree peaks compared with those in 1C. After the second
discharging, the curve of 2D is nearly the same as that of1D. This indicates a nearly reversible structural change ofFigure 3. Fourier transforms of the V

the Li 7 [V 15 O 36 (CO 3 )] molecule during the charging andK-edge EXAFS spectra for 1C, 1D, 2C

discharging processes.

and 2D.

In situ XAFS analyses have revealed the evolution of the oxidation state and the local structure of
vanadium ions of in the Li 7 [V 15 O 36 (CO 3 )] battery. The large capacity can be ascribed to the redox
change in the vanadium ions.
Presentations & Papers:1.”Reversible oxidation of Pt nanoparticles: In situ hard X-ray photoelectron spectroscopy studies underH 2 O and MeOH atmospheres”, H. Wang, Y. Takagi, Y. Uemura, O. Sekizawa, T. Uruga, M. Tada, Y.
Iwasawa, T. Yokoyama, H. Yoshikawa, The International Chemical Congress of Pacific Basin Societies
2015, Hawaii, Dec. 2015, poster presentation.
2.”In situ XAFS studies of high performance vanadium oxide cathode materials: a new phase transition
behavior of vanadium oxide”, H. Wang, J. Isobe, D. Matsumura, H. Yoshikawa, paper in preparation.