http://support.spring8.or.jp/Report_JSR/PDF_JSR_2020A/2020A1621.pdf

2020A1621

BL14B2EXAFS and XANES Study for a Novel Cost-Effective Cathode Material for
Sodium-Ion Battery with Long Cyclability.
Jiwei Ma a, Junji Inukai baTongji University, b University of Yamanashi

The XAS measurements were carried out on Na Super Ionic Conductor (NASICON)-type phosphate materials
synthesized at Tongji University prepared at four different chemical conditions. Ex situ XANES data showedalmost unchange of chemical states at diffent preparation conditions. The chemical states of Fe are close to +3,and the chemical states of V was mostly +3 regardless of the preparation conditions. Structural analysis is inprogress based on EXAFS and XRD.
Keywords: Sodium-ion battery, NASICON, Na intercalation, XAS
Introduction
The development of next-generation batteries, including sodium-ion batteries (SIB) are largely demanded.
Na3V2(PO4)3, which this NASICON-type (Na Super Ionic Conductor) phosphate, shows good cyclability and highenergy density, and a candidate material for the cathode of SIB [1]. Its large 3D tunnels in the structure and multiple
Na-ion intercalation sites enables the large Na-ion diffusion kinetics and the highly reversible capacity. However,for the large-scale applications, V must be substituted by highly abundant and low-cost elements. Since theNASICON type structures are highly tolerant toward substitutions of the transition metal elements, we focused on
the most ideal but unexplored substitution element of Fe3+.
We synthesized a novel NASICON phosphate, Na3.4(+)0.6FeV(PO4)3 (NFVP), where (+) indicates cation
vacancies. Their structure was characterized using the conventional XRD/Rietveld refinement and XPS. Theelectrochemical characterization showed multiple redox couples of Fe2+/Fe3+ and V3+/V4+, and also the part ofV4+/V5+ were confirmed within 1.5–4.4 V. Those preliminary studies have already revealed that this new phosphate
phase (NFVP) is the promising cathode material for SIBs possessing superior cyclability, rate capability with lower
cost and more abundant element. The crystal framework of this phosphate is a typical NASICON structure , andthe Rietveld refinement showed Trigonal system (S.G.: R3 ̅c). The atoms V1 and Fe1 share the special position
12c with 50% occupancy each. Both sodium atoms Na1 and Na2 are located in special positions 6b and 18e
respectively.
Based on the previous our studies, we aim to clarify the oxidation-states variation and the local structuralenvironment of both V and Fe cations by ex situ EXAFS/XANES upon charge/discharge. While XPS measurement
is strictly limited to the surface (<10 nm) and the problems of peak superposition, EXAFS measurement gives
much more precise information on the local environments of our bulk structure such as the coordination mode ofeach atoms and valence change of V and Fe upon charge/discharge, thus clarifying the involved electrochemical
Na+ ions extraction/insertion mechanisms. This will largely contribute to the optimization of SIBs cathodematerials and will be of help to the ommercialization of next-generation SIBs.
Experimental
The samples were prepared under four
different states of charge/discharge (Fig. 1),
namely A (pristine state), B (charge to 3.5 V),
C (charge to 4.4 V), D (discharge to 1.5 V).
The NFVP electrode samples at the different
state of charge/discharge (pristine, charged to
3.5 V, 4.4 V, and discharged to 1.5 V) preparedat Tongji University in China.
The EXAFS/XANES measurements wereconducted at the beamline BL14B2 of SPring8. The reference samples were Fe, FeO andFe2O3 for K-edge Fe measurements and V, andV2O5 for K-edge vanadium measurements.
Fig. 1 Preparation states of Samples A to D.

Results
Differences in XANES are not clear under the different charged-discharged conditions. The exploration ofXANES measurements at different states can confirm the expected oxidation states of iron and vanadium (Fig. 2).
The similar spectra suggest similar chemical states for the different charged-discharged electrodes. The chemicalstates of Fe were close to +3, whereas the chemical states of V were close to +3 as well. The almost unchange ofchemical states of Fe and V was possibly due to the air exposure of electrodes or the data extraction errors. The
raw data should be re-examined in order to avoid the extraction errors.
1.6

Fe - K edge

1.2

0.8
FeO
Fe2O3

0.4

0.0
7100

Pristine
3.5 V Ch.
4.4 V Ch.
1.5 V Dis.

7120

7140Energy (eV)

7160

7180

Normalized intensity

Normalized intensity

1.6

V - K edge

1.20.8

V2O3VO2V2O5

0.4

0.0
5460

Pristine
3.5 V Ch.
4.4 V Ch.
1.5 V Dis.

5470

5480

Energy (eV)

5490

5500

Fig. 2 XANES data of Fe (left) and V (right).
For the future data analysis:
1. For the XANES spectra, not only the while-line position, but the pre-peaks and intensities of spectra changed,
which should be discussed.
2. For the Fourrier transform data of the EXAFS spectra, the first-nearest shells did not much change, but thesecond-nearest shells changed, corresponding possibly to Fe-Na and V-Na. Structural models are needed for theanalysis.
3. The analyses should be performed based on the ex-situ XPS and XRD analyses.
The results of this study will serve for full-cell Na-ion batteries applications through understanding the structuralchange during the charge/discharge of this NASICON cathode material.
Reference[1] M. Hadouchi, N. Yaqoob, P. Kaghazchi, M. Tang, J. Liuc, P. Sang, Yo. Fu, Y. Huang, J. Ma, Energy Storage
Materials, 35, 192-202 (2021).