Photon Factory Activity Report 2014 #32 (2015) B
BL-8B/2013G001,BL-8B/2014G507, BL-9C/2013G068, BL-12C/2014G501,BL-12C/2014G506

Structural and electronic properties of Mn–Co hexacyanoferrates against Li

Yutaro Kurihara1 and Yutaka Moritomo1,2
Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8571, Japan
2
Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba,
Tsukuba305-8571, Japan
1

1 Introduction
The PBAs, AxM[Fe(CN)6]zwH2O (A and M are alkali andtransition metals, respectively.), are promising cathodematerials for LIBs. Imanishi et al.[1,2] reported Li+intercalation behaviors in M[Fe(CN)6]z (M = V, Mn, Fe,Ni, Cu). However, their charge/discharge cyclability is far
from satisfactory. Cyclability is fairly improved in
K0.10MnII[FeIII(CN)6]0.704.2H2O
andRb0.61MnII[FeIII(CN)6]0.87-2.2H2O.[3]Matsuda
andMoritomo[4] synthesized a thin film of manganesehexacyanoferrate, Li1.32MnII[FeII(CN)6]0.833.5H2O, bymeans of electrochemical deposition. They reported thatthe thin-film electrode exhibits a high capacity of128mAh/g and an average operating voltage of 3.6V
against Li with good cyclability. Kurihara et al.[5]improved the capacity of Mn-PBA by increasing the Feconcentration (z): the discharge capacity increases from
115mAh/g at z = 0.83 (Li1.32MnII[FeII(CN)6]0.833.5H2O) to
143mAh/g at z = 0.93 (Li1.72MnII[FeII(CN)6]0.932.3H2O).
On the other hand, Takachi et al.[6] reported that theLi1.60CoII[FeII(CN)6]0.902.9H2O film electrode exhibits ahigh capacity of 132mAh/g and an average operating
voltage of 3.6V against Li with good cyclability.
The high capacity of Mn-PBA is ascribed to the two
reduction processes for Mn and Fe,[7] which form twowell defined plateaus at 3.8V (Mn) and 3.4V (Fe) in thedischarge curve of LIB. On the other hand, the highcapacity of Co-PBA is ascribed to two reduction
processes for Fe and Co,[7] which form two plateaus at
4.0V (Fe) and 3.2V (Co) in the discharge curve of LIB.
We note that the order of redox sites is reversed between
the two systems; MnFe in Mn-PBA and FeCo in CoPBA. We also emphasize that the redox voltage (VFe) ofFe is markedly increased in Co-PBA (4.0 V) as comparedwith that (3.4 V) in Mn-PBA. In this study, we fabricatedthin films of mixed PBAs, LixMn1-yCoy[Fe(CN)6]zwH2O,
and investigated their electrochemical, structural, andelectronic properties against x.
2 ExperimentThin films of NaxMnII1-yCoIIy[FeII(CN)6]zwH2O weresynthesized by electrochemical deposition on an indium–tin oxide (ITO) transparent electrode under potentiostatic
conditions at 10.45V vs a standard Ag/AgCl electrode in
an aqueous solution containing K3[FeIII(CN)6], MnIICl2,CoIICl2, and 5 mol/L NaCl. The deposition time was 5min. The thickness of the films was 0.5 µm. Before thefilm growth, the surface of the ITO electrode was purified
by the electrolysis of water for 5 min. The chemicalcomposition of the films was determined by inductively

coupled plasma (ICP) measurement. The magnitudes of xand z were determined so as to minimize the sum of thesquares of the residual error under the charge neutrality
condition (x + 2 - 4z = 0). The Li compounds, LixMn1wereobtainedby
theyCoy[Fe(CN)6]zwH2O,
electrochemical substitution of Li+ for Na+. The
substitutions was performed by charge/discharge cyclesof the as-grown films in an ethylene carbonate(EC)/diethyl carbonate (DEC) solution containing 1
mol/L LiClO4.
The Li concentration (x) of the film was controlled by
the charge/discharge process described above. The
magnitude of x was calculated from the total current
under the assumption that Li4z-2M[Fe(CN)6]z is in thedischarge state and Li0.00M[Fe(CN)6]z is in the charge
state. Except for the high-x region, the film is stable in air.
The high-x films were carefully and rapidly sealed in aglass capillary. The film was carefully removed from theITO glass substrate with a microspatula, and then fine
powder was filled into a 300µmφ glass capillary. The
powder diffraction pattern was detected with an imaging
plate (IP). The exposure time was 5 min. The X-ray
wavelength was calibrated using the lattice constant (a =5.41112Å) of standard CeO2 powder. Thus-obtained
power diffraction patterns were analyzed by the Rietveld
method (Rietan-FP[8]).
Ex situ XAS measurements were performed at the 9C
beamline of the PF, KEK. XAS spectra were measured ina fluorescent yield mode with a Lytle detector at 300 K.
The X-ray was monochromized with a Si(111) doublecrystal monochromator. In the XAS measurement forrespective elements, e.g., Mn, Co, and Fe, themonochromator was calibrated by the K-edge of therespective metal foils. The background subtraction,normalization, and component decomposition wereperformed with the ATHENA program.[9] The selfabsorption effect is negligible because the penetration
depths (λ> 40µm) of the compounds at the fluorescenceand absorption energies of the Mn, Fe, and Co K-edges
are much larger than the thickness of the films (= 0.5 µm)
3 Results and Discussion
Figure1(a)showstheXRD
patternsofLixMn0.89Co0.11[Fe(CN)6]0.893.1H2O (y = 0.11) against x.
Except at x =0.00, the XRD patterns can be indexed withthe fcc setting. The cell parameter a was refined byRietveld analysis with the fcc (Fm3m: Z = 4) model. At x= 0.00, doublet features are observed in the (200), (220),and (400) reflections. The features indicate a phasePhoton Factory Activity Report 2014 #32 (2015) B
separation into two fcc phases. The cell parameter a of therespective phase was refined with a two-phase model.
Similar two-phase features are observed in the low-x
region at y = 0.22 (not shown).

at x = 0.0 nearly coincides with that of low-spin (LS)Co3+, while the spectrum at 1.56 coincides with that ofHS Co2+.The systematic spectral change in Fig. 2(b)indicates that the average Co valence changes from 3+ to2+ with an increase in x. Figure 6(c) shows the XAS
spectra of the films against x around the Fe K-edge. Thebroken vertical lines represent the peak energies of Fe2+and Fe3+. The red shift of the peak indicates that theaverage Fe valence changes from 3+ to 2+ with anincrease in x.

Fig. 1 XRD patterns of (a) LixMn0.89Co0.11[Fe(CN)6]0.893.1H2O,(b)
LixMn0.18Co0.82[Fe(CN)6]0.913.4H2O,
and(c)
LixCo[Fe(CN)6]0.924.6H2O. Values in parentheses represent
indexes in the fcc setting. The subscript R indicates therhombohedral setting

Figure1(b)showstheXRD
patternsof
LixMn0.18Co0.82[Fe(CN)6]0.913.4H2O (y = 0.82). All theXRD patterns can be indexed with the fcc setting. The
cell parameter a was refined with the fcc (Fm3m: Z = 4)
model. Figure 1(c) shows the XRD patterns of
LixCo[Fe(CN)6]0.924.6H2O (y = 1.00). Except at x = 1.68,
the XRD patterns can be indexed with the fcc setting. The
cell parameter a was refined with the fcc (Fm3m: Z = 4)
model. At x = 1.68, a doublet feature is observed at 11°,indicating a rhombohedral distortion. The cell parameters
a and c at x = 1.68 were refined with the rhombohedral(R3m: Z = 4) model.

Fig.
3.
X-ray
absorption
spectraofLixMn0.18Co0.82[Fe(CN)6]0.913.4H2O against x around the (a) Mn
K-, (b) Co K-, and (c) Fe K-edges. The broken vertical lines in(c) represent the peak energies of Fe2+ and Fe3+.

Figure3(a)showstheXAS
spectraofLixMn0.18Co0.82[Fe(CN)6]0.913.4H2O (y = 0.82) against xaround the Mn K-edge. The XAS spectrum at x = 0.0 isdue to Mn3+, while the spectrum at 1.64 is due to Mn2+.
The spectral change in Fig. 3(a) indicates that the average
Mn valence changes from 3+ to 2+ with an increase in x.
Figure 3(b) shows the XAS spectra of the films against xaround the Co K-edge. The spectral change indicates thatthe average Co valence changes from 3+ to 2+ with anincrease in x. Figure 3(c) shows the XAS spectra of thefilms against x around the Fe K-edge. The red shift of thepeak indicates that the average Fe valence changes from3+ to 2+ with an increase in x.

Fig. 2. XAS of LixMn0.89Co0.11[Fe(CN)6]0.893.1H2O against xaround the (a) Mn K-, (b) Co K-, and (c) Fe K-edges. The
broken vertical lines in (c) represent the peak energies of Fe2+and Fe3+.

Figure2(a)shows
theXAS
spectraofLixMn0.89Co0.11[Fe(CN)6]0.893.1H2O (y = 0.11) against xaround the Mn Kedge. The spectra show negligible
changes with x, indicating that the Mn valence essentiallyremains 2+. Figure 2(b) shows the XAS spectra of thefilms against x around the Co K-edge. The XAS spectrum

Fig. 4. X-ray absorption spectra of
LixCo[Fe(CN)6]0.924.6H2O against x around the (a) Co K- and(b) Fe Kedges. The broken vertical lines in (b) represent thepeak energies of Fe2+ and Fe3+.

Photon Factory Activity Report 2014 #32 (2015) B
Figure4(a)showstheXAS
spectraof
LixCo[Fe(CN)6]0.924.6H2O (y = 1.00) against x around theCo K-edge. The spectral change indicates that the average
Co valence changes from 3+ to 2+ with increase in x.
Figure 4(b) shows the XAS spectra of the films against xaround the Fe K-edge. The red shift of the peak indicatesthat the average Fe valence changes from 3+ to 2+ withan increase in x.
Acknowledgement (option)
We thank M. Takachi for fruitful discussion on the latticeeffects on the redox process. This work was supported by
the Mitsubishi Foundation, the Canon Foundation, and aGrant-in-Aid for Scientific Research (Nos. 25620036 and23249) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. The elementary analysisof the films was performed at the Chemical Analysis
Division, Research Facility Center for Science andEngineering, University of Tsukuba.
References[1] N. Imanishi, et al., J. Power Sources 79, 215 (1999).
[2] N. Imanishi, et al., J. Power Sources 81–82, 530 (1999).
[3] M. Okubo, et al., J. Phys. Chem. Lett. 1, 2063 (2010).

[4] T. Matsuda and Y. Moritomo, Appl. Phys. Express 4,047101 (2011).
[5] Y. Kurihara, T. Matsuda, and Y. Moritomo, Jpn. J. Appl.
Phys. 52, 017301 (2013).
[6] M. Takachi, T. Matsuda, and Y. Moritomo, Jpn. J. Appl.
Phys. 52, 044301 (2013)[7] Y. Moritomo, M. Takachi, Y. Kurihara, and A. Matsuda,Adv. Mater. Sci. Eng. 2013, 967285 (2013).
[8] F. Izumi and K. Momma, Solid State Phenom. 130, 15(2007).
[9] B. Ravel and M. Newville, J. Synchrotron Radiat. 12, 537(2005).

* moritomo.yutaka.gf@u.tsukuba.ac.jp