Photon Factory Activity Report 2014 #32 (2015) B
BL-8A/2010G501,BL-8B/2013G001,BL-7C/2010G502,BL-9C/2013G068

Cobalt Hexacyanoferrate as Cathode Material for N+ Secondary Battery

Masamitsu Takachi1, Tomoyuki Matsuda1 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
Sodium-ion secondary battery (SIB) is a promising
candidate for the next-generation battery beyond thelithium ion secondary battery (LIB) with safe,environmentally friendly, and low-cost characteristics.
The SIB device stores the electric energy utilizing theintercalation/deintercalation process of abundant Nat(Clark number:2.63), instead of rare Lit (0.006), between
the cathode and anode active materials. In this sense, theSIB is suitable for large-scale battery for stable use of thesolar and/or window energies. So far, many researchersinvestigated the cathode materials for SIB. Recently,
Komaba et al. have found that hard carbon exhibits arepeatable Nat intercalation behavior, and demonstrated
thatacoin-type
fullSIBwith
hardcarbon/NaNi0.5Mn0.5O2 configuration exhibits a highcapacity of more than 200 mAh/g (anode basis) and an
average operating voltage of _3V with a good cyclability.
Coordination polymers are promising cathode materials
for LIB and SIB, due to their covalent and nanoporous
host framework. Among them, Prussian blue analogues,
AxM[Fe(CN)6]y (A and M are the alkali and transition
metals, respectively), exhibit a three-dimensional (3D)
jungle-gym-type host framework and cubic nanopores,
0.5 nm at the edge. Moritomo’s group[1] synthesized amanganesehexacyanoferrate
thinfilm,
Li1.32MnII[FeII(CN)6]0.833.5H2O,
withhighLiconcentration. They reported that the thin-film electrodeexhibits a large capacity of 128 mAh/g and an average
operating voltage of 3.6V against Li, with a good
cyclability. Recently, Goodenough’s group [2] havereported Nat intercalation behaviors in a K–M–Fe(CN)6
system (M = Mn, Fe, Co, No, Cu, Zn). However, their
coulomb efficiency (60%), i.e., the ratio of discharge
capacity and charge capacity, is very low. The low
efficiency suggests an irreversible redox process inaddition to the reversible Nat intercalation/deintercalation
process.
2 ExperimentThin films of NaxCo[Fe(CN)6]0.902.9H2O (denoted asNCF90) were synthesized by electrochemical deposition
on an indium tin oxide (ITO) transparent electrode underpotentiostatic conditions at _0:50V vs a standard
Ag/AgCl electrode in an aqueous solution, containing
0.8mM K3[Fe(CN)6], 0.5mM Co(NO3)2, and 5.0M
Na(NO3). The chemical composition of the films wasdetermined by the inductively coupled plasma (ICP)
method and CHN organic elementary analysis. Thethickness of the film was measured with a profilometer(Dektak3030). The typical film thickness was 1.1 µm.
The grain structures of the films were investigated using
scanning electron microscopy (SEM) images (TECHNEX
Mighty-8). The films consist of crystalline grains withdiameters of 400 nm.
The electrochemical properties of the NCF90 films wereinvestigated in a beaker-type cell using of Na as theanode. The electrolyte used was 1M NaClO4 in propylene
carbonate (PC). The active areas of the films were about
2.0 cm2. The cut-off voltage was in the range of 2.0 to 4.0
V. The charge rate was fixed at 0.6 C. The mass of eachfilm was measured using a conventional electronic
weighing machine after the film was carefully removed
from the ITO glass with a microspatula. The experimental
error for the mass, and hence the capacity, is 10%.
The magnitude of x of the LCF90 films was controlled
by the charge/discharge process in a beaker-type cell. We
confirmed that the discharge capacity (= 135mAh/g) ofthe NCF90 films was close to the ideal value (=125mAh/g). Then, the magnitude of x was calculated
from the total current under the assumption that the fullydischarged state is Na1.6Co[Fe(CN)6]0.902.9H2O (x = 1.6)
and the fully charged state is Co[Fe(CN)6]0:902.9H2O (x= 0.0). The films were carefully removed from the ITO
glass with a microspatula in air atmosphere. The fine
powder samples were filled into a 0.3mmφ glass capillary,
and the capillary was sealed in vacuum. In the entire xregion, the films were stable in air at least for 1 h. The
capillary was sealed and placed on the Debye–Scherrercamera. The powder diffraction patterns were detectedwith an imaging plate (IP). The exposure time was 5 min.
The X-ray wavelength (= 0.73255 A) was calibrated by
the lattice constant of standard CeO2 powders.
The valence states of the Co and Mn sites weredetermined by the ex situ XAS measurement around theCo and Fe Kedges. The films were sealed in plastic in aglove box to avoid air exposure. The XAS spectra of theLCF90 films were recorded by a Lytle detector in afluorescent yield mode with a Si(111) double-crystal
monochromator at 300 K. The background subtraction,normalization, and component decomposition were
performed with ATHENA program.[4]3 Results and Discussion

Photon Factory Activity Report 2014 #32 (2015) B

Fig. 1. Discharge curves of thin-film electrode of NCF90
measured at 0.6, 6, 40, and 60 C. The film thickness is 1.1 µm.
The inset shows the jungle-gym-type host framework ofPrussian blue analogues. Small spheres represent Co and Fe,
while bars represent CN-.

Figure 1 shows discharge curves of NCF90 films
measured at various rates. The discharge curve can be
regarded as the open-circuit-voltage (OCV) curve,
because no rate dependence was observed in this rate
region (not shown). The observed capacity (= 135mAh/g)
is close to the ideal value (= 125mAh/g) for the twoelectron reaction. The discharge curve exhibits twoplateaus at 3.8 V (plateau I) and 3.4 V (plateau II). The
thin film electrode exhibits a high capacity of 121 mAh/g(90% of the OCV value) even at 60 C.

Fig. 2. XRD pattern of NCF90 against x. Values in parentheses
represent indexes in the face-centered cubic setting

Fig. 3. (a) Discharge curves of thin-film electrode of NCF90
against x. (b) Lattice constant (a) of NCF90 against x. Open andclosed circles denote data points obtained in the charge anddischarge processes, respectively. Solid and broken lines in (b)indicate the results of the least-squares fittings in plateaus I andII, respectively.

Figure 2 shows the XRD patterns of NCF90 against x.

We confirmed that all the reflections can be indexed withthe face-centered cubic setting, as shown in parentheses.
The lattice constant (a) was refined by Rietveld analysis(Rietan-FP[3]) with the face-centered cubic model(Fm3m: Z = 4). In Fig. 3, we show the (a) discharge curveand (b) a of NCF90 against x. The magnitude of a slightly
decreases in plateau I (0:0 < x < 0.6), while it steeply
increases in plateau II (0.6 < x < 1.6).

Fig. 4. (Color online) XAS spectra around the (a) Co K and (b)Fe K edges of NCF90 film against x.

Figure 4(a) shows the XAS spectra of the LCF90 filmaround the Co K edge against x. Here again, we observed
significant spectral change around x = 1.2. The XAS
spectrum at x = 0.0 coincides with that of the low-spin(LS) Co3+ (S = 0), while the spectrum at 1.6 coincides
with that of the high-spin (HS) Co2+ (S = 3/2). In theintermediate x-region, we estimated the average Co
valence by the spectral decomposition. Figure 4(b) shows
the XAS spectra of the LCF90 film around the Fe K edge
against x. The spectrum at x = 1.6 coincides with that of
the LS Fe2+ (S = 0). Therefore, the electronic
configuration at x = 1.6 is Co2+–Fe2+. The electronic
configuration at x = 0.0 becomes Co3*– Fe2.7+ if we
assume the charge neutrality. With a decrease in x, theXAS spectra exhibit a slight blue-shift [see vertical bar inFig. 4(b)]. The blue-shift indicates partial formation of LS
Fe3+ (S = 1/2). The average Fe valence was determinedby the peak energy (E).
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 a
Grant-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] T. Matsuda and Y. Moritomo, Appl. Phys. Express 4,047101 (2011).

Photon Factory Activity Report 2014 #32 (2015) B[2] Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, Chem.
Commun. 48, 6544 (2012).
[3] F. Izumi and K. Momma, Solid State Phenom. 130, 15(2007).
[4] B. Ravel and M. Newville, J. Synchrotron Radiat. 12, 537(2005).

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