Removal of Interstitial H
2
O in Hexacyanometallates for a Superior
Cathode of a Sodium-Ion Battery
Jie Song,
†
Long Wang,
§
Yuhao Lu,
§
Jue Liu,
∥
Bingkun Guo,
†
Penghao Xiao,
‡
Jong-Jan Lee,
§
Xiao-Qing Yang,
⊥
Graeme Henkelman,
‡
and John B. Goodenough*
,†
†
Materials Science and Engineering Program and Texas Materials Institute and
‡
Department of Chemistry and the Institute for
Computational and Engineering Sciences, The University of Texas at Austin, Austin, Texas 78712, United States
§
Sharp Laboratories of America, Camas, Washington 98607, United States
∥
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794, United States
⊥
Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States
*
S
Supporting Information
ABSTRACT: Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for
large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage,
and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease
of synthesis and rigid open framework that enables fast Na
+
insertion and extraction. Here we report an intriguing effect of
interstitial H
2
O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal
composition Na
2
MnFe(CN)
6
·zH
2
O (Na
2−δ
MnHFC). The newly discovered dehydrated Na
2−δ
MnHFC phase exhibits superior
electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of
150 mAh g
−1
in a sodium half cell and 140 mAh g
−1
in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C,
the half-cell capacity is 120 mAh g
−1
, and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.
■
INTRODUCTION
Large-capacity, cost-effective electrical-energy storage is the
transformational technology needed t o enable large-scale
integration of renewable energy into the grid and to increase
dramatically power generation and transmission efficiency.
1
To
date, pumped-hydro and compressed-air energy storage are
widely used, but both have serious infrastructure requirements
that limit large-scale energy storage to specificsites.
Rechargeable, low-cost batteries would provide distributed
electrical-energy storage. Lithium-ion batteries (LiIBs) are the
leading option for this application, but the use of lithium is
hampered by cost and supply restriction. Therefore, a room-
temperature sodium-ion battery (NaIB) for large-scale storage
of electrical energy is an attractive technical target.
2−4
Recently, hexacyanometallates A
x
M
1
[M
2
(CN)
6
]
y
·z H
2
O (A =
alkaline metal; M
1
and M
2
= transition metal ions; 0 ≤ x ≤ 2; y
≤ 1) have been investigated as NaIB cathodes in both
aqueous
5−8
and nonaqueous electrolytes,
9−14
owing to their
ease of synthesis and rigid open framework with large
interstitial space. The structure of A
x
M
1
[M
2
(CN)
6
]
y
·zH
2
O,
with y = 1, consists of a double perovskite framework with
(CN)
−
anions bridging M
1
N
6
and M
2
C
6
octahedra; A
+
and
H
2
O occupy the large space of the framework’s interstitial sites.
The interstitial water forms (AOH
2
)
+
molecules that H-bond to
the framework.
15,16
The operational voltage with an aqueous
electrolyte is restricted to the 1.5 V range, so the Na
x
M
1
HCF
framework Na-ion cathodes in a nonaqueous electrolytes is an
alternative way to increase the energy density of NaIBs. The
most promising candidate among these Na
x
M
1
HCF Na-ion
Received: December 4, 2014
Revised: January 28, 2015
Accepted: January 29, 2015
Published: February 13, 2015
Article
pubs.acs.org/JACS
© 2015 American Chemical Society 2658 DOI: 10.1021/ja512383b
J. Am. Chem. Soc. 2015, 137, 2658−2664
cathodes is the discharged nominal Na
2
Mn[Fe(CN)
6
]·zH
2
O,
which contains only earth-abundant elements and enables two
Na
+
per formula unit to cycle reversibly. Although Imanishi et
al.
17
explored the affect of the number of coordinating water
molecules on Li
+
intercalation in Fe
4
[Fe(CN)
6
]
3
·zH
2
O, the
details of the c orrelation among interstitial water within
hexacyanometallates and their crystalline structures and
electrochemical properties have remained unknown. In this
paper, we carefully investigated the role of interstitial H
2
O in
determining the structural and electrochemical properties of
Na
2−δ
MnHFC. Removal of interstitial H
2
O leads to the
discovery of a new dehydrated Na
2−δ
MnHFC phase that
exhibits different electrochemical behavior than the hydrated
phase and shows a high reversible capacity at 3.5 V with
excellent rate and cycling performance.
■
RESULTS AND DISCUSSION
The as-prepared Na
2−δ
MnHCF precipitate was separated into
two parts; both parts were dried at 100 °C, one under air and
the other under vacuum. Figure 1 compares the first-cycle
charge/discharge curves of air-dried and vacuum-dried samples
in sodium half cells. The air-dried Na
2−δ
MnHFC electrode
exhibits two steps, at 3.45 and 3.79 V on charge and at 3.17 and
3.49 V on discharge. In contrast, vacuum-dried Na
2−δ
MnHCF
exhibits an apparently single flat plateau at 3.53 V on charge, at
3.44 V on discharge with a higher cycle efficiency, at 100 versus
300 mV between charge and discharge, and a higher reversible
capacity (150 mAh g
−1
) at a rate of 0.1 C over the range of 2.0
≤ V ≤ 4.0 V.
Both white powder samples have the same noncubic shape
with a particle size of 200−400 nm (Figure S1) and the same
molar Na/Fe/Mn ratio of 1.89:0.97:1.00 as determined by
inductively coupled plasma (ICP) analysis. Thermogravimetric
analysis (TGA) of the air-dried sample shows three distinct
weight-loss events (Figure 2a): a first step occurring below 120
°C corresponds to the loss of adsorbed water, the second step
occuring in the range of 120 < T < 190 °C can be assigned to
the elimination of interstitial water,
16,18
and the sharp weight
loss occurring near 190 °C indicates the decomposition of the
MnHCF framework. The weight loss over the second step
corresponds to z = 1.87. TGA of the vacuum-dried sample
shows little weight loss in the interval 120 < T < 225 °C and a
sharp loss in the range 225 < T < 275 °C; its total water
content is considered to be z = 0.3. The infrared (IR) spectra in
Figure 2b also suggest the removal of interstitial water by
vacuum-drying. Sharp absorptions at 1619 and 3532 cm
−1
in
the IR spectra of air-dried Na
2−δ
MnHCF are associated with
the O− H stretching and H−O−H bending modes arising from
interstitial water, and the peak at 3604 cm
−1
is characteristic of
free surface water (nonhydrogen bonded).
19,20
The Synchroton X-ray diffraction (SXRD) data (Figure 3a,c)
of the two powders were first indexed and then refined by the
Rietveld method in Topas software (version 4.2). Although
attempts to determine the locations of H atoms in the hydrated
sample were unsuccessful, a reasonable structure with an
Figure 1. Galvanostatic initial charge and discharge profiles of (a) air-dried and (b) vacuum-dried Na
2−δ
MnHCF at a current of 0.1 C (15 mA g
−1
) in
the voltage range of 2.0 −4.0 V. The derivative curves (dQ/dV) plotted as a function of V are shown as inserts.
Figure 2. (a) TGA curves and (b) IR spectra of air-dried and vacuum-dried Na
2−δ
MnHCF. The TGA test was conducted at a heating rate of 5 °C
min
−1
under N
2
flow.
Journal of the American Chemical Society Article
DOI: 10.1021/ja512383b
J. Am. Chem. Soc. 2015, 137, 2658−2664
2659
oxygen position (in H
2
O) has been obtained by Rietveld
refinement, with atomic positions shown in Table S1a. The
atomic positions in the dehydrated sample were identified by
SXRD and time-of-flight (TOF) neutron diffraction (Figure S2
and Table S1b). The hydrated Na
2−δ
MnHCF structure is
monoclinic (space group = P2
1
/n, a = 10.5864, b = 7.5325, c =
7.3406 Å, and β = 92.119°); the dehydrated Na
2−δ
MnHCF
structure is rhombohedral (space group = R3
, a = b = 6.5800, c
= 18.9293 Å). Their local structures are also illustrated in
Figure 3b,d.
The monoclinic M-Na
2−δ
MnHFC and rhombohedral R-
Na
2−δ
MnHCF samples have the same composition except for
the water content, which shows that the interstitial H
2
O plays a
key role in the structures as well as on the electrochemical
voltage profile of nominal Na
2−δ
MnFe(CN)
6
·zH
2
O. In the
dehydrated rhombohedral phase, the Na
+
are cooperatively
displaced along a cubic [111] axis toward the Fe vertex, and a
cooperative rotation of the linear (CN)
−
anions brings the N
atoms toward the displacement axis to make contact with the
displaced Na
+
ions. Incorporation of interstitial water trans-
forms the rhombohedral structure to a monoclinic one. The
monoclinic dis tortion reflects a cooperative distortion of
(NaOH
2
)
+
groups, with the Na
+
displaced along alternating
cubic [111] and [1−11] axes and the Na moving toward the Fe
vertices in neighboring (010) planes with the oxygen atoms
displaced to near a face bridging neighboring Na atoms.
Although the strong CN bond keeps these anions linear, they
are cooperatively rotated from the cubic Mn−NC−Fe axis.
The rhombohedral structure has a smaller volume and a larger
distortion from the cubic symmetry than does the monoclinic
phase (Figure 3b,d). The shorter Mn···Fe distance in R-
Na
2−δ
MnHCF is due to a larger rotation of the (CN)
−
anion
from the Mn−NC−Fe bond axis, which reduces the Mn···Fe
separation and lowers the volume. Moreover, the energies of
the HS Mn
III
/Mn
II
and LS Fe
III
/Fe
II
couples are controlled by
σ-bonding Mn−N and π-bonding Fe−C interactions, respec-
tively, so that they respond differently to the degree of the
rotation of the linear (CN)
−
anions from the Mn···Fe axes.
This difference can account for the merger of the two voltage
plateaus in the hydrated sample to essentially one plateau in the
dehydrated sample.
Ex situ XRD measurements show that different structural
changes occur during the Na
+
deinsertion/insertion process for
M-Na
2−δ
MnHCF and R-Na
2−δ
MnHCF samples. On Na
+
extraction from M-Na
2−δ
MnHCF, the XRD spectrum shows
amergerofdoubletsintosharpsinglepeaksandthe
disappearance of a peak at 1 4.9°,indicatingaphase
transformation from a monoclinic phase to a cubic one over
the first voltage plateau (Figure 4a); apparently, the (NaOH
2
)
+
and Na
+
displacements become disordered at room temper-
ature with the removal of one Na
+
per formula unit. The XRD
pattern with all Na
+
removed was indexed with tetragonal
symmetry (a = b =10.1186 Å and c = 10.5414 Å), consistent
with a cooperative Jahn−Teller distortion of the Mn
III
N
6
octahedra (Figure 4b). The cubic unit cell expands with one
Na
+
per formula unit; the volume of the M phase with all Na
+
Figure 3. Synchroton X-ray diffraction (SXRD) patterns of (a) M-Na
2−δ
MnHCF and (c) R-Na
2−δ
MnHCF. Local structures of (b) M-Na
2−δ
MnHCF
and (d) R-Na
2−δ
MnHCF, showing the Na
+
displacements and distorted framework. In a and c, the black dots show observed data, the red lines are
the calculated patterns, the purple lines correspond to the difference between the observed and calculated patterns, and the vertical green bars are the
expected positions of Bragg reflections. Asterisks in c show peaks from absorbed water during the sample preparation for SXRD. In-house XRD
pattern (Figure S2a) shows a single rhombohedral Na
2−δ
MnHCF phase. In b and d, high-spin Mn
II
is blue, low-spin Fe
II
is green, N is silver, C is
dark brown, Na is yellow, and O is red.
Journal of the American Chemical Society Article
DOI: 10.1021/ja512383b
J. Am. Chem. Soc. 2015, 137, 2658−2664
2660
removed is smallest (Figure S3). On return of two Na
+
per
formula unit, the single monoclinic phase was not returned;
instead, a mixture of the M and R phases was found, indicating
asegregationintodehydratedandhydratedphases.A
segregation of the water content apparently takes place on
the initial removal of Na
+
ions because a small shoulder from
the rhombohedral phase already appears in the XRD pattern
when the electrode is charged to 3.46 V at the first cycle. On
removal of Na from the dehydrated R phase, the intensity of
the R-phase peaks systematically decrease at the expense of a
second Na-poor phase with a larger volume that evolves to a
tetragonal phase as all Na
+
is extracted (Figure 4c,d). On the
return of two Na
+
ions per formula unit, the XRD evolution is
reversed to a well-developed R-phase XRD pattern.
The phase transition between M and R is reversible: for δ ≈
0, thermal removal of interstitial H
2
O from the hydrated phase
induces the M to R structural change and exposure of the R
phase to air with 80% humidity for 20 h at room temperature
transforms the R phase to the M phase (Figure S4). In addition,
a hydrated M phase showed the onset of a new voltage plateau
after the first charge/discharge cycle at around 3.4 V that grew
in capacity with increased cycling with a complementary
decrease in the capacities of the 3.45/3.17 and 3.79/3.49 V
plateaus. After the 10th cycle, the 3.4 V plateau, characteristic of
the R phase, contributes most of the capacity, as shown in
Figure S5. Figure S6 shows the XRD spectrum of the M-
Na
2−δ
MnHCF electrode after 20 cycles. The R phase is
dominant with a smaller concentration of the M phase, which is
consistent with the segregation into a dehydrated (z = 0) phase
and a z ≈ 2 phase that are caused by the loss of H
2
O on cycling.
Calculation
21
shows that the binding energy of H
2
O in the
lattice becomes much weaker if it is unattached to a Na
+
ion,
dropping from 0.8 to less than 0.1 eV, supporting the
Figure 4. Ex situ XRD patterns of (a) M-Na
2−δ
MnHCF and (c) R-Na
2−δ
MnHCF at different states at the first cycle. Schematic illustration of
structural evolution of (b) M-Na
2−δ
MnHCF and (d) R-Na
2−δ
MnHCF along with Na
+
extraction and insertion.
Journal of the American Chemical Society Article
DOI: 10.1021/ja512383b
J. Am. Chem. Soc. 2015, 137, 2658−2664
2661
conclusion that H
2
O is extracted with Na
+
ions of a hydrated
unit cell containing (NaOH
2
)
+
units.
Figure 5a shows that R-Na
2−δ
MnHCF has an excellent rate
capability in a sodium half cell for both charge and discharge,
retaining 81% of its capacity when the discharge rate increases
from 0.1 to 20 C. At 0.7 C, R-Na
2−δ
MnHCF retained 75%
capacity after 500 cycles with a high Coulombic efficiency of
nearly 100% (Figure 5b). The 150 mAh g
−1
reversible capacity
of R-Na
2−δ
MnHCF with a flat 3.5 V versus Na
+
/Na is
comparable to that of Li FePO
4
in a Li-ion battery; R-
Na
2−δ
MnHCF gives a performance superior, in terms of
reversible capacity, rate capability, and cyclability, to that of the
cathode materials for a reversible Na-ion battery that have been
reported in the literature (Table S2). However, to date, metallic
sodium is not a practical anode for a rechargeable Na-ion
battery. Therefore, rechargeable Na-ion batteries are fabricated
in a discharged state with an insertion-compound host or metal
that alloys reversibly with Na as the anode. We have chosen
hard carbon as the anode for a full-cell test. On the basis of the
mass of R-Na
2−δ
MnHCF, we obtained a reversible capacity of
140 mAh g
−1
over the voltage range 1.5 ≤ V ≤ 3.8 V in the full
cell (Figure 6a). This full cell gave a moderate rate
performance, with 96 mAh g
−1
at 2 C, as shown in Figure
S7. No obvious capacity fade was observed over 30 cycles at
100 mA g
−1
(0.7 C) with a Coulombic efficiency nearly 100%
(Figure 6b).
In summary, an intriguing effect of interstitial H
2
O on the
structure and electrochemical properties of Na
2−δ
MnHCF has
been revealed. The removal of interstitial H
2
O either thermally
or electrochemically from the MnHCF framework induces a
structural transition and a profound electrochemical property
change. Dehydrated rhombohedral Na
2−δ
MnHCF delivers 150
mAh g
−1
with flat charge and discharge plateaus at 3.5 V; it
exhibits excellent rate capability and cycling performance. Full
cells with hard carbon as the anode have also been
demonstrated to have a 140 mAh g
−1
reversible capacity at
moderate charge/discharge rates. The dehydrated
Na
2−δ
MnHCF cathode promises to be a competitive, low-
cost cathode of rechargeable NaIBs for large-scale applications.
Furthermore, understanding the critical effect of interstitial
H
2
O also provides an instructive insight for the development of
other hexacyanometallates for alkali ion batteries.
■
MATERIALS AND METHODS
Materials Synthe sis. Sy nthesis of Na
2−δ
MnHCF was
carried out as described elsewhere in the literature,
11
with
slight modification. Briefly, 3 mmol of Na
4
Fe(CN)
6
and 14.0 g
of NaCl were dissolved in a solution of 75 mL of distilled water
and 25 mL of ethanol. A 100 mL solution containing 6 mmol of
MnCl
2
·4H
2
OwasslowlydroppedintotheNa
4
Fe(CN)
6
solution under strong stirring. The obtained white precipitate
of Na
2−δ
MnHCF was washed with distilled water and was
dried using two conditions: (1) 100 °C in air and (2) 100 °C in
vacuum. Dehydration was achieved by evacuating MnHCF to
15 mTorr at 100 °C for 30 h.
Figure 5. (a) Rate capability of a Na/R-Na
2−δ
MnHCF cell. (b) Capacity retention of R-Na
2−δ
MnHCF cells over 500 cycles.
Figure 6. (a) The initial charge/discharge galvanostatic curves of the R-Na
2−δ
MnHCF/Na half cell, hard-carbon/R-Na
2−δ
MnHCF full cell and hard-
carbon/Na half cell. (b) Charge and discharge curves of a hard-carbon/Na
x
MnHCF full cell for the first 25 cycles. The full cell was cycled in the
range of 3.8−1.5 V at a charge and discharge current of 100 mA g
−1
. The inset of b shows the capacity retention and Coulombic efficiency during
cycling. All the specific capacity and current density in full cells were normalized by the active mass of Na
2−δ
MnHCF.
Journal of the American Chemical Society Article
DOI: 10.1021/ja512383b
J. Am. Chem. Soc. 2015, 137, 2658−2664
2662
