Synthesis of the Metastable Cubic Phase of Li2OHCl by a Mechanochemical Method

Article abstract of DOI:10.1021/acs.inorgchem.0c01631

The oxyhalide-based solid electrolyte Li2OHCl usually forms the thermodynamically stable orthorhombic phase at room temperature and shows poor lithium ionic conductivity. Above 35 °C, a structural phase transition into the cubic phase occurs and ionic conductivity is enhanced. In this work, mechanochemical synthesis of Li2OHCl is reported. The as-prepared Li2OHCl formed a cubic Pm3ˉ m structure and showed an ionic conductivity of 2.6 × 10-6 S cm-1 at 25 °C. Once the cubic phase was treated at 200 °C, the orthorhombic Pmc21 structure appeared at 25 °C and the ionic conductivity decreased down to 1.4 × 10-7 S cm-1. Formation of the metastable cubic phase could be explained in terms of low crystallinity of Li2OHCl derived from mechanochemical synthesis.

Full text of DOI:10.1021/acs.inorgchem.0c01631

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Synthesis of the Metastable Cubic Phase of Li2OHCl by a
Mechanochemical Method
Takayuki Yamamoto,* Hinata Shiba, Naohiro Mitsukuchi, Manoj Krishna Sugumar,
Munekazu Motoyama, and Yasutoshi Iriyama
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ABSTRACT: The oxyhalide-based solid electrolyte Li2OHCl usually forms the thermodynamically stable orthorhombic phase at
room temperature and shows poor lithium ionic conductivity. Above 35 °C, a structural phase transition into the cubic phase occurs
and ionic conductivity is enhanced. In this work, mechanochemical synthesis of Li2OHCl is reported. The as-prepared Li2OHCl
formed a cubic Pm3
̅
m structure and showed an ionic conductivity of 2.6 × 10−6 S cm−1 at 25 °C. Once the cubic phase was treated
at 200 °C, the orthorhombic Pmc21 structure appeared at 25 °C and the ionic conductivity decreased down to 1.4 × 10−7 S cm−1.
Formation of the metastable cubic phase could be explained in terms of low crystallinity of Li2OHCl derived from mechanochemical
synthesis.
ithium-ion conductors have attracted broad interest
because of their potential application to all-solid-state
motion associated with OH bond orientation. In this work, we
report that the cubic phase of Li2OHCl is formed at room
temperature by mechanochemical synthesis.
L
rechargeable lithium batteries.1 The ionic conductivity of the
solid electrolyte strongly affects the performance of the battery.
Although several crystalline oxide-based solid electrolytes such
Li2OHCl used in this work was synthesized by a
mechanochemical method using LiCl and LiOH as starting
materials. LiCl (>99%) was purchased from Fujifilm Wako
Pure Chemical Corp. and used without further purification.
LiOH was prepared by heating LiOH·H2O (>99%, Fluka) at
250 °C for 6 h to remove water. A total of 15.1 mmol of LiCl
and 15.1 mmol of LiOH were mechanically milled in a 12 mL
zirconia pot with 15 g of zirconia balls (5 mm diameter) using
a planetary ball mill (Fritsch, Pulverisette 7) at 700 rpm for 72
h. All processes were carried out under a dry argon
atmosphere.
Variable-temperature synchrotron powder X-ray diffraction
(PXRD) measurements were conducted at beamline BL5S2 of
Aichi Synchrotron Radiation Center with a synchrotron
radiation of λ = 1.033 Å. A powdered sample was loaded
into a soda-lime glass capillary (0.5 mm diameter). The
measurement temperature was controlled between 30 and 200
°C using a nitrogen gas flow. Rietveld analyses were performed
using GSAS-II software.13 The initial structural parameters of
as Li7La3Zr2O12,2 Li1+xAlxTi2−x(PO4)3,3 La2/3‑xLi3xTiO3,4 and
5
LiTa2PO8 have achieved a lithium-ion conductivity of 10−4
S
cm−1 at room temperature, they are hard and fragile ceramics
prepared through high-temperature firing. Thus, it is not easy
to form well-adhered interfaces between those solid electro-
lytes and electrode active materials.
Recently, oxyhalide-based solid electrolytes Li3−xOHxX (X =
Cl and Br) with antiperovskite-type structure have attracted
attention because of their relatively high ionic conductivity and
soft nature.6 In the antiperovskite structure, anions are body-
centered-cubic (bcc)-packed, and theoretical calculations
suggest that such a bcc-packed anion sublattice provides fast
ion conductivity.7 However, there is still uncertainty for
improvement of the ionic conductivity of oxyhalide-based solid
electrolytes. One reason arises from the difficulties in
fabricating pure samples.8,9 To overcome this problem, we
recently found that Li2OHBr was prepared by mechanochem-
ical synthesis at room temperature, which is a simple and easy
method to control the composition.10 However, it is not clear
how the crystalline phase is formed in the sample.
To understand this phenomenon in detail, we focused on
the preparation of Li2OHCl in this work. It has been reported
that Li2OHCl forms the cubic phase above 35 °C and shows
an ionic conductivity of 6 × 10−5 S cm−1 at 37 °C.11 Below 35
°C, however, the ionic conductivity drastically drops to 2 ×
10−9 S cm−1 because of the structural phase transition in the
orthorhombic phase. Theoretical calculations also support such
̅
the cubic Pm3m and orthorhombic Pmc21 structures were
taken from the previous experimental and theoretical
reports.9,12 A pseudo-Voight function was used to fit the
diffraction peaks, and the background was generated using a
Chebyshev polynominal with 18 terms.
Received: June 3, 2020
̅
a phase transition between the cubic Pm3m and orthorhombic
Pmc21 structures.12 According to this calculation, a higher ionic
conductivity of the cubic phase than the orthorhombic phase
can be explained in terms of highly correlated lithium-ion
https://dx.doi.org/10.1021/acs.inorgchem.0c01631
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
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For alternating-current (ac) impedance spectroscopy, a
pelletized sample sandwiched between gold foil (0.025 mm
thickness) was prepared by pressurizing a powdered sample
under 3.6 × 102 MPa for 3 min at room temperature. The
typical thickness and diameter of the pellet were 1 and 7 mm,
respectively. ac impedance spectroscopy was performed using
an impedance analyzer (Bio-Logic, VMP3) in a frequency
range of 500 kHz to 0.1 Hz at an applied voltage of 10 mV.
The measurement temperature was controlled between 25 and
200 °C using a constant-temperature dry-block bath (EYELA,
MG-2200).
larger than 1 (1.08 and 1.06, respectively). The occupancy
factors of lithium and hydrogen were refined under constraint
to maintain a chemical composition of Li3−xOHxCl, and the
refined composition was Li1.88OH1.12Cl. Although a small
discrepancy from the stoichiometry can be expected from the
existence of a tiny amount of unreacted LiCl, it should be
noted that the reliability of the refinements for light-element
parameters using XRD is relatively low and other analyses, for
example, neutron diffraction, are desirable to determine the
actual composition. However, it is worth noting that, although
Li2OHCl usually forms the orthorhombic phase below 35 °C
as mentioned above,11 Li2OHCl synthesized by a mechano-
chemical method in this work forms the cubic phase at 30 °C.
Parts a and b of Figure 2 show the results of variable-
temperature synchrotron PXRD measurements. For the first
cycle between 30 and 200 °C, as shown in Figure 2a, the initial
cubic phase remained during heating to 200 °C and
subsequent cooling to 40 °C. On the other hand, the
diffraction pattern drastically changed at 30 °C upon cooling,
suggesting a structural phase transition. Rietveld analysis on
this newly observed diffraction pattern revealed that the
orthorhombic Pmc21 structure of Li2OHCl with lattice
constants of a = 3.87601(6) Å, b = 3.82796(5) Å, and c =
7.99425(11) Å formed at 30 °C after heating to 200 °C
(Figure 1b and Table 2). The atomic parameters of the minor
cubic phase were fixed to the values in Table 1, and the
occupancy factors of the major orthorhombic phase were fixed
to 1 because of unreliable refined values larger than 1. The
detail of this phase transition was investigated for the second
cycle of variable-temperature synchrotron PXRD measure-
ments between 30 and 40 °C, as shown in Figure 2b. Upon
heating, the orthorhombic phase gradually changed to the
cubic phase between 36 and 40 °C, whereas the cubic phase
changed to the orthorhombic phase between 32 and 30 °C
upon cooling. From Rietveld analysis on these diffraction
patterns, the fractions of the cubic phase were calculated and
plotted against the temperature in Figure 2c. Figure 2c clearly
shows that the transition temperature is between 30 and 40 °C.
The small hysteresis between the heating and cooling
processes is reasonable considering that this phase transition
is thought to be of first order.
Figure 1a shows the result of Rietveld analysis on the
synchrotron PXRD pattern of the as-prepared Li2OHCl
Figure 1. Rietveld refinements on the synchrotron PXRD patterns of
Li2OHCl measured at 30 °C (a) before and (b) after heating up to
200 °C: black circles, experimental; red curves, fitting; blue curves,
residual; gray curves, background; green bars, peak positions for the
cubic phase Li2OHCl; orange bars, peak positions for the
orthorhombic phase Li2OHCl; yellow bars, peak positions for LiCl.
Nyquist plots measured at 25 °C before heating are shown
in Figure S4a. A semiarc was observed in a frequency range of
500 kHz to 300 Hz. The resistance was estimated from the
diameter of this semiarc, and the total ionic conductivity was
calculated from this resistance, based on the pellet thickness
and diameter. The Arrhenius plot of the total ionic
conductivity of Li2OHCl is plotted in Figure 3. The initial
ionic conductivity was 2.6 × 10−6 S cm−1 at 25 °C before
heating. The ionic conductivity increased with increasing
temperature and reached 4.1 × 10−3 S cm−1 at 200 °C. The
activation energy estimated from the Arrhenius equation was
0.54 eV between 25 and 200 °C, showing good agreement with
the previous reports of the cubic phase of Li2OHCl.11,14
During cooling, the ionic conductivity decreased with an
activation energy of 0.53 eV to 40 °C and drastically dropped
to 1.4 × 10−7 S cm−1 at 25 °C. This sudden decrease of the
ionic conductivity originated from the structural phase
transition from the cubic to orthorhombic phase observed in
variable-temperature synchrotron PXRD measurements (Fig-
ure 2).
measured at 30 °C. The observed diffraction pattern could
̅
be well fitted with the cubic Pm3m structure of Li2OHCl with
a lattice constant of a = 3.90317(2) Å, and other parameters
are shown in Table 1. The occupancy factors of oxygen and
chlorine were fixed to 1 because of unreliable refined values
Table 1. Fractional Coordinates, Isotropic Displacement,
and Occupancy Factors for the Cubic Pm3m (No. 221)
̅
Structure of the As-Prepared Li2OHCl Measured at 30 °C
a
by Synchrotron PXRD
atom Wyckoff
x
y
z
Uiso
occupancy
O
Cl
Li
H
1a
1b
3d
8g
0
0
/
0
0.0213(2)
0.0191(1)
0.0691(8)
0.025
1
1
1
1
1
/
/
2
2
2
1
/
0
0
0.628(4)
0.140(2)
2
0.1279
0.1279
0.1279
a
The fractional coordinates of all of the atoms, occupancy factors of
Both PXRD and ac impedance measurements clearly show
that Li2OHCl synthesized by a mechanochemical method
oxygen and chlorine, and isotropic displacement of hydrogen were
fixed in the refinement.
B
https://dx.doi.org/10.1021/acs.inorgchem.0c01631
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pubs.acs.org/IC
Communication
Figure 2. Variable-temperature synchrotron PXRD patterns (a) for the first cycle between 30 and 200 °C and (b) for the second cycle between 30
and 40 °C. (c) Temperature dependence of the cubic phase fraction for the second heating−cooling cycle estimated by Rietveld analysis: red,
heating; blue, cooling.
evaluated from 110 diffraction (around 21.5° in Figure 2a)
using Scherrer’s equation
Table 2. Fractional Coordinates, Isotropic Displacement,
and Occupancy Factors for the Orthorhombic Pmc21 (No.
26) Structure of the Annealed Li2OHCl Measured at 30 °C
Kλ
D =
a
by Synchrotron PXRD
β cos θ
atom Wyckoff
x
y
z
Uiso
occupancy
where D is the crystallite size, K is the shape factor (here 0.9 is
adopted), λ is the wavelength of a radiated X-ray, β is the full
width at half-maximum of the diffraction peak, and θ is the
Bragg angle. D was calculated to be 37 nm for the as-prepared
Li2OHCl, while the value increased to 176 nm after the first
heating−cooling process (Figure S5). This indicates that the
crystallite size increases by the first heating−cooling process. It
has been reported that materials with nanosized crystallites of
less than 100 nm show phase behaviors different from those of
bulk materials, probably originating from their large surface
energy.18 In our Li2OHCl case, stabilization of the cubic phase
is thought to be derived from its small crystallite size. During
the first heating process, crystal growth proceeds and the
resultant Li2OHCl will behave as a bulk material, as has been
reported in previous reports.11
O
2a
2b
2a
2b
2a
0
/
−0.024
0
0.0136(5)
0.0138(3)
0.0304(15)
0.025
1
1
1
1
1
1
1
1
Cl
Li1
Li2
H
/
2
/
/
2
4
4
1
0
0.001
0.086
0.699
1
/
0
0
2
0
0.025
a
The fractional coordinates and occupancies for all of the atoms and
isotropic displacements of Li2 and hydrogen were fixed in the
refinement.
In summary, we report that the cubic phase Li2OHCl
formed at room temperature by mechanochemical synthesis
and obtained 1 order of magnitude higher ionic conductivity at
25 °C than the thermodynamically stable orthorhombic phase
Li2OHCl formed through a heating−cooling cycle between 30
and 200 °C. This stabilization of the metastable phase will
originate from smaller-sized crystallites uniquely formed
through mechanochemical synthesis.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01631.
■
Figure 3. Arrhenius plots of the total ionic conductivity of Li2OHCl
during the first heating (red) and cooling (blue) cycles.
sı
forms the cubic phase at room temperature, although the
orthorhombic phase is usually the most stable phase at room
temperature.11 It has been reported that a metastable phase is
synthesized by a mechanochemical method in various
materials, such as metal alloys,15 metal hydrides,16 and metal
oxides.17 Materials synthesized by a mechanochemical method
generally have low crystallinity compared to those synthesized
by a sintering process, and hence it can be expected that the
crystallinity affects the phase behavior. The temperature
dependence for the crystallite size of the cubic phase of
Li2OHCl during the first heating−cooling process was
Material characterization, Rietveld analysis, Nyquist
plots, and crystallite size (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Takayuki Yamamoto − Department of Materials Design
Innovation Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8603, Japan; orcid.org/0000-0003-
0648-1813; Email: yamamoto.takayuki@material.nagoya-
u.ac.jp
C
https://dx.doi.org/10.1021/acs.inorgchem.0c01631
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Authors
Hinata Shiba − Department of Materials Design Innovation
Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya,
Aichi 464-8603, Japan
Naohiro Mitsukuchi − Department of Materials Design
Innovation Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8603, Japan
Manoj Krishna Sugumar − Department of Materials Design
Innovation Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8603, Japan
Munekazu Motoyama − Department of Materials Design
Innovation Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8603, Japan; orcid.org/0000-0002-
1769-143X
Yasutoshi Iriyama − Department of Materials Design
Innovation Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8603, Japan
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Communication
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant
JP18K14318 and, in part, by Grant JP19H05813 (Grant-in-
Aid for Scientific Research on Innovative Areas “Interface
IONICS”). The synchrotron PXRD measurements were
conducted at beamline BL5S2 of Aichi Synchrotron Radiation
Center, Aichi Science & Technology Foundation, Aichi, Japan
(Proposals 2018L4002 and 2018L5002) with financial support
of the Synchrotron Radiation Research Center, Nagoya
University. Fourier transform infrared measurements were
carried out at the National Institute for Materials Science
Battery Research Platform.
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