Communication
Selective and Facile Synthesis of Sodium Sulfide and Sodium
Disulfide Polymorphs
Hany El-Shinawi,‡ Edmund J. Cussen,† and Serena A. Corr*,‡
‡School of Chemistry, WestCHEM, University of Glasgow, Glasgow G12 8QQ, U.K.
†Department of Pure and Applied Chemistry, WestCHEM, The University of Strathclyde, Glasgow G1 1XL, U.K.
S
* Supporting Information
tetraglyme at room temperature (the S concentration was
ABSTRACT: Na2S and Na2S2 were selectively synthe-
sized using a microwave-assisted thermal treatment of a
Na+/S solution in tetraglyme between 100 and 200 °C,
considerably lower than that of current routes. This novel
synthetic pathway yields the Na2S phase in high purity and
allows for good selectivity between the polymorphs of
Na2S2 (α and β phases). These materials show promising
electrochemical properties and are particularly interesting
for the continued development of Na−S batteries.
approximately 0.1 M). A clear green solution was obtained after
stirring in a glovebox for 30 min. A 5 mL aliquot of this solution
was heated to 200 °C in a sealed 10 mL microwave tube for 30
min using a CEM Discover SP microwave synthesizer (2.45
GHz). The resulting product was washed thoroughly with
tetrahydrofuran (3 × 3 mL) and finally dried at 100 °C for 2 h.
The washing and drying steps were performed under an argon
atmosphere. Powder X-ray diffraction (XRD) showed that the as-
prepared material is phase-pure, crystalline Na2S. No significant
change in the XRD pattern was observed when the as-prepared
material was calcined at 700 °C under argon for 1 h. The XRD
patterns of the synthesized material, compared to commercial
Na2S (anhydrous; Alfa Aesar), are shown in Figure 1. It should be
noted that sulfur readily dissolves in tetraglyme in the presence of
sodium tert-butoxide. This suggests that S likely combines with a
sodium tetraglyme complex6 to form a soluble, green precursor
complex, which subsequently decomposes by microwave-
assisted heating to Na2S. A microwave heat treatment is essential
here because the reaction could not be achieved by conventional
heating in a Teflon-lined autoclave.
Our synthetic approach also proved to be successful in
synthesizing phase-pure Na2S2 (in the form of a mixture of α and
β polymorphs) simply by varying the reaction temperature. At
temperatures below 150 °C, the same sulfur/sodium tert-
butoxide solution yields phase-pure Na2S2 in the form of a
mixture of α (low-temperature) and β (high-temperature)
phases, with no evidence for the formation of Na2S (see the
conditions, we could prepare α and β phases with good
separation. Figure 2 shows the XRD patterns of Na2S2 obtained
after the heat treatment of 10 mL of a Na+/S solution at 110 and
120 °C. These α and β polymorphs are the only phases present,
i.e., there are no traces of Na2S, demonstrating the high selectivity
of our synthesis method. Additionally, we clearly observe that α-
Na2S2 predominates in the material prepared at 110 °C, while β-
Na2S2 predominates in the material prepared at 120 °C,
indicating good separation of the two polymorphs. At higher
treatment temperatures (>150 °C), we begin to see the
emergence of Na2S together with α-Na2S2 (see Figure S1). A
mixture of Na2S and Na2S2 is also obtained when the S/Na ratio
in the initial S/Na+ solution is changed from 1:2 (e.g., to 1:1),
indicating that a 1:2 ratio is crucial for obtaining the precursor
igh-purity sodium sulfide and polysulfides are key
H
reactants in a range of organic and inorganic processes,
with applications across industries including dye manufacturing,
textiles, and the synthesis of organic intermediates. Industrially,
Na2S is prepared by the carbon reduction of Na2SO4. On a
laboratory scale, a reaction between elemental Na and S is
typically carried out. The former synthesis requires temperatures
between 600 and 1100 °C and produces CO2 as a byproduct,
according to the reaction Na2SO4 + 2C → Na2S + 2CO2.1 The
reaction between Na and S (2Na + S → Na2S), on the other
hand, is performed in liquid ammonia and requires extreme
precautions because of the hazardous nature of some of these
reactants.2 Na2S and sodium polysulfides may also be accessed by
the reaction between Na and S, either by a direct molten reaction
or in dimethoxyethane in the presence of catalytic aromatic
hydrocarbons.3−5 These techniques offer a good degree of
selectivity; however, they do not avoid the use of elemental Na,
which again presents a considerable hazard. Beyond the
carboreduction of Na2SO4, Na-metal-free syntheses are limited
to the drying of hydrous sodium sulfide, which is both time- and
energy-consuming because of the requirements of several heat
treatments under controlled pressure. With the considerable use
of these materials in industrial processes and their potential for
applications in energy storage, there is a pressing need to develop
new chemistries to access these compounds through more
benign routes. Here, we report a novel and facile synthesis that
avoids the use of elemental Na and produces high-purity
materials at low cost and at temperatures not exceeding 200 °C.
Additionally, this approach not only is very selective toward
producing either Na2S or Na2S2 but also provides good
separation of the α and β phases of Na2S2.
High-purity anhydrous Na2S was successfully prepared using a
simple microwave-assisted heat treatment of a Na+/S solution in
sodium tert-butoxide were dissolved in a 1:2 molar ratio in
Received: March 23, 2018
© XXXX American Chemical Society
A
Inorg. Chem. XXXX, XXX, XXX−XXX
El-Shinawi, Hany
