Mechanochemical synthesis of rock salt-type Na₂CaSnS₄ as a sodium-ion conductor

Abstract

Na₂CaSnS₄ was prepared by mechanochemical synthesis from a mixture of Na₂S, CaS, and SnS₂. The crystal structure was determined from X-ray powder diffraction data. The chemical composition was confirmed by energy dispersive X-ray spectroscopy, and the ionic conductivity was measured using electrochemical impedance spectroscopy. Na₂CaSnS₄ crystallizes with a rock salt-type structure, space group Fm3̄m, a = 5.6842 (3) Å, V = 183.66 (2) ų, and Z = 1. All the cations are statistically disordered over a unique crystallographic site and are octahedrally coordinated to the sulfur atoms. The ionic conductivity of Na₂CaSnS₄ is 4.2 × 10⁻⁸ S cm⁻¹ (E_a = 0.6 eV) at 33 °C.

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1. Introduction

Solid electrolytes are materials that exhibit proton, lithium, sodium, potassium, silver, fluoride, or oxide ion conduction. They have been widely used in several all-solid-state electrochemical devices such as gas sensors, capacitors, smart windows, fuel cells, and batteries. In the latter, it is desirable to have a solid electrolyte exhibiting high ionic conductivity in the order of 10⁻²–10⁻³ S cm⁻¹ at room temperature to be comparable to liquid electrolytes. Nevertheless, materials with lower ionic conductivities can be implemented in batteries. These solid electrolytes should also be dense with a negligible electronic conductivity and a wide electrochemical stability window.¹

For all solid-state sodium ion batteries (ASSBs), three types of electrolytes are known (i.e. polymers, inorganic compounds, and their combinations). Among the crystalline inorganic compounds, one can find oxides, sulfides, and boron hydrides.² Examples thereof are the oxides sodium-beta-alumina Na-β′′-Al₂O₃,³ the NASICON-type compounds Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3),⁴ the layered-type compounds Na₂M₂TeO₆ (M = Ni, Co, Zn, Mg);⁵ the sulfides Na₆MS₄ (M²⁺ = Mg, Zn),⁶ Na₅MS₄ (M³⁺ = Al, Ga, In),^7–9 Na₄MS₄ (M⁴⁺ = Si, Ge, Sn),^10–12 Na₃BS₃,¹³ Na₃MS₄ (M⁵⁺ = P, As, Sb),^14–16 and Na₂MS₄ (M⁶⁺ = Mo, W);^17,18 and the boron hydrides Na₂(CB₁₁H₁₂)(CB₉H₁₀),¹⁹ Na₃BH₄B₁₂H₁₂,²⁰ or Na₄(CB₁₁H₁₂)₂(B₁₂H₁₂).²¹ The ionic conductivity of these end member materials does not exceed 4 × 10⁻³ S cm⁻¹ at room temperature. However, in binary systems such as that of Na₃SbS₄–Na₂WS₄, it can reach up to 10⁻² S cm⁻¹ at room temperature.²²

The mechanical milling technique has attracted much attention as a procedure for preparing amorphous, crystalline, and composite materials. Indeed, the intensive grinding of particulates, especially through high-energy milling, provides conditions favorable for initiating changes in the particulates. These changes include solid state chemical reactions,^23,24 polymorphic transformations,^25,26 and very often amorphization.²⁷ Several binary, ternary, quaternary, and quinary chalcogenide compounds easily form on high energy grinding.²⁸ Numerous examples thereof exist (see the Zn–S, Se–S; Li–Si–S, Li–P–S; Li–Si–N–S; and Li–P–Si–N–S systems).^29–34

In the A₂⁺M²⁺SnS₄ family of compounds, more than forty compounds were reported so far. None of them contains calcium and only Na₂MnSnS₄ crystallizes in the rocksalt-type structure.³⁵ Therefore, in the present work we report the mechanochemical synthesis of Na₂CaSnS₄, which is the second member of this family that crystallizes with a rocksalt-type structure. The crystal structure was determined from X-ray powder diffraction data. The average composition was confirmed by EDX and the ionic conductivity was measured by electrochemical impedance spectroscopy. The crystallographic data of all the A₂⁺M²⁺SnS₄ compounds were also reviewed.

2. Experimental

2.1. Synthesis

The Na₂CaSnS₄ sample was prepared by the mechanochemical synthesis route from a stoichiometric mixture of Na₂S (>99.1%, Nagao, Japan), CaS (99.95%, Kishida), and SnS₂ (>99.5%, Mitsuwa Chem.). As for Na₂MnSnS₄,³⁵ the starting materials (0.5 g in total) were mixed in an agate mortar in a dry Ar glove box. The mixtures were placed in 45 mL zirconia milling pots with 90 g of zirconia balls of 4 mm diameter and mechanically milled using a planetary ball mill apparatus (Pulverisette 7, Fritsch). The rotational speed and milling time were set at 500 rpm and 48 h, respectively without breaks. The obtained dark green powder is hygroscopic.

2.2. Electron microprobe analyses

Semiquantitative EDX analyses of the green powder were carried out on several particles with a JSM-6610A (JEOL) scanning electron microscope (SEM). The experimentally observed Na/Ca/Sn/S atomic ratios were close to 2 : 1 : 1 : 4 as expected for Na₂CaSnS₄.

2.3. X-ray powder diffraction measurements

XRPD patterns were measured with a Rigaku Smartlab diffractometer using Bragg-Brentano geometry with Cu-K_α radiations. The X-ray data were collected in the 2θ range from 5–80° with a step of 0.01°. The X-ray diffraction data were refined by a Le Bail profile analysis using the Jana2006 program package.³⁶ Since the powder contains an amorphous component, the background was estimated manually, and the peak shapes were described by a pseudo-Voigt function varying four profile coefficients.

2.4. Electrochemical impedance spectroscopy (EIS)

The Na₂CaSnS₄ sample was characterized by EIS. The powder sample was pelletized at 360 MPa by uniaxial pressing. Gold electrodes were deposited on each face of the pellet using a sputtering apparatus (Quick Coater SC-701MKII Advance; Sanyu Electron Corp.) in an Ar-filled glove box. The conductivity of the sample was then measured on this pellet by an alternating current (AC) impedance technique using an impedance analyzer (SI-1260; Solartron, Metrology, UK) in the frequency range 1.0 × 10⁷–0.1 Hz and at an applied voltage of 50 mV.

3. Results and discussion

3.1. Structure refinement

The search and match procedure using the ICDD database revealed that the XRPD pattern of the milled Na₂CaSnS₄ sample was very similar to that of NaCl_0.8Br_0.2. The latter crystallizes in the rock salt NaCl-type structure (S. G. Fm3̄m). Consequently, a full pattern matching was performed which led to the cell parameters a = 5.6842 (3) Å and V = 183.66 (2) ų. For the Rietveld refinement, the cations Na⁺, Ca²⁺, and Sn⁴⁺ were set at the 4b Wyck. position (1/2, 1/2, 1/2) and the anions were set at the 4a Wyck. position (0, 0, 0). Restrictions on the occupancies and displacement parameters of Na1, Ca1, and Sn1 were applied. The final residual factors and the refined atomic positions are given in Tables 1 and 2, respectively. Fig. 1 shows an excellent agreement between the experimental and calculated patterns. Furthermore, the Na/Ca/Sn/S atomic ratios of 2 : 1 : 1 : 4 were confirmed by EDX analyses (Fig. 2). Furthermore, the EDX mapping images showed a homogeneous elemental distribution (Fig. S1†).

Table 1 Crystallographic data and structure refinement for Na₂CaSnS₄

Crystal data
Chemical formula	Na2CaSnS4
M r	333
Crystal system	Cubic
Space group	Fm3̄m
Temperature (K)	293
a (Å)	5.6842 (3)
V (Å^3)	183.66 (2)
Z	1
Radiation type	Cu-Kα

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Data collection
Diffractometer	Rigaku smartlab
Data collection mode	Bragg Brentano
2θ values (°)	2θmin = 10
	2θmax = 110
	2θstep = 0.01

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Refinement
R factors and goodness of fit	R p = 0.013
	R wp = 0.017
	R exp = 0.011
	R(F) = 0.033
	R B = 0.032
	χ ^2 = 2.190
No. of parameters	9

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Table 2 Atomic position and equivalent isotopic displacement parameters for Na₂CaSnS₄

Atom	Wyck.	Occ.	x	y	z	U iso (Å^2)
Na1	4b	0.5	1/2	1/2	1/2	0.0605(4)
Ca1	4b	0.25	1/2	1/2	1/2	0.0605(4)
Sn1	4b	0.25	1/2	1/2	1/2	0.0605(4)
S1	4a	1	0	0	0	0.0555(6)

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Fig. 1 Observed (cross), calculated (solid line) and difference (bottom) XRPD patterns (Cu-K_α radiation) for Na₂CaSnS₄.

Fig. 2 EDX analysis and SEM micrographs of the milled Na₂CaSnS₄ sample.

3.2. Crystal structure of Na₂CaSnS₄

The title compound Na₂CaSnS₄ is isostructural to Ag₂Sn^IISn^IVS₄ (AgSnS₂) which crystallizes in the rock salt-type structure. The crystal structure is depicted in Fig. 3. The sulfur atoms form a cubic close packing with ABC-type arrangement. The statistically disordered sodium, calcium, and tin atoms occupy the octahedral interstices. The Na–S and Ca–S distances of 2.8421(3) Å are in good agreement with 2.86 Å and 2.84 Å, values estimated from the effective ionic radii of the six-coordinated Na⁺, Ca²⁺, and S²⁻.⁷⁴ However, the Sn–S distance of 2.8421(3) Å is too large when compared to the 2.53 Å value estimated from the effective ionic radii of the six-coordinated Sn⁴⁺ and S²⁻. It should be mentioned that for octahedrally coordinated Sn in SnS₂ ⁷⁵ and Cu₄SnS₆,⁷⁶ the Sn–S distances are 2.631 Å and 2.595 Å, respectively. The bond valence sums (BVSs) of 1.17, 2.08, and 1.82 for Na⁺, Ca²⁺, and Sn⁴⁺, respectively confirm that Sn⁴⁺ is strongly under bonded.⁷⁷

Fig. 3 Crystal structure of Na₂CaSnS₄.

The crystallographic data of all the A₂MSnS₄ compounds are reported in Table 3. Only Na₂CaSnS₄ and Ag₂Sn^IISn^IVS₄ crystallize in the rock salt-type structure. Furthermore, in the Na₂MSnS₄ series only 4 compounds are known (M = Mg, Zn, Cd, and Sn). A careful examination of their crystal structures indicated that in the phases with Zn²⁺ or Cd²⁺ the cations are tetrahedrally coordinated, whereas in the phases with Mg²⁺ or Sn²⁺ the cations are octahedrally coordinated. Moreover, a group-subgroup relationship exists between R3̄m and Fm3̄m which suggests the existence of a structural relationship between these phases. Indeed, Heppke & Lerch confirmed that the Na₂MgSnS₄ (NaMg_1/2Sn_1/2S₂; hR4, R3̄m) structure is derived from the NaCl-type structure.⁶⁵ In this structure, the sodium atom occupies the 3b Wyck. position at (0, 0, 1/2) whereas the Mg and Sn are statistically disordered over the 3a Wyck. position at (0, 0, 0). Since in the rock salt structure all cations are disordered over a unique crystallographic site, it is concluded that the change in the crystal structure from the rocksalt-type (cF8, Fm3̄m) to the layered-type (hR4, R3̄m) is mainly due to the partial ordering of the cations, which is driven by the difference in the size of the cations and the synthesis conditions. Such a phase transition from Fm3̄m to R3̄m was observed in many compounds prepared by mechanochemical synthesis and subjected to heat treatments (i.e. LiVO₂, Na₂TiS₃, and Na₂MnSnS₄);^35,78,79 however it was not observed in Na₂CaSnS₄ which decomposed on heating.

Table 3 Crystallographic data of the A₂⁺M²⁺SnS₄ compounds (A⁺ = Li, Na, K, Rb, Cu, Ag, and Tl; M²⁺ = Mg, Mn, Fe, Co, Zn, Cd, Sn, Sr, Ba, Eu, and Hg)

S. G.	Compound	a (Å)	b (Å)	c (Å)	α (°)	β (°)	γ (°)	V (Å^3)	Ref.
a This work.
Pn	Li2MnSnS4	6.4143(6)	6.8475(6)	8.0078(7)	90	89.980(6)	90	351.71	37
Pna21	Li2MnSnS4	13.7036(3)	8.0023(2)	6.4155(2)	90	90	90	703.52	37
Pn	Li2FeSnS4	6.3727(3)	6.7776(3)	7.9113(4)	90	90 207(3)	90	341.70	38
Pn	Li2CoSnS4	6.3432(2)	6.7184(2)	7.9404(3)	90	89.988(2)	90	338.38	39
Pn	Li2ZnSnS4	6.3728(13)	6.7286(13)	7.9621(16)	90	90(3)	90	341.42	40
Pmmn	Li2CdSnS4	7.9654(15)	6.4917(15)	6.9685(12)	90	90	90	360.33	41
Pmn21	Li2CdSnS4	7.9555(3)	6.9684(3)	6.4886(3)	90	90	90	359.71	42
P3̄m1	Li2Sn^IISn^IVS4	3.67	3.67	7.9	90	90	120	92.15	30
R3̄m	Li2Sn^IISn^IVS4	3.741(1)	3.741(1)	23.89(10)	90	90	120	289.39	43
Pmn21	Li2HgSnS4	7.9400(17)	6.9310(15)	6.5122(14)	90	90	90	358.38	44
I4̄2m	Cu2MnSnS4	5.49	5.49	10.72	90	90	90	323.10	45
I4̄2m	Cu2FeSnS4	5.47	5.47	10.747	90	90	90	321.56	46
P4̄	Cu2FeSnS4	5.414(3)	5.414(3)	5.414(3)	90	90	90	158.69	47
I4̄2m	Cu2CoSnS4	5.402	5.402	10.805	90	90	90	315.31	48
I4̄2m	Cu2ZnSnS4	5.4332(2)	5.4332(2)	10.8402(6)	90	90	90	320.00	49
I4̄	Cu2ZnSnS4	5.4335(3)	5.4335(3)	10.8429(10)	90	90	90	320.11	50
I4̄2m	Cu2ZnSnS4	5.436	5.436	10.85	90	90	90	320.62	51
Pmn21	Cu2ZnSnS4	7.5385	6.4304	6.2038	90	90	90	300.73	52
I4̄2m	Cu2CdSnS4	5.582	5.582	10.86	90	90	90	338.38	51
P31	Cu2SrSnS4	6.29	6.29	15.57(8)	90	90	120	533.48	53
P3221	Cu2SrSnS4	6.29	6.29	15.57(8)	90	90	120	533.48	54
Ama2	Cu2SrSnS4	10.514(3)	10.456(3)	6.425(2)	90	90	90	706.32	55
P31	Cu2BaSnS4	6.367	6.367	15.833	90	90	120	555.86	56
P3221	Cu2BaSnS4	6.3711(3)	6.3711(3)	15.8425(12)	90	90	120	556.90	57
Ama2	Cu2EuSnS4	10.47930(10)	10.3610(2)	6.40150(10)	90	90	90	695.05	58
I4̄2m	Cu2HgSnS4	5.566	5.566	10.88	90	90	90	337.07	51
Pc	Ag2MnSnS4	6.6510(10)	6.9430(10)	10.536(2)	90	129.145(3)	90	377.32	59
I4̄2m	Ag2FeSnS4	5.74(3)	5.74(3)	10.96(5)	90	90	90	361.11	60
I4̄2m	Ag2ZnSnS4	5.786(4)	5.786(4)	10.829(6)	90	90	90	362.53	61
Pn	Ag2CdSnS4	6.7036(2)	7.0375(3)	8.2166(3)	90	901 577(9)	90	387.63	62
Pmn21	Ag2CdSnS4	8.2137(4)	7.0403(4)	6.7033(2)	90	90	90	387.63	62
Fm3̄m	Ag2Sn^IISn^IVS4	5.506(2)	5.506(2)	5.506(2)	90	90	90	166.92	63
I222	Ag2BaSnS4	7.127(3)	8.117(3)	6.854(3)	90	90	90	396.50	64
R3̄m	Na2MgSnS4	3.74963(11)	3.74963(11)	19.9130(6)	90	90	120	242.45	65
Fm3̄m	Na2CaSnS4	5.6842 (3)	5.6842 (3)	5.6842 (3)	90	90	90	183.66
Fm3̄m	Na2MnSnS4	5.4368(2)	5.4368(2)	5.4368(2)	90	90	90	160.71	35
R3̄m	Na2MnSnS4	3.7523 (4)	3.7523 (4)	19.883 (2)	90	90	120	242.44	35
I4̄	Na2ZnSnS4	6.4835(6)	6.4835(6)	9.1337(10)	90	90	90	383.94	66
C2	Na2ZnSnS4	9.1749(6)	9.1325(4)	6.4873(5)	90	134.999(4)	90	384.37	67
I4̄2m	Na2ZnSnS4	6.4789(15)	6.4789(15)	9.121(2)	90	90	90	382.86	67
C2	Na2CdSnS4	9.282(1)	9.421(3)	6.593(9)	90	134.83(9)	90	408.88	68
R3̄m	Na2Sn^IISn^IVS4	3.69	3.69	25.54	90	90	120	301.16	69
C2/c	K2CdSnS4	11.021(5)	11.030(5)	15.151(10)	90	100.416(12)	90	1811.42	70
R3	K2Sn^IISn^IVS4	3.67	3.67	25.61	90	90	120	298.73	69
R3	Rb2Sn^IISn^IVS4	3.76	3.76	24.33	90	90	120	297.88	69
P21212	Au2BaSnS4	10.982(4)	11.093(4)	6.652(4)	90	90	90	810.37	71
C2221	Au2BaSnS4	6.6387(3)	11.0605(7)	10.9676(6)	90	90	90	805.32	72
I4̄2m	Tl2HgSnS4	7.8571(6)	7.8571(6)	6.6989(7)	90	90	90	413.5(1)	73

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3.3. Ionic conductivity of Na₂CaSnS₄

AC impedance measurements were performed on a powder compacted pellet of Na₂CaSnS₄. The Nyquist plot, which consists of a semicircle in the high-frequency region and a spike in the low-frequency region, is depicted in Fig. 4a. The profile indicates that the sample is a typical ionic conductor, and the total resistance of the sample, including the bulk and grain boundary resistances, was used to determine its conductivity. Fig. 4b shows the temperature dependence of the conductivities of Na₂CaSnS₄. The ionic conductivity of 4.2 × 10⁻⁸ S cm⁻¹ (E_a = 0.6 eV) at 33 °C is much lower than that of the cubic Na₃PS₄ compound (2 × 10⁻⁴ S cm⁻¹ at RT).¹⁴ Although both compounds crystallize in the cubic system and have similar chemical compositions (M₄S₄ with M = Na, Ca, Sn, P), there is a large difference between the ionic conductivities. This is mainly due to the difference in the crystal structures. It seems that the Ca/Sn/Na disorder within the rock salt structure hinders the Na diffusion, whereas the order of Na and P within Na₃PS₄ enables fast Na diffusion. In the future, the synthesis of cation deficient rock salt-type compounds will be performed to enhance the sodium diffusion as it was done with the rock salt compound Ag_3.8Sn₃S₈.⁸⁰ It should be noted that the relative density of the pellet used for EIS measurements was only 72%. This value could not be improved neither by applying higher pressures nor by sintering the pellets at high temperatures. Indeed, our prepared sample is metastable. It is a composite material formed of a crystalline phase (rocksalt-type) and an amorphous phase (see in Fig. 1 the hallow feature at low angles).

Fig. 4 (a) Typical impedance spectrum acquired at 98.4 °C for the Na₂CaSnS₄ sample and (b) the temperature dependence of the conductivities.

4. Conclusions

The chalcogenide family of compounds A₂BCD₄ (A⁺ = monovalent cation, B²⁺ = divalent cation, C⁴⁺ = tetravalent cation, and D²⁻ = chalcogen) contains at least 150 members. In this work we demonstrated that the facile mechanochemical synthesis route enables the synthesis of the new member Na₂CaSnS₄, which is the first quaternary compound of this family that contains calcium and the second to crystallize with a rock salt-type structure. Its ionic conductivity is relatively low when compared with other sulfide ionic conductors such as Na₃PS₄. Therefore, it would be necessary to prepare other cation deficient phases to enhance the ionic conductivity. This will be conducted in the near future.

Data availability

Our data will be available on request.

Author contributions

H. Ben Yahia designed and performed the experiments and wrote the manuscript and A. Sakuda and A. Hayashi secured the research funds and revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by MEXT/JSPS KAKENHI Grant Numbers JP19H05812, JP21H04625 and JP23H04633 and the MEXT program: Data Creation and Utilization-Type Material Research and Development Project Grant Number JPMXP1122712807. We are grateful to Dr Furukawa for his support with EDX analyses and to the reviewers for providing valuable feedback and constructive criticism on this research paper.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 and EDX mapping images of Na₂CaSnS₄. See DOI: https://doi.org/10.1039/d4mr00028e

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