Synthesis, structural characterization, and hydrogen bonds of Co₉(OH)₁₄[SO₄]₂

1 Introduction

The sulfate-based polyanionic compounds such as AMSO₄X (A=Li, Na, K; M=Fe, Mn, Ni, Co; X=F, OH) and Li₂M(SO₄)₂ (M=Fe, Co, Mn) have been extensively studied as positive electrode materials in lithium and sodium ion batteries. However, they are still immature and cannot compete with LiFePO₄, which went through more than one decade of optimization [1]. These materials are often synthesized using AX or A₂SO₄ and M(SO₄)·xH₂O precursors. With cobalt as the transition metal, only five cobalt sulfate compounds have been reported so far; M(SO₄)·xH₂O (x=1 [2], 4 [3], 6 [4]), Co₅(SO₄)₂[OH]₆[H₂O]₄ [5], and Co₃(SO₄)₂[OH]₂[H₂O]₂ [6].

Transition metal sulfite compounds are less abundant when compared with sulfate-based polyanionic compounds. Therefore, we decided to investigate sulfite compounds as possible positive electrode materials for Li and Na ion batteries. To our knowledge, in the Fe^IIO–S^IVO₃–H₂O system, only the two iron sulfite compounds FeSO₃(H₂O)₃ [7] and FeSO₃(H₂O)_2.5 exist [8], whereas in the Mn^IIO–S^IVO₃–H₂O system, there are four hydrated manganese sulfite compounds MnSO₃(H₂O) [9], and MnSO₃(H₂O)₃ (space groups: P2₁/c [10], P2₁2₁2₁ [11], Pnma [12]). Our recent investigation of the Na₂SO₃–LiF–(CH₃COO)₂Mn·4H₂O system using the hydrothermal synthesis route revealed three new phases Mn₂(OH)₂SO₃, Mn₂F(OH)SO₃, and Mn₅(OH)₄(H₂O)₂[SO₃]₂[SO₄] [13].

In the Co^IIO–S^IVO₃–H₂O system, only one sulfite compound has been reported recently [14]. Therefore, the Na₂SO₃–LiF–(CH₃COO)₂Co·4H₂O system was explored. Interestingly, the new phase Co₉(OH)₁₄[SO₄]₂ was discovered and its crystal structure was solved using single crystal X-ray diffraction data. The results of our studies are presented in the following section. During our investigation, it was brought to our attention that the two sodium cobalt sulfite compounds NaCo₂(SO₃)₂(OH)(H₂O) and Na₄Co₂(SO₃)₄ exist; however, they were synthesized from trivalent cobalt precursors [Co^III(NH₃)₆]Cl₃ and Na₂SO₃ [15].

2 Experimental

3 Discussion

Co₉(OH)₁₄[SO₄]₂ compound crystalizes in a new type of structure (Fig. 1a and b) closely related to the hydrozincite Zn₅(OH)₆[CO₃]₂ [20] and the cianciulliite MgMn₂Zn₂[H₂O]₂(OH)₁₀ [21]. The CoO₆ octahedra share edges and form layers (Fig. 1c) similar to those in the brucite Co(OH)₂ (Fig. 1d) [22]. The empty octahedron, in the middle of the unit cell and centered at the inversion center, is very similar to those observed in Li_0.75CoO₂ [23], K₂Mn₃[VO₄]₂[OH]₂ [24], Cu₅(OH)₄[VO₄]₂ [25], and Na₂Ni₃[PO₄]₂[OH]₂ [26] (Fig. 2). The Co5O₄ tetrahedra share faces with the empty octahedra (Fig. 1f) and corners with the filled octahedra, forming single layers, which are stacked parallel to the (100) plane (Fig. 1e). The successive layers are interconnected through SO₄ tetrahedra forming a 3D framework (Fig. 1a). The interatomic distances are given in Table 2.

Fig. 1:

Views of the structure of Co₉(OH)₁₄[SO₄]₂ along the layers (a) and perpendicular to the layers (b) and views of the octahedral environment of the cobalt atoms in Co₉(OH)₁₄[SO₄]₂ (c), and in the brucite Co(OH)₂ (d). Views of the single layer of CoO₆ octahedra and CoO₄ tetrahedra sharing corners (e), the empty octahedron sharing faces with the Co5O₄ tetrahedra (f), and the Co5O₄ and SO₄ tetrahedra sharing corners (g). The dashed red lines surround a single layer (a).

Fig. 2:

Crystallographic data of a few (A,B)₅[X]₂[OH]_y compounds and Co₉(OH)₁₄[SO₄]₂.

The CoO₆ octahedra are distorted with the Co–O distances ranging from 2.016 to 2.386 Å. The average distances of 2.110, 2.133, 2.110, and 2.118 Å in Co1O₆, Co2O₆, Co3O₆, and Co4O₆, respectively, are in good agreement with the sum of the ionic radii (0.74+1.40=2.14 Å) [27]. Only Co5 is tetrahedrally coordinated. The Co5–O distances range from 1.943 to 2.033 Å, with an average distance of 1.974 Å. The BVSs of 1.988, 1.904, 1.941, 1.929, and 1.876 indicate that Co2 and Co5 are slightly under-bonded.

The S⁶⁺ cations are coordinated by four oxygen atoms forming distorted SO₄ tetrahedra. The S–O distances range from 1.448 to 1.508 Å with an average distance of 1.477 Å. This is a conventional value often observed in sulfate compounds such as Co₅[SO₄]₂[OH]₆[H₂O]₄ (~1.477 Å) [5] or Cmcm-Co[SO₄] (~1.477 Å) [28]. The BVS of 5.96 is in agreement with the expected value of +6 for S⁶⁺.

The presence of the OH⁻ ions, which are known to be hydrogen bond donors, suggests the presence of hydrogen bond acceptors. Indeed, the oxygen atoms forming the SO₄ tetrahedra (O2, O3, and O4) play the role of hydrogen bond acceptors (Table 3). The checkcif indicates alert levels A and B, which are related mainly to the presence of suspicious hydrogen bonds and of missing acceptor atoms (for OH6 and OH10), respectively. However, our careful examination of the different bonds did not show any anomaly. Based on the classification of Jeffrey [29], the O6–H6···O3 and O10–H10···O4 hydrogen bonds are weak and the five other bonds are medium (Table 3 and Fig. 3) [30].

Fig. 3:

Arrangement of the hydrogen bonds in Co₉(OH)₁₄[SO₄]₂.

The layered character of the triclinic structure of Co₉(OH)₁₄[SO₄]₂ is very similar to that of the monoclinic structures of the K₂Mn₃[VO₄]₂[OH]₂, Na₂Ni₃[PO₄]₂[OH]₂, MgMn₂Zn₂[H₂O]₂(OH)₁₀, and Zn₅(OH)₆[CO₃]₂ compounds (Fig. 2). These structures are mainly formed of layers of octahedral metal hydroxides that share corners with tetrahedral Co, V, P, Zn, and Zn cations, respectively. The main difference between these structures resides in the interlayer connections (Fig. 2, last row). In K₂Mn₃[VO₄]₂[OH]₂ and Na₂Ni₃[PO₄]₂[OH]₂, the interlayer space is filled with large seven-coordinated potassium and sodium cations, whereas in MgMn₂Zn₂[H₂O]₂(OH)₁₀, only water molecules were observed between the layers. This suggests that hydrogen bonds most probably stabilize this structure. Unfortunately, the authors did not report the positions of the H atoms. In Zn₅(OH)₆[CO₃]₂ and Co₉(OH)₁₄[SO₄]₂, the layers are held together by the carbonate triangles and sulfate tetrahedra, respectively. Even within the layers of octahedral metal hydroxides, the vacancies are distributed in different fashions (Fig. 4). This explains why no group–subgroup relationships between Co₉(OH)₁₄[SO₄]₂ and Zn₅(OH)₆[CO₃]₂ could be found.

Fig. 4:

Views perpendicular to the layers in the structure of Co₉(OH)₁₄[SO₄]₂ and Zn₅(OH)₆[CO₃]₂.

4 Conclusion

The investigation of the Na₂SO₃–Co(CH₃COO)₂·4H₂O system using a hydrothermal synthesis route under air led to the discovery of the new compound Co₉(OH)₁₄[SO₄]₂. Its structure, which has been solved using single crystal X-ray diffraction data, can be described as a stacking of sheets strongly related to those found in the hydrozincite and brucite structures. Weak and medium O–H···O–S hydrogen bonds have been observed.

5 Supporting information

Tables of bond valence sums, atomic coordinates, and anisotropic displacement parameters of Co₉(OH)₁₄[SO₄]₂ as well as a scanning electron microscope image and the energy dispersive X-ray analysis of the single crystal used for the data collection are given as Supporting Information available online (DOI: 10.1515/znb-2017-0046).