Cobalt Phosphates and Applications

1. Introduction

The search for new inorganic materials with open frameworks formed by tetrahedra and octahedra sharing corners or edges; delimiting cages (1D), interlayer spaces (2D), or tunnels (3D); or communicating by the intermediate of windows where cations are located is an interesting field with intense activity including several disciplines: solid-state chemistry, physics, mechanics, etc. Synthesis and physicochemical studies of metallophosphate compounds are the driving force behind the recent technological development, and studies are progressing through the exchange of points of view between specialists concerned.

Metallophosphates have a promising field for various applications: electrical, electrochemical, magnetic, and catalytic processes [1, 2, 3, 4, 5, 6, 7]. Nevertheless, the introduction of monovalent ions into metallophosphates can lead to materials with interesting properties. This orientation was initiated from the discovery of the ionic conduction properties of NASICON Na₃Zr₂Si₂PO₁₂ (σ_300°C = 0.2 S cm⁻¹ and Ea = 0.29 eV) in 1976 [5] followed by olivine series studies of general formula LiMPO₄ (M = Co²⁺, Fe²⁺, Mn²⁺) usable in the manufacture of cathodes of rechargeable lithium-ion batteries [7]. These materials have a remarkable structural richness: olivine structure [7], zeolitic structure [8], alluaudite structure [9], melilite structure [10], etc. In relation to their structures, these materials have many physicochemical properties: ionic conduction [10], ion exchange [6, 7], etc. In this context, several researcher groups have tried to explore CoO-P₂O₅ and A₂O-CoO-P₂O₅ systems (A: monovalent metals). This chapter is dedicated to treated physicochemical and structural studies of monovalent cation cobalt phosphates (Li, Na, K, and Ag).

2. Synthesis methods and experimental techniques

The most common synthesis method is the solid-state reaction method. Nevertheless, to minimize the energy consumption and to improve quality of the developed materials (particle size, purity, homogeneity, etc.), other techniques such as hydrothermal method are adopted. In this method, the crystalline products are synthesized at low temperature, generally 150–250°C, and under high pressure.

3. Cobalt phosphate

In the literature, there are more than 80 allotropic forms of cobalt phosphates in which cobalt takes different oxidation degrees, sometimes in the same compound. Some cobalt phosphates have distinguishable physical properties in relation to their structures. In this chapter, cobalt monophosphate CoPO₄ will be reported.

CoPO₄ [1] material, like FePO₄ structure, is usable in the manufacture of Li-ion batteries. In fact, the lithium extraction from LiCoPO₄ material leads to CoPO₄ compound. The delithiated sample was prepared by electrochemical Li extraction in galvanostatic mode at a C/5-rate from LiCoPO₄. The latter shows considerable stability during several cycles of charge-discharge of the battery. In fact, CoPO₄ crystallized in the orthorhombic with Pnma space group. The structure is formed by (CoO₆)_n chains connected with PO₄ tetrahedra to form layers in the ab plane. The connection between layers formed a 3D framework showing several types of tunnels according to [001] and [010] directions (Figure 1). In this structure type, the cobalt ion has an oxidation degree of +III.

4. Monovalent metal cobalt phosphate

There are more than 40 monovalent cation cobalt phosphates. The monovalent metal cobalt phosphates will be classified according to the oxygen/phosphor molar ratio.

4.1 Monophosphates

This family is known as orthophosphate or also monophosphate; it is characterized by its high stability compared to other phosphates. In the structure, (PO₄)³⁻ tetrahedra are isolated from each other.

The most famous material is lithium cobalt monophosphate LiCoPO₄ (Figure 2) [1]. It crystallizes in the orthorhombic system, Pnma space group. It belongs to the olivine family of general formula LiMPO₄ (M = Fe, Ni, Co, and Mn). Xiang Huang et al. [11] have proposed hydrothermal synthesis method of this monophosphate which shows performance in terms of reaction yield and product homogeneity versus dry route. The phospho-olivine series is used in the manufacture of cathodes in Li-ion batteries [12]. LiCoPO₄-CoPO₄ system shows high stability during several charge-discharge cycles of the battery at room temperature (Figure 3). The olivine structure can be described as a compact hexagonal stack of A-B-A-B-A-type oxygen layers. The A = Na or Co cations occupy half of the octahedral sites AO₆ and the B = P cations 1/8 of the available tetrahedral P sites of PO₄ tetrahedra.

On the other hand, when lithium is substituted by sodium in different synthesis conditions, the monophosphate NaCoPO₄ may present in four allotropic forms [13, 14]. Figure 4 groups the polymorphisms in sodium cobalt monophosphate. All sodium materials show open anionic frameworks containing tunnels which contain sodium cations. On the other hand, the structure of the P2₁/n form where cobalt is only tetracoordinated is related to zeolite ABW (LiAlSiO₄.H₂O) [14]. In NaCoPO₄ (P2₁/c space group subgroup of Pnma), Stucky et al. [13] report that the structure is also a distortion of the ABW zeolite structure but that it is a little more complex since the cobalt environment is trigonal bipyramidal. Indeed, the main characteristic of ABW zeolites is their spatial structures which contain pores and channels that can absorb or reject various solids, liquids, or gases. The applications of zeolites are numerous: food supplement for animals, additives for detergents, molecular filters, water treatment, catalysis, etc.

The α-NaCoPO₄ (P2₁/n space group) with maricite type is formed by octahedral chains CoO₆ sharing edge and parallel to the a axis. They are interconnected via the PO₄ tetrahedra, which creates large cavities where Na⁺ cations are located [13].

While the phase of the hexagonal system ᵦ-NaCoPO₄ is stuffed tridymite type which is a high temperature variety of quartz SiO₂. These compounds have a lower symmetry than tridymite due to the order of cations within the channels.

The silver cobalt monophosphate AgCoPO₄ [15] has another structure type with a twofold oxygen coordination for silver atoms and a fivefold coordination for cobalt atoms. Indeed, the silver compound crystallizes in the triclinic system, space group P-1. A projection of the structure of this phase is shown in Figure 5.

Another monophosphate is classified as Na-ionic conductor: NaCo₄(PO₄)₃ [16] with activation energy Ea = 0.89 eV and σ = 10⁻⁶ S cm⁻¹. Indeed, cationic sites, located in wide-sectioned channels (Figure 7a), are partially occupied by Na⁺ ions and relatively agitated which may explain the sodium mobility in the anionic framework. This compound crystallizes in the monoclinic system, space group P2₁/n. The isoformula potassium material KCo₄(PO₄)₃ [17] crystallizes, in a different structure, the orthorhombic system, space group Pnnm. The structure projection along the [001] direction is shown in Figure 6(b).

The sodium cobalt monophosphate Na₄Co₇(PO₄)₆ [18] is synthesized by the dry route. This compound is a member of a family of phases including Na₄Ni₇(PO₄)₆ [19] and K₄Ni₇(PO₄)₆ [20]. Previous studies have shown that the material Na₄Ni₇(PO₄)₆ is classified as fast ionic conductor. Several studies relating to the substitution of phosphate by arsenate have led to Na₄Co₇(AsO₄)₆ (Ea = 1.00 eV) [21], Na₄Co_5.63Al_0.96(AsO₄)₆ (Ea = 0.53 eV) [22, 23, 24], Na₄Li_0.62Co_5.67Al_0.71(AsO₄)₆ [25], and Ag₄Co₇(AsO₄)₆ (Ea =0.61 eV) [26].

A projection of the structure of Na₄Co₇(PO₄)₆ according to [100] is given in Figure 7. The anionic framework has both a tetrahedral (CoO₄ and PO₄) and octahedral (CoO₆) environment as well as hexagonal tunnels where the sodium ions lodge.

4.2 Polyphosphates

Short-chain polyphosphates also named n-polyphosphates are characterized by short chains of PO₄³⁻ tetrahedra sharing corners. The general formulas of the phosphate anion are given by [P_nO_3n+1]⁽ⁿ⁺²⁾⁻ with n > 1. Oligophosphates for which n = 2, 3, 4, and 5 are known until now. These compounds are infrequent for n ≥ 4.

The other type corresponds to polyphosphates with long chains. When n tends to infinity, their phosphate anions take the formula [PO₃]_nⁿ⁻, thus forming infinite chains of PO₄ tetrahedra. If the tetrahedron chain closes on itself to form rings, the corresponding phosphates are called cyclophosphates. The general formula of the cyclic anion is [P_nO_3n]ⁿ⁻ with n = 3, 4, 5, 6, 8, 10, and 12.

In this part, a variety of monovalent ion cobalt polyphosphates found in the literature will be mentioned.

4.2.1 Short-chain polyphosphates [P_nO_3n+1]⁽ⁿ⁺²⁾⁻ with n > 1

4.2.1.1 Diphosphates (n = 2)

The formula of the phosphate anion is P₂O₇⁴⁻, known as diphosphate (or pyrophosphate). The group P₂O₇ consists of two PO₄ tetrahedra sharing a single corner.

Concerning diphosphates of formulation A₂CoP₂O₇ (A: Na or K), the sodium compound is presented in three allotropic forms: triclinic (Figure 8a), monoclinic (Figure 8b), and quadratic (Figure 8c) [8, 10].

In the last form, cobalt atoms have purely tetrahedral environment, and the anionic framework is formed by layers formed by [CoP₂O₇]²⁻ groups. The Na⁺ and/or K⁺ cations are located in the interlayer space. Sanz et al. [10] postponed the study of the ionic conductivity of the quadratic form to sodium; their study reveals that it is a fast ionic conductor. Marzouki et al. [27] proposed a modeling of alkaline cation conduction paths in these structures (Figure 9). The conductivity in this type of material is bi-dimensional.

The silver cobalt diphosphates include Ag_3.68Co₂(P₂O₇)₂ [28] and (Ag_0.58Na_1.42)₂Co₂(P₂O₇)₂ [29]. They crystallize in the triclinic system, space group P-1. Projection of the mixed Na/Ag metals is presented in Figure 10. The cobalt, in this case, is purely octahedral. In the anionic framework, the cohesion between two symmetrical units Co(2)P₂O₁₁ is provided by Co(1)O₆ octahedra to form the Co₄P₄O₂₈ unit. According to the three spatial directions, the junction between two Co₄P₄O₂₈ units is provided by two P₂O₇ diphosphates forming 3D anionic framework.

Silver transport pathways in Ag_3.68Co₂(P₂O₇)₂ are simulated using BVSE calculations. The BVSE simulation shows that the material should be moderate 3D ionic conductor with activation energy value of 1.7 eV. The result is described in Figure 11.

The particularity of lithium cobalt diphosphates is the non-stoichiometry in composition. The formulas found in the bibliography are Li_5.88C_o5.06(P₂O₇)₄ [30] where cobalt and lithium share the same crystallographic sites and Li_4.03Co_1.97(P₂O₇)₂ [31] where a fraction of cobalt oxidation degree is +III. The projections of their structures are shown in Figures 12 and 13.

4.2.1.2 Triphosphates (n = 3)

Single monovalent cation cobalt triphosphate is found in the literature. Its formula is LiCo₂P₃O₁₀ [32]. This material crystallizes in the monoclinic system, space group P2₁/m. In the anionic framework, the P₃O₁₀ groups ensure cohesion between the infinite chains formed by Co₂O₁₀ dimers which are linked together by edge sharing. Figure 14 shows a projection of the structure in the direction [100]. The NaCo₂As₃O₁₀ triarsenate [33], isostructural with LiCo₂P₃O₁₀ triphosphate, shows interesting electrical properties (Ea = 0.48 eV; σ_300°C = 1.2 × 10⁻⁵ S cm⁻¹).

4.2.1.3 Polyphosphates with long chains [PO₃]_nⁿ⁻

The first phase seen in the bibliography is tetraphosphate K₂Co(PO₃)₄ [34]. This material is synthesized by the dry route; it crystallizes in the monoclinic system, with non-centrosymmetric space group “Cc.” Cobalt has the oxidation state (+II) and is octacoordinated. Phosphate anions of formulation [PO₃]_nⁿ⁻ (n tends to infinity) thus develop into long chains of PO₄ tetrahedra linked together by CoO₆ octahedra to form a 3D framework (Figure 15).

As for the lithium compound [35] with LiCo₂P₃O₉ formula (Figure 16), this material is at a higher symmetry: orthorhombic system, space group P212121. The Co₂O₁₁ dimers, in this case, are formed by two vertex-linked CoO₆ octahedra. They ensure the cohesion between the nn-infinite tetrahedral (PO₃) chains to lead to a three-dimensional framework.

4.2.1.4 Mixed mono-diphosphate Na₄Co₃(PO₄)₂P₂O₇

Some materials may have more than one type of phosphate group. The material Na₄Co₃(PO₄)₂P₂O₇ [36] has both PO₄ isolated tetrahedra and P₂O₇ diphosphate groups. This mono-diphosphate crystallizes in the orthorhombic system, space group Pn2₁a. Projections of the three-dimensional framework of this material (Figure 17) show that PO₄ monophosphates are bound to CoO₆ octahedra, on the one hand by edge sharing and on the other hand by sharing vertices, while diphosphates join four CoO₁₀ units by pooling vertices.

5. Structural factors involved in ionic conductivity: Correlation of structure-electrical properties

Ionic conductors have been intensively researched since the discovery of properties of ionic superconductors [37] whose conductivity is sufficiently high to consider applications as solid electrolytes in batteries [38, 39] in storage devices of energy and sensors [40]. On the other hand, materials with low ionic conductivity remain interesting to elucidate certain mechanisms of cation transport. In these materials, the charge carriers are cations.

The open framework is an essential factor that governs the mobility of cations within a crystal lattice [6, 41, 42]. Among these structures, there are:

• Three-dimensional frameworks with windows or channels: this type of material has an ionic conduction influenced by the size of the bottlenecks separating two adjacent available sites. The existence of wide-sectioned channels between the cationic sites promotes the passage of cations. According to Hong, for fast ionic conduction, the minimum sections of the windows must be greater than or equal to twice the sum of the radii of the cation and the nearest anion [40].

• Layered structures: in this case, mobile ions move in parallel planes, located in the interlayer space. The conduction in this case is probably two-dimensional [27].

• Structures with isolated tetrahedral groups: these structures consist of tetrahedral groups (SiO₄³⁻, PO₄³⁻, etc.) connected to each other solely by alkaline ions. These independent tetrahedra facilitate the movement of cations [43, 44, 45].

Other factors than the open framework can also promote ionic mobility [40]:

• Site occupation: the partial occupations of the ionic sites (occupancy rate lower than 1) favor the displacement of the mobile ion from one site to another energetically equivalent.

• Coordination polyhedra: the cation environment can play an important role in its mobility. Indeed, mobile ions can cross rectangular faces more easily than triangular faces.

• Ion size: to promote conduction, congestion must be minimized, so the use of small cations is recommended, to facilitate their movement.

• Structural defects: substitution or doping of one or more elements with other(s) having different degrees of oxidation is responsible for the creation of cationic vacancies at the origin of conduction properties in certain materials.

Taking into account the structural factors influencing the conductivity mentioned above, several studies have been devoted to improving the electrical properties of such materials by acting on other factors. This is the case of the total or partial substitution of the mobile species such as in NASICON Na_1+xZr_2−xMg_x/2(PO₄)₃ (0 < x ≤ 2) [46] and in SKELETON phosphates (3D) A₃M₂(PO₄)₃: A = Li, Na, Ag, K, and M = Cr, Fe [47]. The doping of materials by one or more chemical elements can also promote the mobility of cations like the oxides La_1.2Sr_1.8Mn_2−xT_xO₇ with T = Fe, Co, Cr [48].

On the other hand, work on a series of materials is being processed in order to show the effect of microstructure optimization (grain size) on conductivity [49]. Moreover, it has been demonstrated in previous studies, such as for LAMOX ceramics (La_2−xR_xMo_2−yW_yO₉ with R = Nd, Gd, Y) [50, 51] and for β-Xenophyllite-type Na₄Co₇(AsO₄)₆ [21] and Ag₄Co₇(AsO₄)₆ [26], that the electrical properties are related to the relative density of sample (100 porosity), which requires a rigorous control of the microstructure.

6. Conclusion

In this chapter, synthesis methods of cobalt phosphates and metallo-cobalt phosphates in the crystalline form have been described: single crystals and/or polycrystalline powders. The structural studies of the studied compounds show structural diversity with open anionic frameworks showing tunnels (3D) and inter-sheet space (2D). However, it shows that the electrical property is related to the structural characteristics of the material. In order to correlate structure and physical properties especially electrical properties of metallo-cobalt phosphates, structural factors influencing the ionic conductivity have been treated. Based on the structural characteristics, the electrical properties of the crystalline materials can be modeled theoretically, especially in the case of purely ionic conductors. In fact, it is possible to determine the value of the activation energy which corresponds to the minimum energy that must be supplied to an ion to move from one site to another site in the crystal lattice. In addition, these modelizations are based on the structural data of the crystal. Since the measurements are often performed on ceramics, it is also necessary to take into account the effect of the relative density of the ceramic: effect of the microstructure.