**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 *A*O6 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

#### **Figure 2.**

*Projection of LiCoPO4 structure along [010] direction.*

**Figure 3.** *Lithium insertion/extraction in the olivine structure CoPO4/LiCoPO4.*

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<sup>+</sup> 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<sup>−</sup><sup>6</sup> S cm<sup>−</sup><sup>1</sup> . Indeed, cationic sites, located in wide-sectioned channels (**Figure 7a**), are partially occupied by Na<sup>+</sup> 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

**15**

**Figure 4.**

**Figure 5.**

[001] direction is shown in **Figure 6(b)**.

*Projection of AgCoPO4 structure along [010] direction.*

isoformula potassium material KCo4(PO4)3 [17] crystallizes, in a different structure, the orthorhombic system, space group Pnnm. The structure projection along the

*Allotropic forms of NaCoPO4: (a) Pnma, (b) P21/c, (c) P65, and (d) P21/n space groups.*

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

*Cobalt Phosphates and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.86215*

*Cobalt Phosphates and Applications DOI: http://dx.doi.org/10.5772/intechopen.86215*

**Figure 4.**

*Cobalt Compounds and Applications*

*Projection of LiCoPO4 structure along [010] direction.*

**Figure 2.**

**Figure 3.**

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,

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

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

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

Another monophosphate is classified as Na-ionic conductor: NaCo4(PO4)3 [16]

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

S cm<sup>−</sup><sup>1</sup>

symmetry than tridymite due to the order of cations within the channels.

group P-1. A projection of the structure of this phase is shown in **Figure 5**.

located in wide-sectioned channels (**Figure 7a**), are partially occupied by Na<sup>+</sup>

cations are located [13].

. Indeed, cationic sites,

ions

molecular filters, water treatment, catalysis, etc.

*Lithium insertion/extraction in the olivine structure CoPO4/LiCoPO4.*

with activation energy Ea = 0.89 eV and σ = 10<sup>−</sup><sup>6</sup>

PO4 tetrahedra, which creates large cavities where Na<sup>+</sup>

**14**

*Allotropic forms of NaCoPO4: (a) Pnma, (b) P21/c, (c) P65, and (d) P21/n space groups.*

**Figure 5.** *Projection of AgCoPO4 structure along [010] direction.*

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

**Figure 6.** *Projections of (a) NaCo4(PO4)3 and (b) KCo4(PO4)3 structures.*

substitution of phosphate by arsenate have led to Na4Co7(AsO4)6 (Ea = 1.00 eV) [21], Na4Co5.63Al0.96(AsO4)6 (Ea = 0.53 eV) [22–24], Na4Li0.62Co5.67Al0.71(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 PO4 <sup>3</sup><sup>−</sup> tetrahedra sharing corners. The general formulas of the phosphate anion are given by [PnO3n+1] (n+2)<sup>−</sup> with n > 1. Oligophosphates for which n = 2, 3, 4, and 5 are known until now. These compounds are infrequent for n ≥ 4.

**17**

**Figure 8.**

*Cobalt Phosphates and Applications*

cyclic anion is [PnO3n]

literature will be mentioned.

*4.2.1.1 Diphosphates (n = 2)*

and/or K<sup>+</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.86215*

*4.2.1 Short-chain polyphosphates [PnO3n+1]*

The formula of the phosphate anion is P2O7

(**Figure 8b**), and quadratic (**Figure 8c**) [8, 10].

anionic framework is formed by layers formed by [CoP2O7]

*Projections of polymorphs of Na2CoP2O7: (a) triclinic, (b) monoclinic, and (c) tetragonal.*

to infinity, their phosphate anions take the formula [PO3]n

The other type corresponds to polyphosphates with long chains. When n tends

*(n+2)<sup>−</sup> with n > 1*

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

phosphate). 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

In the last form, cobalt atoms have purely tetrahedral environment, and the

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

cations are located in the interlayer space. Sanz et al. [10] postponed

<sup>n</sup><sup>−</sup> with n = 3, 4, 5, 6, 8, 10, and 12. In this part, a variety of monovalent ion cobalt polyphosphates found in the

<sup>n</sup><sup>−</sup>, thus forming infinite

<sup>4</sup><sup>−</sup>, known as diphosphate (or pyro-

<sup>2</sup><sup>−</sup> groups. The Na<sup>+</sup>

**Figure 7.** *Projection of Na4Co7(PO4)6 structure along the a axis.*

#### *Cobalt Phosphates and Applications DOI: http://dx.doi.org/10.5772/intechopen.86215*

The other type corresponds to polyphosphates with long chains. When n tends to infinity, their phosphate anions take the formula [PO3]n <sup>n</sup><sup>−</sup>, 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 [PnO3n] <sup>n</sup><sup>−</sup> 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 [PnO3n+1] (n+2)<sup>−</sup> with n > 1*

*4.2.1.1 Diphosphates (n = 2)*

*Cobalt Compounds and Applications*

and Ag4Co7(AsO4)6 (Ea =0.61 eV) [26].

*Projections of (a) NaCo4(PO4)3 and (b) KCo4(PO4)3 structures.*

phosphate anion are given by [PnO3n+1]

*Projection of Na4Co7(PO4)6 structure along the a axis.*

**4.2 Polyphosphates**

**Figure 6.**

by short chains of PO4

substitution of phosphate by arsenate have led to Na4Co7(AsO4)6 (Ea = 1.00 eV) [21], Na4Co5.63Al0.96(AsO4)6 (Ea = 0.53 eV) [22–24], Na4Li0.62Co5.67Al0.71(AsO4)6 [25],

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.

Short-chain polyphosphates also named n-polyphosphates are characterized

n = 2, 3, 4, and 5 are known until now. These compounds are infrequent for n ≥ 4.

<sup>3</sup><sup>−</sup> tetrahedra sharing corners. The general formulas of the

(n+2)<sup>−</sup> with n > 1. Oligophosphates for which

**16**

**Figure 7.**

The formula of the phosphate anion is P2O7 <sup>4</sup><sup>−</sup>, 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] <sup>2</sup><sup>−</sup> groups. The Na<sup>+</sup> and/or K<sup>+</sup> 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 Ag3.68Co2(P2O7)2 [28] and (Ag0.58Na1.42)2C o2(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.

#### **Figure 9.**

*Bond valence site energy-simulated pathways of Na+ ions within the K0.86Na1.14CoP2O7 structure (Na brown and K gray and the layer at z = 0).*

**19**

**Figure 12.**

*Projection of Li5.88Co5.06(P2O7)4 structure along b direction.*

**Figure 11.**

*(i.e., ~1.7 eV).*

*Cobalt Phosphates and Applications*

*4.2.1.2 Triphosphates (n = 3)*

*DOI: http://dx.doi.org/10.5772/intechopen.86215*

Silver transport pathways in Ag3.68Co2(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 Li5.88Co5.06(P2O7)4 [30] where cobalt and lithium share the same crystallographic sites and Li4.03Co1.97(P2O7)2 [31] where a fraction of cobalt oxidation degree is +III. The

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

*3D silver transport pathways in Ag3.68Co2(P2O7)2 with bond valence mismatch of |ΔV(Ag)| = 1.3 u.v* 

projections of their structures are shown in **Figures 12** and **13**.

**Figure 10.** *Projection of (Ag0.58Na1.42)2Co2 (P2O7)2 structure along the a axis.*

#### *Cobalt Phosphates and Applications DOI: http://dx.doi.org/10.5772/intechopen.86215*

Silver transport pathways in Ag3.68Co2(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 Li5.88Co5.06(P2O7)4 [30] where cobalt and lithium share the same crystallographic sites and Li4.03Co1.97(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)*

*Cobalt Compounds and Applications*

in this type of material is bi-dimensional.

*Bond valence site energy-simulated pathways of Na+*

*and K gray and the layer at z = 0).*

by two P2O7 diphosphates forming 3D anionic framework.

alkaline cation conduction paths in these structures (**Figure 9**). The conductivity

The silver cobalt diphosphates include Ag3.68Co2(P2O7)2 [28] and (Ag0.58Na1.42)2C o2(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

 *ions within the K0.86Na1.14CoP2O7 structure (Na brown* 

**18**

**Figure 10.**

**Figure 9.**

*Projection of (Ag0.58Na1.42)2Co2 (P2O7)2 structure along the a axis.*

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

#### **Figure 11.**

*3D silver transport pathways in Ag3.68Co2(P2O7)2 with bond valence mismatch of |ΔV(Ag)| = 1.3 u.v (i.e., ~1.7 eV).*

**Figure 12.** *Projection of Li5.88Co5.06(P2O7)4 structure along b direction.*

**Figure 13.** *Projection of Li4.03Co1.97(P2O7)2 structure along a direction.*

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; σ300°C = 1.2 × 10<sup>−</sup><sup>5</sup> S cm<sup>−</sup><sup>1</sup> ).

#### *4.2.1.3 Polyphosphates with long chains [PO3]n n−*

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]n <sup>n</sup><sup>−</sup> (n tends to

**21**

**Figure 16.**

**Figure 15.**

*Projection of K2Co(PO3)4 structure along the c axis.*

*Projection of LiCo2P3O9 structure along the a axis.*

*Cobalt Phosphates and Applications*

three-dimensional framework.

*DOI: http://dx.doi.org/10.5772/intechopen.86215*

octahedra to form a 3D framework (**Figure 15**).

*4.2.1.4 Mixed mono-diphosphate Na4Co3(PO4)2P2O7*

infinity) thus develop into long chains of PO4 tetrahedra linked together by CoO6

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

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

**Figure 14.** *Projection of LiCo2P3O10 structure along a direction.*

### *Cobalt Phosphates and Applications DOI: http://dx.doi.org/10.5772/intechopen.86215*

*Cobalt Compounds and Applications*

σ300°C = 1.2 × 10<sup>−</sup><sup>5</sup>

**Figure 13.**

 S cm<sup>−</sup><sup>1</sup> ).

*Projection of Li4.03Co1.97(P2O7)2 structure along a direction.*

*4.2.1.3 Polyphosphates with long chains [PO3]n*

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;

and is octacoordinated. Phosphate anions of formulation [PO3]n

*n−*

<sup>n</sup><sup>−</sup> (n tends to

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)

**20**

**Figure 14.**

*Projection of LiCo2P3O10 structure along a direction.*

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

**Figure 15.** *Projection of K2Co(PO3)4 structure along the c axis.*

**Figure 16.** *Projection of LiCo2P3O9 structure along the a axis.*

**Figure 17.** *Projections of Na4Co3(PO4)2P2O7 structure in (a) a, (b) b, and (c) c directions.*

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.
