**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.

#### **2.1 Solid-state reaction method**

Solid-state reaction route is the most adopted method to prepare single crystals or polycrystalline materials. The essential steps are:


#### **2.2 Hydrothermal route**

The hydrothermal or solvothermal method consists of preparing an aqueous solution containing the reagents dissolved totally or partially. The aqueous solution is transferred either into a Teflon autoclave, both enclosed in metal autoclave.

The preparation in the autoclave is brought to a temperature between 373 and 573 K maintained for a few days in order to obtain single crystals. The maximum temperature is imposed by the resistance of the material constituting the Teflon.

*Note*: In this chapter, structures have been determined using X-ray diffraction (on single crystal or on powder). Electrical measurements are carried out using often complex impedance spectroscopy.

#### **2.3 Experimental techniques**

In this chapter, the structural studies of the studied materials were carried out by X-ray diffraction on single crystals or in some cases X-ray powder diffraction.

Electrical measurements are often performed using the complex impedance spectroscopy technique.

#### **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.

**13**

*Cobalt Phosphates and Applications*

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

the cobalt ion has an oxidation degree of +III.

*Projection of CoPO4 structure along the (a) c axis and (b) b axis.*

**4. Monovalent metal cobalt phosphate**

tetrahedra are isolated from each other.

molar ratio.

**Figure 1.**

**4.1 Monophosphates**

CoPO4 [1] material, like FePO4 structure, is usable in the manufacture of Li-ion

There are more than 40 monovalent cation cobalt phosphates. The monovalent

This family is known as orthophosphate or also monophosphate; it is character-

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

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

3−

metal cobalt phosphates will be classified according to the oxygen/phosphor

ized by its high stability compared to other phosphates. In the structure, (PO4)

*B* = P cations 1/8 of the available tetrahedral P sites of PO4 tetrahedra.

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,

*Cobalt Compounds and Applications*

**2.1 Solid-state reaction method**

**2.2 Hydrothermal route**

often complex impedance spectroscopy.

**2.3 Experimental techniques**

spectroscopy technique.

**3. Cobalt phosphate**

high pressure.

**2. Synthesis methods and experimental techniques**

or polycrystalline materials. The essential steps are:

volatile compounds (NH3, H2O, CO2, etc.).

(usually porcelain, alumina, or platinum crucibles).

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

Solid-state reaction route is the most adopted method to prepare single crystals

• Mixing and grinding solid reagents and placing the mixture in a container

• Calcination: a first heat treatment at 573–673 K for a few hours to remove the

• Grinding another time the remaining mixture to homogenize and reduce the size of the particles which will increase the contact area between the grains.

• Second heat treatment by gradually increasing the temperature to a so-called "pasty" state of the mixture (partially melted mixture). Maintain this temperature for a few days, and then slowly lower it to room temperature.

The hydrothermal or solvothermal method consists of preparing an aqueous solution containing the reagents dissolved totally or partially. The aqueous solution is transferred either into a Teflon autoclave, both enclosed in metal autoclave. The preparation in the autoclave is brought to a temperature between 373 and 573 K maintained for a few days in order to obtain single crystals. The maximum temperature is imposed by the resistance of the material constituting the Teflon. *Note*: In this chapter, structures have been determined using X-ray diffraction (on single crystal or on powder). Electrical measurements are carried out using

In this chapter, the structural studies of the studied materials were carried out by X-ray diffraction on single crystals or in some cases X-ray powder diffraction. Electrical measurements are often performed using the complex impedance

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.

**12**

**Figure 1.** *Projection of CoPO4 structure along the (a) c axis and (b) b axis.*

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.
