Anodic ZnO-Graphene Composite Materials in Lithium Batteries

Herrera-Pérez Gabriel, Pérez-Zúñiga Germán, Verde-Gómez Ysmael, Valenzuela-Muñiz Ana María and Vargas-Bernal Rafael

### Abstract

An important area to cope with in the implementation of technologies for the generation of energy from renewable sources is storage, so it is a priority to develop new ways of storing energy with high efficiency and storage capacity. Experimental reports focused on ZnO-graphene composite materials applied to the anode design which indicated that they show low efficiencies of around 50 %, but values very close to the theoretical capacity have already been reported in recent years. The low efficiency of the materials for the anode design of the Li-ion battery is mainly attributed to the pulverization and fragmentation of the material or materials, caused by the volumetric changes and stability problems during the charge/discharge cycles. In this chapter, we will discuss the development of composite materials such as ZnOgraphene in its application for the design of the anode in the Li-ion battery.

Keywords: ZnO, batteries, graphene, Li-ion, composites, ZnO-Graphene

### 1. Introduction

The search for new and efficient energy storage systems has been mainly aimed at batteries, which have positioned themselves as one of the best options for this purpose [1]. The development and innovation in such systems has been slow compared to other technologies since the relevant innovations have taken even centuries, from the invention of the battery in 1800 by Alessandro Volta [2], then the first lead-acid batteries partially rechargeable in 1860 by Gaston Planté [3], up to the lithium-ion batteries, which Sony introduced to the market in 1991 [4].

The batteries can be classified as primary (not rechargeable) and secondary (rechargeable). The classification of the batteries is made in relation to the active materials, in the components of the battery. At present, there are still few batteries with the ability to have reversible electrochemical reactions. Table 1 shows some of the main physical and chemical properties of current commercial batteries. The Liion batteries, clearly positioned today, as one of the best options for energy storage, are far above the other batteries in terms of the number of cycles.

### 1.1 Overview of batteries

The first Li-based batteries used LiCoO2 cathode, anode carbon, and LiPF6 electrolytes, with a capacity of around 140 mAh and 3.7 V and an efficiency of 50 %,


temperature. The theoretical capacity of a cell is determined by the amount of active materials; this expresses the amount of electricity involved in the electrochemical reaction, which can be expressed in terms of coulombs or

Within the characteristics of the electrodes, two phenomena mainly occur: diffusion and adsorption. The diffusion phenomenon happens in the cathode, and the carbon that is commonly used in the anode presents a phenomenon of adsorption. Nowadays the diversity of existing materials that are used or that are proposed for the electrodes and the great majority are governed by diffusion phenomena or

The first Li-ion batteries had been based on components as LiCoO2 electrodes in

A wide variety of materials has been used for the cathode since the introduction of the first Li-ion batteries. Specifically, Li-ion batteries are not governed by the chemical potentials of the materials involved, so the Li/Li<sup>+</sup> potential for both electrodes is taken as a reference. This means that the material of the cathode that will interact with the Li must have a high potential (+) in relation to that of Li/Li<sup>+</sup> [8, 9]. One of the most significant innovations that was made in the first batteries was the replacement of the Co, which was expensive and also toxic, being replaced by the Mn, which significantly decreased the price and at the same time increased the power capacity to 250 mAhg<sup>1</sup> and a voltage of 4.6–2.5 V [6]. After the innovation that occurred with the LiMn2O4, new proposals like LiFePO4 [10] were developed, and recently high ionic conductivity systems were investigated, in which the S is

in a composite material (CM) in which both phenomena are involved [9].

mainly involved, which has a theoretical capacity of 1675 mAhg<sup>1</sup> [6].

problems when metallic Li is used as an anode [5, 8].

The graphite that was used in the first Li-ion batteries had a capacity that was theoretically limited to 372 mAhg<sup>1</sup> [4], and as a solution to these limitations, various materials have emerged such as Si, Sn, Sb, Ge, and new forms allotropic carbon [11]. The materials used at the anode, unlike the cathode, must have a potential no greater than that of Li/Li<sup>+</sup> and less than the potential of the cathode. That is, the materials that are used as electrodes should not have a greater potential than Li/Li<sup>+</sup> since the Li-ions could be reduced by forming metallic Li and in certain circumstances dendrites that considerably reduce the cycles and especially generate problems of safety since they can cause short circuit, which is one of the main

In recent years, transition metal oxides (TMO) have been used, such as Ni, Sn, Mn, and Zn, among others. In a fully charged Li battery, the anode contains an excess of Li-ions that have a chemical potential to diffuse through the electrolyte and into the vacancies of the cathode structure [5]. Figure 2 shows a representative diagram of a charging and discharging process for a Li battery, and as can be seen

the cathode and carbon in the anode. The development and innovation in the components of a rechargeable battery have considerably improved characteristics, such as the capacity of gravimetric and volumetric power, which has led to the development of batteries with equal or greater power but of lower weight

ampere/hour [8].

and volume [6].

1.2.1 Cathode

1.2.2 Anode

115

1.2 Materials for the design of electrodes

DOI: http://dx.doi.org/10.5772/intechopen.86169

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

Table 1.

General characteristics of the most commonly used commercial batteries [5].

Figure 1. Schematic of a Li-ion cell.

which was relatively low. The innovations that followed in the Li-ion batteries revolved around the three main components that are the anode, cathode, and electrolyte, in order to improve the characteristics and problems that currently accompany this technology, such as the capacity of gravimetric and volumetric power, safety, cost, efficiency, and the search for materials that do not harm the environment [4, 6, 7].

A Li-ion battery is a set of cells, connected in series or in parallel in Figure 1. A diagram of the main components of a Li cell is shown, which are a negative electrode or anode and the positive electrode or cathode and between them a Li-ion conductive electrolyte. The principle of the operation of a cell is based on the transport of Li-ions between the two electrodes, capable of storing Li [1].

During the charge and discharge processes, the electrons flow from the anode to an external load and to the cathode in a discharge process, through an oxidation reaction at the anode and a flow of Li-ions from the anode to the cathode through the electrolyte. In a charging process, the flow of electrons is from the cathode to a source and to the anode, followed by a reduction reaction at the cathode and a flow of Li-ion to the anode through the electrolyte, forming a circuit. The collectors of charge or substrates that are used regularly are copper and aluminum for the anode and cathode, respectively [8].

The standard potential of a cell is determined by the type of active materials in the cell and can be calculated in relation to the Gibbs free energy or obtained experimentally. This potential can be calculated from the standard potential of the electrodes in the following way:

anode oxi ð :potentialÞ þ cathode red ð :potentialÞ ¼ standard cell potential (1)

The oxidation potential is the negative value of the reduction potential; it is worth mentioning that it is dependent on other factors such as concentration and temperature. The theoretical capacity of a cell is determined by the amount of active materials; this expresses the amount of electricity involved in the electrochemical reaction, which can be expressed in terms of coulombs or ampere/hour [8].

### 1.2 Materials for the design of electrodes

Within the characteristics of the electrodes, two phenomena mainly occur: diffusion and adsorption. The diffusion phenomenon happens in the cathode, and the carbon that is commonly used in the anode presents a phenomenon of adsorption. Nowadays the diversity of existing materials that are used or that are proposed for the electrodes and the great majority are governed by diffusion phenomena or in a composite material (CM) in which both phenomena are involved [9].

The first Li-ion batteries had been based on components as LiCoO2 electrodes in the cathode and carbon in the anode. The development and innovation in the components of a rechargeable battery have considerably improved characteristics, such as the capacity of gravimetric and volumetric power, which has led to the development of batteries with equal or greater power but of lower weight and volume [6].

### 1.2.1 Cathode

which was relatively low. The innovations that followed in the Li-ion batteries revolved around the three main components that are the anode, cathode, and electrolyte, in order to improve the characteristics and problems that currently accompany this technology, such as the capacity of gravimetric and volumetric power, safety, cost, efficiency, and the search for materials that do not harm the

Battery type Anode Cathode Electrolyte Voltage (V) Duration (cycles) Li-ion Graphite LiCoO2 LiPF6(nonaqueous) 3.7 >1000 Pb-acid Pb PbO2 H2SO4(aqueous) 2.1 <500 Ni-Cd Cd NiOOH KOH(aqueous) 1.2 2000 NMH Ti2Ni+TiNi NiOOH KOH(aqueous) 1.2 500–1000

General characteristics of the most commonly used commercial batteries [5].

Zinc Oxide Based Nano Materials and Devices

A Li-ion battery is a set of cells, connected in series or in parallel in Figure 1. A diagram of the main components of a Li cell is shown, which are a negative electrode or anode and the positive electrode or cathode and between them a Li-ion conductive electrolyte. The principle of the operation of a cell is based on the transport of Li-ions between the two electrodes, capable of storing Li [1].

During the charge and discharge processes, the electrons flow from the anode to an external load and to the cathode in a discharge process, through an oxidation reaction at the anode and a flow of Li-ions from the anode to the cathode through the electrolyte. In a charging process, the flow of electrons is from the cathode to a source and to the anode, followed by a reduction reaction at the cathode and a flow of Li-ion to the anode through the electrolyte, forming a circuit. The collectors of charge or substrates that are used regularly are copper and aluminum for the anode

The standard potential of a cell is determined by the type of active materials in

anode oxi ð :potentialÞ þ cathode red ð :potentialÞ ¼ standard cell potential (1)

The oxidation potential is the negative value of the reduction potential; it is worth mentioning that it is dependent on other factors such as concentration and

the cell and can be calculated in relation to the Gibbs free energy or obtained experimentally. This potential can be calculated from the standard potential of the

environment [4, 6, 7].

Schematic of a Li-ion cell.

Table 1.

Figure 1.

and cathode, respectively [8].

electrodes in the following way:

114

A wide variety of materials has been used for the cathode since the introduction of the first Li-ion batteries. Specifically, Li-ion batteries are not governed by the chemical potentials of the materials involved, so the Li/Li<sup>+</sup> potential for both electrodes is taken as a reference. This means that the material of the cathode that will interact with the Li must have a high potential (+) in relation to that of Li/Li<sup>+</sup> [8, 9].

One of the most significant innovations that was made in the first batteries was the replacement of the Co, which was expensive and also toxic, being replaced by the Mn, which significantly decreased the price and at the same time increased the power capacity to 250 mAhg<sup>1</sup> and a voltage of 4.6–2.5 V [6]. After the innovation that occurred with the LiMn2O4, new proposals like LiFePO4 [10] were developed, and recently high ionic conductivity systems were investigated, in which the S is mainly involved, which has a theoretical capacity of 1675 mAhg<sup>1</sup> [6].

### 1.2.2 Anode

The graphite that was used in the first Li-ion batteries had a capacity that was theoretically limited to 372 mAhg<sup>1</sup> [4], and as a solution to these limitations, various materials have emerged such as Si, Sn, Sb, Ge, and new forms allotropic carbon [11]. The materials used at the anode, unlike the cathode, must have a potential no greater than that of Li/Li<sup>+</sup> and less than the potential of the cathode. That is, the materials that are used as electrodes should not have a greater potential than Li/Li<sup>+</sup> since the Li-ions could be reduced by forming metallic Li and in certain circumstances dendrites that considerably reduce the cycles and especially generate problems of safety since they can cause short circuit, which is one of the main problems when metallic Li is used as an anode [5, 8].

In recent years, transition metal oxides (TMO) have been used, such as Ni, Sn, Mn, and Zn, among others. In a fully charged Li battery, the anode contains an excess of Li-ions that have a chemical potential to diffuse through the electrolyte and into the vacancies of the cathode structure [5]. Figure 2 shows a representative diagram of a charging and discharging process for a Li battery, and as can be seen

Figure 2. Reversible conversion reaction of an metallic oxide (MO) with Li.

during the lithiation process, the metallic oxide (MO) is reduced to its metallic state, inside a Li2O matrix according to the following reaction:

$$\text{MO}\_{\text{x}} + 2\text{xL}\text{i}^{+} + 2\text{xe}^{-} \leftrightarrow \text{M} + \text{xL}\text{i}\_{2}\text{O} \tag{2}$$

generally poor electrical conductors or in certain cases semiconductors, being one of the causes for which they are commonly used in CM, with carbonaceous materials that compensate for these deficiencies of conductivity and volumetric changes [5]. One of the MO that is being investigated as an anode is ZnO, which theoretically

The theoretical capacity of ZnO can be calculated as follows:

¼ ð Þ z � 96485 A s ð Þ =81:39 gð Þ¼ ¼ ð Þ z � 26801 mAh ð Þ =ð Þ¼ Mwð Þg

<sup>¼</sup> ð Þ <sup>3</sup> � 26801 mAh ð Þ <sup>=</sup>81:39gÞ ¼ <sup>987</sup>:87 mAhg�<sup>1</sup>

whereCt is the theoretical specific capacity, F is the Faraday constant, z is the number of electrons transferred from each structural unit, and Mw is the molecular weight [16]. The theoretical capacity of ZnO is one of the reasons why this material is interesting to be used as an anode in Li-ion battery. However, it is not only of interest because of its theoretical capacity, since it is also a material that, unlike the other transition metal oxides, has certain advantages such as high chemical stability, low cost, nontoxic, and relative ease of synthesis with a large number of methods

ZnO is a material of great interest in various research areas and a large number of technological applications, such as in ceramics, piezoelectric, transducers, chemical sensors, catalysis, optical applications, photovoltaics, and lithium batteries, among many others. For this reason, the interest in this material continues to be latent since it is also one of the few oxides that shows effects of quantum confine-

ZnO is a material with great versatility, in terms of synthesis methods and nanostructures, such as nanobars [20], nanosheets [25, 26], 3D nanostructures [27, 28], and nanocrystals [18, 29]. Within the synthesis methods, relatively complex methods are found such as the chemical vapor deposition (CVD) [30], Vapor Phase Transport (VPT) [31] for its acronym in English. These techniques are relatively expensive and complex, compared to others such as chemical precipitation [28, 32], sol-gel [23, 33–35], hydrothermal [29, 36–41], or solvothermal [19, 42, 43]. The Wurtzite phase that is defined by a hexagonal crystalline system is the most stable phase of ZnO at environmental conditions, but this ZnO can also be obtained in cubic phase forming the zinc blende; and using Si substrates and with high pressures, the rock of salt (NaCl) form is obtained. The structure of the orthorhombic unit cell of the main precursor phase of the Wurtzite, which is the Wülfingite, that chemically is Zn(OH)2, is presented in Figure 3a, b. For wurzite

Ct mAhg�<sup>1</sup> ¼ ð<sup>z</sup> � <sup>F</sup> Cmol�<sup>1</sup> <sup>=</sup> Mw gmol�<sup>1</sup> <sup>¼</sup>

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

DOI: http://dx.doi.org/10.5772/intechopen.86169

and a great variety of structures and morphologies [17–21].

; the charge and discharge reactions can be written as

Zn þ Li<sup>þ</sup> þ e� \$ LiZn (4)

(6)

ZnO þ 2Li<sup>þ</sup> þ 2e� \$ Zn þ Li2O (3)

ZnO þ 3Li<sup>þ</sup> þ 3e� \$ LiZn þ Li2O (5)

has a capacity of 987 mAhg�<sup>1</sup>

Complete reaction:

2.1 Zinc oxide

117

ment to certain particle sizes [22–24].

follows:

In the lithiation process, metal nanoparticles (M) embedded in a Li2O matrix are formed (this generates volumetric expansion), and in the first discharge the nanoparticles of metal (M) are oxidized again into smaller particles.

Graphene is a material that in recent years has attracted a lot of attention due to its exceptional properties and its potential application in many areas, being one of them in Li-ion batteries, where it is used in the negative electrode. However, it has been seen that it cannot be used in a pure manner, because after the first discharge, it presents great problems of reversibility, so it is common for it to be widely used with other materials in a composite, generally with MO [5, 12].

### 2. Materials for the design of the ZnO-graphene system

Currently, graphene is one of the most researched and promising materials to replace graphite in Li-ion battery anode. When using graphene in an CM, it is expected to take advantage of the properties of high conductivity and high surface area, and in the specific case when used with an MO, it works as a buffer of the volumetric changes that it undergoes in lithiation [13]. One of the pioneers in the study of the intercalation of Li in various carbonaceous materials was Dahn et al. [14] where they concluded that graphite is limited to a certain amount of Li retention and that, in addition most of the processes of interaction with lithium, it is dominated by mechanisms of physical interaction.

Graphene, which is usually used as CM, usually comes from graphite and is obtained as GO. In several tests that have been carried out as anodes, irreversible capacities of up to 1250 mAhg�<sup>1</sup> have been obtained in the first cycle; this is due to the formation of a solid interface with the electrolyte (SEI), which is one of the main causes of the decrease in capacity, which at the same time is due to the high surface area [5].

Carbon/metal CMs are made with metals that are capable of forming alloys with Li or transition metal oxides (TMO), a term introduced by Tarascon et al. in the year 2000 and where they show several MO reporting reversible capacities of up to 700 mAhg�<sup>1</sup> [15]. One of the main disadvantages when using an MO is that they are Anodic ZnO-Graphene Composite Materials in Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.86169

generally poor electrical conductors or in certain cases semiconductors, being one of the causes for which they are commonly used in CM, with carbonaceous materials that compensate for these deficiencies of conductivity and volumetric changes [5].

One of the MO that is being investigated as an anode is ZnO, which theoretically has a capacity of 987 mAhg�<sup>1</sup> ; the charge and discharge reactions can be written as follows:

$$\text{ZnO} + 2\text{Li}^+ + 2\text{e}^- \leftrightarrow \text{Zn} + \text{Li}\_2\text{O} \tag{3}$$

$$\text{Zn} + \text{Li}^+ + \text{e}^- \leftrightarrow \text{LiZn} \tag{4}$$

Complete reaction:

during the lithiation process, the metallic oxide (MO) is reduced to its metallic

formed (this generates volumetric expansion), and in the first discharge the

nanoparticles of metal (M) are oxidized again into smaller particles.

with other materials in a composite, generally with MO [5, 12].

2. Materials for the design of the ZnO-graphene system

dominated by mechanisms of physical interaction.

surface area [5].

116

Figure 2.

In the lithiation process, metal nanoparticles (M) embedded in a Li2O matrix are

Graphene is a material that in recent years has attracted a lot of attention due to its exceptional properties and its potential application in many areas, being one of them in Li-ion batteries, where it is used in the negative electrode. However, it has been seen that it cannot be used in a pure manner, because after the first discharge, it presents great problems of reversibility, so it is common for it to be widely used

Currently, graphene is one of the most researched and promising materials to replace graphite in Li-ion battery anode. When using graphene in an CM, it is expected to take advantage of the properties of high conductivity and high surface area, and in the specific case when used with an MO, it works as a buffer of the volumetric changes that it undergoes in lithiation [13]. One of the pioneers in the study of the intercalation of Li in various carbonaceous materials was Dahn et al. [14] where they concluded that graphite is limited to a certain amount of Li retention and that, in addition most of the processes of interaction with lithium, it is

Graphene, which is usually used as CM, usually comes from graphite and is obtained as GO. In several tests that have been carried out as anodes, irreversible capacities of up to 1250 mAhg�<sup>1</sup> have been obtained in the first cycle; this is due to the formation of a solid interface with the electrolyte (SEI), which is one of the main causes of the decrease in capacity, which at the same time is due to the high

Carbon/metal CMs are made with metals that are capable of forming alloys with Li or transition metal oxides (TMO), a term introduced by Tarascon et al. in the year 2000 and where they show several MO reporting reversible capacities of up to 700 mAhg�<sup>1</sup> [15]. One of the main disadvantages when using an MO is that they are

MOx þ 2xLi<sup>þ</sup> þ 2xe� \$ M þ xLi2O (2)

state, inside a Li2O matrix according to the following reaction:

Reversible conversion reaction of an metallic oxide (MO) with Li.

Zinc Oxide Based Nano Materials and Devices

$$\text{ZnO} + \text{3Li}^+ + \text{3e}^- \leftrightarrow \text{LiZn} + \text{Li}\_2\text{O} \tag{5}$$

The theoretical capacity of ZnO can be calculated as follows:

$$\begin{aligned} \mathbf{C}\_{l} \text{(mAhg}^{-1}) &= (\mathbf{z} \times \mathbf{F} \text{ (Cmol}^{-1}) / (\text{M}\_{\text{w}} \text{(gmol}^{-1})) = \\ &= (\mathbf{z} \times \mathbf{96485} \text{ (A s)}) / \text{81.39 (g)} = \\ &= (\mathbf{z} \times \mathbf{26801} \text{ (mAh)}) / (\text{M}\_{\text{w}} \text{(g)}) = \\ &= (\mathbf{3} \times \mathbf{26801} \text{ (mAh)}) / \text{81.39g}) = \mathbf{987.87} \text{ (mAhg}^{-1}) \end{aligned} \tag{6}$$

whereCt is the theoretical specific capacity, F is the Faraday constant, z is the number of electrons transferred from each structural unit, and Mw is the molecular weight [16].

The theoretical capacity of ZnO is one of the reasons why this material is interesting to be used as an anode in Li-ion battery. However, it is not only of interest because of its theoretical capacity, since it is also a material that, unlike the other transition metal oxides, has certain advantages such as high chemical stability, low cost, nontoxic, and relative ease of synthesis with a large number of methods and a great variety of structures and morphologies [17–21].

### 2.1 Zinc oxide

ZnO is a material of great interest in various research areas and a large number of technological applications, such as in ceramics, piezoelectric, transducers, chemical sensors, catalysis, optical applications, photovoltaics, and lithium batteries, among many others. For this reason, the interest in this material continues to be latent since it is also one of the few oxides that shows effects of quantum confinement to certain particle sizes [22–24].

ZnO is a material with great versatility, in terms of synthesis methods and nanostructures, such as nanobars [20], nanosheets [25, 26], 3D nanostructures [27, 28], and nanocrystals [18, 29]. Within the synthesis methods, relatively complex methods are found such as the chemical vapor deposition (CVD) [30], Vapor Phase Transport (VPT) [31] for its acronym in English. These techniques are relatively expensive and complex, compared to others such as chemical precipitation [28, 32], sol-gel [23, 33–35], hydrothermal [29, 36–41], or solvothermal [19, 42, 43].

The Wurtzite phase that is defined by a hexagonal crystalline system is the most stable phase of ZnO at environmental conditions, but this ZnO can also be obtained in cubic phase forming the zinc blende; and using Si substrates and with high pressures, the rock of salt (NaCl) form is obtained. The structure of the orthorhombic unit cell of the main precursor phase of the Wurtzite, which is the Wülfingite, that chemically is Zn(OH)2, is presented in Figure 3a, b. For wurzite

form is presented in Figure 3c, d the Hexagonal crystalline system and that belongs to the space group P63mc, with the network parameters a = 0.35 nm and c = 0.52 nm [44]. Wurtzite is a material with a relatively simple atomic crystalline structure, in which it can be seen that each atom of Zn is surrounded by four atoms of O forming

a tetrahedron and likewise with each atom of O surrounded by four atoms of Zn

Figure 4 shows a relatively simple transformation of a metal hydroxide to a metal oxide, by dehydroxylation of Wülfingite only Wurtzite can be obtained; in the scientific literature, there is a very extensive information on the relative simplicity of this transformation, this time a temperature of 600 °C is a maximum referent, but it is known that depending on the synthesis techniques this temperature can be reduced to promote a smaller crystal growth, as well as contribute to decrease the possible secondary processes at low temperatures this is between about

The interest in ZnO and its application as an anode in lithium-ion batteries is due

material as an anode in lithium batteries [46]. In addition, its theoretical capacity puts it ahead of other transition metals such as Cu, Ni, Fe, or Ti, and even on the Sn of group IV, where there are elements such as Ge and Si of greater theoretical capacities but that have certain disadvantages in terms of their relatively complex

one of the big problems is that in a few cycles (10 cycles), it loses its capacity until

process (400 %) [47]. Similarly, ZnO undergoes volumetric changes of 228 %, which is reflected in a decrease in its efficiency and capacity to 400 mAhg<sup>1</sup>

however, the ZnO has the novelty of having a wide variety of synthesis processes, morphologies, and structures that could help solve these problems [20, 48] as

chosen to perform CM of ZnO with a great variety of carbonaceous materials, graphite [25], nanotubes [21], or graphene [32, 35, 49, 50]. When using carbonaceous materials, two problems with ZnO are compensated: the low electrical con-

ductivity and as a buffer for the volumetric changes of ZnO [48, 50].

The solution to these problems has motivated several research groups that have

Graphene is a crystalline, two-dimensional (2D) material of a single atom in thickness, with covalent bonds σ, sp<sup>2</sup> between carbon and carbon, with a junction

Crystal structure of (a) graphite and (b) after the oxidation process, a structure equivalent to the GO.

, due to the great volumetric expansion it undergoes in the lithiation

) [25], this with respect to graphite that

, which is the most commonly used

); however,

;

forming tetrahedral indicating the one-to-one ratio Zn:O [24, 45].

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

DOI: http://dx.doi.org/10.5772/intechopen.86169

synthesis processes and in the case of Ge, rarity and toxicity [47].

Si is the material with the highest theoretical capacity (4200 mAhg<sup>1</sup>

90 and 250 °C.

200 mAhg<sup>1</sup>

shown in Figure 4.

2.2 Graphene

Figure 5.

119

to its great theoretical capacity (987 mAhg<sup>1</sup>

has a theoretical capacity of around 372 mAhg<sup>1</sup>

### Figure 3.

(a) Unitary cell of ε-Zn(OH)2, (b) representation of polyhedra of the unitary cell of the wülfingite, (c) unitary cell of ZnO, and (d) representation of polyhedra of the unit cell of the wurtzite.

Figure 4. Thermal transformation of zinc hydroxide (wülfingite) for zinc oxide (wurtzite).

### Anodic ZnO-Graphene Composite Materials in Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.86169

a tetrahedron and likewise with each atom of O surrounded by four atoms of Zn forming tetrahedral indicating the one-to-one ratio Zn:O [24, 45].

Figure 4 shows a relatively simple transformation of a metal hydroxide to a metal oxide, by dehydroxylation of Wülfingite only Wurtzite can be obtained; in the scientific literature, there is a very extensive information on the relative simplicity of this transformation, this time a temperature of 600 °C is a maximum referent, but it is known that depending on the synthesis techniques this temperature can be reduced to promote a smaller crystal growth, as well as contribute to decrease the possible secondary processes at low temperatures this is between about 90 and 250 °C.

The interest in ZnO and its application as an anode in lithium-ion batteries is due to its great theoretical capacity (987 mAhg<sup>1</sup> ) [25], this with respect to graphite that has a theoretical capacity of around 372 mAhg<sup>1</sup> , which is the most commonly used material as an anode in lithium batteries [46]. In addition, its theoretical capacity puts it ahead of other transition metals such as Cu, Ni, Fe, or Ti, and even on the Sn of group IV, where there are elements such as Ge and Si of greater theoretical capacities but that have certain disadvantages in terms of their relatively complex synthesis processes and in the case of Ge, rarity and toxicity [47].

Si is the material with the highest theoretical capacity (4200 mAhg<sup>1</sup> ); however, one of the big problems is that in a few cycles (10 cycles), it loses its capacity until 200 mAhg<sup>1</sup> , due to the great volumetric expansion it undergoes in the lithiation process (400 %) [47]. Similarly, ZnO undergoes volumetric changes of 228 %, which is reflected in a decrease in its efficiency and capacity to 400 mAhg<sup>1</sup> ; however, the ZnO has the novelty of having a wide variety of synthesis processes, morphologies, and structures that could help solve these problems [20, 48] as shown in Figure 4.

The solution to these problems has motivated several research groups that have chosen to perform CM of ZnO with a great variety of carbonaceous materials, graphite [25], nanotubes [21], or graphene [32, 35, 49, 50]. When using carbonaceous materials, two problems with ZnO are compensated: the low electrical conductivity and as a buffer for the volumetric changes of ZnO [48, 50].

### 2.2 Graphene

form is presented in Figure 3c, d the Hexagonal crystalline system and that belongs to the space group P63mc, with the network parameters a = 0.35 nm and c = 0.52 nm [44]. Wurtzite is a material with a relatively simple atomic crystalline structure, in which it can be seen that each atom of Zn is surrounded by four atoms of O forming

Zinc Oxide Based Nano Materials and Devices

(a) Unitary cell of ε-Zn(OH)2, (b) representation of polyhedra of the unitary cell of the wülfingite, (c) unitary

cell of ZnO, and (d) representation of polyhedra of the unit cell of the wurtzite.

Thermal transformation of zinc hydroxide (wülfingite) for zinc oxide (wurtzite).

Figure 3.

Figure 4.

118

Graphene is a crystalline, two-dimensional (2D) material of a single atom in thickness, with covalent bonds σ, sp<sup>2</sup> between carbon and carbon, with a junction

angle of 120° and a length of a = 0.14 nm, resulting in a hexagonal honeycomb lattice, which can be simplified to a trigonal unit cell with two atoms per unit, as shown in Figure 5.

Several authors such as Kundu et al. [20] and Liu et al. [48] obtained from chemical precipitation at normal conditions nanobars and ZnO nanoparticles, respectively. According to these same authors, they obtained the nanoparticles or nanobars individually or separately, and in the case of Lui et al. [48], they

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

DOI: http://dx.doi.org/10.5772/intechopen.86169

performed an extra process of carbon coating or as Giri et al. [28] who performed a hydrothermal procedure to coat the nanoparticles. On the other hand, Ramadoss et al. [32] obtained an CM where they precipitated nanoparticles of ZnO in situ, on

The method or technique of hydrothermal synthesis is one of the most used for the synthesis of ZnO, and with the same principle of the technique of chemical precipitation, this technique gets its name because the conditions of synthesis, which is carried out in water and temperatures above 25 °C up to 430 °C, reaching autogenous pressures of over 221 bar at 375 °C, which is the critical point where the liquid phase and gas are not distinguished from water [56]. This type of synthesis is also governed by the precipitation equation but adding temperature and

In various works such as that of Alver et al. [40], they synthesized ZnO doped with boron by the hydrothermal method and subsequently formed a nanocomposite with graphene using the same technique. Yoo et al. [41] with the same method obtained hemispherical nanoparticles of 25 nm. Wang et al. [57] synthesized ZnO in flower form, doped with Mn, with the hydrothermal method. Bøjesen et al. [36] conducted an in situ study of the growth of ZnO nanoparticles by hydrothermal synthesis. In this work, they conclude that temperature is one of the biggest factors that influences the size and shape characteristics of glass, this, without forgetting

The solvothermal synthesis method is a variant of the hydrothermal one, since, in this technique, solvents different to water are used, but with the same principles of hydrothermal synthesis and governed by the precipitation equation. The said solvents can be alcohols, acids, bases, or mixtures, since, with this, a greater dissolution or changes in the pressures generated in the synthesis are sought [56]. With the solvothermal synthesis technique, authors such as Wi et al. [19] and Wang et al. [42] have obtained nanoparticle agglomerates, of spherical shape and porous sheet type, respectively. In the case of Wang et al. [42], the synthesis process was carried

out at room temperature reaching thicknesses of up to 10 nm in the sheets.

On the other hand, the sol-gel synthesis technique consists of a chemical synthesis in which, from a colloidal solution or "sol", small precipitates of a solid phase gradually form inside a continuous network called "gel". The peculiarity of the technique is the nanometric size of the particles that can be obtained by this technique as shown in the work of Spanhel et al. [32], who obtained ZnO

nanoparticles of colloidal sizes between 3 and 6 nm. Hjiri et al. [33] obtained sizes between 20 and 40 nm for ZnO and up to 3 nm for ZnO doped with Al. Li et al. [34] used the technique to obtain in situ a nanocomposite of nanoparticles deposited in

sheets of graphene, whose reported particle size was an average of 9 nm.

graphene sheets.

pressure variables.

other factors such as time.

3.3 Solvothermal method

3.4 Sol-gel

121

3.2 Hydrothermal method

Graphene can have other allotropic forms at the same time, since it is the basic building structure of the other forms such as graphite (3D), nanotubes (1D), and fullerenes (0D). In all its structures, it has very good properties of thermal, electrical, and other properties such as mechanical, optical, and magnetic properties. The movement of electrons through a sheet of graphene is governed mainly by the relativistic law of Dirac. For a perfect sheet of graphene, it is estimated that its conductivity reaches 200,000 cm<sup>2</sup> /Vs; however, this intrinsic property is greatly affected by defects that generate dispersion centers generated mainly by substrates, dopants, and quiralities.

Since the nineteenth century, oxidized graphite has been produced by different methods: the Brodie method in 1859 [51], the Hummers method in 1958 [52], and the Hummers method modified in July 2004; and in October of the same year, Andre Geim and Kostya Novoselov, professors at the University of Manchester, London, managed to isolate a sheet of graphite a few atoms thick, by mechanical exfoliation, from which they discovered the exceptional properties of this material that has attracted enormous attention since then [53]. However, the synthesis of graphene remains a great challenge, since, to be considered high purity, it is necessary to take into account certain characteristics such as the quality, size, quantity, complexity, and control of the synthesis method. For this reason, it is very important to know the different methods, degrees of purity, and qualities expected for graphene [54]. In the work presented by Hirata et al. [55], they obtained sheets of several nanometers thick and 20 μm wide on average. In the same work, they propose several categories of graphene: oxidized graphene, reduced graphene, functionalized graphene, and graphene in its pure state.

### 3. Methods of synthesis of ZnO

The most used synthesis processes for ZnO are chemical precipitation at normal conditions, synthesis by hydrothermal, solvothermal techniques, and the sol-gel method. The advantage and novelty of these techniques is the obtaining of a great diversity of geometries, with relatively simple processes and with very accessible precursors.

### 3.1 Chemical precipitation at normal conditions

The technique is used specifically for ZnO, part of a Zn precursor, normally a salt or inorganic compound, soluble in water commonly and as a reducing agent either an acid or a base, which, when reacting in solution with the salt, will form a precipitate or solid of insoluble that will commonly require a calcination process for crystallization. This type of reactions could be expressed in a general equation in the following way:

$$\text{AC} + \text{BD} \to \text{AD}\_{(\text{insolvable})} + \text{BC} \tag{7}$$

This type of reactions usually occurs between ionic compounds where one of the products is insoluble, and because each component exchanges pairs, these types of reactions are usually called double-displacement reactions.

Several authors such as Kundu et al. [20] and Liu et al. [48] obtained from chemical precipitation at normal conditions nanobars and ZnO nanoparticles, respectively. According to these same authors, they obtained the nanoparticles or nanobars individually or separately, and in the case of Lui et al. [48], they performed an extra process of carbon coating or as Giri et al. [28] who performed a hydrothermal procedure to coat the nanoparticles. On the other hand, Ramadoss et al. [32] obtained an CM where they precipitated nanoparticles of ZnO in situ, on graphene sheets.
