# A Review of High-Energy Density Lithium-Air Battery Technology: Investigating the Effect of Oxides and Nanocatalysts

Authors: Asep Suryatna; Indah Raya; Lakshmi Thangavelu; Firas Rahi Alhachami; Mustafa M. Kadhim; Usama S. Altimari; Zaid H. Mahmoud; Yasser Fakri Mustafa; Ehsan Kianfar
Journal: Journal of Chemistry
 (2022)

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## Abstract

In vehicles that require a lot of electricity, such as electric vehicles, it is necessary to use high-energy batteries. Among the developed batteries, the lithium-ion battery has shown better performance. This battery has an energy density of 10 equal to that of a lithium-ion battery and uses air oxygen as the active material of the cathode and anode like a lithium-ion battery made of lithium metal. The cathode used in these batteries must have special properties such as strong catalytic activity and high conductivity, and nanotechnology has greatly helped to improve the materials used in the cathode of lithium-air batteries. The importance of proper catalyst distribution and the relationship between the oxide product and the catalyst and the indirect effect of the ORR catalyst on the OER reaction is not present in the fuel cell. The maximum capacity of lithium-air battery theory using graphene under optimal electron conduction conditions and the experimental maximum obtained for graphene by optimizing the structure geometry, examples of structural engineering using carbon fiber and carbon nanotubes in cathode fabrication with the ability to perform the reaction properly while providing space for lithium oxide placement, are examined. This article describes the mechanism of this battery, and its components are examined. The challenges of using this battery and the application of nanotechnology to solve these challenges are also discussed.


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## Body

## 1. Introduction

The next generation battery, according to many researchers, is a lithium-ion battery, because this battery has a very high-energy density compared to a lithium battery (lithium ion) [1, 2]. This feature will transform many industries, including the electric vehicle industry, as high-energy densities enable electric cars to travel much longer distances with each charge. Figure 1 shows a comparison of energy density of different types of batteries and the mileage of the car that can be traveled after charging different batteries. In lithium-ion batteries, what limits the energy and capacity of the battery is the cathode, which has a capacity of less than 300 mAh/g. On the other hand, there are anodes with high capacities such as silicon (with a capacity of 4000 mAh/g). To solve this problem, we have to look for cathodes other than intercalation cathodes.Figure 1
(a) Comparison of energy density of different types of batteries. (b) Thorough-paced distance of the car after charging for different batteries [3–5].
(a)(b)The exhaustion of fossil oil resources and serious global warming issues, accompanied by quickly developing portable electronic devices, have forced us to look for sustainable, clean, and renewable energy storage systems. There has been substantial interest in wind and solar energy; however, these types of energy sources have random and variable energy output, which makes them difficult to manage. Electrochemical energy storage systems with high-energy density, simplicity, and reliability are expected to have a critical role in clean energy generation and related applications. Up to now, lithium-ion batteries (LIBs) exhibit the best performance among current batteries, with their high theoretical energy density. To date, LIBs have dominated the portable electronic market because of their attractive performance in terms of a long life cycle and rate capability. On the other hand, the battery has a limited energy density owing to electrode materials that have intercalation chemistry. Although LIBs can power electrical vehicles up to a driving range of 160 km on a single charge, that is still far from practical applications. Metal (Zn, Al, Fe, Mg, Ca, and Li)-air/O2 batteries have attracted much attention owing to their relatively high-energy density because the cathode can obtain oxygen readily from the surrounding environment as a reactant. Among all kinds of metal-air battery systems, lithium (Li)-air batteries (LABs) show the highest specific energy with respect to the anode (13,000 Wh kg −1), which is highly comparable to that of gasoline (13,200 Wh kg −1). The first LABs were introduced by Littauer and Tsai in 1976. Because of the undesirable reaction of Li with water through the use of aqueous electrolytes, progress was limited. LABs started to attract more attention after Abraham and Jiang presented a successful secondary LAB in the 1996 based on a nonaqueous electrolyte (polyacrylonitrile-based polymer electrolyte). In 2008, the recharge ability of the system was explored by Bruce [6]; those pioneering works led to intensive research in the field of LieO2 batteries all over the world. In particular, IBM and its partners launched the Battery 500 project to evaluate LABs for automotive applications. Bruce et al. estimated the practical specific energy of LABs to range from 500 to 900 Wh kg −1, which is much advanced compared with that of alternative rechargeable batteries [7]. Compared with current LIBs, the specific energy of LABs is at least enhanced by two to three times and would be sufficient to deliver a driving range of more than 550 km.Two important cathodes, which have been proposed as alternatives to incoming cathodes, are sulfur and oxygen. Apart from the capacity advantage, sulfur and oxygen are less expensive compared to the elements cobalt, manganese, etc. [8–10] found in traditional cathodes. These types of batteries are very different from lithium-ion batteries in terms of internal operation mechanism and limitations and are therefore known by another name. For example, in this type of battery, the cathode is a porous structure through which oxygen is transferred and energy generation reactions are performed, and unlike lithium ion batteries, lithium ions are not stored in it.The energy of these batteries is due to the oxidation of lithium and the reduction of oxygen and varies depending on the type of aqueous or organic electrolyte, the type of reactions, and the mechanism of action. Figure2 shows the types of cathode reactions from left to right for lithium-ion, lithium-air with organic electrolyte, lithium-air with aqueous electrolyte, and lithium-sulfur batteries, respectively. In all four types of batteries, the anode reaction is the same (during the oxidation reaction, the lithium atom is converted to an ion).Figure 2
The reactions performed at the cathode for different types of new generation batteries [3–5].The mechanism of action of the lithium-air battery is shown in Figure3. The cathode has a porous structure and is often made of carbon, through which oxygen reaches the reaction site. Lithium ions are transported through the electrolyte and electrons through the cathode, followed by an oxygen reduction reaction (Figure 2).Figure 3
Charge, discharge, overall reaction, and voltage theory of lithium-air batteries [11].In this type of battery, the cathode electrode needs to be porous to provide oxygen penetration, to be conducive to provide electron transfer, and not to react itself, but only to act as a conductor of oxygen and electrons. Due to these factors, porous carbon is one of the options used, which has a low price [12–14].Today, due to the problems of carbon cathodes, new research papers using nanotechnology and nanocarbon cathodes have been proposed that provide better performance that is addressed. These batteries are very similar to fuel cells, because in the fuel cell, oxygen and hydrogen are supplied from an external source to perform the electrochemical reaction, and here, too, oxygen is supplied through an external source, which is air. Thus, like fuel cells, they have high-energy densities and low power (as shown in the Ragone curve for fuel cells in early battery articles) [15–17].Compared to fuel cells, these batteries can have advantages: hydrogen is more dangerous than lithium, hydrogen storage is difficult, and in the end, the fuel cells are not rechargeable, while some types of lithium-air batteries (organic electrolyte type) have the ability to recharge, but these types of batteries have to overcome many problems in order to be commercialized [18, 19].For example, the capacity of these batteries is much lower than the theoretical capacity. They also have a cycle life of less than 30 cycles, so nanotechnology is used to solve these problems as we look at it. Organic electrolyte lithium-ion batteries have a higher status than water due to their safety and recharge ability. In these batteries, the capacity limiter is the cathode, not the anode [20, 21]. Although oxygen is supplied through an unlimited source of air, because the discharge reaction products (lithium oxide) are solid, they are electrically insulating, clogging cavities and preventing the transfer of oxygen, electrons, and ions.Therefore, before the lithium anode is depleted, oxygen transfer is limited or the thickness of the insulating oxide layer is so high that there is no electron conduction. These limitations and other problems, such as potentials and formation of lithium carbonate deposits, underscore the importance of research on the cathode [22, 23].A cavity cathode is installed that reacts with the entry of air and electrolytes. An important point about the cathode is the effective surface of these cavities. On the one hand, the more effective the surface, the more suitable places for electrochemical reaction and the higher the energy density and but increasing the surface by a large amount means a structure with smaller cavities and if the size of these cavities is very small and easily by-products The reduction reaction (Li2O2) is blocked and the battery performance decreases [18–20, 24–26].Therefore, it can be concluded that the dimensions of the cavities in the design of the cathode should be such that the state optimal of effective increase surface and the size of the cavities to prevent the obstruction of the cavities in the early stages [27, 28]. On the other hand, the excessive size of the surface causes a relatively thick layer to be deposited on the electrode, which is insulating, prevents the transfer of load to the appropriate location, and impairs battery performance. From the N2 adsorption–desorption isotherms and pore-size distribution, the specific surface area and size of the pores of porous NWs were, respectively, 87.47 m2 g−1 and 5–10 nm. In the electrochemical analysis, the hybrid Co3O4 NWs/carbon black (Super P) inherited excellent catalytic activity compared with other catalysts including pristine Super P and Co3O4 nanoparticles (NPs)/Super P. In the first discharge-charge curves, the charging potential of Co3O4 NWs or NPs exhibited lower voltage than that of the pure carbon cathode. The first discharge capacity at a current density of 100 mA g−1 of Co3O4 NWs/Super P was 11,160 mAh g−1, which was higher than pure Super P (7205 mAh g−1) and NP-based Co3O4 NPs/Super P (9764 mAh g−1). Most importantly, the durability of the cycling performance increased from 32 to 61 cycles when Co3O4 NPs were doped on SP, and further increased to 73 cycles for Co3O4 NWs/Super P (Figures 4(d)–4(i)). The results clearly indicated that NWs had superior catalytic activity and stability in oxygen electrochemical reactions [29]. Due to this point, the importance of nanotechnology in preparing the appropriate cathode structure is determined [30, 31].Figure 4
(a) Scanning electron microscopy, (b) transmission electron microscopy, and (c) high-resolution transmission electron microscopy images of discharge-charge curves at a discharge depth of 1000 mAh g−1 at a current density of 100 mA g−1. (d) Super P (SP), (e) Co3O4 nanoparticles (NPs)/SP, (f) Co3O4 nanowires (NW)/SPs, cycling performances of (g) Super P (SP), (h) Co3O4 NPs/SP, and (i) Co3O4 NW/SP [29].
(a)(b)(c)(d)(e)(f)Unfortunately, conventional carbon cathodes do not have proper cavity dimensions and distribution. This draws researchers’ attention to important carbon nanostructures such as carbon nanotubes and graphene. Figure5 compares the discharge and charge curves of different cathodes. The lower curves are for discharge and the upper curves are for charging. The figure illustrates the charge and discharge curves for the graphene cathode (graphene nanosheets) (“a”) and for the two types of black carbon cathodes (“b” and “c”). As this figure shows, the capacity of a graphene cathode is much higher than that of black carbon cathodes. Although it can be seen that the discharge capacity of graphene is much higher than that of black carbon, the charge curve for graphene is still very low, as for other carbons [32–34]. In fact, using nanomaterial alone is not enough to get the right result. Instead, optimizing the use of nanomaterial, including its geometry, using a suitable catalyst and so on is among the variables that should be considered. Graphene and carbon nanotubes are two materials with high electrical conductivity and good mechanical strength with catalytic properties (in case of defects) [35, 36]. These materials, if used at the cathode, improve battery performance. However, according to previous articles, the use of these materials alone is not enough, but the optimal use of these materials (to provide the structure of a cathode with proper performance) is more important [37–39]. Fortunately, because these types of structures have atomic dimensions and are flexible, the cathode structure can be built with the precision of engineering these atomic components. In other words, with these components, the geometric structure of the cathode was properly adjusted for the proper delivery of oxygen and the placement of Li2O2 (without clogging the cavities) and other factors [40–42].Figure 5
Comparison between charge and discharge curves of graphene (red) and conventional carbon cathodes [5].


## 2. Types of Lithium-Air Batteries

Because the type of electrolyte has a major effect on the reactions and performance of the lithium-air battery, in this type of batteries, classification is based on the type of electrolyte. These batteries include four types of electrolytes: aqueous, organic, hybrid, and solid-state [11, 43–52]. Figure 6 shows the types of these batteries based on electrolytes.Figure 6
The types of lithium-air batteries according to the type of electrolyte [3, 4].
(a)(b)(c)(d)


### 2.1. Batteries with Aqueous Electrolytes

The basis of this group of electrolytes is lithium salts dissolved in water. Due to the strong reaction of lithium with water, safety precautions must be taken [53–56]. For this purpose, an intermediate layer of ceramic is usually formed between the anode (lithium) and the electrolyte [57, 58]. The environment of these types of electrolytes can be acidic or alkaline [59].The basic reaction in this type of battery is as follows:(1)2Li+12O2+H2O↔2LiOH.In this type of battery, due to the solubility of the reaction product, there is no problem with organic electrolytes [60–62]. However, the disadvantage of this type of battery is that the electrolyte is consumed during the reaction (according to the above reaction, some water is used to form lithium hydroxide) [63, 64]. These batteries are known as primary batteries, so much research has focused on rechargeable organic electrolyte batteries. Therefore, our discussion also focuses on the effect of nanotechnology on batteries with organic electrolytes [65,66]. The efficiency of different aqueous lithium-air batteries is summarized in Table 1.Table 1
Performance of aqueous lithium-air batteries.
Solid electrolyteAnolyteCatholyteAir electrodeCurrent density (mA cm−2)Area capacity (mAh cm−2)Energy efficiency (%)Ref.LATPLiPONSat. LiOH0.10.290 (1st)-90 (100th)[99]LATP1 M LiClO4 in EC/DECSat. LiOH−10 M LiClKB/RuO20.645.860 (1st)-1.53 (8th)[100]LATP1 M LiClO4 in EC/DEC1 M LiNO3-0.5 M LiOHGNS0.51.070 (1st)-60 (50th)[101]LATPLiFSI-2G4Sat. LiOH-10 M LiClKB/RuO20.641270 (1st)-50 (12th)[102]LANTP4 M LiFSI in DME10 M LiCl-1.5 M LiOHMnO20.253.263 (1st)-51 (30th)[23]LAGTP4.5 M LiFSI10 M LiCl−1.5 M LiOHMnO21.01.085 (1st)-85 (10th)[103]LLTOLiPON1 M LiNO3MnO20.50.579 (1st)-54 (40th)[104]LATP0.1 M LiPF6 in EC/DEC0.1 M H3PO4 1 M LiH2PO3C/Pt0.50.2472 (1st)-67 (20th)[105]LATP1 M LiPF6 in EC/DEC0.1 M H3PO4 -1 M LiH2PO4Pt/C–IrO21.00.582 (1st)[67]LATPPEO18LiTFSI −10 wt%(HAc (90%) BaTiO3–H2O)-LiAcKB0.50.250 (1st)-70 (15th)[67]


### 2.2. Batteries with Nonaqueous Electrolytes

The electrolytes of this group are nonaqueous solutions (organic, of protic type); this type of electrolytes is much safer than aqueous electrolytes due to the lack of reaction of lithium with water. Another advantage of this type of battery is the ability to recharge [68–75].The two reactions that lead to the production of energy in this type of battery are as follows:(2)2Li+12O2↔Li2O2Li+O2↔Li2O2.At the anode, during discharge, lithium oxidizes. Lithium is converted to ions and travels to the cathode via the electrolyte. At the cathode, oxygen is reduced to form an oxide with a lithium ion. The following two formulas show the reactions [76–78]:(3)Anode:Li↔Li++eCathode:2Li++O2+2e↔Li2O2.The cathode reaction can be much more complex than this, but in general it forms a lithium oxide, which is mainly of a different chemical formula. An oxide product, regardless of its type and method of formation, is formed by oxygen reduction. These cathode oxide reaction products (regardless of chemical formula and so on) are all insoluble in organic electrolytes as well as electron insulators [79, 80].


### 2.3. Hybrid Electrolytes

As mentioned, the previous two types of electrolytes had disadvantages. Nonaqueous electrolytes are not soluble in lithium oxide, and aqueous electrolytes react strongly with water [81–86]. To address these drawbacks, a new type of battery that has the two types of aqueous and nonaqueous electrolytes was designed. In this type, the anode (lithium metal) is in contact with the nonaqueous electrolyte, and the cathode is in contact with the aqueous electrolyte, in order to prevent the deposition of lithium oxide and to block the cathode cavities. These two parts are separated by an intermediate membrane (LISICON) through which lithium ions can pass [87–94].


### 2.4. Solid Electrolytes

To increase the safety of batteries and prevent the reaction of active substances inside the battery such as lithium with water, a solid type of electrolyte, in which a combination of solid layers replaces the liquid electrolyte, was invented [95, 96]. Solids electrolytes are key components for all-solid-state lithium-air battery. Advantages of using solids electrolytes to improve the battery performance are the following:(i)
Metal-oxides-based inorganic materials are stable at elevated temperatures. The conductivity of some solid electrolytes may even exceed that of liquid electrolytes [2].(ii)
Undesirable “cross-talk” effects, occurring between the cathode and the anode in some LIB systems, can be avoided or greatly repressed. For example, in liquid electrolyte-based LSBs, the maligned polysulfide “hasp” result will significantly lower the coulomb efficiency [8]. These problems can be solved by using solid electrolytes, because these hasp substances are immobile in solid electrolytes [2].(iii)
The chemical stability of solid-state electrolytes can, in principle, stand much higher, allowing high voltage lithium cathode materials in lithium-air batteries. High electrochemical stability windows of up to 6 V versus metallic lithium have been reported. The standard Li reference electrode will be used throughout this paper [9].


## 2.1. Batteries with Aqueous Electrolytes

The basis of this group of electrolytes is lithium salts dissolved in water. Due to the strong reaction of lithium with water, safety precautions must be taken [53–56]. For this purpose, an intermediate layer of ceramic is usually formed between the anode (lithium) and the electrolyte [57, 58]. The environment of these types of electrolytes can be acidic or alkaline [59].The basic reaction in this type of battery is as follows:(1)2Li+12O2+H2O↔2LiOH.In this type of battery, due to the solubility of the reaction product, there is no problem with organic electrolytes [60–62]. However, the disadvantage of this type of battery is that the electrolyte is consumed during the reaction (according to the above reaction, some water is used to form lithium hydroxide) [63, 64]. These batteries are known as primary batteries, so much research has focused on rechargeable organic electrolyte batteries. Therefore, our discussion also focuses on the effect of nanotechnology on batteries with organic electrolytes [65,66]. The efficiency of different aqueous lithium-air batteries is summarized in Table 1.Table 1
Performance of aqueous lithium-air batteries.
Solid electrolyteAnolyteCatholyteAir electrodeCurrent density (mA cm−2)Area capacity (mAh cm−2)Energy efficiency (%)Ref.LATPLiPONSat. LiOH0.10.290 (1st)-90 (100th)[99]LATP1 M LiClO4 in EC/DECSat. LiOH−10 M LiClKB/RuO20.645.860 (1st)-1.53 (8th)[100]LATP1 M LiClO4 in EC/DEC1 M LiNO3-0.5 M LiOHGNS0.51.070 (1st)-60 (50th)[101]LATPLiFSI-2G4Sat. LiOH-10 M LiClKB/RuO20.641270 (1st)-50 (12th)[102]LANTP4 M LiFSI in DME10 M LiCl-1.5 M LiOHMnO20.253.263 (1st)-51 (30th)[23]LAGTP4.5 M LiFSI10 M LiCl−1.5 M LiOHMnO21.01.085 (1st)-85 (10th)[103]LLTOLiPON1 M LiNO3MnO20.50.579 (1st)-54 (40th)[104]LATP0.1 M LiPF6 in EC/DEC0.1 M H3PO4 1 M LiH2PO3C/Pt0.50.2472 (1st)-67 (20th)[105]LATP1 M LiPF6 in EC/DEC0.1 M H3PO4 -1 M LiH2PO4Pt/C–IrO21.00.582 (1st)[67]LATPPEO18LiTFSI −10 wt%(HAc (90%) BaTiO3–H2O)-LiAcKB0.50.250 (1st)-70 (15th)[67]


## 2.2. Batteries with Nonaqueous Electrolytes

The electrolytes of this group are nonaqueous solutions (organic, of protic type); this type of electrolytes is much safer than aqueous electrolytes due to the lack of reaction of lithium with water. Another advantage of this type of battery is the ability to recharge [68–75].The two reactions that lead to the production of energy in this type of battery are as follows:(2)2Li+12O2↔Li2O2Li+O2↔Li2O2.At the anode, during discharge, lithium oxidizes. Lithium is converted to ions and travels to the cathode via the electrolyte. At the cathode, oxygen is reduced to form an oxide with a lithium ion. The following two formulas show the reactions [76–78]:(3)Anode:Li↔Li++eCathode:2Li++O2+2e↔Li2O2.The cathode reaction can be much more complex than this, but in general it forms a lithium oxide, which is mainly of a different chemical formula. An oxide product, regardless of its type and method of formation, is formed by oxygen reduction. These cathode oxide reaction products (regardless of chemical formula and so on) are all insoluble in organic electrolytes as well as electron insulators [79, 80].


## 2.3. Hybrid Electrolytes

As mentioned, the previous two types of electrolytes had disadvantages. Nonaqueous electrolytes are not soluble in lithium oxide, and aqueous electrolytes react strongly with water [81–86]. To address these drawbacks, a new type of battery that has the two types of aqueous and nonaqueous electrolytes was designed. In this type, the anode (lithium metal) is in contact with the nonaqueous electrolyte, and the cathode is in contact with the aqueous electrolyte, in order to prevent the deposition of lithium oxide and to block the cathode cavities. These two parts are separated by an intermediate membrane (LISICON) through which lithium ions can pass [87–94].


## 2.4. Solid Electrolytes

To increase the safety of batteries and prevent the reaction of active substances inside the battery such as lithium with water, a solid type of electrolyte, in which a combination of solid layers replaces the liquid electrolyte, was invented [95, 96]. Solids electrolytes are key components for all-solid-state lithium-air battery. Advantages of using solids electrolytes to improve the battery performance are the following:(i)
Metal-oxides-based inorganic materials are stable at elevated temperatures. The conductivity of some solid electrolytes may even exceed that of liquid electrolytes [2].(ii)
Undesirable “cross-talk” effects, occurring between the cathode and the anode in some LIB systems, can be avoided or greatly repressed. For example, in liquid electrolyte-based LSBs, the maligned polysulfide “hasp” result will significantly lower the coulomb efficiency [8]. These problems can be solved by using solid electrolytes, because these hasp substances are immobile in solid electrolytes [2].(iii)
The chemical stability of solid-state electrolytes can, in principle, stand much higher, allowing high voltage lithium cathode materials in lithium-air batteries. High electrochemical stability windows of up to 6 V versus metallic lithium have been reported. The standard Li reference electrode will be used throughout this paper [9].


## 3. Battery Problems with Organic Electrolytes

These batteries, as mentioned, account for the bulk of the research due to their recharge ability. However, for commercial use of these batteries, we need to overcome a series of problems. In this type of batteries, there is a problem of dendritic growth of lithium metal; methods to solve this problem include placing a mechanical layer on the anode. However, most of the problems with this type of battery come from the cathode; the three main problems with these batteries are [97–99] as follows.


### 3.1. Blocking the Transfer Paths

Because the reaction products (lithium oxide) are insoluble in the electrolyte when the reaction takes place in the form of sediment, they block the cathode cavities and prevent the transfer of lithium ions, electrons, and oxygen, thus reducing the capacity. Figure7 illustrates this well [100–102].Figure 7
The deposition of insoluble oxide particles on cathode cavities and blocking cavities [48].
(a)(b)


### 3.2. Polarization

Like fuel cells, polarization (about the types of polarization described in the original articles of lithium-ion batteries) plays an important role in this battery, which can cause significant potentials; therefore, charging and discharging potentials are very different from equilibrium. Oxygen reduction and oxygen evolution are two reactions that occur during discharge and charge, respectively, as shown below [103, 104].Oxygen reduction reaction, or ORR for short, is as follows:(4)2Li++O2+2e↔Li2O2.Oxygen evolution reaction, or OER for short, is as follows:(5)Li2O2↔2Li++O2+2e.These two reactions have high activation energies, which is why they cause activation polarization. In addition to activation polarization, ohm and concentration polarization also play an important role due to the problem of oxygen, ion, and electron transfer [105]. Despite the similarity to the fuel cell, there is one important difference here; that is, the product of the discharge reaction is a solid compound (lithium oxide) that is an electron insulator and can limit transmissions. Charging potential is worse than discharge potential [67]. The charging voltage of lithium-ion batteries is 0.8 to 1.5 volts higher than their discharge voltage. This causes the return to be lower than the acceptable return (90% efficiency) [106]. Figure 8 shows the equilibrium potential with the dashed line, the value of U0, and the potentials with η. In addition to reducing battery life, the potential causes the voltage required for charging to be high, thus leading to the possibility of electrolyte oxidation.Figure 8
Charging and discharging curves of a lithium-air battery with charging and discharging potentials [49].


### 3.3. Calcium Carbonate Formation

In this type of batteries, carbon and binder are somewhat unstable and cause lithium carbonate formation. This compound, like oxide, inhibits transitions. Lithium carbonate is worse in conductivity than lithium oxide and has much lower reversibility when charging, thus reducing battery capacity [107, 108]. Figures 9–13 showing chemical scheme carbonate and producing lithium carbonate illustrates this well.Figure 9
Chemical scheme for the manufacture of calcined sodium carbonate [109].Figure 10
Chemical scheme for producing lithium carbonate by carbonizing a lithium chloride solution in the presence of ammonia [109].Figure 11
A technological scheme for producing battery-quality lithium using the ammonia process [109].Figure 12
Chemical scheme for producing lithium carbonate from a lithium sulfate solution in the presence of ammonia [109].Figure 13
Chemical scheme for producing lithium carbonate from a lithium nitrate solution in the presence of ammonia [109].


## 3.1. Blocking the Transfer Paths

Because the reaction products (lithium oxide) are insoluble in the electrolyte when the reaction takes place in the form of sediment, they block the cathode cavities and prevent the transfer of lithium ions, electrons, and oxygen, thus reducing the capacity. Figure7 illustrates this well [100–102].Figure 7
The deposition of insoluble oxide particles on cathode cavities and blocking cavities [48].
(a)(b)


## 3.2. Polarization

Like fuel cells, polarization (about the types of polarization described in the original articles of lithium-ion batteries) plays an important role in this battery, which can cause significant potentials; therefore, charging and discharging potentials are very different from equilibrium. Oxygen reduction and oxygen evolution are two reactions that occur during discharge and charge, respectively, as shown below [103, 104].Oxygen reduction reaction, or ORR for short, is as follows:(4)2Li++O2+2e↔Li2O2.Oxygen evolution reaction, or OER for short, is as follows:(5)Li2O2↔2Li++O2+2e.These two reactions have high activation energies, which is why they cause activation polarization. In addition to activation polarization, ohm and concentration polarization also play an important role due to the problem of oxygen, ion, and electron transfer [105]. Despite the similarity to the fuel cell, there is one important difference here; that is, the product of the discharge reaction is a solid compound (lithium oxide) that is an electron insulator and can limit transmissions. Charging potential is worse than discharge potential [67]. The charging voltage of lithium-ion batteries is 0.8 to 1.5 volts higher than their discharge voltage. This causes the return to be lower than the acceptable return (90% efficiency) [106]. Figure 8 shows the equilibrium potential with the dashed line, the value of U0, and the potentials with η. In addition to reducing battery life, the potential causes the voltage required for charging to be high, thus leading to the possibility of electrolyte oxidation.Figure 8
Charging and discharging curves of a lithium-air battery with charging and discharging potentials [49].


## 3.3. Calcium Carbonate Formation

In this type of batteries, carbon and binder are somewhat unstable and cause lithium carbonate formation. This compound, like oxide, inhibits transitions. Lithium carbonate is worse in conductivity than lithium oxide and has much lower reversibility when charging, thus reducing battery capacity [107, 108]. Figures 9–13 showing chemical scheme carbonate and producing lithium carbonate illustrates this well.Figure 9
Chemical scheme for the manufacture of calcined sodium carbonate [109].Figure 10
Chemical scheme for producing lithium carbonate by carbonizing a lithium chloride solution in the presence of ammonia [109].Figure 11
A technological scheme for producing battery-quality lithium using the ammonia process [109].Figure 12
Chemical scheme for producing lithium carbonate from a lithium sulfate solution in the presence of ammonia [109].Figure 13
Chemical scheme for producing lithium carbonate from a lithium nitrate solution in the presence of ammonia [109].


## 4. Nanotechnology and Batteries with Organic Electrolyte

So far, different types of batteries with different electrolytes have been introduced. Most research, as mentioned earlier, focuses on batteries with organic electrolytes [110–116]. In this type of battery, there are various characteristics for the electrolyte such as the ability to wet the cathode cavities, and stability [117–125]. Most of the nanotechnology research into the components of this battery is done on the cathode to effectively solve the problems mentioned, so in this collection of articles, all the material is about the cathode of these batteries. In conventional lithium-ion batteries, the cathode electrode is made from a type of carbon powder, such as carbon black or activated carbon under various names such as Super P and Ketjenblack, which is used with a binder to bind the powder particles and the catalyst to increase speed [126–131]. Nanotechnology can be effective in removing the binder and using a method other than mechanical mixing [132–137].


## 5. Li2O2 Oxide Product

The product of the discharge reaction, as mentioned, is an oxide insulator (Li2O2 or Li2O) that closes the cavities and limits the transitions. However, to use the battery, there is no escape from the formation of this product [138]. Therefore, its formation should be optimized during discharge as much as possible so that the cathode cavities are not blocked and oxygen transfer can take place, because if oxygen is not transferred, the discharge process will stop [139]. Electron isolation also limits charge and discharge, as electron transfer does not occur to carry out reactions. There is disagreement about the dimensions and shape of the oxide product. Depending on the type of electrode, cavity geometry, type of electrolyte, current intensity, catalyst, etc., the oxide product will be different. Dimensions and morphology of this product are one of the current research issues that are examined in numerous articles, both experimentally and in the form of simulations and calculations [140]. Various shapes such as layers, disks, donuts, and particles have been reported, but the dimensions of these sediments are mostly submicron and even in the nanometer range. In most cases, the particle size is even below 100 nanometers, although in some studies the particle size has reached one micron [141]. Oxides are nanometers because they are insulators as they are not good electron conductors. Figure 14 shows a comparison between charge and discharge curves of graphene and conventional carbon cathodes.Figure 14
Comparison between charge and discharge curves of graphene and conventional carbon cathodes [50].The electron must be transferred from the cathode to the reaction site, because the oxide is insulated [142]. This is why oxide particles cannot grow very large. On the other hand, it has been shown that Li2O2 is more stable in nanoscale between two product oxides (Li2O and Li2O2) [143]. The same dimensions that have been reported for oxide in the articles are still very high due to the oxide insulation, because in order for such dimensions to grow, electrons must be transferred to continue the oxide growth reaction, which, as we move forward, disturbs the thickness of the oxide itself.Therefore, a lot of research, both simulation-computational and recently even experimental, is done to study the conductivity of Li2O2, and one of the fields of research in nanoscale is to study the electronic and ionic conductivity of this material in different dimensions, morphology, and crystallinity [144–148]. The importance of studying the conductivity of this nanoparticle oxide stems from the fact that until our knowledge of the optimal conductivity of this oxide for different dimensions and morphologies is complete, it is not possible to optimize this oxide product by controlling battery performance such as current, geometry, cathode, and catalyst [149–154].


## 6. Nanocatalyst and Its Effects

To solve the problem of potentials of this battery, like fuel cells, the use of a catalyst should be considered. The catalyst reduces the activation energy, so it plays an essential role in reducing the potential. Because the effect of catalysts is completely surface-related, nanomaterials are very effective because they have a high surface-to-volume ratio [155–159]. Furthermore, due to the nanometer dimensions of the cathode cavities, nanocatalysts must be used, whether we like it or not. Types of nanocatalysts include metal oxide and precious metals. Despite the similarity to fuel cells, because the product is a solid discharge reaction, very different conditions are created compared to fuel cells [160, 161].For example, from Figure15, it is clear that when the oxide layer covers the catalyst, the catalyst is no longer exposed to the reactants to show its performance. Therefore, it happens many times that, in these batteries, nanoelectrochemical factors such as blockage of cavities by oxide and nontransfer of reactants due to oxidation of the catalyst affect the reaction and do not allow the catalyst to function, hence the importance of cathode structure engineering and sediment optimization [162, 163]. From this discussion, it can be concluded that if the nanocatalyst is well distributed, it will show better performance because it will have better access to the reactors (oxygen, electrons, and ions). Compared to fuel cells, the use of catalysts here has other effects besides potential [3].Figure 15
Comparison between charge and discharge curves of carbon with palladium catalyst (a) and without palladium catalyst (b) [4].
(a)(b)In battery discharge (the same as the ORR), the higher the level of the catalyst, the more the reaction points, and so the more buds and the faster the reaction, which makes the lithium oxide formed smaller [164]. This smaller size of the oxide prevents clogging of the cavities and, due to its smaller size, leads to better electronic and ionic conductivity, which in turn prevents the capacity from dropping rapidly [4]. For example, the use of manganese oxide catalyst has increased the capacity from 850 mAh/g to 1000 mAh/g.If the catalyst is well distributed, it shows better performance because in this case the reaction is distributed in all areas of the cathode, so the reaction has taken place in all areas. Therefore, the oxide product is evenly distributed and does not accumulate in one place to block cathode cavities or prevent electron transfer [165–168]. Conversely, if the catalyst is not well distributed, the catalyst is rendered useless by performing the reaction and accumulating the product at one point as in Figure 16. Due to all this, there is a mutual relationship of the catalyst with its proper distribution on the one hand and with the oxide product and the geometry of the cathode cavities on the other hand [169]. The type of catalyst and more importantly its proper distribution are one of the topics of the day. Proper distribution geometry and how to prepare this proper geometry are one of the hot topics in the articles. The ALD method has been used to create a uniform distribution of palladium catalysts that uniformly cover the entire surface of the porous carbon cathode.Figure 16
The defects and their catalytic properties in grapheme for oxygen reduction, with red dots indicating catalytic locations [51].Thinking about how to use ALD, how to use ALD of palladium properly, how many cycles of ALD work best, and how to create fine particles with ALD are all important issues in the relationship between designs, showing the method of preparation and performance in articles. One of the advantages of the method is that the catalyst is properly distributed [170].Figure17 shows the effect of palladium on battery performance. The capacity provided by the ALD method prepared by the ALD method shows a capacity of about 6700 mAh/g, and the capacity of the same carbon sample without palladium is only 1500 mAh/g. Ordinary carbon itself has a catalytic property; in comparison, carbon nanotubes and graphene have more catalytic properties [171]. In the case of graphene, for example, this catalytic property is caused by defects in graphene, so the more defects graphene has, the more catalytic it is. Defects, such as the formation of polygons other than hexagons in the structure of graphene, as well as the edges of graphene, cause sites of adsorption and reaction. Figure 18 shows these types of defects and their catalytic performance.Figure 17
Comparison of voltage-time characteristics for three cathodes: red for graphene sheet, blue for carbon black with platinum, and black for carbon without catalyst [52].Figure 18
Demonstration of the effect of nitrogen doping in increasing the discharge capacity of graphene; water without doping and red with nitrogen doping [73].Figure19 shows that a cathode made of graphene nanosheet, or GNS for short, due to its good catalytic properties, has a performance comparable to a black carbon cathode. It has up to 20% of weight of rare and expensive platinum catalysts [2]. Specified to improve the catalytic performance of graphene and carbon nanotubes, doping specifically nitrogen can be very effective. Nitrogen provides more active points, thus improving catalytic performance. Figure 20 shows the discharge behavior for doped graphene with nitrogen doping; both types of graphene cathodes are prepared in the same way; both types of graphene are mixed with PVDF binder and then molded [9]. The structure of the cathodes is similar according to the same production method.Figure 19
Oxide product from discharge: (a) without nitrogen doping and (b) with doping [3, 4, 109].
(a)(b)Figure 20
The effect of dual-function catalyst in increasing the capacity and decreasing the charge and discharge potentials [3, 4, 109].Therefore, Figure21 shows only the effect of the catalyst on increasing the capacity. As can be seen, nitrogen doping has a significant effect on increasing capacity. Figure 22 shows the discharge results of the two graphene samples mentioned above. As can be seen, the doped sample contains finer particles. The finer particles confirm exactly what has been said before, that is, the presence of a catalyst and its uniform distribution, because it encourages germination and produces finer oxide particles. The finer the oxide particles, the lower the transfer potentials and the less likely the cavities to become clogged, thus justifying the better capacity shown in Figure 23 for the nitrogen doping state.Figure 21
Schematic diagram of the different formation mechanisms of RuO2 on purified multiwalled carbon nanotubes and oxidized multiwalled carbon nanotubes in the preparation process and their actual transmission electron microscopy images scale bar (20 nm) [3, 4, 109].Figure 22
Comparison of the effect of charge and discharge curves for CNT carbon nanotubes only and another carbon nanotube with RuO2/CNT catalyst (charge curves are high) [3, 4, 109].Figure 23
Transmission electron microscopy images of RuO2/carbon nanotubes at low (a), medium (b), and high (c) magnifications and the size distribution of nanoparticles supported on carbon nanotubes (d) [109].
(a)(b)(c)(d)Therefore, in addition to reducing the potential, catalysts are also effective in increasing the capacity. So far, there has been talk about the catalytic properties of carbon nanomaterials, but these nanomaterials are just good catalysts for the ORR. To improve catalytic performance, other types of nanocatalysts can be used, such as metal oxides, precious metals, and metal alloys. These catalysts can be grown on a substrate of graphene, nanofibers, or carbon nanotubes [168, 172–176].It is important to note that the OER (charge reaction) is not independent of the ORR (discharge reaction) and that the ORR catalyst has an indirect effect on the OER. Because the shape and crystallinity of oxide particles change according to the ORR catalyst, their decomposition in OER varies. Bifunctional catalysts that have a direct effect on reducing the activation energy of both ORR and OER, such as the catalyst in Figure24 (Pt-Au nanocatalyst), perform better than single-function catalysts (such as graphene, which acts only on ORR) [10].Figure 24
XRD patterns of functionalized carbon nanotube and RuO2/CNT [109].It has been suggested that the ORR catalyst indirectly affects the OER. In a valid article, a comparison was made between the performance of a cathode made of single carbon nanotube and a cathode of carbon nanotube with ruthenium oxide catalyst (RuO2) [12]. The RuO2 catalyst is homogeneously distributed in the form of particles in the range of one nanometer on carbon nanotubes (Figure 25). As Figure 19 shows, when the catalyst is used, the charging potential is effectively reduced. Interestingly, this catalyst effectively reduced the charge potential but did not affect the activation energy of the charge reaction. As can be seen in the box and also with the arrow sign in Figure 19, this catalyst affects the ORR activation energy in the very early stages. According to this theorem (and also based on other studies not presented here), it is clear that the presence of a catalyst affects the structure and morphology of the oxidative product of the discharge reaction by reducing the activation energy of the ORR and more places to perform the reaction. According to Figure 21, in the presence of a catalyst, the oxide product is more uniform. More importantly, the presence of this catalyst results in the formation of an amorphous oxide product.Figure 25
Size distribution of RuO2 NPS prepared without adding carbon nanotube [109].This oxide product is a good electron and ion transmitter, which in turn causes the amount of to be significantly reduced during charging. This is what was first observed in Figure20, in which, despite the lack of reduction in OER activation energy by the catalyst, the amount of charge potential is significantly reduced. This is an important demonstration of the importance of the structure and morphology of the oxide, as well as the indirect relationship of the ORR catalyst with the OER, both of which have been previously discussed.Figure22 shows transmission electron microscopy images of RuO2/CNT, Figure 23 shows XRD patterns of functionalized carbon nanotubes and RuO2/carbon nanotube, and Figure 24 shows size distribution of RuO2 nanoparticles prepared without adding carbon nanotubes. Various alcohols were used to investigate the activity of RuO2/carbon nanotube in aerobic oxidation, as listed in Table 2. In a control experiment, functionalized carbon nanotubes were used as a “catalyst” for the aerobic oxidation of benzyl alcohol.Table 2
Aerobic oxidation results of alcohols using RuO2/carbon nanotube catalyst [109].
SubstratesProductsTime (h)Conversion (%)Selectivity (%)TOFa (h1)4———1.5100>9992100>99140.580>99230.593>99242100>9971.5100>999443>991.5498>993.3


## 7. Maximum Theoretical Capacity

Figure26 shows that, between the two carbon walls, we can have a maximum of two 5 nm oxide particles (5 nm because we get the upper limit of capacity by considering the maximum allowable size), until the conduction is well maintained. Because graphene is geometrically the smallest thickness of a carbon wall, we consider each of these walls to be a single layer of graphene. On the other hand, the calculation indicates that a volume of lithium oxide equal to 26.3 cm3/gr of graphene can be stored [177–180].Figure 26
The maximum amount of Li2O2 between two carbon walls while maintaining very good electron conduction [3, 4, 109].Given that the specific surface area of graphene theory is 2693m2/g, of course, about 40% of the graphene surface can be used (due to shrinkage). According to all these topics, a capacity of 28409mAh is created per gram of graphene while the capacity shown in Figure 4 for graphene was close to 9000mAh/gr [181–184].Note that here the capacity for the cathode was calculated only for the weight of carbon (here graphene); another type of calculation is based on the weight of the carbon cathode and lithium oxide, in which case the value of the capacitance is much lower [185–190]. For example, if we assume that there is no need for a cathode at all and that oxygen itself can react directly to produce the oxide, a capacitance of 1169 mAh/g of Li2O2 is obtained, which is the ideal theoretical capacity limit per gram of oxide [3, 191].


## 8. Graphene Is Engineered in Cathode Fabrication

Achieving the capacity of the above theory is a goal of researchers that requires easy passage of oxygen and proper conduction of electrons and ions with no blockage of cavities [192–195]. It has been found that a suitable geometric structure with two pore dimensions, one with large dimensions for the passage of oxygen gas and the other with small mesoporous cavities, provides a suitable result for the reaction and placement of lithium oxide [191, 196, 197].Because in this type of structure only the reaction takes place and oxide deposition takes place inside the small cavities, the oxygen transfer path, which is the large cavities, remains for the passage of oxygen [5, 198–200]. Given the advantages expressed for graphene due to its atomic and engineering dimensions, it can be used to provide such a structure. In a valid article [9], a hierarchical structure of graphene is prepared in which micrometer cavities cause excellent penetration of oxygen gas, and nanometer cavities increase the effective surface area and catalytic role and increase the reaction areas. Figure 27 shows a realistic and schematic illustration of this structure, which resembles a broken egg with interconnected cavities. In this capacity structure, a significant amount of 15,000 mAh per gram of grapheme has been reported, which is very close to the theoretical value of the above paragraph and, as far as the authors know, the highest reported value for lithium-air battery capacity.Figure 27
(a) Image of the actual hierarchical structure of graphene and (b) schematic representation of this structure [3, 4, 109].The method of preparing this structure itself is of great scientific discussion and importance. In short, due to the hydrophobicity of graphene and by micro emulsion method, this structure has been prepared. This prepared structure has other arguments that are not discussed here. Interestingly, the same synthesis method was applied to Ketjenblack (KB), which is a type of carbon black, but such an excellent result was not obtained, and in fact the formation of the hierarchical structure is due to the atomic dimensions of graphene [162, 180, 201].


## 9. Carbon Nanofiber and Structural Engineering

These nanofibers are grown vertically, wrapped together, and doped with nitrogen. This structure has a coral state. In the prepared cathode, it provides a good path for oxygen and ion transfer and holds lithium oxide well without closing the oxygen path. This structure is first grown on silicon and then transferred on steel [202, 203].There is no need for a binder in this structure, and therefore we do not have any harmful reactions related to the binder which is one of the problems of the cathode. According to Figure28, this coral structure has branches and leaflets that increase the effective surface area between the electrode and the oxide compared to when the carbon is branched. Figure 29(a) shows the cycle life of this cathode. It is observed that no capacitance reduction is observed in this cathode. In Figure 29(b), a comparison is made between this cathode, the cathodes made from the vertical growth of carbon nanotubes on steel, and the cathode made of carbon nanotube powder. It is clear from the figure that a vertically grown carbon nanotube cathode has less potential than a carbon nanotube powder [204, 205].Figure 28
(a, b) Coral nanofibers with side branches and leaflets in different dimensions. (c) Schematic of the structure showing how lithium oxide is formed [3, 4, 109].
(a)(b)(c)Figure 29
(a) The cathode life cycle of the cathode. (b) The charge-discharge behavior of powder made of carbon nanotubes (black), carbon nanotubes grown vertically (purple), and structure under study (red) (shown in Figure28) [3, 4, 109].
(a)(b)However, compared to the nanofiber cathode and the vertical carbon nanotube cathode, the nanofiber has a better result, due to first the nitrogen doping and second the leaf and branch structure, which improves the effective surface and the relationship between oxide and nanofiber. This example shows that the importance of structure is even greater than the type of material used [206–212].


## 10. Carbon Nanotubes and Structural Engineering

In another type of structural engineering, a hierarchical structure as shown in Figure30 is provided by a carbon nanotube forest. The structure is made of ten fibrous plates. Fibrils are seen in Figures 30(a) and 30(b). Each fibril is made of hundreds of strands (Figure 29(c)), each of which is itself a multiwalled carbon nanotube (Figure 30(d)). Due to the empty spaces and architectural cavities, this structure passes oxygen and electrolytes well, and the oxide product is formed easily (without any problem in blocking the transmission path). Figure 31 shows a comparison between the built-in battery performance of the hierarchical structure in question (Figures 31(a), 31(b)), ordinary carbon (Figures 31(c), 31(d)), and carbon nanotube powder (Figures 31(e), 31(f)) [163]. It can be seen that the capacitance, cyclic longevity, potential, and power for a cathode with a hierarchical structure are much better than those of the other two. The result of this hierarchical sample is better than that of the electrode made of carbon nanotube powders (powder made of carbon nanotube), because in this structure regular cavities provide oxygen, electron, and ion transfers without being blocked by the oxide product. The superiority of the results of the hierarchical structure over those of the cathode made of powdered carbon nanotubes shows how important the engineering and architecture of the structure are [109]. In this hierarchical structure, due to open spaces, the catalyst can be easily deposited well on carbon nanotubes by spattering method [213–216]. Figure 32 shows the same structure this time with the catalyst, which in turn improves battery performance.Figure 30
The magnification of the structure increases from left to right, and the inset shows the magnification of the red box [3, 4, 109].Figure 31
Comparisons between the performance of batteries made (a, b) for the hierarchical structure in question, (c, d) for ordinary carbon (e, f), and for powder made of carbon nanotubes [3, 4, 109].
(a)(b)(c)(d)(e)(f)Figure 32
The same structure this time with the catalyst, which in turn improves battery performance [3, 4, 109].


### 10.1. Carbon Nanotubes in the Battery Anode

Carbon nanotubes have been proposed by many research groups as the active ingredient of lithium-ion battery electrodes since 1990 [2, 8–10]. The capacity of the anode electrode made of carbon nanotubes is greater than the capacity of the anode electrode made of graphite (which is LiC6 and is equal to 372 mAh/g). Figure 33 shows the curve for lithium insertion (discharge) and lithium desorption (charge) from an electrode containing carbon nanotubes with a very regular graphite structure [12]. As can be seen, a large amount of irreversible capacity is observed in the first stage of the charge-discharge process. This high irreversible capacity is attributed to the formation of the SEI layer, due to the high specific surface area of carbon nanotubes [217–219].Figure 33
Curve for lithium insertion (discharge) and lithium desorption (charge) from an electrode containing carbon nanotubes with a very regular graphite structure [12].


### 10.2. Morphological Effect

The morphology of carbon nanotubes plays an essential role in the electrochemical properties of lithium-ion batteries with carbon nanotubes as anode electrodes. Parameters such as defects, length, and diameter can affect the morphology of these materials and consequently affect the electrochemical properties. In general, two methods of chemical etching and mechanical milling are widely used to modify the morphology of carbon nanotubes. These methods change the structure and create functional groups on the surface of carbon nanotubes. This structural change causes defects on the lateral surface of carbon nanotubes, shortens the length of carbon nanotubes, and increases the lithium-ion insertion capacity [12].


### 10.3. Effects of Carbon Nanotube Defects

One of the most important and effective parameters that can be considered for carbon nanotubes as an anode electrode of lithium-ion battery is the effect of defects. Figure34 shows two types of carbon nanotubes, one with defects and the other with regular graphite walls [13]. Increased defects increase the storage capacity of lithium ions. For example, milling operations on carbon nanotubes increase the density of defects, reduce the length of carbon nanotubes, and can affect the performance of the electrode [14, 220–224], and high irreversible capacity due to SEI layer formation and other side effects suffers severely [15, 225, 226]. Therefore, chemical etching and milling methods are suggested to eliminate these defects. Chemical etching and milling with a strong acid or conventional chemical methods can be used to create holes in the walls of carbon nanotubes. Figure 33 shows another carbon nanotube that is carbonized at 2000°C and has almost no oxygen. This carbon nanotube shows a low charge capacity of 282 mAh/g in the first charge-discharge process. However, after 20 charge-discharge cycles, carbonized carbon nanotubes at 900°C lose 65.3% of their capacity, while carbonized carbon nanotubes at 2000°C retain 91.5% of their charge capacity [16].Figure 34
Image TEM shows two types of carbon nanotubes, one with defects and the other with regular graphite walls [13].
(a)(b)


### 10.4. Composite of Carbon Nanotubes with Carbon Nanofibers

The mechanism for inserting lithium ions into carbon nanofibers is similar to that of carbon nanotubes. The performance of carbon nanofibers is similar to that of carbon nanotubes depending on their microstructure. High-graphite carbon nanofibers show a capacity of 297–431 mAh/g (especially at low potential), but coulomb efficiency in the first cycle is low (approximately 60%), which should be addressed [23, 227–229]. Poor graphite nanocarbons have a high initial storage capacity, but they also have a high irreversible capacity and suffer from large volumetric changes during the charge-discharge process. This will cause the SEI layer to disappear and eventually the electrodes to flake off. On the other hand, carbon materials with high-graphite structure show small volume changes in the charge-discharge process [230–232]. Therefore, these materials are mechanically stable. A combination of carbon nanotubes and carbon nanofibers can ideally increase storage capacity and improve longevity. Figure 35 shows a composite of carbon nanotubes with carbon nanofibers in which a 5–6 nm diameter carbon nanofiber is placed inside a 50 nm diameter carbon nanotube [25, 233–237]. The result of the charge-discharge test indicates an increase in capacity (Figure 36). A clear oxidation-reduction peak is observed in the curve, which is attributed to the insertion and excretion of lithium ions between the graphene layers.Figure 35
A composite of carbon nanotubes with carbon nanofibers [23].Figure 36
Result of the charge-discharge test indicates an increase in capacity [25].
(a)(b)


### 10.5. Lithium-Oxygen Batteries

Nonaqueous lithium-oxygen/air batteries have great potential for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their extremely high-energy density [250–256]. The air electrode plays an important role in the battery performance; however, it is still a critical challenge to develop an optimum carbon cathode with appropriate surface area, porosity, and morphology. Graphene nanosheets (GNSs) have been reported as ideal cathode materials for lithium-oxygen batteries because of their unique morphology and structure which provide both diffusion channels for O2 and active sites for cathode reactions. Chemical doping with nitrogen atoms into GNSs can modify the electronic property, provide more active sites, and enhance the interaction between carbon structure and other molecules, thus improving the performance in various applications, such as fuel cells, lithium-ion batteries, and supercapacitors. Recent studies reported that nitrogen-doped carbon powder and carbon nanotubes showed higher discharge capacities than the pristine counterparts; however, there is no report on nitrogen-doped graphene nanosheets (N-GNSs) as cathode materials for lithium-oxygen batteries [181, 238]. In this study, for the first time, N-GNSs were employed in lithium-oxygen batteries, and it was found that they show excellent electrocatalytic activity for oxygen reduction, thereby increasing about 40% of the discharge capacity compared to GNSs. This finding not only shows that N-GNSs are promising electrode materials, but also gives a rational direction to modify other carbon materials for application in lithium-oxygen batteries [239].As shown in Figure37(a), the initial discharge capacity of GNS electrodes is 8530 mAh g−1 at a current density of 75 mA g−1, while it is 11,660 mAh g−1 for N-GNSs, which is about 40% higher than that of GNSs. With the current density increase, the discharge capacities of both samples decrease, which are 5333 and 3090 mAh g−1 for GNSs and 6640 and 3960 mAh g−1 for N-GNSs at current densities of 150 and 300 mA g−1, respectively. N-GNS electrodes not only exhibit higher discharge capacity but also show higher average discharge plateau than GNS electrodes at various discharge current densities, indicating a higher catalytic activity for cathode reaction which may be due to the higher binding energy of oxygen after nitrogen doping. The cyclic voltammogram (CV) curves of GNSs and N-GNSs are shown in Figure 37(b). Featureless voltammetric currents were observed in the Ar-saturated solution (inset of Figure 37(b)), while in the O2-saturated solution, the onset potentials of oxygen reduction reaction (ORR) for GNSs and N-GNSs are at 2.76 V and 2.8 V, respectively. The positive shift of the onset potential and the great and well-defined enhanced reduction peak at 2.29 V of N-GNSs indicate a superior electrocatalytic activity for ORR. From the rotating-disk electrode (RDE) voltammetry measurements, it can be seen that the onset potential of ORR for N-GNSs (2.84 V) is still positive shift compared to GNSs (2.79 V) (Figure 1(c)). There is no well-defined diffusion limiting current plateau for both samples, which is similar to other non-noble catalysts especially pure carbon for ORR; however, the current density of N-GNSs at 2 V is about 2.5 times that of GNSs [22, 23]. Furthermore, the numbers of electron transferred in ORR are 0.99 for N-GNSs and 0.80 for GNSs, respectively, according to the Tafel slops shown in Figure 37(d). These results also suggest that N-GNSs have higher catalytic activity for ORR in a nonaqueous solution than GNSs [240].Figure 37
(a) Voltage profiles of GNSs and N-GNS electrodes at various current densities. (b) CVs of GNS and N-GNS electrodes in O2-saturated 0.1 mol dm−3 LiPF6 in TEGDME solution at a scan rate of 5 mV s−1; inset is the CVs in Ar-saturated solution. (c) Rotating-disk electrode voltammograms recorded for GNS and N-GNS electrodes for ORR at a rotating speed of 100 rpm in O2-saturated 0.1 mol dm−3 LiPF6 in TEGDME solution at a scan rate of 5 mV s−1. (d) Tafel slops for the ORR on the GNS and N-GNS electrodes [181].
(a)(b)(c)(d)N-GNSs and GNSs show no obvious difference from the structure and morphology but the same micro/meso-porosity and pore-size distribution (Figure38(a), b and c). XPS confirms that three types of nitrogen (pyridinic-N, pyrrolic-N, and graphitic-N) are formed in N-GNSs and the nitrogen percentage is about 2.8% (Figure 38(d)). The structural defects introduced by nitrogen doping increase the amount of unsaturated carbon atoms (Figure 38(e)); these atoms are very active in reacting with oxygen and forming oxygen-containing groups (C–O, C=O, and O=C–O), which can catalyze the ORR (Figure 38(f).). The Raman spectra of GNSs and N-GNSs are shown in Figure 38(g). The intensity ratio of D band (∼1350 cm−1) to G band (∼1595 cm−1) for GNSs and N-GNSs are 0.96 and 1.10, respectively, confirming that there are more defects after nitrogen doping. There are more defects and functional groups after nitrogen doping, and the carbon atoms adjacent to nitrogen dopants possess higher positive charge density, resulting in an enhanced adsorption of oxygen and reactive intermediates, which enhances the ORR [241, 242].Figure 38
TEM images of (a) GNSs and (b) N-GNSs. (c) N2 adsorption–desorption isotherms at 77 K; inset is the pore-size distribution. XPS (d) survey, (e) C1s, and (f) O1s spectra of GNSs and N-GNSs; inset of (d) is N 1s spectra of two samples. (g) Raman spectra of GNSs and N-GNSs [181].
(a)(b)(c)(d)(e)(f)(g)


## 10.1. Carbon Nanotubes in the Battery Anode

Carbon nanotubes have been proposed by many research groups as the active ingredient of lithium-ion battery electrodes since 1990 [2, 8–10]. The capacity of the anode electrode made of carbon nanotubes is greater than the capacity of the anode electrode made of graphite (which is LiC6 and is equal to 372 mAh/g). Figure 33 shows the curve for lithium insertion (discharge) and lithium desorption (charge) from an electrode containing carbon nanotubes with a very regular graphite structure [12]. As can be seen, a large amount of irreversible capacity is observed in the first stage of the charge-discharge process. This high irreversible capacity is attributed to the formation of the SEI layer, due to the high specific surface area of carbon nanotubes [217–219].Figure 33
Curve for lithium insertion (discharge) and lithium desorption (charge) from an electrode containing carbon nanotubes with a very regular graphite structure [12].


## 10.2. Morphological Effect

The morphology of carbon nanotubes plays an essential role in the electrochemical properties of lithium-ion batteries with carbon nanotubes as anode electrodes. Parameters such as defects, length, and diameter can affect the morphology of these materials and consequently affect the electrochemical properties. In general, two methods of chemical etching and mechanical milling are widely used to modify the morphology of carbon nanotubes. These methods change the structure and create functional groups on the surface of carbon nanotubes. This structural change causes defects on the lateral surface of carbon nanotubes, shortens the length of carbon nanotubes, and increases the lithium-ion insertion capacity [12].


## 10.3. Effects of Carbon Nanotube Defects

One of the most important and effective parameters that can be considered for carbon nanotubes as an anode electrode of lithium-ion battery is the effect of defects. Figure34 shows two types of carbon nanotubes, one with defects and the other with regular graphite walls [13]. Increased defects increase the storage capacity of lithium ions. For example, milling operations on carbon nanotubes increase the density of defects, reduce the length of carbon nanotubes, and can affect the performance of the electrode [14, 220–224], and high irreversible capacity due to SEI layer formation and other side effects suffers severely [15, 225, 226]. Therefore, chemical etching and milling methods are suggested to eliminate these defects. Chemical etching and milling with a strong acid or conventional chemical methods can be used to create holes in the walls of carbon nanotubes. Figure 33 shows another carbon nanotube that is carbonized at 2000°C and has almost no oxygen. This carbon nanotube shows a low charge capacity of 282 mAh/g in the first charge-discharge process. However, after 20 charge-discharge cycles, carbonized carbon nanotubes at 900°C lose 65.3% of their capacity, while carbonized carbon nanotubes at 2000°C retain 91.5% of their charge capacity [16].Figure 34
Image TEM shows two types of carbon nanotubes, one with defects and the other with regular graphite walls [13].
(a)(b)


## 10.4. Composite of Carbon Nanotubes with Carbon Nanofibers

The mechanism for inserting lithium ions into carbon nanofibers is similar to that of carbon nanotubes. The performance of carbon nanofibers is similar to that of carbon nanotubes depending on their microstructure. High-graphite carbon nanofibers show a capacity of 297–431 mAh/g (especially at low potential), but coulomb efficiency in the first cycle is low (approximately 60%), which should be addressed [23, 227–229]. Poor graphite nanocarbons have a high initial storage capacity, but they also have a high irreversible capacity and suffer from large volumetric changes during the charge-discharge process. This will cause the SEI layer to disappear and eventually the electrodes to flake off. On the other hand, carbon materials with high-graphite structure show small volume changes in the charge-discharge process [230–232]. Therefore, these materials are mechanically stable. A combination of carbon nanotubes and carbon nanofibers can ideally increase storage capacity and improve longevity. Figure 35 shows a composite of carbon nanotubes with carbon nanofibers in which a 5–6 nm diameter carbon nanofiber is placed inside a 50 nm diameter carbon nanotube [25, 233–237]. The result of the charge-discharge test indicates an increase in capacity (Figure 36). A clear oxidation-reduction peak is observed in the curve, which is attributed to the insertion and excretion of lithium ions between the graphene layers.Figure 35
A composite of carbon nanotubes with carbon nanofibers [23].Figure 36
Result of the charge-discharge test indicates an increase in capacity [25].
(a)(b)


## 10.5. Lithium-Oxygen Batteries

Nonaqueous lithium-oxygen/air batteries have great potential for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their extremely high-energy density [250–256]. The air electrode plays an important role in the battery performance; however, it is still a critical challenge to develop an optimum carbon cathode with appropriate surface area, porosity, and morphology. Graphene nanosheets (GNSs) have been reported as ideal cathode materials for lithium-oxygen batteries because of their unique morphology and structure which provide both diffusion channels for O2 and active sites for cathode reactions. Chemical doping with nitrogen atoms into GNSs can modify the electronic property, provide more active sites, and enhance the interaction between carbon structure and other molecules, thus improving the performance in various applications, such as fuel cells, lithium-ion batteries, and supercapacitors. Recent studies reported that nitrogen-doped carbon powder and carbon nanotubes showed higher discharge capacities than the pristine counterparts; however, there is no report on nitrogen-doped graphene nanosheets (N-GNSs) as cathode materials for lithium-oxygen batteries [181, 238]. In this study, for the first time, N-GNSs were employed in lithium-oxygen batteries, and it was found that they show excellent electrocatalytic activity for oxygen reduction, thereby increasing about 40% of the discharge capacity compared to GNSs. This finding not only shows that N-GNSs are promising electrode materials, but also gives a rational direction to modify other carbon materials for application in lithium-oxygen batteries [239].As shown in Figure37(a), the initial discharge capacity of GNS electrodes is 8530 mAh g−1 at a current density of 75 mA g−1, while it is 11,660 mAh g−1 for N-GNSs, which is about 40% higher than that of GNSs. With the current density increase, the discharge capacities of both samples decrease, which are 5333 and 3090 mAh g−1 for GNSs and 6640 and 3960 mAh g−1 for N-GNSs at current densities of 150 and 300 mA g−1, respectively. N-GNS electrodes not only exhibit higher discharge capacity but also show higher average discharge plateau than GNS electrodes at various discharge current densities, indicating a higher catalytic activity for cathode reaction which may be due to the higher binding energy of oxygen after nitrogen doping. The cyclic voltammogram (CV) curves of GNSs and N-GNSs are shown in Figure 37(b). Featureless voltammetric currents were observed in the Ar-saturated solution (inset of Figure 37(b)), while in the O2-saturated solution, the onset potentials of oxygen reduction reaction (ORR) for GNSs and N-GNSs are at 2.76 V and 2.8 V, respectively. The positive shift of the onset potential and the great and well-defined enhanced reduction peak at 2.29 V of N-GNSs indicate a superior electrocatalytic activity for ORR. From the rotating-disk electrode (RDE) voltammetry measurements, it can be seen that the onset potential of ORR for N-GNSs (2.84 V) is still positive shift compared to GNSs (2.79 V) (Figure 1(c)). There is no well-defined diffusion limiting current plateau for both samples, which is similar to other non-noble catalysts especially pure carbon for ORR; however, the current density of N-GNSs at 2 V is about 2.5 times that of GNSs [22, 23]. Furthermore, the numbers of electron transferred in ORR are 0.99 for N-GNSs and 0.80 for GNSs, respectively, according to the Tafel slops shown in Figure 37(d). These results also suggest that N-GNSs have higher catalytic activity for ORR in a nonaqueous solution than GNSs [240].Figure 37
(a) Voltage profiles of GNSs and N-GNS electrodes at various current densities. (b) CVs of GNS and N-GNS electrodes in O2-saturated 0.1 mol dm−3 LiPF6 in TEGDME solution at a scan rate of 5 mV s−1; inset is the CVs in Ar-saturated solution. (c) Rotating-disk electrode voltammograms recorded for GNS and N-GNS electrodes for ORR at a rotating speed of 100 rpm in O2-saturated 0.1 mol dm−3 LiPF6 in TEGDME solution at a scan rate of 5 mV s−1. (d) Tafel slops for the ORR on the GNS and N-GNS electrodes [181].
(a)(b)(c)(d)N-GNSs and GNSs show no obvious difference from the structure and morphology but the same micro/meso-porosity and pore-size distribution (Figure38(a), b and c). XPS confirms that three types of nitrogen (pyridinic-N, pyrrolic-N, and graphitic-N) are formed in N-GNSs and the nitrogen percentage is about 2.8% (Figure 38(d)). The structural defects introduced by nitrogen doping increase the amount of unsaturated carbon atoms (Figure 38(e)); these atoms are very active in reacting with oxygen and forming oxygen-containing groups (C–O, C=O, and O=C–O), which can catalyze the ORR (Figure 38(f).). The Raman spectra of GNSs and N-GNSs are shown in Figure 38(g). The intensity ratio of D band (∼1350 cm−1) to G band (∼1595 cm−1) for GNSs and N-GNSs are 0.96 and 1.10, respectively, confirming that there are more defects after nitrogen doping. There are more defects and functional groups after nitrogen doping, and the carbon atoms adjacent to nitrogen dopants possess higher positive charge density, resulting in an enhanced adsorption of oxygen and reactive intermediates, which enhances the ORR [241, 242].Figure 38
TEM images of (a) GNSs and (b) N-GNSs. (c) N2 adsorption–desorption isotherms at 77 K; inset is the pore-size distribution. XPS (d) survey, (e) C1s, and (f) O1s spectra of GNSs and N-GNSs; inset of (d) is N 1s spectra of two samples. (g) Raman spectra of GNSs and N-GNSs [181].
(a)(b)(c)(d)(e)(f)(g)


## 11. Comparing Rechargeable Batteries

Figure39 shows a comparison between the features of conventional rechargeable batteries.Figure 39
Comparison between the features of conventional rechargeable batteries.The following can be seen in Figure39 [79]:(i)
The internal resistance of the battery is a factor that depends on items such as cell rate, number of cells, and type of protection circuit(ii)
Battery life largely depends on the proper maintenance of the battery and its regular charge(iii)
Battery life depends on complete discharge of charge at each use(iv)
Capacity for current pulses is high(v)
Battery cost for portable electronics is shown(vi)
Voltage 1.2 and 1.25 batteries have very little difference


## 12. Summary and Perspectives

(1)
In this paper, the advantages of lithium-air batteries, the differences in mechanisms, and the importance of nanotechnology for these batteries were identified. Types of lithium-air batteries were introduced. Problems with batteries with organic electrolytes were accurately described. The cathode structure of conventional lithium-air batteries was described; in addition, nanotechnology areas including optimization of cathode geometry, catalysts, and nano-mechanized and nanocarbon cathodes were introduced, and their close relationship was expressed.(2)
This article examines important aspects of the battery; the importance of the type and product of nanoparticle oxide discharge and the necessity of research on it were determined. The effect of different nanocatalysts such as carbon nanotubes, graphene, and dual function on reducing the activation energy of reactions was determined. The importance of optimizing the morphology and distribution of nanocatalysts was explained, and finally the effect of the discharge reaction nanocatalyst on the oxide product and then the effect on the charge reaction were determined.(3)
In this article, due to the novelty and lack of regular and categorized materials in the field of cathode structural engineering, we used the best examples taken from valid and educational articles to help create a proper view of structural engineering and the importance of this field and help to complete projects. In the first part, the maximum theoretical capacity that can be created with graphene is calculated. In the second part of the nanotechnology, flexibility and strength have led to a special structure of graphene that is closer to the theoretical capacity. An example of carbon fiber with coral morphology is given to illustrate the importance of increasing the effective surface area. The last example also shows the importance of proper and regular cavities compared to irregular cavities. Taken together, these examples highlight the importance of structure optimization for oxygen permeability, oxide conductivity, and improved battery performance.(4)
In the present era, the electronics industry has made significant progress, while providing easy and desirable energy for the development of portable devices is an important and undeniable necessity. This is to the extent that the battery can be considered the heart of portable electronics. The best option to address this is to use rechargeable batteries to store electrical energy. The most commonly used material for anode is commercial lithium-ion batteries, which have a relatively small capacity and are still not large enough for use in systems such as electric hybrid cars. Among these, carbon nanotubes with properties such as unique structural, electrical, and mechanical properties seem to be a suitable alternative to graphite carbon. On the other hand, if the length of carbon nanotubes can be reduced without milling during synthesis, this will increase the cologne efficiency and increase the capacity. Even if the various composites of carbon nanotubes are examined with the right materials, we can see an improvement in the performance and capacity of these batteries. In general, if carbon nanotubes can be synthesized by low-cost methods and the morphology of these materials can be properly controlled, carbon nanotubes will be a suitable alternative to graphite as the anode electrode in lithium-ion batteries.



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*Source: 2762647-2022-03-12.xml*