6. Nanoporous carbon xerogels in energy storage

Electrical energy storage is required in numerous applications, not only as a result of the increasing number of electronic devices that we use in our daily life but also as a means to ensure a more rational and sustainable use of energy resources. Thus, there is an increasing demand for more efficient energy storage devices in telecommunications, stand-by power systems, uninterruptible power supply systems, electric/hybrid vehicles, energy recovery systems, burst and regenerative power in industry and transportation, electric grid modulation or as complements to renewable energies.

The specifications for the energy storage devices are given in terms of energy stored (W h) and power (W) as well as size and weight, cost and durability. The most common electrical energy storage device is the battery. Batteries can store large amounts of energy within relatively small confines of volume and weight. However, the problem with battery systems is their life-span as their energy storage method is based on a chemical process that degrades the components of the battery and reduces the cycling life [65]. Furthermore, the current development of electronic applications demands devices with high power requirements, far beyond the capability of standard batteries. Supercapacitors, also known as electrochemical double layer capacitors (EDLC), store energy via a physical process based on the formation of an electric double layer at the electrode/electrolyte interface [65]. Therefore, the charge/discharge process is very fast and totally reversible with a very low level of degradation of the components. As a consequence, supercapacitors offer a high power density and a very long cycle life.

In general, batteries are used for their relatively high energy density, while supercapacitors are employed for their high power density. Therefore, both devices, if combined, could fulfill complementary functions. Currently both technologies require further optimization of their performance and the incorporation of safer and more cost-effective components. Research and industrial developments for storage devices are currently oriented toward improving energy and power density with new electrode materials, new electrolytes and new electrochemical concepts. Moreover, some research is currently focused on developing hybrid devices where in one electrode energy storage is based on a chemical process, while in the other, it is based on a physical double layer [66, 67].

An EDLC consists of two porous carbon electrodes in contact with the current collector, separated by a porous film and impregnated with an electrolyte solution. When there is an electric potential difference between the electrodes, the electrons in the negatively polarized electrode (anode) are balanced by an equal number of positive cations at the electrode/electrolyte interface, while the storage holes in the positively polarized electrode (cathode) are electrically balanced by the anions from the electrolyte. Figure 9 shows the scheme of the basic principle of energy storage in this kind of device.

The capacitance of a single electrode (Ce) is proportional to the surface area of the electrode (S) according to Eq. (1),

$$\mathbf{C}\_{\mathbf{e}} = \varepsilon\_0 \,\,\varepsilon \,\, (\mathbf{S}/\mathbf{d})\tag{1}$$

electrolyte are accommodated [68, 69]. In this respect, exohedral surfaces are more receptive, as

For a capacitor, two electrodes are needed, which is the equivalent to two capacitors in series.

where C+ and C� are the capacitance of the cathode and the anode, respectively. Consequently, the difference between the capacitance values of a single electrode or complete capacitor is of great importance because if both electrodes have the same thickness, size, mass and material,

in the bibliography) will be a factor of four. The maximum energy stored in a supercapacitor

where ΔE is the operation voltage. The most commonly used operation voltage value is 1 V for aqueous electrolytes and around 2.7 V for organic electrolytes. Higher voltages would lead to the degradation of the components. It is clear that increasing the operation voltage is the most effective way to increase the energy density of a supercapacitor, which is why organic electrolytes are widely used in commercial devices, even though their conductivity, and their capacitance values (see Eq. (1)), are lower. However, the most common organic electrolytes (quaternary ammonium salts in polycarbonate or acetonitrile) have a number of disadvantages that can cause environmental problems; they are highly inflammable and chemically unstable, etc. In order to overcome these problems, research has been oriented toward developing organic electrolytes and ionic liquids, the latter being able to work up to 3.5 V. The wider range of electrolytes available will lead to a correspondingly wider range of porosities in the active

1=C ¼ 1=C<sup>þ</sup> þ 1=C� (2)

<sup>E</sup> <sup>¼</sup> <sup>C</sup> ð Þ <sup>Δ</sup><sup>E</sup> <sup>2</sup> h i=<sup>2</sup> (3)

, i.e., the most common parameter employed

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 79

The resultant capacitance C can be expressed by the following equation Eq. (2),

Figure 9. Mechanism of charge/discharge of an electrochemical double layer capacitor.

it was mentioned above.

the difference between both capacitances (in F g�<sup>1</sup>

material used, which is the topic of this chapter.

(E) is proportional to its capacitance and is given by Eq. (3),

where ε<sup>0</sup> is the vacuum permittivity (8.854 10�<sup>12</sup> F m�<sup>1</sup> ), ε is the relative permittivity of the dielectric electrolyte used and d is the effective thickness of the double layer. According to this equation, the capacitance of a single electrode will increase as the effective surface area of the electrode increases and the thickness of the double layer decreases (i.e., better contact between electrode and ions of electrolyte). This is the reason for using carbons with well-developed porosity (i.e., a high effective surface area, which means a higher surface area accessible to the electrolyte) as active materials in supercapacitors. Moreover, a greater or lesser ability to accommodate electrolyte ions on the surface of the electrode will also influence the thickness (d) of the double layer. Some studies show that the curvature of the surface (i.e., whether it is positive or negative) will influence the way in which solvated and desolvated ions of the

Figure 9. Mechanism of charge/discharge of an electrochemical double layer capacitor.

The specifications for the energy storage devices are given in terms of energy stored (W h) and power (W) as well as size and weight, cost and durability. The most common electrical energy storage device is the battery. Batteries can store large amounts of energy within relatively small confines of volume and weight. However, the problem with battery systems is their life-span as their energy storage method is based on a chemical process that degrades the components of the battery and reduces the cycling life [65]. Furthermore, the current development of electronic applications demands devices with high power requirements, far beyond the capability of standard batteries. Supercapacitors, also known as electrochemical double layer capacitors (EDLC), store energy via a physical process based on the formation of an electric double layer at the electrode/electrolyte interface [65]. Therefore, the charge/discharge process is very fast and totally reversible with a very low level of degradation of the components. As a conse-

In general, batteries are used for their relatively high energy density, while supercapacitors are employed for their high power density. Therefore, both devices, if combined, could fulfill complementary functions. Currently both technologies require further optimization of their performance and the incorporation of safer and more cost-effective components. Research and industrial developments for storage devices are currently oriented toward improving energy and power density with new electrode materials, new electrolytes and new electrochemical concepts. Moreover, some research is currently focused on developing hybrid devices where in one electrode energy storage is based on a chemical process, while in the other, it is based on a

An EDLC consists of two porous carbon electrodes in contact with the current collector, separated by a porous film and impregnated with an electrolyte solution. When there is an electric potential difference between the electrodes, the electrons in the negatively polarized electrode (anode) are balanced by an equal number of positive cations at the electrode/electrolyte interface, while the storage holes in the positively polarized electrode (cathode) are electrically balanced by the anions from the electrolyte. Figure 9 shows the scheme of the basic

The capacitance of a single electrode (Ce) is proportional to the surface area of the electrode (S)

dielectric electrolyte used and d is the effective thickness of the double layer. According to this equation, the capacitance of a single electrode will increase as the effective surface area of the electrode increases and the thickness of the double layer decreases (i.e., better contact between electrode and ions of electrolyte). This is the reason for using carbons with well-developed porosity (i.e., a high effective surface area, which means a higher surface area accessible to the electrolyte) as active materials in supercapacitors. Moreover, a greater or lesser ability to accommodate electrolyte ions on the surface of the electrode will also influence the thickness (d) of the double layer. Some studies show that the curvature of the surface (i.e., whether it is positive or negative) will influence the way in which solvated and desolvated ions of the

Ce ¼ ε<sup>0</sup> ε ð Þ S=d (1)

), ε is the relative permittivity of the

quence, supercapacitors offer a high power density and a very long cycle life.

physical double layer [66, 67].

78 Porosity - Process, Technologies and Applications

according to Eq. (1),

principle of energy storage in this kind of device.

where ε<sup>0</sup> is the vacuum permittivity (8.854 10�<sup>12</sup> F m�<sup>1</sup>

electrolyte are accommodated [68, 69]. In this respect, exohedral surfaces are more receptive, as it was mentioned above.

For a capacitor, two electrodes are needed, which is the equivalent to two capacitors in series. The resultant capacitance C can be expressed by the following equation Eq. (2),

$$1/\mathbb{C} = 1/\mathbb{C}\_{+} + 1/\mathbb{C}\_{-} \tag{2}$$

where C+ and C� are the capacitance of the cathode and the anode, respectively. Consequently, the difference between the capacitance values of a single electrode or complete capacitor is of great importance because if both electrodes have the same thickness, size, mass and material, the difference between both capacitances (in F g�<sup>1</sup> , i.e., the most common parameter employed in the bibliography) will be a factor of four. The maximum energy stored in a supercapacitor (E) is proportional to its capacitance and is given by Eq. (3),

$$\mathbf{E} = \left[ \mathbf{C} \left( \Delta \mathbf{E} \right)^{2} \right] / 2 \tag{3}$$

where ΔE is the operation voltage. The most commonly used operation voltage value is 1 V for aqueous electrolytes and around 2.7 V for organic electrolytes. Higher voltages would lead to the degradation of the components. It is clear that increasing the operation voltage is the most effective way to increase the energy density of a supercapacitor, which is why organic electrolytes are widely used in commercial devices, even though their conductivity, and their capacitance values (see Eq. (1)), are lower. However, the most common organic electrolytes (quaternary ammonium salts in polycarbonate or acetonitrile) have a number of disadvantages that can cause environmental problems; they are highly inflammable and chemically unstable, etc. In order to overcome these problems, research has been oriented toward developing organic electrolytes and ionic liquids, the latter being able to work up to 3.5 V. The wider range of electrolytes available will lead to a correspondingly wider range of porosities in the active material used, which is the topic of this chapter.

As already mentioned and in accordance with Eq. (1), the capacitance of an EDLC is proportional to the surface area of the electrode. However, this surface area is the effective surface area required for the interaction between the electrode-electrolyte, and it might not be the same as, for example, the most common specific surface area values determined by N2 adsorption-desorption isotherms at 196C. The presence of microporosity in the electrode is necessary, as micropores are the main contributors to the surface area of the electrode. However, the electrolyte must have access to all the microporosity. Therefore, the presence of feeder pores, i.e., mesopores, is also a determinant. Moreover, the ability to adjust the size of the mesopores to the electrolyte is important in order to favor its movement without penalizing the volumetric capacitance of the electrode. Apart from this, Gogotsi and col. [70] showed that the size of micropores also has a relevant role to play as if this is optimized, the electrolyte ions can be adsorbed in a desolvated or partially desolvated form, thereby minimizing the thickness of the double layer and increasing the capacitance of the electrode (see Eq. (1)). In summary, the ideal electrode will have a very large surface area with the size of micro- and mesopores optimized for the specific electrolyte that is going to be used in the EDLC. Carbon xerogels would appear to be the most suitable candidates for this kind of application, as their micro- and mesoporosity can be tailored to have both the required porosity and a good electrical conductivity.

conductivity of the active material, its chemical composition, the technique used to fabricate the electrodes and even the design and engineering aspects involved in the fabrication of the device. Also important is the final application of the supercapacitor: whether it is to be used for energy or power applications or portable or stationary applications (where the volume of the

Another factor that has a considerable influence on the capacitance of the supercapacitors is the presence of heteroatoms such as O, N, P or B in the structure of the electrode materials. On the one hand, these surface groups improve the wettability of the electrodes, facilitating contact and diffusion of the aqueous electrolyte in the electrode. On the other hand, if the electrode material has too many surface groups, faradaic reactions are likely to take place (redox reactions). This phenomenon widely known as pseudocapacitance may contribute substantially to increase the total capacitance of the EDLC. However, the presence of some functional groups (e.g., oxygen functional groups) can considerably reduce the electrical conductivity of the active material to the detriment of the overall performance of the supercapacitor, especially in relation to the power supply, as shown below. Moreover, the presence of pseudocapacitance, which is a chemical process, favors the degradation of the components, shortening the life span of the supercapacitor,

The other main feature that distinguishes supercapacitors from batteries is that they are able to supply great power densities. The maximum power density of the supercapacitor also depends on the maximal applicable voltage, see Eq. (4), but it is inversely proportional to the equivalent series resistance (ESR) of the system. The ESR corresponds to the sum of the (i) resistance of the active material used as electrode (i.e., electronic resistance and ionic diffusion resistance), (ii) resistance of the electrolyte selected and (iii) resistance of the assembly of the supercapacitor cell (electrode/current collector contact, type of current collectors and sheet separators, etc.) [41].

<sup>P</sup> <sup>¼</sup> ð Þ <sup>Δ</sup><sup>E</sup> <sup>2</sup>

Organic electrolytes and ionic liquids have another advantage over aqueous electrolytes that affects the operating voltage of the supercapacitor, namely that they result in higher power density values. In contrast, aqueous electrolytes have considerably higher electrical conductivities, i.e., several orders of magnitude higher, compared to organic and ionic liquids, besides lower viscosities that facilitate their diffusion in the porous structure and the ion movement during charge/discharge process. These characteristics may give higher capacitance values, compensating only partially the lower operating voltage used. It is clear that the properties of the active materials, in terms of porosity and electrical conductivity, will be the key to achieving a high power density, if the same electrolyte and supercapacitor cell configuration are used. Carbon materials with high electrical conductivities combined with an appropriate pore size distribution in order to facilitate ionic diffusion are essential for achieving a first-rate EDLC. Carbon xerogels are able to combine both characteristics, which is not usual in other porous carbons. They have therefore become the most sought-after active materials in supercapacitors. The most common representation of the relation between the main characteristics of a supercapacitor (i.e., energy and power) is the Ragone plot. Figure 10 shows the typical Ragone plot where the role of supercapacitors occupying the gap left to be filled by the conventional capacitors and batteries is highlighted. It is clear that the development of new carbon materials,

=ð Þ 4 ESR (4)

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 81

which is one of the features that distinguishes it from batteries.

device will be important), etc.

Clearly, each electrolyte will have different cation and anion sizes. Table 1 shows some recorded values for the most common electrolytes used in supercapacitors. However, it must be taken into account that these sizes refer to desolvated ions, and it is a known fact that ions move and may be adsorbed in solvated or partially solvated form. Thus, for example, the sizes of TEA+ and BF4 are 0.69 and 0.46 nm, respectively, but the same ions solvated in acetonitrile double that size (i.e., 1.30 and 1.16 nm for TEA+ and BF4 , respectively) [45]. Moreover, new electrolytes are continuously being developed in order to increase the operation voltage of EDLCs and these have a great influence on the maximum energy storage of the device (see Eq. (3)). For these reasons, certain materials susceptible to porosity manipulation, such as carbon xerogels, appear to be the key to future supercapacitor developments.

What is more, there is no general agreement about the optimum micro- and mesopore size, as many factors influence the final performance of the supercapacitor, such as the electrical


Table 1. Desolvated ion sizes for different electrolytes used in supercapacitors [71, 72].

conductivity of the active material, its chemical composition, the technique used to fabricate the electrodes and even the design and engineering aspects involved in the fabrication of the device. Also important is the final application of the supercapacitor: whether it is to be used for energy or power applications or portable or stationary applications (where the volume of the device will be important), etc.

As already mentioned and in accordance with Eq. (1), the capacitance of an EDLC is proportional to the surface area of the electrode. However, this surface area is the effective surface area required for the interaction between the electrode-electrolyte, and it might not be the same as, for example, the most common specific surface area values determined by N2 adsorption-desorption isotherms at 196C. The presence of microporosity in the electrode is necessary, as micropores are the main contributors to the surface area of the electrode. However, the electrolyte must have access to all the microporosity. Therefore, the presence of feeder pores, i.e., mesopores, is also a determinant. Moreover, the ability to adjust the size of the mesopores to the electrolyte is important in order to favor its movement without penalizing the volumetric capacitance of the electrode. Apart from this, Gogotsi and col. [70] showed that the size of micropores also has a relevant role to play as if this is optimized, the electrolyte ions can be adsorbed in a desolvated or partially desolvated form, thereby minimizing the thickness of the double layer and increasing the capacitance of the electrode (see Eq. (1)). In summary, the ideal electrode will have a very large surface area with the size of micro- and mesopores optimized for the specific electrolyte that is going to be used in the EDLC. Carbon xerogels would appear to be the most suitable candidates for this kind of application, as their micro- and mesoporosity can be tailored to have

Clearly, each electrolyte will have different cation and anion sizes. Table 1 shows some recorded values for the most common electrolytes used in supercapacitors. However, it must be taken into account that these sizes refer to desolvated ions, and it is a known fact that ions move and may be adsorbed in solvated or partially solvated form. Thus, for example, the sizes of TEA+ and BF4

are 0.69 and 0.46 nm, respectively, but the same ions solvated in acetonitrile double that size (i.e.,

ously being developed in order to increase the operation voltage of EDLCs and these have a great influence on the maximum energy storage of the device (see Eq. (3)). For these reasons, certain materials susceptible to porosity manipulation, such as carbon xerogels, appear to be the

What is more, there is no general agreement about the optimum micro- and mesopore size, as many factors influence the final performance of the supercapacitor, such as the electrical

) 0.69 0.46

H2SO4 0.10 0.53 KOH 0.26 0.11

Table 1. Desolvated ion sizes for different electrolytes used in supercapacitors [71, 72].

EMImBF4 0.95 0.68 0.52 0.52 EMImTFSI 0.95 0.68 1.13 0.84

) 0.65 0.46

, respectively) [45]. Moreover, new electrolytes are continu-

Cation size (nm) Anion size (nm)

both the required porosity and a good electrical conductivity.

1.30 and 1.16 nm for TEA+ and BF4

80 Porosity - Process, Technologies and Applications

Organic electrolytes (C2H5)4NBF4, (TEA<sup>+</sup>

Aqueous electrolytes

Ionic liquids (long wide)

(C2H5)3(CH3)NBF4 (TEMA<sup>+</sup>

key to future supercapacitor developments.

BF4

BF4

Another factor that has a considerable influence on the capacitance of the supercapacitors is the presence of heteroatoms such as O, N, P or B in the structure of the electrode materials. On the one hand, these surface groups improve the wettability of the electrodes, facilitating contact and diffusion of the aqueous electrolyte in the electrode. On the other hand, if the electrode material has too many surface groups, faradaic reactions are likely to take place (redox reactions). This phenomenon widely known as pseudocapacitance may contribute substantially to increase the total capacitance of the EDLC. However, the presence of some functional groups (e.g., oxygen functional groups) can considerably reduce the electrical conductivity of the active material to the detriment of the overall performance of the supercapacitor, especially in relation to the power supply, as shown below. Moreover, the presence of pseudocapacitance, which is a chemical process, favors the degradation of the components, shortening the life span of the supercapacitor, which is one of the features that distinguishes it from batteries.

The other main feature that distinguishes supercapacitors from batteries is that they are able to supply great power densities. The maximum power density of the supercapacitor also depends on the maximal applicable voltage, see Eq. (4), but it is inversely proportional to the equivalent series resistance (ESR) of the system. The ESR corresponds to the sum of the (i) resistance of the active material used as electrode (i.e., electronic resistance and ionic diffusion resistance), (ii) resistance of the electrolyte selected and (iii) resistance of the assembly of the supercapacitor cell (electrode/current collector contact, type of current collectors and sheet separators, etc.) [41].

$$\mathbf{P} = (\Delta \mathbf{E})^2 / (\mathbf{4} \text{ ESR}) \tag{4}$$

Organic electrolytes and ionic liquids have another advantage over aqueous electrolytes that affects the operating voltage of the supercapacitor, namely that they result in higher power density values. In contrast, aqueous electrolytes have considerably higher electrical conductivities, i.e., several orders of magnitude higher, compared to organic and ionic liquids, besides lower viscosities that facilitate their diffusion in the porous structure and the ion movement during charge/discharge process. These characteristics may give higher capacitance values, compensating only partially the lower operating voltage used. It is clear that the properties of the active materials, in terms of porosity and electrical conductivity, will be the key to achieving a high power density, if the same electrolyte and supercapacitor cell configuration are used. Carbon materials with high electrical conductivities combined with an appropriate pore size distribution in order to facilitate ionic diffusion are essential for achieving a first-rate EDLC. Carbon xerogels are able to combine both characteristics, which is not usual in other porous carbons. They have therefore become the most sought-after active materials in supercapacitors.

The most common representation of the relation between the main characteristics of a supercapacitor (i.e., energy and power) is the Ragone plot. Figure 10 shows the typical Ragone plot where the role of supercapacitors occupying the gap left to be filled by the conventional capacitors and batteries is highlighted. It is clear that the development of new carbon materials,

maximum specific capacity is limited to 372 mAhg�<sup>1</sup>

Figure 11. Scheme of the performance of a lithium-ion battery.

7. Conclusions

carbon atoms. Moreover, changes in volume that occur during the successive insertion-deinsertion cycles can cause cell failure when an active material such as graphite is used [1, 73, 78]. One possible remedy is to use alternative materials that combine electrical conductivity, an appropriate porous structure that permits a rapid charge-discharge and free space to accommodate

Carbon xerogels are porous materials whose porosity can be designed and adapted for use in ion-Li batteries as anodes, or at least to give some light on the mechanisms involved in the

Nanoporous carbon xerogels have a great potential in numerous applications since their nanoporosity can be tailored quite accurately. Mesoporosity can be predetermined during the synthesis of the organic xerogel relatively easily by controlling the proportion of each component of the solution precursor. However, the numerous variables involved, i.e., pH and type of catalyst used, degree of dilution, proportion of methanol that comes with the formaldehyde and R/F ratio, require a better knowledge of how their interrelation influences the resulting mesoporosity. The mesoporosity formed in this stage is relatively well preserved during the subsequent carbonization and/or activation processes, during which the micropososity of the carbon xerogel is formed. Depending on the carbonization or activation method selected for obtaining the carbon xerogel, the microporosity can also be tailored. The versatility of these nanoporous carbons makes them suitable for use in numerous applications. Additionally, carbon xerogels possess a good balance between porosity and electrical conductivity, two opposite properties. Thus, high surface area carbon xerogels have an electrical conductivity that is superior to other high surface area carbons. This makes carbon xerogels the ideal materials for use as electrodes in storage devices like supercapacitors where a high surface area and high conductivity are required. The potential of carbon xerogels for serving as electrode in lithium-ion batteries is also currently under investigation with promising results.

variations in volume. To achieve this combination of properties is no easy task.

reactions occurring inside the carbon anode, which are still unclear [79].

, corresponding to one lithium atom per six

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 83

Figure 10. Ragone plot of several active materials and carbon xerogels.

with the appropriate porosity and electrical conductivity for use in supercapacitors, should be focused on the upper right zone of the plot. To this end, carbon xerogel used as an active material offers a truly unique polymeric structure with a high electrical conductivity combined with a modulable micro- and mesoporosity and therefore optimizable supercapacitor energy and power supply. Figure 10 shows the location on the Ragone plot of a series of commercial carbons available for the supercapacitor market for use in EDLCs with aqueous electrolyte, in addition to the performances of different carbon xerogels. The advantage of the bespoke nanoporous carbon xerogels discussed in this study is evident.

Batteries are energy storage devices whose performance is based on the conversion of chemical energy into electrical energy via reversible reactions that take place between the anode and cathode electrodes. The working principle is based on the reversible migration of ions from the cathode during the charge stage, after which they migrate through the electrolyte to intercalate into the structure of a carbon-based anode material [73–77]. Figure 11 shows a schematic representation of a lithium-ion battery.

However, during charging, secondary reactions also take place leading to the formation of a stable passivation layer, known as the solid-electrolyte interface that entraps the ions, reducing the energy storage over time. As a consequence, the service life of batteries is limited. The formation of this layer and the diffusion of the ions through the anode material are complex mechanisms that depend largely on the chemical composition and porous properties of the active material used as anode.

Batteries provide high capacitances, i.e., they supply great energy densities, which is their most advantageous characteristic. However, the chemical process is relatively slow and so, unlike supercapacitors, the main challenge of batteries is to achieve high power densities. Different compounds can be used as electrodes: NaS, Ni-Cd, ion-Li, etc. Graphite is the most commonly used active material as anode material in lithium-ion batteries [1, 73, 78]. Nevertheless, its

Figure 11. Scheme of the performance of a lithium-ion battery.

maximum specific capacity is limited to 372 mAhg�<sup>1</sup> , corresponding to one lithium atom per six carbon atoms. Moreover, changes in volume that occur during the successive insertion-deinsertion cycles can cause cell failure when an active material such as graphite is used [1, 73, 78]. One possible remedy is to use alternative materials that combine electrical conductivity, an appropriate porous structure that permits a rapid charge-discharge and free space to accommodate variations in volume. To achieve this combination of properties is no easy task.

Carbon xerogels are porous materials whose porosity can be designed and adapted for use in ion-Li batteries as anodes, or at least to give some light on the mechanisms involved in the reactions occurring inside the carbon anode, which are still unclear [79].
