**2.2 Siloxene application in batteries**

Like supercapacitors, rechargeable batteries (e.g., lithium-ion batteries, sodium-ion batteries, lead-acid, etc.) are primary power sources for large-scale portable and wearable electronic devices. They have received significant consideration due to their high energy density and long cyclic stability [27, 28]. However, the current battery technologies cannot meet the advanced application requirement as the result of confined energy storage capacity. Thus, the development of commercial electrodes in the existing technologies is highly needed.

The theoretical capacity of silicon (Si) is 4200 mA hg−1 [29], which is higher than the capacity of graphite (372 mA hg−1), has considered being an active anode material for the future lithium-ion batteries (LIBs). However, the severe capacity degradation and the high-volume change during the lithiation-delithiation process may lead to lower Coulombic efficiency. Making 2D Si nanosheets with oxygen functional groups provides a high specific surface area, resulting in fast lithium storage and preventing volume changes. As mentioned in the previous section, the siloxene oxidation level can be controlled by the various synthesis conditions such as temperature, oxidants, concentrations, etc. The oxidation level may influence the lithiation-delithiation process. Xu and co-workers have demonstrated the siloxene preparation with different oxidation levels in the various oxidants and the temperature [30]. Three types of siloxene oxidation level have been achieved by altering the oxidants and temperature: (i) CuCl2 aqueous solution used to prepare fully oxidized siloxene nanosheet (FO-SNS) at room temperature; (ii) partially oxidized siloxene nanosheet (PO-SNS) made in SnCl2 ethanol solution at 60°C and (iii) hardly oxidized siloxene nanosheet (HO-SNS) synthesized in a LiCl-KCl molten salt at 400°C (**Figure 5(i)**). The FO-SNS, PO-SNS, and HO-SNS electrodes delivered the lithiation capacity of 298, 1218, and 1450 mA hg−1. Besides, the HO-SNS presented a higher Coulombic efficiency of 66%, which is higher than FO-SNS (24%) and PO-SNS (56%). The improved performance of HO-SNS has associated with the presence of a higher atomic percentage (64%) of bulk Si (Si0) and the lower percentage (7%) of SiO2 (Si4+) in HO-SNS, which were estimated from the XPS analysis (**Figure 5(ii)**). Besides, the hierarchical nanostructure of HO-SNS could buffer the volume expansion and contribute to the good rate performance. Fu and coworkers have remarked that bare siloxene is an unsuitable anode material for LIBs due to its inadequate electrochemical capacity, resulting in the lower Coulombic efficiency. However, Si- derivatives such as silicon suboxides (SiOx), carbon-coated SiO2, etc., from siloxene can meet higher capacity requirements with satisfactory Coulombic efficiency. Fu et al., have demonstrated the carbon-coated 2D SiOx nanocomposites (nano-Si/α-SiO2) from siloxene to moderate the volume expansion during the electrochemical lithiation-delithiation process [31]. The carbon-coated nano-Si/α-SiO2 anode materials showed the limited volume change, fast electrons

**163**

**Figure 5.**

**2.3 Siloxene based electrochemical sensor**

*Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor…*

transport, and more significant Li-ion kinetics, resulting in high initial Coulombic efficiency (72.5%) with a capacity of 946 mA hg−1. On the other hand, the value of x in SiOx can influence Li storage's electrochemical performance. Thus, controlling the oxidation level of the SiOx is a crucial process to achieve higher capacity than bare Si structures. Many previous studies showed that SiOx with x = 1.0 presented the specific capacity value higher than 1000 mA hg−1 [32]. However, unsatisfactory cyclic life has limited its practical usage. Thus, turning the oxygen content in SiOx is a proper way to improve the electrochemical performance in LIBs. After investigating carbon-coated nano-Si/α-SiO2, Fu and co-workers have prepared siloxene with different levels of oxidation in SiOx and used as anode material for LIB. They controlled the SiOx oxidation level in the siloxene via stepwise oxidizing of the siloxene precursor at various times [32]. SiOx with four different oxidation levels, such as SiO1.01, SiO1.25, SiO1.47, and SiO1.78 has been tailored through siloxene oxidation and investigated their Li-storage capacity. The sample SiO1.47 exhibited optimal electrochemical behavior due to the synergistic effect of electrical conductivity and Li-ion diffusivity. The higher oxygen level in SiOx caused a larger polarization effect, resulting in the poor Coulombic efficiency and smaller reversible capacity. Similar to the graphene-siloxene composite electrode in supercapacitors, the incorporation of siloxene sheets between the graphene layers enhances the specific surface area, facilitating the fast Li-storage. In the siloxene-graphene (SiG) composite, the siloxene sheets have provided higher Li-storage, and the encapsulated graphene sheets prevented the volume expansion during lithium insertion-extraction process. SiG anode material exhibited the initial cycle charge and discharge capacities of 3016 mA hg−1 and 3880 mA hg−1 with a capacity decay of 78%, which were higher than the bare siloxene and graphene electrodes. The synergistic effect of graphene and siloxene and the excellent electrical conductivity of graphene in the composite contributed to the higher electrochemical performance for LIBs [29].

*(i) SEM images of (a, b) FO-SNS, (c, d) PO-SNS, (e, f) HO-SNS; (ii) XPS spectrum of siloxene samples at* 

*different oxidation (reproduced from [30] with permission from Springer).*

The 2D siloxene sheets not only possessed the excellent electrochemical characteristics towards electrochemical energy application. Besides, due to the large

*DOI: http://dx.doi.org/10.5772/intechopen.93958*

*Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor… DOI: http://dx.doi.org/10.5772/intechopen.93958*

#### **Figure 5.**

*Novel Nanomaterials*

**Figure 4.**

**2.2 Siloxene application in batteries**

Like supercapacitors, rechargeable batteries (e.g., lithium-ion batteries, sodium-ion batteries, lead-acid, etc.) are primary power sources for large-scale portable and wearable electronic devices. They have received significant consideration due to their high energy density and long cyclic stability [27, 28]. However, the current battery technologies cannot meet the advanced application requirement as the result of confined energy storage capacity. Thus, the development of

The theoretical capacity of silicon (Si) is 4200 mA hg−1 [29], which is higher than the capacity of graphite (372 mA hg−1), has considered being an active anode material for the future lithium-ion batteries (LIBs). However, the severe capacity degradation and the high-volume change during the lithiation-delithiation process may lead to lower Coulombic efficiency. Making 2D Si nanosheets with oxygen functional groups provides a high specific surface area, resulting in fast lithium storage and preventing volume changes. As mentioned in the previous section, the siloxene oxidation level can be controlled by the various synthesis conditions such as temperature, oxidants, concentrations, etc. The oxidation level may influence the lithiation-delithiation process. Xu and co-workers have demonstrated the siloxene preparation with different oxidation levels in the various oxidants and the temperature [30]. Three types of siloxene oxidation level have been achieved by altering the oxidants and temperature: (i) CuCl2 aqueous solution used to prepare fully oxidized siloxene nanosheet (FO-SNS) at room temperature; (ii) partially oxidized siloxene nanosheet (PO-SNS) made in SnCl2 ethanol solution at 60°C and (iii) hardly oxidized siloxene nanosheet (HO-SNS) synthesized in a LiCl-KCl molten salt at 400°C (**Figure 5(i)**). The FO-SNS, PO-SNS, and HO-SNS electrodes delivered the lithiation capacity of 298, 1218, and 1450 mA hg−1. Besides, the HO-SNS presented a higher Coulombic efficiency of 66%, which is higher than FO-SNS (24%) and PO-SNS (56%). The improved performance of HO-SNS has associated with the presence of a higher atomic percentage (64%) of bulk Si (Si0) and the lower percentage (7%) of SiO2 (Si4+) in HO-SNS, which were estimated from the XPS analysis (**Figure 5(ii)**). Besides, the hierarchical nanostructure of HO-SNS could buffer the volume expansion and contribute to the good rate performance. Fu and coworkers have remarked that bare siloxene is an unsuitable anode material for LIBs due to its inadequate electrochemical capacity, resulting in the lower Coulombic efficiency. However, Si- derivatives such as silicon suboxides (SiOx), carbon-coated SiO2, etc., from siloxene can meet higher capacity requirements with satisfactory Coulombic efficiency. Fu et al., have demonstrated the carbon-coated 2D SiOx nanocomposites (nano-Si/α-SiO2) from siloxene to moderate the volume expansion during the electrochemical lithiation-delithiation process [31]. The carbon-coated nano-Si/α-SiO2 anode materials showed the limited volume change, fast electrons

commercial electrodes in the existing technologies is highly needed.

*Synthesis of siloxene-reduced graphene oxide hydrogel and its specific capacitance plot [26].*

**162**

*(i) SEM images of (a, b) FO-SNS, (c, d) PO-SNS, (e, f) HO-SNS; (ii) XPS spectrum of siloxene samples at different oxidation (reproduced from [30] with permission from Springer).*

transport, and more significant Li-ion kinetics, resulting in high initial Coulombic efficiency (72.5%) with a capacity of 946 mA hg−1. On the other hand, the value of x in SiOx can influence Li storage's electrochemical performance. Thus, controlling the oxidation level of the SiOx is a crucial process to achieve higher capacity than bare Si structures. Many previous studies showed that SiOx with x = 1.0 presented the specific capacity value higher than 1000 mA hg−1 [32]. However, unsatisfactory cyclic life has limited its practical usage. Thus, turning the oxygen content in SiOx is a proper way to improve the electrochemical performance in LIBs. After investigating carbon-coated nano-Si/α-SiO2, Fu and co-workers have prepared siloxene with different levels of oxidation in SiOx and used as anode material for LIB. They controlled the SiOx oxidation level in the siloxene via stepwise oxidizing of the siloxene precursor at various times [32]. SiOx with four different oxidation levels, such as SiO1.01, SiO1.25, SiO1.47, and SiO1.78 has been tailored through siloxene oxidation and investigated their Li-storage capacity. The sample SiO1.47 exhibited optimal electrochemical behavior due to the synergistic effect of electrical conductivity and Li-ion diffusivity. The higher oxygen level in SiOx caused a larger polarization effect, resulting in the poor Coulombic efficiency and smaller reversible capacity.

Similar to the graphene-siloxene composite electrode in supercapacitors, the incorporation of siloxene sheets between the graphene layers enhances the specific surface area, facilitating the fast Li-storage. In the siloxene-graphene (SiG) composite, the siloxene sheets have provided higher Li-storage, and the encapsulated graphene sheets prevented the volume expansion during lithium insertion-extraction process. SiG anode material exhibited the initial cycle charge and discharge capacities of 3016 mA hg−1 and 3880 mA hg−1 with a capacity decay of 78%, which were higher than the bare siloxene and graphene electrodes. The synergistic effect of graphene and siloxene and the excellent electrical conductivity of graphene in the composite contributed to the higher electrochemical performance for LIBs [29].

#### **2.3 Siloxene based electrochemical sensor**

The 2D siloxene sheets not only possessed the excellent electrochemical characteristics towards electrochemical energy application. Besides, due to the large

surface area and the unique 2D structure of siloxene, the heterogeneous electron transfer (HET) is high, which beneficial for selective electrochemical bio-marker detections. We have recently demonstrated the siloxene-based novel electrochemical dopamine sensor and obtained remarkable achievements in dopamine detection by the siloxene modified sensor [11]. Dopamine (DA) is an important neurotransmitter that plays a crucial role in the central nervous system and cardiovascular systems. A variety of materials have been employed for electrochemical DA detection in the past decades. However, the high selectivity of DA is limited to the existing materials. As a result of high HET rates, large surface area, and improved mass transportation, siloxene possessed high selectivity for DA detection (**Figure 6**). Siloxene modified glassy carbon electrode showed a well-defined redox peak in the cyclic voltammetry technique towards DA detection. Excellent linearity has been achieved for the siloxene electrode in the presence of a different concentration of DA, and the modified electrode exhibited a detection limit of 0.327 μM. Besides, the proposed sensor revealed a wide linear range from 10 to 1100 μM (**Figure 6(b)**).

The DA detection performance by the 2D siloxene sheets is remarkably higher than that of other reported 2D graphene and g-C3N4 modified electrodes. Siloxene sheets owned a higher response for the detection limit and showed high selectivity for DA detection. The stronger π-π interaction between the siloxene planar structure and the dopamine phenyl structure enables faster electron transportation during the DA oxidation process, making the high selectivity characteristic of the siloxene modified electrode. On the other hand, the π-π interaction of the siloxene structure with other biomolecules such as ascorbic acid, uric acid, etc., is weak, resulting in

**Figure 6.**

*(a-d) Electrochemical differential pulsed voltammetry response and linear range of siloxene modified electrode for DA detections (reproduced from [11] with permission from Elsevier).*

**165**

near future.

**Acknowledgements**

*Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor…*

the inactive oxidation. However, the thickness of the siloxene sheets can affect the electron conduction during the electrochemical reactions similar to graphene [33]. Reducing the size and the layer thickness of siloxene could tremendously enhance

In conclusion, this chapter deals with the comprehensive review of the raising star 2D siloxene based electrochemical energy and sensor applications. The siloxene synthesis process and the siloxene structure affecting parameters have been reviewed in detail. The functional groups in siloxene and the oxidation level can be varied at different synthesis times and the annealing temperature. Compared to pristine siloxene, high temperature treated siloxene possessed an excellent performance in the electrochemical supercapacitors because of its reduced functional groups. Besides, the siloxene and its composite have been used as anode materials for LIBs and showed a significant capacity and Coulombic efficiency. Li-storage has influenced by the oxidation level in siloxene due to the presence of different atomic percentages of Si functional groups. However, both supercapacitors and LIBs applications, siloxene derivatives such as SiOx, SiOC showed improved performance as the results of its better electrical conductivity and Li-ion diffusivity compared to the bare siloxene. The reported siloxene works have focused on the performance of siloxene in supercapacitors and LIBs. But many works failed to investigate the insight of the electrochemistry of siloxene and its derivatives for better energy density, capacity, and cyclic stability. Thus, the research direction should be focused more on the study of electrochemistry of siloxene. On the other hand, the 2D siloxene sheets proved as a novel electrochemical sensor for highly selective dopamine detection. Moreover, the size and thickness of the layer can influence the HET rate, specific surface area, and active sites for DA detection, which need to be optimized in the

This work was supported in part by the National Natural Science Foundation of China (Project No. 51950410598), in part by Shenzhen Science and Technology Innovation Committee (Projects No. JCYJ20170412154426330), and in part by Guangdong Natural Science Funds (Project No.: 2016A030306042 and 2018A050506001). Also, this work was supported by the Major Program of

Guangdong Basic and Applied Research (No. 2019B030302009).

*DOI: http://dx.doi.org/10.5772/intechopen.93958*

**3. Summary and future research direction**

its performance for DA detection.

*Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor… DOI: http://dx.doi.org/10.5772/intechopen.93958*

the inactive oxidation. However, the thickness of the siloxene sheets can affect the electron conduction during the electrochemical reactions similar to graphene [33]. Reducing the size and the layer thickness of siloxene could tremendously enhance its performance for DA detection.
