*4.2.2 Galvanostatic charge and discharge*

**Figure 8(a)** shows the first charge and discharge curves of raw MWCNTs and graphite half-cells at 1C rate; for graphite half-cells, the voltage plateau of SEI film formation is at about 0.7 V [36]. In comparison, for MWCNT half-cells, the

#### **Figure 7.**

*Illustration of lithium-ion capacitors and corresponding SEM images of the electrode materials. (a) SEM image of AC anode, (b) SEM image of graphite cathode, (c) SEM image of MWCNTs/graphite composite cathode.*

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than pure graphite.

**Figure 8.**

*of 60 min (c).*

*Performance and Applications of Lithium Ion Capacitors*

voltage plateau of SEI film formation is at about 0.7 V too. Meanwhile, MWCNTs have a higher irreversible capacity and first discharge capacity than graphite. **Figure 8(b)** shows the differential capacity versus voltage (dQ/dV) curves of MWCNTs and graphite half-cells. Three stages of lithium-ion intercalation voltage were local on 0.16, 0.08, and 0.055 V, respectively. **Figure 8(c)** shows the first delithiation (charge) capacity of CNT0, CNT25, CNT50, CNT75, and CNT100 at 60 min prelithiation time. In the same prelithiation time, the opencircuit voltage (OCV) of pure graphite half-cell was significantly superior to other half-cells. The delithiation capacity increases with the gradual increase of MWCNTs, which indicates the kinetics of intercalation of MWCNTs is higher

*The first charge-discharge curves of MWCNTs and graphite electrodes before being predoping (a), the differential capacity versus voltage (dQ/dV) curves of the MWCNTs/Li and graphite/Li coin cells (b) and the first charge curves of MWCNTs/graphite electrodes with different content of MWCNT at a prelithiation time* 

**Figure 9(a–e)** showed the galvanostatic charge-discharge curves of LIC0, LIC25, LIC50, LIC75, and LIC100 at different current densities, respectively. The tests were performed using two-electrode system at voltage profile of 2–4 V. The energy density of LICs can be calculated by Esp = (Csp\*V2)/2 (Csp represents the specific capacitance and V represents the discharge potential excluding IR drop). The power density of LICs can be calculated by Psp = Esp/t (t represents the discharge time), and the specific capacitance Csp can be calculated by the formula C = (2I\*t)/(m\*ΔV) (I represents the discharge current, m is the active material mass of a single pole, ΔV is the potential of discharge, and t is the discharge time). The charge-discharge curves of LIC25 showed a good linear relationship and exhibited a shape of isosceles triangle. The LIC25 had the longest discharge time than other LICs and showed good capacitance characteristics. Meanwhile, the charge-discharge curves of LIC75 also showed a good linear relationship and

*DOI: http://dx.doi.org/10.5772/intechopen.80353*

**Figure 8.**

*Science, Technology and Advanced Application of Supercapacitors*

total mass of the MWCNTs, graphite, AC, and SP.

*4.1.4 Characterizations*

**4.2 Results and discussion**

lithium-ion intercalation.

*4.2.2 Galvanostatic charge and discharge*

*4.2.1 The SEM of anode and cathodes*

on the content of MWCNTs, the half-cells were signed as CNT0, CNT25, CNT50, CNT75, and CNT100, respectively. The two-electrode LICs were assembled with AC cathode and MWCNTs anode, and the corresponding LICs were recorded as LIC0, LIC25, LIC50, LIC75, LIC100. All cells were assembled in an argon-filled glove box.

The SEM of anode and that of cathode were characterized by FE-SEM (JSM-6701F). The electrochemical characterization of the LICs was performed by a cell tester (CT-3008W-5V5mA-S4). The specific capacitance was calculated based on

**Figure 7(a)** shows the SEM image of AC anode, which shows irregular structure and occupies the vast majority of space. Meanwhile, SP uniformly dispersed between gaps of AC particles can provide good conductivity. **Figure 7(b)** shows the SEM image of graphite cathode, and **Figure 7(c)** shows the SEM image of MWCNTs/graphite composite cathode; comparison shows that MWCNTs and graphite are well connected and present a web-like network structure and three-dimensional conduction system. This structure was applied to the negative electrode to shorten the diffusion path of lithium ions and improve the kinetics of

**Figure 8(a)** shows the first charge and discharge curves of raw MWCNTs and graphite half-cells at 1C rate; for graphite half-cells, the voltage plateau of SEI film formation is at about 0.7 V [36]. In comparison, for MWCNT half-cells, the

*Illustration of lithium-ion capacitors and corresponding SEM images of the electrode materials. (a) SEM image of AC anode, (b) SEM image of graphite cathode, (c) SEM image of MWCNTs/graphite composite* 

**80**

**Figure 7.**

*cathode.*

*The first charge-discharge curves of MWCNTs and graphite electrodes before being predoping (a), the differential capacity versus voltage (dQ/dV) curves of the MWCNTs/Li and graphite/Li coin cells (b) and the first charge curves of MWCNTs/graphite electrodes with different content of MWCNT at a prelithiation time of 60 min (c).*

voltage plateau of SEI film formation is at about 0.7 V too. Meanwhile, MWCNTs have a higher irreversible capacity and first discharge capacity than graphite. **Figure 8(b)** shows the differential capacity versus voltage (dQ/dV) curves of MWCNTs and graphite half-cells. Three stages of lithium-ion intercalation voltage were local on 0.16, 0.08, and 0.055 V, respectively. **Figure 8(c)** shows the first delithiation (charge) capacity of CNT0, CNT25, CNT50, CNT75, and CNT100 at 60 min prelithiation time. In the same prelithiation time, the opencircuit voltage (OCV) of pure graphite half-cell was significantly superior to other half-cells. The delithiation capacity increases with the gradual increase of MWCNTs, which indicates the kinetics of intercalation of MWCNTs is higher than pure graphite.

**Figure 9(a–e)** showed the galvanostatic charge-discharge curves of LIC0, LIC25, LIC50, LIC75, and LIC100 at different current densities, respectively. The tests were performed using two-electrode system at voltage profile of 2–4 V. The energy density of LICs can be calculated by Esp = (Csp\*V2)/2 (Csp represents the specific capacitance and V represents the discharge potential excluding IR drop). The power density of LICs can be calculated by Psp = Esp/t (t represents the discharge time), and the specific capacitance Csp can be calculated by the formula C = (2I\*t)/(m\*ΔV) (I represents the discharge current, m is the active material mass of a single pole, ΔV is the potential of discharge, and t is the discharge time). The charge-discharge curves of LIC25 showed a good linear relationship and exhibited a shape of isosceles triangle. The LIC25 had the longest discharge time than other LICs and showed good capacitance characteristics. Meanwhile, the charge-discharge curves of LIC75 also showed a good linear relationship and

#### **Figure 9.**

*Galvanostatic charge-discharge curves of lithium-ion capacitors with different content of WCNT at a different current density, (a) LIC0, (b) LIC25, (c) LIC50, (d) LIC75, (e) LIC100, and specific capacitance with different current density (f), the ragone plots for the LICs (g).*

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**Figure 10.**

*Performance and Applications of Lithium Ion Capacitors*

exhibited high power performance. On the contrary, the charge-discharge curves of LIC0 and LIC100 presented a distorted shape, and the internal resistance obviously increases with the improving current density and the discharge time is obviously shortened, which related a poor power density. Generally, the power density of LICs is determined by the negative materials; when the negative electrode consists of pure graphite, the rate of intercalation and deintercalation of lithium ions is slow, resulting in a poor power density. The intercalation and deintercalation rate of lithium ions will be accelerated with the addition of MWCNTs. However, excessive amounts of carbon nanotubes will consume large amounts of lithium ions, and the formation of thick solid electrolyte interface (SEI) film will greatly impede the migration of lithium ions. That is, the appropriate MWCNTs content to improve the

**Figure 9(f )** showed the specific capacitance of LICs at various current densities. The LIC25 showed higher discharging specific capacitance and rate performance than other LICs. **Figure 9(g)** showed the ragone plots of LICs. LIC25 presented the best electrochemical performance. The maximal energy density and power density of LIC25 reached 96 Wh/kg and 10.1 kW/kg in the range of current density from

**Figure 10** showed the charge and discharge cycle performance of LIC0 and LIC25. The 3000 cycles test was performed in the range of 2.2~3.8 V at the current density of 0.8 A/g. After 5000 cycles of constant current charge and discharge, the cycling performance of LIC0 drops significantly, which is related to the cracking and pulverization of graphite materials, lithium, and organic solvents common into the graphite layer, and then influences the performance of cycle. As opposed to LIC0, the capacitance retention of LIC25 still holds 86%, the charge and discharge curves without twist and distortion, which still maintained a good isosceles triangle

In the chapter, lithium-ion capacitors have been assembled with prelithiated MWCNTs/graphite composite as anode and activated carbon as cathode. The results showed that LICs with prelithiated exhibit excellent electrochemical performance. Especially, the LIC25 exhibited optimal electrochemical performance, with a specific capacitance of 58.2 F/g at current density of 0.1 A/g, and the maximal energy and power density reached 96 Wh/kg and 10.1 kW/kg in the range of current

*Long-term cycle performance for the LIC in the voltage range of 2.2~3.8 V at 800 mA/g current density.*

*DOI: http://dx.doi.org/10.5772/intechopen.80353*

power density is of crucial importance.

shape and shows good cycle performance.

0.1 to 8 A/g.

**4.3 Conclusions**

#### *Performance and Applications of Lithium Ion Capacitors DOI: http://dx.doi.org/10.5772/intechopen.80353*

*Science, Technology and Advanced Application of Supercapacitors*

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**Figure 9.**

*Galvanostatic charge-discharge curves of lithium-ion capacitors with different content of WCNT at a different current density, (a) LIC0, (b) LIC25, (c) LIC50, (d) LIC75, (e) LIC100, and specific capacitance with* 

*different current density (f), the ragone plots for the LICs (g).*

exhibited high power performance. On the contrary, the charge-discharge curves of LIC0 and LIC100 presented a distorted shape, and the internal resistance obviously increases with the improving current density and the discharge time is obviously shortened, which related a poor power density. Generally, the power density of LICs is determined by the negative materials; when the negative electrode consists of pure graphite, the rate of intercalation and deintercalation of lithium ions is slow, resulting in a poor power density. The intercalation and deintercalation rate of lithium ions will be accelerated with the addition of MWCNTs. However, excessive amounts of carbon nanotubes will consume large amounts of lithium ions, and the formation of thick solid electrolyte interface (SEI) film will greatly impede the migration of lithium ions. That is, the appropriate MWCNTs content to improve the power density is of crucial importance.

**Figure 9(f )** showed the specific capacitance of LICs at various current densities. The LIC25 showed higher discharging specific capacitance and rate performance than other LICs. **Figure 9(g)** showed the ragone plots of LICs. LIC25 presented the best electrochemical performance. The maximal energy density and power density of LIC25 reached 96 Wh/kg and 10.1 kW/kg in the range of current density from 0.1 to 8 A/g.

**Figure 10** showed the charge and discharge cycle performance of LIC0 and LIC25. The 3000 cycles test was performed in the range of 2.2~3.8 V at the current density of 0.8 A/g. After 5000 cycles of constant current charge and discharge, the cycling performance of LIC0 drops significantly, which is related to the cracking and pulverization of graphite materials, lithium, and organic solvents common into the graphite layer, and then influences the performance of cycle. As opposed to LIC0, the capacitance retention of LIC25 still holds 86%, the charge and discharge curves without twist and distortion, which still maintained a good isosceles triangle shape and shows good cycle performance.

## **4.3 Conclusions**

In the chapter, lithium-ion capacitors have been assembled with prelithiated MWCNTs/graphite composite as anode and activated carbon as cathode. The results showed that LICs with prelithiated exhibit excellent electrochemical performance. Especially, the LIC25 exhibited optimal electrochemical performance, with a specific capacitance of 58.2 F/g at current density of 0.1 A/g, and the maximal energy and power density reached 96 Wh/kg and 10.1 kW/kg in the range of current

density from 0.1 to 8 A/g, respectively. After 3000 charge-discharge cycles, the LIC25 maintained about 86% capacity retention rate. Therefore, the LICs with the prelithiated MWCNTs/graphite composite materials have a potential application for energy storage device.
