*3.2.2 XRD and Raman spectroscopy analysis of MWCNTs*

**Figure 3(a)** showed the X-ray diffraction (XRD) pattern of MWCNTs. The main diffraction peaks of MWCNTs were both at 2θ = 26°, which coincide with the (002) planes. The main diffraction peak of MWCNTs is sharp and narrow, which indicates that the MWCNTs have a more regular and orderly arrangement of carbon atoms. Moreover, MWCNTs have a higher degree of crystallinity and conductivity. The

**77**

**Figure 4.**

*TG curve of MWCNTs.*

*Performance and Applications of Lithium Ion Capacitors*

are great. In addition, the 2D peak appears at 2752 cm<sup>−</sup><sup>1</sup>

which confirm that the purity of these MWCNTs is great.

The energy density of LICs can be calculated by Esp = (Csp\*V2

*3.2.4 Galvanostatic charge and discharge*

MWCNTs have higher degree of crystallinity.

*3.2.3 TG of the MWCNTs*

(100) and (004) diffraction peaks are the catalyst components in the preparation of MWCNTs. **Figure 3(b)** showed the Raman spectroscopy of MWCNTs. There exhibited two distinct peaks corresponding to about 1351 cm-1D band and about 1585 cm-1G band, respectively. The MWCNTs have higher and sharper G peaks, which indicate that the degree of crystallinity and structure integrity of MWCNTs

**Figure 4** showed the TG curves of MWCNTs. The TG test was performed under

air atmosphere with the heating rate of 5°C/min to 1000°C. The TG curves of MWCNTs were divided into two stages. In the first stage, the weight loss of 0.11% is caused by the oxidation of a small amount of amorphous carbon during the synthesis of MWCNTs. The weight loss of the second stage is caused by the ablation of impurities in the MWCNTs. The initial reaction temperature of MWCNTs was 585°C, which indicates that the antioxidant capacity and thermal stability of MWCNTs were great. Meanwhile, the residual amounts of MWCNTs were 0.2%,

**Figure 5(a)** and **(b)** showed the galvanostatic charge-discharge curves of none-lithiated and prelithiated LICs at different current densities, respectively. The tests were performed using two-electrode system at voltage profile of 2–4 V.

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 prelithiated LICs showed a good linear relationship and exhibited a shape of isosceles triangle. On the contrary, the charge-discharge curves of nonlithiated LICs 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

, which indicates that the

)/2 (Csp represents the

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

**Figure 2.** *Micromorphology of (a) SLMP and (b) AC.*

**Figure 3.** *XRD (a) and Raman spectroscopy (b) of MWCNTs.*

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

(100) and (004) diffraction peaks are the catalyst components in the preparation of MWCNTs. **Figure 3(b)** showed the Raman spectroscopy of MWCNTs. There exhibited two distinct peaks corresponding to about 1351 cm-1D band and about 1585 cm-1G band, respectively. The MWCNTs have higher and sharper G peaks, which indicate that the degree of crystallinity and structure integrity of MWCNTs are great. In addition, the 2D peak appears at 2752 cm<sup>−</sup><sup>1</sup> , which indicates that the MWCNTs have higher degree of crystallinity.

### *3.2.3 TG of the MWCNTs*

*Science, Technology and Advanced Application of Supercapacitors*

**3.2 Results and discussion**

*3.2.1 The micromorphology of SLMP and AC*

The MWCNTs were characterized by field-emission scanning electron micros-

**Figure 2(a)** showed the micromorphology of SLMP. The diameters of the SLMP range from 30 μm, and outside coated with a thin layer of Li2CO3 protective coating, which can exist in a relatively low air humidity environment. **Figure 2(b)** showed the micromorphology of AC; it was observed that AC particles show irregular

**Figure 3(a)** showed the X-ray diffraction (XRD) pattern of MWCNTs. The main diffraction peaks of MWCNTs were both at 2θ = 26°, which coincide with the (002) planes. The main diffraction peak of MWCNTs is sharp and narrow, which indicates that the MWCNTs have a more regular and orderly arrangement of carbon atoms. Moreover, MWCNTs have a higher degree of crystallinity and conductivity. The

copy (FE-SEM, JSM-6701F), transmission electron microscopy (TEM, JEOL JEM-2010FEF), X-ray diffraction (XRD, DI SYSTEM), Raman spectrometer (SENTERRA), and thermogravimetry (TGA, PYRIS DIAMOND). The galvanostatic charge-discharge test of lithium-ion capacitors was performed after placed at

room temperature for 24 h by a cell tester (CT-3008W-5V5mA-S4).

morphology and the average size of the particles is about 4 μm.

*3.2.2 XRD and Raman spectroscopy analysis of MWCNTs*

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**Figure 3.**

**Figure 2.**

*XRD (a) and Raman spectroscopy (b) of MWCNTs.*

*Micromorphology of (a) SLMP and (b) AC.*

**Figure 4** showed the TG curves of MWCNTs. The TG test was performed under air atmosphere with the heating rate of 5°C/min to 1000°C. The TG curves of MWCNTs were divided into two stages. In the first stage, the weight loss of 0.11% is caused by the oxidation of a small amount of amorphous carbon during the synthesis of MWCNTs. The weight loss of the second stage is caused by the ablation of impurities in the MWCNTs. The initial reaction temperature of MWCNTs was 585°C, which indicates that the antioxidant capacity and thermal stability of MWCNTs were great. Meanwhile, the residual amounts of MWCNTs were 0.2%, which confirm that the purity of these MWCNTs is great.

## *3.2.4 Galvanostatic charge and discharge*

**Figure 5(a)** and **(b)** showed the galvanostatic charge-discharge curves of none-lithiated and prelithiated LICs 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 prelithiated LICs showed a good linear relationship and exhibited a shape of isosceles triangle. On the contrary, the charge-discharge curves of nonlithiated LICs 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

**Figure 4.** *TG curve of MWCNTs.*

**Figure 5.**

*Galvanostatic charge/discharge curves of LICs with nonlithiated (a) and prelithiated (b) specific capacitance with different current density (c) and the ragone plots (d) for the LICs.*

power density. Generally, the power density of lithium-ion capacitors is determined by the negative materials; when the negative electrode consists of nonlithiated MWCNTs, 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 SLMP. **Figure 5(c)** showed the discharging specific capacity at different rates. The prelithiated LICs showed higher discharging specific capacity and rate performance than those of nonlithiated LICs. The nonlithiated and prelithiated LICs exhibited discharging specific capacity of 10.74 and 85.18 F/g at current density of 0.1 A/g. **Figure 5(d)** showed the ragone plots of LICs. Prelithiated LICs presented the best electrochemical performance. The maximal energy density and power density of prelithiated LICs reached 140.4 Wh/kg and 5.25 W/kg in the range of current density from 0.05 to 4 A/g [34, 35].

**Figure 6(a)** showed the charge and discharge cycle performance of LICs with nonlithiated and prelithiated. The 3000 cycles test was performed in the range of 2~4 V at the current density of 0.4 A/g. After 3000 cycles of constant current charge and discharge, the cycling performance of LICs with nonlithiated drops significantly. In contrast, **Figure 6(b)** showed the discharge cycle performance of LICs with prelithiated after 3000 cycles. The capacitance retention still holds 82%, the charge and discharge curves without twist and distortion, which still maintained a good isosceles triangle shape and shows good cycle performance.

#### **3.3 Conclusions**

In the chapter, lithium-ion capacitors have been assembled with SLMP/ MWCNTs composite as anode and activated carbon as cathode, respectively. The results showed that prelithiated LICs exhibit excellent electrochemical performance. The addition of SLMP to anode can increase the electrochemical

**79**

storage device.

**Figure 6.**

**4.1 Experiment**

**composite as anode**

was controlled by contact time.

*4.1.2 The preparation of cathode*

*Performance and Applications of Lithium Ion Capacitors*

performance of the LICs and eliminate irreversible capacity. Especially, the prelithiated LICs exhibited optimal electrochemical performance, with a specific capacity of 85.18 F/g at current density of 0.1 A/g, and the maximal energy and power density reached 140.4 Wh/kg and 5.25 W/kg in the range of current density from 0.05 to 4 A/g, respectively. After 3000 charge-discharge cycles, the prelithiated LICs maintained about 82% capacity retention rate, Therefore, the prelithiated LICs with a SLMP addition in the anode have a potential application for energy

*Charge and discharge cycle performance of LICs with nonlithiated (a) and prelithiated (b).*

**4. Lithium-ion capacitors using prelithiated MWCNTs/graphite** 

The slurry of composite active material (MWCNTs/graphite) was prepared by ultrasonically dispersing and high-speed shearing with super carbon black (SP) as conductive agent, polyvinylidene difluoride (PVDF) as binder, and NMP as solvent, with the ratio of 8:1:1. The slurry was coated on the copper foil. Then, the anode was dried at 60°C under vacuum for 12 h. The MWCNTs content in composite active material was 0, 25, 50, 75, 100 wt%, respectively. The prelithiation was accomplished through direct physical contact between as-prepared MWCNTs/graphite electrode and lithium metal with electrolyte in pressure; the degree of prelithiation

The ratio of AC:SP:PVDF is 8:1:1, subsequently followed by ultrasonically dispersing, high-speed shearing, and coating on aluminum foil. Then, the cathode was dried at 60°C under vacuum for 12 h and was cut into a disc of 14 mm diameter.

The tailored MWCNTs/graphite anodes were used as working electrodes. Lithium foil was used as the counterelectrode and Celgard 2300 was used as the separator. The solution of 1.0 M LiPF6 in EC:DMC (1:1, vol.) was utilized as the electrolyte. Based

*4.1.3 The fabrication of MWCNTs/Li half-cells and lithium-ion capacitors*

*4.1.1 The preparation of anode and prelithiation procedure*

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

*Science, Technology and Advanced Application of Supercapacitors*

power density. Generally, the power density of lithium-ion capacitors is determined by the negative materials; when the negative electrode consists of nonlithiated MWCNTs, 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 SLMP. **Figure 5(c)** showed the discharging specific capacity at different rates. The prelithiated LICs showed higher discharging specific capacity and rate performance than those of nonlithiated LICs. The nonlithiated and prelithiated LICs exhibited discharging specific capacity of 10.74 and 85.18 F/g at current density of 0.1 A/g. **Figure 5(d)** showed the ragone plots of LICs. Prelithiated LICs presented the best electrochemical performance. The maximal energy density and power density of prelithiated LICs reached 140.4 Wh/kg and 5.25 W/kg in the range of

*Galvanostatic charge/discharge curves of LICs with nonlithiated (a) and prelithiated (b) specific capacitance* 

**Figure 6(a)** showed the charge and discharge cycle performance of LICs with nonlithiated and prelithiated. The 3000 cycles test was performed in the range of 2~4 V at the current density of 0.4 A/g. After 3000 cycles of constant current charge and discharge, the cycling performance of LICs with nonlithiated drops significantly. In contrast, **Figure 6(b)** showed the discharge cycle performance of LICs with prelithiated after 3000 cycles. The capacitance retention still holds 82%, the charge and discharge curves without twist and distortion, which still maintained a

good isosceles triangle shape and shows good cycle performance.

In the chapter, lithium-ion capacitors have been assembled with SLMP/ MWCNTs composite as anode and activated carbon as cathode, respectively. The results showed that prelithiated LICs exhibit excellent electrochemical performance. The addition of SLMP to anode can increase the electrochemical

current density from 0.05 to 4 A/g [34, 35].

*with different current density (c) and the ragone plots (d) for the LICs.*

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**3.3 Conclusions**

**Figure 5.**

**Figure 6.** *Charge and discharge cycle performance of LICs with nonlithiated (a) and prelithiated (b).*

performance of the LICs and eliminate irreversible capacity. Especially, the prelithiated LICs exhibited optimal electrochemical performance, with a specific capacity of 85.18 F/g at current density of 0.1 A/g, and the maximal energy and power density reached 140.4 Wh/kg and 5.25 W/kg in the range of current density from 0.05 to 4 A/g, respectively. After 3000 charge-discharge cycles, the prelithiated LICs maintained about 82% capacity retention rate, Therefore, the prelithiated LICs with a SLMP addition in the anode have a potential application for energy storage device.
