**3. SLMP/MWCNTs composite anode for lithium-ion capacitors**

#### **3.1 Experiment**

*Science, Technology and Advanced Application of Supercapacitors*

lated lithium anodes.

capacitors was investigated.

**2. Fabrication and characterization of MWCNTs**

In the carbon-based lithium-ion capacitors, the lithium ions are mainly derived from the electrolyte. But the solid-electrolyte interface (SEI) film formed during cycles will consume an amount of lithium ions which are irreversibly embedded in negative materials. That will bring down the capacity and cycle performance of LICs. So it is particularly important for the lithium predoping in negative electrode [21]. MWCNTs composed of unique one-dimensional systems with nanostructure have better stability, excellent conductivity, and lithium capacitance. It has become a popular research object for lithium-ion batteries [22]. SLMP applied to negative electrode can effectively prevent the problem of lithium ions deficiency and increase the capacity and rate performance of the LICs [23]. There is a potential difference between carbon electrode and lithium metal, which will promote the continuous flow of lithium ions into the carbon electrode when the carbon electrode and lithium metal are connected by short circuiting [9, 24, 25]. The final potential of the carbon anode will drop close to 0 V (vs. Li/Li+). Here, we introduce two new type LICs with different preinterca-

It is generally known that graphite has a high theoretical Li intercalation capacity and widely was used as anode materials for lithium-ion capacitors because of natural abundance and relatively low cost [26–30]. However, lithiumion intercalation tended to the same direction, and the dynamics of lithium-ion intercalation is slow. So it is difficult to perform charge/discharge work for lithium-ion capacitor at high current density with a poor rate performance [31, 32]. Compared to graphite, MWCNTs have higher stability. In this chapter, we report internal short circuit (ISC) approach was applied to high-performance LICs with activated carbon as cathode and prelithiated multiwalled carbon nanotubes/graphite composite as anode. Electrochemical performance of lithium-ion

MWCNTs were prepared by chemical vapor deposition (CVD), and benzene (Aladdin Co. Ltd., Shanghai) was used as carbon source, ferrocene (Aladdin Co. Ltd., Shanghai) as catalyst, and thiophene (Aladdin Co. Ltd., Shanghai) as accelerant. Ferrocene and thiophene were added into benzene and stirred uniformly; the flow rate was controlled by a micropump. Hydrogen and argon were used as carrier gas. The flow rate was controlled by a mass flow meter. The carbon source was fed into reactor with carrier gas. The MWCNTs were synthesized in a tube furnace with appropriate contents of ferrocene and thiophene and the ratio of benzene to

**74**

**Figure 1.**

*(a) FESEM and (b) HRTEM images of MWCNTs.*

### *3.1.1 Preparation of cathode electrode*

The activated carbon (AC) was dispersed by sonication in N-methyl-2 pyrrolidone (NMP) for 2 h. The surfactant of polyvinylpyrrolidone (PVP, YueMei chemical Co. Ltd., Guangzhou) was added to improve the dispersion performance. The polyvinylidene fluoride (PVDF) was used as binder. The super carbon black (SP) was added to improve the conductivity. The activated carbon slurry was completed after a high-speed (FA25) cutting under 10,000 r/min for 1 h, and the mass ratio of AC:SP:PVDF was 85:5:10. The prepared slurry was coated on Al foil and dried at 60°C under vacuum, and cut into a disc of 14 mm diameter.

#### *3.1.2 Preparation of anode electrode*

The MWCNT powders were dispersed by sonication in N-methyl-2-pyrrolidone (NMP) for 2 h. The surfactant of polyvinylpyrrolidone was added to improve the dispersion performance. The polyvinylidene fluoride (PVDF) was used as binder. The slurry was completed after a high-speed (FA25) cutting under 10,000 r/min for 1 h. The mass ratio of MWCNTs:SP:PVDF was 8:1:1. The prepared slurry was coated on Cu foil and dried at 60°C under vacuum. A mixture of 0.5% polystyrene (PS) and 0.5% styrene butadiene rubber (SBR) was selected as a polymer binder, xylene as a solvent, and two groups were mixed to produce a binder solution. SLMP (FMC Corporation) was dispersed in the binder solution to obtain SLMP suspension with 0.5 wt%. Then the SLMP suspension was evenly coated on anode. After dried in vacuum, the SLMP coating is pressed between two glass plates for activating SLMP and then cut into a disc of 14 mm diameter.

#### *3.1.3 Fabrication and characterization of lithium-ion capacitors*

The two-electrode CR-2025 button lithium-ion capacitors were assembled with activated carbon as cathode and SLMP/MWCNT composites as anode in an argonfilled dry glove box. The electrolyte was 1 mol/L LiPF6 in a mixed solvent system of EC/DMC (ethylene carbonate/diethyl carbonate) at a ratio of 1:1, and polypropylene microporous membrane was used as the separator.

The MWCNTs were characterized by field-emission scanning electron microscopy (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).
