Effect of S-doped carbon nanotubes as a positive conductive agent in lithium-ion batteries

doi:10.21203/rs.3.rs-4117338/v1

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Effect of S-doped carbon nanotubes as a positive conductive agent in lithium-ion batteries

https://doi.org/10.21203/rs.3.rs-4117338/v1

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published 24 Apr, 2024

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In this paper, sulfur-doped carbon nanotubes were synthesized and modified at 600, 700 and 800°C. The results showed that the amount of sulfur doped in carbon nanotubes increased with the increase of temperature, which were 0.78%, 0.98%, and 1.07%, respectively, but the carbon/sulfur binding mode did not change. At the same time, sulfur doping significantly increased the specific surface area, which was conducive to improving the infiltration of the electrolyte into the electrode piece. Sulfur-doped carbon nanotubes are used as conductive agents for the cathode NCM523 of lithium-ion batteries, and compared with untreated carbon nanotubes, they effectively improve the battery polarization, reduce the internal resistance, and greatly improve the ratio performance, and in terms of cycling performance, the capacity retention rate of the battery is increased from 71.3% to 81 ~ 85%.

carbon nanotubes

post-synthesis modification

sulfur drop

Lithium ion batteries have the characteristics of high energy density, high rate, and long cycle, which makes their market share in fields such as digital 3C and power vehicles increasingly widespread^[1]. With the increasing demand for applications, some problems of lithium-ion batteries also urgently need to be solved. Most active materials used in cathode materials for lithium-ion batteries have poor conductivity, resulting in relatively large electrode internal resistance and low utilization of active material, which seriously affects the performance of the battery in terms of rate capability, cycling stability, and safety^{[2, 3]}, the contraction and expansion of materials in the process of charging and discharging also greatly affect the safety and life of the battery^{[4, 5]}. Conductive additives can increase the conductive contact between active materials, improve the electronic conductivity, generate micro-current between active materials and collector surfaces, reduce electrode contact resistance, accelerate electron mobility^[5-7].Therefore, it is often necessary to add additional conductive materials to improve battery performance. Due to the requirements for stability and resistance to acid and alkaline corrosion, conductive additives in lithium-ion batteries are mostly carbon-based materials^[8-11], including conductive carbon black, conductive graphite, carbon fibers, graphene, and carbon nanotubes. In addition to their excellent electrical conductivity, carbon nanotubes have a fiber-like structure that provides good flexibility and mechanical stability^[12-14], It is conducive to improving the processability of the battery plate and the cycle life of the battery. Therefore, they are favored by researchers and manufacturers.

Common preparation methods for carbon nanotubes include arc discharge, laser evaporation and chemical vapor deposition^[15-17]. Due to the need for batch preparation, the chemical vapor deposition method is currently the most widely used. The resulting carbon nanotubes contain residual catalysts such as iron, cobalt, nickel, and aluminum. To remove these impurities, purification using strong acids such as nitric acid or sulfuric acid is a common method^{[18, 19]}, but this process can result in lattice defects and the formation of oxygen functional groups on the surface of the carbon nanotubes, which reduces their electrical conductivity. However, lattice defects and oxygen functional groups on carbon nanotubes are suitable for use as active sites for modifying the material.

Post-synthetic modification is an easy method for modifying carbon nanotubes. Common dopants include nitrogen, phosphorus, sulfur, boron, etc^[20-25]. The introduction of impurities can improve the insertion of metal ions into carbon materials, change the crystal structure and surrounding charge distribution, and reduce the migration and diffusion obstacles that metal ions must overcome during doping processes^[26-28]. There are few reports on sulfur doped carbon nanotubes as conductive materials for lithium-ion batteries

In this report, we prepared CNTs with different amounts of S doping at 600, 700, and 800℃ and found that when used as a conductive additive for NCM523 batteries, they significantly reduced battery polarization and internal resistance, improving high-rate charge/discharge performance and cycle life.

Preparation of S-droped CNTs

Carbon nanotubes (CNTs) were synthesized via chemical vapor deposition (CVD). SO₂ gas was used as the sulfur source during synthesis.

Sulfur-doped carbon nanotubes were prepared via a post-synthesis method. CNTs were placed in a ceramic crucible, which was then placed in a furnace with an N₂ atmosphere. The crucible was heated to the final temperature [600°C, 700°C, and 800℃] at a heating rate of 5 ℃·min^− 1. SO₂ was fed into the furnace at a rate of 10 mL·min^− 1. The final temperature of the furnace was maintained for 1 h. Then, the furnace was allowed to cool to 25℃. The resulting materials were named CNTs@S-1, CNTs@S-2, and CNTs@S-3 respectively, and the initial sample is called CNTs. For the sake of convenience, we will refer to them collectively as S-droped CNTs and the original sample later on.

Material characterization

X-ray diffraction (XRD; Rigaku, Miniflex-600), Raman spectroscopy, transmission electron microscopy (TEM; FEI, Titan G260-300), scanning electron microscopy (SEM; ZEISS, EVO18), X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha), energy-dispersive X-ray spectroscopy (EDS), specific surface area and pore size analysis (Beishide Insteument, PS2-M025-B) were used to characterize the materials.

Battery preparation

The ratio of the cathode material/conductive agent/binder in the battery is 90:4:6. LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) was used as the cathode material, and the conductive agent is composed of 50% carbon nanotubes(CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3) and 50% Super-P. Polyvinylidene fluoride (PVDF) was dissolved in N-methyl-2-pyrrolidone (NMP). Then, carbon nanotubes, Super P., and dispersant(Polyvinylpyrrolidone) were added to NMP solution and ultrasonicated and magnetically stirred to obtain a conductive slurry. NCM523 was added to the conductive slurry, and the electrode slurry was obtained after high-speed dispersion. The electrode slurry was coated on an aluminum foil, dried, and cut into circles with a diameter of 12 mm, which was used for a button cell. The electrolyte comprised 1 M LiPF₆ in a 1:1 (v/v) mixture of EC and DMC, and a lithium metal sheet was used as the negative electrode. Assemble the battery in the order of negative electrode shell, lithium sheet, separator, positive electrode sheet, gasket, spring sheet, and positive electrode shell in a glove box with an inert atmosphere.For the sake of convenience, we refer to the batteries prepared with CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3 as conductive agents as C-1, C-2, C-3, and C-4, respectively

The batteries were left for 24 h and then electrochemically tested at 25℃. The constant current charge and discharge performance of the batteries in the voltage range of 2.75–4.2 V was analyzed (Neware, BTS-5 V/10 mA). Electrochemical impedance spectroscopy (EIS) was performed between 0.01 Hz and 100 kHz (Metronhm, PGSTAT204), and cyclic voltammetry (CV; Ivium-Stat) was performed at 0.1–1 mV/s in the voltage range of 2.75–4.3 V.

The EDS maps in Fig. 1 show that sulfur is efficiently and uniformly incorporated into carbon nanotubes.

The XRD characteristic peaks of the carbon nanotubes are (002) and (100) peaks with 2-Theta angles around 26° and 42.9°, as shown in Fig. 2, which we can observe in all samples. Compared to the original samples (CNTs), S-droped CNTs have no significant deviation in peak intensity and peak position, indicating that the doping sites of sulfur atoms are mainly concentrated on the surface layer of carbon nanotubes.

Figure 2b shows the Full Spectrum XPS spectrum of all samples,it can be seen that CNTs are primarily composed of two elements, i.e., C and O. In the XPS spectra of S-droped CNTs the S_2p hybrid characteristic peak is observed at ~ 160 eV. The sulfur contents of CNTs@S-1, CNTs@S-2, and CNTs@S-3 are 0.78%, 0.98%, and 1.07%, respectively, consistent with EDS results, further proving the successful sulfur doping of CNTs. The S_2p peak fitting (Figs. 2(c–e)) reveals that the S_2p peaks observed in the XPS spectra of samples obtained at different temperatures[600,700,800℃] appear at approximately 163.9 and 165.1 eV correspond to the C–S bond (C–S) and C = S bond (165.5 eV), respectively, and the peak of the carbon–sulfur oxide bond of C–SO_X (x = 2–4); 168.9 eV) is not observed in the XPS spectrum of S-droped CNTs, indicating that under the experimental conditions, the temperature affects the sulfur doping amount of CNTs, but not the carbon and sulfur binding mode.

, CNTs@S-1, CNTs@S-2 and CNTs@S-3, (b) XPS XPS Full Spectrum of CNTs

, CNTs@S-1, CNTs@S-2 and CNTs@S-3, (c) S2p of CNTs@S-1, (d) S2p of CNTs@S-2, (e) S2p of CNTs@S-3.

Raman spectroscopy is an efficient technique to identify electronic or structural rearrangements and the atomic properties of defects in carbon nanotubes. In the Raman spectrum, the D peak at ~ 1310 cm^− 1 represents the C atom lattice defect or disorder site, and the G peak at ~ 1600 cm^− 1 is the characteristic peak of sp² carbon atoms. The intensity of the D peak to the G peak (called the I_D/I_G ratio) is usually used to represent the degree of defects in carbon nanotubes or graphene. The Raman spectra of CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3 are shown in Fig. 3a. The I_D/I_G ratios of CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3 are 0.62, 0.63, 0.64, and 0.57, respectively.

Figures 3(b–d) show the TEM images of CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3, it can be seen that there is no significant change in the structure and morphology of the carbon nanotubes before and after doping, indicating that sulfur doping does not damage the structure of carbon nanotubes.

The pore structures of CNTs and S-droped CNTs were studied via N₂ adsorption–desorption isotherms. Figure 4 shows that specific surface areas of CNTs, CNTs@S-1, CNTs@S-2, and CNTs@S-3 are 285.69, 295.77, 308.52, and 354.51 m²·g^− 1, respectively. This means that as the sulfur doping amount in carbon nanotubes increases, the specific surface area of the S-droped CNTs considerably increases. This is because the atomic radius of sulfur is much greater than that of carbon, and the C–S bond is longer than the C–C bond; hence, the combination of sulfur and carbon distorts the six-ring structure of carbon nanotubes in the same plane. Such changes are beneficial to the diffusion of lithium ions.

CV was performed in the range of 0.1–1 mV. The potential difference between the oxidation/reduction peaks is usually used to indicate the degree of polarization of a cell. As shown in Fig. 5, the battery (C-1) manufactured using untreated carbon nanotubes as conductive agents (carbon nanotubes) exhibits the most severe polarization, compared with C-1, the polarization of batteries (C-2, C-3, C-4) made with sulfur doped carbon nanotubes as conductive agents has significantly decreased, with C1, C4, C3, C2 in descending order from high to low. Polarization decreases the battery output power and energy utilization and affects the service life and safety of the battery. Reduced polarization can increase the effective current and improve the rate performance of the cells.This indicates that sulfur doping can effectively improve the rate, cycling, and safety of batteries

Chart 1.ΔE of C-1, C-2, C-3 and C-4 in different scan rates

Scan rate/mV·s^− 1	Materials
Scan rate/mV·s^− 1	CNTs	CNTs@S-1	CNTs@S-2	CNTs@S-3
0.1	0.317	0.145	0.172	0.171
0.2	0.332	0.152	0.248	0.268
0.3	0.425	0.272	0.327	0.315
0.4	0.534	0.325	0.392	0.367
0.5	0.576	0.378	0.446	0.426
0.6	0.642	0.441	0.495	0.472
0.7	0.696	0.45	0.547	0.516
0.8	0.759	0.488	0.589	0.56
0.9	0.812	0.521	0.632	0.591
1	0.855	0.553	0.672	0.637

EIS results in the frequency range of 0.1–100 kHz are shown in Fig. 6a. The high-frequency region is controlled by the electrode reaction dynamics (charge transfer process), and the low-frequency region is controlled by the diffusion of reactants or products produced at the electrode. From the perspective of the semicircle diameter in the high-frequency region, as conductive agents, sulfur-doped carbon nanotubes considerably reduce the charge transfer resistance (R_ct) of the battery, which further decreases with an increase in the doping amount. Furthermore, the oblique line in the low-frequency region in the EIS plots of CNTs also significantly deviates from 45°, indicating that sulfur doping is conducive to the diffusion of lithium ions.

Figure 6(b) shows taht the first charge and discharge performance of the battery, which was studied at 25°C and 0.2 C. The first discharge capacities of C-1, C-2,C-3 and C-4 are 156.3, 157.0, 160.5, and 162.9 mAh·g^− 1, respectively, and the first coulombic efficiencies are 84.4%, 85.0%, 87.0%, and 89.2%, respectively, both of which increase as the S doping amount increases, consistent with EIS results.

Figure 6(d) shows that rate performance shows a different regularity from EIS and first discharge specific capacity. Firstly, at a low rate (< 1 C), there is almost no difference in the discharge specific capacity of C-1, C-2, C-3, and C-4. However, as the discharge rate increases(2 C, 3 C, 5 C ), the influence of polarization on the battery performance increases, the discharge specific capacity and stability of batteries (C-2, C-3, C-4) using S-droped CNTs as conductive agents is significantly higher than that of original sample(C-1).

The cycle performance of cells was tested at 3 C. Figure 6(d) shows that the initial discharge specific capacity of batteries (C-2, C-3, C-4) using S-droped CNTs as conductive agents is significantly higher than that of original sample(C-1). The capacity retention rates of C-1, C-2, C-3, and C-43 are 71.3%, 83.1%, 84.7%, and 81.8%, respectively. The cycle performance of cells is affected by battery internal resistance and polarization.

Fiugre 6. Electrochemical properties.(a)EIS, (b)First charge and discharge, (c)Rate performance, (d)Cyclic performance at 3 C

In this paper, we prepared sulfur-doped carbon nanotubes with different doping amounts at 600,700, and 800℃ after synthetic modification, and found that the doping amount of sulfur elements also increased with the increase of temperature. The specific surface area and pore of carbon nanotubes increased significantly with increasing sulfur doping. Button cells were prepared by adding each carbon nanotube to NCM523, and it was found that sulfur-doped carbon nanotubes greatly reduced the polarization and internal resistance of the cells. In terms of electrochemical performance, cells with higher sulfur doping showed higher first-discharge specific capacity and coulombic efficiency. The original samples, 0.78%, 0.98 and 1.07% sulfur-doped carbon nanotubes were 156.3 mAh·g^− 1 (84.4%), 157 mAh·g^− 1 (85%), 160.5 mAh·g^− 1 (87%) and 162.9 mAh·g^− 1 (89.2%), respectively. In addition to this, the discharge capacity of the battery at high magnifications has also been increased.The cycle life test of the battery was carried out at 3C rate, and it was found that the capacity retention rate of the battery was also greatly improved after 100 cycles, The original samples, 0.78%, 0.98 and 1.07% sulfur-doped carbon nanotubes were 71.3%, 83.1%, 84.7% and 81.8%. This is of great significance for the research and development of fast-charging and discharging lithium-ion batteries.

Funding

PRC Ministry of Industry and Information Technology (TC220H06P)

Author Contribution

Jinchao Huang is mainly responsible for the experiment of the thesis and the writing of the thesis. Shengwen Zhong provide financial support for experiments. Ziting Guo, Qingmei Xiao and Min Zeng are mainly responsible for the editing of pictures. All authors reviewed the manuscript.

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No competing interests reported.

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You are reading this latest preprint version

Effect of S-doped carbon nanotubes as a positive conductive agent in lithium-ion batteries

Status:

Journal Publication

Version 1

Abstract

Figures

Intorduction

Experimental Methods and Sample Characterization

Preparation of S-droped CNTs

Material characterization

Battery preparation

Results and Discussion

, CNTs@S-1, CNTs@S-2 and CNTs@S-3, (b) XPS XPS Full Spectrum of CNTs

Chart 1.ΔE of C-1, C-2, C-3 and C-4 in different scan rates

Conclusions

Declarations

Funding

Author Contribution

References

Additional Declarations

Status:

Journal Publication

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