NiCo2S4 Nanocrystals on Nitrogen-Doped Carbon Nanotubes as High-Performance Anode for Lithium-Ion Batteries

In recent years, the development of lithium-ion batteries (LIBs) with high energy density has become one of the important research directions to fulfill the needs of electric vehicles and smart grid technologies. Nowadays, traditional LIBs have reached their limits in terms of capacity, cycle life, and stability, necessitating their further improvement and development of alternative materials with remarkably enhanced properties. A nitrogen-containing carbon nanotube (N-CNT) host for bimetallic sulfide (NiCo2S4) is proposed in this study as an anode with attractive electrochemical performance for LIBs. The prepared NiCo2S4/N-CNT nanocomposite exhibited improved cycling stability, rate performance, and an excellent reversible capacity of 623.0 mAh g–1 after 100 cycles at 0.1 A g–1 and maintained a high capacity and cycling stability at 0.5 A g–1. The excellent electrochemical performance of the composite can be attributed to the unique porous structure, which can effectively enhance the diffusivity of Li ions while mitigating the volume expansion during the charge–discharge processes.


Background
Lithium-ion battery (LIB) is a leading battery technology used in portable electronic devices, electric vehicles, and renewable energy storage [1,2]. Therefore, the development of LIBs with a high energy density has become a research direction crucial for the sustainable development of various sectors of economics and industry [3][4][5]. For instance, the specific energy density of a commercial graphite anode material reached its theoretical capacity of 372 mAh g -1 , which does not leave much room for its further enhancement to satisfy the performance requirements of emerging electronics and electric vehicle technologies [6,7]. As a result, it is essential to develop alternative anode materials for LIBs to satisfy the needs of the modern society.

Open Access
*Correspondence: yongguangzhang@hebut.edu.cn 1 School of Materials Science and Engineering, Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Hebei University of Technology, Tianjin 300130, China Full list of author information is available at the end of the article reversibility of charge/discharge processes due to the accompanying volume variation, leading to material disintegration and consequently severe capacity fading [18]. Another serious problem originates from the shuttle effect of polysulfides produced by the dissolution of lithium polysulfide (LPS) in the electrolyte, resulting in a low capacity retention as well [19,20].
To overcome the issues of NiCo 2 S 4 anodes related to the volume change and LPS dissolution, various approaches including nanostructuring and use of carbonaceous additives and carbon-based hosts have been developed with promising results. Nanostructuring and its combination with carbon/graphene networks can increase the electrode-electrolyte interface contact area and shorten the Li-ion pathways, leading to a higher specific capacity [18]. Therefore, this study reports the in situ growth of NiCo 2 S 4 nanoparticles onto carbon nanotubes (CNT) structure using a hydrothermal method. Furthermore, to increase the electroactivity of the electrode material, the nitrogen (N) heteroatoms were incorporated into the CNT matrix. Such a processing makes N-CNT more conducive, leading to the uniform growth of NiCo 2 S 4 and thus improving the crystallinity of NiCo 2 S 4 /N-CNT anode. In this unique structure, CNT forms an elastic matrix that enhances the structural stability, improves the ionic conductivity of the composite, and mitigates the volume variation of NiCo 2 S 4 particles. The NiCo 2 S 4 /N-CNT material maintains good capacity retention during cycling and significantly restrains the voltage fading. The NiCo 2 S 4 /N-CNT composite anode exhibits an initial discharge capacity of 1412.1 mAh g -1 at 0.1 A g -1 , and the discharge capacity remains at 623.0 mAh g -1 after 100 cycles.

Synthesis of NiCo 2 S 4
First, 0.074 g of Co(AC) 2 ·4H 2 O and 0.037 g of Ni(Ac) 2 ·4H 2 O were dissolved in 40 mL ethanol. The solution was stirred on a water bath at 80 °C for 2 h and at room temperature for another 2 h. Then, 0.078 g of thiourea was added to the mixture, which was further continuously stirred for 20 h before transferring the reaction mixture to a 100 mL autoclave. The hydrothermal reaction was carried out at 170 °C for 3 h. After cooling to room temperature, the product was washed several times with deionized water and freeze-dried under reduced pressure.

Synthesis of NiCo 2 S 4 /N-CNT Nanocomposites
First, 68 mg of mildly oxidized CNT was ultrasonically dispersed in 40 mL of ethanol. Then, 0.074 g of Co(AC) 2 ·4H 2 O and 0.037 g of Ni(Ac) 2 ·4H 2 O were added, and the mixture was stirred on a water bath at 80 °C for 2 h. Next, 2 mL of NH 3 ·H 2 O and 0.078 g of thiourea were added to the solution, and the reaction mixture was stirred for 2 h. The reaction mixture was transferred to a 50-mL autoclave, followed by a hydrothermal reaction at 170 °C for 3 h. The product was cooled to room temperature and centrifuged with deionized water several times and freeze-dried. NiCo 2 S 4 /CNT was synthesized following the same method but without the addition of NH 3 ·H 2 O.

Characterization of Materials
The crystal structure of the as-synthesized samples was characterized by powder X-ray diffraction (XRD, D8 Discover Bruker). X-ray photoelectron spectrometry (XPS) was performed to analyze the elemental composition of the samples using a K-Alpha 1063 analyzer. The morphology of the samples was studied using a scanning electron microscope (SEM, JSM-7100F, JEOL) and a transmission electron microscope (TEM, JEM-2100F). The specific surface area of the samples was calculated using the Brunauer-Emmett-Teller (BET) method based on the N 2 adsorption-desorption isotherms obtained by using a V-Sorb 2800P. Thermogravimetric analysis (TGA) was carried out in air with a heating rate of 10 °C min −1 .

Electrochemical Measurements
The electrochemical performance of NiCo 2 S 4 /N-CNT samples was evaluated in CR 2032 coin-type cells. To prepare the electrode slurry, 70 wt% of NiCo 2 S 4 /N-CNT composite, 15 wt% of carbon black (Super P), and 15 wt% of polyvinylidene fluoride (PVDF) binder were mixed in 1-methyl-2-pyrrolidinone (NMP). The slurry was uniformly spread onto a Cu foil using a doctor blade technique and then dried at 70 °C for 8 h in air. Circular disk electrodes were cut after drying, and the cells were assembled in a high-purity Ar-gas (99.9995%) filled glove box (MBraun). The mass loading of NiCo 2 S 4 /N-CNT in the electrodes was about 2 mg cm -2 . Pure Li foils were used as reference and counter electrodes, and microporous polypropylene Celgard 2300 was used as a separator. The electrolyte was 1 mol L -1 LiPF 6 (Aladdin, CAS number: 21324-40-3) in a mixture of ethylene carbonate (EC, Aladdin, CAS number: 96-49-1) and dimethyl carbonate (DMC, CAS number: 616-38-6) with a volume ratio of 1:1. The galvanostatic charge/discharge measurements were conducted using a multichannel battery testing system (Neware BTS4000) at a potential window of 0.01-3.00 V (vs. Li + /Li). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using an electrochemical workstation (Princeton, VersaState4).

Results and Discussion
Scheme 1 shows the preparation route of NiCo 2 S 4 /N-CNT composite. Initially, the surface of CNT was pretreated with a solution of Ni 2+ and Co 2+ . Then, the N atoms were doped into the CNTs via a hydrothermal reaction at 170 °C, while NiCo 2 S 4 was grown in situ on the surface of CNTs. The crystal structures of NiCo 2 S 4 , NiCo 2 S 4 /CNT, and NiCo 2 S 4 /N-CNT composites were characterized by XRD (Fig. 1a). The characteristic diffraction peaks of NiCo 2 S 4 (JCPDS 20-0728) were observed in all the three samples. Moreover, the peaks in NiCo 2 S 4 /N-CNT were more pronounced and sharper than those in NiCo 2 S 4 /CNT [21]. It is believed that N-CNT can be used as active nucleation sites to promote the uniform and dense growth of NiCo 2 S 4 [22]. Figure 1b shows the BET results for the NiCo 2 S 4 /N-CNT nanocomposites. The specific surface area of NiCo 2 S 4 /N-CNT nanocomposites is 62.67 m 2 g −1 . As shown in the TGA analysis data (Fig. 1c), the NiCo 2 S 4 /N-CNT nanocomposite exhibited a weight loss at a temperature range of 400-600 °C, which was caused by the combustion of CNTs. Therefore, the content of NiCo 2 S 4 in the NiCo 2 S 4 /N-CNT composite was determined as ~ 30 wt%.
The SEM results of the samples are shown in Fig. 2a, b. The as-synthesized NiCo 2 S 4 nanoparticles appear to be more tightly packed and agglomerated. On the other hand, through the introduction of CNT and N-CNT, the NiCo 2 S 4 nanoparticles were uniformly distributed and deposited to form NiCo 2 S 4 /CNT composite (Fig. 2c, d) and NiCo 2 S 4 /N-CNT (Fig. 2e, f ), respectively. However, the density of NiCo 2 S 4 nanoparticles on the N-CNT surface in NiCo 2 S 4 /N-CNT was significantly higher than that in the NiCo 2 S 4 /CNT composite. This confirms that the introduction of N atoms in CNTs promotes the denser growth of NiCo 2 S 4 nanoparticles.
The TEM images in Fig. 3a show that the NiCo 2 S 4 particles have an average diameter of ~ 5 nm and are uniformly distributed on the surface of N-CNTs. In the highresolution TEM (HRTEM) image of NiCo 2 S 4 /N-CNT shown in Fig. 3b, the nanoparticles of about 5 nm in diameter exhibit a clear lattice fringe of 0.35 nm, corresponding to the (220) plane of NiCo 2 S 4 . Besides, many crooked graphitic lattice fringes were observed around Scheme 1 Schematic representation of NiCo 2 S 4 /N-CNT composite  Fig. 3b further confirmed the incorporation of NiCo 2 S 4 nanoparticles into the N-CNT structure.
Further, XPS was used to determine the bonding characteristics and surface chemical composition of NiCo 2 S 4 /N-CNT. The Co 2p spectra (Fig. 4a) can be divided into two peaks at 778.8 eV and 793.0 eV, corresponding to Co 3+ and Co 2+ , respectively [23,24]. In the N 1s spectrum (Fig. 4b), the peaks at 398.3, 399.7, and 400.9 eV can be assigned to the pyridinic, pyrrolic, and graphitic N, respectively [25,26]. In the XPS spectrum  (Fig. 4c), the S 2p 3/2 and S 2p 1/2 can be clearly observed at 161.2 and 163.1 eV, respectively, and the peak at 163.8 eV corresponds to the metal-sulfur bond [27,28]. As shown in Fig. 4d, in addition to the satellite peaks, the binding energies of Ni 2p centered at 854.6 and 856.9 eV correspond to Ni 2p 3/2 , and those at 871.1 and 875.5 eV correspond to Ni 2p 1/2 . This indicates the presence of both Ni 3+ and Ni 2+ in the sample [29,30]. As shown in Fig. 4e, three fitting peaks are present in the C1s profile at 284.9, 285.7, and 290.4 eV, which can be attributed to C-C, C-N, and -C=O bonds, respectively. In summary, the XPS of NiCo 2 S 4 /N-CNT indicated the formation of a highly ordered crystal structure of NiCo 2 S 4 and demonstrated the successful introduction of N element into the structure of compounds.
The electrochemical characteristics of NiCo 2 S 4 /N-CNT for Li storage were evaluated by CV and charge-discharge cycling, as shown in Fig. 5 at a potential range of 0.01-3.00 V (vs. Li + /Li). The cathodic process consisted of three reduction peaks (Fig. 5a) situated at 1.71 V, 1.33 V, and 0.70 V. The strongest peak is positioned at 1.33 V, and two weaker peaks correspond to the reduction of NiCo 2 S 4 to Ni and Co. In comparison, the peaks at 1.71 V and 0.70 V correspond to the formation of Li 2 S and the SEI film, respectively. In the anodic process, the oxidation peaks at 1.33 V and 2.05 V can be attributed to the oxidation of metallic Co to CoS x . In addition, there is an intensive peak at 2.32 V resulting from the oxidation reactions of metallic Ni and Co to NiS x and CoS x , respectively. The shape of the curve, peak position, and the intensity of peaks are relatively stable in the following cycles, indicating that NiCo 2 S 4 /N-CNT has good stability and reversibility. Figure 5b shows the charge-discharge curves of NiCo 2 S 4 /N-CNT at 0.1 A g -1 for the 1st, 2nd, and 10th cycles. The first charge and discharge capacities of the NiCo 2 S 4 /N-CNT electrode reached 807.6 and 1412.1 mAh g -1 , respectively, with the initial coulombic efficiency of 57.2%. The discharge capacities of the 2nd and 10th cycles are 970.7 mAh g -1 and 891.1 mAh g -1 , respectively. The reversibility of the charge/discharge process improved with the cycle number accompanied with an increased coulombic efficiency. The obtained To further study the electrochemical performance of NiCo 2 S 4 /N-CNT, the rate capability was evaluated at current densities from 0.1 to 5 A g -1 (Fig. 5c). The results indicate that the capacity of NiCo 2 S 4 /N-CNT decreased with the increase in current density. When the current density was returned to 0.1 A g −1 , the capacity of NiCo 2 S 4 /N-CNT returned to a value of 796.1 mAh g -1 , exhibiting about 84% capacity retention and demonstrating that NiCo 2 S 4 /N-CNT exhibits an excellent rate performance. The cycling performance data of NiCo 2 S 4 /N-CNT, NiCo 2 S 4 /CNT, and NiCo 2 S 4 for 100 cycles at 0.1 A g -1 are shown in Fig. 5d. For the initial 50 cycles, the anode undergoes a slight capacity fading. Then, NiCo 2 S 4 /N-CNT anode stabilized its capacity for the rest of the cycles and demonstrated a value of 623.0 mAh g -1 after 100 cycles. These results show that compared with the NiCo 2 S 4 /CNT and NiCo 2 S 4 electrodes, the NiCo 2 S 4 /N-CNT electrode exhibited a remarkably higher discharge specific capacity and better cycle stability. Figure 5e shows the EIS data. The highfrequency semicircles in the Nyquist plots correspond to the charge transfer resistance (R ct ) of the electrodes. The NiCo 2 S 4 /N-CNT electrode clearly exhibits the lowest R ct values, suggesting a remarkably enhanced charge/ mass transfer kinetics. Figure 5f shows the cycling performance of NiCo 2 S 4 /N-CNT electrode at 0.5 A g −1 over 500 cycles. The NiCo 2 S 4 /N-CNT electrode delivers an initial specific discharge capacity of 750.2 mAh g −1 and maintains a reversible capacity of 539.3 mAh g −1 after 500 cycles, further confirming an excellent cycling and rate capability of this high capacity anode for lithium batteries.

Conclusions
In summary, a NiCo 2 S 4 /N-CNT composite was prepared using a one-pot facile hydrothermal synthesis route. By introducing the N atoms into the CNT structure, uniformly distributed NiCo 2 S 4 nanoparticles with reduced particle sizes were obtained. The assembled cells with the NiCo 2 S 4 /N-CNT anode demonstrated a high specific capacity of about 623.0 mAh g -1 and excellent cycling stability at 0.1 A g -1 after 100 cycles. Furthermore, this electrode exhibited an excellent cycling property at 0.5 A g −1 over 500 cycles, confirming its ability to maintain its high performance at elevated current densities. Our study shows that this synthesis method is a feasible way to grow NiCo 2 S 4 nanoparticles with uniform distribution on the surface of a CNT substrate as a high-performance anode for LIBs.