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.

Transition-metal sulfides (TMSs) offer remarkably higher specific capacity than traditional electrode materials [8,9,10,11,12]. Recently, TMSs have been reported as anodes with excellent conductivity and catalytic activity. Among them, binary nickel–cobalt sulfide (NiCo2S4) exhibits a high theoretical specific capacity (703 mAh g–1), an excellent electronic conductivity (1.26 × 106 S m–1), and a greater abundance of redox reaction sites [13,14,15,16,17]. The reported general charge/discharge mechanism of NiCo2S4 with lithium (Li) involves the following reactions:

$${\text{NiCo}}_{2} {\text{S}}_{4} + 8{\text{Li}}^{ + } + 8{\text{e}}^{{-}} \to {\text{Ni}} + {\text{Co}} + 4{\text{Li}}_{2} {\text{S}}$$
(1)
$${\text{Ni}} + x{\text{Li}}_{2} {\text{S}} \leftrightarrow {\text{NiS}}_{x} + 2x{\text{Li}}^{ + } + 2x{\text{e}}^{{-}}$$
(2)
$${\text{Co}} + x{\text{Li}}_{2} {\text{S}} \leftrightarrow {\text{CoS}}_{x} + 2x{\text{Li}}^{ + } + 2x{\text{e}}^{{-}}$$
(3)

However, despite the large Li storage capacity of NiCo2S4, there are still problems related to the low 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 NiCo2S4 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 NiCo2S4 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 NiCo2S4 and thus improving the crystallinity of NiCo2S4/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 NiCo2S4 particles. The NiCo2S4/N-CNT material maintains good capacity retention during cycling and significantly restrains the voltage fading. The NiCo2S4/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.

Methods

Synthesis of NiCo 2 S 4

First, 0.074 g of Co(AC)2·4H2O and 0.037 g of Ni(Ac)2·4H2O 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)2·4H2O and 0.037 g of Ni(Ac)2·4H2O were added, and the mixture was stirred on a water bath at 80 °C for 2 h. Next, 2 mL of NH3·H2O 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. NiCo2S4/CNT was synthesized following the same method but without the addition of NH3·H2O.

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 N2 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 NiCo2S4/N-CNT samples was evaluated in CR 2032 coin-type cells. To prepare the electrode slurry, 70 wt% of NiCo2S4/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 NiCo2S4/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 LiPF6 (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 NiCo2S4/N-CNT composite. Initially, the surface of CNT was pretreated with a solution of Ni2+ and Co2+. Then, the N atoms were doped into the CNTs via a hydrothermal reaction at 170 °C, while NiCo2S4 was grown in situ on the surface of CNTs. The crystal structures of NiCo2S4, NiCo2S4/CNT, and NiCo2S4/N-CNT composites were characterized by XRD (Fig. 1a). The characteristic diffraction peaks of NiCo2S4 (JCPDS 20-0728) were observed in all the three samples. Moreover, the peaks in NiCo2S4/N-CNT were more pronounced and sharper than those in NiCo2S4/CNT [21]. It is believed that N-CNT can be used as active nucleation sites to promote the uniform and dense growth of NiCo2S4 [22]. Figure 1b shows the BET results for the NiCo2S4/N-CNT nanocomposites. The specific surface area of NiCo2S4/N-CNT nanocomposites is 62.67 m2 g−1. As shown in the TGA analysis data (Fig. 1c), the NiCo2S4/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 NiCo2S4 in the NiCo2S4/N-CNT composite was determined as ~ 30 wt%.

Scheme 1
scheme 1

Schematic representation of NiCo2S4/N-CNT composite

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Fig. 1
figure 1

a XRD patterns of NiCo2S4, NiCo2S4/CNT, and NiCo2S4/N-CNT; b N2 adsorption–desorption isotherms of NiCo2S4/N-CNT; c TGA of NiCo2S4/N-CNT

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The SEM results of the samples are shown in Fig. 2a, b. The as-synthesized NiCo2S4 nanoparticles appear to be more tightly packed and agglomerated. On the other hand, through the introduction of CNT and N-CNT, the NiCo2S4 nanoparticles were uniformly distributed and deposited to form NiCo2S4/CNT composite (Fig. 2c, d) and NiCo2S4/N-CNT (Fig. 2e, f), respectively. However, the density of NiCo2S4 nanoparticles on the N-CNT surface in NiCo2S4/N-CNT was significantly higher than that in the NiCo2S4/CNT composite. This confirms that the introduction of N atoms in CNTs promotes the denser growth of NiCo2S4 nanoparticles.

Fig. 2
figure 2

SEM images of a, b NiCo2S4; c, d NiCo2S4/CNT; and e, f NiCo2S4/N-CNT

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The TEM images in Fig. 3a show that the NiCo2S4 particles have an average diameter of ~ 5 nm and are uniformly distributed on the surface of N-CNTs. In the high-resolution TEM (HRTEM) image of NiCo2S4/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 NiCo2S4. Besides, many crooked graphitic lattice fringes were observed around the nanoparticles. The fast Fourier transform (FFT) and lattice spacing profiles in Fig. 3b further confirmed the incorporation of NiCo2S4 nanoparticles into the N-CNT structure.

Fig. 3
figure 3

a TEM image; b HRTEM image of NiCo2S4/N-CNT

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Further, XPS was used to determine the bonding characteristics and surface chemical composition of NiCo2S4/N-CNT. The Co 2p spectra (Fig. 4a) can be divided into two peaks at 778.8 eV and 793.0 eV, corresponding to Co3+ and Co2+, 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 of S 2p (Fig. 4c), the S 2p3/2 and S 2p1/2 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 2p3/2, and those at 871.1 and 875.5 eV correspond to Ni 2p1/2. This indicates the presence of both Ni3+ and Ni2+ 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 NiCo2S4/N-CNT indicated the formation of a highly ordered crystal structure of NiCo2S4 and demonstrated the successful introduction of N element into the structure of compounds.

Fig. 4
figure 4

XPS spectra of a Co 2p, b N 1s, c S 2p, d Ni 2p, and e C 1s in NiCo2S4/N-CNT

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The electrochemical characteristics of NiCo2S4/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 NiCo2S4 to Ni and Co. In comparison, the peaks at 1.71 V and 0.70 V correspond to the formation of Li2S 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 CoSx. In addition, there is an intensive peak at 2.32 V resulting from the oxidation reactions of metallic Ni and Co to NiSx and CoSx, respectively. The shape of the curve, peak position, and the intensity of peaks are relatively stable in the following cycles, indicating that NiCo2S4/N-CNT has good stability and reversibility.

Fig. 5
figure 5

a CVs of NiCo2S4/N-CNT at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V versus Li+/Li; b charge/discharge curves of NiCo2S4/N-CNT at 0.1 A g−1; c rate capability of NiCo2S4/N-CNT electrodes at various current rates; d cycling performance of NiCo2S4/N-CNT, NiCo2S4/CNT, and NiCo2S4 at 0.1 A g–1; e EIS plots of NiCo2S4/N-CNT, NiCo2S4/CNT, and NiCo2S4; f cycling performance at a current density of 0.5 A g−1

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Figure 5b shows the charge–discharge curves of NiCo2S4/N-CNT at 0.1 A g–1 for the 1st, 2nd, and 10th cycles. The first charge and discharge capacities of the NiCo2S4/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 CV profiles correspond to the charge/discharge curves of NiCo2S4/N-CNT.

To further study the electrochemical performance of NiCo2S4/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 NiCo2S4/N-CNT decreased with the increase in current density. When the current density was returned to 0.1 A g−1, the capacity of NiCo2S4/N-CNT returned to a value of 796.1 mAh g–1, exhibiting about 84% capacity retention and demonstrating that NiCo2S4/N-CNT exhibits an excellent rate performance. The cycling performance data of NiCo2S4/N-CNT, NiCo2S4/CNT, and NiCo2S4 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, NiCo2S4/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 NiCo2S4/CNT and NiCo2S4 electrodes, the NiCo2S4/N-CNT electrode exhibited a remarkably higher discharge specific capacity and better cycle stability. Figure 5e shows the EIS data. The high-frequency semicircles in the Nyquist plots correspond to the charge transfer resistance (Rct) of the electrodes. The NiCo2S4/N-CNT electrode clearly exhibits the lowest Rct values, suggesting a remarkably enhanced charge/mass transfer kinetics. Figure 5f shows the cycling performance of NiCo2S4/N-CNT electrode at 0.5 A g−1 over 500 cycles. The NiCo2S4/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 NiCo2S4/N-CNT composite was prepared using a one-pot facile hydrothermal synthesis route. By introducing the N atoms into the CNT structure, uniformly distributed NiCo2S4 nanoparticles with reduced particle sizes were obtained. The assembled cells with the NiCo2S4/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 NiCo2S4 nanoparticles with uniform distribution on the surface of a CNT substrate as a high-performance anode for LIBs.