1 Introduction

In recent years, energy storage and conversion have drawn a great deal of attention due to the employment of various sustainable energy technologies [1,2,3,4,5,6]. A lot of technologies have been developed, such as lithium–air batteries (LABs) [5, 6], lithium–sulfide batteries (LSBs) [7,8,9,10,11], lithium-ion batteries (LIBs) [12,13,14,15,16], sodium-ion batteries (SIBs) [17,18,19,20,21,22,23], potassium-ion batteries (PIBs) [24,25,26,27,28], zinc-ion batteries [29, 30], zinc–air batteries [31, 32] and supercapacitors (SCs) [33,34,35,36]. Among them, SCs (also named electrochemical capacitors, ECs) have great potential to fulfill the challenges for energy storage and conversion due to the fast charging and discharging, more power density, good cycle stability, besides relatively low cost [37,38,39,40,41,42,43]. The superior high power density makes SCs a promising electrochemical device for high-power harvesting applications featuring ultralong life span [44,45,46]. It is known that the capacitance characteristics of SCs are influenced by electrodes, electrolytes and assembly techniques, the most important of which is the electrode materials. Accordingly, it is extremely important to introduce capable electrode substances with the highest specific capacitance, magnificent power density to meet superior cycle stability [47,48,49].

Depending on the energy storage mechanism, SCs can be categorized into two types: electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs). EDLCs energy storage is built upon an electric double-layer effect, occurring at the electrode–electrolyte interaction achieved through transferring the electron charge between electrolyte and electrode via adsorption/desorption and ion transfer in electrochemical reactions [48,49,50]. PCs are based on an electrochemical storage mechanism by virtue of reversible Faradaic redox reaction on/near the electrode surface. Compared with EDLCs, PCs usually show higher specific capacitance and higher energy density [50, 51]. The specific capacitance of PCs depends mainly on the surface area of active materials and the structure of electrodes [52,53,54]. Transition-metal oxide/hydroxides/sulfides [55,56,57,58,59,60] and conducting polymers [41, 61] are broadly investigated as pseudocapacitive electrode materials. Transition-metal oxides/hydroxides deliver high energy density but are hampered by their low rate capability. Conducting polymers show high specific capacitance, yet suffer from poor cycle stability [62,63,64,65]. Transition-metal sulfides are proved to be one of the most potential electrode materials for SCs [66,67,68,69]. Based on their favorable Faradaic redox reaction, metal sulfides, such as NiSx, CoSx, FeSx, MnSx, CuSx and MoS2, are the desired choice [70,71,72,73,74,75]. Group of nickel sulfides (NixSy) is comprised of Ni and S with various stoichiometric proportions, including NiS, NiS2, Ni3S2, Ni3S4, Ni6S5, Ni7S6 and Ni9S8 due to the low electronegativity of sulfur easily constructed with nickel. Due to the rich chemical composition, natural abundance, large capacitance and high conductivity as well as environmental benignity, nickel sulfides have been widely used in SCs [76,77,78]. In particular, their vital specific capacitance (several times higher than that of carbon/graphite-attribute substances) makes them stand out from other electrode materials [79, 80]. These merits of nickel sulfides drive their rapid development for SCs. There are various reviews on metal sulfides for energy storage and conversion [58, 59, 70, 71]; however, there is no systematic review on nickel sulfides for SCs. Therefore, it is necessary and timely to review the latest advances in nickel sulfides. This review summarizes the synthesis methods, the morphology/composite-dependent electrochemical properties, challenges and perspectives of various nickel sulfides for SCs.

2 Reaction mechanism

The redox reactions of nickel sulfides in alkaline electrolytes are generally as follows [81]:

$${\text{NiS}}_{x} + {\text{OH}}^{ - } \leftrightarrow {\text{NiS}}_{x} {\text{OH}} + {\text{e}}^{ - }$$
(1)
$${\text{Ni}}_{3} {\text{S}}_{2} + 3{\text{OH}}^{ - } \leftrightarrow {\text{Ni}}_{3} {\text{S}}_{2} \left( {\text{OH}} \right)_{3}\,+\,3{\text{e}}^{ - }$$
(2)

Semi-infinite diffusion-limited reaction process verifies the electrochemical responses of active electrode materials to differentiate the redox mechanism of battery (value of the exponent, b = 0.5) or PCs-type electrodes, for the kinetic process of the electrode and redox reaction is controlled by a semidefinite diffusion. The peak current (I) versus scan rate (v) at a constant voltage window in cyclic voltammograms (CV) can be acknowledged and described as follows (a and b are adjustable parameters) [48]:

$$I = a\nu^{b}$$
(3)

However, the poor cycling execution of NixSy could usually be accompanied by irreversible reactions:

$${\text{NiS}}_{x} + {\text{H}}_{2} {\text{O}} + 0.5{\text{O}}_{2} \to {\text{Ni}}\left( {\text{OH}} \right)_{2}\,+\,x{\text{S}}$$
(4)
$${\text{Ni}}_{3} {\text{S}}_{4} + 3{\text{H}}_{2} {\text{O}} + 1.5{\text{O}}_{2} \to 3{\text{Ni}}\left( {\text{OH}} \right)_{2}\,+\,4{\text{S}}$$
(5)
$${\text{Ni}}\left( {\text{OH}} \right)_{2}\,+\,{\text{OH}}^{ - } \to {\text{NiOOH}} + {\text{H}}_{2} {\text{O}} + {\text{e}}^{ - }$$
(6)

According to previous reports [81,82,83], the battery-type redox mechanism of NixSy electrodes and the lower rate performance and poor cycling stability thereof will significantly hamper their extensive applications. The main drawback of nickel sulfides is their fast-decayed capacitance at high rates (viz. relatively low cycle stability), which results from their low intrinsic electrical conductivity and inadequate amount of electroactive sites. The proposed strategies to overcome the abovementioned problems include designed synthesis of nanostructures, binder-free electrodes grown on conductive substrates [84,85,86,87] and fabrication of nanohybrids with good conductive supports such as porous carbons [88,89,90,91], graphene [92,93,94,95] and conducting polymers [96,97,98,99,100,101]. It is well known that the electrochemical performances largely depend on the morphology and surface area of electrode materials; thus, the controlled synthesis of nickel sulfide nanomaterials with stable structure and large surface area is of great importance [102,103,104,105].

3 NiS

3.1 Pristine NiS

Among the various NixSy compositions, NiS possesses the best easiest stoichiometry occurring in two possible phases of α-NiS (hexagonal, P63/mmc) and β-NiS (rhombohedral, R3m). NiS is extremely beneficial owing to its huge theoretical capacity (1060 F·g−1) [106, 107], good electrical conductivity, low cost, easy fabrication process and environmental sustainability. Hou et al. [108] firstly used NiS as electrode material for SCs and revealed the main drawback to be the low rate capability, which arises from the poor electrical conductivity and limited electron transport rate. An effective method to resolve the abovementioned problems is to construct diverse nanostructured NiS with different dimensions, including nanorods [109], nanoplates [110], nanowires [111], porous nanomaterials, etc., as well as hierarchical architectures including core–shell, complex hollow structures via various synthesis methods such as hydrothermal method, anion exchange reaction and potentiodynamic deposition.

As electrode materials, hollow nanostructures possess the advantages of huge electrolyte accessible surface area and abundant electroactive sites. For example, Chen et al. [112] designed a facile template-engaged route to prepare nickel sulfides besides copper sulfide hierarchical hollow spheres. The NiS hollow structure (Fig. 1a) exhibits high BET specific surface area of 124 m2·g−1 with a standard pore size of 3.5 nm (Fig. 1b) and delivers great specific capacitances of 1460 and 727 F·g−1 at current densities of 4 and 20 A·g−1, respectively (Fig. 1c), as well as excellent capacitance retention (93.6% after 5000 charge–discharge cycles at 12 A·g−1) (Fig. 1d). Considering the advantages of hollow structure, Wang et al. [113] developed NiS hierarchical hollow microspheres through a template-free sulfidation process using α-Ni(OH)2 microspheres as precursor. The unique hierarchical hollow spherical structure can provide more electroactive sites and efficiently smooth the contact between the active materials and electrolyte. The as-synthesized NiS exhibited a high specific capacity of 153.6 mAh·g−1 at a current density of 0.5 A·g−1 at high mass loading, which can be attributed to its small crystal size, hierarchically porous structure and high electrical conductivity.

Fig. 1
figure 1

(reproduced from Ref. [112], Copyright 2017 Springer Nature)

Structural characterization and electrochemical performance of NiS hollow nanostructure: a SEM image; b nitrogen adsorption–desorption isotherm (inset being distribution of pore size); c GCD curves at different current densities; d cycling performance at 12 A·g−1 (inset being first three charge–discharge curves).

Full size image

The construction of hierarchical structures will guarantee the electrode stability compared to that of discrete/isolated nanostructures for enhanced performance. For example, the heterogeneous reactions at the substrate surface can cause nucleation, aggregation and coalescence, as well as the growth of thin film [114]. The unique β-NiS thin films with different morphologies of nanoneedles, nanoplates and nanoflowers are fabricated by Patil et al. [114] via a hydrothermal process with Ni(OH)2 thin films as precursors on stainless strip substrate. Among these nanostructures, stacked nanoneedle morphology of β-NiS thin film exhibits the highest specific capacitance of 416 F·g−1 at 0.5 mA·cm−2. This superior performance was credited with its open porous structure that offers huge electroactive surface area and straightforward path for insertion/de-insertion of electrolyte ions within the active electrode. Furthermore, the flexible solid-state symmetric supercapacitors (FSS-SCs) assembled with β-NiS thin films and polyvinyl alcohol–lithium perchlorate (PVA–LiClO4) as a polymer gel electrolyte exhibit a capacitance of 55.8 F·g−1 with a scan rate of 10 mV·s−1 at potential window of 1.2 V, as well as good cycle stability. Yu et al. [115] synthesized NiS nanoframes utilizing Ni–Co Prussian blue analogue nanocubes as a matrix. These NiS nanostructures showed a superior capacitance of 2384 F·g−1 at 1 A·g−1. This extraordinary pseudocapacitive behavior is possibly credited to the high surface area/volume ratio with the distinctive 3D frame-like hollow porous structure, which provides a huge electrode–electrolyte interaction. You et al. [116] prepared hierarchical porous NiS superstructures (h-NiSx) by the electrodeposition method and obtained an excellent areal specific capacitance of 6104 mF·cm−2 at a scan rate 10 mV·s−1, besides the amazing cycle stability. The h-NiSx accompanied by three-dimensional (3D) hierarchical structures composed of diffused, interconnected macropores and mesopores enables bountiful electrolyte-accessible channels, and the reduced ion diffusion length is responsible for high capacitive performance. The hierarchical NiS microflowers designed to utilize Ni(OH)2 as a precursor by sulfuration method [117] delivered a remarkable specific capacitance of 1123 F·g−1 at a current density of 1 A·g−1 along with 97.8% retention in specific capacitance at 10 A·g−1 over 1000 charge–discharge cycles. High porosity and huge specific surface area created by unique NiS microflower structure, which promote the electron transfer and electrolyte insertion/de-insertion, are accountable for the excellent capacitive performance. Surendran et al. [118] prepared the arrays of flower-like β-NiS nanostructures on flexible carbon cloth (β-NiS@CC), which showed an extraordinary areal capacity of 1.654 C·cm−2 (827 C·g−1) at 1 mA·cm−2. Du et al. [119] synthesized NiS double-shelled hollow structures using α-Fe2O3 as template. The nanosheet-assembled shells produced open huge mesopores, which could enable transportation of the electrolyte and ions. Among them, capsule-shaped NiS exhibited an outstanding capacitance of 1159 F·g−1 at a rate of 2 A·g−1 in three-electrode system. The corresponding NiS//rGO@Fe3O4 hybrid asymmetric supercapacitor (ASC) showed a specific capacitance of 122.8 F··g−1 at a current density of 0.83 A·g−1. Yang et al. [120] introduced the synthesis of hierarchical flower-like β-NiS spheres through a solvothermal template-free approach, which delivered specific capacitances of 972.3 and 729.7 F·g−1 at current densities of 2 and 15 A·g−1, respectively, and showed a capacitance of 778.8 F·g−1 after 3000 cycles at the current density of 4 A·g−1, indicating excellent rate capability. This remarkable capacitive performance is due to the hierarchical flower-like framework assembled by nanoplates, which offers plenty of reactive sites for charge–discharge route. Uniform NiS hierarchical hollow cubes are also designed from Ni-formate framework through a hydrothermal process based on Kirkendall effect and Ostwald ripening [121]. These electrodes reveal a capacitance of 874.5 F·g−1 at 1 A·g−1 along with 90.2% retaining capacitance even 3000 charge–discharge cycles. Additionally, NiS//CNFs in asymmetric configuration showed an energy density of 34.9 W·kg−1 at a power density of 387.5 W·kg−1.

3.2 NiS composites

Aforementioned research investigations on nickel sulfide materials illustrate that those special nanostructures are skilled to raise the electrochemical performance for SCs. Nickel sulfides generally exhibit higher specific capacitance than carbon-based electrodes, but they have some drawbacks, such as poor electrical conductivity and particle agglomeration, which crucially limit their demands in SCs [122]. To extensively combine the carbonaceous materials with potential benefits and pseudocapacitive materials, the construction of functional nanomaterials on carbonaceous materials including graphene, carbon nanotubes and carbon fiber is an impressive strategy to enhance the electrochemical behavior described with cycling life and rate capability [123, 124]. Up to date, various NiS hybrids such as NiS/hollow carbon spheres (HCSs) have been proved to be potential electrode materials [125], where the internal space provides ion channels for fast electrochemical reactions, the carbon shell assists in electron transport and the huge surface area recommends the discharge of active materials. Accordingly, a narrow and porous layer of active material expanded on the hollow carbon spheres surface could raise energy storage and rate capacity. Structure and synthesis of NiS porous nanosheets coated on N-doped hollow carbon spheres (NHCSs) by well-organized template-assisted route are investigated for SCs [125]. Core–shell structures of NHCSs@SiO2 serving as a template for nickel silicates further yield NiS/NHCS hollow structures followed by the formation of NHCS-reinforced NiS nanosheets (NiS/NHCS). Charge–discharge profiles of NiS/NHCS and NiS hollow spheres (NiS/HS) exhibited reversible Faradaic redox processes. The NiS/NHCS electrode shows a favorable capacitance of 1150 F·g−1 at a current density of 1 A·g−1 and a capacitance retention of 52.2% at 20 A·g−1, as well as 76% at 5 A·g−1 over 4000 charge–discharge cycles. Furthermore, the hybrid supercapacitor of NiS/NHCS//AC performed a high capacitance of 120 F·g−1 at 0.2 A·g−1 and maintained a capacitance of 46 F·g−1 at 5 A·g−1.

The incorporation of hierarchically porous electrodes and conductive substrates/frameworks may exhibit synergistic effects for enhanced performances [126, 127]. Recently, metal–organic frameworks (MOFs) are widely used as an emerging template for the fabrication of active materials because MOFs could be easily transformed into battery-type materials or carbon-based materials with large surface areas [126, 127]. The combination of MOFs-derived composites and graphene sheets is expected to provide adequate electrical affinity between the discrete electroactive particles and shorten the ionic transport path. Qu et al. [109] designed and fabricated a hierarchically porous hybrid electrode of α-NiS nanorods decorated on reduced graphene oxide (R-NiS/rGO) (Fig. 2a), deriving from water-refluxed Ni-MOF-74/rGO template. The obtained R-NiS/rGO shows a low charge-transfer resistance of 1.41 Ω, presenting good conductivity, which is attributed to the intimate contact of conductive rGO with NiS (Fig. 2b). The R-NiS/rGO hybrid composite electrode delivers a capacity of 744 C·g−1 at 1 A·g−1 and an extraordinary rate performance (600 C·g−1 at 50 A·g−1), and it preserves more than 89% of the initial capacity over 20,000 cycles (Fig. 2c). These results indicate that the MOF-extracted α-NiS nanorods with abundantly available electrolyte-accessible surfaces are beneficial to fast redox reactions in an alkaline electrolyte. Furthermore, when coupled with a negative electrode of N-doped graphene aerogel (C/NG-A), the hybrid supercapacitor exhibits a high energy density of 93 W·kg−1 at a power density of 962 W·kg−1 (Fig. 2d).

Fig. 2
figure 2

(reproduced from Ref. [109], Copyright 2018 The Royal Society of Chemistry)

a TEM image of R-NiS/rGO; b Nyquist plots and c cycling performances of R-NiS/rGO and N-NiS; d comparison of Ragone plots of R-NiS/rGO//C/NG-A and other recently reported high-performance hybrid/asymmetric SCs.

Full size image

AbdelHamid et al. [123] synthesized graphene-wrapped NiS nanoprisms that showed excellent capacitance of 1337 and 778 F·g−1 at current densities of 3 and 20 A·g−1, respectively, indicating remarkable rate capability. This excellent capacitive performance results from the direct anchoring of NiS nanoprisms on the surface of graphene sheets, which ultimately facilitates electronic transport; furthermore, the larger surface area and porosity of the nanocomposite expedite the electrolyte transport and ionic diffusion, which enhances the rate capability of the electrode. Sun et al. [106] demonstrated a simple in situ sulfuration and phase-controlled process for α-NiS nanoparticles inserted in carbon nanorods (α-NiS/CRs). This hybrid composite electrode material exhibits high specific capacitance (1092 and 740 F·g−1 at 1 and 10 A·g−1, respectively) with superior cycling life (with no capacitance decrease after 2000 cycles). Singh et al. [124] hydrothermally synthesized MWCNTs/NiS (MWNS) composite and graphene nanoplatelets, which were used as electrode materials for the positive and negative electrodes. The MWCNT/NiS//graphene nanoplatelets-based ASC exhibited a specific capacitance of 181 F·g−1 at a current density of 1 A·g−1. The ASC also presented 92% retention of initial capacitance after 1000 cycles at 2 A·g−1. The improved capacitive performance results from the enhanced conductivity and large surface area of MWNS, which enable deeper penetration of electrolyte ions into electrode materials and electrochemical activity enhancement of the graphene sheet film. Li et al. [128] chose the commercial flexible makeup cottons (MCs) as skeleton to fabricate 3D interconnected graphene-wrapped macro-networks with NiS (MCs@GNS@NiS), which were synthesized by “dip and dry” and electrodeposition technique. This MCs@GNS@NiS electrode demonstrates a high specific capacitance of 775 F·g−1 at a current density of 0.5 A·g−1 and 88.1% retention in capacitance after 1000 cycles at 2 A·g−1; it also delivers a high energy density of 11.2 W·kg−1 at a high power density of 1008 W·kg−1. This high capacitive achievement is assigned to its special homogeneous 3D networks that are favorable for the access of electrolytes to active electrode materials. Zhang et al. [129] reported a one-step solvothermal reaction for NiS/N-doped carbon fiber aerogel (N-CFA) nanocomposite. The optimized NiS/N-CFA nanocomposite delivers a large specific capacitance of 1613 F·g−1 at 1 A·g−1 and shows 87.0% retention in capacitance after 5000 cyclic voltammetry cycles at a scan rate of 20 mV·s−1 as well as 66.7% retention in capacitance at 20 A·g−1.

4 NiS2

4.1 Pristine NiS2

Recently, nonnoble metal chalcogenides including NiS2 have attracted great interest owing to their large energy storage capacity and catalytic activity [130]. However, the inferior cycling performance of NiS2-based electrode materials hampers their application in SCs [131]. To solve these problems especially durability and sluggish redox kinetics, metal doping into the lattice of NiS2 is proved to be an effective strategy. Well-organized charge transfer between the doped metal element and host as well as the modification of the electronic environment can decrease the kinetic resistance during the charge–discharge process and favor an enhancement in electrochemical performance [132]. Xie et al. [132] fabricated a Co-doped nickel disulfide (NixCo1−xS2) nanostructure via a facile solvothermal process. The fluffy structure of Ni0.75Co0.25S2 facilitates the contact between the electrolyte and electrode material during the charging/discharging process. Transmission electron microscopy (TEM) image of Ni0.75Co0.25S2 reveals that most hollow spheres were broken into uniform nanoparticles with numerous active edge sites (Fig. 3a). The Ni0.75Co0.25S2 sample demonstrates an excellent specific capacitance of 2141.9 F·g−1 at a current density of 2 A·g−1 (Fig. 3b), as well as a highest energy density of 54.9 W·kg−1 and enhanced cycle life (85.1% capacitance retained over 4000 cycles).

Fig. 3
figure 3

(reproduced from Ref. [134]. Copyright 2015 Elsevier)

a TEM and HRTEM images of Ni0.75Co0.25S2 sample (inset being fast Fourier transform (FFT) image) and b galvanostatic charge/discharge (GCD) curves of Ni0.75Co0.25S2. (reproduced from Ref. [132], Copyright 2018 The Royal Society of Chemistry and the Partner Organisations); c schematic diagram representing electronic transport pathway and electrolyte ions dissemination in porous NiS2 electrode; d TEM image of NiS2 and e Ragone plots of NiS2//rGO ASC and the rGO//rGO symmetric SC.

Full size image

Low-dimensional nanomaterials have high surface area and short electron transport pathway for enhanced performance [133]. Ruan et al. [134] developed a simple facile solution process to fabricate square rod-like NiS2 precursor with open ends (Fig. 3c). In contradiction to the traditional hydrothermal route for specific materials structure with numerous conditional inhibitions, this process can be smoothly conducted in favorable innovative circumstances and extended to other metal sulfide precursors. Self-assembly synthesis of the square rod-like NiS2 precursor depends on the reaction time, which is represented by TEM image (Fig. 3d). The porous NiS2 square rods achieved via an annealing process showed high specific capacitances of 1020.2, 534.9 F·g−1 at 1 and 10 A·g−1, respectively, as well as long cycle stability (93.4% retention of the initial specific capacitance over 1000 cycles). Moreover, ASC device using NiS2 as the cathode and rGO as the anode reveals a high energy density of 32.8 W·kg−1 at a power density of 954 W·kg−1 (Fig. 3e). Lu et al. [130] reported that NiS, NiO and NiSe2 with the unique hollow structure could be obtained based on NiS2 hollow spheres through the l-cysteine-assisted hydrothermal method. Specific capacitances of the NiS2, NiS, NiO and NiSe2 electrodes were 1643, 1076, 581 and 341 F·g−1, respectively, at a rate of 1 A·g−1. The consequences indicated that both the NiS2 and NiS electrodes displayed the highest specific capacitance, while the NiO and NiSe2 electrodes exhibited long cycling stability. To achieve larger specific surface area with more exposed active sites for fast ion/electrolyte transfer, Ni et al. [135] synthesized foam-like hierarchical structures of ultrathin NiS2 nanosheets epitaxially grown on their homogeneous nanowire backbones by one-pot reaction (i.e., controlled nucleation and growth process) and subsequent sulfidation (Fig. 4a–d). The as-synthesized hierarchical NiS2 demonstrated a high pseudocapacitance of 1788 F·g−1 at a current density of 0.3 A·g−1, as well as superior rate capacitance of 1223 and 750 F·g−1 at 10 and 30 A·g−1, respectively (Fig. 4e) [135].

Fig. 4
figure 4

(reproduced from Ref. [135], Copyright 2014 The Royal Society of Chemistry)

a Schematic diagram of one-pot synthesis for foam-like hierarchical structure of Ni nanosheets-wrapped nanowires (i.e., Ni ultrathin lamella wrapped chainlike backbone), and b corresponding TEM image; c SEM images and d X-ray diffraction (XRD) pattern of as-synthesized hierarchical NiS2 nanofoam, and e related specific capacitance at various current densities.

Full size image

4.2 NiS2 composites

Some other transition-metal-based composite electrode materials for supercapacitors have been reported for improving the electrical conductivity and cycle stability of Ni-based materials due to the strong coupling effects between different components [55, 104, 136, 137]. Furthermore, the phase boundaries in these composites could create highly active sites for redox reaction and thus lead to improved electrochemical performance [138]. Gou et al. [138] prepared hollow spherical Ni2P/NiS2 composite (Fig. 5a) by a simple hydrothermal route, which shows capacitances of 212.7, 181.7, 159.4, 148.7 and 120.9 mAh·g−1 at various current densities of 2, 4, 8, 10, and 20 A·g−1, respectively (Fig. 5b). Ni2P/NiS2 composite demonstrates higher pseudocapacitance than bare NiS2 and Ni2P due to their synergistic contribution (Fig. 5c), exhibiting a fair cycling life (maintaining a gravimetric capacity of 118.9 mAh·g−1 after 5000 cycles at a current density of 2 A·g−1), as well as high rate capacitance with 83.6% retention of the starting capacity up to 10 A·g−1 (Fig. 5d). Li et al. [139] synthesized a 3D graphene-modified NiS2 nanocomposite (graphene/NiS2) with high surface area, improved electrical conductivity, good mechanical strength and stable crystal structure by a template-free solvothermal method. The graphene/NiS2 composite displays a specific capacitance of 478.1 F·g−1 at 0.5 A·g−1. After 2000 cycles, the composite decays only 10.7% of its initial value. Ji et al. [140] established a heterostructure material of core–shell MnO2@NiS2/Ni(OH)2 by a two-step hydrothermal route at various temperatures. The as-prepared heterostructure composite consists of 1D MnO2 nanosticks and 2D NiS2/Ni(OH)2 sheets as pseudocapacitive materials, exhibiting a large specific capacitance of 1010 F·g−1 at 1 A·g−1 and an enhanced cycling capacitance of 785 F·g−1, i.e., 78% capacity retention after 3000 cycles.

Fig. 5
figure 5

(reproduced from Ref. [138], Copyright 2017 The Electrochemical Society)

a SEM image and b GCD curves at various current densities of Ni2P/NiS2 composite; c gravimetric capacities of Ni2P, NiS2 and Ni2P/NiS2; d cycle performance of Ni2P/NiS2 composite at current density of 10 A·g−1 over 5000 cycles.

Full size image

5 Ni3S2

5.1 Pristine Ni3S2

Ni3S2 is one of the most crucial phases of nickel sulfides, and it provides many virtues such as excellent theoretical capacitance (2412 F·g−1), outstanding redox features and high conductivity, which are promising for practical energy storage systems [141]. In addition, Ni3S2 is of low cost, natural abundance and environmental benignity. Ni3S2 shows intrinsic metallic behavior resulting from its extended network of Ni–Ni bond structures [142]. Furthermore, these nickel sulfide nanostructures can be easily obtained through general chemical reactions with controllable morphologies including nanowires, nanorods, nanoneedles and nanosheets [143, 144]. Therefore, to generate an enhanced supercapacitive performance, a rational design of Ni3S2 nanostructures with abundant surface electroactive sites is required. For example, as a novel category of flexible energy storage devices, fiber-type supercapacitors have gained intense awareness due to their advantages of fairly large capacitance density, flexibility and facile incorporation with different electronic devices, viz., they could be precisely used as wearable and portable device units and can be simply designed into different structures for new design and readily combined with a variety of electronic devices [145]. For example, Wen et al. [145] fabricated a new coaxial-type flexible fiber asymmetrical supercapacitor (ASC) using Ni3S2 nanorod arrays and pen ink as positive and negative electrode materials, respectively. The Ni3S2 nanorod array used as electrode material in fiber SCs was directly spread on Ni wire via hydrothermal route, and a simple dip-coating method was used to develop the pen ink/Ni electrode. At the voltage range of 0–0.5 V, the Ni3S2 nanorod arrays/nickel wire shows similar redox peaks in CV curves at scan rates from 10 to 100 mV·s−1, revealing high reversible reaction on the surface of Ni3S2 electrode. The constructed ASC device shows a specific capacitance of 34.9 F·g−1 (87.3 F·cm−1) at a scan rate of 10 mV·s−1. The ASC device can work robustly within a voltage window of 0–1.4 V and deliver a high energy density of 8.2 W·kg−1 (0.81 mWh·cm−3) and a power density of 214.6 W·kg−1 (21.1 mW·cm−3). To further improve the application of metal sulfides, different nanostructures have been instantly grown on 3D porous current collectors to construct large surface areas, short electron- and ion-transport pathways and outstanding rate capability and cycle performance. Xiong et al. introduced a unique electrode framework made up of Ni3S2 nanosheet-onto-Ni3S2 nanorods spread on nickel foam through a simple one-step hydrothermal method (Fig. 6a) [146]. The Ni3S2 nanosheet@nanorods electrode exhibited an initial capacity of 489 F·g−1 and maintained 89.3% retention after 5000 cycles (Fig. 6b) at a cycling rate of 120 mA·cm−2, as well as a specific capacitance decrease by < 42% with increased current density from 20 to 240 mA·cm−2, indicating good rate capability. Ni3S2 nanosheet@Ni3S2 nanorods electrode for ASC, with a high loading density of 5.8 mg·cm−2, displays an optimized volumetric energy density of 1.96 mWh·cm−3 at 1.2 A·g−1 and a power density of 0.6 W·cm−3 at 28.3 A·g−1 (bridging the performance gap between thin-film Li batteries and commercial AC//AC supercapacitors) (Fig. 6c), as well as an excellent cycle stability over 5000 cycles (Fig. 6d). This remarkable capacitive achievement is assigned to the adequate mechanical friction and electrical attachment with the substrate, superior contact area with electrolyte and prevented structural pulverization through the cyclic process. To clearly indicate various parameters on performance, a series of typical Ni3S2 nanostructures with different morphologies obtained by different synthesis methods is shown in Table 1 [147,148,149,150,151,152,153,154,155,156].

Fig. 6
figure 6

(reproduced from Ref. [146]. Copyright 2016 Springer Nature)

a SEM image and b specific capacitance at various current densities of Ni3S2 nanosheet@nanorods (NS@NRs); c comparison of volumetric power density of Ni3S2 NS@NRs ASC electrode with other reported ECs/batteries data (i.e., laser-scribed graphene (LSG) EC, Li thin-film battery and commercial AC//AC EC with typical parameters); d cycle stability of Ni3S2 nanosheet@nanorods//AC.

Full size image
Table 1 Electrochemical performances of typical Ni3S2 and their hybrid nanostructures
Full size table

5.2 Ni3S2/TMC composites

Numerous techniques have been practiced to enhance the behavior of energy storage devices through the evolution of new electrode materials. The fabrication of transition-metal compound (TMC) composite electrodes plays a vital role in improving the performance of energy storage devices [157, 158]. Bimetallic sulfides possess richer redox reactions than their single-component counterparts, resulting in superior specific capacitance [157], and bimetallic sulfide species can also be readily restored from their metal oxide/hydroxide precursors via anion exchange reactions or Kirkendall effects and finally lead to multiform and controllable morphologies (e.g., nanowires, nanotubes and nanosheets). Thus, developing rational nanostructures for upgraded pseudocapacitive electrodes can not only increase the electrical conductivity and the utilization of atoms, but also shorten the electron/ion diffusion path. In this manner, ultrathin and porous Ni3S2/CoNi2S4 3D network grown on Ni foam (Ni3S2/CoNi2S4/NF) was synthesized by a sulfidation process using 3D-networked Ni–Co precursor (Ni–Co precursor/NF) [157]. The SEM images show that surfaces of sulfuretted nanosheets become rough due to the formation of nanoparticles on the surface (Fig. 7a, b). Ni3S2/CoNi2S4/NF 3D networked electrode can deliver a specific capacitance of 2435 F·g−1 at 2 A·g−1 (Fig. 7c) and a notable rate capability of 80% retention at 20 A·g−1. Additionally, the as-prepared hybrid capacitor demonstrates a superior capacitance (175 F·g−1 at 1 A·g−1) and an energy density of 40.0 W·kg−1 at a power density of 17.3 kW·kg−1.

Fig. 7
figure 7

(reproduced from Ref. [158], Copyright 2016 The Royal Society of Chemistry)

a SEM image and b structural diagram of Ni3S2/CoNi2S4/NF; c specific capacitances of Ni3S2/CoNi2S4/NF, Ni–Co oxide/NF and Ni–Co precursor/NF at different current densities (reproduced from Ref. [157]. Copyright 2017 WILEY–VCH); d schematic diagram of NiCo2O4@Ni3S2 hybrid nanostructure; e cycling performance of NiCo2O4 and NiCo2O4@Ni3S2 electrodes; f Ragone plot of Ni3S2/CoNi2S4/NF based ASC device compared with other reported ASC devices.

Full size image

An emerging approach to increase the electrochemical performance of nickel sulfides is to combine with NiCo2O4 arrays [158]. For instance, 3D hierarchical NiCo2O4@Ni3S2 core/shell arrays on Ni foam were prepared via a stepwise process (Fig. 7d) [158]. The 3D heterogeneous NiCo2O4 nanoarchitecture generates an interconnected web-like platform and acts as the core inside Ni3S2 shell. The NiCo2O4@Ni3S2 nanowire array electrodes demonstrate significant electrochemical performance, e.g., optimal specific areal capacitance of 3.0 F·cm−2 at a current density of 5 mA·cm−2 and good cycling stability of 93.3% retention in capacitance after 10,000 cycles (Fig. 7e). The NiCo2O4@Ni3S2//AC solid-state ASC device displays a high energy density of 1.89 mW·cm−3 at 5.81 W·cm−3 and a high power density of 56.33 W·cm−3 at 0.94 mW·cm−3 (Fig. 7f). The synthesis of TMC composites usually integrates the benefits of their constituent materials, resulting in a synergistic effect. The phase interface existing in a composite often leads to lattice mismatch and therefore creates more active sites for energy storage, allowing the electrodes to exceed their theoretical capacity. The synergistic effect generated from the composite interface boundaries and pseudocapacitance contributions could display excellent diffusion-controlled capacity performance even at a higher rate [157,158,159]. In consideration of these favorable conditions, Long et al. [159] synthesized Ni3N@Ni3S2 nanosheets composite by calcinating Ni-based precursor in ammonia followed by sulfuration. Ni3S2 is preferred over NiO due to the better conductivity of sulfides than oxides. Ni3N@Ni3S2 nanosheets showed an excellent capacitance of 849 F·g−1 at a current density of 6.25 A·g−1 and 89% retention in capacitance after 15,000 long-term charge–discharge cycles.

5.3 Ni3S2/carbon composites

Fabrication of hybrid capacitors by direct growth of pseudocapacitive materials on a conducting matrix, e.g., graphene, porous Ni foam, is a promising alternative to increase the electrical conductivity and cyclic stability of pseudocapacitor electrodes [160, 161], which will not only eliminate the side effects and complications arising from the use of binders but also produce a promising freestanding electrode of large contact area with the electrolyte, potentially contributing to the improved electrochemical performance [162]. Three-dimensional nanostructures are considered as one of the most prominent electrode materials for SCs, which possess a short path for ion diffusion and huge surface area enabling adequate association between the electrolyte ions and the active materials [161]. Three-dimensional graphene network (3DGN) grown on nickel foam is an ideal template for the construction of graphene-based composite electrodes and pseudocapacitor electrode materials [161]. Inspired from the advantages of 3DGN integration with pseudocapacitive materials including nickel sulfides and layer-structured nickel hydroxide, Zhou et al. [161] prepared a Ni3S2 nanorod@Ni(OH)2 nanosheet core–shell nanostructure spread on a 3DGN/nickel foam through a facile hydrothermal process, and morphology evolution and growth mechanism were discussed. This Ni3S2@Ni(OH)2/3DGN electrode delivers a high capacitance of 1037.5 F·g−1 at 5.1 A·g−1 and 3.85 F·cm−2 at 19.1 mA·cm−2 along with 99.1% retention in capacitance after 2000 cycles. In addition, core–shell nanostructures with a controllable material constitution and utility could also help to increase both conductivity and charge storage capability in electrodes of energy storage devices. For example, a conductive core could enhance the charge conduction within the material, while a shell made of porous materials could provide preferable contact with ions at the electrode–electrolyte interface and buffer the volume changes during the charge–discharge process. Hierarchical carbon@Ni3S2@MoS2 double core–shell nanorods have been designed and prepared with the assistance of carbon/Ni nanorods serving as template and precursor [160]. This unique architecture improves the connection area with electrolyte and significantly enhances the capacitance along with the protection from the conductive core material in redox reactions. As a result, the C@Ni3S2@MoS2 double core–shell nanorods display a specific capacitance of 1544 F·g−1 at a current density of 2 A·g−1, and outstanding cycle stability (92.8% retained capacitance after 2000 cycles at 20 A·g−1).

6 Ni3S4

6.1 Pristine Ni3S4

Among all kinds of NixSy, Ni3S4 shows outstanding redox reversibility, safety and marvelous specific capacitance [82, 163]. Pristine Ni3S4 nanoparticles can deliver a specific capacitance of 1226.5 F·g−1 and 71.8% capacitance retention after 1000 cycles [164]. Wang et al. [165] synthesized rigid 3D Ni3S4 nanosheet frames via the hydrothermal method, demonstrating a high specific capacitance of 1213 F·g−1 and a capacitance retention of about 60% over 2000 cycles. In addition, to improve their supercapacitive properties, design and fabrication of hybrid nanomaterials are considered to be successful approaches. By hybridizing other active materials with nickel sulfides, cycling stability can be increased. However, in the practical approach, those electrodes are restricted in flexibility. Huang et al. [166] synthesized Ni3S4 on the surface of carbon cloth (CC) through a facile one-step hydrothermal process. CC will provide not only superior device flexibility but also a 3D scaffold for the diffusion of ions. The as-prepared flexible Ni3S4/CC electrode demonstrates an excellent specific performance of 1340 F·g−1 at 1 A·g−1 in 2 mol·L−1 KOH aqueous electrolyte, as well as long cyclic stability with 94.5% maintained capacitance after 5000 cycles of charge–discharge at a current density 5 A·g−1. The fabricated ASC using the Ni3S4//CC as positive electrode and active carbon (AC) as negative electrode demonstrates an enlarged cell voltage to 1.5 V in polyvinyl alcohol–potassium hydroxide (PVA/KOH) electrolyte. The capacitance retention of ASC device is around 84.7% over 5000 cycles at the current density of 1 A·g−1 along with a superior energy density (14.6 W·kg−1 at a power density of 750.8 W·kg−1) and high power density (6750 W·kg−1 at an energy density of 5.4 W·kg−1).

6.2 MoS2/Ni3S4 composites

The incorporation of 2D MoS2 with nickel sulfides could boost the synergetic effect and will be beneficial to further high-performance application [167,168,169]. Zhang et al. [164] designed Ni3S4@MoS2 hierarchical nanostructures for SCs via a well-organized strategy to totally utilize the merits of each component. For the coated crystalline Ni3S4 core with the amorphous MoS2 shell (Ni3S4@MoS2) fabricated via a facile one-pot route, amorphous MoS2 (indicating a specific capacitance of 1.6 times as that of the crystalline counterpart) can raise the electrochemical performance of the electrode materials, achieving a superior specific capacitance of 1441 F·g−1 at 2 A·g−1. The core–shell structure showed 90.7% initial retention capacitance at 10 A·g−1 over 3000 cycles and 76.2% maintained capacitance after 10,000 cycles. Luo et al. [110] constructed Ni3S4–MoS2 heterojunction (Fig. 8a) electrode that displays a specific capacitance of 985 F·g−1 at 1 A·g−1 and 85% capacitance recovering (i.e., 573 F·g−1) after 20,000 cycles at 10 A·g−1 (Fig. 8b). The Ni3S4 support provides enhanced conductivity in the heterojunction and shows additional affinity with MoS2, permitting a superior cycle life span. The Ni3S4/MoS2//AC (ASC) device demonstrates a high energy density of 58.43 W·kg−1 when the power density extends 385.95 W·kg−1 and retains an energy density of 18.75 Wh·kg−1 even at a large power density of 7500 W·kg−1. Huang et al. [136] synthesized Ni3S4@MoS2 nanosheets decorated on carbon fiber paper (Ni3S4@MoS2/CFP) by a facile one-step hydrothermal process. The Ni3S4@MoS2/CFP exhibits a remarkable specific capacitance (1296 F·g−1), excellent rate capability and long cycling stability (96.2% retention over 5000 cycles at 5 A·g−1). The Ni3S4@MoS2/CFP electrode delivers a high specific performance of 1296 F·g−1 at 1 A·g−1 in 2 mol·L−1 KOH solution. Wang et al. [82] fabricated an advanced rose-like Ni3S4 microflower by a direction exchange from the as-synthesized Ni(OH)2 microflowers with sulfur based on the mechanism of self-assembly and oriented attachment crystal growth process under hydrothermal conditions [170]. The Ni3S4 microflower electrode displays a high specific capacitance of 1703 F·g−1 at 1 A·g−1 and even 1165 F·g−1 when the current density is raised to 10 A·g−1. Furthermore, the Ni3S4//AC device exhibited superior cycling performance (~ 93% retained capacitance after 5000 cycles). To enhance the cycling stability of Ni3S4 at high current densities, an electrode of controlled MoS2/Ni3S4 composite nanosheets wrapped on interconnected carbon shells (C@MoS2/Ni3S4) was designed (Fig. 8c) [137] by a multistep process. The as-prepared composite reveals excellent cycling performance at large current densities due to its synergistic effects. After 10,000 cycles at 20 A·g−1 (Fig. 8d), a specific capacitance of 640.7 F·g−1, even much larger than the starting value, can be displayed. Electrochemical tests demonstrate that the interconnected C shells can decrease the equivalent series resistance (ESR) and the incorporation of MoS2 can exceptionally raise the cycling stability. In addition, the MoS2/Ni3S4 nanosheets spread around the C shells have greater contacts with the electrolyte and meanwhile buffer the volume change during the charge–discharge process.

Fig. 8
figure 8

(reproduced from Ref. [137], Copyright 2018 Elsevier)

a SEM image of Ni3S4–MoS2; b cycling performances of MoS2, Co3S4–MoS2 and Ni3S4–MoS2 at a current density of 10 A·g−1 (reproduced from Ref. [110], Copyright 2017 The Royal Society of Chemistry); c SEM image of C@MoS2/Ni3S4 composite; d cycling performance of C@MoS2/Ni3S4, C@Ni3S4, C@MoS2, MoS2/Ni3S4 and MoS2 samples at 20 A·g−1.

Full size image

6.3 NixSy/Ni3S4 composites

Apart from the hierarchical design and electronic conductivity, the reversible Faradaic reaction included in the charging/discharging process also has effect on the cycle performance [163, 171]. Transition-metal oxides or hydroxides of the similar metal but with double edges as hybrid supercapacitor electrode materials can assist in the cycling life [163, 171]. Moreover, morphology and composition are significant features for boosting the specific capacity, rate capability and cycling stability of electrode materials in electrochemical energy storage devices. With the beneficial particle size (200–400 nm) and composition, the NiS/Ni3S4 composite synthesized by Gou et al. [171] can deliver a favored gravimetric capacity (194.4 mAh·g−1 at 2 A·g−1, and 133.1 mAh·g−1 at 10 A·g−1) and enhanced cycling stability (89.5 mAh·g−1 at 10 A·g−1 after 5000 cycles). The nanoparticles inside could buffer the volume changes during the long-term cycling, and for the pseudocapacitive mechanism, redox reaction is attributed to the reformation of Ni2+/Ni3+ couple. And the synergistic effect is obtained from an intimate contact between NiS and Ni3S4, and the pinning/interaction of Ni3S4 with NiS could inhibit nickel sulfide particles from aggregating and avoid electrode material dissolution into the electrolyte. Cheng et al. [163] rationally designed and synthesized size-tunable hierarchical hollow core–shell submicrospheres based on nickel sulfide via a one-step hydrothermal process, in which Ni3S2/NiS hollow submicrosphere works as a core and Ni3S4 nanoflakes as shell. SEM and TEM images revealed the 3D hollow submicrospheres with lots of nanoflakes uniformly wrapped on the surface of hollow sphere with a diameter of 230 nm (Fig. 9a–c). The diameter of the hierarchical sphere can be managed by cetyltrimethylammonium bromide (CTAB). The unique structure design and mechanism is consistently investigated, and it can be assigned to a synergetic process of cage effect, Kirkendall effects and Ostwald ripening. When accommodated as electrodes for SCs, the optimized hybrids revealed a large specific capacity of 1031 C·g−1 (286.5 mAh·g−1) at a current density of 2 A·g−1. Even as the current density rises to 40 A·g−1, the hierarchical core–shell nanosphere still retains 614 C·g−1, showing an extraordinary rate capacity; it also exhibits a high retained capacitance of 90.3% after 3000 cycles at 10 A·g−1, due to the synergistic effects of multiphase nickel sulfides. Furthermore, the optimized ASC based on the Ni3S2/NiS@Ni3S4 hierarchical hollow core–shell submicrospheres and rGO (NHS//rGO) shows good cycling stability (94.4% of the initial capacitance value after 14,000 cycles at 10 A·g−1), high power density and energy density (Fig. 9d, e).

Fig. 9
figure 9

(reproduced from Ref. [163]. Copyright 2018 Elsevier)

a TEM image (inset being diameter distribution histogram) and b SEM image of Ni3S2/NiS@Ni3S4core/shell hollow submicrospheres (NHS); c hybrid NHS-3 nanostructure diagram; d schematic illustration of NHS-3//rGO ASC device configuration, and e cycling stability of the ASC (insets being GCD curves of NHS-3//rGO ASC at different current densities and photograph showing that two SCs in series can light up a red LED).

Full size image

Composition and morphology control of electrode materials is a successful route to boost the specific capacity, rate performance and cycling stability of electrochemical energy storage devices. Dai et al. [83] synthesized delicate nickel sulfide nanostructures as battery-type electrode for hybrid capacitor, where the phase structure and morphology are tuned by changing the initial NiCl2/S mole ratio and hydrothermal synthesis conditions to produce a three-phase nickel sulfide (NiS–Ni3S2–Ni3S4, indicated as TP-NixSy) with 3D flower-like design made up of interconnected nanoflakes. The as-synthesized TP-NixSy shows a specific capacity of 724 C·g−1 at a current density of 1 A·g−1. When incorporated with rGO, the TP-NixSy/rGO composite electrode demonstrates not only higher specific capacity (807 C·g−1 at 1 A·g−1) but also better rate capability (~ 72% capacity recovered as the current density accumulated from 1 to 20 A·g−1). Moreover, the hybrid capacitor, fabricated from a TP-NixSy/rGO positive electrode and a graphene-based negative electrode, exhibits a high energy density of 46 W·kg−1 at a power density of 1.8 kW·kg−1, and retains an energy density of 32 W·kg−1 at a power density of 17.2 kW·kg−1, signifying a promising potential for practical application.

7 Conclusion and perspectives

Supercapacitors are a prominent energy storage technology in developing the renewables and electric vehicles. Numerous synthetic approaches have been prospected to upgrade the electrochemical performance of electrode materials for SCs. Nickel sulfides and their hybrids are promising and competent candidates to alleviate the challenges for SCs because of their specific structure and properties.

We here propose some future perspectives of nickel sulfide-based materials for high-performance SCs listed as follows:

  1. 1.

    To enhance the performance of nickel sulfides through preparing the nanostructures with tunable morphologies that possess high surface area, good conductivity and appropriate pore properties. Nanostructures with large specific surface area will build effective contact between electrode and electrolyte ions, furnishing abundant electroactive sites for energy storage, specifically at high discharge currents.

  2. 2.

    To enhance the conductivity by constructing nanocomposites. Nickel sulfides/carbon composites not only illustrate the merits of the components but also conquer the defects of single constituents. The integration of nickel sulfides with carbon materials (e.g., graphene, carbon nanotubes and carbon nanofibers) can combine the advantages from all the constituents, and this strategy has been considered as one of the most prominent techniques to enhance the conductivity of nickel sulfide-based materials.

  3. 3.

    To obtain high energy density beyond high power density through fabricating a hybrid capacitor. As we know, the operational potential window of nickel sulfides is restricted by its intrinsic electrochemical properties. Integrating nickel sulfide materials as the cathode and carbon materials as the anode for hybrid capacitors will realize a larger potential window and higher energy/power densities thereof.

  4. 4.

    It is noteworthy that there is hardly any report on SCs with nickel sulfides electrodes based on organic electrolytes probably due to that the adoption of organic electrolytes in this kind of cell with nickel sulfides as cathode/positive electrode will not significantly extend the potential window but reduce the performance of SCs, especially power density due to lowered electrolyte conductivity. Moreover, as we know, in a related battery-type cell of NiS//Li, the discharge plateau is around 1.5 V using an organic electrolyte [172]; when the Li anode/negative electrolyte is replaced by a carbon electrode which is similar to the hybrid capacitor system, lithium plating may occur in an extended potential window and the rate/power-output performance may be a shortcoming. However, the adoption of some specific electrolytes such as redox-active electrolytes, water-in-salt electrolytes (including the saturated NaClO4 aqueous electrolyte with a wide potential window over 3 V [173]) and ionic liquids could be a promising trend [174].

  5. 5.

    To design electrodes with enhanced structural and cyclic stability. It is of great importance to design and synthesize advanced electrodes and cells with cost-efficient strategies based on further insight into the electrochemical mechanism of nickel sulfides amid the large transition-metal chalcogenide family. Furthermore, avoiding the usage of inactive components such as binders, current collectors for self-supporting electrodes will also enhance the energy densities of devices.