Last few centuries the availability of energy has transformed the course of humanity. New sources of energy from fossil fuels have been identified and the fear of exhausting the fossil in couple decades and concern caused by the pollution from nuclear and thermal power stations lead to the search for renewable energy resources. Fossil fuels account for seventy per cent of the global emission of greenhouse gases and play a major role in climate change and air pollution.[1–3] To address the task of CO2 emissions and control air pollution, a rapid shift towards sources that have smaller CO2 emissions are examined and can be obtained through renewable energy resources. In renewable energy, energy transfers from biomass to electrical, solar to electrical, wind energy to electrical, the potential of water to electrical, biogas to electrical and chemical to electrical are largely employed.[4] Of these only a few provide continuous and constant supply of energy. Due to change in weather conditions and vagaries of nature the energy output fluctuates. In case of photovoltaic conversion, the energy is not available during night. To circumvent these difficulties, there is a need to store the generated energy an efficient, reliable, long-lasting, clean and eco-friendly energy storage device is important.[5–7] The electrical energy can be stored electrochemically through a chemical reaction. Among different electrochemical storage devices, large number of energy conversion devices and storage technology techniques i.e., Rechargeable batteries, conventional capacitors and Supercapacitors, have been attempted.[5, 8] Batteries are useful for systems that require a larger energy backup since it can provide high energy density. The conventional capacitors were used in a high-power system where higher power density is required. Supercapacitors can deliver energy at a faster rate than conventional rechargeable lead acid batteries but at the same time can store million times larger energy than the conventional capacitors. Thus, act as a via media between batteries and conventional capacitors.[5, 8, 9] Distinctive characteristics like high-power density, specific capacitance, stability, reversibility, moderate energy density, rapid charge capability, safe, eco-friendly and zero maintenance capability are some of the factors that favour the use of supercapacitors in a range of applications.[10] Researchers are taking greater efforts in improving the specific energy of supercapacitors as the energy density is still below the batteries.[6]
The supercapacitor performance is dependent on the electrode material. A good electrode combined with an appropriate electrolyte/ reservoir of charges, can exhibit high specific capacitance, high working potential, electrical conductivity and greater cyclic stability. The morphology of the electrode material gives a huge impact on the supercapacitor performance. Metal sulfide-based electrode materials are suitable for fulfilling the above needs to increase the performance of supercapacitors.[2, 10]
Metal sulfides such as CoS, ZnS, FeS, CuS, CdS, SnS[11–17] exhibit profitable qualities for use in optical, electrical, magnetic and catalytic applications. They have been examined as plausible candidates towards the development of devices for hydrogen storage[6, 16], sensor[18], supercapacitors[19], solar cells[20, 21], hydrogen evolving reaction catalysts[16], photoconductors and batteries[22]. Due to their controlled nanostructure, high theoretical capacity, and affordability, metal sulphides have been looked at as possible choice for supercapacitors.[23][10] Metal sulphides are endowed with good mechanical and thermal properties compared to polymers and oxides of metals, but still, their potential as electrochemical energy storage material is yet to be thoroughly and extensively examined.[14, 24] NiS is one of the metal sulphides with crucial constituents. The profitable features like good electronic conduction, low-price, ease of production and the existence of multiple valance states could turn the attention of a large number of scientific communities towards them.[25] Nickel sulphides are capable of existing in a wide variety of chemical compositions such as NiS, Ni3S2, Ni7S6, Ni9S2 and NiS2.[14, 26–36] These are unemployed in the supercapacitor application to a larger extent. Among other metal sulphides, the NiS exhibits a high redox reaction. Nickel sulphides have been investigated and have a widespread application in supercapacitors, batteries, catalysts, etc.,[37–41] The lower cycling stability of metal sulphides is a major hindrance towards practical applications [42]
One of the most successful strategies for enhancing a material's electrochemical characteristics is doping a metal element with good electrical conductivity. [15] V-doped NiS (V/NiS) has enhanced the redox reaction and exhibited better specific capacitance. Li has attracted huge interest in electrode material as it has high reactivity and is widely used in batteries.[43]
Bhardwaj et al. synthesised β-NiS nanoparticles through the hydrothermal method.[44] Balayeva et al. prepared β-NiS through the SILAR route.[45] Shombe et al. reported a solvent-free and phase synthesis β-NiS for the supercapacitor application.[46] Yang et al. could realise a specific capacitance of 857 F g− 1 (at 2 A g− 1) in the case of hierarchical flower-shaped β-NiS.[47] NiS2 nanostructure prepared via microwave-assisted synthesis resulted in a specific capacitance of 695 F g− 1 at 1.25 A g− 1.[48] Fu S et al. reported hydrothermal sulfidation of the facial and a low-cost method of synthesising Ni3S2 nanosheet in Ni- foam for ultra-high capacitance. Fu S et al. have fabricated popcorn-shaped morphology Sn-doped Ni3S2 by a simple two-step hydrothermal reaction.[49] Anand et al. reported that lithium incorporated SnS via a sonochemical approach.[50]
Lithium metal is known to react intensely with aqueous solutions, forming lithium hydroxide and highly flammable hydrogen. With the downside properties of lithium, it seems impossible to use aqueous solution synthesis methods to prepare lithium-based materials. The problem is more pronounced with the formation and dissolution of lithium. In spite of this difficulty, many workers have prepared composites containing lithium compounds. This is feasible as we do not deal with metallic lithium but the ions of lithium. Hence, this work did not encounter any violent reactions associated with metallic lithium in an aqueous medium. Li-based nano-composites were synthesised via hydrothermal and microwave-assisted hydrothermal methods[51, 52] and in this way hydrothermal method has been employed in synthesising Li-rich nanomaterials such as Li0.94[LiNiMn]O2, LiFeO2, LiCoO2, LiFePO4, Li3V2PO4, LiNaMnNiPO4 and LiMnO2 as a one-step process for supercapacitors and battery application.[53–60]
In the present work, lithium nitrate is taken in the precursor. The processing of the precursor has resulted in the formation of dilithium, which has been mostly reported in gaseous form.[61] Probably, this is the first report on the formation of dilithium species in the formation of a composite. The structural and textural composition and purity of the nanocomposite were examined. The suitability of the prepared composite has been examined via cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The cyclic stability of the electrode is carried out. The symmetric device was fabricated using the Li2: β-NiS nanocomposite, and its electrochemical studies proceeded. The energy storage capability of the Li2: β-NiS nanocomposite electrode has been reported with the help of graphical plots.