Rapid synthesis of binary α-NiS–β-NiS by microwave autoclave for rechargeable lithium batteries
Highlights
► NiS has been synthesized by a rapid, one-pot, hydrothermal microwave autoclave method. ► The α-NiS–β-NiS sample synthesized at 160 °C yielded good electrochemical performance in terms of high reversible capacity (320 mAh g−1 at 0.1C up to 100 cycles). ► At high rates, the sample operated at a good fraction of its capacity.
Introduction
There is an urgent need for renewable clean energy sources at a much higher level than is presently in force. The CO2 issue, and the consequent air pollution in large urban areas, may be only solved by replacing internal combustion engine (ICE) cars with ideally, zero emission vehicles, i.e. electric vehicles (EVs) or, at least, by controlled emission vehicles, i.e. hybrid electric vehicles (HEVs) and/or plug-in hybrid electric vehicles (PHEVs). In this respect, lithium-ion battery technology could be the most promising approach. Thus, achieving the goals of low cost combined with higher energy density, better cycling stability, and non- or less toxic and more environmentally friendly materials as electrodes for lithium ion batteries has become mandatory if clean renewable technologies are to be developed for the future [1], [2]. Presently, LiMO2 or LiM2O4 (M: transition metal) compounds are widely employed as positive electrode for lithium-ion batteries due to their high reversibility in Li-ion intercalation/de-intercalation processes [3], [4], [5], [6], [7]. Although LiCoO2 is still being used as a successful cathode material in most commercial lithium-ion batteries, it is less available in terms of raw materials, more toxic, and more costly than other transition metals, such as manganese, nickel, and iron. In addition, LiCoO2 is not as stable as other potential electrode materials and can undergo performance degradation or failure when overcharged [8], [9], [10]. LiNiO2, which also forms a distorted rock-salt structure, is lower in cost and has a higher energy density [11], but is less stable [12], [13] and less ordered [14], as compared to LiCoO2. LiMn2O4 has shown excellent cycling performance at room temperature in the region of 4 V, but still suffers from capacity loss, particularly at elevated temperature [15]. Various strategies have been proposed and tested to avoid some of these drawbacks, and scientists are still struggling to minimize these problems. Besides these types of positive materials, sulfur and metal sulfides are known to be promising cathode materials because of their low cost and high theoretical capacity, assuming complete discharge to Li2S [16], [17], [18], [19], [20], [21]. Among the various metal sulfides, nickel sulfide is one of the most promising cathode materials. Nickel sulfide (NiS) has a high theoretical capacity of 590 mAh g−1 with good electronic conductivity [22]. At present, only a few studies on NiS as a battery material have been reported [22], [23], [24], [25], [26], [27]. Studies based on the ex situ X-ray diffraction method by Han et al. [25] indicated that the cathode reaction of NiS could be explained as follows:3NiS + 2Li ↔ Ni3S2 + Li2SNi3S2 + 4Li ↔ 3Ni + 2Li2S
The total charge–discharge mechanism is represented as:NiS + 2Li ↔ Ni + Li2S
To this end, various synthesis techniques have been adopted to synthesize NiS in order to improve the electrochemical performance of NiS electrodes, such as ball-milling [22], [25], [26], high-boiling-point solvent [27], polyol [24], spark plasma-sintering [28], and solvothermal [23] techniques, in which most of the methods require extensive mechanical mixing, high temperature, and long reaction time. Wang et al. [23] have prepared NiS by the solvothermal method, which needs 24 h to complete the reaction. In addition, homogeneous NiS readily forms after 12 h of ball milling, as reported by Han et al. [25]. Quite recently, microwave-assisted synthesis methods have been particularly proved to be effective for many types of cathode materials [29], [30], [31], [32]. Microwave-assisted synthesis requires much shorter reaction times and saves energy, which is favorable for industrial manufacturing when compared with the conventional solvothermal and hydrothermal synthesis. Manthiram et al. have successfully synthesized a series of LiMPO4 (M: Fe, Mn, Co, and Ni) nanocrystallites within 5–15 min at temperatures below 300 °C [29], [30], [33], [34], and the preparation of LiMnO2 using conventional and microwave hydrothermal routes has also been reported by Ji et al. [32]. It was observed that the electrochemical performance of LiMnO2 obtained from the microwave hydrothermal method was improved compared to the conventional method.
It is well known that electronic transport properties can be tuned by interfacial design and by varying the spacing of interfaces down to the nano-regime [35], [36], [37]. In recent reports, the interface in some dual-phase materials is demonstrated to be more sensitive for storing the extra lithium [23], [27], [35], [38], [39]. There are many hybrid systems reported due to some great advantages which possibly could not be found in the single system, such as higher catalytic activity, capability to absorb the volume variation of the active material during lithium insertion, and ability to react reversibly with a larger amount of lithium, and these factors can greatly improve the electrochemical performance compared to the single component systems [23], [35], [38]. Morphological stability can also be further improved if several materials are combined in an appropriately structured composite [40], [41]. When two compounds with different crystal structures combine and form a composite, the crystal lattice mismatch causes numerous lattice defects in the structure of the composite. As a result, channels for Li+ transfer in the composite will be much more abundant than in either of the constituent compounds [35]. Based on this approach, we attempted to incorporate β-NiS phase into α-NiS, with the aim of improving the electrochemical performance of α-NiS cathode.
In this study, we report binary α-NiS–β-NiS compound prepared via the microwave hydrothermal method within a short period of time. In addition, an electrolyte consisting of 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in poly(ethylene glycol) dimethyl ether 500 g/mol (PEGDME 500) is also reported in this work. It was previously reported that the electrochemical performances of nickel sulfide electrodes were improved by using this electrolyte when compared with conventional electrolyte composed of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) [23], [24].
Section snippets
Materials preparation
In a typical procedure for the preparation of NiS powders [41], 0.01 mol nickel (II) acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%, Sigma–Aldrich), 0.03 mol thiourea (N2SCH4, ≥99%, Sigma–Aldrich), and 0.01 mol citric acid trisodium salt dehydrate (Na3C6H5O7·2H2O, 99%, Aldrich) were added to 40 mL deionized water under stirring. Afterward, 20 mL ammonia solution (28–30%, Merck) was added dropwise to tune the pH value to 12, and the dark blue solution was further stirred for 30 min. Then, the solution
Results and discussion
The XRD patterns of the samples synthesized at 140 °C, 160 °C, and 180 °C for 15 min are shown in Fig. 1. Microwave hydrothermal treatment of the mixture at 140 °C produced a pure hexagonal NiAs-type α-NiS phase, which shows good agreement with JCPDS data (JCPDS File Card No. 75-0613, space group: P63/mmc, a = 3.4200 Å and c = 5.3000 Å). Liu has reported that the synthesis of single-phase α-NiS was obtained at a longer reaction time of 24 h by a conventional hydrothermal route [40]. In our case, α-NiS
Conclusions
Nanocrystalline α-NiS–β-NiS powder has been successfully synthesized in a simple microwave autoclave within a short period of time. The results indicate that hexagonal NiAs-type α-NiS phase can be partially converted to a β-NiS millerite-like secondary phase with increasing heat-treatment temperature. In a sample prepared at 160 °C, a duplex crystallite size (α-NiS ∼ 47 nm and high surface area β-NiS ∼ 53 nm) is observed. Electrochemical testing demonstrates that the sample synthesized at 160 °C has a
Acknowledgements
Financial support provided by the Australian Research Council (ARC) through a Linkage Project (LP 100100802) and ARC Centre of Excellence funding are gratefully acknowledged. Nurul Hayati Idris is grateful to the Ministry of Higher Education of the Government of Malaysia for scholarship support. The authors would like to thank Dr. Tania Silver for critical reading and correction of this manuscript.
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