Elsevier

Energy Storage Materials

Volume 24, January 2020, Pages 594-601
Energy Storage Materials

Nickel disulfide nanosheet as promising cathode electrocatalyst for long-life lithium–oxygen batteries

https://doi.org/10.1016/j.ensm.2019.06.017Get rights and content

Abstract

Lithium–oxygen batteries (LOBs) are considered as next-generation energy storage systems owing to their high energy densities. In order to achieve high-performance LOBs, it is necessary to develop efficient electrocatalysts that exhibit reversible formation and decomposition of discharge products on the oxygen-electrode side. In this study, single-crystalline NiS2 nanosheets (NiS2-NSs) are fabricated as an efficient electrocatalyst in an oxygen-electrode for high-performance LOBs. Ni(OH)2-NSs are prepared through a hydrothermal reaction and subsequently reacted with sulfur by a solid/gas phase reaction process to form NiS2-NSs. As an electrocatalyst in an oxygen-electrode, the single-crystalline NiS2-NSs can reversibly form and decompose the discharge products during the discharging and charging processes, respectively. In particular, the NiS2-NSs more effectively decompose the discharge products compared to the Ni(OH)2-NSs owing to its high affinity to oxygenated species. In addition, the NiS2-NSs exhibit a long-term cyclability over 300 cycles at a current density of 1000 mA g−1 with a cut-off capacity of 1000 mA h g−1. Moreover, NiS2-NSs without conducting agent exhibit an electrocatalytic activity and its LOB performance can be further maximized through addition of a redox mediator.

Introduction

Rechargeable lithium–oxygen batteries (LOBs) attract significant attention for application in next-generation automobile technologies owing to their high theoretical specific energy density of 3505 W h kg−1, which is approximately 5–10 times higher than those of conventional lithium-ion batteries (LIBs) [[1], [2], [3]]. In an LOB system using an aprotic electrolyte, the electrochemical reaction between Li-ion and oxygen can be expressed as: 2Li+ + 2e ​+ ​O2 ↔ Li2O2 (E0 = 2.96 V vs. Li/Li+), during which an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) occur upon the discharging and charging processes, respectively [1,2]. However, some critical issues such as the low energy efficiency and the short cycle life hinder their applications [4]. In particular, the sluggish kinetics of the ORR and OER lead to a high potential gap between the ORR and OER. In order to overcome this obstacle, it is necessary to use an effective electrocatalyst in an oxygen-electrode (or cathode) [5,6]. Extensive studies have been carried out to develop suitable electrocatalysts for LOBs, such as noble metals (Au, Pt, Ru) and transition-metal oxides, carbides, and nitrides [[7], [8], [9], [10], [11], [12]]. However, considering the significant drawbacks, including the high cost and the low catalytic activity and stability, it is required to develop novel electrocatalysts for LOBs.

Transition-metal sulfides (TMSs) attract interest as energy conversion and storage materials owing to their low costs, earth-abundance, and higher electron conductivities and electrochemical activities compared to those of their oxide counterparts [[13], [14], [15], [16]]. Recently, they have been reported as LOB electrodes. For example, Sennu et al. synthesized Co3S4 with a high reversibility of 95.72% during the first discharging and charging process [17]. Dou et al. and Lin et al. reported an excellent intrinsic oxygen affinity of Co9S8; nanocage- and sisal-shaped Co9S8 structures exhibited discharge capacities of 7000 and 6875 mA h g−1, respectively, at a current density of 50 mA g−1 [13,18]. Additionally, Ma et al. reported a flower-like NiS, which exhibited a full-discharge capacity of 6733 mA h g−1 at a current density of 75 mA g−1 [19]. However, although NiS2 has been reported in diverse applications such as water splitting electrocatalysts and supercapacitors [20,21], it has not been reported as an electrocatalyst for LOBs.

To the best of our knowledge, for the first time, we utilized a two-dimensional (2D) single-crystalline NiS2 nanosheets (NiS2-NSs) as an effective ORR/OER electrocatalyst for LOBs. As the number of crystal boundaries in a single-crystalline structure is smaller than that of a polycrystalline structure, the single-crystal structure promotes electron movement and exhibits an excellent catalytic activity [22,23]. In addition, the 2D morphology is suitable for large numbers of Li-ion and O2 species [24,25]. Single-crystalline NiS2-NSs were obtained through a hydrothermal reaction and a subsequent solid/gas phase reaction. The synthesized NiS2-NSs electrode exhibited effective formation and decomposition of the discharge products and excellent cycle stability. In addition, a NiS2-NSs electrode without conducting agent exhibited a good electrocatalytic performance.

Section snippets

Synthesis of a Ni(OH)2-NSs precursor

The Ni(OH)2-NSs precursor was synthesized by a simple hydrothermal method. First, Ni(OCOCH3)2·4H2O (8 mmol, 98%, Sigma-Aldrich) was completely dissolved in deionized water (100 mL) under magnetic stirring. Second, the pH of the solution was adjusted to 9.1–9.2 using an ammonia solution (28–30%). Third, the solution was transferred to a 200-mL Teflon-lined autoclave and heated at 170 °C for 12 h. After the hydrothermal reaction, the product was washed several times with deionized water and

Results and discussion

Fig. 1a shows the XRD patterns of the Ni(OH)2-NSs and NiS2-NSs. After the hydrothermal reaction, a crystalline Ni(OH)2 phase (hexagonal polymorph, JCPDS No. 14–0117) was obtained; no secondary peaks were observed (lower panel in Fig. 1a). After the thermal reaction of the Ni(OH)2-NSs precursor with the sulfur powder, the Ni(OH)2 phase was completely transformed to the NiS2 phase (cubic polymorph, JCPDS No. 11–0099); no secondary peaks were observed (upper panel in Fig. 1a). However, there were

Conclusion

In summary, we developed single-crystalline NiS2-NSs as an electrocatalyst in an oxygen-electrode for rechargeable LOBs. The single-crystalline Ni(OH)2-NSs were synthesized by the hydrothermal synthesis and then reacted with evaporated sulfur through the gas/solid phase reaction to form the single-crystalline NiS2-NSs. As an electrocatalyst in the oxygen-electrode, the NiS2-NSs exhibited excellent ORR/OER performances with a low potential gap of 1.42 V after 100 cycles. Particularly, they

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT, South Korea (2019R1A2B5B02070203) and by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT, South Korea (2018M3D1A1058744).

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