Carbon paper with attached hollow mesoporous nickel oxide microspheres as a sulfur-hosting material for rechargeable lithium-sulfur batteries

Electrochimica Acta 327 (2019) 135028

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Carbon paper with attached hollow mesoporous nickel oxide microspheres as a sulfur-hosting material for rechargeable lithium-sulfur batteries Jyun-Wei Guo, Mao-Sung Wu* Department of Chemical and Materials Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, 807, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 August 2019 Received in revised form 28 September 2019 Accepted 7 October 2019 Available online 9 October 2019

Hollow nickel hydroxide microspheres built of nanosheets could be obtained by hydrothermal synthesis in the presence of glycine. After heating to 400
C in air, the hexagonal b-Ni(OH)2 was converted into cubic NiO which showed better electrical conductivity and greater surface area compared with other annealing temperatures. The surface morphology remained almost unchanged, while the Ni(OH)2 nanosheets with solid interior were changed into NiO nanosheets composed of interconnected nanoparticles and mesopores after annealing at 400
C. The sulfur-filled hollow mesoporous NiO microspheres obtained after annealing at 400
C was loaded into the carbon paper as cathode (NiO/S) for lithium-sulfur batteries. The NiO/S cathode could deliver a large specific capacity of 1300 mAh g1 at 0.2C (335 mA g1), much greater than bare sulfur loaded into the carbon paper (1000 mAh g1). Moreover, the NiO/S cathode showed superior capacity recovery after C-rate test and coulombic efficiency during cycling test than S cathode. The improved performance for NiO/S could be attributed to the conductive NiO microspheres with mesoporous nanosheets that provided intimate electrical contact for boosting the charge-transfer reaction between lithium ions and sulfur materials, limited the polysulfide shuttle, and buffered volumetric expansion/contraction during charging and discharging. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Lithium-sulfur batteries Sulfur-hosting matrix Hollow microspheres Mesopores

1. Introduction There has been a growing demand for portable energy in recent years because of the rapid growth of mobile electronics including smart phones, laptop computers, aviation, and electric vehicles [1]. Typically, the rechargeable batteries are essential electrochemical energy-storage units to fulfill the power requirements for the mobile electronics. Lithium-sulfur (LieS) batteries have attracted growing interest among the prevailing batteries and are deliberated as the probable candidates for next-generation batteries [2,3]. Much effort has been made to solve the essential concerns which hamper the implementation of LieS batteries. Rechargeable LieS batteries typically consist of lithium anode, electrolyte-filled porous separator, and sulfur cathode. Lithium metal can be electrochemically oxidized to lithium ions that migrate to the sulfur cathode and react with sulfur, forming lithium polysulfides with various chain lengths in discharging process. The reverse

* Corresponding author. E-mail addresses: [email protected], [email protected] (M.-S. Wu). https://doi.org/10.1016/j.electacta.2019.135028 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

electrochemical processes arise and consequently convert the lithium polysulfides back to sulfur in charging process [1]. Sulfur is abundant in the earth and possesses theoretical specific capacity as high as 1675 mAh g1, making it promising as the cathode material with low cost and high energy density for LieS batteries [4]. Nevertheless, there are several critical issues that need to be improved further in the sulfur cathode [4]. First, the poor electrical conductivity reduces the electrochemical utilization of sulfur, leading to decreased capacities of LieS batteries particularly in high-rate charge/discharge processes [5,6]. Second, the longchain lithium polysulfides may dissolve in the electrolyte and then diffuse to anode (lithium metal) surface through electrolyte, resulting in the growth of short-chain polysulfides on the lithium anode. These short-chain intermediates can diffuse back to the cathode surface and react with sulfur or polysulfides through the shuttling effect, leading to a decrease in the coulombic efficiency and cycling stability of sulfur cathode [7]. Third, the drastic variation in volume between sulfur and lithium polysulfides in repeated charging/discharging processes may destroy the integrality and stability of cathode, resulting in the pulverization and structural disintegration of the active cathode materials [1,6].

2

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

To improve drawbacks which arise from the sulfur cathode, some strategies, such as using the host materials for the confinement of sulfur, changing the electrolyte’s composition, and employing a barrier layer for polysulfide shuttle, have been made [4]. Among them, the sulfur-hosting materials function as the key parts in improving the gravimetric and volumetric energy densities of LieS batteries [1]. On account of the insulating characteristic for sulfur and polysulfide materials, some carbonaceous materials have been adopted as the sulfur-hosting matrix to enhance the electrical conductivity of sulfur cathode and mitigate polysulfide shuttle by increasing the chemical attraction between carbon and polysulfides [4,8e12]. Carbonaceous materials with porous structures are particularly evident for their huge surface areas, abundant pores, and adjustable pore configurations to improve the sulfur loading and sulfur utilization [4,13e16]. More recently, there has been an increasing tendency for LieS batteries to develop the noncarbonaceous materials as the composite sulfur cathodes [2]. Porous materials, such as metal oxide/hydroxide and conducting polymer, have been reported and shown superior properties compared with sulfur-containing carbonaceous cathodes [17e25]. These non-carbonaceous porous materials turn out to be very successful in improving the conductivity of sulfur cathodes owing to their three-dimensional (3D) porous structure for trap of lithium polysulfides. In addition, the metal oxide host can alleviate the dissolution of polysulfides by forming a superior interface to adsorb the hydrophilic polysulfide intermediates compared with some carbon-based materials [26]. The rational design of cathode matrices (current collectors) including physical confinement and chemical adsorption has become one of the most straightforward methods in the development of numerous sulfur cathodes. Carbon and metal/metal oxide matrices such as foam and sponge have been shown to significantly enhance the lithium-storage characteristics of sulfur cathode [27e37]. A pioneer study on sulfur cathode using carbon paper has demonstrated good electrical conductivity between the carbon and the confined sulfur [38]. Lithium-sulfur cathodes based on carbonpaper current collectors gain several advantages compared to the conventional ones [39e41]. A combination of cathode matrix and sulphiphilic hollow/porous metal oxide materials offers a promising and valuable cathode structure for promoting the electrochemical performance of LieS batteries. In the present study, the carbon paper composed of carbon-fiber scaffolds is used as the porous and conductive matrix to support the hollow nickel oxide microspheres with mesoporous shells for facilitating the kinetic behavior of sulfur cathode in the rechargeable LieS batteries.

oven and kept at this temperature for 24 h. After cooling down to ambient temperature, green precipitates (nickel hydroxide) were separated from the solution by filtration, rinsed several times with DI water until a neutral pH was reached, and then dried at 60
C for 12 h. After drying, the powder was ground into the fine particles by an agate mortar. Hollow nickel oxide microspheres were prepared by heating the as-prepared nickel hydroxide powder to 400
C or 600
C for 4 h in air in a muffle furnace. The nickel oxide/sulfur composite could be obtained using a classical melt-diffusion process. Typically, the sulfur powder (Showa, 99%) and nickel oxide microspheres in a mass ratio of 7:3 were mixed together in Teflonlined autoclave (45 mL capacity) and sealed with a screw cap under nitrogen atmosphere. The mixture was heated to 155
C at 5
C min1 in an electric oven for 12 h, and then allowed to cool to ambient temperature under natural conditions. The sulfur-filled nickel oxide powder was ground into fine particles using an agate mortar. Small particles were collected after sieving through a 150mesh sieve. 2.2. Materials characterization The surface characteristics and interior microstructure of the nickel oxide/hydroxide powder were observed using SEM (scanning electron microscopy, Carl Zeiss Auriga) and TEM (transmission electron microscopy, Jeol JEM-1400), respectively. XRD (X-ray diffraction) spectra of powder were measured by a diffractometer (Bruker D8) with Cu Ka radiation at a wavelength of 0.15406 nm. The chemical composition of nickel oxide powder was measured using an X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). The electrical conductivity of nickel oxide/hydroxide was measured by an electrochemical impedance spectroscopy (EIS) with a sinusoidal amplitude of 0.01 V in a frequency range of 0.1e106 Hz under open-circuit condition at ambient temperature (Autolab PGSTAT302N). Powder sample was placed in between two stainless steel cylinders (15 mm in diameter) by applying a constant force of 3 tonf (tons-force long UK) in the direction perpendicular to the insulating base of the cylinders using a hydraulic machine. The thickness of pressed pellets was measured to be approximately 0.45 mm. The thermal properties and sulfur content of the composite samples were examined by a thermogravimetric analyzer (TGA, TA Instruments SDT Q600). Nitrogen adsorption and desorption curves of powder samples at 77 K were recorded by means of Micromeritics ASAP 2020 equipment, and the corresponding specific surface area and pore size distribution could be analyzed by the BET (Brunauer-Emmett-Teller) and BJH (BarrettJoyner-Halenda) methods, respectively.

2. Experimental 2.3. Electrochemical measurements All chemicals were of analytical grade unless otherwise stated and were used as received without any further purification. All aqueous solutions were prepared by means of the DI (de-ionized) water with resistivity of about 18.2 MU cm. 2.1. Synthesis of NiO microspheres and NiO/sulfur composite The hollow nickel hydroxide microspheres were prepared by a hydrothermal process according to the literature with some modifications [42]. Typically, the glycine (8.0 g), sodium sulfate (8.0 g), and nickel nitrate hexahydrate (3.7 g) were homogeneously dissolved in 100 mL of DI water under ultrasonic conditions for 10 min, and then 40 mL of sodium hydroxide solution (5 M) was dropped at a regulated rate into the stirred solution by a magnetic stir bar for another 10 min. This mixed solution (blue in color) was placed in a Teflon-lined autoclave (225 mL capacity), and then sealed with a screw cap, followed by heating to 160
C at 5
C min1 in an electric

Active material (sulfur or sulfur-filled nickel oxide), conductive carbon powder (Super P, Timcal), and poly(vinylidene fluoride) binder (Kuraha Chemical, KF-1300) in a mass ratio of 8:1:1 were blended together using N-methyl-2-pyrrolidone as solvent by a slurry homogenizer. The slurry was loaded into a commercial carbon paper (CP) with porosity of 75% (HCP030N, 1.2 cm in diameter and 0.03 cm in thickness), and then dried at 60
C in a vacuum chamber to form sulfur cathode. The sulfur loading in each cathode was controlled to be about 1.5 mg cm2. The sulfur contents on the S and NiO/S cathodes were 53.4 and 80.0 wt%, respectively. The porosity of electrodes was measured to be about 45% by imbibition method using n-butanol under vacuum at 27
C. The densities of S and NiO/S cathodes were 0.73 and 0.76 g cm3, respectively. Celgard 2400 and lithium sheet served as the porous separator and anode (and reference), respectively. The electrolyte consisted of 1 M lithium bis(trifluoromethane sulfonyl) imide dissolved in a

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

3

galvanostatic charge/discharge cycles were measured by a charger/ discharger unit (Scribner Associates, 580 Battery Test System) in a potential range of 1.8e2.8 V vs. Liþ/Li. EIS of the coin cells was examined using an electrochemical working station (Autolab PGSTAT302N) with an sinusoidal amplitude of 0.01 V in a frequency range of 0.1e105 Hz under open-circuit conditions. 3. Results and discussion

Fig. 1. SEM and TEM micrographs of the nickel hydroxide (a,b) before and after annealing at (c,d) 400
C and (e,f) 600
C.

solution mixture of 1,3-dioxolane and dimethoxymethane (1:1 by volume) with 0.2 M lithium nitrate additive [21]. The coin cell (CR2032) constructed with lithium anode, separator, and sulfur cathode was filled with an electrolyte (45 mL) and then assembled in an argon-filled glove box. The ratio of electrolyte to sulfur was 30 mL mg1. Cyclic voltammetry was carried out using a potentiostat/galvanostat (CH Instruments, 608C) in a potential range of 1.8e2.8 V vs. Liþ/Li under a sweep rate of 0.1 mV s1. The

The physicochemical properties of nickel hydroxide rely on the heat-treatment temperature remarkably. Fig. 1 shows the SEM and TEM micrographs of nickel hydroxide microspheres before and after annealing at 400
C and 600
C in air. The as-prepared nickel hydroxide reveals a spherical morphology with diameters ranging from 3 to 5 mm. Obviously, the microsphere is built of nanosheets, and the nanosheets arrange to form a hollow sphere with porous shell. The shell thickness is approximately one-third of the sphere in diameter. Previous report has indicated that the glycine and strong alkaline condition are vital to the formation of hollow sphere [42]. In the absence of glycine, nickel hydroxide tends to form solid particles rather than hollow microspheres. The change in surface morphology of hollow microspheres after annealing at temperatures lower than 600
C is not significant, while the change in microstructure is considerable as revealed in the TEM micrographs. Before annealing, the nanosheets show solid interior, while the nanosheets are composed of interconnected nanoparticles (about 15 nm in size) and pores after annealing at 400
C. The small nanoparticles assemble into large nanoparticles as the temperature increases up to 600
C, resulting in the formation of discrete large pores on the nanosheets. Fig. 2a displays the XRD spectra of as-prepared nickel hydroxide powder before and after annealing at different temperatures. The diffraction peaks of as-prepared nickel hydroxide could be assigned to the b-Ni(OH)2 with well-crystalline hexagonal structure (JCPDS 14e0117) in space group P3m1 (164) and lattice parameters a ¼ 3.126 Å and c ¼ 4.605 Å. When the temperature is higher than 400
C, the XRD spectra are characterized as the NiO with cubic

Fig. 2. (a) XRD spectra of the nickel hydroxide powder before and after annealing at different temperatures. (b) XRD patterns of the sulfur and sulfur-filled nickel oxide powder (NiO was annealed at 400
C before filling with sulfur).

4

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

structure (JCPDS 71e1179) in space group Fm3m (225) and lattice parameter a ¼ 4.1771 Å [43]. The XRD peaks become sharper when the temperature increases from 400
C to 600
C. This reflects that the degree of crystallinity for the NiO after annealing at 400
C is not high and it increases with elevating the temperature. Wider XRD peaks indicate that the NiO exhibits lower crystallinity and smaller grains. The average grain size of NiO powder can be calculated using Scherrer’s equation at 2q ¼ 43.3
. The grain size of NiO annealed at 400, 500, and 600
C are 16, 25, and 34 nm, respectively. The higher temperature leads to the formation of NiO microspheres with larger grain size. This result confirms the change in particle size of NiO nanosheets shown in TEM micrographs. Fig. 2b displays the XRD patterns of sulfur (S) and sulfur-filled NiO (NiO/S) powder. NiO was obtained by annealing the Ni(OH)2 microspheres at 400
C in air. Clearly, the characteristic peaks of sulfur appear in the NiO/S sample, indicating that the melt-diffusion process allows for the infiltration of sulfur into hollow NiO microspheres through the porous shells. Fig. 3 shows the XPS spectra of NiO powder obtained after annealing at 400
C and 600
C. Three

Fig. 3. XPS spectra of the nickel oxide powder after annealing at (a) 400
C and (b) 600
C.

deconvoluted 2p3/2 peaks could be found in each sample. A peak at around 853.5 eV comes from the Ni2þ species in the NiO crystal, while the neighboring peak at 855.3 eV corresponds to surface Ni3þ species caused by Ni2þ vacancies in NiO crystal lattices [44,45]. A wide satellite peak at about 860.9 eV results from a shakeup process in NiO structure [46]. The x value in NiOx is decreased from 1.22 (400
C) to 1.14 (600
C), indicating that the stoichiometric NiO (x ¼ 1) may be obtained after annealing at higher temperature in air. As revealed in literature, the heat-treatment temperature affects the electrical conductivity of nickel oxide/hydroxide considerably [47]. EIS is a useful technique to unravel whether the electrical conductivity in materials is controlled by bulk or grain boundary [48]. Fig. 4 shows the EIS curves and equivalent circuits of Ni(OH)2 powder before and after annealing at 400 and 600
C in air. A schematic diagram of the apparatus for the electrical conductivity measurement is shown in the inset of Fig. 4a. The Nyquist plot of Ni(OH)2 powder (Fig. 4a) reveals only one intercept at Z’ corresponding to conduction resistance through the bulk of the Ni(OH)2 (Rb). A vertical line in low frequency region caused by the charge build-up at the blocking metal electrodes is represented by a constant phase element (CPEdl) [48]. As illustrated in Fig. 4b and c, a semicircle appears in the Nyquist plots of NiO samples after annealing at 400
C and 600
C, indicating the contribution of grain-boundary capacitance (represented by another constant phase element, CPEgb) in parallel with grain-boundary resistance (Rgb) to the impedance [48]. The equivalent circuits corresponding to the EIS spectra of Ni(OH)2 and NiO are shown in Fig. 4d. The electrical conductivity (s) can be calculated by s ¼ l/(RA), where R represents the electrical resistance (U) including Rb and Rgb, A denotes the base area of the cylinder (cm2), and l indicates the distance between two cylinders (cm) corresponding to the thickness of powder sample under compression. The electrical conductivities of Ni(OH)2, NiO (400
C), and NiO (600
C) are calculated to be 1.5  105, 1.2  103, and 4.6  105 S cm1, respectively. Clearly, the electrical conductivity is sensitive to the annealing temperature. When the heating temperature is lower than the conversion temperature of Ni(OH)2 to NiO, the material has very low

Fig. 4. EIS spectra of the nickel hydroxide powder (a) before and after annealing at (b) 400
C and (c) 600
C. (d) Equivalent circuits for the nickel hydroxide/oxide powder. Inset shows a schematic diagram of the apparatus for the conductivity measurement.

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

conductivity due to the presence of Ni(OH)2 which is poorly conductive per se. The high electrical conductivity can be obtained at 400
C as a result of the chemical conversion from Ni(OH)2 to non-stoichiometric NiO which exhibits better conductivity than stoichiometric one [47]. The conductivity of NiO annealed at 400
C could be enhanced by two orders compared to that of Ni(OH)2 and is far greater than that of S and Li2S at room temperature [1]. When the temperature exceeds 400
C, the stoichiometric NiO powder are in the majority and become insulating, showing a decreasing conductivity with increasing the temperature [47]. The electrical resistances of CP substrate, S-, and NiO(400
C)/S-coated CP electrodes were measured to be 0.08, 0.16, and 0.05 U, respectively using the same apparatus and process, while the applied force was reduced to 1.3 tonf to avoid the pulverization of CP. Clearly, the presence of NiO (400
C) improves the electrical conductivity of the composite sulfur/CP cathodes. TGA curve of the Ni(OH)2 powder in air is shown in Fig. 5a. About 20% weight loss can be detected at temperatures ranging from 25 to 600
C. A significant weight loss from 25 to 250
C is due to the evaporation of hydrated water. A drastic weight loss at about 300
C results from the chemical conversion of Ni(OH)2 into NiO and gaseous water [43]. XRD results endorse that this drastic weight loss is due to the phase conversion from hexagonal bNi(OH)2 into cubic NiO. Previous report has indicated that heating Ni(OH)2 in 300e400
C forms a non-stoichiometric NiO with high electrical conductivity [47]. Above that range, the conductive NiO is converted into insulating material and is not applicable for electrochemical applications [47]. On the other hand, the water content remains in the NiO/Ni(OH)2 at low heat-treatment temperature, which is detrimental to the electrochemical operation of nonaqueous lithium batteries. The pore-size distribution curves of Ni(OH)2 powder before and after annealing at 400
C and 600
C are shown in Fig. 5b. The majority of pores are located at about 50 nm for the Ni(OH)2 and NiO (600
C) powder, while the NiO (400
C) sample obtains an additional pore size of about 8 nm. The specific surface area of Ni(OH)2, NiO (400
C), and NiO (600
C) is determined by BET method to be approximately 6.1, 42.0, and 7.2 m2 g1, respectively. Clearly, the heat-treatment temperature also impacts the specific surface area of resultant NiO significantly. A sevenfold increase in BET surface area after annealing at 400
C could be

5

attributed to the transformation of solid nanosheet structure to mesoporous nanosheet with interconnected nanoparticles deduced from TEM results. Much higher temperature (600
C) leads to the self-assembly of nanoparticles into large nanoparticles, decreasing the number of mesopores and the specific surface area. Thus, in this work, the annealing temperature of Ni(OH)2 microspheres was set at 400
C in terms of high electrical conductivity, less water content, and large surface area. The amount of sulfur in the sulfur-filled NiO could be evaluated by the TGA method as revealed in Fig. 5a. The NiO microspheres after annealing at 400
C in air were used the hosting material for sulfur. Evidently, about 66.7 wt% of sulfur could be filled in the hollow mesoporous NiO microspheres. To investigate the electrochemical characteristics of S and NiO (400
C)/S cathodes, cyclic voltammetry was implemented under a slow sweep rate of 0.1 mV s1 in the potential range of 1.8e2.8 V vs. Li/Liþ. Fig. 6 displays the cyclic voltammograms (CVs) of S and NiO (400
C)/S electrodes for the first three cycles. Obviously, the first cycle, particularly for the cathodic branch, is slightly different from other cycles for both electrodes. The cathodic peak potential at 2.2 V in the first lithiation cycle is shifted to 2.3 V in the subsequent cycles. This may result from the structural and textural changes of active sulfur material as a result of the continuous formation of lithium polysulfides with different chain lengths. After the first cycle, the S electrode (Fig. 6a) reveals two cathodic peaks at 2.3 and 2.0 V and two corresponding anodic peaks at 2.3 and 2.4 V in the subsequent scan. These redox peaks result from the multistep conversion of sulfur into Li2S and the reverse reaction from Li2S into sulfur [16]. Changes in the CV profiles are not significant in the cathodic and anodic peaks in the subsequent cycles, implying that the loss of active sulfur could be considerably mitigated by using the porous carbon paper. CVs of NiO (400
C)/S electrode shown in Fig. 6b are similar to those of S electrode, showing stable CV profiles after the first cycle. Except for the CV shape, the NiO (400
C)/S electrode shows higher integrated CV area than S electrode in each cycle, reflecting the higher lithium-storage capacity of NiO (400
C)/S by the hollow mesoporous structure of NIO that offers enough space to accommodate the lithium polysulfides. SEM micrographs of CP and NiO-coated CP (CP/NiO) are shown in the insets of Fig. 6. CP is composed carbon microfibers with an average

Fig. 5. (a) TGA curves of the as-prepared nickel hydroxide, sulfur, and sulfur-filled nickel oxide powder (NiO was obtained after annealing at 400
C). (b) Pore size distributions of the nickel hydroxide before and after annealing at 400 and 600
C obtained from the desorption branches of nitrogen adsorption/desorption isotherms.

6

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

Fig. 6. CVs of (a) the sulfur and (b) NiO (400
C)/S electrodes under a sweep rate of 0.1 mV s1. Insets show SEM micrographs of the carbon paper (a) without and (b) with NiO microspheres.

diameter of 10 mm and ultrathin carbon sheets. CP acts as a conducting substrate to distribute current to various parts of the electrode and a hosting matrix for polysulfides. The CVs of bare CP and CP/NiO (400
C) substrate electrodes are also shown Fig. 6. Clearly, the current density of substrate electrodes based on the loading mass of sulfur is very small when compared to that of sulfur-embedded electrodes, reflecting the major contribution of sulfur to the lithium storage in the composite sulfur cathodes. Fig. 7 displays the GCD (galvanostatic charge/discharge) curves of S and NiO (400
C)/S electrodes for the first three cycles at 0.2C (1C current ¼ 1675 mA g1). During discharging, sulfur (S8) is electrochemically reduced by lithium ions into various polysulfide intermediates, and sulfur ring could be opened and shortened until the ultimate Li2S is formed [3]. In the first discharging (lithiation) process, potentials in both electrodes are slightly different from the subsequent cycles due to the considerably structural and textural changes of sulfur materials in the first lithiation process. Two potential plateaus appear during the discharging process. The first plateau at 2.3 V comes from the electrochemical reduction of S8 to Li2S4 which further reduces to Li2S at the second plateau (2.1 V). The reverse reactions occur during charging (delithiation) and two plateaus are merged together owing to the coupled conversion of Li2S back into S8 [49]. S and NiO (400
C)/S exhibit a small loss of capacity in each sequential cycle. Obviously, the reversible capacity of NiO (400
C)/S electrode could reach as high as about 1300 mAh g1, far greater than that of S electrode (about 1000 mAh g1). In this work, the specific capacity of sulfur supported by porous carbon paper is much higher than that of sulfur materials coated on flat aluminum substrates [17,50]. An improvement in the reversible capacity of NiO (400
C)/S results from the redox conversion

between Li2S4 and Li2S (the second plateau) facilitated by the hollow mesoporous NiO structure. During discharging, sulfur cathode is electrochemically reduced stepwise by lithium ions to a series of soluble lithium polysulfides (especially long-chain lithium polysulfides) in an organic electrolyte. These polysulfide intermediates may participate in the polysulfide shuttle, resulting in a significant loss of active sulfur. The hollow mesoporous NiO structure acts as a hosting matrix for long-chain lithium polysulfides to alleviate the loss of active sulfur materials. In addition, hollow conductive NiO microspheres with high surface area can escalate the electrical conductivity of NiO (400
C)/S cathode, overcoming the conductivity limitation of pure sulfur. Previous reports have demonstrated that the transport pathways for electrons and lithium ions could be significantly shortened by reducing the particle size of sulfur, leading to a higher sulfur utilization [3]. In this work, the sulfur loss could be considerably suppressed by confining the sulfur in a large number of small spaces through the hollow NiO microspheres with mesoporous shells and the porous carbon paper. Fig. 8 illustrates the rate capabilities of S and NiO/S electrodes at various GCD current densities ranging from 0.2 to 5.0C. Clearly, the NiO (400
C)/S electrode shows a better rate capability than the S electrode, revealing 1300, 1138, 978, 723, and 257 mAh g1 at 0.2, 0.5, 1.0, 2.0, and 5.0C, respectively. The NiO (400
C)/S electrode also exhibits superior capacity recovery and coulombic efficiency. Once the current density is decreased back to 0.2C after 5.0C tests, the specific capacity of the NiO (400
C)/S electrode could be recovered to 1285 mAh g1, representing superior electrochemical and mechanical stability of the electrode structure. Besides, the S electrode shows the capacities of 1000, 847, 730, 423, and 115 mAh g1 at 0.2, 0.5, 1.0, 2.0, and 5.0C, respectively. Except for a higher initial capacity, the NiO (600
C)/S electrode has a similar capacity to the S electrode under various current densities. Obviously, the S electrode has lower capacities than the NiO/S electrode at each current density, resulting from its polysulfide shuttle, poor kinetics, and low conductivity. Whereas, it can be found that the NiO/S and S electrodes reveal a suitable cycling stability at a small current density of 0.2C owing to the porous carbon paper that help trap the lithium polysulfides. The S and NiO (600
C)/S electrodes display the worse rate capability, likely owing to poor conductivity of sulfur and NiO (600
C) compared with NiO (400
C). The interactions between lithium polysulfides and hosting materials conspicuously influence the coulombic efficiency and cycling stability of sulfurbased composite cathode [51]. The hosting material of metallic oxide has been shown to mitigate the dissolution of polysulfides by forming an appropriate interface with Li2S compared to that of carbon materials due to its binding preference for different lithium polysulfides [26]. Therefore, the superior rate capability and coulombic efficiency of NiO (400
C)/S could be ascribed to the network structure of carbon paper with attached hollow mesoporous NiO microspheres that enhances the electrical conductivity of composite electrode and facilitates surface-mediated attraction for trapping lithium polysulfides. The conductive carbon paper with porous structure is used to immobilize part of lithium polysulfides and distribute electrons along the carbon fiber framework. Meanwhile, the hollow NiO microspheres with mesoporous shells offer the conductive pathways to further increase the conductivity of sulfur and allow the rapid transport of electrons along the fibers of carbon paper and around the NiO shells [52]. Fig. 9 illustrates the GCD curves of S and NiO (400
C)/S electrodes using aluminum foil as substrate for 5 cycles at 0.2C (1C current ¼ 1675 mA g1). The S-coated Al electrode reveals a lower capacity and faster decay in capacity than the NiO/S-coated Al electrode, showing the significant contribution of hollow NiO microspheres to the electrochemical performance of sulfur cathode. As compared with Fig. 7, the lithium-storage capacity of sulfur

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

7

Fig. 7. GCD profiles of (a) the S and (b) NiO (400
C)/S electrodes for the first three cycles under a current density of 0.2C.

Fig. 8. Specific capacity and coulombic efficiency of the S and NiO/S electrodes obtained at various GCD current densities.

cathode can be largely improved by using the CP substrate. This reflects that the CP may contribute the most to the conductivity of bulk electrode and the mesoporous NiO microspheres trap the lithium polysulfides. Previous reports on carbon/hydroxide composites for LieS batteries have shown the effectiveness of nonconductive polar materials like Ni(OH)2 with the presence of other conductive agents [53e55]. Therefore, the difference between NiO and Ni(OH)2 in conductivity may be small for the battery

performance when there is a sufficient amount of additional conductive agents. In addition, longer cycling test is necessary to evaluate the stability of the composite sulfur electrodes in the future by optimizing the electrolyte/sulfur ratio, porosity, density, and sulfur content of the composite electrodes. Fig. 10 displays typical Nyquist plots of S and NiO (400
C)/S electrodes before cycling tests. The equivalent circuit adopted to fit the Nyquist plots and the obtained resistance values after fitting are

8

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

Fig. 9. GCD curves of S and NiO (400
C)/S electrodes using aluminum foil as substrate for the first five cycles at 0.2C (1C current ¼ 1675 mA g1).

Fig. 10. Measured and fitted EIS plots for the S and NiO (400
C)/S electrodes before cycling test. Inset displays an equivalent circuit for fitting the plots and the resistance terms obtained by fitting to the experimental data.

shown in the inset of Fig. 10. Rs is attributed to the series resistance of electrode and electrolyte, which could be obtained from the intercept of the curve on Z0 -axis in the high frequency end. Rs value of NiO/S electrode (6.0 U) is smaller than that of S electrode (9.1 U), revealing an improved conductivity of the NiO/S electrode since the same electrolyte was used for each coin cell. A semicircle in high frequency region results from the charge-transfer resistance (Rct) in parallel with an electric double-layer capacitor (CPEdl, constantphase element) at the electrolyte-electrode interface. A decrease in Rct value is obvious when employing the hollow mesoporous NiO microspheres. The NiO/S electrode shows a much lower Rct (about 62.6 U) than the S electrode (117.9 U) owing to the hollow

mesoporous NiO microspheres which provide high surface area for the adsorption of polysulfides and superior electrical conductivity for boosting the charge-transfer reaction. An inclined line in low frequency end comes from the finite-length Warburg impedance (Zw) by the ion diffusion within the electrode. It is noteworthy that the mesoporous NiO shell slightly impedes the diffusion of lithium ions and polysulfides, resulting an increased diffusion resistance (Rw) from 44.0 to 54.4 U. The superior specific capacity, coulombic efficiency, and rate performance of the NiO (400
C)/S electrode result from the hollow NiO structure with mesoporous shell that acts as an adsorption layer for spatial restriction of polysulfides and affords abundant transport pathways for electrons to facilitate the

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028

charge-transfer kinetics within the NiO (400
C)/S composite cathode. The sulfur nanomaterials could be engineered through nanoscale confinement in the mesoporous shell of NiO microspheres, which shorten the transport pathways for lithium ions and electrons in solid sulfur and consequently increase the electroactive surface area for lithium storage. 4. Conclusion Hollow mesoporous NiO microspheres with nanosheets were prepared by hydrothermal method in the presence of glycine followed by heating at different temperatures in air. The annealing temperature remarkably affected the chemical composition, pore size distribution, surface area, crystal structure, and conductivity of nickel hydroxides/oxides but did not change their surface morphologies significantly. TGA and XRD results showed that a drastic weight loss at 300
C was caused by the phase transition from hexagonal b-Ni(OH)2 to cubic NiO. Before annealing, the nanosheets revealed a solid interior, while the nanosheets are composed of loosely electrically connected nanoparticles leaving abundant mesopores in the nanosheets after annealing above the conversion temperature. An optimum annealing temperature for the formation of NiO was found to be 400
C in terms of less water content, high surface area, and appropriate electrical conductivity. The hollow NiO microspheres obtained after annealing at 400
C were filled with sulfur using melt-diffusion method. The CVs and GCD curves revealed that the first peak (plateau) at 2.3 V could be ascribed to the electrochemical formation of S8 to Li2S4 which could be further reduced to Li2S at the second peak (plateau) at 2.1 V in cathodic process. The reverse reactions occurred during the anodic process and two peaks (plateaus) were merged together, likely due to the coupled conversion of Li2S back into S8. The NiO (400
C)/S electrode exhibited better rate performance, capacity recovery performance, and coulombic efficiency than the S electrode. Most of the problems for the sulfur cathode arise from the polysulfide shuttle and poor conductivity. Therefore, the mesoporous NiO shell was important for physicochemical confinement of polysulfides, overcoming the insulating nature of sulfur materials. Carbon paper could act as scaffolds to electrically connect different hollow NiO microspheres for facilitating the charge-transfer reaction in different dimensions. Acknowledgments The authors acknowledge financial support from the Ministry of Science and Technology, Taiwan (Project No: 107-2221-E-992 -028 -MY3). References [1] X. Yang, X. Li, K. Adair, H. Zhang, X. Sun, Structural design of lithium-sulfur batteries: from fundamental research to practical application, Electrochem. Energy Rev. 1 (2018) 239. [2] A.N. Arias, A.Y. Tesio, V. Flexer, Review-Non-carbonaceous materials as cathodes for lithium-sulfur batteries, J. Electrochem. Soc. 165 (2018) A6119. [3] S. Evers, L.F. Nazar, New approaches for high energy density lithium-sulfur battery cathodes, Acc. Chem. Res. 46 (2013) 1135. [4] A. Fu, C. Wang, F. Pei, J. Cui, X. Fang, N. Zheng, Recent advances in hollow porous carbon materials for lithium-sulfur batteries, Small 15 (2019) 1804786. [5] D.-W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.-M. Cheng, I.R. Gentle, G.Q.M. Lu, Carbon-sulfur composites for Li-S batteries: status and prospects, J. Mater. Chem. A 1 (2013) 9382. [6] K. Zhang, Q. Zhao, Z. Tao, J. Chen, Composite of sulfur impregnated in porous hollow carbon spheres as the cathode of Li-S batteries with high performance, Nano Res 6 (2013) 38. [7] M. Helen, M.A. Reddy, T. Diemant, U. Golla-Schindler, R.J. Behm, U. Kaiser, M. Fichtner, Single step transformation of sulphur to Li2S2/Li2S in Li-S batteries, Sci. Rep. 5 (2015) 12146. [8] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur

9

cathode for lithium-sulphur batteries, Nat. Mater. 8 (2009) 500. [9] M. Wang, X. Xia, Y. Zhong, J. Wu, R. Xu, Z. Yao, D. Wang, W. Tang, X. Wang, J. Tu, Porous carbon hosts for lithium-sulfur batteries, Chem. Eur J. 25 (2019) 3710. [10] Y. Yan, M. Shi, Y. Wei, C. Zhao, M. Carnie, R. Yang, Y. Xu, Process optimization for producing hierarchical porous bamboo-derived carbon materials with ultrahigh specific surface area for lithium-sulfur batteries, J. Alloy. Comp. 738 (2018) 16. [11] J. Luo, Y. Dong, H. Xiaoqin, H. Liu, X. Rao, S. Zhong, R. Liu, Hierarchical N/P-cooped porous carbon as host materials for high-performance lithium sulfur battery, J. Electrochem. Soc. 166 (2019) A880. [12] J. Yu, X. Li, Y. Shu, L. Ma, X. Zhang, Y. Ding, Anchoring polysulfides in hierarchical porous carbon aerogel via electric-field-responsive switch for lithium sulfur battery, Electrochim. Acta 293 (2019) 458. [13] D. Yang, W. Ni, J. Cheng, Z. Wang, T. Wang, Q. Guan, Y. Zhang, H. Wu, X. Li, B. Wang, Flexible three-dimensional electrodes of hollow carbon bead strings as graded sulfur reservoirs and the synergistic mechanism for lithium-sulfur batteries, Appl. Surf. Sci. 413 (2017) 209. [14] S. Liu, X. Hong, D. Wang, Y. Li, J. Xu, C. Zheng, K. Xie, Hollow carbon spheres with nanoporous shells and tailored chemical interfaces as sulfur host for long cycle life of lithium sulfur batteries, Electrochim. Acta 279 (2018) 10. [15] Y. Zhang, X. Zong, L. Zhan, X. Yu, J. Gao, C. Xun, P. Li, Y. Wang, Double-shelled hollow carbon sphere with microporous outer shell towards high performance lithium-sulfur battery, Electrochim. Acta 284 (2018) 89. [16] J. Cai, Z. Zhang, S. Yang, Y. Min, G. Yang, K. Zhang, Self-conversion templated fabrication of sulfur encapsulated inside the N-doped hollow carbon sphere and 3D graphene frameworks for high-performance lithium-sulfur batteries, Electrochim. Acta 295 (2019) 900. [17] Y. Sun, Y. Zhao, Y. Cui, J. Zhang, G. Zhang, W. Luo, W. Zheng, A facile synthesis of mesoporous TiO2 sub-microsphere host for long life lithium-sulfur battery cathodes, Electrochim. Acta 239 (2017) 56. [18] J. Wang, C. Fu, X. Wang, Y. Yao, M. Sun, L. Wang, T. Liu, Three-dimensional hierarchical porous TiO2/graphene aerogels as promising anchoring materials for lithium-sulfur batteries, Electrochim. Acta 292 (2018) 568. [19] R. Zhuang, S. Yao, X. Shen, T. Li, Hydrothermal synthesis of mesoporous MoO2 nanospheres as sulfur matrix for lithium sulfur battery, J. Electroanal. Chem. 833 (2019) 441. [20] L. Hu, C. Dai, H. Liu, Y. Li, B. Shen, Y. Chen, S.-J. Bao, M. Xu, Double-shelled NiONiCo2O4 heterostructure@carbon hollow nanocages as an efficient sulfur host for advanced lithium-sulfur batteries, Adv. Energy Mater. 8 (2018) 1800709. [21] Y. Li, B. Shi, W. Liu, R. Guo, H. Pei, D. Ye, J. Xie, J. Kong, Hollow polypyrrole @ MnO2 spheres as nano-sulfur hosts for improved lithium-sulfur batteries, Electrochim. Acta 260 (2018) 912. [22] J. Zhang, Z. Li, Y. Chen, S. Gao, X.W. Lou, Nickel-iron layered double hydroxide hollow polyhedrons as a superior sulfur host for lithium-sulfur batteries, Angew. Chem. Int. Ed. 57 (2018) 10944. [23] J. Ni, L. Jin, M. Xue, J. Zheng, J.P. Zheng, C. Zhang, TiO2 microboxes as effective polysufide reservoirs for lithium sulfur batteries, Electrochim. Acta 296 (2019) 39. [24] M. Zhu, S. Li, J. Liu, B. Li, Promoting polysulfide conversion by V2O3 hollow sphere for enhanced lithium-sulfur battery, Appl. Surf. Sci. 473 (2019) 1002. [25] Y. Liu, W. Yan, X. An, X. Du, Z. Wang, H. Fan, S. Liu, X. Hao, G. Guan, A polypyrrole hollow nanosphere with ultra-thin wrinkled shell: synergistic trapping of sulfur in Lithium-Sulfur batteries with excellent elasticity and buffer capability, Electrochim. Acta 271 (2018) 67. [26] Q. Pang, D. Kundu, M. Cuisinier, L.F. Nazar, Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries, Nat. Commun. 5 (2014) 4759. [27] R. Fang, G. Li, S. Zhao, L. Yin, K. Du, P. Hou, S. Wang, H.-M. Cheng, C. Liu, F. Li, Single-wall carbon nanotube network enabled ultrahigh sulfur-content electrodes for high-performance lithium-sulfur batteries, Nano Energy 42 (2017) 205. [28] H.T. Lin, G. Yang, Y.-Y.T. Tsao, Y. Liu, C. Yu, Ionic liquid treated carbon nanotube sponge as high areal capacity cathode for lithium sulfur batteries, J. Appl. Electrochem. 48 (2018) 487. [29] Y. Zhang, P. Wang, H. Tan, X. Fan, K. Huang, Free-standing sulfur and graphitic porous carbon nanofibers composite cathode for high electrochemical performance of lithium-sulfur batteries, J. Electrochem. Soc. 165 (2018) A741. [30] Y. Liu, A.K. Haridas, Y. Lee, K.-K. Cho, J.-H. Ahn, Freestanding porous sulfurized polyacrylonitrile fiber as a cathode material for advanced lithium sulfur batteries, Appl. Surf. Sci. 472 (2019) 135. [31] M. Zhang, K. Amin, M. Cheng, H. Yuan, L. Mao, W. Yan, Z. Wei, A carbon foamsupported high sulfur loading composite as a self-supported cathode for flexible lithiumesulfur batteries, Nanoscale 10 (2018) 21790. [32] Q. Zhao, Q. Zhu, J. Miao, Z. Guan, H. Liu, R. Chen, Y. An, F. Wu, B. Xu, Threedimensional carbon current collector promises small sulfur molecule cathode with high areal loading for lithium-sulfur batteries, ACS Appl. Mater. Interfaces 10 (2018) 10882. [33] Q. Zhu, H. Deng, Q. Su, G. Du, Y. Yu, S. Ma, B. Xu, A free-standing nitrogendoped porous carbon foam electrode derived from melaleuca bark for lithium-sulfur batteries, Electrochim. Acta 293 (2019) 19. [34] C. Li, S. Dong, D. Guo, Z. Zhang, M. Wang, L. Yin, Ternary NiO/RGO-Sn hybrid flexible freestanding film as interlayer for lithium-sulfur batteries with improved performance, Electrochim. Acta 251 (2017) 43. [35] L. Lu, J.T.M. De Hosson, Y. Pei, Three-dimensional micron-porous graphene

10

[36]

[37]

[38]

[39] [40]

[41]

[42] [43]

[44]

[45]

J.-W. Guo, M.-S. Wu / Electrochimica Acta 327 (2019) 135028 foams for lightweight current collectors of lithium-sulfur batteries, Carbon 144 (2019) 713. J. Cheng, H. Song, Y. Pan, Y. Ou, Q. Liu, L. Liu, 3D copper foam/bamboo charcoal composites as high sulfur loading host for lithium-sulfur batteries, Ionics 24 (2018) 4093. N. Liu, L. Wang, Y. Zhao, T. Tan, Y. Zhang, Hierarchically porous TiO2 matrix encapsulated sulfur and polysulfides for high performance lithium/sulfur batteries, J. Alloy. Comp. 769 (2018) 678. R. Elazari, G. Salitra, A. Garsuch, A. Panchenko, D. Aurbach, Sulfur-impregnated activated carbon fiber cloth as a binder-free cathode for rechargeable Li-S batteries, Adv. Mater. 23 (2011) 5641. S.-H. Chung, A. Manthiram, A Li2S-TiS2-electrolyte composite for stable Li2Sbased lithium-sulfur batteries, Adv. Energy Mater. 9 (2019) 1901397. S.-H. Chung, K.-Y. Lai, A. Manthiram, A facile, low-cost hot-pressing process for fabricating lithium-sulfur cells with stable dynamic and static electrochemistry, Adv. Mater. 30 (2018) 1805571. S.-H. Chung, A. Manthiram, Low-cost, porous carbon current collector with high sulfur loading for lithium-sulfur batteries, Electrochem. Commun. 38 (2014) 91. Y. Wang, Q. Zhu, H. Zhang, Fabrication of b-Ni(OH)2 and NiO hollow spheres by a facile template-free process, Chem. Commun. (2005) 5231. M.-S. Wu, H.-H. Hsieh, Nickel oxide/hydroxide nanoplatelets synthesized by chemical precipitation for electrochemical capacitors, Electrochim, Acta 53 (2008) 3427. B.P. Payne, M.C. Biesinger, N.S. McIntyre, Use of oxygen/nickel ratios in the XPS characterisation of oxide phases on nickel metal and nickel alloy surfaces, J. Electron. Spectrosc. Relat. Phenom. 185 (2012) 159. X. Xu, L. Li, F. Yu, H. Peng, X. Fang, X. Wang, Mesoporous high surface area NiO synthesized with soft templates: remarkable for catalytic CH4 deep oxidation, Molecular Catalysis 441 (2017) 81.

[46] K.S. Kim, N. Winograd, X-ray photoelectron spectroscopic studies of nickeloxygen surfaces using oxygen and argon ion-bombardment, Surf. Sci. 43 (1974) 625. [47] C.H. Comniellis, I.A. Groza, Influence of heat treatment on the electrochemical behaviour of nickel, Thermochim. Acta 136 (1988) 19. [48] J.T.S. Irvine, D.C. Sinclair, A.R. West, Electroceramics: characterization by impedance spectroscopy, Adv. Mater. 2 (1990) 132. [49] X. Yang, N. Yan, W. Zhou, H. Zhang, X. Li, H. Zhang, Sulfur embedded in onedimensional French fries-like hierarchical porous carbon derived from a metal-organic framework for high performance lithium-sulfur batteries, J. Mater. Chem. A 3 (2015) 15314. [50] H. Wu, Y. Huan, D. Wang, M. Li, X. Cheng, Z. Bai, P. Wu, W. Peng, R. Zhang, Z. Ji, M. Zou, X. Yan, Hierarchical VS2 nano-flowers as sulfur host for lithium sulfur battery cathodes, J. Electrochem. Soc. 166 (2019) A188. [51] Y. Zhong, K.R. Yang, W. Liu, P. He, V. Batista, H. Wang, Mechanistic insights into surface chemical interactions between lithium polysulfides and transition metal oxides, J. Phys. Chem. C 121 (2017) 14222. [52] Z. Gao, Y. Zhang, N. Song, X. Li, Towards flexible lithium-sulfur battery from natural cotton textile, Electrochim. Acta 246 (2017) 507. [53] H.-J. Peng, Z.-W. Zhang, J.-Q. Huang, G. Zhang, J. Xie, W.-T. Xu, J.-L. Shi, X. Chen, X.-B. Cheng, Q. Zhang, A cooperative interface for highly efficient lithiumsulfur batteries, Adv. Mater. 28 (2016) 9551. [54] J. Zhang, H. Hu, Z. Li, X.W. Lou, Double-shelled nanocages with cobalt hydroxide inner shell and layered double hydroxides outer shell as highefficiency polysulfide mediator for lithium-sulfur batteries, Angew. Chem. Int. Ed. 55 (2016) 3982. [55] J. Jiang, J. Zhu, W. Ai, X. Wang, Y. Wang, C. Zou, W. Huang, T. Yu, Encapsulation of sulfur with thin-layered nickel-based hydroxides for long-cyclic lithiumesulfur cells, Nat. Commun. 6 (2015) 8622.