Co₃O₄ nanoneedle arrays as a multifunctional “super-reservoir” electrode for long cycle life Li–S batteries

Abstract

Lithium–sulfur (Li–S) batteries are highly attractive as energy storage devices due to their low cost and high energy density. The undesired capacity degradation caused by the polysulfide shuttle, however, has hindered their commercialization. Herein, a Co₃O₄ nanoneedle array on carbon cloth (CC@Co₃O₄) nanocomposite has been prepared and demonstrated for the first time as a multifunctional “super-reservoir” electrode to prolong the cycle life of Li–S batteries. Owing to the polar surface of the Co₃O₄ nanoneedle array, soluble lithium polysulfides (Li₂S_n, 4 < n < 8) can be effectively absorbed and then transformed to insoluble Li₂S₂/Li₂S which evenly covers the surface of the Co₃O₄ nanoneedle during the discharge process. Further, during the charge process, the Co₃O₄ nanoneedle can catalyze the electrochemical transformation of Li₂S₂/Li₂S into soluble polysulfides. A high initial capacity of 1231 mA h g⁻¹ at 0.5C and a slow capacity decay of 0.049%/cycle at 2.0C over 500 cycles were achieved; excellent rate performance was also obtained.

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1. Introduction

Lithium–sulfur (Li–S) batteries are emerging as one of the most promising energy storage devices due to their high specific capacity (1675 mA h g⁻¹) and theoretical energy density (2600 W h kg⁻¹).^1–3 However, sulfur cathodes still suffer from several inherent challenges that hinder their commercialization.^4–6 For example, Li–S batteries are known to suffer from low utilization of active materials and rapid capacity decay during the cycling process. The notable “shuttle effects” caused by the dissolution of lithium polysulfides in the organic electrolyte combined with the large volumetric expansion during cycling and the uncontrolled deposition of Li₂S₂/Li₂S are believed to be the main reasons for the low coulombic efficiency and rapid capacity fade.^7–9

Tremendous efforts have been devoted to circumventing the problem caused by soluble polysulphides. The most prevalent approach is to encapsulate sulfur in the pores of different host materials, including mesoporous carbon,^10–12 microporous carbon spheres,^13–15 carbon nanotubes (CNTs),^16–20 graphene oxides,^21–26 hollow porous carbon spheres^27,28 and MXene.²⁹ This strategy improves the kinetics/reversibility and alleviates the capacity fading; however, it still fails to adequately resolve the short cycle life problem.³⁰ The weak interaction between non-polar carbon materials and Li₂S_n (4 < n < 8) leads to loss of the active material from the cathode and finally reduces the cycle life of the battery.^31,32 Another promising approach is to bind the polysulfides with functional groups (such as oxygen-containing and/or nitrogen-containing groups) on the surface of the host materials.^21,33,34 Nevertheless, the electronic conductivity of these functionalised materials can be much lower than that of the corresponding carbon host, which is deleterious to battery performance. Moreover, the interaction between the functional groups and the polysulfides is not strong enough to completely entrap soluble polysulphides. The pioneering work by Cui and Nazar et al. on oxides for confining sulfur suggested that the efficiently reversible conversion of Li₂S₂/Li₂S into soluble polysulfides also has great significance for the electrochemical performance of Li–S batteries.^31,32,35 Therefore, multifunctional host materials that can promote the even deposition and decomposition of Li₂S₂/Li₂S are highly desired to develop long-life Li–S batteries.

Co₃O₄ is a promising transition metal oxide electrocatalyst in a number of reactions. For example, it was reported that Co₃O₄ nanocrystals on graphene displayed high catalytic activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).³⁶ Co₃O₄ nanocrystals with different shapes and crystal planes also showed catalytic properties for methane catalytic combustion.³⁷ Co₃O₄ used as a catalytic electrode for Li–S batteries has not been reported so far.

Herein, an electrode consisting of bottlebrush-like Co₃O₄ nanoneedle arrays on flexible carbon cloth fibers (CC@Co₃O₄) has been prepared and demonstrated for the first time as a multifunctional “super-reservoir” to prolong the cycle life of Li–S batteries. The strong affinity of Co₃O₄ for the soluble lithium polysulfides (Li₂S_n) can effectively accelerate the polysulfide conversion during the discharge process. The widely distributed active sites of the nanoneedle array afford the continuous deposition of Li₂S₂/Li₂S during the discharge process. The Li₂S₂/Li₂S can be readily transformed to the polysulfide species during the charge process due to the possible electrocatalytic activity of the Co₃O₄ nanoneedles. We show that, as a multifunctional “super reservoir” for Li–S batteries, this special design of CC@Co₃O₄ combining the polar surface, massive active sites and good conductivity exhibits high initial capacity, slow capacity decay and excellent rate performance.

2. Experimental

2.1 Material synthesis

Preparation of Co₃O₄ nanoneedle arrays on carbon cloth (CC@Co₃O₄). The CC@Co₃O₄ nanoneedle arrays were fabricated using a simple hydrothermal approach in combination with a heat treatment.^38,39 Typically, 5 mmol of CoCl₂·6H₂O, 7 mmol of NH₄F, and 9 mmol of urea were dissolved in 60 mL of deionized water under strong stirring to form a transparent pink solution. A piece of acid treated carbon cloth (2 × 4 cm²) was placed in the obtained solution. The mixture was transferred into a Teflon-lined stainless steel autoclave and kept for 12 h at 100 °C. After reaction, the substrate was thoroughly washed with deionized water and ethanol, and then dried in a vacuum oven at 60 °C for 12 h to give the Co-precursor nanoneedle array on carbon cloth (CC@Co-precursor). Then, the CC@Co-precursor was placed into a quartz tube and calcined at 350 °C for 2 h in air to obtain the CC@Co₃O₄ nanoneedle arrays. The mass loading of Co₃O₄ on CC is about 1.3 mg cm⁻². In addition, a control sample denoted as CC@Co₃O₄-1 in which the carbon cloth is not fully covered with Co₃O₄ nanowires was prepared by holding for only 6 h at 100 °C with the same reaction system as CC@Co₃O₄ except 6 mmol of CoCl₂·6H₂O was applied.

2.2 Material characterization

The morphologies were characterized by scanning electron microscopy (SEM, JEOL JSM-6380LV FE-SEM) and transmission electron microscopy (TEM, FEI, Tecnai-20). X-ray diffraction (XRD) measurements were carried out with a Bruker-AXS D8 DISCOVER diffractometer using Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI 550 spectrometer with Al Kα (1486.6 eV) as the X-ray source. The relative content of polysulfides was recorded using an ultraviolet-visible spectrophotometer (Cary 60 UV-Vis). Raman spectra were recorded on a HORIBA Scientific LabRAMHR Raman spectrometer system with a 532.4 nm laser. The N₂ adsorption–desorption isotherm was obtained by using a Micromeritics BK122T-B analyzer. The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) method. The pore size distribution was obtained from the Barrett–Joyner–Halenda (BJH) desorption branch of the isotherm.

2.3 Electrochemical measurements

The CC@Co₃O₄ nanoneedle arrays were directly used as the cathode, with Li foil as the anode in CR2016 coin cells, assembled in an argon-filled glovebox, in which both the moisture and oxygen contents were controlled to be less than 1 ppm. The CC@Co₃O₄ was cut into a piece of 0.5 × 0.5 cm² and was dried in a vacuum oven at 60 °C overnight prior to use. 20 μL of Li₂S₈ electrolyte was introduced into the coin cells, corresponding to about 4.1 mg cm⁻² of sulfur mass loading, which is comparable with that in the previous work (Table S1†). The coin cells were galvanostatically charged/discharged at different current densities between 1.7 and 3.0 V (vs. Li/Li⁺) by using a CT2001A cell test instrument (LAND Electronic Co.). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted with a CHI 600A electrochemical workstation at a scanning rate of 0.02 mV s⁻¹.

3. Results and discussion

The CC@Co₃O₄ nanoneedle arrays were prepared with a facile hydrothermal method, in combination with a thermal treatment, as shown in Fig. 1a. Owing to its excellent mechanical strength and high electrical conductivity, flexible carbon cloth was selected as a substrate. SEM and TEM were used to identify the morphology of the CC@Co₃O₄. As shown in Fig. 1b and c, the SEM images show that bottlebrush-like Co₃O₄ nanoneedles uniformly grow on the carbon fibers. The Co₃O₄ nanoneedle displays a length of about 3 μm. The structure of the Co-precursor nanoneedle arrays on carbon cloth (CC@Co-precursor) was perfectly retained after heat treatment (Fig. S1†). The bottlebrush-like structure of the Co₃O₄ nanoneedle array enlarges the specific area, exposes more active sites and allows fast diffusion of the electrolyte to the active surface of the electrode.⁴⁰ The TEM image elucidates that the Co₃O₄ nanoneedle consists of nanoparticles of about 30 nm (Fig. 1d). The CC@Co₃O₄ exhibits a BET surface area of 75.6 m² g⁻¹ and a pore volume of 0.26 cm³ g⁻¹ (Fig. S2†). The crystalline structure of the Co₃O₄ nanoneedle was characterized by XRD. Fig. 1e shows the XRD pattern of the CC@Co₃O₄, which contains eight diffraction peaks attributed to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of the cubic Co₃O₄ phase (JCPDS card no. 42-1467). The peaks at 26.21° and 43.61° are typical graphite (002) and (100) reflection ascribed to the carbon cloth.³⁸ The mechanism of the CC@Co₃O₄ as a multifunctional “super-reservoir” to prolong the cycle life of Li–S batteries is schematically demonstrated in Fig. 1f. In the discharge process, the active material Li₂S₈ in the form of catholyte can be absorbed on the surface of the Co₃O₄ nanoneedle arrays due to the strong affinity of Co₃O₄ for the soluble polysulfides. Then Li₂S₈ was effectively reduced to insoluble Li₂S₂/Li₂S and evenly distributed because of widely exposed active sites of the Co₃O₄ nanoneedle. The diameter of the Co₃O₄ nanoneedle increases due to the continuous accumulation of Li₂S₂/Li₂S. During the subsequent charge process, the deposited Li₂S₂/Li₂S could be readily transformed to the polysulfide species, which makes the diameter of the Co₃O₄ nanoneedle decrease and morphology of the Co₃O₄ nanoneedle recovers. Meanwhile, the resulting soluble polysulfides could be again absorbed on the surface of the Co₃O₄ nanoneedle, eventually realizing a highly reversible redox process and effectively prolonging the cycle life.

Fig. 1 (a) Schematic illustration of the preparation route of the CC@Co₃O₄. Physical characterization of the CC@Co₃O₄: (b and c) SEM images. (d) TEM images (inset: under higher magnification at 100 nm) and (e) XRD pattern. (f) The electrochemical process of the CC@Co₃O₄ during the cycling process.

The free-standing CC@Co₃O₄ “super reservoir” was directly used as an electrode in a coin cell with 0.2 M Li₂S₈ catholyte as the electrolyte. As shown in Fig. 2a, the galvanostatic charge/discharge profiles of the first cycle for the CC@Co₃O₄ at current rates of 0.2, 0.5 and 1C (1C = 1675 mA h g⁻¹) show well defined plateaus. The low polarization of the CC@Co₃O₄ electrode with increasing current rate and cycling number (Fig. S3†) indicates favourable kinetics and good electrolyte accessibility.^41,42Fig. 2b, c and e display the cycling performance at different rates. For comparison, the CC and CC@Co₃O₄-1 in which the carbon cloth is not fully covered with Co₃O₄ nanoneedles were also used as the reservoirs and evaluated under the same conditions. Despite the relatively high discharge capacities of 900 mA h g⁻¹ in the first two cycles at 0.5C, the capacities of the CC reservoir rapidly decay to less than 400 mA h g⁻¹ after 160 cycles (Fig. S4†). The severe capacity fading is attributed to the continued shuttle effect of polysulfides and the random and irreversible deposition of the insoluble lithium sulfides (Li₂S/Li₂S₂).⁷ The CC@Co₃O₄-1 reservoir shows enhanced performance due to the effect of the Co₃O₄ (Fig. 2b). By sharp contrast, the CC@Co₃O₄ “super reservoir” shows an initial capacity of 1231 mA h g⁻¹ at 0.5C and sustains about 987 mA h g⁻¹ even after 200 cycles with a capacity fade of only 0.14% per cycle. At a higher current density of 1C, the CC@Co₃O₄ “super reservoir” delivers an exceptional reversible capacity of nearly 700 mA h g⁻¹ after 280 cycles with a very small capacity degradation rate of 0.06% (Fig. 2c). Even at a current density as high as 2C (Fig. 2e), the CC@Co₃O₄ “super reservoir” still delivers an initial discharge capacity of 621 mA h g⁻¹, and stabilizes at 476 mA h g⁻¹ after 500 cycles. Rate performances of the CC@Co₃O₄ and CC@Co₃O₄-1 reservoirs are summarized in Fig. 2d. The CC@Co₃O₄ delivers a reversible discharge capacity of 1120, 1010, 830, and 610 mA h g⁻¹ at 0.2, 0.5, 1 and 2C, respectively. When the current density was switched abruptly from 2 to 0.2C, the original capacity is largely recovered. The rate performance of the CC@Co₃O₄ is much better than that of the CC@Co₃O₄-1. Furthermore, the electrochemical performance of the CC@Co₃O₄ achieved in this work is competitive compared to other excellent studies reported previously (Table S1†).^31,43,44 The excellent electrochemical performance can be ascribed to the advantages of the CC@Co₃O₄. The conductive bottlebrush-like structure of CC@Co₃O₄ is robust and flexible, which could accommodate the precipitation/decomposition of the active material during cycling as a reservoir. More importantly, the Co₃O₄ nanoneedle shows strong affinity to the polysulfides. As shown in the results of the adsorption experiment (Fig. S5†), the color of the Li₂S₈ solution treated with CC@Co₃O₄ fades more quickly than the one treated with CC@Co₃O₄-1, while the color of the Li₂S₈ treated with CC is nearly unchanged, which indicates that CC@Co₃O₄ is a super polysulfide reservoir. The strong affinity of Co₃O₄ nanoneedles to the polysulfides can effectively improve the reversibility of the redox process and promote the precipitation/decomposition of Li₂S₂/Li₂S; it can thus further enhance the stability and prolong the cycling life.

Fig. 2 Electrochemical performance of Li–S batteries using CC@Co₃O₄ and CC@Co₃O₄-1 as reservoirs. (a) Galvanostatic charge/discharge profiles of the first cycle for CC@Co₃O₄ at current rates of 0.2, 0.5, and 1C. (b, c, and e) Cycling performance and coulombic efficiency at 0.5, 1 and 2C. (d) Rate performance of CC@Co₃O₄ and CC@Co₃O₄-1.

From the viewpoint of electrochemical transitions, the charge/discharge profile of the CC@Co₃O₄ “super reservoir” can be described in terms of the following voltage states: fresh cathode, the partly discharged state of intermediate polysulfides Li₂S_n (n < 8) at 2.0 V, fully discharged state of Li₂S₂/Li₂S at 1.7 V, partly charged state of intermediate polysulfides Li₂S_n (n < 8) at 2.4 V and fully charged state of Li₂S₈ (even S₈) at 2.8 V (Fig. 3a). The electrochemical transition of the active material during the first cycle at 0.5C was characterized by ex situ Raman and UV-Vis absorption spectra. It is clear that all CC@Co₃O₄ electrodes in different voltages show obvious Raman peaks from Co₃O₄ (Fig. 3b). Compared with the fresh CC@Co₃O₄ electrode, the electrode in the partly discharged state of 2.0 V displays four new Raman peaks ascribed to Li₂S₈ (456 cm⁻¹), Li₂S₆ (510 cm⁻¹ and 398 cm⁻¹) and Li₂S₄ (203 cm⁻¹) respectively.⁴² In the fully discharged state of 1.7 V, the Raman spectrum shows strong signals from Li₂S₂ (450 cm⁻¹) and Li₂S (381 cm⁻¹). In addition, no signals from other sulfur species are discovered, which validates the fact that the polysulfides are completely transformed to the solid Li₂S₂ and Li₂S.⁴⁵ In the partly charged state of 2.4 V, the Raman peaks from Li₂S₂ and Li₂S become weaker than those at 1.7 V, and peaks from polysulfides appear again due to the decomposition of the solid Li₂S₂ and Li₂S. In the fully charged state of 2.8 V, only peaks from polysulfides can be found and no peaks from Li₂S₂ and Li₂S exist, implying thorough decomposition of Li₂S₂/Li₂S. It is notable that the intensity of the Co₃O₄ peaks undergoes a periodic change during the cycling process. With the proceeding of the discharge/charge process, the intensity of the Co₃O₄ peaks gradually becomes weak/strong. This could be ascribed to the improved reversible deposition/decomposition of the Li₂S₂/Li₂S on the Co₃O₄ nanoneedle. In the discharged state, the Co₃O₄ nanoneedle is evenly covered with the Li₂S₂/Li₂S induced by the strong affinity of Co₃O₄ for the soluble polysulfides; thus the intensity of Co₃O₄ peaks becomes weak. In the charged state, the solid Li₂S₂/Li₂S decomposes, and the Co₃O₄ nanoneedle exposes and hence the intensity of Co₃O₄ peaks becomes strong. The content of polysulfides in the extracts of CC@Co₃O₄ at different voltages was characterized by UV-Vis absorption spectra. The concentration of the polysulfides decreases/increases gradually during the discharge/charge process (Fig. S6†). It is the lowest in the fully discharged voltage, and nearly equals to that of the fresh electrode in the fully charged voltage. Digital photos of the CC@Co₃O₄ extracts show the same trend (Fig. S7†). The colour of the extracts fades/deepens gradually. The extract is nearly colourless in the fully discharged state. The Raman and UV-Vis results indicate a fact of highly reversible electrochemical transition of the active material.

Fig. 3 (a) The first galvanostatic charge/discharge profile of the CC@Co₃O₄ at 0.5C (brown, blue, green, red, and purple dots represent electrodes in fresh, partly discharged, fully discharged, partly charged and fully charged states, respectively). (b) Raman spectra of the washed CC@Co₃O₄ in different discharge/charge voltages. (c–j) SEM images of the CC@Co₃O₄ in different charge/discharge voltages ((c, d) 2.0 V; (e, f) 1.7 V; (g, h) 2.4 V; (i, j) 2.8 V).

Morphologies of the CC@Co₃O₄ “super reservoir” in different charge/discharge voltages are shown in Fig. 3c–j. Compared with the fresh CC@Co₃O₄ “super reservoir” (Fig. 1c and d), it can be observed that a few tiny particles of Li₂S₂/Li₂S deposit on the CC@Co₃O₄ nanoneedle at 2.0 V (Fig. 3c and d). It is notable that Co₃O₄ nanoneedles could afford the uniform deposition of Li₂S₂/Li₂S due to the excellent affinity to polysulfides and high surface area, which is similar to the uniform deposition of Li₂S₂/Li₂S on the conductive tin-doped indium oxide and Ti₄O₇ as reported in previous work.^31,32 As observed in Fig. 3e and f, deposited Li₂S₂/Li₂S forms a spider web-like structure in the fully discharged state of 1.7 V. The three-dimensional (3D) deposition of Li₂S₂/Li₂S could improve the electrochemical performance.⁴⁶ Meanwhile, the original bottlebrush-like morphology of CC@Co₃O₄ can be clearly identified. In the partly charged state of 2.4 V, it is interesting that the 3D spider web-like Li₂S₂/Li₂S disappears (Fig. 3g and h). In the fully charged state at 2.8 V, the Co₃O₄ nanoneedles recover the original morphology with a nearly smooth surface as shown in Fig. 3i and j. This indicates effective decomposition of Li₂S₂/Li₂S, which can be ascribed to the possible catalytic effect of Co₃O₄.

To further understand the role of the Co₃O₄ nanoneedle array, the morphologies of CC and CC@Co₃O₄-1 in different charged/discharged states at 0.5C were checked (Fig. S8a†). In the case of CC@Co₃O₄-1, in the fully discharged state of the first cycle the solid Li₂S₂/Li₂S mostly accumulates on the Co₃O₄ nanoneedle array, and not on the void space of the CC fibres. This indicates that Co₃O₄ is favourable for the preferential and selective deposition of the sulfur species (Fig. S8b and c†), which can be ascribed to the strong affinity of polar Co₃O₄ to the polysulfides and widely distributed active sites of the Co₃O₄ nanoneedle array.^31,32,47 In the case of CC, the Li₂S₂/Li₂S randomly deposits and accumulates on the electrode and is present even in the fully charged state as shown in Fig. S8d;† this again emphasizes the crucial role of the Co₃O₄ nanoneedle in accelerating the electrochemical process.

XPS was used to confirm the chemical interaction between the sulfur species and Co₃O₄. The S, C, O and Co signals are present in the XPS survey of the CC@Co₃O₄ in the fully discharged state in Fig. S9a.† Different sulfur bindings in the fully discharged state are observed in the S 2p XPS spectrum (Fig. S9b†). The peak at 170.1 eV is assigned to sulfate species arising from sulfur oxidation in air.³⁴ The peak at 169.4 eV corresponds to Co–S bonding, indicating the presence of chemical interaction between solid Li₂S₂/Li₂S and Co₃O₄.^48,49 Peaks at 168.7 and 167.3 eV correspond to S–O and S–S bonding. Peaks at 163.8 and 165.2 eV are assigned to S 2p1/2 and S 2p3/2, respectively.⁵⁰ In the Co 2p3/2 spectra, as shown in Fig. 4a, the peak at 783.0 eV in both the charged and discharged states can be ascribed to the shakeup satellite. Co 2p3/2 in the discharged state exhibits two major peaks at 781.7 and 780.1 eV corresponding to Co²⁺ and Co³⁺, in good accordance with the reported data for Co₃O₄.^51,52 However, in the fully charged state, the Co 2p3/2 peaks shift to a lower binding energy of 780.9 and 779.8 eV, respectively, which could be ascribed to the electron transfer from polysulfides to the Co atoms, indicating the strong chemical interaction between Co₃O₄ and the polysulfides.⁵³ The chemical interaction between Co₃O₄ and polysulfides could effectively avoid the diffusion of the resulting polysulfides into the electrolyte, therefore increasing the utility of the active material.

Fig. 4 (a) The Co 2p3/2 XPS spectra of the CC@Co₃O₄ in the fully charged and discharged states of the first cycle at 0.5C. (b) CV curves of CC and CC@Co₃O₄. (c) Peak voltages of CC and CC@Co₃O₄; (d) onset voltages of CC and CC@Co₃O₄.

CV curves were recorded at a scan rate of 0.02 mV s⁻¹ (Fig. 4b) in order to investigate the electrocatalytic activity of the Co₃O₄. The CV curve of the CC@Co₃O₄ “super reservoir” displays two cathodic peaks at 2.36 (peak 1) and 1.96 V (peak 2) and one anodic peak at 2.48 V (peak 3), consistent with the charge/discharge profiles. Compared with the CC, the CC@Co₃O₄ “super reservoir” indicates sharper redox peaks, suggesting better redox reversibility of polysulfides. In addition, the cathodic peak 2 for the CC@Co₃O₄ has a positive shift of 0.04 V (Table S2†), and the anodic peak 3 negatively shifts 0.05 V compared with those for the CC reservoir, indicating greatly suppressed electrochemical polarization of the CC@Co₃O₄ “super reservoir” (Fig. 4c). Moreover, in the case of the CC@Co₃O₄, the increased onset voltage of 0.02 and 0.04 V for the cathodic peaks 1 and 2 and the decreased onset voltage of 0.05 V for the anodic peak 3 (Fig. 4d and Table S3†) further verify the better kinetics of polysulfides. These accelerated kinetics of polysulfides could be ascribed to not only the strong adsorption of Co₃O₄ nanoneedle arrays to polysulfides, but also the catalytic effect of Co₃O₄ to promote the decomposition of Li₂S₂/Li₂S. Recently, Dong et al. also reported improved performance of sulfur cathodes due to the catalytic effect of Co.⁵⁴ The dual functions of Co₃O₄ nanoneedles to accelerate the redox process highly improve the stability and prolong the cycling life of Li–S batteries.

To get a more comprehensive understanding of the effects of CC@Co₃O₄ nanoneedle arrays on the stability of Li–S batteries, the electrodes after long-term cycling were further characterized. The EIS spectra of the CC and CC@Co₃O₄ “super reservoir” cycled for 300 cycles at 0.5C were recorded. Obviously, the CC electrode exhibits an extra-flattened semicircle, which could be due to the severe irreversible deposition of insoluble products on the surface of the CC electrode (Fig. S10 and Table S4†). By contrast, the CC@Co₃O₄ “super reservoir” shows lower charge-transfer resistance and Warburg diffusion resistance, indicating the more rapid electronic/ionic transport thanks to the efficiently reversible deposition/decomposition of the Li₂S₂/Li₂S. The R_ct is 336 Ω for the CC reservoir, while 116 Ω for the CC@Co₃O₄ “super reservoir”. The obviously decreased arc radius of impedance of the CC@Co₃O₄ “super reservoir” shows that the energy barrier for the electrode reactions is smaller than that of the CC reservoir, that is, the impedance value of the Faraday current is decreased and the reaction speed is increased. In order to evaluate the catalytic effect of Co₃O₄, R_ct values at different temperatures for CC and CC@Co₃O₄ as reservoirs were measured (Fig. S11†) and then the Arrhenius plots (ln(1/R_ct) ∼ 1/T) (Fig. S12†) were drawn based on the R_ct of CC (Table S5†) and CC@Co₃O₄ (Table S6†). The reaction activation energies for CC and CC@Co₃O₄ are respectively 226.5 and 87.5 kJ mol⁻¹ according to the Arrhenius plots (Fig. S12†). This indicates that the reaction activation energy for Li–S batteries is greatly reduced due to the catalytic effect of Co₃O₄. This is mainly due to the possible catalytic activity of Co₃O₄, which can decrease the energy barrier of the electrode reaction and accelerate the reaction of the electrode. Charge transfer at the CC@Co₃O₄ interface is much faster than that at the CC interface, indicating considerable enhancement of the redox kinetics of polysulfides, which could be ascribed to the possible catalytic effect of the CC@Co₃O₄ electrode.^41,55

SEM images of the CC@Co₃O₄ “super-reservoir” in fully discharged and charged states of the 100^th, 200^th, 300^th and 500^th cycles at 0.5C are shown in Fig. S13.† For all the CC@Co₃O₄ electrodes after cycling for different cycles, Li₂S₂/Li₂S uniformly deposits on the Co₃O₄ nanoneedle and retains a similar spider web-like structure to that of the first cycle in the fully discharged state. The original bottlebrush-like morphology of the CC@Co₃O₄ can be clearly identified. While in fully charged states, the spider web-like Li₂S₂/Li₂S disappears, and the Co₃O₄ nanoneedles just display a little rough surface but without obvious accumulation of Li₂S₂/Li₂S bulks even after 500 cycles. TEM was further employed to monitor the morphology of the CC@Co₃O₄ “super-reservoir”. As shown in Fig. 5a and b, there is a layer of Li₂S₂/Li₂S uniformly distributed on the Co₃O₄ nanoneedle in the fully discharged state of the 100^th cycle, and it nearly disappears in the fully charged state. With prolonged cycles, the morphology does not change significantly. Even at the 500^th cycle, the layer of Li₂S₂/Li₂S is uniform in the fully discharged state, and mostly disappears in the charged state. These SEM and TEM results further indicate effective decomposition of insoluble Li₂S₂/Li₂S ascribed to the catalytic effect of Co₃O₄ in the long cycling process. The conductive bottlebrush-like structure of the CC@Co₃O₄ is robust and flexible, which can not only accommodate the solubilization/precipitation of the active material, but also effectively improve the reversibility of the precipitation/decomposition process during cycling, and therefore enhance the utilization of the active materials and maintain the long cycling stability of the Li–S batteries.

Fig. 5 TEM images of the CC@Co₃O₄ at 0.5C (a, c, e, and g) in fully discharged states of the 100^th, 200^th, 300^th and 500^th cycles. (b, d, f, and h) in fully charged states of the 100^th, 200^th, 300^th and 500^th cycles.

4. Conclusions

In conclusion, the CC@Co₃O₄ nanoneedle array with a bottlebrush-like structure was employed as an efficient multifunctional “super-reservoir” to prolong the cycle life of Li–S batteries. The special 3D structure array provides uniform active sites for the continuous deposition of insoluble Li₂S₂/Li₂S during the discharge process. The inherent polar surface of Co₃O₄ nanoneedles has a high affinity to polysulfides, which is beneficial to the kinetics of the polysulfide conversion and improves the stability of the batteries. Moreover, the decomposition of insoluble Li₂S₂/Li₂S can be accelerated due to the possible electrocatalytic activity of the Co₃O₄ and therefore further prolong the cycling life of Li–S batteries. In addition, the carbon cloth as a backbone employed here can act as a conductive core to provide efficient transport of electrons for the Co₃O₄ nanoneedle array. The characterization for the CC@Co₃O₄ in different charge/discharge voltages convincingly indicates highly reversible precipitation/decomposition of the active material, validating the dual functions of Co₃O₄ nanoneedles to accelerate the redox process. This special “super-reservoir” electrode is robust and flexible, leading to greatly improved performance of Li–S batteries. A high initial capacity of 1231 mA h g⁻¹ at 0.5C and a slow capacity decay of 0.049% per cycle at 2.0C for 500 cycles were achieved. Excellent rate performance was also obtained. This work demonstrates that the concept of employing a catalytic host material will improve the electrochemical performance of high-energy Li–S batteries.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (No. 2014CB239701), National Natural Science Foundation of China (No. 51372116, 51672128), Natural Science Foundation of Jiangsu Province (BK20151468, BK2011030) and Fundamental Research Funds for the Central Universities of NUAA (NJ20160104). Z. Chang is grateful to the Foundation of Graduate Innovation Centre in NUAA (kfjj20150612).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta07202j

‡ Both authors contributed equally to this work.

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