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In situ transformation of LDH into hollow cobalt-embedded and N-doped carbonaceous microflowers as polysulfide mediator for lithium-sulfur batteries
Journal Pre-proofs In Situ Transformation of LDH into Hollow Cobalt-Embedded and N-doped Carbonaceous Microflowers as Polysulfide Mediator for Lithium-SulfurBatteries Shixia Chen, Xinxin Han, Junhui Luo, Jing Liao, Jun Wang, Qiang Deng, Zheling Zeng, Shuguang Deng PII: DOI: Reference:
S1385-8947(19)32870-0 https://doi.org/10.1016/j.cej.2019.123457 CEJ 123457
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
15 September 2019 23 October 2019 9 November 2019
Please cite this article as: S. Chen, X. Han, J. Luo, J. Liao, J. Wang, Q. Deng, Z. Zeng, S. Deng, In Situ Transformation of LDH into Hollow Cobalt-Embedded and N-doped Carbonaceous Microflowers as Polysulfide Mediator for Lithium-SulfurBatteries, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123457
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In Situ Transformation of LDH into Hollow Cobalt-Embedded and N-doped Carbonaceous Microflowers as Polysulfide Mediator for Lithium-Sulfur Batteries Shixia Chena†, Xinxin Hana†, Junhui Luo a, Jing Liao a, Jun Wanga*, Qiang Denga, Zheling Zenga, Shuguang Dengb*
a
Key Laboratory of Poyang Lake Environment and Resource Utilization, Ministry of Education,
School of Resources Environmental & Chemical Engineering, Nanchang University, Nanchang, 330031, China b
School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler
Mall, Tempe, AZ 85287, USA
*Corresponding author: 1. E-mail: [email protected] (S. Deng) 2. E-mail: [email protected] (J. Wang) †S.
Chen and X. Han are equally contributed to this work.
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Abstract: The shuttle effect of soluble lithium polysulfides (LiPSs), accompanying with sluggish redox kinetics has severely impeded the implementation of lithium-sulfur (Li-S) batteries. Herein, a novel hollow cobalt-embedded and nitrogen-doped carbonaceous microflower (H-Co-NCM) is fabricated via in situ transformation of metanilic anions intercalated Co-Al layered double hydroxides (CoAl LDHs). The as-obtained S@H-Co-NCM electrode exhibits superior electrocatalytic performances to boost the kinetics of LiPSs conversion and Li2S nucleation. Consequently, the assembled Li-S batteries with a high sulfur loading of 82% display a remarkable initial capacity of 1374 mAh g-1 at 0.1 C, excellent rate capability (611 mAh g-1 at 2 C), and superb cycle stability (cyclic decay rate of 0.069% over 500 cycles at 0.5 C). The integrated strategy of strong chemisorption and fast conversion of LiPSs provides deeper insights to suppress the shuttle effect.
Key Words：Layered double hydroxide (LDH); hollow, sulfur host; lithium-sulfur batteries
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1. Introduction Over the past few years, the dramatically expansive researches towards next-generation energy-storage devices have highlighted lithium sulfur (Li-S) batteries because of their superior theoretical specific capacity (1675 mAh g-1) and ultrahigh energy density (2600 Wh kg-1). Meanwhile, the appealing features of sulfur, such as cost-effective, environment benignity, and earth abundance, have also driven the practical application progress [1-5]. However, the implementation of Li−S batteries is still impeded by some intractable obstacles, such as the insulating nature of sulfur and lithium sulfides, significant volume changes (~80%) during cycling, and notorious shuttle effect caused by the dissolved intermediate lithium polysulfides (LiPSs) [6-8]. The shuttle effect greatly induces the loss of active sulfur, corrosion of lithium anode, rapid capacity fading, and low Coulombic efficiency [3,9,10]. To address the above issues, considerable efforts have been dedicated to designing novel sulfur host cathodes [11-14]. Porous and conductive carbon scaffolds are the most investigated sulfur hosts that could physically confine LiPSs within the nanopores. However, during long-term charge/discharge cycling, the polar LiPSs tend to diffuse outward due to their poor affinity with non-polar carbon hosts [15-17]. Therefore, multiple polar materials, e.g. metal oxides [18,19], metal sulfides [20,21], and N,O-heteroatoms [22,23], have been introduced into the nonpolar carbon substrates to strengthen the carbon-LiPSs attraction forces. However, the finite attractive sites and limited inner space of host materials are not able to fully realize the theoretical performances. Notably, three-quarters of the theoretical capacity of Li-S batteries are released from the sluggish liquid-solid conversion of soluble Li2S4 to solid Li2S [24-26]. The poor kinetics of LiPSs reduction and transformation lead to the accumulation of soluble LiPSs surrounding the cathode and eventually 3
causes the arbitrary precipitation of solid Li2S2/Li2S on the cathode/anode surface [24,27]. Thus, the electrocatalytic regulation of LiPSs is expected to be highly efficient to mitigate the shuttle effect, especially in high sulfur loading and long cycling conditions [28,29]. Recently, in-situ electrocatalysts, including Ni [30], Co [31], and their metal sulfides/nitrides [27,32], have been proposed as an integrated kinetics-promotion strategy of enhancing the transformation of LiPSs to Li2S2/Li2S and anchoring LiPSs. Particularly, Co and N codoped carbon (Co-N-C) materials that serve as effective electrocatalysts [33,34] have evoked a tremendous attention in the application of Li-S batteries. For example, Dong et al. [35] revealed that Co could facilitate the transformation of LiPSs to Li2S2/Li2S and the N heteroatoms can accelerate the oxidation of LiPSs to S8. Wu et al.[36] demonstrated that the encapsulated Co nanoparticles as well as the N heteroatoms in the graphitic carbon frameworks could synergistically reserve the soluble LiPSs and facilitate the redox reaction kinetics. However, the Co-N-C catalysts are mostly derived from metal-organic frameworks (MOFs) with relatively complex and time-consuming preparation processes [33,35-38], thus it is highly desired to develop novel synthesis method from simple templates. Moreover, efficient void space to load sulfur as well as accommodate the volume expansion is also an important criterion to design sulfur cathode. Ding et al. reported a yolk-shell structure with conductive carbon shells that can provide comfortable accommodation of the volume fluctuation, achieve a high sulfur loading of 74.8 wt% and deliver a high initial capacity of 1400 mAh g-1 [39]. Thus, we proposed a facile LDHs template to construct micro-size hollow carbonaceous flower structures with in-situ electrocatalytic activity as sulfur host. Herein, we report a novel and low-cost route for the fabrication of H-Co-NCM sulfur host as Li-S battery cathode by facile carbonization of the metanilic acid intercalated cobalt-aluminum layered 4
double hydroxides (CoAl-M-LDHs) without additional carbon sources and conductive additives. The metanilic acid molecules insert into the hydroxide layers simultaneously in situ guide the self-assembly of micoflower-like carbon framework and the functionalities incorporation. In the subsequent acid etch process, excessive Co and Al-based nanoparticles are removed, but strongly coupled Co functionalities with N heteroatoms are preserved, thus crafting a Co- and N-codoped hollow porous microflower structure. The heteroatoms doping of Co/N further enhances the conductivity of carbon matrix and acts as the synergetic adsorption and catalysis sites for LiPSs capture and conversion to improve the electrochemistry performance of Li-S batteries. Moreover, the hierarchical pore systems enable the H-Co-NCM microflower structure with a high specific BET surface area of 571.4 m2 g-1. As a result, the as-obtained H-Co-NCM could contain 82 wt% S and deliver an excellent initial discharge capacity of 1374 mAh g-1 at 0.1 C. Furthermore, the interfacial affinity and kinetically conversion of H-Co-NCM towards LiPSs are clarified by the visualized adsorption experiment, Li2S6 symmetric tests, Tafel, and Li2S nucleation measurements. Consequently, the assembled Li-S battery exhibited a superior rate capacity of 611 mAh g-1 at 2 C and a remarkable lifespan of 500 cycles with a decay rate of 0.069% per cycle at 0.5 C.
2. Experimental section 2.1 Synthesis of CoAl-M-LDH precursor The metanilic acid intercalated CoAl-LDH precursor was prepared by a minor modification of previous literature [40]. Typically, 2 mmol aluminum nitrate nonahydrate (Al(NO3)3·9H2O), 6 mmol cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 8 mmol ammonium fluoride (NH4F), and 10 mmol hexamethylene tetramine (HMT) were dissolved in 20 mL degassed deionized (DI) water, denoted as solution A . Then, 20 mmol metanilic acid was dissolved in 20 mL 1 M NaOH solution, named as 5
solution B. Afterward, solution B was added dropwise into solution A with vigorous stirring. The resultant mixed solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 100 °C for 12 h. After cooling to room temperature, the obtained products were rinsed with excessive DI water and ethanol, and dried at 100 °C for 12 h, denoted as CoAl-M-LDH. The DI water used in the entire synthesis experiment was degassed by high-purity N2 to avoid the competitive intercalation of carbonate anions into CoAl-LDHs. 2.2 Synthesis of hollow cobalt-embedded and nitrogen-doped carbonaceous microflower (H-Co-NCM) The as-prepared CoAl-M-LDHs were calcined at 700 °C for 2 h with a ramp of 3 °C min−1 under N2 flow. After cooling, the obtained c-LDH was etching with 18 wt% HCl solution for 24 h and then rinsed with enough DI water and ethanol. The obtained samples were collected through the filter and followed by drying at 60 °C overnight and denoted as H-Co-NCM. 2.3 Sulfur impregnation in H-Co-NCM materials Typically, 0.2 g H-Co-NCM and 1.0 g sublimed sulfur were dispersed in 20 mL N, N-dimethylformamide and 50 mL of carbon disulfide (CS2), respectively by ultra-sonication. The two solutions were subsequently fully mixed and kept at 50 °C under stirring for 12 h to completely evaporate CS2 [41]. The mixture was then loaded to a sealed glass vial and maintained at 200 °C for 30 min in an N2 atmosphere. Acetylene black and S composites (S@CB) were prepared using the same method. 2.4 Material characterization The morphology was conducted by field-emission scanning electron microscope (FESEM, JSM-6701F, and XL30 Environmental FEG coupled with an energy dispersive X-ray spectroscopy 6
(EDX)) and transmission electron microscope (TEM, JEM-2100, Japan and ARM200F, Japan). The N2 adsorption-desorption isotherms were measured by the Brunauer-Emmett-Teller (BET, ASAP 2460, USA) at 77 K. The BET surface area and total pore volume were calculated from the BET equation and the pore size distribution (PSD) was derived from the non-local density functional theory (NL-DFT) method. The sulfur content of the cathode was estimated from 30 to 500 ℃ under N2 atmosphere through thermogravimetric analysis (TGA; STA2500, NETZSCH, German). The crystal structures of the materials were recorded on X-ray powder diffraction (XRD, PANalytical empyrean series2, Netherlands) with Cu-Kα radiation. The C, H, N, S and O contents of the samples were measured by an elemental analyzer (Elementar Vario Micro, German). Infrared spectra were collected by a Nicolet 5700 Fourier transformed infrared (FT-IR) spectrometer, using the KBr pellet method and X-ray photoelectron spectroscopy (XPS) measurements were obtained by an Escalab 250Xi spectrometer. Raman spectra were collected using a LabRAM HR Evolution Raman microscope with 532 nm laser source. The electronic conductivity was conducted by the ST-2258C Four-Point Probe Resistance Tester. 2.5 Electrochemical measurements The working electrodes consisted of the active material, acetylene black, and polyvinylidene difluoride were prepared with a weight ratio of 7:2:1 in N-methyl-pyrrolidone (NMP) under vigorous stirring for 8 h. The uniform slurry was cast onto a carbon paper current collector and vacuum dried at 60 °C for 12 h and punched into disks with a diameter of 12 mm. The batteries were assembled using CR2032 coin cells in an Ar-filled dry glove box (<0.1 ppm H2O/O2) with Celgard 2500 separators. The sulfur content in the working electrode was about 2 mg cm-2 and 40 μL of the electrolyte containing 1 M bis(trifluoromethane)sulfonimide lithium (LiTFSI) in a mixed solvent of 7
1,3-dioxolane (DOL) and 1,2-dimethoxy-ethane (DME) (v/v =1:1), with 1 wt% LiNO3, was used for each coin cell. Electrochemical measurements were performed on a Neware battery analyzer with a voltage window of 1.7-2.8 V vs. Li+/Li at room temperature. Cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and Tafel polarization were measured on a CHI 660E electrochemical workstation (CH Instrument, China). All specific capacity values were obtained based on the mass of elemental sulfur. The experiment details of visualized adsorption experiment, Li2S6 symmetric cells, Li2S nucleation and, Tafel measurements were provided in supporting information.
3. Results and discussion 3.1 Materials synthesis and characterization The fabrication procedure of microflower-like carbon frameworks (H-Co-NCM) and the responding morphological evolution is illustrated in Scheme 1. The H-Co-NCM material was fabricated through a hydrothermal method and followed by annealing at 700°C to obtain amorphous N-doped carbon skeletons, denoted as c-LDH. After acid etching treatment, the H-Co-NCMs with uniform particle size and ordered morphology were successfully prepared. Finally, sulfur was encapsulated into the H-Co-NCM by a CS2 solvent method to yield S@H-Co-NCM composite.
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Scheme 1. Schematic illustration of the synthesis process of S@H-Co-NCM electrode. The morphologies of CoAl-M-LDH, c-LDH, and H-Co-NCM were demonstrated by SEM and TEM images. As shown in Fig. 1a, the CoAl-M-LDH presents as a uniformly distributed 3D flower-like morphology assembled with an average diameter of ~2 μm. Closer examination indicates that each assembly consists of multiple curved nanosheets with a smooth surface and the thickness is about 30 nm (Fig. 1d and 1g). These primary nanosheets are oriented and interconnected at different angles that act as efficient secondary packing blocks, which could stabilize the structural integrity during the sulfur impregnation and cycling processes [42]. Upon calcination, metanilic acid molecules are in situ converted to N-doped carbon frameworks, meanwhile, the LDH layers are converted to uniform Co- and Al-based nanoparticles (1-20 nm) that are uniformly dispersed on the petals of microflower (Fig. 1e, 1h and S1). Impressively, the microflower morphology of H-Co-NCM could be well retained after HCl leaching (Fig. 1c). The nanoparticles on the microflower petals are etched away and result in numerous dents (Fig. 1f and 1i) that can provide abundant channels for the fast electrolyte infiltration. Moreover, the hollow structure nature of H-Co-NCM can be demonstrated by Fig. 1i and further confirmed by the damaged structure (Fig. S2). The unique features of hollow micro-/nanostructures with hierarchical pores can not only achieve high loading of sulfur but also physically block the LiPSs diffusion pathways [43]. The selected area 9
electron diffraction (SAED) pattern of H-Co-NCM confirms the embedded Co in the H-Co-NCM (Fig. S3). EDX mapping of H-Co-NCM also reveals the carbonaceous nature of the obtained microflower and partial Co nanoparticles were preserved after acidic etching (Fig. S4). Moreover, the content of Co is measured to be 3.62 % by the ICP, while the contents of C and N are 43.36% and 5.32%, respectively, quantified by the ultimate element analysis (Table S1).
Fig. 1 SEM images of (a)(d) CoAl-M-LDH, (b)(e) c-LDH, and (c)(f) H-Co-NCM; TEM images of (g) CoAl-M-LDH, (h) c-LDH and (i) H-Co-NCM with the corresponding enlarged images. The structure evolution process from the CoAl-M-LDH to H-Co-NCM is further verified by the XRD analysis. As illustrated in Fig. 2a, the precursor CoAl-M-LDH could be indexed to a typical LDH phase with an (003) interlayer distance of 1.55 nm following the metanilic anions-intercalating LDH [40]. After calcination, the crystalline phases of Co and Co9S8 nanoparticles on c-LDH could 10
be observed. For H-Co-NCM materials, the diffraction peaks of (111) peak at 44° and (003) peak at a 25° correspond to the crystalline Co and amorphous carbon, respectively [36]. The preservation of Co particles could be ascribed to the strong chemical bonds with N functionalities. The Raman and FT-IR spectra further substantiated to the successful synthesis of H-Co-NCM (Fig. S5 and S6). Moreover, the electronic conductivity of H-Co-NCM is estimated to be 716 S m-1 which is higher than that of CB (578 S m-1) using the four-probe method, implying the highly conductive nature of the obtained carbon skeletons. Due to the unique structure and acid etching, the H-Co-NCM showed much larger BET surface area and pore volume (571.4 m2 g-1 and 0.535 cm3 g-1) than those of pristine CoAl-M-LDH (6.4 m2 g-1 and 0.014 cm3 g-1) and c-LDH (151.3 m2 g-1 and 0.163 cm3 g-1) (Fig. 2b). The N2 adsorption/desorption isotherm of H-Co-NCM at 77 K exhibited typical features of type IV isotherm with a hysteresis loop, indicating the hierarchical pore systems with abundant micropores, mesopores, and macropores. The pore size distribution plot depicted that micropores and mesopores with a pore size of 1 to 20 nm are dominating and correspond to the sizes of Co and Co9S8 nanoparticles (Fig. 2c). Such hierarchical pore systems provide sufficient internal space for efficient sulfur loading, physical entrapment of LiPSs, and volume expansion during charge and discharge [40,44]. Owing to these merits, the prepared H-Co-NCM could contain a high sulfur mass loading of 82% as determined by the TGA (Fig. 2d). Upon sulfur infiltration, the specific BET surface area of S@H-Co-NCM drastically decreased to 12.3 m2 g-1, suggesting that the sulfur has been successfully accommodated into the hierarchical pores (Fig. S7).
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Fig. 2 (a) XRD patterns, (b) N2 adsorption-desorption isotherms, (c) PSD and pore volume of CoAl-M-LDH, c-LDH, and H-Co-NCM, (d) TGA curves of S@H-Co-NCM. After sulfur loading, the morphology could be well maintained and no surface sulfur particles are observed, demonstrating the successful encapsulation of sulfur into the H-Co-NCM frameworks (Fig. 3a). Furthermore, the TEM image (Fig. 3b) clearly showed that a clear contrast of the dark inner spaces of S@H-Co-NCM with the transparent hollow structure of H-Co-NCM, revealing the successful accommodation of sulfur into the hollow host. The presence of Co is further confirmed by the high-resolution TEM (HRTEM) image in Fig. 3c, which shows the lattice fringe spacing of 0.204 nm, corresponding to the (111) crystal planes of Co. The uniformly dispersed Co nanoparticles with 12
an average size of ~5 nm construct the uniformly distributed sulfiphilic active sites and regulate the rapid nucleation and growth of Li2S [45]. EDS element mapping of the S@H-Co-NCM composite showed the uniform distribution of N and Co throughout the H-Co-NCM framework as well as the encapsulation of sulfur within the cathode materials (Fig. 3d). The impregnated sulfur in the S@H-Co-NCM composite was α-S8 (PDF 78-1889), as revealed by the XRD pattern (Fig. S8).
Fig. 3 Morphology of S@H-Co-NCM composite. (a) FESEM image, (b) TEM image, (c) HRTEM image and (d) EDS elemental mappings of the S@H-Co-NCM composite. 3.2 Electrochemical performance To demonstrate the superiority of H-Co-NCM for promoting electrochemical kinetics in a working Li−S battery, the influence of H-Co-NCM on the electrochemical reactions was firstly investigated by the CV and EIS profiles and the S@CB electrode was adopted as a control. As shown in Fig. 4a, the CV curves of S@H-Co-NCM and S@CB composites display two obvious pairs of redox peaks, indicating a reversible reduction/oxidation process. The peak currents of two cathodic and anodic processes are denoted as IC1, IC2 and IA1, IA2, respectively, corresponding to the two-step reduction of 13
sulfur to Li2S2/Li2S and reversible oxidation of Li2S2/Li2S to S8 [25]. Compared to the two broad cathodic peaks of S@CB cathode, S@H-Co-NCM exhibits much sharper peaks with an obvious positive shift. Whereas the anodic peaks of both cathodes appear at the almost same position, these observations strongly imply that the doped Co- and N-functionalities could significantly decrease the polarization and facilitate the polysulfide redox kinetics [36,46]. This catalytic effect is further demonstrated by the EIS tests (Fig. 4b), the charge-transfer resistance (Rct) represents a parameter closely related to the chemical reaction activation energy, the Rct of S@H-Co-NCM (62.11 Ω) is much lower than that of S@CB electrode (80.09 Ω), suggesting faster electrochemical kinetics with Co- and N- functionalities [25,47]. Moreover, the less difference of the median voltage difference on the galvanostatic charge/discharge profile at 0.1C of S@H-Co-NCM (152 mV) than S@CB cathodes (156 mV) can further verify the less polarization of S@H-Co-NCM (Fig. 4c). However, when the current rate was raised to 1 C, the S@H-Co-NCM still displayed two obvious discharge plateaus, while the S@CB suffered severe polarization that the two-step discharge processes were blurred. Benefiting from the improved polysulfide conversion kinetics, the S@H-Co-NCM cathode exhibits the superior rate capability under gradually increased current densities, as shown in Fig. 4d. With the stepwise increasing current rates from 0.1, 0.2, 0.5, 1 to 2C, the S@H-Co-NCM cathode delivers the corresponding discharge capacity of 1374, 957, 841,738 and 611 mAh g-1, respectively. After switching current density back to 1.0, 0.5, 0.2, and 0.1 C in turns, a reversible discharge capacity of 768, 797, 813, and 829 mAh g-1 could be recovered, indicating the outstanding structure stability of S@H-Co-NCM composite [36,48]. In contrast, the S@CB cathode displays a rapid capacity decay with increasing rate current, namely, an inferior discharge capacity of 168 mAh g-1 was observed at 2.0 C. The outstanding kinetic promotion of S@H-Co-NCM towards LiPSs redox chemistry was 14
further confirmed through the long-term cycling performance that was evaluated at 0.5 C (Fig. 4e). After 500 cycles, a low capacity fading rate of 0.069% per cycle with high coulombic efficiency near 100% was obtained, indicating the shuttle effect of LiPSs has been efficiently restrained [49]. On the contrary, without Co and N doping, S@CB suffered rapid capacity decay (a capacity fading rate of 0.113% per cycle over 300 cycles) and unsatisfactory coulombic efficiency (98% at 300th cycle). The long-term cycling experiments performed of S@H-Co-NCM at 1C and 2C could also obtain a low capacity fading rate of 0.078% and 0.076% with a stable coulombic efficiency near 100% after 200 cycles (Fig. S9). Notably, the electrochemistry performance is highly competitive with the state of the art sulfur cathodes (Table S2). Moreover, 48 light-emitting diodes (LED) could be lighted with one assembled Li-S cell for 3 hours (Fig. S10). The exhibited excellent overall performance could be attributed to the strong chemisorption and fast electrocatalytic effects provided by the Co and N functionalities.
Fig. 4 (a) CV curves and (b) Nyquist plots of S@CB and S@H-Co-NCM cells; (c) The discharge-charge profiles of S@CB and S@H-Co-NCM cells at 0.1 and 1C; (d) Rate performance of 15
S@CB and S@H-Co-NCM cells ranging from 0.1 to 2C; (e) The cycling performance of S@H-Co-NCM and S@CB cells at 0.5C . The strong chemical LiPSs adsorption ability is the prerequisite for the rapid redox reactions on the H-Co-NCM mediator. Thus, the chemisorption of H-Co-NCM toward LiPSs was initially examined by the visual adsorption experiment by immersing H-Co-NCM into a 0.05 M Li2S4 solution in 1,3-dioxolane (DOL)/dimethoxyethane (DME). As depicted in Fig. 5a, a brownish Li2S4 solution was entirely decolored by H-Co-NCM after static adsorption for 4 h, while the Li2S4 solution with CB still displayed brownish color. Furthermore, the colorless separators of S@Co/N-C cells disassembled after cycling also indicate the strong capture ability of Co/N-C with LiPSs (Fig. S11). To further reveal the mechanism behind the strong interaction forces of H-Co-NCM towards LiPSs, the surface chemistry of H-Co-NCM before and after the adsorption experiment was examined by XPS measurement. The N 1s spectrum in Fig. 5b can be deconvoluted into four peaks: pyridinic N (397.8 eV, 11.6%), Co-N (398.5 eV, 10.1%), pyrrolic N (400.3 eV, 68.4%), and graphitic N (401.3 eV, 9.9% ) [47,50,51]. The dominance of pyrrolic N and pyridinic plays a key role in enabling strong Li-N bonds to trap LiPSs, while graphitic N could improve the conductivity of host materials [47,52]. Upon adsorption of Li2S4, the decreased intensities of pyridinic N (397.8 eV, 7.7%), Co-N (398.5 eV, 7.8%) and pyrrolic N (400.3 eV, 63.9%) imply the strong adsorption ability towards LiPSs originating from these doped nitrogen groups [52]. The existence of Co-N bond demonstrates the embedded Co functionalities are strongly coupled with N, which could effectively modify the electronic structure and distribution of the carbon matrix [47]. The high-resolution Co 2p XPS spectrum could be split into two characteristic resonances of 2p3/2 and 2p1/2 with the satellite peaks (785.4 and 801.8 eV), further suggesting the existence of Co nanoparticle (Fig. 5c). In comparison to 16
the pristine H-Co-NCM, four characteristic peaks of Co 2p3/2 and 2p1/2 all upshifted to higher binding energies, suggesting the enhanced interaction of exposed Co nanoparticles with surrounding strong electronegative sulfur ligand [45]. Obviously, the Co 2p3/2 spectrum of H-Co-NCM-Li2S4 has an additional peak at 778.9 eV, indicating the existence of a Co-S bond, further evidencing the rich adsorption sites for LiPSs [32,53]. The above evidence firmly confirms the existence of Co- and N-species that could potentially be applied as an efficient catalyst for LiPSs conversions [24,31]. Moreover, the appearance of the polythionate complex in the S 2p spectrum unveiled the H-Co-NCM could actively promote polysulfides conversion other than only passively adsorb LiPSs (Fig. S12). To gain deeper insights into the electrocatalytic effect of Co- and N- functionalities on the redox kinetics of polysulfide, symmetrical cells with two identical H-Co-NCM electrodes in Li2S6-containing electrolytes were assembled. For comparison, one symmetrical cell based on CB was also assembled. As shown in Fig. 5d, the Li2S6-free cell reveals little contribution to the capacitive current, while the current density of the H-Co-NCM electrode in the Li2S6 electrolyte is much larger than that of CB, indicating significantly improved redox kinetics than that of CB in the soluble phases. Fig. S13 shows the CV curves of symmetrical H-Co-NCM and CB cells at different scan rates that all redox curves of Co/N-C reveal a good shape and retain good reversibility, even at a scan rate as high as 100 mV s-1, demonstrating the expeditious kinetics feature of a series of polysulfide conversions facilitated by the Co/N-C. Notably, the high superimposition of the first eight CV cycles of the H-Co-NCM electrode suggests the excellent stability of H-Co-NCM electrode (Fig. 5e). Recently, Co single atom has been reported as an efficient catalyst in LiPSs conversion [24,54], in order to unveil the role of Co single atom, the H-Co-NCM was further etched by concentrated hydrochloric acid to remove metallic Co nanoparticles. As shown in Fig.S14a, no 17
obvious Co diffraction peaks were observed after HCl treatment in the XRD pattern, suggesting that most of the metallic Co nanoparticles have been etched. Whereas, the current density of the H-Co-NCM after HCl treatment with Li2S6 electrode significantly decreased (Fig.S14b), manifesting the importance of both metallic Co nanoparticles and Co single atom in mediating LiPSs conversion, which is in accordance with the previous literature [35,36]. The superior catalytic activity afforded by the H-Co-NCM with regards to LiPSs conversion could be further revealed by its much lower Tafel slope (53 mV dec-1) and obviously higher exchange current density (0.225 mA cm-2), comparing with the corresponding 934 mV dec-1 and 0.019 mA cm-2 on CB as obtained from the Tafel plots (Fig. 5f) [55,56]. To demonstrate the conversion kinetics of soluble LiPSs into solid Li2S on the electrode/electrolyte interfaces, the Li2S precipitation experiment was monitored. According to the following Avrami formula (Eq.(1)) [57], a larger fraction (η) represents a faster kinetic nucleation and growth rate of Li2S. η =1−exp(−Btn)
(1)
where η is the transformed fraction of Li2S, B is the kinetic constant, and n is the Avrami exponent. As shown in Fig. 5g and 5h, the potentiostatic discharge curves undergo three stages i.e. the reduction of Li2S8, reduction of Li2S6, and precipitation of Li2S [57,58]. The responsivity of Li2S nucleation was earlier (5816 s) on H-Co-NCM over that on CB (15542 s). Moreover, the H-Co-NCM also presented a higher maximum current (0.32 mA) and precipitation capacity (175.8 mAh g-1) than those on CB (0.21 mA and 148.1 mAh g-1) even at a shorter nucleation and growth time [45,57]. The obtained results strongly confirm that the H-Co-NCM microflower could serve as an efficient electrocatalyst and promote the conversion from Li2Sx (4≤x≤8) to Li2S. In summary, H-Co-NCM 18
with the rational design of the porous structure and heterogeneous chemistry surfaces of Co-N as sulfur host could effectively suppress the LiPSs shuttling via the following routes (Fig. 5i): (i) the highly conductive porous carbon skeleton enables the facile electron transport and fast ion diffusion; (ii) the Co- and N-heteroatoms provide strong affinity towards LiPSs to effectively capture the LiPSs and (iii) the Co- and N-coordination centers also serve as an electrocatalyst to promote the redox kinetics
of
LiPSs.
Fig. 5 Kinetic behaviors for sulfur redox reactions. (a) Visualized adsorption of Li2S4 by CB and H-Co-NCM; High-resolution XPS spectra of (b) N 1s and (c) Co 2p of H-Co-NCM before and after adsorption of Li2S4; (d) Polarization curves of symmetrical H-Co-NCM and CB cells; (e) Polarization curves of symmetrical H-Co-NCM for 8 cycles; (f) Tafel plots; Fitting of current vs. 19
time curve for a potentiostatic discharge at 2.05 V on (g) CB and (h) H-Co-NCM; (i) Illustration of the shuttle suppress mechanism in an S@H-Co-NCM cathode.
4. Conclusions In summary, we proposed a novel intercalated LDH derived strategy for in situ controllable synthesis of H-Co-NCM microflower with uniform distribution of dual-functional Co- and N- active sites for capture and catalytic conversion of LiPSs to significantly suppress the shuttle effect. Moreover, the precise morphology modulation of hollow conductive carbon matrix and highly tortuous nanosheets with tailored mesopores endow H-Co-NCM commodious space for an ultrahigh sulfur loading of 82% and synchronously guarantee the unimpeded ion/electron transport channels. Thus, the Li-S batteries employing H-Co-NCM as cathode exhibited a superior initial capacity of 1374 mAh g-1 at 0.1C and impressive rate capability (611 mAh g-1 at 2C), and outstanding capacity decay of 0.069% per cycle over 500 cycles at 0.5C. Systematic kinetic characterizations demonstrate H-Co-NCM could serve as an advanced electrocatalyst to boost the process of soluble LiPSs conversion and lithium sulfide nucleation. This study provides a novel integrated strategy of physical confine and chemical conversion of LiPSs and develops a facile and low-cost method for the design and synthesis of advanced electrodes for Li-S batteries.
Acknowledgements This research work was supported by the National Natural Science Foundation of China (No. 51672186). The authors would like to acknowledge the support from Nanchang University and Arizona State University. S.X Chen gratefully acknowledges support from the Chinese Scholarship 20
Council (CSC, No. 201806820004) to undertake this research. Note †Shixia Chen and Xinxin Han contribute equally to this work.
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Declaration of interests ☑The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
29
Graphic Abstract
30
Highlights
A novel intercalated-LDH template method to fabricate Co-embedded sulfur host;
The unique hollow porous microflower-like cathode enables fast electron/ion transport and high sulfur loading;
Systematic validation of synergistic effect of electrocatalytic boosted redox kinetics of LiPSs and well-defined physical confinement.
31
S1385-8947(19)32870-0 https://doi.org/10.1016/j.cej.2019.123457 CEJ 123457
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
15 September 2019 23 October 2019 9 November 2019
Please cite this article as: S. Chen, X. Han, J. Luo, J. Liao, J. Wang, Q. Deng, Z. Zeng, S. Deng, In Situ Transformation of LDH into Hollow Cobalt-Embedded and N-doped Carbonaceous Microflowers as Polysulfide Mediator for Lithium-SulfurBatteries, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123457
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In Situ Transformation of LDH into Hollow Cobalt-Embedded and N-doped Carbonaceous Microflowers as Polysulfide Mediator for Lithium-Sulfur Batteries Shixia Chena†, Xinxin Hana†, Junhui Luo a, Jing Liao a, Jun Wanga*, Qiang Denga, Zheling Zenga, Shuguang Dengb*
a
Key Laboratory of Poyang Lake Environment and Resource Utilization, Ministry of Education,
School of Resources Environmental & Chemical Engineering, Nanchang University, Nanchang, 330031, China b
School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler
Mall, Tempe, AZ 85287, USA
*Corresponding author: 1. E-mail: [email protected] (S. Deng) 2. E-mail: [email protected] (J. Wang) †S.
Chen and X. Han are equally contributed to this work.
1
Abstract: The shuttle effect of soluble lithium polysulfides (LiPSs), accompanying with sluggish redox kinetics has severely impeded the implementation of lithium-sulfur (Li-S) batteries. Herein, a novel hollow cobalt-embedded and nitrogen-doped carbonaceous microflower (H-Co-NCM) is fabricated via in situ transformation of metanilic anions intercalated Co-Al layered double hydroxides (CoAl LDHs). The as-obtained S@H-Co-NCM electrode exhibits superior electrocatalytic performances to boost the kinetics of LiPSs conversion and Li2S nucleation. Consequently, the assembled Li-S batteries with a high sulfur loading of 82% display a remarkable initial capacity of 1374 mAh g-1 at 0.1 C, excellent rate capability (611 mAh g-1 at 2 C), and superb cycle stability (cyclic decay rate of 0.069% over 500 cycles at 0.5 C). The integrated strategy of strong chemisorption and fast conversion of LiPSs provides deeper insights to suppress the shuttle effect.
Key Words：Layered double hydroxide (LDH); hollow, sulfur host; lithium-sulfur batteries
2
1. Introduction Over the past few years, the dramatically expansive researches towards next-generation energy-storage devices have highlighted lithium sulfur (Li-S) batteries because of their superior theoretical specific capacity (1675 mAh g-1) and ultrahigh energy density (2600 Wh kg-1). Meanwhile, the appealing features of sulfur, such as cost-effective, environment benignity, and earth abundance, have also driven the practical application progress [1-5]. However, the implementation of Li−S batteries is still impeded by some intractable obstacles, such as the insulating nature of sulfur and lithium sulfides, significant volume changes (~80%) during cycling, and notorious shuttle effect caused by the dissolved intermediate lithium polysulfides (LiPSs) [6-8]. The shuttle effect greatly induces the loss of active sulfur, corrosion of lithium anode, rapid capacity fading, and low Coulombic efficiency [3,9,10]. To address the above issues, considerable efforts have been dedicated to designing novel sulfur host cathodes [11-14]. Porous and conductive carbon scaffolds are the most investigated sulfur hosts that could physically confine LiPSs within the nanopores. However, during long-term charge/discharge cycling, the polar LiPSs tend to diffuse outward due to their poor affinity with non-polar carbon hosts [15-17]. Therefore, multiple polar materials, e.g. metal oxides [18,19], metal sulfides [20,21], and N,O-heteroatoms [22,23], have been introduced into the nonpolar carbon substrates to strengthen the carbon-LiPSs attraction forces. However, the finite attractive sites and limited inner space of host materials are not able to fully realize the theoretical performances. Notably, three-quarters of the theoretical capacity of Li-S batteries are released from the sluggish liquid-solid conversion of soluble Li2S4 to solid Li2S [24-26]. The poor kinetics of LiPSs reduction and transformation lead to the accumulation of soluble LiPSs surrounding the cathode and eventually 3
causes the arbitrary precipitation of solid Li2S2/Li2S on the cathode/anode surface [24,27]. Thus, the electrocatalytic regulation of LiPSs is expected to be highly efficient to mitigate the shuttle effect, especially in high sulfur loading and long cycling conditions [28,29]. Recently, in-situ electrocatalysts, including Ni [30], Co [31], and their metal sulfides/nitrides [27,32], have been proposed as an integrated kinetics-promotion strategy of enhancing the transformation of LiPSs to Li2S2/Li2S and anchoring LiPSs. Particularly, Co and N codoped carbon (Co-N-C) materials that serve as effective electrocatalysts [33,34] have evoked a tremendous attention in the application of Li-S batteries. For example, Dong et al. [35] revealed that Co could facilitate the transformation of LiPSs to Li2S2/Li2S and the N heteroatoms can accelerate the oxidation of LiPSs to S8. Wu et al.[36] demonstrated that the encapsulated Co nanoparticles as well as the N heteroatoms in the graphitic carbon frameworks could synergistically reserve the soluble LiPSs and facilitate the redox reaction kinetics. However, the Co-N-C catalysts are mostly derived from metal-organic frameworks (MOFs) with relatively complex and time-consuming preparation processes [33,35-38], thus it is highly desired to develop novel synthesis method from simple templates. Moreover, efficient void space to load sulfur as well as accommodate the volume expansion is also an important criterion to design sulfur cathode. Ding et al. reported a yolk-shell structure with conductive carbon shells that can provide comfortable accommodation of the volume fluctuation, achieve a high sulfur loading of 74.8 wt% and deliver a high initial capacity of 1400 mAh g-1 [39]. Thus, we proposed a facile LDHs template to construct micro-size hollow carbonaceous flower structures with in-situ electrocatalytic activity as sulfur host. Herein, we report a novel and low-cost route for the fabrication of H-Co-NCM sulfur host as Li-S battery cathode by facile carbonization of the metanilic acid intercalated cobalt-aluminum layered 4
double hydroxides (CoAl-M-LDHs) without additional carbon sources and conductive additives. The metanilic acid molecules insert into the hydroxide layers simultaneously in situ guide the self-assembly of micoflower-like carbon framework and the functionalities incorporation. In the subsequent acid etch process, excessive Co and Al-based nanoparticles are removed, but strongly coupled Co functionalities with N heteroatoms are preserved, thus crafting a Co- and N-codoped hollow porous microflower structure. The heteroatoms doping of Co/N further enhances the conductivity of carbon matrix and acts as the synergetic adsorption and catalysis sites for LiPSs capture and conversion to improve the electrochemistry performance of Li-S batteries. Moreover, the hierarchical pore systems enable the H-Co-NCM microflower structure with a high specific BET surface area of 571.4 m2 g-1. As a result, the as-obtained H-Co-NCM could contain 82 wt% S and deliver an excellent initial discharge capacity of 1374 mAh g-1 at 0.1 C. Furthermore, the interfacial affinity and kinetically conversion of H-Co-NCM towards LiPSs are clarified by the visualized adsorption experiment, Li2S6 symmetric tests, Tafel, and Li2S nucleation measurements. Consequently, the assembled Li-S battery exhibited a superior rate capacity of 611 mAh g-1 at 2 C and a remarkable lifespan of 500 cycles with a decay rate of 0.069% per cycle at 0.5 C.
2. Experimental section 2.1 Synthesis of CoAl-M-LDH precursor The metanilic acid intercalated CoAl-LDH precursor was prepared by a minor modification of previous literature [40]. Typically, 2 mmol aluminum nitrate nonahydrate (Al(NO3)3·9H2O), 6 mmol cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 8 mmol ammonium fluoride (NH4F), and 10 mmol hexamethylene tetramine (HMT) were dissolved in 20 mL degassed deionized (DI) water, denoted as solution A . Then, 20 mmol metanilic acid was dissolved in 20 mL 1 M NaOH solution, named as 5
solution B. Afterward, solution B was added dropwise into solution A with vigorous stirring. The resultant mixed solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 100 °C for 12 h. After cooling to room temperature, the obtained products were rinsed with excessive DI water and ethanol, and dried at 100 °C for 12 h, denoted as CoAl-M-LDH. The DI water used in the entire synthesis experiment was degassed by high-purity N2 to avoid the competitive intercalation of carbonate anions into CoAl-LDHs. 2.2 Synthesis of hollow cobalt-embedded and nitrogen-doped carbonaceous microflower (H-Co-NCM) The as-prepared CoAl-M-LDHs were calcined at 700 °C for 2 h with a ramp of 3 °C min−1 under N2 flow. After cooling, the obtained c-LDH was etching with 18 wt% HCl solution for 24 h and then rinsed with enough DI water and ethanol. The obtained samples were collected through the filter and followed by drying at 60 °C overnight and denoted as H-Co-NCM. 2.3 Sulfur impregnation in H-Co-NCM materials Typically, 0.2 g H-Co-NCM and 1.0 g sublimed sulfur were dispersed in 20 mL N, N-dimethylformamide and 50 mL of carbon disulfide (CS2), respectively by ultra-sonication. The two solutions were subsequently fully mixed and kept at 50 °C under stirring for 12 h to completely evaporate CS2 [41]. The mixture was then loaded to a sealed glass vial and maintained at 200 °C for 30 min in an N2 atmosphere. Acetylene black and S composites (S@CB) were prepared using the same method. 2.4 Material characterization The morphology was conducted by field-emission scanning electron microscope (FESEM, JSM-6701F, and XL30 Environmental FEG coupled with an energy dispersive X-ray spectroscopy 6
(EDX)) and transmission electron microscope (TEM, JEM-2100, Japan and ARM200F, Japan). The N2 adsorption-desorption isotherms were measured by the Brunauer-Emmett-Teller (BET, ASAP 2460, USA) at 77 K. The BET surface area and total pore volume were calculated from the BET equation and the pore size distribution (PSD) was derived from the non-local density functional theory (NL-DFT) method. The sulfur content of the cathode was estimated from 30 to 500 ℃ under N2 atmosphere through thermogravimetric analysis (TGA; STA2500, NETZSCH, German). The crystal structures of the materials were recorded on X-ray powder diffraction (XRD, PANalytical empyrean series2, Netherlands) with Cu-Kα radiation. The C, H, N, S and O contents of the samples were measured by an elemental analyzer (Elementar Vario Micro, German). Infrared spectra were collected by a Nicolet 5700 Fourier transformed infrared (FT-IR) spectrometer, using the KBr pellet method and X-ray photoelectron spectroscopy (XPS) measurements were obtained by an Escalab 250Xi spectrometer. Raman spectra were collected using a LabRAM HR Evolution Raman microscope with 532 nm laser source. The electronic conductivity was conducted by the ST-2258C Four-Point Probe Resistance Tester. 2.5 Electrochemical measurements The working electrodes consisted of the active material, acetylene black, and polyvinylidene difluoride were prepared with a weight ratio of 7:2:1 in N-methyl-pyrrolidone (NMP) under vigorous stirring for 8 h. The uniform slurry was cast onto a carbon paper current collector and vacuum dried at 60 °C for 12 h and punched into disks with a diameter of 12 mm. The batteries were assembled using CR2032 coin cells in an Ar-filled dry glove box (<0.1 ppm H2O/O2) with Celgard 2500 separators. The sulfur content in the working electrode was about 2 mg cm-2 and 40 μL of the electrolyte containing 1 M bis(trifluoromethane)sulfonimide lithium (LiTFSI) in a mixed solvent of 7
1,3-dioxolane (DOL) and 1,2-dimethoxy-ethane (DME) (v/v =1:1), with 1 wt% LiNO3, was used for each coin cell. Electrochemical measurements were performed on a Neware battery analyzer with a voltage window of 1.7-2.8 V vs. Li+/Li at room temperature. Cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and Tafel polarization were measured on a CHI 660E electrochemical workstation (CH Instrument, China). All specific capacity values were obtained based on the mass of elemental sulfur. The experiment details of visualized adsorption experiment, Li2S6 symmetric cells, Li2S nucleation and, Tafel measurements were provided in supporting information.
3. Results and discussion 3.1 Materials synthesis and characterization The fabrication procedure of microflower-like carbon frameworks (H-Co-NCM) and the responding morphological evolution is illustrated in Scheme 1. The H-Co-NCM material was fabricated through a hydrothermal method and followed by annealing at 700°C to obtain amorphous N-doped carbon skeletons, denoted as c-LDH. After acid etching treatment, the H-Co-NCMs with uniform particle size and ordered morphology were successfully prepared. Finally, sulfur was encapsulated into the H-Co-NCM by a CS2 solvent method to yield S@H-Co-NCM composite.
8
Scheme 1. Schematic illustration of the synthesis process of S@H-Co-NCM electrode. The morphologies of CoAl-M-LDH, c-LDH, and H-Co-NCM were demonstrated by SEM and TEM images. As shown in Fig. 1a, the CoAl-M-LDH presents as a uniformly distributed 3D flower-like morphology assembled with an average diameter of ~2 μm. Closer examination indicates that each assembly consists of multiple curved nanosheets with a smooth surface and the thickness is about 30 nm (Fig. 1d and 1g). These primary nanosheets are oriented and interconnected at different angles that act as efficient secondary packing blocks, which could stabilize the structural integrity during the sulfur impregnation and cycling processes [42]. Upon calcination, metanilic acid molecules are in situ converted to N-doped carbon frameworks, meanwhile, the LDH layers are converted to uniform Co- and Al-based nanoparticles (1-20 nm) that are uniformly dispersed on the petals of microflower (Fig. 1e, 1h and S1). Impressively, the microflower morphology of H-Co-NCM could be well retained after HCl leaching (Fig. 1c). The nanoparticles on the microflower petals are etched away and result in numerous dents (Fig. 1f and 1i) that can provide abundant channels for the fast electrolyte infiltration. Moreover, the hollow structure nature of H-Co-NCM can be demonstrated by Fig. 1i and further confirmed by the damaged structure (Fig. S2). The unique features of hollow micro-/nanostructures with hierarchical pores can not only achieve high loading of sulfur but also physically block the LiPSs diffusion pathways [43]. The selected area 9
electron diffraction (SAED) pattern of H-Co-NCM confirms the embedded Co in the H-Co-NCM (Fig. S3). EDX mapping of H-Co-NCM also reveals the carbonaceous nature of the obtained microflower and partial Co nanoparticles were preserved after acidic etching (Fig. S4). Moreover, the content of Co is measured to be 3.62 % by the ICP, while the contents of C and N are 43.36% and 5.32%, respectively, quantified by the ultimate element analysis (Table S1).
Fig. 1 SEM images of (a)(d) CoAl-M-LDH, (b)(e) c-LDH, and (c)(f) H-Co-NCM; TEM images of (g) CoAl-M-LDH, (h) c-LDH and (i) H-Co-NCM with the corresponding enlarged images. The structure evolution process from the CoAl-M-LDH to H-Co-NCM is further verified by the XRD analysis. As illustrated in Fig. 2a, the precursor CoAl-M-LDH could be indexed to a typical LDH phase with an (003) interlayer distance of 1.55 nm following the metanilic anions-intercalating LDH [40]. After calcination, the crystalline phases of Co and Co9S8 nanoparticles on c-LDH could 10
be observed. For H-Co-NCM materials, the diffraction peaks of (111) peak at 44° and (003) peak at a 25° correspond to the crystalline Co and amorphous carbon, respectively [36]. The preservation of Co particles could be ascribed to the strong chemical bonds with N functionalities. The Raman and FT-IR spectra further substantiated to the successful synthesis of H-Co-NCM (Fig. S5 and S6). Moreover, the electronic conductivity of H-Co-NCM is estimated to be 716 S m-1 which is higher than that of CB (578 S m-1) using the four-probe method, implying the highly conductive nature of the obtained carbon skeletons. Due to the unique structure and acid etching, the H-Co-NCM showed much larger BET surface area and pore volume (571.4 m2 g-1 and 0.535 cm3 g-1) than those of pristine CoAl-M-LDH (6.4 m2 g-1 and 0.014 cm3 g-1) and c-LDH (151.3 m2 g-1 and 0.163 cm3 g-1) (Fig. 2b). The N2 adsorption/desorption isotherm of H-Co-NCM at 77 K exhibited typical features of type IV isotherm with a hysteresis loop, indicating the hierarchical pore systems with abundant micropores, mesopores, and macropores. The pore size distribution plot depicted that micropores and mesopores with a pore size of 1 to 20 nm are dominating and correspond to the sizes of Co and Co9S8 nanoparticles (Fig. 2c). Such hierarchical pore systems provide sufficient internal space for efficient sulfur loading, physical entrapment of LiPSs, and volume expansion during charge and discharge [40,44]. Owing to these merits, the prepared H-Co-NCM could contain a high sulfur mass loading of 82% as determined by the TGA (Fig. 2d). Upon sulfur infiltration, the specific BET surface area of S@H-Co-NCM drastically decreased to 12.3 m2 g-1, suggesting that the sulfur has been successfully accommodated into the hierarchical pores (Fig. S7).
11
Fig. 2 (a) XRD patterns, (b) N2 adsorption-desorption isotherms, (c) PSD and pore volume of CoAl-M-LDH, c-LDH, and H-Co-NCM, (d) TGA curves of S@H-Co-NCM. After sulfur loading, the morphology could be well maintained and no surface sulfur particles are observed, demonstrating the successful encapsulation of sulfur into the H-Co-NCM frameworks (Fig. 3a). Furthermore, the TEM image (Fig. 3b) clearly showed that a clear contrast of the dark inner spaces of S@H-Co-NCM with the transparent hollow structure of H-Co-NCM, revealing the successful accommodation of sulfur into the hollow host. The presence of Co is further confirmed by the high-resolution TEM (HRTEM) image in Fig. 3c, which shows the lattice fringe spacing of 0.204 nm, corresponding to the (111) crystal planes of Co. The uniformly dispersed Co nanoparticles with 12
an average size of ~5 nm construct the uniformly distributed sulfiphilic active sites and regulate the rapid nucleation and growth of Li2S [45]. EDS element mapping of the S@H-Co-NCM composite showed the uniform distribution of N and Co throughout the H-Co-NCM framework as well as the encapsulation of sulfur within the cathode materials (Fig. 3d). The impregnated sulfur in the S@H-Co-NCM composite was α-S8 (PDF 78-1889), as revealed by the XRD pattern (Fig. S8).
Fig. 3 Morphology of S@H-Co-NCM composite. (a) FESEM image, (b) TEM image, (c) HRTEM image and (d) EDS elemental mappings of the S@H-Co-NCM composite. 3.2 Electrochemical performance To demonstrate the superiority of H-Co-NCM for promoting electrochemical kinetics in a working Li−S battery, the influence of H-Co-NCM on the electrochemical reactions was firstly investigated by the CV and EIS profiles and the S@CB electrode was adopted as a control. As shown in Fig. 4a, the CV curves of S@H-Co-NCM and S@CB composites display two obvious pairs of redox peaks, indicating a reversible reduction/oxidation process. The peak currents of two cathodic and anodic processes are denoted as IC1, IC2 and IA1, IA2, respectively, corresponding to the two-step reduction of 13
sulfur to Li2S2/Li2S and reversible oxidation of Li2S2/Li2S to S8 [25]. Compared to the two broad cathodic peaks of S@CB cathode, S@H-Co-NCM exhibits much sharper peaks with an obvious positive shift. Whereas the anodic peaks of both cathodes appear at the almost same position, these observations strongly imply that the doped Co- and N-functionalities could significantly decrease the polarization and facilitate the polysulfide redox kinetics [36,46]. This catalytic effect is further demonstrated by the EIS tests (Fig. 4b), the charge-transfer resistance (Rct) represents a parameter closely related to the chemical reaction activation energy, the Rct of S@H-Co-NCM (62.11 Ω) is much lower than that of S@CB electrode (80.09 Ω), suggesting faster electrochemical kinetics with Co- and N- functionalities [25,47]. Moreover, the less difference of the median voltage difference on the galvanostatic charge/discharge profile at 0.1C of S@H-Co-NCM (152 mV) than S@CB cathodes (156 mV) can further verify the less polarization of S@H-Co-NCM (Fig. 4c). However, when the current rate was raised to 1 C, the S@H-Co-NCM still displayed two obvious discharge plateaus, while the S@CB suffered severe polarization that the two-step discharge processes were blurred. Benefiting from the improved polysulfide conversion kinetics, the S@H-Co-NCM cathode exhibits the superior rate capability under gradually increased current densities, as shown in Fig. 4d. With the stepwise increasing current rates from 0.1, 0.2, 0.5, 1 to 2C, the S@H-Co-NCM cathode delivers the corresponding discharge capacity of 1374, 957, 841,738 and 611 mAh g-1, respectively. After switching current density back to 1.0, 0.5, 0.2, and 0.1 C in turns, a reversible discharge capacity of 768, 797, 813, and 829 mAh g-1 could be recovered, indicating the outstanding structure stability of S@H-Co-NCM composite [36,48]. In contrast, the S@CB cathode displays a rapid capacity decay with increasing rate current, namely, an inferior discharge capacity of 168 mAh g-1 was observed at 2.0 C. The outstanding kinetic promotion of S@H-Co-NCM towards LiPSs redox chemistry was 14
further confirmed through the long-term cycling performance that was evaluated at 0.5 C (Fig. 4e). After 500 cycles, a low capacity fading rate of 0.069% per cycle with high coulombic efficiency near 100% was obtained, indicating the shuttle effect of LiPSs has been efficiently restrained [49]. On the contrary, without Co and N doping, S@CB suffered rapid capacity decay (a capacity fading rate of 0.113% per cycle over 300 cycles) and unsatisfactory coulombic efficiency (98% at 300th cycle). The long-term cycling experiments performed of S@H-Co-NCM at 1C and 2C could also obtain a low capacity fading rate of 0.078% and 0.076% with a stable coulombic efficiency near 100% after 200 cycles (Fig. S9). Notably, the electrochemistry performance is highly competitive with the state of the art sulfur cathodes (Table S2). Moreover, 48 light-emitting diodes (LED) could be lighted with one assembled Li-S cell for 3 hours (Fig. S10). The exhibited excellent overall performance could be attributed to the strong chemisorption and fast electrocatalytic effects provided by the Co and N functionalities.
Fig. 4 (a) CV curves and (b) Nyquist plots of S@CB and S@H-Co-NCM cells; (c) The discharge-charge profiles of S@CB and S@H-Co-NCM cells at 0.1 and 1C; (d) Rate performance of 15
S@CB and S@H-Co-NCM cells ranging from 0.1 to 2C; (e) The cycling performance of S@H-Co-NCM and S@CB cells at 0.5C . The strong chemical LiPSs adsorption ability is the prerequisite for the rapid redox reactions on the H-Co-NCM mediator. Thus, the chemisorption of H-Co-NCM toward LiPSs was initially examined by the visual adsorption experiment by immersing H-Co-NCM into a 0.05 M Li2S4 solution in 1,3-dioxolane (DOL)/dimethoxyethane (DME). As depicted in Fig. 5a, a brownish Li2S4 solution was entirely decolored by H-Co-NCM after static adsorption for 4 h, while the Li2S4 solution with CB still displayed brownish color. Furthermore, the colorless separators of S@Co/N-C cells disassembled after cycling also indicate the strong capture ability of Co/N-C with LiPSs (Fig. S11). To further reveal the mechanism behind the strong interaction forces of H-Co-NCM towards LiPSs, the surface chemistry of H-Co-NCM before and after the adsorption experiment was examined by XPS measurement. The N 1s spectrum in Fig. 5b can be deconvoluted into four peaks: pyridinic N (397.8 eV, 11.6%), Co-N (398.5 eV, 10.1%), pyrrolic N (400.3 eV, 68.4%), and graphitic N (401.3 eV, 9.9% ) [47,50,51]. The dominance of pyrrolic N and pyridinic plays a key role in enabling strong Li-N bonds to trap LiPSs, while graphitic N could improve the conductivity of host materials [47,52]. Upon adsorption of Li2S4, the decreased intensities of pyridinic N (397.8 eV, 7.7%), Co-N (398.5 eV, 7.8%) and pyrrolic N (400.3 eV, 63.9%) imply the strong adsorption ability towards LiPSs originating from these doped nitrogen groups [52]. The existence of Co-N bond demonstrates the embedded Co functionalities are strongly coupled with N, which could effectively modify the electronic structure and distribution of the carbon matrix [47]. The high-resolution Co 2p XPS spectrum could be split into two characteristic resonances of 2p3/2 and 2p1/2 with the satellite peaks (785.4 and 801.8 eV), further suggesting the existence of Co nanoparticle (Fig. 5c). In comparison to 16
the pristine H-Co-NCM, four characteristic peaks of Co 2p3/2 and 2p1/2 all upshifted to higher binding energies, suggesting the enhanced interaction of exposed Co nanoparticles with surrounding strong electronegative sulfur ligand [45]. Obviously, the Co 2p3/2 spectrum of H-Co-NCM-Li2S4 has an additional peak at 778.9 eV, indicating the existence of a Co-S bond, further evidencing the rich adsorption sites for LiPSs [32,53]. The above evidence firmly confirms the existence of Co- and N-species that could potentially be applied as an efficient catalyst for LiPSs conversions [24,31]. Moreover, the appearance of the polythionate complex in the S 2p spectrum unveiled the H-Co-NCM could actively promote polysulfides conversion other than only passively adsorb LiPSs (Fig. S12). To gain deeper insights into the electrocatalytic effect of Co- and N- functionalities on the redox kinetics of polysulfide, symmetrical cells with two identical H-Co-NCM electrodes in Li2S6-containing electrolytes were assembled. For comparison, one symmetrical cell based on CB was also assembled. As shown in Fig. 5d, the Li2S6-free cell reveals little contribution to the capacitive current, while the current density of the H-Co-NCM electrode in the Li2S6 electrolyte is much larger than that of CB, indicating significantly improved redox kinetics than that of CB in the soluble phases. Fig. S13 shows the CV curves of symmetrical H-Co-NCM and CB cells at different scan rates that all redox curves of Co/N-C reveal a good shape and retain good reversibility, even at a scan rate as high as 100 mV s-1, demonstrating the expeditious kinetics feature of a series of polysulfide conversions facilitated by the Co/N-C. Notably, the high superimposition of the first eight CV cycles of the H-Co-NCM electrode suggests the excellent stability of H-Co-NCM electrode (Fig. 5e). Recently, Co single atom has been reported as an efficient catalyst in LiPSs conversion [24,54], in order to unveil the role of Co single atom, the H-Co-NCM was further etched by concentrated hydrochloric acid to remove metallic Co nanoparticles. As shown in Fig.S14a, no 17
obvious Co diffraction peaks were observed after HCl treatment in the XRD pattern, suggesting that most of the metallic Co nanoparticles have been etched. Whereas, the current density of the H-Co-NCM after HCl treatment with Li2S6 electrode significantly decreased (Fig.S14b), manifesting the importance of both metallic Co nanoparticles and Co single atom in mediating LiPSs conversion, which is in accordance with the previous literature [35,36]. The superior catalytic activity afforded by the H-Co-NCM with regards to LiPSs conversion could be further revealed by its much lower Tafel slope (53 mV dec-1) and obviously higher exchange current density (0.225 mA cm-2), comparing with the corresponding 934 mV dec-1 and 0.019 mA cm-2 on CB as obtained from the Tafel plots (Fig. 5f) [55,56]. To demonstrate the conversion kinetics of soluble LiPSs into solid Li2S on the electrode/electrolyte interfaces, the Li2S precipitation experiment was monitored. According to the following Avrami formula (Eq.(1)) [57], a larger fraction (η) represents a faster kinetic nucleation and growth rate of Li2S. η =1−exp(−Btn)
(1)
where η is the transformed fraction of Li2S, B is the kinetic constant, and n is the Avrami exponent. As shown in Fig. 5g and 5h, the potentiostatic discharge curves undergo three stages i.e. the reduction of Li2S8, reduction of Li2S6, and precipitation of Li2S [57,58]. The responsivity of Li2S nucleation was earlier (5816 s) on H-Co-NCM over that on CB (15542 s). Moreover, the H-Co-NCM also presented a higher maximum current (0.32 mA) and precipitation capacity (175.8 mAh g-1) than those on CB (0.21 mA and 148.1 mAh g-1) even at a shorter nucleation and growth time [45,57]. The obtained results strongly confirm that the H-Co-NCM microflower could serve as an efficient electrocatalyst and promote the conversion from Li2Sx (4≤x≤8) to Li2S. In summary, H-Co-NCM 18
with the rational design of the porous structure and heterogeneous chemistry surfaces of Co-N as sulfur host could effectively suppress the LiPSs shuttling via the following routes (Fig. 5i): (i) the highly conductive porous carbon skeleton enables the facile electron transport and fast ion diffusion; (ii) the Co- and N-heteroatoms provide strong affinity towards LiPSs to effectively capture the LiPSs and (iii) the Co- and N-coordination centers also serve as an electrocatalyst to promote the redox kinetics
of
LiPSs.
Fig. 5 Kinetic behaviors for sulfur redox reactions. (a) Visualized adsorption of Li2S4 by CB and H-Co-NCM; High-resolution XPS spectra of (b) N 1s and (c) Co 2p of H-Co-NCM before and after adsorption of Li2S4; (d) Polarization curves of symmetrical H-Co-NCM and CB cells; (e) Polarization curves of symmetrical H-Co-NCM for 8 cycles; (f) Tafel plots; Fitting of current vs. 19
time curve for a potentiostatic discharge at 2.05 V on (g) CB and (h) H-Co-NCM; (i) Illustration of the shuttle suppress mechanism in an S@H-Co-NCM cathode.
4. Conclusions In summary, we proposed a novel intercalated LDH derived strategy for in situ controllable synthesis of H-Co-NCM microflower with uniform distribution of dual-functional Co- and N- active sites for capture and catalytic conversion of LiPSs to significantly suppress the shuttle effect. Moreover, the precise morphology modulation of hollow conductive carbon matrix and highly tortuous nanosheets with tailored mesopores endow H-Co-NCM commodious space for an ultrahigh sulfur loading of 82% and synchronously guarantee the unimpeded ion/electron transport channels. Thus, the Li-S batteries employing H-Co-NCM as cathode exhibited a superior initial capacity of 1374 mAh g-1 at 0.1C and impressive rate capability (611 mAh g-1 at 2C), and outstanding capacity decay of 0.069% per cycle over 500 cycles at 0.5C. Systematic kinetic characterizations demonstrate H-Co-NCM could serve as an advanced electrocatalyst to boost the process of soluble LiPSs conversion and lithium sulfide nucleation. This study provides a novel integrated strategy of physical confine and chemical conversion of LiPSs and develops a facile and low-cost method for the design and synthesis of advanced electrodes for Li-S batteries.
Acknowledgements This research work was supported by the National Natural Science Foundation of China (No. 51672186). The authors would like to acknowledge the support from Nanchang University and Arizona State University. S.X Chen gratefully acknowledges support from the Chinese Scholarship 20
Council (CSC, No. 201806820004) to undertake this research. Note †Shixia Chen and Xinxin Han contribute equally to this work.
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Declaration of interests ☑The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphic Abstract
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Highlights
A novel intercalated-LDH template method to fabricate Co-embedded sulfur host;
The unique hollow porous microflower-like cathode enables fast electron/ion transport and high sulfur loading;
Systematic validation of synergistic effect of electrocatalytic boosted redox kinetics of LiPSs and well-defined physical confinement.
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