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Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes
Journal Pre-proof Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes Jin Kyu Kim, Hyemin Park, Sun Sook Lee, Seung Uk Son, Yongku Kang, Won Bin Im, Sungho Choi PII:
S0254-0584(19)31377-X
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122567
Reference:
MAC 122567
To appear in:
Materials Chemistry and Physics
Received Date: 14 October 2019 Revised Date:
4 December 2019
Accepted Date: 17 December 2019
Please cite this article as: J.K. Kim, H. Park, S.S. Lee, S.U. Son, Y. Kang, W.B. Im, S. Choi, Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/ j.matchemphys.2019.122567. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement
Jin Kyu Kim is “Formal Analysis” and “Investigation”. Hyemin Park is “Validation” and “Investigation”. Sun Sook Lee is “Data Curation”. Seung Uk Son is “Supervision”. Yongku Kang is “Supervision” and “Project Administration”. Won Bin Im is “Data Curation” and “Writing-Review & Editing”. Sungho Choi is “Conceptualization”, “Writing-Original Draft” and “Supervision”.
Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes
Jin Kyu Kima#, Hyemin Parkb,c#, Sun Sook Leeb, Seung Uk Sonc, Yongku Kangb, Won Bin Imd*, and Sungho Choib*
a
Advanced Automotive Battery Division, LG Chem Research Park, 188 Munji-ro, Yuseong,
Daejeon, Republic of Korea b
Advanced Materials Division, Korea Research Institute of Chemical Technology, 141
Gajeongro, Yuseong, Daejeon, Republic of Korea c
Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea
d
Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro,
Seongdong-gu, Seoul 04763, Republic of Korea
Keywords: porous nanoparticle, cobalt oxide, sulfur reservoir, cathode, lithium–sulfur battery
*Corresponding authors. Tel: 82–42–860–7372. E-mails: [email protected] (Won Bin Im) [email protected] (Sungho Choi)
Abstract Hole-embedded one-directional two-dimensional sheet-assembly cobalt oxide microspheres are synthesized and assessed as effective encapsulation media for polysulfides in cathodes of lithium–sulfur batteries. The cathode of fishing-net-shaped cobalt oxide compounds incorporated with sulfur provides exceptional electrochemical properties such as a high Coulombic efficiency (99%), stable capacity fading, and a reversible capacity of 925 mA h g−1 at 1 C, which is 22% higher than that of the conventional nanoparticulate cobalt oxide/sulfur electrode. The enhanced electrochemical behavior is attributed to the effective polysulfide adsorbent and hierarchically designed porous cobalt oxide with a coaggregated framework. The controlled pore-embedded particle size is an essential parameter determining the capture of the mobile polysulfides, rather than the use of numerous mere highly porous encapsulants, such as nanoparticulate cobalt oxides.
1. INTRODUCTION The current lithium ion batteries (LIBs) have limited energy densities and thus could not meet the increasing demands for various portable devices and electric vehicles. In this context, the electrochemical reactions between lithium metal and chalcogen ions in Li batteries based on sulfur (S) and oxygen (O) are promising for next-generation energy storage devices. Recently, there has been a remarkable development of novel materials as alternatives to the conventional intercalation electrodes. Graphite and layered oxides for state-of-the-art LIBs, i.e., S cathode for the Li–S battery and air electrode for the Li–air battery and stable Li metal, have facilitated the commercialization of high-energy-density Li rechargeable batteries.[1,2] The rechargeable Li–S battery is considered for application in the upcoming energy storage device with an elemental sulfur cathode and anode, which is electrochemically reactive with metallic Li. It has various significant advantages, such as high theoretical specific capacity (~ 1675 mA h g-1) and energy density (~ 2600 W h kg-1), which is approximately four times higher than those of the current LIBs, low cost, and abundance in the nature along with the environmental benignity of S.[3–6] For commercialization of Li−S batteries, several issues need to be overcome including the low electronic conductivity and detrimental volume change of the S cathode in the repetitive charge/discharge cycles as well as the dissolution of mobile polysulfide byproducts and related shuttle effects.[3,7] Therefore, the utilization of activated S followed by the reversible electrochemical reaction with Li+ cannot be commercialized with the current techniques. Extensive studies have been carried out to address the above issues.[7–12] Representative issues are the controlled design of nanostructured cathodes using large-surface-area conductive carbon-based materials, infiltration of S, and addition of reactive reservoirs capturing the mobile polysulfide intermediates. Generally, the latter can be achieved using a
physically/chemically active porous host matrix with a high affinity to the polysulfides, which leads to an enhanced capacity retention. In principle, the large surface areas achieved by nanoparticles and large pore volumes can provide many reactive sites through surface and/or interface-related reactions such as gas adsorption/desorption, radiation protection, and energy conversion.[13-15] Nevertheless, the extraordinary particle property does not fully guarantee the optimal performances in various applications. Additionally, a cost-effective large-scale production is needed to obtain monodisperse/uniform nanoparticles.[16] To better understand the shuttle effect, various compositions and morphology-controlled inorganic compounds, such as TiO2, MgO, Mn3O4, and Co3O4,[17–21] were applied to the shuttling entrapments through strong chemical bondings. Among them, the spinel-structure cobalt oxide is a representative electrochemically active material within the Co–O binary system with a mixed valence state of Co2+/Co3+ and diverse polymorphic crystalline structures. Characteristic nanocrystals with various shapes and/or composite forms, such as graphene-supported nanoparticle cobalt oxide, and very recently, we reported an in-situ synthesis of bicomposition CoOx, which provided unique electrochemical behaviors of porous CoO/Co3O4 compounds for Li rechargeable battery electrodes.[22] Nevertheless, cobalt oxides, used as catalytic agents for Li–S batteries with a morphology effect and electrochemical behavior, have rarely been reported. In this study, we synthesized microspherical cobalt oxide particles, which had omnidirectional coaggregated platelets with numerous geometrical holes acting as effective encapsulation media for polysulfides in lithium−sulfur battery cathodes. The morphology of the electrode material normally affects the surface/volume ratio, effective density of electrochemical reaction sites, drift/diffusivity of ions and electrons, and volume expansion during the Li+ transport within the reversible electrochemical reactions. Additionally, the utilization of specific precursors with coordination polymers forming the desired
nanomaterials can be achieved by a self-assembly process of the compound based on the coordination interactions between metal ions and organic surfactants in the presence of coordinating modulating agents.[23–25] We aimed to determine the efficiencies of the porosities and/or incorporated pore structures of the encapsulants, which exhibited strong chemical interactions with polysulfides and alleviated the shuttle effects caused by long-chain soluble species. The high chemical affinity of the cobalt oxide particles to the polysulfide intermediates effectively suppressed the diffusion of LixSn byproducts and improved the electrochemical stability with controlled capacity fading of the considered cathode. The poreembedded controlled particle morphology was more important for the effective polysulfide capturing center than the use of only a medium of nanoparticulate high-porosity encapsulants as in mesoporous cobalt oxides.
2. EXPERIMENTAL METHODS 2.1 Syntheses of cobalt oxide and composite Fishing-net-like and/or nanocrystal cobalt oxides were prepared by a solvothermal reaction. The appropriate amount of cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98%, Junsei Chemical) was dissolved in 30 mL of methanol (reagent grade, 99.9%, Samchun). We then prepared organic-surfactant-mixed methanol solutions with different amine surfactants, 1.4diazabicyclo [2,2,2] octane (C6H12N2, ≥99%, Sigma Aldrich, DABCO), pyrazine (C4H4N2, ≥99%, Sigma Aldrich), and pyridazine (C4H4N2, 98%, Sigma Aldrich). Finally, the two stock solutions were thoroughly mixed and stirred for 1 h at room temperature. The as-prepared precursor solution was transferred to a teflon tube for a treatment in a microwave reactor. Microwaves were applied to proceed a solvothermal reaction at 160 °C for 30 min. The resulting powder was centrifuged to remove the methanol solvent and washed several times with a distilled water/ethanol solution. After the drying process, the as-obtained powder was
calcined at a temperature of 250–500 °C in nitrogen for 30 min and then in oxygen for 30 min. The as-synthesized cobalt oxides with different molar ratios (pyrazine:pyridazine) of the surfactants are denoted as PP11 (1:1), PP13 (1:3), and PP31 (3:1). Specific synthetic procedures of the as-synthesized Co3O4 particles with using various surfactants are summarized in sTable. 1. To form the composite with sulfur, we synthesized the nanoparticle sulfur compound in advance. The appropriate amounts of polyvinylpyrrolidone ((C6H9NO)n, MW ~ 40,000, Sigma Aldrich) and sodium thiosulfate (Na2S2O3, ≥99.99%, Sigma Aldrich) were sequentially dissolved in 40 mL of distilled water. Subsequently, a 5% dilute HCl solution was added to the as-prepared solution to synthesize nanoscale sulfur, followed by stirring for 30 min. The solvent was then removed by centrifugation and the acidic solvent was washed with distilled water until it became neutral. A drying process was then carried out at a low temperature of 60 °C to prevent vaporization of the sulfur. The corresponding cobalt oxide, dispersed in 40 mL of distilled water, was mixed with as-prepared sulfur followed by stirring for 30 min. Finally, the whole solvent was removed by centrifugal separation and then dried at room temperature in vacuum for 12 h. The resulting sulfur–cobalt oxide composite powder was subjected to melt diffusion to obtain a final composited powder. The powder was placed in a sealed vial and heated in a vacuum container at 150 °C for 12 h, yielding a powder used as a cathode material for a lithium–sulfur battery. 2.2 Material characterization The crystalline structures of the as-prepared particles were analyzed by powder X-ray diffraction (XRD, Rigaku D/Max-2200V Diffractometer) with Cu Kα radiation. The particle sizes and morphologies of the cobalt oxide compounds were observed by field-emissionscanning electron microscopy (FE-SEM, Tescan Mira 3 LMU FEG) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F30 S-Twin, 200 kV). X-ray
photoelectron spectroscopy (XPS, K-Alpha Thermo Scientific) was used to evaluate the chemical states of the elements. A thermogravimetric analysis (TGA) was carried out to evaluate the sulfur content at a heating rate of 5 °C/min in nitrogen. Synchrotron diffraction data of the pristine and aged samples were acquired in the Pohang Accelerator Laboratory using radiation at a calibrated wavelength of 1.5178 Å over the angular range of 15° ≤ 2θ ≤ 80°, with a step size of 0.01°. Structural data of the samples were extracted using the diffraction data through Rietveld refinement using the general structure analysis system (GSAS) program.[26] A split pseudo-Voigt-function was used to fit the diffraction profile, while a shifted Chebyshev polynomial was used to define the background. Refinements were carried out using least-square methods until reasonable goodness-of-fit parameters were obtained. 2.3 Preparation of a coin cell and electrochemical analysis To evaluate the electrode behavior of the as-prepared cobalt oxide/sulfur composite, the cathode material was composed of cobalt oxide/sulfur powder, conductive carbon (Super-P) and polymer binder (polyvinylidene fluoride, Kureha KF-1100) in a weight ratio of 7:2:1 and then dissolved in an organic solvent (99.5%, N-methyl-2-pyrrolidone, Sigma Aldrich). Lithium metal was used as the counter electrode (anode) while 1 M of lithium bis(trifluoromethylsulfonyl)imide in a mixture of 1,3-dioxolane and dimethoxyethane (equivolume ratio) was used as a Li+ electrolyte. The components were assembled into 2032type coin cells in an argon-filled glove-box. Galvanostatic charge–discharge cycles were carried out under specific current densities in the voltage window of 1.8 to 2.6 V vs. Li/Li+ using a TOYO Toscat 3100U battery test system. Electrochemical impedance spectroscopy (EIS) was performed by applying an AC signal from 0.01 Hz to 100 kHz with 5 mVpp using an impedance analyzer (Biologic, Netherland).
3. RESULTS AND DISCUSSION The crystal phases of the as-synthesized cobalt oxide compounds with various organic surfactants were analyzed by powder XRD, as shown in Fig. 1. Based on the diffraction angles and intensities, all diffraction peaks were assigned to the cubic Co3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) 073-1701), while the crystallinities of the samples slightly differed. For the samples obtained with mixed surfactants, pyrazine : pyridazine = 1:1, 1:3 and 3:1 molar ratio, we obtained sharper diffraction peaks, which indicated the high crystallinities of the powder samples. remove references results. Therefore, we speculate that the crystallinities of the novel particles as well as the morphology-inherited shapes substantially changed by choosing the precursors with metal ions coordinated to specific organic molecules. We evaluated the characteristic particle morphology for the as-synthesized compounds with various organic surfactants. As mentioned above, the employed surfactants, pyrazine, pyridazine, and diazabicyclo octane, are rigid organic molecules formed with directionally coordinated nitrogen atoms. Therefore, we expect that they can form specific sacrificial templates, which effectively capture elemental sulfur. As preliminary results, as shown in sFig. 1 (a)–(d), the particle morphologies substantially change with the structural diversity of the organic moieties particularly in the precursors with and without heat treatment. A rather porous morphology was obtained in the case of pyrazine, while a spherical morphology with rough-faced surface shapes was observed in the case of the pyridazine surfactant. By mixing the surfactants in different molar ratios, we synthesized surface-controlled particles, as shown in Fig. 2 (a)–(f). It is worth noting that the particles with porous, spherical, and texturedsurface-inherited morphologies could be modified and that uniformly distributed particles were successfully obtained. We observed a textured surface with numerous platelets (thickness < 100 nm) packed in all directions and composed of tiny nanopores with sizes of
approximately 50 nm. According to the molar ratio, Co3O4 should have a symmetric spherical shape constructed by thin platelets with interparticle nanopores distributed throughout the surfaces. This was observed more clearly for the equimolar mixing of the two organic surfactants, pyrazine and pyridazine. We can understand the possible formation mechanism in line with the solvothermal selfassembly process proposed in other studies. Wang et al. reported a porous spherical shellstructured Co3O4 by oligomerization with glycol followed by a heat treatment, of which selfassembled nanosheet cobalt-oligomers gradually transformed into the desired structures and finally converted to the spherical-shell-coated CoOx particles.[25] Such an omnidirectional fishing-net-shaped structure constructed by thin moieties is beneficial to provide sufficient reaction sites to facilitate the reversible electrochemical reactions and efficient scraping of electrolyte soluble polysulfide byproducts. We also prepared highly porous (irregular and roughly agglomerated) Co3O4 nanoparticles using diazabicyclo octane, as shown in Fig. 2 (g), (h), and sFig. 1 (e). The corresponding Co3O4 particles are porous particles; therefore, it was useful to evaluate the surface area by a gas adsorption/desorption Brunauer–Emmett–Teller (BET) analysis. The measured BET specific surface areas of the corresponding nanoparticles are presented in Table 1. N2 adsorption/desorption isotherms and pore size distribution graphs are also presented in sFig. 2. The as-prepared Co3O4 particles obtained using only diazabicyclo octane exhibited a significantly larger (three times) specific surface area than those of the other samples with pyrazine and pyridazine mixed precursor solutions, 138 m2 g-1 and smaller than 43 m2 g-1, respectively. Intuitively, the porous media are very effective to grab the residual polysulfides during the reversible electrochemical reactions with a small diffusion length for a rapid lithium-ion and electron transport. Nevertheless, the controlled surface morphology with appropriate pore volumes of the particles was more effective for the polysulfide
reservoirs in Li–S battery cathodes, as illustrated by the following electrochemical data. Therefore, below, we focus mainly on the PP11 compound having a rather spherical shape with the appropriate surface/volume ratio compared to the sample grown using diazabicyclo octane. To maximize the contact area between the active sulfur and polysulfide reservoir (Co3O4), the Co3O4 particles were thoroughly mixed with elemental sulfur particles followed by heating in a sealed reactor. In the melt procedures, the vaporization and/or quantification of the residual sulfur content are important to implement the controlled Co3O4– sulfur composite cathode. As shown in Fig. 3 and sFig. 3, enlarged at low angle diffraction angles and wide angel scan ranges respectively, the peaks of pure elemental sulfur are attributed to an orthorhombic structure (JCPDS 01-078-1889) with traceable amounts of Co3O4 matched to the spinel-structured Co3O4, as shown in Fig. 1. The main peaks at 31.3 and 36.8° (denoted as ♦ in red) are attributed to the Co3O4 particles in the composite, while some peaks of highindex planes became weak and even disappeared within the Co3O4– sulfur composite suggesting that meltdown sulfur was incorporated into the Co3O4 matrix. To confirm the phase stability of the composite after the melt procedure, we carried out a synchrotron XRD analysis followed by Rietveld refinements. In order to obtain structural details of the composites, Rietveld refinement was performed on both samples, as shown in Fig. 4. In the refinement cycles, all parameters except the occupancy factors of the elements were iteratively refined. The refinement was converged to obtain reasonable goodness-of-fit parameters; the obtained structural parameters are presented in Table 2. The prominent phase in both samples was elemental S8 assuming an orthorhombic Fddd space group, with estimated weight percentages of 88.6% and 86.5%, respectively. The Co3O4 in the composite was assumed to have a cubic Fd-3m structure. The lattice parameter of the Co3O4 structure
significantly increased with the aging, compared to that of the pristine sample. The lattice expansion was due to the S incorporation into the Co3O4 lattice. After the sulfur was loaded and then annealed, a substantial change in morphology was observed, while the composite maintained the microspherical morphology with geometrical holes (Fig. 5 (a) and 5 (b)). The amount of sulfur in the Co3O4–sulfur composite was approximately 90%, according to the TGA of the composite, as shown in Fig. 5 (c). The samples with Co3O4 obtained using PP11 and diazabicyclo octane exhibited abrupt weight changes around 250 °C. The mass losses of the two samples were similar when the temperature increased to 500 °C, which well agrees with previous results.[21,27] In our result, the weight of fishing-net-shaped Co3O4/S was substantially change after 250 oC with the sulfur loading was set to 90 wt%. This suggests that the porous Co3O4 incorporated composite cathode is an efficient structure as a sulfur host that can load a high amount sulfur and support the maximal amount of sulfur for effective participation in the electrochemical reaction. Nevertheless, the individual Co3O4/S particles were distinctly separated, unlike in the composite sample obtained using diazabicyclo octane in which the particles strongly melt-agglomerated with elemental sulfur. Fig. 6 shows galvanostatic discharge profiles of the first cycle at different current rates and corresponding charge/discharge profiles with cycles. For comparison, the composite cathode with Co3O4 particles prepared using diazabicyclo octane, in which sulfur was fully aggregated with Co3O4, was also evaluated under the same conditions. The rate-variable charge/discharge profiles, measured at 0.1, 0.5, and 1 C (1 C = 1675 mA h g-1), for the corresponding composite cathodes showed well-defined plateaus. Although a relatively high discharge capacity of 1450 mA h g-1 was observed at 0.1 C, the capacities of the composite cathode of Co3O4 obtained using diazabicyclo octane rapidly decreased to 960 mA h g-1 at 0.5 C and 770 mA h g-1 at 1 C, corresponding to a capacity retention of ~50% compared to
the current density of 0.1 C. The significantly decreased charge capacity is attributed to the highly activated shuttle phenomena for the LiSx polysulfides where the polysulfide reservoirs, Co3O4 particles, were not fully activated within the agglomerated Co3O4/ sulfur composite cathodes. On the other hand, the fishing-net-shaped Co3O4-incorporated composite cathode had an initial capacity of 1175 mA h g-1 at 0.1 C, which was lower than that of the sulfur cathode with Co3O4 particles prepared using diazabicyclo octane. It provided a stable discharge capacity above 1000 mA h g-1 at 0.5 C with a significantly smaller capacity degradation even at a current density as high as 1 C (930 mA h g-1). Additionally, it is clear that the capacity fading with cycles for the given composites incorporating fishing-net-shaped Co3O4 were much stable during the successive electrochemical reactions as shown in Fig. 6 (c). To confirm the cycle stability for the given composite electrodes, we investigate the comparative analysis between the cycling capacity retention and the EIS spectra. We used the EIS results to visualize the constituent resistance components (diameter of the semicircular arc in the Nyquist plot of complex impedance) for the given Co3O4/S composites within the successive cyclic repetitions. As shown in Fig. 7, spectra for the two samples are similar with a semicircle in the high-frequency range and a linear plot for the Warburg region (for low-frequency range). Actually, the diameter of semicircle (followed by the intersection in Zreal-axis) for the PP11/sulfur electrode is much smaller overall cyclic reaction than the composite sample obtained using diazabicyclo octane electrode, showing improved electron conductivity. Rather stable diameter change in each semicircles in the high-frequency region denotes enhanced Li+ ion migration induced by the homogeneous mixture of the active center (sulfur) and polysulfide adsorbent (Co3O4). The results clearly show that the stable electrochemical reaction within the Co3O4/sulfur composite electrode with cyclic redox reactions may be due to the
controlled designed porous polysulfide adsorbent, Co3O4, with a coaggregated framework, which maximize the capturing efficiency for the mobile polysulfides. More effective capturing area for the fishing-net-shaped polysulfide adsorbent inhibiting the shuttle effect leads to small charge transfer resistance across the composite electrode, which finally boosted the electrochemical kinetics of the lithium insertion/extraction processes. Fig. 8 shows the cycling stabilities of the composite cathodes measured at 0.1 C. The corresponding galvanostatic charge/discharge profiles, as shown in sFig. 4, of the composite electrodes exhibited two discharging plateaus, around 2.3 and 2.1 V, and two overlapping charging plateaus, similar to the results in previous studies.[14,18] The PP11/sulfur composite cathode provided a relatively low initial discharge capacity of 1175 mA h g-1 compared to that of the sulfur cathode with Co3O4 particles prepared using diazabicyclo octane (1450 mA h g-1), while the capacity fading behavior was favorable, reaching a value above 300 mA h g-1 after 75 cycles. We can only succeed in getting the SEM image of the sulfur cathode after 75 cycles for PP11 samples. As you can see it, sFigure. 5, the hole-embedded morphology is still preserved, while the overall particles are swollen compared to the cathode before cycle, Fig. 5(a). Similar to the C-rate behavior, even when the initial capacity was lower, the capacity retention of the composite cathode incorporated with the fishing-net-shaped Co3O4 particles was satisfactory. The enhanced cycling behavior can be attributed to the fishing-net-like structure of the polysulfide reservoir/sulfur composite, which could alleviate the detrimental precipitation/decomposition of the active material, elemental sulfur, during cycling as an “effective” reservoir with a high capture efficiency for the residual polysulfides. The inner spacious microspheres not only prevented the agglomeration with sulfur upon the melt diffusion but also structurally restricted soluble lithium polysulfides within the spherical structure, while the pore-embedding nanosheets with a large surface area chemically entrapped polysulfides.
Further, an XPS analysis was carried out to confirm the absorption ability of the fishingnet-shaped Co3O4 for lithium polysulfides (Fig. 9). All peaks were calibrated using the C 1s peak at 284.3 eV and were fitted to a Gaussian spectrum. Both composite electrodes were tested after 50 cycles (in the same way as the electrodes after cycle reactions, as shown in Fig. 8). Four major peaks around 162.7, 164.0, 167.5, and above 169.0 eV were detected, corresponding to Co-S, bridging sulfur, polysulfides and LiTFSI, respectively.[8,11,27] The peaks at high binding energies, particularly at 167.5 eV, were enhanced after the cycling of the PP11/S composite cathode, which indicated conversion of most of the lithium polysulfides and increased ratio of low-order polysulfides. A substantially increased contribution from a higher bonding energy above 167.0 eV, which showed the existence of S in higher valence states induced by the strong chemical adsorption between metal oxides (acting as a polysulfide trap) and LixSn.[9,11] Therefore, the tested composite cathode with the fishing-net-shaped-Co3O4-embedded sulfur nanoparticles acted as an efficient polysulfide reservoir, improving the electrochemical reactions between Li+ ions and elemental S.
4. CONCLUSIONS Composites consisting of fishing-net-shaped cobalt oxide microspheres incorporated with a highly loaded sulfur (approximately ~90 wt%) were fabricated and evaluated as sulfur cathodes. With the numerous nanopore-embedded cobalt oxide microspheres, the corresponding hierarchical composites and phase-pure crystal structure facilitated the adsorption of polysulfides and Li+ electrochemical reaction. The characteristic Co–O nano/microarchitectures could be induced by the directional electrostatic interaction within the single lone pair in the nitrogen group of the organic surfactants considered as structure-directing agents, which had a crucial role in the formation of the unique particle morphology. The composite cathode exhibited a stable capacity retention with a high Coulombic efficiency
(99%), controlled capacity fading, and reversible capacity of 925 mA h g−1 at 1 C, which was 22% higher than that of the conventional nanoparticulate cobalt oxide/sulfur composite electrode. Therefore, the strong chemical adsorption of polysulfides to the reservoir medium, cobalt oxide, in the composite mixed with sulfur could be realized by adjusting the synergistic effect of the morphology and composition, in which the active pore sites are sufficiently stable during the repetitive cyclic reaction.
Acknowledgements This work was financially supported by the R&D Convergence Program (CAP-15-02KBSI) of NST (National Research Council of Science & Technology) of Republic of Korea, Republic of Korea and also supported by the key projects of Korea Research Institute of Chemical Technology
References
[1] B. Liu, J. G. Zhang, W. Xu, Advancing Lithium Metal Batteries, Joule 2 (5) (2018) 833845. [2] J. W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nature Reviews Materials 1 (4) (2016) 16013. [3] A. Manthiram, Y. Fu, Y. S. Su, Challenges and Prospects of Lithium-Sulfur Batteries, Accounts of Chemical Research 46 (5) (2013) 1125-1134. [4] G. Xu, B. Ding, J. Pan, P. Nie, L. Shen, X. Zhang, High performance lithium-sulfur batteries: advances and challenges, J. Mater. Chem. A 2 (32) (2014) 12662-12676. [5] Z. Lin, C. Liang, Lithium-sulfur batteries: from liquid to solid cells, J. Mater. Chem. A (3) (2015) 936-958. [6] G. Li, Z. Chen, J. Lu, Lithium-Sulfur Batteries for Commercial Applications, Chem 4 (1) (2018) 3-7. [7] J. Wang, Y. S. He, J. Yang, Sulfur-Based Composite Cathode Materials for High-Energy Rechargeable Lithium Bateries, Adv. Mater 27 (3) (2015) 569-575. [8] C. Zha, D. Wu, T. Zhang, J. Wu, H. Chen, A facile and effective sulfur loading method: Direct drop of liquid Li2S8 on carbon coated TiO2 nanowire arrays as cathode towards commercializing lithium-sulfur battery, Energy Storage Mater. 17 (2019) 118-125. [9] C. Zha, F. Yang, J. Zhang, T. Zhang, S. Dong, H. Chen, Promoting polysulfide redox reactions and improving electronic conductivity in lithium-sulfur batteries via hierarchical cathode materials of graphene-wrapped porous TiO2 microspheres with exposed (001) facets, J. Mater. Chem. A 6 (2018) 16574-16582. [10] X. Liang, C. Y. Kwok, F. L. Marzano, Q. Pang, M. Cuisinier, H. Huang, C. J. Hart, D. Houtarde, K. Kaup, H. Sommer, T. Brezesinski, J. Janek, L. F. Nazar, Lithium-Sulfur
Batteries: Tuning Transition Metal Oxide-Sulfur Interactions for Long Life Lithium Sulfur Batteries: The “Goldilocks” Principle, Adv. Energy Mater 6 (6) (2016) 1501636. [11] C. Zha, X. Gu, D. Wu, H. Chen, Interfacial active fluorine site-induced electron transfer on TiO2 (001) facets to enhance polysulfide redox reactions for better liquid Li2S6-Based lithium-sulfur batteries, J. Mater. Chem. A 7 (2019) 6431-6438. [12] X. Zeng, X. Gao, G. Li, M. Sun, Z. Lin, M. Ling, J. Zheng, C. Liang, Conductive molybdenum carbide as the polysulfide reservoir for lithium-sulfur batteries, J. Mater. Chem. A 6 (35) (2018) 17142-17147. [13] X. Zhou, C. Zhang, M. Zhang, A. Feng, S. Qu, Y. Zhang, X. Liu, Z. Jia, G. Wu, Synthesis of Fe3O4/carbon foams composites with broadened bandwidth and excellent electromagnetic wave absorption performance, Composites Part A 127 (2019) 105627. [14] Z. Jia, Z. Gao, A. Feng, Y. Zhang, C. Zhang, G. Nie, K. Wang, G. Wu, Laminated microwave absorbers of A-site cation deficiency perovskite La0.8FeO3 doped at hybrid RGO carbon, Composites Part B 176 (2019) 107246. [15] A. Feng, M. Ma, Z. Jia, M. Zhang, G. Wu, Fabrication of NiFe2O4@carbon fiber coated with phytic acid-doped polyaniline composite and its application as an electromagnetic wave absorber, RSC Adv. 9 (2019) 25932-25941. [16] W. Li, J. Liu, D. Zhao, Mesoporous materials for energy conversion and storage devices, Nature Reviews. Materials 1 (6) (2016) 1-17. [17] H. Wang, T. Zhou, D. Li, H. Gao, A. Du, H. Liu, Z. Guo, Ultrathin Cobaltosic Oxide Nanosheets as an Effective Sulfur Encapsulation Matrix with Strong Affinity Toward Polysulfides, ACS Appl. Mater. Interfaces 9 (5) (2017) 4320-4325. [18] Z. Chang, H. Dou, B. Ding, J. Wang, Y. Wang, X. Hao, D. R .MacFarlane, Co3O4 nanoneedle arrays as a multifunctional “super-reservoir” electrode for long cycle life LiS batteries, J. Mater. Chem. A 5 (1) (2017) 250-257.
[19] J. Guo, X. Zhang, X. Du, F. Zhang, A Mn3O4 nano-wall array based binder-free cathode for high performance lithium-sulfur batteries, J. Mater. Chem. A 5 (14) (2017) 64476454. [20] S. Evers, T. Yim, L. F. Nazar, Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li-S Battery, J. Phys. Chem. C 116 (37) (2012) 19653-19658. [21] R. Ponraj, A. G. Kannan, J. H. Ahn, D. W. Kim, Improvement of Cycling Performance of Lithium-Sulfur Batteries by Using Magnesium Oxide as a Functional Additive for Trapping Lithium Polysulfide, ACS Appl. Mater. Interfaces 8 (6) (2016) 4000-4006. [22] J. K. Kim, J. Y. Ju, S. K. Choi, S. Unithrattil, S. S. Lee, Y. K. Kang, Y. S. Kim, W. B. Im, S. H. Choi, In-situ preparation and unique electrochemical behavior of poreembedding CoO/Co3O4 intermixed composite for Li+ rechargeable battery electrodes, J. of Power Sources 378 (2018) 562–570. [23] R. Wu, X. Qian, K. Zhou, J. Wei, J. Lou, P. M. Ajayan, Porous Spinel ZnxCo3-xO4 Hollow Polyhedra Templated for High-Rate Lithium-Ion Batteries, ACS Nano 8 (6) (2014) 6297-6303. [24] G. Li, C. Xie, Z. Shen, Z. Chang, X. Bu, Cobalt oxide 2D nano-assemblies from infinite coordination polymer precursors mediated by a multidentate pyridyl ligand, Dalton Trans., 45 (2016) 7866-7874. [25] F. Wang, Z. Wen, C. Shen, K. Rui, X. Wu, C. Chen, Open mesoporous spherical shell structured Co3O4 with highly efficient catalytic performance in Li-O2 batteries, J. Mater. Chem. A, 3 (2015) 7600-7606. [26] A. C. Larson, R. B. Von Dreele, General Structure Analysis System(GSAS), Los Alamos National Laboratory Report LAUR, 1994, 86.
[27] X. Qian, L. Jin, L. Zhu, S. Yao, D. Rao, X. Shen, X. Xi, K. Xiao, S. Qin, CeO2 nanodots decorated ketjen black for high performance lithium-sulfur batteries, RSC Advances 6 (112) (2016) 111190-111196.
Figures
Figure 1. XRD patterns of the as-synthesized cobalt oxide compounds with various organic surfactants: (a) PP11, (b) PP13, (c) PP31, and (d) diazabicyclo octane. (e) Reference data of the cubic Co3O4 (JCPDS 073-1701).
Figure 2. SEM images of the cobalt oxides synthesized with the different organic surfactants: (a)(b) PP11, (c)(d) PP13, (e)(f) PP31, and (g)(h) diazabicyclo octane.
Figure 3. Enlarge XRD patterns of the sulfur cathodes with and without Co3O4 after the melt-diffusion annealing: with (a) PP11, (b) diazabicyclo octane and (c) without encapsulants. Reference data of elemental S8 (JCPDS 01-078-1889) and traceable Co3O4 phase (marked as ♦) also denoted.
Figure 4. Rietveld refinement results of the synchrotron diffraction profiles of S8 + Co3O4 (a) before and (b) after the aging. The dots represent the observed intensities, while the solid line represents the calculated intensities. A difference (observed − calculated) plot is shown below them. The panels on the top show the calculated Bragg positions of the Co3O4 and S8 structures.
Figure 5. SEM images of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane. (c) Corresponding TGA curves of the composites.
Figure 6. Discharge voltage profiles of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane measured at different current densities. Corresponding charge/discharge profiles with cycles are presented at (c) and (d).
Figure 7. Representative EIS curves of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane (@ 1C current density).
Figure 8. Comparison of the cycle performances of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane and (c) without encapsulants.
Figure 9. Deconvoluted sulfur 2p XP spectra of the sulfur composite cathode after 50 charge/discharge cycles; with (a) PP11 and (b) diazabicyclo octane. Each spectrum is fitted with deconvoluted peaks (blue, green and magenta) for differently bonded sulfur atoms; the sum of the fitting curves (red) is consistent with the raw data (black).
Tables
Table 1. Specific surface areas and pore volumes of the polysulfide reservoirs synthesized with the different organic surfactants.
Table 2. Rietveld refinement and crystal parameter data of Co3O4. The numbers in parentheses are the estimated standard deviations of the least-significant figure.
Table 1.
Surface Area
Pore Volume
Single point adsorption total
surfactant* BET model (m2 g-1)
Langmuir model 2
-1
(m g )
pore volume of pores (cm2 g-1)
PP 1:1
43.1206
60.0857
0.089529
PP 1:3
18.3077
20.1243
0.123514
PP 3:1
17.1381
19.4505
0.099781
DABCO
138.2581
195.7793
0.510006
Table 2.
S8 + Co3O4 formula pristine
after melt-diffusion
radiation type
synchrotron
2θ range (degree)
10-75
T/K
295
symmetry
cubic
space group
Fd-3m
Z
8
a=b=c/Å
8.07782(15)
8.08297(14)
V/Å3
527.088(29)
528.096(27)
Rp
8.83 %
8.91 %
Rwp
12.41 %
11.86 %
χ2
6.15
4.28
Research Highlights
Optimum pore and morphology controlled spherical Co3O4 for polysulfide reservoir Stable capacity retention for the given Co3O4 microsphere incorporated S cathode Propose conceptual design of cobalt oxide compounds for the novel electrode material
Footnote
# These authors contributed equally to this work. * Corresponding authors. Tel: 82–42–860–7372. E-mails: [email protected] (Won Bin Im) [email protected] (Sungho Choi)
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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DOI:
https://doi.org/10.1016/j.matchemphys.2019.122567
Reference:
MAC 122567
To appear in:
Materials Chemistry and Physics
Received Date: 14 October 2019 Revised Date:
4 December 2019
Accepted Date: 17 December 2019
Please cite this article as: J.K. Kim, H. Park, S.S. Lee, S.U. Son, Y. Kang, W.B. Im, S. Choi, Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/ j.matchemphys.2019.122567. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement
Jin Kyu Kim is “Formal Analysis” and “Investigation”. Hyemin Park is “Validation” and “Investigation”. Sun Sook Lee is “Data Curation”. Seung Uk Son is “Supervision”. Yongku Kang is “Supervision” and “Project Administration”. Won Bin Im is “Data Curation” and “Writing-Review & Editing”. Sungho Choi is “Conceptualization”, “Writing-Original Draft” and “Supervision”.
Fishing-net-shaped cobalt oxide microspheres for effective polysulfide reservoirs of rechargeable Li–S battery cathodes
Jin Kyu Kima#, Hyemin Parkb,c#, Sun Sook Leeb, Seung Uk Sonc, Yongku Kangb, Won Bin Imd*, and Sungho Choib*
a
Advanced Automotive Battery Division, LG Chem Research Park, 188 Munji-ro, Yuseong,
Daejeon, Republic of Korea b
Advanced Materials Division, Korea Research Institute of Chemical Technology, 141
Gajeongro, Yuseong, Daejeon, Republic of Korea c
Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea
d
Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro,
Seongdong-gu, Seoul 04763, Republic of Korea
Keywords: porous nanoparticle, cobalt oxide, sulfur reservoir, cathode, lithium–sulfur battery
*Corresponding authors. Tel: 82–42–860–7372. E-mails: [email protected] (Won Bin Im) [email protected] (Sungho Choi)
Abstract Hole-embedded one-directional two-dimensional sheet-assembly cobalt oxide microspheres are synthesized and assessed as effective encapsulation media for polysulfides in cathodes of lithium–sulfur batteries. The cathode of fishing-net-shaped cobalt oxide compounds incorporated with sulfur provides exceptional electrochemical properties such as a high Coulombic efficiency (99%), stable capacity fading, and a reversible capacity of 925 mA h g−1 at 1 C, which is 22% higher than that of the conventional nanoparticulate cobalt oxide/sulfur electrode. The enhanced electrochemical behavior is attributed to the effective polysulfide adsorbent and hierarchically designed porous cobalt oxide with a coaggregated framework. The controlled pore-embedded particle size is an essential parameter determining the capture of the mobile polysulfides, rather than the use of numerous mere highly porous encapsulants, such as nanoparticulate cobalt oxides.
1. INTRODUCTION The current lithium ion batteries (LIBs) have limited energy densities and thus could not meet the increasing demands for various portable devices and electric vehicles. In this context, the electrochemical reactions between lithium metal and chalcogen ions in Li batteries based on sulfur (S) and oxygen (O) are promising for next-generation energy storage devices. Recently, there has been a remarkable development of novel materials as alternatives to the conventional intercalation electrodes. Graphite and layered oxides for state-of-the-art LIBs, i.e., S cathode for the Li–S battery and air electrode for the Li–air battery and stable Li metal, have facilitated the commercialization of high-energy-density Li rechargeable batteries.[1,2] The rechargeable Li–S battery is considered for application in the upcoming energy storage device with an elemental sulfur cathode and anode, which is electrochemically reactive with metallic Li. It has various significant advantages, such as high theoretical specific capacity (~ 1675 mA h g-1) and energy density (~ 2600 W h kg-1), which is approximately four times higher than those of the current LIBs, low cost, and abundance in the nature along with the environmental benignity of S.[3–6] For commercialization of Li−S batteries, several issues need to be overcome including the low electronic conductivity and detrimental volume change of the S cathode in the repetitive charge/discharge cycles as well as the dissolution of mobile polysulfide byproducts and related shuttle effects.[3,7] Therefore, the utilization of activated S followed by the reversible electrochemical reaction with Li+ cannot be commercialized with the current techniques. Extensive studies have been carried out to address the above issues.[7–12] Representative issues are the controlled design of nanostructured cathodes using large-surface-area conductive carbon-based materials, infiltration of S, and addition of reactive reservoirs capturing the mobile polysulfide intermediates. Generally, the latter can be achieved using a
physically/chemically active porous host matrix with a high affinity to the polysulfides, which leads to an enhanced capacity retention. In principle, the large surface areas achieved by nanoparticles and large pore volumes can provide many reactive sites through surface and/or interface-related reactions such as gas adsorption/desorption, radiation protection, and energy conversion.[13-15] Nevertheless, the extraordinary particle property does not fully guarantee the optimal performances in various applications. Additionally, a cost-effective large-scale production is needed to obtain monodisperse/uniform nanoparticles.[16] To better understand the shuttle effect, various compositions and morphology-controlled inorganic compounds, such as TiO2, MgO, Mn3O4, and Co3O4,[17–21] were applied to the shuttling entrapments through strong chemical bondings. Among them, the spinel-structure cobalt oxide is a representative electrochemically active material within the Co–O binary system with a mixed valence state of Co2+/Co3+ and diverse polymorphic crystalline structures. Characteristic nanocrystals with various shapes and/or composite forms, such as graphene-supported nanoparticle cobalt oxide, and very recently, we reported an in-situ synthesis of bicomposition CoOx, which provided unique electrochemical behaviors of porous CoO/Co3O4 compounds for Li rechargeable battery electrodes.[22] Nevertheless, cobalt oxides, used as catalytic agents for Li–S batteries with a morphology effect and electrochemical behavior, have rarely been reported. In this study, we synthesized microspherical cobalt oxide particles, which had omnidirectional coaggregated platelets with numerous geometrical holes acting as effective encapsulation media for polysulfides in lithium−sulfur battery cathodes. The morphology of the electrode material normally affects the surface/volume ratio, effective density of electrochemical reaction sites, drift/diffusivity of ions and electrons, and volume expansion during the Li+ transport within the reversible electrochemical reactions. Additionally, the utilization of specific precursors with coordination polymers forming the desired
nanomaterials can be achieved by a self-assembly process of the compound based on the coordination interactions between metal ions and organic surfactants in the presence of coordinating modulating agents.[23–25] We aimed to determine the efficiencies of the porosities and/or incorporated pore structures of the encapsulants, which exhibited strong chemical interactions with polysulfides and alleviated the shuttle effects caused by long-chain soluble species. The high chemical affinity of the cobalt oxide particles to the polysulfide intermediates effectively suppressed the diffusion of LixSn byproducts and improved the electrochemical stability with controlled capacity fading of the considered cathode. The poreembedded controlled particle morphology was more important for the effective polysulfide capturing center than the use of only a medium of nanoparticulate high-porosity encapsulants as in mesoporous cobalt oxides.
2. EXPERIMENTAL METHODS 2.1 Syntheses of cobalt oxide and composite Fishing-net-like and/or nanocrystal cobalt oxides were prepared by a solvothermal reaction. The appropriate amount of cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98%, Junsei Chemical) was dissolved in 30 mL of methanol (reagent grade, 99.9%, Samchun). We then prepared organic-surfactant-mixed methanol solutions with different amine surfactants, 1.4diazabicyclo [2,2,2] octane (C6H12N2, ≥99%, Sigma Aldrich, DABCO), pyrazine (C4H4N2, ≥99%, Sigma Aldrich), and pyridazine (C4H4N2, 98%, Sigma Aldrich). Finally, the two stock solutions were thoroughly mixed and stirred for 1 h at room temperature. The as-prepared precursor solution was transferred to a teflon tube for a treatment in a microwave reactor. Microwaves were applied to proceed a solvothermal reaction at 160 °C for 30 min. The resulting powder was centrifuged to remove the methanol solvent and washed several times with a distilled water/ethanol solution. After the drying process, the as-obtained powder was
calcined at a temperature of 250–500 °C in nitrogen for 30 min and then in oxygen for 30 min. The as-synthesized cobalt oxides with different molar ratios (pyrazine:pyridazine) of the surfactants are denoted as PP11 (1:1), PP13 (1:3), and PP31 (3:1). Specific synthetic procedures of the as-synthesized Co3O4 particles with using various surfactants are summarized in sTable. 1. To form the composite with sulfur, we synthesized the nanoparticle sulfur compound in advance. The appropriate amounts of polyvinylpyrrolidone ((C6H9NO)n, MW ~ 40,000, Sigma Aldrich) and sodium thiosulfate (Na2S2O3, ≥99.99%, Sigma Aldrich) were sequentially dissolved in 40 mL of distilled water. Subsequently, a 5% dilute HCl solution was added to the as-prepared solution to synthesize nanoscale sulfur, followed by stirring for 30 min. The solvent was then removed by centrifugation and the acidic solvent was washed with distilled water until it became neutral. A drying process was then carried out at a low temperature of 60 °C to prevent vaporization of the sulfur. The corresponding cobalt oxide, dispersed in 40 mL of distilled water, was mixed with as-prepared sulfur followed by stirring for 30 min. Finally, the whole solvent was removed by centrifugal separation and then dried at room temperature in vacuum for 12 h. The resulting sulfur–cobalt oxide composite powder was subjected to melt diffusion to obtain a final composited powder. The powder was placed in a sealed vial and heated in a vacuum container at 150 °C for 12 h, yielding a powder used as a cathode material for a lithium–sulfur battery. 2.2 Material characterization The crystalline structures of the as-prepared particles were analyzed by powder X-ray diffraction (XRD, Rigaku D/Max-2200V Diffractometer) with Cu Kα radiation. The particle sizes and morphologies of the cobalt oxide compounds were observed by field-emissionscanning electron microscopy (FE-SEM, Tescan Mira 3 LMU FEG) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F30 S-Twin, 200 kV). X-ray
photoelectron spectroscopy (XPS, K-Alpha Thermo Scientific) was used to evaluate the chemical states of the elements. A thermogravimetric analysis (TGA) was carried out to evaluate the sulfur content at a heating rate of 5 °C/min in nitrogen. Synchrotron diffraction data of the pristine and aged samples were acquired in the Pohang Accelerator Laboratory using radiation at a calibrated wavelength of 1.5178 Å over the angular range of 15° ≤ 2θ ≤ 80°, with a step size of 0.01°. Structural data of the samples were extracted using the diffraction data through Rietveld refinement using the general structure analysis system (GSAS) program.[26] A split pseudo-Voigt-function was used to fit the diffraction profile, while a shifted Chebyshev polynomial was used to define the background. Refinements were carried out using least-square methods until reasonable goodness-of-fit parameters were obtained. 2.3 Preparation of a coin cell and electrochemical analysis To evaluate the electrode behavior of the as-prepared cobalt oxide/sulfur composite, the cathode material was composed of cobalt oxide/sulfur powder, conductive carbon (Super-P) and polymer binder (polyvinylidene fluoride, Kureha KF-1100) in a weight ratio of 7:2:1 and then dissolved in an organic solvent (99.5%, N-methyl-2-pyrrolidone, Sigma Aldrich). Lithium metal was used as the counter electrode (anode) while 1 M of lithium bis(trifluoromethylsulfonyl)imide in a mixture of 1,3-dioxolane and dimethoxyethane (equivolume ratio) was used as a Li+ electrolyte. The components were assembled into 2032type coin cells in an argon-filled glove-box. Galvanostatic charge–discharge cycles were carried out under specific current densities in the voltage window of 1.8 to 2.6 V vs. Li/Li+ using a TOYO Toscat 3100U battery test system. Electrochemical impedance spectroscopy (EIS) was performed by applying an AC signal from 0.01 Hz to 100 kHz with 5 mVpp using an impedance analyzer (Biologic, Netherland).
3. RESULTS AND DISCUSSION The crystal phases of the as-synthesized cobalt oxide compounds with various organic surfactants were analyzed by powder XRD, as shown in Fig. 1. Based on the diffraction angles and intensities, all diffraction peaks were assigned to the cubic Co3O4 (Joint Committee on Powder Diffraction Standards (JCPDS) 073-1701), while the crystallinities of the samples slightly differed. For the samples obtained with mixed surfactants, pyrazine : pyridazine = 1:1, 1:3 and 3:1 molar ratio, we obtained sharper diffraction peaks, which indicated the high crystallinities of the powder samples. remove references results. Therefore, we speculate that the crystallinities of the novel particles as well as the morphology-inherited shapes substantially changed by choosing the precursors with metal ions coordinated to specific organic molecules. We evaluated the characteristic particle morphology for the as-synthesized compounds with various organic surfactants. As mentioned above, the employed surfactants, pyrazine, pyridazine, and diazabicyclo octane, are rigid organic molecules formed with directionally coordinated nitrogen atoms. Therefore, we expect that they can form specific sacrificial templates, which effectively capture elemental sulfur. As preliminary results, as shown in sFig. 1 (a)–(d), the particle morphologies substantially change with the structural diversity of the organic moieties particularly in the precursors with and without heat treatment. A rather porous morphology was obtained in the case of pyrazine, while a spherical morphology with rough-faced surface shapes was observed in the case of the pyridazine surfactant. By mixing the surfactants in different molar ratios, we synthesized surface-controlled particles, as shown in Fig. 2 (a)–(f). It is worth noting that the particles with porous, spherical, and texturedsurface-inherited morphologies could be modified and that uniformly distributed particles were successfully obtained. We observed a textured surface with numerous platelets (thickness < 100 nm) packed in all directions and composed of tiny nanopores with sizes of
approximately 50 nm. According to the molar ratio, Co3O4 should have a symmetric spherical shape constructed by thin platelets with interparticle nanopores distributed throughout the surfaces. This was observed more clearly for the equimolar mixing of the two organic surfactants, pyrazine and pyridazine. We can understand the possible formation mechanism in line with the solvothermal selfassembly process proposed in other studies. Wang et al. reported a porous spherical shellstructured Co3O4 by oligomerization with glycol followed by a heat treatment, of which selfassembled nanosheet cobalt-oligomers gradually transformed into the desired structures and finally converted to the spherical-shell-coated CoOx particles.[25] Such an omnidirectional fishing-net-shaped structure constructed by thin moieties is beneficial to provide sufficient reaction sites to facilitate the reversible electrochemical reactions and efficient scraping of electrolyte soluble polysulfide byproducts. We also prepared highly porous (irregular and roughly agglomerated) Co3O4 nanoparticles using diazabicyclo octane, as shown in Fig. 2 (g), (h), and sFig. 1 (e). The corresponding Co3O4 particles are porous particles; therefore, it was useful to evaluate the surface area by a gas adsorption/desorption Brunauer–Emmett–Teller (BET) analysis. The measured BET specific surface areas of the corresponding nanoparticles are presented in Table 1. N2 adsorption/desorption isotherms and pore size distribution graphs are also presented in sFig. 2. The as-prepared Co3O4 particles obtained using only diazabicyclo octane exhibited a significantly larger (three times) specific surface area than those of the other samples with pyrazine and pyridazine mixed precursor solutions, 138 m2 g-1 and smaller than 43 m2 g-1, respectively. Intuitively, the porous media are very effective to grab the residual polysulfides during the reversible electrochemical reactions with a small diffusion length for a rapid lithium-ion and electron transport. Nevertheless, the controlled surface morphology with appropriate pore volumes of the particles was more effective for the polysulfide
reservoirs in Li–S battery cathodes, as illustrated by the following electrochemical data. Therefore, below, we focus mainly on the PP11 compound having a rather spherical shape with the appropriate surface/volume ratio compared to the sample grown using diazabicyclo octane. To maximize the contact area between the active sulfur and polysulfide reservoir (Co3O4), the Co3O4 particles were thoroughly mixed with elemental sulfur particles followed by heating in a sealed reactor. In the melt procedures, the vaporization and/or quantification of the residual sulfur content are important to implement the controlled Co3O4– sulfur composite cathode. As shown in Fig. 3 and sFig. 3, enlarged at low angle diffraction angles and wide angel scan ranges respectively, the peaks of pure elemental sulfur are attributed to an orthorhombic structure (JCPDS 01-078-1889) with traceable amounts of Co3O4 matched to the spinel-structured Co3O4, as shown in Fig. 1. The main peaks at 31.3 and 36.8° (denoted as ♦ in red) are attributed to the Co3O4 particles in the composite, while some peaks of highindex planes became weak and even disappeared within the Co3O4– sulfur composite suggesting that meltdown sulfur was incorporated into the Co3O4 matrix. To confirm the phase stability of the composite after the melt procedure, we carried out a synchrotron XRD analysis followed by Rietveld refinements. In order to obtain structural details of the composites, Rietveld refinement was performed on both samples, as shown in Fig. 4. In the refinement cycles, all parameters except the occupancy factors of the elements were iteratively refined. The refinement was converged to obtain reasonable goodness-of-fit parameters; the obtained structural parameters are presented in Table 2. The prominent phase in both samples was elemental S8 assuming an orthorhombic Fddd space group, with estimated weight percentages of 88.6% and 86.5%, respectively. The Co3O4 in the composite was assumed to have a cubic Fd-3m structure. The lattice parameter of the Co3O4 structure
significantly increased with the aging, compared to that of the pristine sample. The lattice expansion was due to the S incorporation into the Co3O4 lattice. After the sulfur was loaded and then annealed, a substantial change in morphology was observed, while the composite maintained the microspherical morphology with geometrical holes (Fig. 5 (a) and 5 (b)). The amount of sulfur in the Co3O4–sulfur composite was approximately 90%, according to the TGA of the composite, as shown in Fig. 5 (c). The samples with Co3O4 obtained using PP11 and diazabicyclo octane exhibited abrupt weight changes around 250 °C. The mass losses of the two samples were similar when the temperature increased to 500 °C, which well agrees with previous results.[21,27] In our result, the weight of fishing-net-shaped Co3O4/S was substantially change after 250 oC with the sulfur loading was set to 90 wt%. This suggests that the porous Co3O4 incorporated composite cathode is an efficient structure as a sulfur host that can load a high amount sulfur and support the maximal amount of sulfur for effective participation in the electrochemical reaction. Nevertheless, the individual Co3O4/S particles were distinctly separated, unlike in the composite sample obtained using diazabicyclo octane in which the particles strongly melt-agglomerated with elemental sulfur. Fig. 6 shows galvanostatic discharge profiles of the first cycle at different current rates and corresponding charge/discharge profiles with cycles. For comparison, the composite cathode with Co3O4 particles prepared using diazabicyclo octane, in which sulfur was fully aggregated with Co3O4, was also evaluated under the same conditions. The rate-variable charge/discharge profiles, measured at 0.1, 0.5, and 1 C (1 C = 1675 mA h g-1), for the corresponding composite cathodes showed well-defined plateaus. Although a relatively high discharge capacity of 1450 mA h g-1 was observed at 0.1 C, the capacities of the composite cathode of Co3O4 obtained using diazabicyclo octane rapidly decreased to 960 mA h g-1 at 0.5 C and 770 mA h g-1 at 1 C, corresponding to a capacity retention of ~50% compared to
the current density of 0.1 C. The significantly decreased charge capacity is attributed to the highly activated shuttle phenomena for the LiSx polysulfides where the polysulfide reservoirs, Co3O4 particles, were not fully activated within the agglomerated Co3O4/ sulfur composite cathodes. On the other hand, the fishing-net-shaped Co3O4-incorporated composite cathode had an initial capacity of 1175 mA h g-1 at 0.1 C, which was lower than that of the sulfur cathode with Co3O4 particles prepared using diazabicyclo octane. It provided a stable discharge capacity above 1000 mA h g-1 at 0.5 C with a significantly smaller capacity degradation even at a current density as high as 1 C (930 mA h g-1). Additionally, it is clear that the capacity fading with cycles for the given composites incorporating fishing-net-shaped Co3O4 were much stable during the successive electrochemical reactions as shown in Fig. 6 (c). To confirm the cycle stability for the given composite electrodes, we investigate the comparative analysis between the cycling capacity retention and the EIS spectra. We used the EIS results to visualize the constituent resistance components (diameter of the semicircular arc in the Nyquist plot of complex impedance) for the given Co3O4/S composites within the successive cyclic repetitions. As shown in Fig. 7, spectra for the two samples are similar with a semicircle in the high-frequency range and a linear plot for the Warburg region (for low-frequency range). Actually, the diameter of semicircle (followed by the intersection in Zreal-axis) for the PP11/sulfur electrode is much smaller overall cyclic reaction than the composite sample obtained using diazabicyclo octane electrode, showing improved electron conductivity. Rather stable diameter change in each semicircles in the high-frequency region denotes enhanced Li+ ion migration induced by the homogeneous mixture of the active center (sulfur) and polysulfide adsorbent (Co3O4). The results clearly show that the stable electrochemical reaction within the Co3O4/sulfur composite electrode with cyclic redox reactions may be due to the
controlled designed porous polysulfide adsorbent, Co3O4, with a coaggregated framework, which maximize the capturing efficiency for the mobile polysulfides. More effective capturing area for the fishing-net-shaped polysulfide adsorbent inhibiting the shuttle effect leads to small charge transfer resistance across the composite electrode, which finally boosted the electrochemical kinetics of the lithium insertion/extraction processes. Fig. 8 shows the cycling stabilities of the composite cathodes measured at 0.1 C. The corresponding galvanostatic charge/discharge profiles, as shown in sFig. 4, of the composite electrodes exhibited two discharging plateaus, around 2.3 and 2.1 V, and two overlapping charging plateaus, similar to the results in previous studies.[14,18] The PP11/sulfur composite cathode provided a relatively low initial discharge capacity of 1175 mA h g-1 compared to that of the sulfur cathode with Co3O4 particles prepared using diazabicyclo octane (1450 mA h g-1), while the capacity fading behavior was favorable, reaching a value above 300 mA h g-1 after 75 cycles. We can only succeed in getting the SEM image of the sulfur cathode after 75 cycles for PP11 samples. As you can see it, sFigure. 5, the hole-embedded morphology is still preserved, while the overall particles are swollen compared to the cathode before cycle, Fig. 5(a). Similar to the C-rate behavior, even when the initial capacity was lower, the capacity retention of the composite cathode incorporated with the fishing-net-shaped Co3O4 particles was satisfactory. The enhanced cycling behavior can be attributed to the fishing-net-like structure of the polysulfide reservoir/sulfur composite, which could alleviate the detrimental precipitation/decomposition of the active material, elemental sulfur, during cycling as an “effective” reservoir with a high capture efficiency for the residual polysulfides. The inner spacious microspheres not only prevented the agglomeration with sulfur upon the melt diffusion but also structurally restricted soluble lithium polysulfides within the spherical structure, while the pore-embedding nanosheets with a large surface area chemically entrapped polysulfides.
Further, an XPS analysis was carried out to confirm the absorption ability of the fishingnet-shaped Co3O4 for lithium polysulfides (Fig. 9). All peaks were calibrated using the C 1s peak at 284.3 eV and were fitted to a Gaussian spectrum. Both composite electrodes were tested after 50 cycles (in the same way as the electrodes after cycle reactions, as shown in Fig. 8). Four major peaks around 162.7, 164.0, 167.5, and above 169.0 eV were detected, corresponding to Co-S, bridging sulfur, polysulfides and LiTFSI, respectively.[8,11,27] The peaks at high binding energies, particularly at 167.5 eV, were enhanced after the cycling of the PP11/S composite cathode, which indicated conversion of most of the lithium polysulfides and increased ratio of low-order polysulfides. A substantially increased contribution from a higher bonding energy above 167.0 eV, which showed the existence of S in higher valence states induced by the strong chemical adsorption between metal oxides (acting as a polysulfide trap) and LixSn.[9,11] Therefore, the tested composite cathode with the fishing-net-shaped-Co3O4-embedded sulfur nanoparticles acted as an efficient polysulfide reservoir, improving the electrochemical reactions between Li+ ions and elemental S.
4. CONCLUSIONS Composites consisting of fishing-net-shaped cobalt oxide microspheres incorporated with a highly loaded sulfur (approximately ~90 wt%) were fabricated and evaluated as sulfur cathodes. With the numerous nanopore-embedded cobalt oxide microspheres, the corresponding hierarchical composites and phase-pure crystal structure facilitated the adsorption of polysulfides and Li+ electrochemical reaction. The characteristic Co–O nano/microarchitectures could be induced by the directional electrostatic interaction within the single lone pair in the nitrogen group of the organic surfactants considered as structure-directing agents, which had a crucial role in the formation of the unique particle morphology. The composite cathode exhibited a stable capacity retention with a high Coulombic efficiency
(99%), controlled capacity fading, and reversible capacity of 925 mA h g−1 at 1 C, which was 22% higher than that of the conventional nanoparticulate cobalt oxide/sulfur composite electrode. Therefore, the strong chemical adsorption of polysulfides to the reservoir medium, cobalt oxide, in the composite mixed with sulfur could be realized by adjusting the synergistic effect of the morphology and composition, in which the active pore sites are sufficiently stable during the repetitive cyclic reaction.
Acknowledgements This work was financially supported by the R&D Convergence Program (CAP-15-02KBSI) of NST (National Research Council of Science & Technology) of Republic of Korea, Republic of Korea and also supported by the key projects of Korea Research Institute of Chemical Technology
References
[1] B. Liu, J. G. Zhang, W. Xu, Advancing Lithium Metal Batteries, Joule 2 (5) (2018) 833845. [2] J. W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nature Reviews Materials 1 (4) (2016) 16013. [3] A. Manthiram, Y. Fu, Y. S. Su, Challenges and Prospects of Lithium-Sulfur Batteries, Accounts of Chemical Research 46 (5) (2013) 1125-1134. [4] G. Xu, B. Ding, J. Pan, P. Nie, L. Shen, X. Zhang, High performance lithium-sulfur batteries: advances and challenges, J. Mater. Chem. A 2 (32) (2014) 12662-12676. [5] Z. Lin, C. Liang, Lithium-sulfur batteries: from liquid to solid cells, J. Mater. Chem. A (3) (2015) 936-958. [6] G. Li, Z. Chen, J. Lu, Lithium-Sulfur Batteries for Commercial Applications, Chem 4 (1) (2018) 3-7. [7] J. Wang, Y. S. He, J. Yang, Sulfur-Based Composite Cathode Materials for High-Energy Rechargeable Lithium Bateries, Adv. Mater 27 (3) (2015) 569-575. [8] C. Zha, D. Wu, T. Zhang, J. Wu, H. Chen, A facile and effective sulfur loading method: Direct drop of liquid Li2S8 on carbon coated TiO2 nanowire arrays as cathode towards commercializing lithium-sulfur battery, Energy Storage Mater. 17 (2019) 118-125. [9] C. Zha, F. Yang, J. Zhang, T. Zhang, S. Dong, H. Chen, Promoting polysulfide redox reactions and improving electronic conductivity in lithium-sulfur batteries via hierarchical cathode materials of graphene-wrapped porous TiO2 microspheres with exposed (001) facets, J. Mater. Chem. A 6 (2018) 16574-16582. [10] X. Liang, C. Y. Kwok, F. L. Marzano, Q. Pang, M. Cuisinier, H. Huang, C. J. Hart, D. Houtarde, K. Kaup, H. Sommer, T. Brezesinski, J. Janek, L. F. Nazar, Lithium-Sulfur
Batteries: Tuning Transition Metal Oxide-Sulfur Interactions for Long Life Lithium Sulfur Batteries: The “Goldilocks” Principle, Adv. Energy Mater 6 (6) (2016) 1501636. [11] C. Zha, X. Gu, D. Wu, H. Chen, Interfacial active fluorine site-induced electron transfer on TiO2 (001) facets to enhance polysulfide redox reactions for better liquid Li2S6-Based lithium-sulfur batteries, J. Mater. Chem. A 7 (2019) 6431-6438. [12] X. Zeng, X. Gao, G. Li, M. Sun, Z. Lin, M. Ling, J. Zheng, C. Liang, Conductive molybdenum carbide as the polysulfide reservoir for lithium-sulfur batteries, J. Mater. Chem. A 6 (35) (2018) 17142-17147. [13] X. Zhou, C. Zhang, M. Zhang, A. Feng, S. Qu, Y. Zhang, X. Liu, Z. Jia, G. Wu, Synthesis of Fe3O4/carbon foams composites with broadened bandwidth and excellent electromagnetic wave absorption performance, Composites Part A 127 (2019) 105627. [14] Z. Jia, Z. Gao, A. Feng, Y. Zhang, C. Zhang, G. Nie, K. Wang, G. Wu, Laminated microwave absorbers of A-site cation deficiency perovskite La0.8FeO3 doped at hybrid RGO carbon, Composites Part B 176 (2019) 107246. [15] A. Feng, M. Ma, Z. Jia, M. Zhang, G. Wu, Fabrication of NiFe2O4@carbon fiber coated with phytic acid-doped polyaniline composite and its application as an electromagnetic wave absorber, RSC Adv. 9 (2019) 25932-25941. [16] W. Li, J. Liu, D. Zhao, Mesoporous materials for energy conversion and storage devices, Nature Reviews. Materials 1 (6) (2016) 1-17. [17] H. Wang, T. Zhou, D. Li, H. Gao, A. Du, H. Liu, Z. Guo, Ultrathin Cobaltosic Oxide Nanosheets as an Effective Sulfur Encapsulation Matrix with Strong Affinity Toward Polysulfides, ACS Appl. Mater. Interfaces 9 (5) (2017) 4320-4325. [18] Z. Chang, H. Dou, B. Ding, J. Wang, Y. Wang, X. Hao, D. R .MacFarlane, Co3O4 nanoneedle arrays as a multifunctional “super-reservoir” electrode for long cycle life LiS batteries, J. Mater. Chem. A 5 (1) (2017) 250-257.
[19] J. Guo, X. Zhang, X. Du, F. Zhang, A Mn3O4 nano-wall array based binder-free cathode for high performance lithium-sulfur batteries, J. Mater. Chem. A 5 (14) (2017) 64476454. [20] S. Evers, T. Yim, L. F. Nazar, Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li-S Battery, J. Phys. Chem. C 116 (37) (2012) 19653-19658. [21] R. Ponraj, A. G. Kannan, J. H. Ahn, D. W. Kim, Improvement of Cycling Performance of Lithium-Sulfur Batteries by Using Magnesium Oxide as a Functional Additive for Trapping Lithium Polysulfide, ACS Appl. Mater. Interfaces 8 (6) (2016) 4000-4006. [22] J. K. Kim, J. Y. Ju, S. K. Choi, S. Unithrattil, S. S. Lee, Y. K. Kang, Y. S. Kim, W. B. Im, S. H. Choi, In-situ preparation and unique electrochemical behavior of poreembedding CoO/Co3O4 intermixed composite for Li+ rechargeable battery electrodes, J. of Power Sources 378 (2018) 562–570. [23] R. Wu, X. Qian, K. Zhou, J. Wei, J. Lou, P. M. Ajayan, Porous Spinel ZnxCo3-xO4 Hollow Polyhedra Templated for High-Rate Lithium-Ion Batteries, ACS Nano 8 (6) (2014) 6297-6303. [24] G. Li, C. Xie, Z. Shen, Z. Chang, X. Bu, Cobalt oxide 2D nano-assemblies from infinite coordination polymer precursors mediated by a multidentate pyridyl ligand, Dalton Trans., 45 (2016) 7866-7874. [25] F. Wang, Z. Wen, C. Shen, K. Rui, X. Wu, C. Chen, Open mesoporous spherical shell structured Co3O4 with highly efficient catalytic performance in Li-O2 batteries, J. Mater. Chem. A, 3 (2015) 7600-7606. [26] A. C. Larson, R. B. Von Dreele, General Structure Analysis System(GSAS), Los Alamos National Laboratory Report LAUR, 1994, 86.
[27] X. Qian, L. Jin, L. Zhu, S. Yao, D. Rao, X. Shen, X. Xi, K. Xiao, S. Qin, CeO2 nanodots decorated ketjen black for high performance lithium-sulfur batteries, RSC Advances 6 (112) (2016) 111190-111196.
Figures
Figure 1. XRD patterns of the as-synthesized cobalt oxide compounds with various organic surfactants: (a) PP11, (b) PP13, (c) PP31, and (d) diazabicyclo octane. (e) Reference data of the cubic Co3O4 (JCPDS 073-1701).
Figure 2. SEM images of the cobalt oxides synthesized with the different organic surfactants: (a)(b) PP11, (c)(d) PP13, (e)(f) PP31, and (g)(h) diazabicyclo octane.
Figure 3. Enlarge XRD patterns of the sulfur cathodes with and without Co3O4 after the melt-diffusion annealing: with (a) PP11, (b) diazabicyclo octane and (c) without encapsulants. Reference data of elemental S8 (JCPDS 01-078-1889) and traceable Co3O4 phase (marked as ♦) also denoted.
Figure 4. Rietveld refinement results of the synchrotron diffraction profiles of S8 + Co3O4 (a) before and (b) after the aging. The dots represent the observed intensities, while the solid line represents the calculated intensities. A difference (observed − calculated) plot is shown below them. The panels on the top show the calculated Bragg positions of the Co3O4 and S8 structures.
Figure 5. SEM images of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane. (c) Corresponding TGA curves of the composites.
Figure 6. Discharge voltage profiles of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane measured at different current densities. Corresponding charge/discharge profiles with cycles are presented at (c) and (d).
Figure 7. Representative EIS curves of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane (@ 1C current density).
Figure 8. Comparison of the cycle performances of the sulfur cathodes with (a) PP11 and (b) diazabicyclo octane and (c) without encapsulants.
Figure 9. Deconvoluted sulfur 2p XP spectra of the sulfur composite cathode after 50 charge/discharge cycles; with (a) PP11 and (b) diazabicyclo octane. Each spectrum is fitted with deconvoluted peaks (blue, green and magenta) for differently bonded sulfur atoms; the sum of the fitting curves (red) is consistent with the raw data (black).
Tables
Table 1. Specific surface areas and pore volumes of the polysulfide reservoirs synthesized with the different organic surfactants.
Table 2. Rietveld refinement and crystal parameter data of Co3O4. The numbers in parentheses are the estimated standard deviations of the least-significant figure.
Table 1.
Surface Area
Pore Volume
Single point adsorption total
surfactant* BET model (m2 g-1)
Langmuir model 2
-1
(m g )
pore volume of pores (cm2 g-1)
PP 1:1
43.1206
60.0857
0.089529
PP 1:3
18.3077
20.1243
0.123514
PP 3:1
17.1381
19.4505
0.099781
DABCO
138.2581
195.7793
0.510006
Table 2.
S8 + Co3O4 formula pristine
after melt-diffusion
radiation type
synchrotron
2θ range (degree)
10-75
T/K
295
symmetry
cubic
space group
Fd-3m
Z
8
a=b=c/Å
8.07782(15)
8.08297(14)
V/Å3
527.088(29)
528.096(27)
Rp
8.83 %
8.91 %
Rwp
12.41 %
11.86 %
χ2
6.15
4.28
Research Highlights
Optimum pore and morphology controlled spherical Co3O4 for polysulfide reservoir Stable capacity retention for the given Co3O4 microsphere incorporated S cathode Propose conceptual design of cobalt oxide compounds for the novel electrode material
Footnote
# These authors contributed equally to this work. * Corresponding authors. Tel: 82–42–860–7372. E-mails: [email protected] (Won Bin Im) [email protected] (Sungho Choi)
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: