Exfoliated 2D Lepidocrocite Titanium Oxide Nanosheets for High

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Exfoliated 2D Lepidocrocite Titanium Oxide Nanosheets for High Sulfur Content Cathodes with Highly Stable Li–S Battery Performance Sharad B. Patil, Hyeon Jin Kim, Hyung-Kyu Lim, Seung Mi Oh, Jiheon Kim, Jaeho Shin, Hyungjun Kim, Jang Wook Choi, and Seong-Ju Hwang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01202 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Energy Letters

Exfoliated 2D Lepidocrocite Titanium Oxide Nanosheets for High Sulfur Content Cathodes with Highly Stable Li−S Battery Performance Sharad B. Patil,†,§ Hyeon Jin Kim,‡,§ Hyung-Kyu Lim,‡,ǁ Seung Mi Oh,† Jiheon Kim,‡ Jaeho Shin,┴ Hyungjun Kim,‡,ǁ Jang Wook Choi,┴,* and Seong-Ju Hwang†,* †

Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 03760, Republic

of Korea ‡

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ǁ

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),

Daejeon 34141, Republic of Korea ┴

School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul

National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

AUTHOR INFORMATION, Corresponding Author

ACS Paragon Plus Environment

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* [email protected] (J.W.C.); * [email protected] (S.-J.H.)

ABSTRACT: Despite the high energy densities, lithium−sulfur (Li−S) batteries suffer from insufficient cycle life originating from the shuttling process involving lithium polysulfides (LiPSs). Various approaches have been introduced to resolve the shuttling problem, but they are not usually effective for electrodes with high sulfur contents. Here, we report exfoliated 2D lepidocrocite titanium oxide nanosheet as a component for sulfur cathodes to suppress polysulfide dissolution markedly.

In particular, the Lewis acidity originating from under-

coordinated Ti species as well as the large surface area associated with the 2D structure endow 2D lepidocrocite titanium oxide with the efficient interaction with LiPSs. As a result, even with a sulfur content of 80 wt%, the Li−S cell exhibits 1023.5 mAh g−1 at 50 mA g−1 and a capacity retention of 82.3% after 300 cycles measured at 1000 mA g−1. The considerably improved cycling performance provides useful insight for designing sulfur cathodes, that is, the incorporation of acidic 2D metal oxide nanosheets.

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The ever-increasing demand for electrochemical energy storage systems to power green sustainable transportation represented by electric vehicles and drones has evoked a great deal of research activity for rechargeable lithium−sulfur (Li−S) batteries because of their high theoretical specific energy density (2567 Wh kg−1).1,2 Although there have recently been significant advances in Li−S batteries,2 the low efficiency of sulfur utilization and rapid capacity fading caused by the dissolution of lithium polysulfides (LiPSs) and uncontrolled deposition of Li2S leave the given technology lingering at a premature research stage.1−3 In particular, the ‘shuttling’ effect of LiPSs results in the loss of active material from the cathode and destabilizes the interface at the Li metal anode, leading to low Coulombic efficiency (CE) in each cycle.1−3 From an electrode design viewpoint, the confinement of LiPSs has been most widely pursued to suppress the dissolution of LiPSs. Along this direction, in contrast with non-polar carbonaceous hosts, homologues with polar surfaces are more effective in suppressing the shuttling process owing to enhanced electrostatic interaction between host and LiPSs.4−7 Accordingly, diverse polar inorganic materials such as oxides,8−12 hydroxides,13 chalcogenides,14−16 MXenes,17,18 and metal organic frameworks,19 have been introduced in sulfur cathodes because LiPSs can be readily adsorbed. Among these materials, TiO2 is unique in its strong affinity with LiPSs,20 low material cost, and environmentally benign nature. Not only has TiO2 been reported to have different morphologies such as yolk-shell21 and reduced inverse opal22 structures, but its derivatives with different compositions including heteroatom-doped hollow spheres,23 magnéli Ti4O7 and Ti6O11 phases,24 and titanium monoxide25 have also been employed for sulfur cathodes. Despite their promising role in the efficient entrapment of LiPSs, a majority of the available approaches that have adopted 0D nanoparticle or 0D hollow sphere morphologies for titanium oxides have only been validated for sulfur contents below 70 wt%. Since it is well-


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known26 that the sulfur content is critical for the energy densities of Li−S batteries, it is essential to explore approaches that are effective for a sulfur cathode with a high sulfur content. 2D nanosheet morphology plays an important role in suppressing LiPSs dissolution because their bifacial surfaces can provide plenty of sites for physisorption of LiPSs. In fact, 2D inorganic nanosheets made of MnO2 and MAX phases were reported to induce reversible interaction with LiPSs through chemisorption process.8,14 For this reason, compared to 2D bulk layered, 0D nanoparticle, and 0D hollow structures, exfoliated 2D nanosheets can offer abundant binding sites with LiPSs at a given material amount (Figure 1a). Unlike the synthesis of many other nanostructures that relies on crystal growth from seeds, a soft-chemical exfoliation route yields highly anisotropic 2D titanium oxide nanosheets without depending on delicate crystal growth processes.27 Especially, the exfoliated layered titanate materials synthesized by proton-exchange process have attracted much attention due to their good catalytic performance derived from high surface Lewis and Brønsted acid sites.28−30 Thus, the surface structures and physicochemical properties of the exfoliated titanium oxide nanosheets can be tailored by altering the crystal structures of the parent layered materials.31,32 As depicted in Figure 1b, two types of titanium oxide sheet structures are feasible: trititanate-type titanium oxide (TT) and lepidocrocite-type titanium oxide (LT). The TT nanosheet possesses a puckered layer structure composed of corner-shared [Ti3O14] units with two kinds of oxygen sites, i.e. bridging and terminal oxygens. By contrast, the LT nanosheet has an unpuckered flat layer structure composed of an infinitely-extended, edge-shared TiO6 octahedral array. The TT nanosheet tends to have Lewis basicity arising from the terminal oxygens with high electron density.29 In comparison, owing to the absence of basic terminal oxygens, the LT nanosheet exhibits Lewis acidity originating from the penta-coordinated Ti species.33 The distinct surface


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structure of the LT can result in enhancing the affinity with LiPSs via Lewis acid−base interactions. Here we report that the exfoliated 2D titanium oxide nanosheets, namely the LT, with Lewis acidity can effectively suppress LiPSs dissolution even at a high sulfur content of ~80 wt%, leading to decent cyclability, such as 82.3% capacity retention after 300 cycles. The present investigation provides a valuable design principle for highly sustainable sulfur cathodes with high sulfur contents: the incorporation of acidic 2D nanosheets for mitigation of LiPS dissolution.

Figure 1. (a) Schematic illustrations of LiPSs adsorption on the surfaces of various titanium oxide morphologies including the bulk layered and its exfoliated nanosheets, nanoparticles, and hollow spheres. (b) Crystal structures of two titanium oxide nanosheets: LT and TT. The parent layered titanium oxides of LT (Cs0.7Ti1.830.17O4, where  is a vacancy) and TT (Na2Ti3O7) were obtained by a heat-treatment of stoichiometric mixture of anatase TiO2 and


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Cs2CO3/Na2CO3 at 800/900 °C.31,32 As shown in the powder X-ray diffraction (XRD) patterns (Figure 2a), the Cs0.7Ti1.830.17O4 and Na2Ti3O7 showed well-defined Bragg reflections of LT phase under the space group Immm (JCPDS-84-1226) and TT phase under the space group P21/m (JCPDS-72-0148), respectively, verifying the formation of single-phase layered titanium oxides. The protonated derivatives of H0.7Ti1.830.17O4 and H2Ti3O7 were prepared by the reaction of the aforementioned layered titanium oxides with 1 M HCl solution.29 The exfoliated LT nanosheets were prepared by the reaction of the protonated H0.7Ti1.830.17O4 with aqueous tetrabutylammonium hydroxide (TBA⋅OH) solution. For the synthesis of TT nanosheets, the H2Ti3O7 was first reacted with methylamine (MA) and then with propylammonium (PA) cations, leading to a gradual increase in interlayer distance from 0.8 to 1.5 nm. An ultrasonic treatment of PA-intercalated trititanate led to delamination into TT nanosheets. As presented in Figure 2a, both the exfoliated LT and TT nanosheets did not show distinct Bragg reflections except a broad feature at 2θ = ~15−25° corresponding to the file of exfoliated nanosheets,34 confirming the complete exfoliation of the bulk layered titanium oxides into their individual nanosheets. Field emission-scanning electron microscopic (FE-SEM) and transmission electron microscopic (TEM) analyses clearly showed highly anisotropic 2D morphologies of both exfoliated LT and TT nanosheets (Figure S1). Two kinds of the titanium oxide−sulfur nanocomposites with different sulfur contents were prepared by direct growth of sulfur on the exfoliated LT or TT nanosheets for use as sulfur cathodes. According to thermogravimetric (TG) analysis (Figure S2), the amounts of sulfur loaded in the LT−sulfur nanocomposites were 88 and 80 wt%, which are denoted as LTS88 and LTS80, respectively. Similarly, TT−sulfur nanocomposites contained 90 and 80 wt% of sulfur


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ACS Energy Letters

and denoted as TTS90 and TTS80, respectively. To evaluate the structural and morphological effect of the exfoliated 2D titanium oxide nanosheets, two control samples, hollow TiO2 (HT) microspheres and commercial anatase TiO2 (AT) nanoparticles, were employed. The HT microspheres synthesized by solvothermal reaction crystallized with anatase TiO2 phase and displayed monodispersed microsphere morphology with uniform diameter of ~1.5 µm (Figure S3).35 The nanocomposites based on these control samples, denoted as HTS80 and ATS80, respectively, were also prepared with a sulfur content of 80 wt% (Figure S4).

Figure 2. (a) Powder XRD patterns of the parent layered titanium oxides and their derivatives. (b) FE-SEM micrographs of the titanium oxide−sulfur nanocomposites; scale bar: 1 µm. (c) TEM micrographs of the same nanocomposites; scale bars: 200 nm.


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According to Rietveld refinement analysis (Figure S5, Table S1), all the sulfur nanocomposites containing LT and TT nanosheets consist mainly of elemental sulfur (JCPDS-89-2600) and reveal absence of layered titanium oxide related Bragg reflections, suggesting homogeneous dispersion of exfoliated LT and TT nanosheets without phase segregation. In contrast, consistent with previous reports,22,24,25 the control HTS80 and ATS80 exhibited the Bragg reflections of the anatase TiO2 phase (Figure S6a−b), indicating phase segregation between titanium oxide and sulfur. This finding underscores the unique advantage of exfoliated nanosheets in producing homogeneous sulfur nanocomposites, which is attributed to their 2D morphologies and highly reactive surface characteristics. The homogeneous distributions of sulfur and nanosheets in all LTS and TTS nanocomposites were reflected in their FE-SEM images (Figure 2b) in which 2D lamella morphologies are invisible. Conversely, the control samples HTS80 and ATS88 showed aggregated titanium oxide components (Figure S6c-d). The TEM (Figure 2c) and energy dispersive spectroscopy (EDS)−elemental mapping analyses (Figure S7) reconfirm the homogenous dispersions of the 2D titanium oxide nanosheets in these nanocomposites. As shown in Fourier transformed-infrared (FT-IR) spectra (Figure 3a), the TT nanosheet solely exhibited a distinct band at 970 cm−1, which is assigned as the bending mode of hydroxyl group.29 Since the terminal oxygen tends to hold a greater electron density and shows higher Lewis basicity than the bridging oxygen, the TT nanosheets possess hydroxyl groups on their surfaces via the protonation of the terminal oxygens. In contrast, this hydroxyl group-related peak was not observed for the LT nanosheets. After the composite formation with sulfur, the IR band related to Ti−O vibration remained intact, confirming the maintained integrity of the titanium oxide nanosheets. In the Ti K-edge X-ray absorption near-edge structure (XANES) spectra (Figure 3b), the spectral features and energies of the pre-edge peaks provide information


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on the local atomic environment of the Ti ion.36 In comparison with anatase and rutile TiO2, the TT and LT nanosheets showed a strong peak at P2, indicative of the layered structures of these materials.36 In contrast to the TT, the LT displayed a distinct shoulder peak P2’, indicating the presence of pentacoordinated Ti ions.37 The unsaturated coordination of this Ti imbues the LT nanosheets with Lewis acidity. While titanium ions in most of titanium oxides including TT exist in octahedral sites with coordination number (CN) of 6, the LT has pentacoordinated titanium ions (CN = 5). Due to their coordinatively-unsaturated nature, pentacoordinated titanium ions have a strong tendency to form additional coordinative bond with basic ligand species like LiPSs, providing Lewis acidity for the LT nanosheet.33 The coordination of basic LiPSs to pentacoordinated titanium ions of LT layer leads to the formation of octahedral symmetry around Ti ions, resulting in the efficient chemisorption of LiPSs. Upon the desorption of LiPSs, the coordinatively-unsaturated nature of the pentacoordinated titanium ions of LT lattice is restored, which makes possible the reversible chemisorption/desorption of LiPSs on the LT nanosheet. Conversely, since the TT layer has no pentacoordinated titanium ion and terminal oxygen with large electron density, this material shows negligible Lewis acidity and thus cannot react efficiently with basic LiPSs.33 The composite formation of the LT and TT with sulfur did not give rise to any significant change in the XANES spectrum. Higher surface acidity of LT nanosheets was confirmed by NH3−temperature programmed desorption (TPD) analysis (Figure 3c). In contrast to TT nanosheets, LT showed a significantly higher amount of desorbed NH3 in the temperature range of 100−250 and 380−800 °C, which correspond to relatively weak and strong acid sites present on the surface of titanate nanosheet.23 The higher surface acidity of LT than TT originates both


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from the presence of penta-coordinated Ti species and the absence of basic terminal oxygens in the lattice.

Figure 3. (a) FT-IR spectra, (b) Ti K-edge XANES spectra, and (c) profiles of NH3-TPD of titanium oxide nanosheets and their nanocomposites with sulfur. The interfacial interaction between titanium oxide nanosheets and LiPSs was examined by Xray photoelectron spectroscopy (XPS). LT and TT nanosheets were reacted with LiPSs (Li2Sn) and are denoted as LT−Li2Sn and TT−Li2Sn. The Li2Sn species showed two strong peaks at 161.6 and 163.1 eV in S 2p branch, assigned as the terminal (ST−1) and bridging (SB0) sulfur atoms, respectively (Figure 4a).23,24 The area ratio of ST−1 to SB0 is approximately 1:1.4, suggesting the LiPSs are in the higher order forms (8 ≥ n ≥ 4), i.e. Li2S5.24 After the interaction with LT, the area ratio of ST−1 to SB0 changed significantly from 1:1.4 to 1.3:1, suggesting the high order-tolow order (1 < n < 4) conversion of LiPSs. Two more sulfur species were revealed in S 2p branch of the LT−Li2Sn at 166.9 and 168.4 eV, which are attributed to thiosulfate and polythionate complexes, respectively.8 The thiosulfates (Figure 4b) are formed by the redox


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reaction of Li2Sn with LT. The initially-formed thiosulphates react with higher order LiPSs (i.e. Li2S5), resulting in the formation of polythionate complexes (Figure 4c) and lower order LiPSs.8 By contrast, the TT−Li2Sn showed weak contribution of thiosulphate groups to S 2p spectrum and absence of polythionate complexes. Similar results were observed for both the HT−Li2Sn and AT−Li2Sn, suggesting the negligible interaction with Li2Sn species. The XPS spectra in Ti 2p branch (Figure 4d) constitute a consistent picture with those in S 2p branch. The LT nanosheets showed a higher binding energy at 458.6 eV than those of the TT and AT counterparts at 458.4 and 458.1 eV, respectively (Figure S8), owing to the higher surface acidity of the LT nanosheets.23,33 Upon the interaction with Li2Sn, the peak of the LT in the Ti 2p3/2 was red-shifted by 0.6 eV, due to an electron transfer from sulfur species to the Ti in the LT.24 Meanwhile, the TT nanosheets displayed a lower binding energy at 458.4 eV arising from its lower surface acidity,33 resulting in a smaller red-shift of 0.4 eV for the TT−Li2Sn. Both the control samples did not show any peak-shift in their Ti 2p3/2 spectra (Figure S8), confirming their negligible interaction with Li2Sn. As depicted in Figure 4e, two kinds of interactions of LiPSs with nanosheets are possible. Possessing high surface acidity and penta-coordinated Ti species, LT nanosheets exhibited strong interaction with LiPSs through both physisorption and chemisorption process. Chemisorption of LiPSs involves conversion of higher order LiPSs into the polythionates, which occurs at the penta-coordinated Ti sites. On the contrary, TT nanosheets demonstrated only physisorption of LiPSs. Moreover, physisorption of LiPSs is less likely at each corner-shared Ti sites with terminal oxygens owing to its high Lewis basicity.


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Figure 4. (a) S 2p XPS spectra of (i) Li2Sn, (ii) LT−Li2Sn, (iii) TT−Li2Sn, (iv) HT−Li2Sn, and (v) AT−Li2Sn. Molecular structures of (b) thiosulfate and (c) polythionate complexes: yellow = sulfur in the thiosulfate, green = bridging sulfur in the polythionates, and red = oxygen. (d) Ti 2p XPS spectra of (i) LT nanosheet, (ii) LT−Li2Sn, (iii) TT nanosheet, and (iv) TT−Li2Sn. (e) Schematic illustration of LiPSs interaction with 2D exfoliated LT and TT nanosheets. To provide further details of Li2Sn adsorption behaviors on complex titanate surfaces, density functional theory (DFT) calculations were carried out. We generated 3 types of surface structures based on the 2D layered structure of LT (Figure S9a), a perfectly oxygenated surface with fully occupied Ti sites (TiO2), a protonated surface derived by Ti vacancy having hydroxyl functional groups (H0.67Ti1.830.17O4), and a fully dehydrated previous structure (Ti1.830.17O3.66) with exposed penta-coordinated Ti species. The binding energy (BE) of Li2S4 molecule on these systematically designed surfaces was calculated. As a result, it was found that weak physisorption occurred on a fully oxygenated surface (TiO2, BE = −0.43 eV) and a hydroxylated surface (H0.67Ti1.830.17O4, BE = −0.65 eV). On the other hand, a strong chemisorption occurred


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on a dehydrated surface (Ti1.830.17O3.66, BE = −2.04 eV) with close contact between pentacoordinated Ti sites and sulfur atoms (Figure S9b). These theoretical calculation results indicate that the strong interaction between LiPSs and LT surface is derived by the high Lewis acidity of penta-coordinated Ti sites. A crucial role of Ti vacancy in the Lewis acidity of lepidocrocitetype titanate is well reported by Kitano et al.33 Thus, the presence of Ti vacancy in lepidocrocite-type titanate layer is supposed to significantly contribute to the promising electrode performance of lepidocrocite titanate−sulfur nanocomposites for Li−S batteries through the enhanced chemisorption of polysulfides in terms of Lewis acid−base interaction. The impact of titanium oxide components on the electrochemical performance was evaluated by carrying out galvanostatic measurements of the corresponding Li−S cells. In this experiment, the LTS and TTS with different sulfur loadings were first tested (Figures 5a and b). All of the specific capacities hereafter were calculated based on the mass of sulfur only. When measured at 50 mA g−1, LTS80, TTS80, LTS88, and TTS90 exhibited reversible capacities of 1023.5, 1172.3, 1094.1, and 1135.2 mAh g−1, respectively, in their first cycles. The initial Coulombic efficiencies (ICEs, defined as charging capacities/discharging capacities) of these samples were 94.3%, 93.7%, 91.7%, and 88.9%, respectively. The higher ICEs of the nanocomposites with 80 wt% sulfur than those with 88 and 90 wt% indicate that 80 wt% is a critical point above which the titanium oxides become less effective in suppressing LiPS dissolution. These capacity values reveal that at the given sulfur contents, TTS delivers higher discharge capacities than those of LTS. For example, at the sulfur content of 80 wt%, TTS80 and LTS80 showed discharge capacities of 1172.3 and 1023.5 mAh g−1, respectively, which might be attributed to the fact that in the case of LTS80, upon LiPSs generation from the beginning of discharge, a certain portion of sulfur is spent for the formation of thiosulfate and polythionates so that a relatively less


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amount of sulfur participates in the lithiation toward Li2S even though the same 80 wt% of sulfur was introduced for the synthesis of the titanium oxide−sulfur nanocomposite. This interpretation is indeed supported by the distinct discharge capacities during the upper plateau (∆C1 in Figure 5a); the ∆C1 values of LTS80 and TTS80 were 143.1 and 154.5 mAh g−1, respectively. The larger portion of sulfur involvement in the formation of thiosulfate and polythionates is consistent with the peak intensities of the XPS spectra in Figure 4a. These titanium oxide−sulfur nanocomposites showed distinct cycling performance (Figure 5b). Notably, LTS80 demonstrated better cyclability than that of the other three samples; while LTS80 and TTS80 retained 82.3% and 57.9% after 300 cycles, respectively, LTS88 and TTS90 preserved 61.5% and 54.6% after 250 cycles, respectively. These results indicate that at various sulfur contents, LT is clearly superior to TT in retaining the original capacity of the electrode. Once again, this phenomenon can be explained by the acidic reaction of the LT with LiPSs, as observed in the XPS data in Figure 4a. However, as the sulfur loading increases, such an effect is weakened, implying that there might exist a critical point in sulfur content beyond which the scavenging of polysulfides by the acidic reaction with LT begins to be less effective, as demonstrated with the comparison of ICE values for varying sulfur contents.


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Figure 5. Electrochemical performance of various titanium oxide−sulfur nanocomposites. (a) The first discharge−charge profiles and (b) cycling performance and CEs of LTS and TTS with different sulfur loadings. (c) The first discharge−charge profiles and (d) cycling performance of LTS80, its derivatives based on control titanium oxide components, and ACS70. (a) and (c) were measured at 50 mA g−1. (e) Rate performance of LTS80. (f) The first discharge−charge profiles of LTS80 with different sulfur loadings.


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The effect of LT was unveiled more clearly in comparison with AT and HT with the fixed sulfur content of 80 wt% (Figures 5c and d). While LTS80, ATS80, and HTS80 exhibited similar discharge−charge profiles with reversible capacities of 1023.5, 975.7, and 1112.7 mAh g−1, respectively, at 50 mA g−1, their capacity retentions after 300 cycles at 1000 mA g−1 were quite distinct, such as 82.3%, 69.1%, and 49.7%. Both ATS80 and HTS80 gradually lost their capacities from the outset of the cycling, in sharp contrast with LTS80. These titanium oxide−sulfur nanocomposites were also compared with a conventional carbon−sulfur composite (ACS70, 0.8 mg cm−1)38 in which 70 wt% of sulfur was infused into activated carbon (1295 m2 g−1). The capacity retention of ACS70 after 100 cycles was only 68.8%, which is ascribed to the relatively weaker adsorption capability of activated carbon with LiPSs. The specific capacities of LTS80 at different current densities are presented in Figures 5e and S10. The superior cyclability of LTS80 at the same content of sulfur can consistently be explained by the acidity of LT that mitigates the polysulfide dissolution. Additional tests without LiNO3 additive and focusing on self-discharge further support the role of LT nanosheets (Figures S11 and S12). In addition, LTS80 showed lower polarization at precycling than that of ATS80 and HTS80 (Figure S13); while the polarization of LTS80, marked with ∆E in Figure S14, was 0.101 V, those of ATS80 and HTS80 were similarly 0.129 V. This observation may point to the fact that LTS80 contains relatively uniform sulfur domains as compared to those of the other control samples, which is mainly responsible for the good rate performance of LTS80. To understand the role of LT nanosheet in the excellent capacities of LTS nanocomposites, we measured the electrode activity of LT under the same condition as described in the experimental section. Although anatase TiO2 is known to show voltage plateaus below 1.7 V (vs. Li/Li+) with a specific capacity of approximately 250−300 mAh g−1,39 the present LT nanosheet exhibited a


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discharge capacity of 46.2 mAh g−1 only with high irreversibility (Figure S14). Furthermore, this capacity became negligible after the first cycle (Figure S14). This negligible capacity is indeed beneficial for the performance of Li−S batteries, as the LT sheets can be dedicated chiefly to the acidic reaction to form thiosulfate and polythionate complexes instead of being sacrificed to the reaction with Li ions. Although the electrical conductivity of LT is not very high due to its wide bandgap semiconducting nature, this material should be able to support a substantial level of electron transport from its semiconducting character. While the conductivity of conductive components is crucial, the distribution of sulfur (thus the size of sulfur domain) is more critical in determining the rate performance. Also, similar to the TiO2 nanowires and metal sulfides,40,41 the LT nanosheets can promote the redox activity of LiPSs, which can contribute to the improved rate performance. For reference, the specific capacity, cycling performance, sulfur content, and energy density of LTS80 are compared with those of other reported works employing metal oxide−sulfur composites and the standard carbon-containing cells (Tables S2−S4).

In addition, the

measurement was expanded to higher sulfur loadings of 1.5 and 2.2 mg cm−1 while the sulfur content was retained at 80 wt% of the active material. The first reversible capacities with these loadings were 996.3 and 942.1 mAh g−1 at a current density of 50 mA g−1, respectively (Figure 5f), along with decent cyclability (Figure S15). The increased overpotentials upon the increase in sulfur loading reflect the increased resistance. In addition, the areal capacity of LTS80 is compared with those of the other nanocomposites (HTS80, ATS80, and ACS70), confirming the advantageous role of LT nanosheet in optimizing the areal cycling performance of Li−S batteries (Figure S16). Also, the areal capacities of LTS80 are plotted in Figure S17 with different loadings of active material for reference.


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The role of the acidity of LT on the cycling performance was further elucidated by analyzing LTS80 at different discharging points (Figure 6a) using XPS. At pristine state (point i), two peaks at 164.5 and 165.7 eV were observed in S 2p branch, which reflect S−S bonds in S 2p3/2 and S 2p1/2 sub-regimes, respectively (Figure 6b). More importantly, at point ii, the peaks at 166.7 and 167.5 eV assigned as thiosulfate as well as the peaks at 168.3 and 169.7 eV assigned as polythionates grew conspicuously, which is consistent with the spectra ii in Figure 4a. Consistent with Nazar et al.,8 the emergence of polythionates reflects the conversion of longchain LiPSs to short-chain ones through the sulfur polymerization of thiosulfate to polythionates, representing thiosulfate as a ‘polysulfide mediator.’ While the presence of LiPSs was revealed by the S 2p3/2 peaks at 161.8 and 163.1 eV corresponding to ST−1 and SB0 respectively, the integration of these two peaks indicates that the ratio between ST−1 and SB0 is 2:1, reconfirming the conversion from long-chain to short-chain LiPS in the plateau at 2.15 V. At the same time, the peak at 164.5 eV, a signature of elemental sulfur, still remained, implying that sulfur was not fully converted to Li2S8. On the other hand, the spectrum at point iii indicates that the polythionates almost vanished, but the thiosulfate still remained to some extent. This phenomenon can be explained by the fact that the further reduction of the polythionates toward Li2S2 and Li2S decreases the amount of polythionates while leaving some thiosulfate behind at the end of discharge. At point iii, the integration ratio of the peaks corresponding to ST−1 and SB0 was 2.25:1, indicating Li2S2 as the main product.8 The spectra in Ti 2p branch portray a consistent picture (Figure 6c). As a result of the formation of the thiosulfate at point ii, the peak of Ti4+ was down-shifted by 1.5 eV compared to that at point i. This peak shift was more significant than that (0.6 eV) in Figure 4d during the experiment with the synthetic polysulfides. The greater peak shift in the actual battery cell is


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attributed to a more significant charge transfer to the Ti, which also indicates that the improved cyclability of the LTS80 is associated with efficient formation of thiosulfate and polythionates via the charge transfer from LiPSs to the Ti of LT nanosheets.

Figure 6. Ex-situ XPS spectra of LTS80 at different cycling points. (a) The first discharge profile. (b) S 2p branches at (i) pristine state and (ii) discharged state at 2.15 V, and (iii) discharged state at 1.7 V. (c) Ti 2p branches at (i) pristine state and (ii) discharged state at 2.15 V. The key to the success of Li−S batteries lies with achieving long cycle life while securing a high sulfur loading in sulfur cathodes. To this end, suppressing LiPS dissolution with a minimal content of LiPS-capturing components is crucial. The present study demonstrates that welldispersed 2D materials with intimate interactions with LiPSs are good candidates in fulfilling


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such a mission. In particular, surface atomic configuration with acidity plays a critical role. The given design principle can be applied to other 2D nanomaterials with properly designed surface properties in the context of mitigating LiPS dissolution more efficiently and thus further improving the cyclability of Li−S cells.

ASSOCIATED CONTENT Supporting Information. Experimental and computational details, supporting figures, tables, and references AUTHOR INFORMATION Author Contributions § These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). S. B. P. was supported by RP-Grant 2016 of Ewha Womans University. J. W. C. acknowledges the support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (NRF2015R1A2A1A05001737).


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