Bronze TiO2 as a cathode host for lithium-sulfur batteries

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Bronze TiO2 as a cathode host for lithium-sulfur batteries Wenjing Dong , Di Wang , Xiaoyun Li , Yuan Yao , Xu Zhao , Zhao Wang , Hong-En Wang , Yu Li , Lihua Chen , Dong Qian , Bao-Lian Su PII: DOI: Reference:

S2095-4956(20)30037-1 https://doi.org/10.1016/j.jechem.2020.01.022 JECHEM 1075

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

20 December 2019 15 January 2020 16 January 2020

Please cite this article as: Wenjing Dong , Di Wang , Xiaoyun Li , Yuan Yao , Xu Zhao , Zhao Wang , Hong-En Wang , Yu Li , Lihua Chen , Dong Qian , Bao-Lian Su , Bronze TiO2 as a cathode host for lithium-sulfur batteries, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.022

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Bronze TiO2 as a cathode host for lithium-sulfur batteries Wenjing Donga,1, Di Wanga,1, Xiaoyun Lib,1, Yuan Yaoc, Xu Zhaod, Zhao Wanga, Hong-En Wanga,*, Yu Lia, Lihua Chena, Dong Qiane, Bao-Lian Sua,f,* a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, China b State Key Laboratory of Silicate Material for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei, China c Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China d Institute of Chemical Materials, Chinese Academy of Engineering Physics, Mianyang 621900, Sichuan, China e College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China f Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium *Corresponding authors. E-mail addresses: [email protected], (H.-E. Wang); [email protected] (B.-L. Su). 1

These authors contributed equally to this work.

ABSTRACT Lithium-sulfur batteries (LSBs) are very promising for large-scale electrochemical energy storage. However, dissolution and shuttling of lithium polysulfides (LiPSs) intermediates have severely affected their overall electrochemical properties and limited their practical application. Designing polar cathode hosts that can effectively bind LiPSs and simultaneously promote their redox conversion is crucial for realizing high-performance LSBs. Herein, we report bronze TiO2 (TiO2-B) nanosheets (~5 nm in thickness) chemically bonded with carbon as a novel multifunctional cathode host for advanced LSBs. Experimental observation and first-principles density functional theory (DFT) calculations reveal that the TiO2-B with exposed (100) plane and Ti3+

ions exhibited high chemical affinity toward polysulfides and effectively confined them at surface. Meantime, Ti3+ ions and interface coupling with carbon promoted electronic conductivity of the composite cathode, leading to enhanced redox conversion kinetics of LiPSs during charge/discharge. Consequently, the as-assembled TiO2-B/S cathode manifested high capacity (1165 mAh/g at 0.2 C), excellent rate capability (244 mAh/g at 5 C) and outstanding cyclability (572 mAh/g over 500 cycles at 0.2 C). This work sheds new insights on rational design and fabrication of novel functional electrode materials for beyond Li-ion batteries.

Keywords: Titanium dioxide; Cathode; Polysulfides; Shuttle effect; Lithium-sulfur batteries; Electrochemistry

1. Introduction Lithium-sulfur batteries (LSBs) are an emerging electrochemical energy storage system compared to current commercial lithium-ion batteries (LIBs) mainly due to their ultrahigh theoretical energy density (2600 Wh/kg) and natural abundance of sulfur with non-toxicity [1–3]. However, their practical application has been largely deferred by the insulating nature of sulfur cathode and discharge product (Li2S2/Li2S), shuttling of soluble polysulfide intermediates (Li2Sx, 4≤x≤8, LiPSs) [4], and large volume expansion of sulfur during discharge (up to ~80%) [5–9]. Various strategies have been proposed to tackle the intractable issues of LSBs, such as tuning cathode structure/composition [10–14] and electrolyte components [15,16], employing novel polymer binders [17] or modified separators [18], using

functional interlayers [19], redox-mediated growth of Li2S [20] and lithium-anode protection [21]. Among these, manipulation of cathode composition/nanostructures provides a versatile approach for LSBs performance improvement [22]. Various polar metal oxides and sulfides [23,24], nitrides etc. [25], with tunable compositions and structures have been devised to serve as novel hosts for sulfur cathode. These transition metal compounds possess higher affinity to LiPSs than nonpolar carbon and some can catalyze fast redox conversion of LiPSs [26–28]. Particularly, anatase-TiO2, a well-studied anode for LIBs [29], has been testified either alone or coupled with other compounds as functional host material for LSBs [24,30,31]. Compared to anatase-TiO2, bronze-TiO2 (TiO2-B) owns more opened channels for fast Li+-ion diffusion and manifests higher practical capacity as anode in LIBs [32]. TiO2-B can serve as a novel functional host for LSBs, though its practical application has not been explored much yet. Inspired by this, we have designed and synthesized ultrathin TiO2-B nanosheets as a novel cathode host for LSBs. First-principles density functional theory (DFT) calculations indicate that the superior electrochemical performance originates from the high affinity of exposed (100) plane of TiO2-B to both LiPSs and Li-ions, which can reduce the shuttling of LiPSs and concentrate Li+ ions on the cathode surface. In addition, the existence of Ti3+ in TiO2-B and coupling with carbon via C-Ti bonds effectively boost the electron transport of the composite cathode, leading to enhanced redox reaction kinetics.

2. Experimental 2.1. Materials All the reagents were analytical pure and used without further purification. 2.2. Sample preparation 2.2.1 Synthesis of TiO2-B nanosheets The TiO2-B nanosheets were synthesized by a hydrothermal reaction following post-treatment employing TiO2/oleylamine as precursor [33]. The precursor was synthesized by adding tetrabutyl titanate (4.5 mL) into ethanol (200 mL) solvent with oleylamine (1.8 mL) and H2O (0.8 mL) under intensive stirring at room temperature. The resultant white TiO2/oleylamine (Scheme 1a) precipitates were kept at static overnight, then collected by configuration after washing with ethanol several times, and finally dried at 80 ℃ in air for 12 h. Then, TiO2-B nanosheets were prepared by hydrothermal treatment of the TiO2/oleylamine precursor powders (0.2 g) dispersed in NaOH aqueous solution (60 mL, 5 M) in a sealed Teflon-lined autoclave (100 mL capacity) at 150 ℃ for 48 h, leading to the formation of Na-titanates intermediates. The Na-titanates were then soaked in dilute HCl solution (200 mL, 0.12 M) for 2 h to obtain H-titanates, followed by washing thoroughly, drying in air, and finally annealing at 300 ℃ in air for 2 h to decompose the H-titanates into TiO2-B nanosheets (Scheme 1b). For comparison, TiO2-B nanoribbons were also synthesized by a similar process by using commercial TiO2 nanoparticles (~30 nm size) as starting materials.

2.2.2 Fabrications of TiO2-B/S composite cathode The TiO2-B/S composite (Scheme 1c) cathode was prepared by a conventional melt-diffusion method [31]. Typically, the TiO2-B and sulfur powders with a weight ratio of 2:3 were thoroughly blended, transferred into a sealed autoclave with a Teflon liner, and maintained at 155 oC for 12 h. The resultant TiO2-B/S composite cathodes with TiO2-B nanosheets and nanoribbons as hosts were termed as “Sheets/S” and “Ribbons/S”, respectively, for convenience.

2.2.3 Physical characterizations Powder X-ray diffraction (XRD) patterns were collected on a Bruker diffractometer with Cu Kα radiation (40 kV/mA). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) micrographs were recorded on a JEM-2100F TEM operated at 200 kV. The surface element composition and electronic states of the samples were analyzed by an X-ray photoelectron spectroscope (XPS, Thermo Fisher Scientific, Alpha) using a monochromatic Al Kα radiation source at 15 kV. The XPS peak positions locations were calibrated using C 1s peak of adventitious carbon at 284.8 eV as reference. The specific surface area and pore size distribution were determined using Brunner-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods through N2 adsorption/desorption isotherms at 77 K on a Tri Star II 3020 surface area and porosity analyzer. Prior to sorption experiments, the samples were degassed at 80 ℃ for 24 h under vacuum. The thermogravimetric analysis (TGA) data were collected with a Pyris1 thermogravimetric analyzer at a heating rate of

5 °C·min-1 in N2. For polysulfide adsorption experiments, Li2S4 solution (20 mM) was prepared by dissolving sulfur and Li2S with a stoichiometric molar ratio of 3:1 in 1,3-dioxalane (DOL) and dimethyl ether (DME) (1:1, v:v) in an Ar-filled glovebox. The solution was stirred for 24 h at 50 °C. Then, 25 mg active materials (TiO2-B nanosheets or nanoribbons) were separately added into 3 mL of the above Li2S4 solution for 6 h.

2.2.4 Electrochemical performance measurements The electrochemical properties were tested using coin-type CR2025 cells with lithium foils as counter and reference electrodes at room temperature. The working electrodes were made by mechanical blending active materials (Sheets/S or Ribbons/S), carbon black (super-P) and polyvinylidene difluoride (PVDF) binder with 8:1:1 mass ratio in N-methyl-2-pyrrolidone (NMP). The slurry was uniformly spread on an aluminum foil, dried at 55 ℃ for 12 h in vacuum and then cut into circular disk electrodes. 1.0 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) dissolved in 1,3-dioxalane (DOL) and dimethyl ether (DME) (1:1 v/v) with 1 wt% LiNO3 additive was used as the working electrolyte. The Li-S cells were assembled in an argon-purged glove-box with H2O and O2 contents <1 ppm. Cyclic voltammetry (CV) sweeps were recorded on a CHI660E (Chenhua, Shanghai) electrochemical workstation within a potential window of 1.8~2.8 V vs. Li+/Li at 0.1 mV s-1. Galvanostatic charge/discharge tests were carried out on a battery testing instrument (LAND CT2001A, Wuhan) at current rates of 0.2~5 C (1 C = 1675 mA g-1). The

specific capacities were calculated based on the weight of sulfur in the composite cathodes. The mass loading of active materials on each electrode is ca. 2 mg cm-2. The electrolyte to sulfur ratio is ca. 10 mL/g. Electrochemical impedance spectra (EIS) were measured at open-circuit potential with an electrochemical workstation (Autolab PGSTAT 302N) in a frequency range of 100 kHz to 10 mHz with an amplitude of 10 mV. To estimate the capacity contribution of TiO2-B host to the Li-S cells, Li-half cells were individually assembled using TiO2-B as working electrode and Li foils as reference/counter electrode. The working electrodes were made by mechanical blending active materials (TiO2-B nanosheets or nanoribbons), carbon black (super-P) and polyvinylidene

difluoride (PVDF) binder with

8:1:1 mass

ratio in

N-methyl-2-pyrrolidone (NMP). The slurry was uniformly spread on an aluminum foil, dried at 55 ℃ for 12 h in vacuum. The Li-half cells (CR2025) were assembled in an Ar-purged glove-box with H2O and O2 contents <1 ppm. Galvanostatic charge/discharge tests were carried out on a battery testing instrument (LAND CT2001A, Wuhan) at 335 mA/g (equivalent to 0.2 C of that for Li-S cell test where 1 C = 1675 mA g-1).

2.3 Theoretical calculations First-principles density functional theory (DFT) calculations were performed using CASTEP implemented in Materials Studio [34,35]. The exchange and correlation energies were calculated using OTFG ultrasoft pseudopotential and

Perdew-Burke-Ernzerhof

(PBE)

functional

within

the

generalized

gradient

approximation (GGA). The electron-ion interactions were described within a plane-wave basis set with an energy cutoff of 571.4 eV for bulk and 400 eV for surface adsorption calculations, respectively. A 121 supercell containing 16 Ti and 32 O atoms was used to model the electronic structures of bulk TiO2-B. To correct the on-site electron correlation, GGA plus Hubbard model (GGA+U) has been employed with a U value of 4.5 eV. To simulate the surface electronic structures, a TiO2-B (100) surface slab was cleaved and a vacuum layer of 15 Å was added to avoid the unwanted interactions between neighboring cells along z-direction. For simulations of oxygen-deficient TiO2-B, one O atom was removed from the 121 supercell. The convergence tests of the total energy with respect to the k-points sampling and the energy-cutoff were carefully examined, using 1×2×2 Monkhorst-Pack k-points grid for TiO2-B bulk and 1×1×1 k-points for the surface calculations. Ionic relaxations were performed using a conjugate gradient algorithm until the net force on all individual atoms was less than 0.03 eV Å−1 for bulk and 0.1 eV Å−1 for surface slabs, the SCF tolerance was set to 1×e-6 eV·atom−1 for bulk geometry optimization and 1×e-5 eV·atom−1 for surface calculations. The adsorption energies (Eads) for LiPSs or Li on TiO2-B surface were determined using the following equation: Eads = Etotal – Esurf – Emol where Etotal is the total energy of the system containing TiO2-B (100) surface with adsorbed LiPSs or Li, Esurf is the energy of the clean TiO2-B (100) surface, Emol is the energy of a free LiPSs molecule or Li in vacuum. During Eads calculations, van der

Waals interactions were included.

3. Results and discussion

Scheme 1. Schematic synthesis procedure of the TiO2-B nanosheets and TiO2-B/S composite. (a) TiO2/oleylamine precursor, (b) TiO2-B nanosheets, (c) TiO2-B/S composite.

Fig. 1. (a) XRD patterns of Sheets/S and Ribbons/S products, (b) HRTEM micrograph of TiO2-B nanosheets, high-resolution XPS spectra of (c) C 1s and (d) Ti 2p core

levels from TiO2-B nanosheets surface.

In the X-ray diffraction (XRD) patterns (Fig. S1a, Supporting Information) of the two TiO2 samples, two strong diffraction peaks can be well indexed to the (110) and (020) planes of monoclinic bronze phase (TiO2-B, space group P63/mcm, JCPDS card No. 74-1940) [36]. After sulfur loading, additional diffration peaks for orthorhombic S8 (space group Fddd, JCPDS card No. 42-1278) [37] can also be observed (Fig. 1a). Scanning electron microscopy (SEM) image (Fig. S1b, Supporting Information) reveals that the TiO2-B nanosheets aggregate together, forming a hierarchically porous nanoarchitecture. The hierarchical porous network of the TiO2-B nanosheets product endows it with a high specific surface area of ca. 209.1 m2/g as determined by N2 adsorption/desorption isotherms experiments (Fig. S2, Supporting Information), higher than that of the TiO2-B nanoribbons counterpart (90.4 m2/g). In Fig. 1(b), high-resolution transmission electron micrograph (HRTEM) combined with fast Fourier transform (FFT) pattern (inset, top right) further confirm the bronze phase of the as-received TiO2 nanosheets. The nanosheets are very thin with a thickness of ~5 nm. The resolved lattice with interplanar distance of 0.58 nm (inset, bottom right) suggests the preferential exposure of (200) plane (Fig. 1b and inset) and the fluctuation of intensity in the line profile indicates the presence of some structural distortion, probably introduced during the thermal treatment process. Additional proof for the presence of (200) plane was shown in Fig. S1(c and d) (Supporting Information). The FFT pattern (white square region in inset of Fig. S1c) derived from

orange square region (inset of Fig. S1c) shows clear diffraction spot of (200) plane. In addition, the measurement of line profile (Fig. S1d) derived from olive rectangular region (inset of Fig. S1c) yields ca. 1.17 nm, corresponding to twice of interplanar distance of (200) plane. Surface composition and electronic states of elements on the sample surface were investigated by X-ray photoelectron spectroscopy (XPS). The high resolution C 1s core level spectrum (Fig. 1c) can be deconvoluted into three species of C-C bond (285.4 eV), C-O bond (288.6 eV) and C-Ti bond (283.2 eV) [38], respectively. The presence of C-C bonds hints the formation of amorphous graphitic carbon species derived from carbonization of trace oleylamine adsorbed on TiO2-B surface. The formation of C-Ti chemical bond confirms the strong interfacial coupling between carbon and TiO2-B, leading to enhanced electrical conductivity of TiO2-B and interface charge transfer from carbon to TiO2-B during charge/discharge. Deconvolution of Ti 2p core level XPS spectrum (Fig. 1d) discloses the presence of two kinds of Ti species. The first pair of bands with binding energies (BE) of 463.8 and 458.1 eV correspond to the Ti4+ ions in TiO2 lattice [39]. The second pair of bands with lower BE of 462.5 and 456.8 eV imply the existence of Ti3+ ions on the surface [37], which can be ascribed to the formation of oxygen vacancies (Vo) in TiO2-B [39]. The existence of Ti4+/Ti3+ pairs can considerably enhance the electronic conductivity of TiO2 via iterant electron delocalization in Ti 3d orbitals. Based on the fitted areas of Ti4+ and Ti3+ components, the concentration of surface Vo in the TiO2-B sample can be roughly estimated to be ca. 27%. In addition, two pairs of sulfur species can be noted in the high-resolution XPS core level spectrum of S 2p (Fig. S3, Supporting Information). The first pair of bands with BE at 164.6 and 163.5 eV stem from S8 in the Sheets/S composite. The other pair of bands with higher BE at 169.6 and 168.4 eV

correspond to some sulfate formed due to the oxidation of surface sulfur species.

Fig. 2. Electrochemical properties of TiO2-B/S composite cathode in LSBs. (a) cyclic voltammetry (CV) curves, (b) galvanostatic charge-discharge profiles, (c) rate property, (d) electrochemical impedance spectra (EIS), and (e) cycling performance of Sheets/S and Ribbons/S electrodes.

The TiO2-B nanosheets were used as host to load sulfur species to prepare composite cathode for Li-S cells. For comparison, TiO2-B nanoribbons were also synthesized and tested at the same conditions. The resultant two cathodes were termed

as “Sheets/S” and “Ribbons/S”, respectively, for convenience. The mass contents of sulfur species in the composite cathodes were determined to be 61% (for Sheets/S) and 61.4% (for Ribbons/S), respectively (Fig. S4, Supporting Information). Cyclic voltammetry (CV) curves (Fig. 2a) reveal both cathodes undergo a stepwise reduction reaction during the cathodic scan. The first peak at 2.28 V signifies the reduction of sulfur species (S8) into long-chain Li2Sx (3
interface. Electrochemical impedance spectra (EIS, Fig. 2d) further unravel the enhanced reaction kinetics of the Sheets/S electrode. After fitting with an equivalent circuit (inset), the derived charge transfer resistance (Rs) for Sheets/S is 44.8 , much smaller than that of Ribbons/S (124.1 ). In addition, the cycling performance testing is shown in Fig. 2(e). The composite cathode displays a continuous and fast capacity drop in the initial a few cycles, which can possibly be ascribed to the uneven distribution of sulfur species loaded on the TiO2-B nanosheets host by a conventional melt-diffusion process. In this case, some sulfur species can be lost during the cycling after being converted into soluble polysulfide during discharge. Next, the Sheets/S electrode gradually cycles stably, maintaining a high capacity of 572 mAh/g over 100 cycles with a good capacity retention of 66% (relative to the 4th cycle), better than that of Ribbons/S electrode (52%) and some recent literature (Table S2, Supporting Information). In addition, the overall performance of the Li-S cells with TiO2-B host can be further improved by (1) tuning its structure and morphology to possess a higher surface area and pore volume with more exposed active sites and (2) compositing with highly conductive porous carbon substrates having high surface area.

Fig. 3. Optimized geometry structures (a, d), corresponding electronic band structures (b, e) and density of states (DOS) (c, f) of anatase 121 supercells without (a-c) and with one oxygen vacancy (d-f). First-principles density functional theory (DFT) calculations were performed to gain more mechanistic insights. A 121 supercell model was adopted to calculate the band structure and density of states (DOS) of TiO2-B with/without oxygen vacancy (Fig. 3a and d). The calculations indicate that pristine TiO2-B is an indirect semiconductor with a wide bandgap (Eg) of ~3.12 eV (Fig. 3b). Partial DOS (PDOS) analyses further reveal the valance band (VB) and conduction band (CB) were mainly composed of O 2p and Ti 3d orbitals, respectively. The incorporation of Vo into TiO2-B leads to the formation of a mid-gap state with a narrowed Eg of 1.1 eV (Fig. 3e). The PDOS plots (Fig. 3f) of the TiO2-B containing Vo show that the gap states stemmed from partial localized Ti 3d orbital of Ti3+ and came across the Fermi level (Ef). The reduction of Eg and increase of DOS near Ef can boost electronic conductivity of the TiO2-B.

Fig. 4. Optimized geometry structures of (a) Li2S4 and (b) Li2S molecules adsorbed on TiO2-B (100) surface; (c) Li insertion in TiO2-B slab, and (d) Li2S adsorbed on TiO2-B co-inserted with Li. (The green and blue contours in the charge density difference distribution plot in (d) represent the electron accumulation and depletion, respectively.)

The adsorption of LiPSs, Li2S and Li on TiO2-B surface was further simulated using (100) plane as a modeling surface and Li2S4 as representative LiPS species. Herein, the choice of (100) plane was mainly based on the experimental finding that it is a preferentially exposed crystal plane. Therefore, it can have a high probability to contact with sulfur and polysulfide during charge/discharge. Fig. 4(a) depicts the optimized geometry construction of Li2S4 molecule on TiO2-B (100) surface. The results demonstrate that Li2S4 molecule can be chemically adsorbed on TiO2-B (100)

surface with an adsorption energy (Eads) of -1.06 eV via interfacial Li-O and S-Ti bonds (with bond lengths of 2.622 Å and 2.471 Å, respectively) (Fig. 4a), leading to reduced dissolution and shuttling. The chemical affinity of TiO2-B surface towards polysulfide has been further verified by the polysulfide adsorption experiments (Fig. S6, Supporting Information). Clearly, the Li2S4 electrolytes after addition of TiO2-B nanosheets or nanoribbons decolored after 6 h, indicating the high adsorption capability of TiO2-B to polysulfide. Similarly, the discharge product Li2S molecule can also be tightly anchored on the TiO2-B (100) surface with a higher Eads of -2.53 eV by forming two Li-O and one S-Ti bonds (with bond lengths of 2.686, 2.669 and 2.459 Å, respectively) (Fig. 4b). As TiO2-B is a polar compound, thus it can be deduced that the surface exposed undercoordinated Ti or O atoms of other (110) and (001) planes can also anchor polysulfides by interfacial chemical bonding albeit with some differences in exact adsorption energies to that of (100) plane. In addition, simulation results unravel that Li can be chemically adsorbed on several atop or bridge sites on the TiO2-B surface (Figs. 5 and S5), suggesting that Li can be concentrated on the TiO2-B surface for enhanced redox conversion reactions of LiPSs during charge/discharge [40]. More interestingly, our simulation results also suggest that Li can be inserted into TiO2-B lattice (Fig. 4c). Similar co-intercalation has been reported in anatase TiO2 as cathode host in LSBs [37]. This phenomenon can be assigned to the overlapped potential regions for the electrochemical testing (1.8~2.8 V for LSBs and 1~3 V vs. Li+/Li for TiO2-B as anode in LIBs). The additional capacity provided by the TiO2-B nanosheet host was estimated as ca. 47.5

mA/g for TiO2-nanosheets and 36.6 mA/g for TiO2-B nanoribbons, respectively (Fig. S7, Supporting Information). Hence, the TiO2-B host only contributes very limited capacity to the total capacity of the Li-S batteries. The co-intercalation of Li+ ions can lead to the formation of electronically conductive LixTiO2 phase [37] for favored redox conversion of LiPSs on/near TiO2-B surface. Meantime, the TiO2-B surface with Li co-insertion can still maintain a high affinity to LiPSs with an Eads of -3.14 eV via interfacial chemical bonding (Fig. 4d) as further elucidated by colored charge density difference plot, effectively suppressing the undesired dissolution and loss of LiPSs species. These simulation results reveal that the TiO2-B surface can tightly anchor the LiPSs species and simultaneously propel their rapid conversion into Li2S during discharge.

Fig. 5. Optimized geometry constructions of Li adsorption on different sites on TiO2-B (100) surface with different Eads: (a) -2.47 eV, (b) -2.74 eV, (c) -4.01 eV, and

(d) -3.91 eV. These results indicate that Li can be effectively concentrated on the TiO2-B and participate in redox reactions during charge/discharge.

4. Conclusions Ultrathin TiO2-B nanosheets chemically bonded with amorphous carbon have been successfully synthesized and employed as a novel functional cathode host for Li-S cells. The presence of Ti3+ ions and carbon significantly boosted the electrical conductivity of the TiO2-B host. The ultrathin TiO2-B nanosheets with exposed (100) facets showed high affinity to lithium polysulfides, afforded abundant activation sites for lithium polysulfide immobilization and propelled their fast redox conversion into Li2S. The resultant TiO2-B/S composite cathode delivered high capacity, superior rate and excellent cyclability. The current work can make an important advance in the design and controllable fabrication of novel multifunctional cathode host materials for high-performance Li-S batteries.

Conflict of Interest The authors declare that they have no conflict of interest.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (21902122), Postdoctoral Science Foundation of China (2019M652723), National Key R&D Program of China (2016YFA0202602) and Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R52). B.-L. Su thanks the Chinese Central Government for an Expert of the “Thousand Talents” award. H.-E. Wang acknowledges the Hubei Provincial Department of Education for the “Chutian Scholar” program.

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Graphical Abstract Bronze TiO2 nanosheets were used as a novel multifunctional cathode host for LSBs that could tightly anchor lithium polysulfides and simultaneously catalyze their fast redox conversion.