Altering the reaction mechanism to eliminate the shuttle effect in lithium-sulfur batteries

Journal Pre-proof Altering the Reaction Mechanism to Eliminate the Shuttle Effect in Lithium-Sulfur Batteries Huanxin Li, Shuai Ma, Jiawen Li, Fuyu Liu, Haihui Zhou, Zhongyuan Huang, Shuqiang Jiao, Yafei Kuang PII:

S2405-8297(20)30002-7

DOI:

https://doi.org/10.1016/j.ensm.2020.01.002

Reference:

ENSM 1052

To appear in:

Energy Storage Materials

Received Date: 5 October 2019 Revised Date:

21 December 2019

Accepted Date: 2 January 2020

Please cite this article as: H. Li, S. Ma, J. Li, F. Liu, H. Zhou, Z. Huang, S. Jiao, Y. Kuang, Altering the Reaction Mechanism to Eliminate the Shuttle Effect in Lithium-Sulfur Batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.01.002. 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. © 2020 Elsevier B.V. All rights reserved.

Credit Author Statement Y. K., S. J., H. Z. and Z. H. conceived and designed the idea and co-wrote the paper and super vised the entire project and is responsible for the infrastructure and project direction. H. L. d iscussed the idea, experimentally realized the study, analyzed the data and co-wrote the pa per, while S. M. cooperated and repeated the experiments, and these work were assisted by J. L. and F. L. All the authors discussed the results, commented on and revised the manuscrip t. Dr. W. S. from Beijing Institute of Technology, J. X. from the University of Science and Tech nology Beijing and L. A. from King's College London have given helpful advise in the preparat ion of this work. Dr. Y. T. were appreciated for acquiring the Ti K-edge EXAFS data.

Altering the Reaction Mechanism to Eliminate the Shuttle Effect in Lithium-Sulfur Batteries Huanxin Li,§ab Shuai Ma,§a Jiawen Li,a Fuyu Liu,a Haihui Zhou,*a Zhongyuan Huang,*a Shuqiang Jiao*b and Yafei Kuang*a a

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and

Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China b

State Key Laboratory of Advanced Metallurgy, University of Science and Technology

Beijing, Beijing 100083, P. R. China §

*

These authors contributed equally to this work Corresponding

authors:

[email protected]; [email protected])

([email protected];

[email protected];

Altering the Reaction Mechanism to Eliminate the Shuttle Effect in Lithium-Sulfur Batteries ABSTRACT: Lithium-sulfur (Li-S) battery is one of the most promising energy storage devices. However, the development of Li-S battery is seriously hindered by the “shuttle effect” of polysulfides. Up to now, almost in all the researches related to sulfur cathode, the polysulfide motion restricting strategy is used to suppress the “shuttle effect”. However, this issue still cannot be thoroughly solved. Here, we report a new polysulfide generation restricting strategy to eliminate the “shuttle effect” in Li-S batteries by turning the S8 molecules into stably adsorbed small sulfur species with suitable solid redox mediators (RMs) to generate an S2σ--RMσ+ at the very beginning. In this way, the mediators (S2σ--RMσ+) are reduced into Li2S2 and Li2S directly without soluble polysulfide forming during the discharging process. Therefore, the “shuttle effect” of polysulfides (Li2S8, Li2S6, and Li2S4) is absolutely eliminated. This new polysulfide generation restricting strategy is realized by using a TiOxNy-TiO2 quantum dots@carbon composite (TiONQDs@C) as a sulfur host. The TiONQDs@C is proved to be an efficient RM to convert S8 molecules into stably adsorbed S2σ--RMσ+ species, eliminating the formation of lithium polysulfide completely. Owing to the new mechanism, the Li-S battery with TiONQDs@C host achieves a capacity of 869 mA h g1

(96% of the initial capacity) after 200 cycles with a low capacity decay of 0.02% per cycle.

This strategy provides a new way to thoroughly solve the “shuttle effect” in Li-S batteries. KEYWORDS: lithium-sulfur batteries, new mechanism, eliminating shuttle effect, small sulfur species, redox mediators

The pursuit for advanced next-generation energy storage devices with higher energy densities, higher power densities and better long-term stabilities is important to fulfil the increasing requirements of our future society, among which lithium-sulfur (Li-S) battery is considered to be one of the most promising candidates due to its high theoretical specific capacity (1672 mA h g-1) and energy density (2600 W h kg-1), low cost and environmental friendliness.1,

2, 3

Although Li-S battery has such advantages, there are still technical

challenges on its application. During the charge-discharge process, the multi-electron reaction of Li-S batteries is very complex with large amounts of soluble polysulfides produced during the reaction process, which will leads to the main obstacles impeding the practical

applications of Li-S batteries: the dissolution and shuttling of the soluble polysulfides (Li2Sx, 2
but is still very difficult to be settled completely with previously reported strategies.

Now we propose a new pathway for the reduction of sulfur by directly transforming S8 molecules into small sulfur mediators (S2σ--RMσ+) followed by further conversion to Li2S2/Li2S in discharge process (The intermediate pathway is shown in Figure 1a), which is realized by introducing TiON solid solution quantum dots (TiONQDs) composed of TiOxNy (0≤x≤y≤1, x+y=1), TiO2 and their heterojunctions as activator and adsorbent in sulfur cathode. Figure 1b presents the schematic diagram of TiONQDs@C composite, in which large amount of TiONQDs are anchored on N-doped carbon materials. The major component of the quantum dots is proved to be TiOxNy solid solution, with some micro-regions occupied by smaller TiO2 islands (Figure 1c). On the surfaces of TiONQDs, plenty of TiOxNy-TiO2 heterostructures are formed as shown in Figure 1d, which is believed to have played an important role in the transformation from S8 molecules into S2σ-Tiσ+-TiON. The proposed mechanism on sulfur reduction might be a new strategy to eliminate the “shuttle effect” completely by restricting the formation of polysulfides in Li-S batteries.

Previously, the reduction mechanism of S8 → Li2S8 → Li2S6 → Li2S4 → Li2S2/Li2S and reversed steps in charging process have been accepted generally. In the recognized charge discharge reaction pathway, the discharge reaction contains multiple intermediate steps, which always results in a dual platform in the discharge curve, corresponding to two reduction peaks in the charge/discharge CV curve (Figure 1e).15 During the discharge process, sulfur is firstly reduced to a series of soluble intermediate long-chain polysulfides (S8 → Li2S8 → Li2S6 → Li2S4), corresponding to the upper voltage plateau and the first reduction peak at around 2.3 V. As the reaction proceeds, these long-chain polysulfides will decompose into short-chain sulphide species (Li2S4 → Li2S2 → Li2S), and re-precipitate onto the electrode, which is represented by the lower voltage plateau and the second reduction peak at around 2.1 V. The specific reaction steps are as follows: S8 + 2Li+ + 2 e- ↔ Li2S8 +

(1) control step 1

-

Li2S8 + 2Li + 2 e ↔ Li2S6 + Li2S2 (2) Li2S6 + 2Li+ + 2 e- ↔ Li2S4 + Li2S2 (3)

Li2S4 + 2Li+ + 2 e- ↔ 2Li2S2

(4) control step 2

Li2S2 + 2Li+ + 2 e- ↔ 2Li2S

(5)

It can be seen that the whole reaction process of sulfur (S8 ↔ Li2S) is determined by two control steps: control step 1 (S8 ↔ Li2S8) and control step 2 (Li2S4 ↔ Li2S2), which is corresponding to the two obvious platforms in the discharge curve. Generally, the two platforms reflect the two control steps as presented in most of Li-S batteries and the control step 2 (Li2S4 ↔ Li2S2) would cause the accumulation of dissolved lithium polysulfides in the electrolyte (Figure 1f).

Following this reaction mechanism, most of researchers tended to focus on increasing the adsorption of polysulfides on sulfur host material, such as heteroatomdoped (B16, N17, P18, S19) carbon materials (graphene, meso-/microporous carbons, carbon nanotubes/fibers), or transition metal compounds (Fe3C,20 Co9S8,21 NiS,22 Fe2O3,23 MnO2,24 etc.) and their composites with carbon materials in previous reports.25 Besides, many efforts have also been made in improving the catalytic ability of host materials to accelerate the reaction rate of the control steps, which could largely shorten the lifetime of lithium polysulfides (e.g. TiN could promote the conversion from Li2S4 to Li2S2), and thereby reduce the possibility of “shuttle effect”.26, 27 However, the dissolution and shuttling of the intermediate lithium polysulfides are still inevitable.

Actually, another pathway has been once noticed that the “shuttle effect” can be eliminated by transforming S8 molecules into stabilized small sulfur molecules (S2-4).28 Li-Jun Wan et al. reported small sulfur molecules (S2-4) wrapped in multiwalled CNTs layers with dimension less than 0.5 nm, where the small sulfur molecules were directly reduced into Li2S2 and Li2S without formation of lithium polysulfides during the discharging process.29 However, it is still a big challenge to stabilize small sulfur molecules through physical space constraints. It is possible that we can stabilize the S2 molecules with effective adsorbents/mediators and finally eliminate the polysulfides by directly altering the reaction mechanism of sulfur reduction (Figure 1g, h). Inspired by this idea, we dedicated to transferring S8 molecules into stably adsorbed S2σ--RMσ+ with suitable adsorbents at the very beginning, and then they can be directly converted into Li2S2 and Li2S to eliminate the “shuttle effect” completely.

Figure 1. (a) The schematic diagram of possible conversion pathway of sulfur species, (b) TiONQDs@C composite and (c) TiOxNy-TiO2 quantum dots; (d) Structure of TiOxNy-TiO2 heterojunction; (e) A typical 2-plateau charge/discharge profile of lithium–sulfur batteries with two control steps; (f) The relative contents of sulfur species in typical 2-plateau charge/discharge process of lithium-sulfur batteries; (g) Schematic of the electrochemical reaction in lithium-sulfur batteries with stable sulfur cathode; (h) The relative contents of sulfur species in lithium-sulfur batteries with a single control step; (i) Schematic of TiOxNyTiO2 heterojunction convert S8 molecule into stable S2σ--RMσ+; (j) Single-plateau charge/discharge profile, (k) Single-peak CV profile and (i) Charge/discharge curves of lithium-sulfur batteries with TiONQDs@C/S cathode at different current densities.

Although S2 molecule is very unstable under common condition, it might be stabilized by some suitable redox mediators (RMs), and the mechanism is shown as follows: (i) S8 + RM → 4S2-RM (adsorbed) → 4S2σ--RMσ+ (ii) S2σ--RMσ+ + 2Li+ + 2e- → Li2S2-RM (iii) Li2S2-RM + 2Li+ + 2e- → 2Li2S + RM TiOxNy, TiO2 and their heterojounctions and vacancies in TiONQDs are proved to be efficient adsorbents and RMs, which might tear S8 molecule into small S2 molecules and stabilize them by converting S2 molecules to S2σ-Tiσ+-TiON (Figure 1i). Interestingly, when the mixture of TiONQDs@C and sulfur (TiONQDs@C/S) served as cathode in Li-S battery, it displays only a single platform in the charge and discharge processes (Figure 1j). Meanwhile, the cyclic voltammetry (CV) curves of TiONQDs@C/S battery present a single peak in each oxidation/reduction process (Figure 1k). Moreover, the charge-discharge curves maintain single platform at different current densities (Figure 1l). It is because that the reactions from S8 to S2σ-Tiσ+-TiON and S2σ-Tiσ+-TiON to Li2S2 are occurred at almost the same voltage of ~2.3 V, and thus the two peaks are difficult to be distinguished. These electrochemical behaviors different from previous reports are ascribed to our newly proposed reaction mechanism.

The TiONQDs@C composite was obtained by a simple annealing method (Supporting Information). Several characterizations were performed to investigate the morphology, structure and composition of TiONQDs@C. Firstly, the morphology of TiONQDs@C composite was observed using scanning electron microscope (SEM). As shown in Figure S1, large amount of carbon nanosheets in TiONQDs@C are stacked together and present a 3D network structure. Transmission electron microscope (TEM, Figure 2a, Figure S2 and S3) images reveal that plenty of quantum dots (~4 nm) uniformly distribute in carbon nanosheets. Further, the spherical aberration corrected transmission electron microscope (ACTEM) images (Figure 2b, Figure S4 and S5) confirm the existence of TiOxNy-TiO2 heterojunctions in TiONQDs@C composite.30 Figure 2c (HRTEM and the SAED pattern (inset)) shows the obvious lattice fringes with spacing of 0.241 nm, corresponding to the (111) plane of TiOxNy solid solution,31 while the lattice fringes of TiO2 are hard to be observed due to their extremely small size. X-ray diffraction (XRD, Figure 2d) was used to characterize the

composition of TiONQDs@C. The first diffraction peak at 2θ=~21° reflects the existence of carbon component, and the other peaks at 2θ=36.66°, 42.59°, 61.81°, 74.07° and 77.96° correspond to (111), (200), (220), (311) and (222) planes of TiN (PDF#38-1420) and TiO (PDF#08-0117) in TiOxNy (Space Group: Fm-3m [225]) solid solution since their crystal parameters are almost the same.32, 33 Then the surface chemical composition of the composite was investigated by X-ray photoelectron spectroscopy (XPS), and the wide-scan survey XPS spectrum in Figure S6 shows the dominant peaks at ~284.6, ~397.2, ~532.1 and ~454.8 eV corresponding to C1s, N1s, O1s and Ti2p, respectively. The signals of Ti, N, C and O reveal the existence of TiOxNy solid solution quantum dots in carbon matrix. The spectrum of Ti2p (Figure 2e) indicates the presence of TiN, TiO2 and TiOxNy on the surface of the quantum dots. The strongest peak at around 454.8 eV and the peak near 455.8 eV represent the majority of Ti-N bond in the composite. Peaks at about 457.1 eV and 463.2 eV are corresponding to TiOxNy and the peak at 460.8 eV reveals the existence of TiO2, indicating the incomplete reduction at annealing process.34 In the spectrum of N1s (Figure S7), the peak at around 397.2 eV represents the Ti-N bond, and the other two peaks at about 396.8 eV and 400.4 eV are ascribed to the presence of O-N bond, which also indicates the existence of TiOxNy solid solution. As for C1s spectrum (Figure S8), the main peak at 284.6 eV corresponds to sp2-hydridized carbon, while the peak at 285.8 eV indicates the formation of C-N bond during the annealing process.35 The typical TEM and corresponding EDS mapping images of Ti, N, C and O elements are demonstrated in Figure 2f, indicating a uniform distribution of each element. The Raman spectroscopy (Figure S9) of TiONQDs@C presents a strong characteristic peak of TiN at ~155 cm-1, and three wide peaks at ~258, ~415 and ~605 cm-1 corresponding to TiO2.36 TGA curve shown in Figure S10 demonstrates the content of TiN quantum dots in the composite is about 37.97 wt. %. In addition, the ICP analysis reveals the accurate concentration of Ti element in TiONQDs@C is 27.94 wt. %, corresponding to a weight ratio of 36.12 wt. % for TiN. The BET analysis in Figure S11 indicates the composite possesses a specific surface area of 78 m2 g-1 with an average pore size of 13.68 nm. The surface atomic structures of the Ti centers in TiN, TiO2 and TiONQDs@C were characterized by X‐ray absorption near-edge structure (XANES) spectroscope. Ti K-edge XANES spectra of TiN, TiO2 and TiONQDs@C are shown in Figure 2g, all of which demonstrate similar K-edge signals of Ti element at ~4960 eV. The Ti K-edge intensity of TiONQDs@C is between that of TiN and TiO2, indicating the electronic structure change of the Ti species in TiONQDs@C with a mixture of TiOxNy solid

solution and small part of TiO2. Moreover, the corresponding Ti K-edge Fourier‐ transformed (FT) EXAFS k3χ(k) oscillation curves are shown in Figure 2h. The oscillation of Ti K-edge FT k3χ(k) in TiONQDs@C is similar with that in TiN since the structure of TiOxNy solid solution is almost the same with that of TiN. However, there is strengthened oscillation amplitude in Ti-N bond related signal and a reduction in Ti-Ti bond oscillation, revealing a structural change in the coordination environment of Ti atoms due to the formation of TiOxNy-TiO2 heterojunctions in TiONQDs@C. The bond lengths of TiN, TiO2 and TiONQDs@C are obtained by fitting the FT k3χ(k)-R space curves. The Ti-N and Ti-O bonds in TiONQDs@C are shorter than that in TiN and TiO2, especially Ti-O bond is 0.14 Å shorter (Figures 2i), indicating the obvious size effects of those quantum dots (Ti-O species shall be smaller than 3 nm in the TiONQDs@C composite). The Ti-Ti distance is 2.99 Å, which is slightly larger than that in TiN (2.98 Å) but smaller than that in TiO2 (3.01 Å), indicating the hybrid structure of TiO2-TiN heterojunctions in TiONQDs@C (Figures 2j). Meanwhile, the coordination number of Ti (11.4) in TiONQDs@C is higher than that in TiN and TiO2, and the N coordination number (12.2) is also higher than that in TiN, but the O coordination number (3.2) in TiONQDs@C is much lower than that in TiO2 (Figures 2k), suggesting that the Ti-N species occupy the dominant position in the TiOxNy solid solution (x
Figure 2. (a) TEM image of TiONQDs@C composite; (b) ACTEM image of TiONQDs@C composite; (c) HRTEM image of TiONQDs@C composite; (d) XRD pattern of TiONQDs@C composite; (e) XPS Ti2p spectra of TiONQDs@C composite; (f) Typical TEM and corresponding elemental mapping images of Ti, N, C, O; (g) Ti K-edge XANES spectra; (h) Ti K-edge Fourier‐ transformed (FT) EXAFS k3χ data of TiN, TiO2 and TiONQDs@C; (i) Corresponding bond length of Ti-N, Ti-O and (i) Ti-Ti bonds and coordination number in TiN, TiO2 and TiONQDs@C.

Density functional theory (DFT) calculations were performed to verify the possible adsorption and transformation behavior of S8 molecules on TiONQDs@C composite. Since previous literature reported that S2 molecules could be stably adsorbed on Cu/ZnO surfaces,38 it was a rational deduction that suitable hybrid of metal/metal compound heterostructural interface might turn the S8 molecules into adsorbed S2σ- species and stabilized by forming S2σRMσ+ mediators. It is verified that the TiONQDs@C composite contains plenty of TiOxNyTiO2 heterojunctions. Our first hypothesis is that the TiOxNy-TiO2 quantum dots might activate the S8 molecules and transformed them into stably adsorbed S2σ--Tiσ- species. To simplify the model of TiOxNy-TiO2 heterojunctions, the value of x was set as 0 and then the TiN-TiO2 heterojunction model was built and fully optimized.39 Since the Ti-Ti bonds in TiO2 and TiN only have slight difference and their crystal structure can be well matched, it is easy to form a stable TiN-TiO2 heterostructure. The optimized TiN-TiO2 heterostructural model is shown in Figure 3a, indicating the theoretical presence of TiN-TiO2 heterojunction. After that, an annular S8 molecule was placed above the TiN-TiO2 heterojunction to investigate this particular adsorption behavior. Remarkably, the structure of S8 molecule after adsorption on TiN-TiO2 heterojunction significantly changed (Figure 3a), with an annular S8 molecule divided into four S2 molecules. For comparison, the TiN and TiO2 unit cells were also built to evaluate their adsorption behaviors on S8 molecules. As it can be seen in Figure 3b and Figure 3c, both S8 molecules adsorbed on TiN and TiO2 unit cells have certain deformation in shape, but still retaining the structure of annular molecule without obvious break of S-S bond. Therefore, it is verified that the TiN-TiO2 heterojunction interface possesses unique catalytic activity in transforming S8 molecules into stably adsorbed Tiσ+S2σ-TiON. Meanwhile, as we mentioned, there was a certain proportion of Ti vacancies in the TiOxNy solid solution. Vacancies have been proved to exhibit unique and significant activity in many catalytic systems,40 which might also be potential active sits for the transformation of S8 molecules. Therefore, we also built a model of TiOxNy solid solution with existence of Ti vacancy (TiOxNy-VTi). The VTi surrounding was always saturated by oxygen atoms to form stable structure.41 As shown in Figure 3d, the structures of S8 on TiOxNy-VTi before and after adsorption have very large difference in shape. Owing to the strong electronegativity of oxygen atoms around the VTi, S8 molecule would be subject to the strong local repulsion and tend to be adsorbed on the Ti atoms nearby, resulting in four separated S2σ- species. Thus, the VTi in TiOxNy solid solution is also an efficient active site for the transformation of S8 molecules to S2σ- molecules. It is obviously that the adsorbed S2 species on both TiN-TiO2 and TiOxNy-VTi interfaces were tightly bonded with Ti atoms, forming an S2σ-Tiσ+-TiON

mediator. Besides, Zhong etc. revealed that the O vacancies in H-TiO2 promoted the formation of Ti-S bonds.42 Therefore, the O vacancies in TiO2 might be another efficient active sites to stabilize the formed S2σ-Tiσ+-TiON species, which remained to be validated. Moreover, the huge surface energy and unique quantum size effects of TiNxOy-TiO2 QDs could also be conducive to tearing the annular S8 molecules into small adsorbed S2σ-Tiσ+TiON species. It is hard to say which factor is the most critical one yet, maybe the combination and synergism of all these factors. Also, it is difficult to distinguish whether the S2σ-Tiσ+-TiON was formed before or during the discharging process, or might be both. But what is certain is that when the TiONQDs@C composite served as sulfur host, it could eliminate the “shuttle effect” at the very beginning since no lithium polysulfide produced during the whole process, which is a totally new strategy to restrain the “shuttle effect” in LiS batteries. The possible reactions might be: (i) S8 + TiON → 4S2-TiON → 4Tiσ+S2σ--TiON (ii) Tiσ+S2σ--TiON + 2Li+ + 2e- → Li2S2-TiON (iii) Li2S2-TiON + 2Li+ + 2e- → 2Li2S + TiON It is worth mentioning that there are several reported TiN/TiOx and carbon composites used as sulfur host for Li-S batteries but no similar results has been revealed previously, which might be due to that the much large TiN/TiOx particles/spheres (50-1000 nm) reported in these literature cannot provide sufficient charges and active sites to generate enough Tiσ+S2σmediator, and thus the lithium polysulfide still generated.43, 44, 45, 46, 47, 48, 49, 50 In our work, the large number of TiOxNy solid solution quantum dots (2-5 nm) enable the rapid formation of stable Tiσ+S2σ--TiON mediator, also, it is possible that the adsorbed mediator would be continously converted into titanium sulfide during discharge process and then transferred into Li2S2 and Li2S, which completely eliminates the polysulfide mediators.

Figure 3. (a) Structures of S8 on TiN-TiO2 quantum dots before and after adsorption; (b) Structures of TiN@S8 and (c) TiO2@S8 before and after adsorption; (d) Structures of S8 on TiOxNy-VTi before and after adsorption.

Then the morphology and structure of TiONQDs@C/S composite were investigated. It remains a nanosheet morphology, without large sulfur particles being observed (Figure 4a, b). In the enlarged TEM image of TiONQDs@C/S composite, plenty of TiONQDs dispersed on the composite, which were not covered by large sulfur particles (Figure 4c). Meanwhile, the HRTEM image of TiONQDs@C/S composite also shows clear TiONQDs (Figure 4d). The typical TEM and corresponding mapping images of C, N, O, Ti and S elements of TiONQDs are shown in Figure 4e-j, which indicate that the C, N, O and Ti elements kept the original state in TiONQDs@C/S and sulfur was in a relatively uniform distribution. XRD patterns of pure sulfur and TiONQDs@C/S are shown in Figure S12a. The pure sulfur demonstrates sharp peaks at 2θ=20-30°. In the XRD pattern of TiONQDs@C/S composite, the peaks for TiON remain, indicating that the composition of TiONQDs@C did not change at this state, however, the peaks for sulfur in TiONQDs@C/S composite were weakened compared to those of pure sulfur, which suggests that sulfur was almost uniformly dispersed on the TiONQDs@C. XPS survey (Figure S12b) demonstrated the existence of S, C, N, Ti and O elements in the TiONQDs@C/S composite. In the high-resolution XPS of Ti 2p, the strongest peak at around 458.3 eV is corresponding to TiOxNy as well as Ti-S bond formed in TiONQDs@C/S composite, indicating the strong interaction between TiONQDs@C and sulfur (Figure S12c). In the S 2p curves of C/S and TiONQDs@C/S composite, the peaks of TiONQDs@C/S and C/S were located almost at the same position (Figure S12d), suggesting the majority of sulfur in TiONQDs@C/S composite remains in the state of S-S bond. TGA curves (Figure S13) demonstrated that both of C/S and TiONQDs@C/S maintained weight ratio of ~35 wt.% at 400 °C, but the curve for TiONQDs@C/S displayed a relatively slow decline, again indicating the strong interaction between TiONQDs@C and sulfur. Therefore, we tend to believe that during the loading process the sulfur was chemically adsorbed on TiONQDs@C and the interaction between Ti and S atoms was strong intermolecular attraction force. Then in the discharging process, since the TiONQDs@C was more conductive than sulfur, the TiONQDs would be in an electron-rich state. The electrons would continously transfer from Ti to S atoms, forming a Tiσ+S2σ--TiON mediator. Finally, with the continuous supply of lithium ions and electrons, the Tiσ+S2σ--TiON mediator further transferred into Li2S and TiON, and the charging process is reversible (Detailed schematic diagram see Figure S14).

Figure 4. (a, b) SEM images of TiONQDs@C/S; (c) TEM and (d) HRTEM images of TiONQDs@C/S; (e-j) TEM and corresponding mapping images of C, N, O, Ti and S elememts.

To further investigate the adsorption and transformation capability of TiONQDs@C composite to S8 molecules, 2025-type coin cells were assembled to observe the electrochemical behaviors of TiONQDs@C/S cathode in Li-S batteries with a lithium sheet as anode. For comparison, a mixture of conductive carbon and sulfur (C/S) was also prepared and served as cathode in Li-S batteries. Firstly, cyclic voltammogram (CV) experiments at a low scan rate of 0.1 mV s-1 were performed for both TiONQDs@C/S battery and C/S battery. As shown in Figure 5a, the C/S battery displayed typical reduction peaks at ~2.3 and ~2.0 V, corresponding to the two control steps of S8 to Li2S8 and Li2S4 to Li2S2 in discharge process. While the TiONQDs@C/S battery presented only a single reduction peak at ~2.35 V, indicating that the sulfur reduction in TiONQDs@C/S battery was a one-step process which might be ascribed to the direct conversion from adsorbed S2σ-Tiσ+ species into Li2S2/Li2S. Since the S2σ-Tiσ+ species were in an activated state, the overpotential of the conversion from S2 to Li2S2 was very low. The charge-discharge curves of TiONQDs@C/S battery and C/S battery are shown in Figure 5b, which also clarified the one-step transformation from S2σTiσ+ to Li2S2 in TiONQDs@C/S battery and multiple steps in C/S battery since the discharge process of TiONQDs@C/S battery presented only one discharge platform at ~2.35 V while the C/S battery displayed two typical discharge platforms (~2.3 V and ~2.0 V). Previously, Zhang et al. reported a kind of porous carbon spheres as Li-S batteries cathode support materials which can inhibit the formation of long-chain polysulfides (Li2S8 and Li2S6) due to its micropore confinement effect, and thus only the low voltage of ~2.0 V corresponding to the conversion of Li2S4 → Li2S2 was observed in the discharge curve.44 The research verified that the charge-discharge curves would change as the reaction mechanism changed. In our work, the research results revealed that the TiONQDs@C composite significantly activated the S8 molecules and transferred them into stably adsorbed S2σ-Tiσ+, thus the intermediate state of lithium polysulfide would not appear and the "shuttle effect" during discharge process could be totally eliminated, which verified our former hypothesis. In order to further experimentally verify the reaction mechanisms of TiONQDs@C/S battery and C/S battery, in-situ XRD analysis was conducted on soft packing Li-S batteries with a window sealed by tape on the TiONQDs@C/S side as shown in Figure 5c. Three electrodes were connected from the XRD operating room to an electrochemical workstation which provided sustainable current signal for the charge and discharge processes of Li-S batteries. The sharp peak at 2θ=~23.8° corresponds to the (-110) plane of Li2S2,45 which is a characteristic product produced during charge and discharge processes (Figure 5d). The continuous XRD pattern of TiONQDs@C/S battery within a charge-discharge cycle is shown

in Figure 4e, f. The in-situ XRD patterns in discharge process (Figure 5e) demonstrated that the Li2S2 appeared when discharged to 2.3 V. Then the peak intensity increased as the discharging proceeded until 2.0 V owing to the accumulation of Li2S2. After that, the peak intensity of Li2S2 started to decline because of the transformation into Li2S. Meanwhile, during the charge process (Figure 5f), firstly Li2S reconverted into Li2S2 before 2.4 V, then turned into S2 rapidly and the signal of Li2S2 disappeared after that. However, in the C/S battery the situation of Li2S2 peaks was different that only a weak XRD peak of Li2S2 presented when discharged to 1.9 V (Figure 5g), and it became much stronger at potential of 1.7 V. The rational reason might be that in C/S battery S8 molecules were firstly reduced into lithium polysulfides (Li2S8, Li2S6 and Li2S4 etc.) before 2.0 V, and then further reduced to be solid Li2S2 after 2.0 V. In charging process, the produced Li2S2 remained until 2.5 V (Figure 5h), indicating the slow kinetics of multi-step oxidation process. To visually observe the onestep conversion of solid to solid (S2 to Li2S2) process in TiONQDs@C/S battery, we construct a two-electrode system to simulate the reaction of the Li-S battery (Figure 5i). The entire system was fabricated in a glove box with an argon atmosphere. Typically, TiONQDs@C/S electrode and lithium sheet were fixed by conductive titanium wires, then inserted into the rubber plug with holes, and installed in a beaker containing Li-S battery electrolyte, at last the entire device was completely sealed for testing. The one with C/S electrode was used for comparison. After that we connected this system to the electrochemical workstation and the discharging test was performed to simulate the reduction process of sulfur species in the Li-S battery. As we can see in Figure 5j, the electrolyte in both beakers with TiONQDs@C/S and C/S electrodes was colorless and transparent before test. As the test proceeding, it can be observed clearly that the electrolyte on the side of the C/S electrode turned yellow, while that near the TiONQDs@C/S electrode showed no significant change. After the experiment was completed, the electrolyte with C/S electrode turned yellow overall, while the electrolyte with TiONQDs@C/S electrode was still transparent. In addition, Raman tests were carried out with the two electrolytes after test as well as the pure electrolyte, and the results are shown in Figure 5k. The pure electrolyte demonstrated typical Raman peaks, and the electrolyte in TiONQDs@C/S system showed almost the same peaks as the pure electrolyte. However, the Raman curve of the electrolyte in C/S system is quite different from the former two. Two excessive peaks at about ~244 and ~487 eV were clearly detected, indicating the presence of polysulfides.10, 51, 52, 53, 54, 55, 56 This phenomenon further revealed that the TiONQDs@C composite could transform S8 molecules into stably adsorbed S2 molecules and completely eliminate the possibility of production of

lithium polysulfide in the electrolyte during reduction process. As a result, the electrochemical performances of TiONQDs@C/S battery were significantly enhanced.57, 58, 59 The AC impedance spectroscopies of TiONQDs@C/S and C/S batteries shown in Figure S16 indicated a much faster electron transfer rate in TiONQDs@C/S battery, which ensured the fast conversion process of sulfur species. Meanwhile, the TiONQDs@C/S battery achieved an excellent rate capacity of 917, 822, 747, 684, 622 and 537 mA h g-1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5C, respectively, which is much better than that of C/S battery (Figure 5l). At the current density of 0.1 C, the initial capacity of TiONQDs@C/S battery was 906 mA h g-1 and remained 869 mA h g-1 after 200 cycles (All the capacities are calculated based on the weight of sulfur in the cathode), corresponding to a low capacity decay of 0.02% per cycle. However, the C/S battery delivered an initial capacity of 685 mA h g-1 and only remained 169 mA h g-1 after 200 cycles (Figure 5m). Therefore, the cycle stability of TiONQDs@C/S battery was significantly improved due to the solid-solid reaction process. The capacity retention rates of Li-S batteries with conventional pathway as well as our strategy are summarized in Figure 5n. The capacity retention rates of Li-S batteries using previous physical restriction, chemical adsorption and catalytic strategy are 50-60% (Ref. S13), 60-70% (Ref. S4-8) and 60-80% (Ref. S9-18), respectively. In our strategy, the retention of 96% of the initial capacity after 200 cycle is the best ever reported in Li-S batteries.

Figure 5. (a) CV curves at a scan rate of 0.1 mV s-1and (b) charge-discharge curves of TiONQDs@C and C/S batteries; (c) In-situ XRD equipment; (d) Typical XRD pattern of TiONQDs@C battery when discharged at 2.0 V; (e) XRD patterns of TiONQDs@C battery during discharge process and (f) Charge process; (g) XRD patterns of C/S battery during discharge process and (h) charge process; (i) Optical image of the two-electrodes system; (j) Optical images of the systems with C/S and TiONQDs@C electrodes before, during and after the testing; (k) Raman analyses of pure electrolyte, and the electrolytes with C/S and TiONQDs@C electrodes after testing; (l) Rate performances and (m) Cycle performances of

TiONQDs@C and C/S batteries; (n) The capacity retention rate of this strategy compared to reported strategies. In summary, we proposed a new idea of transforming S8 into stable S2σ--RMσ+ mediators to completely eliminate the “shuttle effect” in lithium-sulfur batteries and thereby the capacity loss was reduced. A TiONQDs@C composite with plenty of TiN, TiOxNy, TiO2 quantum dots and their heterojunctions anchored in nitrogen-doped carbon material was synthesized, which has been proven capable of activating S8 molecules to be stably adsorbed Tiσ+S2σ-TiON. The results show that when TiONQDs@C/S serves as cathode in Li-S battery, it demonstrates only one platform at ~2.35 V in the discharge curve, and the CV curve also shows a single reduction peak, which means a one-step conversion from Tiσ+S2σ--TiON to Li2S2 molecules, resulting in a complete elimination of “shuttle effect”. Owing to the new mechanism, the TiONQDs@C/S battery achieved a capacity of 869 mA h g-1 (96% of the initial capacity) after 200 cycles at 0.1 C with a low capacity decay of 0.02% per cycle in LiS batteries. The superior stability is ascribed to the solid-solid reaction process in the conversion of sulfur species. This strategy opens up a new road for eliminating the "shuttle effect" in Li-S batteries, which significantly promotes the development of Li-S batteries. It should be pointed out that the structure of TiN-TiNxOy-TiO2 is not very easy to be accurately controlled yet through this one-step annealing method. In the future, it is a meaningful work to search for more precisely designed approaches and more effective adsorbents to stabilize the S2 molecules, which could be a novel strategy to eliminate the "shuttle effect" in Li-S battery completely.

Acknowledgements The authors are grateful to Dr. Weili Song from Beijing Institute of Technology, Jiusan Xiao from the University of Science and Technology Beijing and Leigh Aldous from King's College London for their assistance during the preparation of this work. Beijing Synchrotron Radiation Facility (BSRF) 4B9A Experimental Station as well as Dr. Yuanyuan Tan were appreciated for acquiring the Ti K-edge EXAFS data. National Supercomputing Center in CHANGSHA is also acknowledged for allowing the use of computational resources including TIANHE-1. This work was financially supported by National Natural Science Foundation of China (Grant nos. 51974114, 51672075, 21271069, 51772092, 51704106 and 21908049) and the Fundamental Research Funds for the Central Universities.

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The authors declare no competing financial interest.