Synchronous Gains of Areal and Volumetric Capacities in Lithium

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Synchronous Gains of Areal and Volumetric Capacities in LithiumSulfur Batteries Promised by Flower-Like Porous Ti3C2Tx Matrix Zhubing Xiao, Zhi Yang, Zhonglin Li, Pengyue Li, and Ruihu Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09296 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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Synchronous Gains of Areal and Volumetric Capacities in Lithium-Sulfur Batteries Promised by Flower-Like Porous Ti3C2Tx Matrix Zhubing Xiao†,§, Zhi Yang*, ‡, Zhonglin Li†,§, Pengyue Li†, and Ruihu Wang*,† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

†

Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡

Nanomaterials & Chemistry Key Laboratory, Wenzhou University, Wenzhou, 325027, China. University of Chinese Academy of Sciences, Beijing, 100049, China.

§

*E-mail: [email protected]; [email protected]

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ABSTRACT The areal and volumetric capacities are important metrics in practical deployment of advanced energy storage systems with the imposed constraints by device volume and chip area. Conductive carbons are promising sulfur host materials for improving areal capacity in lithium-sulfur (Li-S) batteries, but they face a few congenital deficiencies, such as low tap density and weak polysulfides entrapment ability, resulting in poor volumetric performance. Here, we report one type of cathode systems based on flower-like porous Ti3C2Tx (FLPT) without the incorporation of any carbon hosts or conductive additives. The resultant FLPT-S electrode synchronously acquires high areal capacity of 10.04 mAh cm-2 and ultrahigh volumetric capacity of 2009 mAh cm-3. Furthermore, ex-situ electron paramagnetic resonance and UV-visible spectra have demonstrated that FLPT enables a fast dynamic equilibrium between S62− anion and S3*− radical during cycling, which promotes the redox reactions of sulfur species.

KEYWORDS: lithium-sulfur batteries, MXene, shuttle effect, volumetric capacity, areal capacity

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Lithium-sulfur (Li-S) batteries hold great promises in next-generation energy storage systems due to overwhelming advantages of the cathode material sulfur, such as high theoretical capacity, low cost and environmental benignity.1,2 However, their practical applications are still hinged by several intractable issues, such as poor electric conductivity of sulfur and its insoluble discharge products, and large volume fluctuation of the active materials during cycling. Worse still, the shuttle effect resulting from the dissolution and migration of long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) usually leads to rapid capacity decay, low Coulombic efficiency and serious self-discharge, which further exacerbates the serious problems.3-5 In response to these daunting challenges, considerable efforts have been devoted to designing rational cathodes with elaborate compositions and structures.6-12 Nevertheless, most of reported scenarios show low overall sulfur content and low areal sulfur loading (ASL, < 4 mg cm-2), the corresponding areal capacities are typically less than 4 mAh cm-2, a representative energy density value for electrical vehicle design. Although the freestanding cathodes have been reported recently to achieve both high sulfur loading and high areal capacity even more than 10 mAh cm-2,8,13 compared with traditional slurry-based cathode, the tedious synthetic procedures and time-consuming film formation processes make the freestanding cathodes impractical for their large-scale application. In order to fabricate high sulfur loading cathodes using traditional slurry-coating technique that fits commercial protocols, two typical strategies are utilized: 1) engineering three-dimensional (3D) conductive networks to accommodate high content of sulfur;14-18 2) constructing compact and high-tap-density cathodes to aggrandize particle packing and decrease void space by integrating individual nanocomposites into large secondary particles.19,20 These tactics improve ASL and 3 ACS Paragon Plus Environment

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corresponding areal capacity to some extent, but most of reports typically rely on the conductive and porous nanocarbons, which have some negative effects by way of parenthesis. Firstly, due to large fraction of low-tap-density nanocarbons, high areal loading of sulfur is achieved at the expense of volumetric energy density, another important metrics for practical application, where the space is grimly limited.20 Moreover, these carbon-based cathodes with high ASL suffer from some tough issues induced by the spikes in electrode thickness, including lengthy ion transport paths and sluggish reaction kinetics. As such, the volumetric capacity over than 2000 mAh cm-3 has rarely been reported in the carbon-based Li-S batteries. Secondly, the reported carbon-based cathodes are usually lack of sufficient polysulfide adsorptivities because the weak interactions between non-polar carbons and highly polar polysulfides can only physically confine sulfur species in the cathode region, which in principle aggravates the shuttle effect of polysulfides. The weak interfacial interaction also impedes the redox of sulfur species and slows down the reaction kinetics on carbon hosts.21,22 Therefore, exploiting one type of neoteric carbon alternatives with high tap density, polarity and conductivity are highly desirable for realizing high-energy-density Li-S batteries with high areal and volumetric capacities. The inorganic conductive materials, such as transition metal sulfides,21-24 carbides25-28 and nitrides,29,30 are well known to possess abundant polar sites to chemically entrap and electrocatalyze polysulfides. These capabilities could be well retained as sulfur host materials in Li-S batteries. In particular, transition metal carbides are preferable candidates owing to their strong polysulfide adsorptivities and high electrical conductivity (104 S cm-1), which is much higher than most of carbon and inorganic materials.26-28, 31, 32 As a proof-of-concept study to address above-mentioned challenges in the carbon-based electrodes, herein, we have introduced the flower-like porous Ti3C2Tx 4 ACS Paragon Plus Environment

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(FLPT, Tx stands for the surface terminations, such as hydroxyl and oxygen) as a sulfur host material, where no conductive carbon hosts and carbon additives are employed. The micrometre-sized FLPT particle composed of well-organized conductive Ti3C2Tx nanosheets not only allows for rapid electron/ion transfer, but also enables a high tap density after sulfur loading. Moreover, apart from strong chemical interactions between FLPT and polysulfides, it is certified that a faster dynamic equilibrium between S62− anion and S3*− radical exists in this FLPT-S system when compared with traditional carbon-based cathode materials, which is highly favorable for the improving electrochemical performance in Li-S batteries. The resultant FLPT-S electrode delivers an areal capacity of 10.04 mAh cm-2 with ASL up to 10.5 mg cm-2, corresponding to an ultrahigh volumetric capacity of 2009 mAh cm-3.

RESULTS AND DISCUSSION The synthetic routine of FLPT is schematically depicted in Figure 1a. Brieﬂy, the etched Ti3C2Tx was firstly obtained through selective removal of the aluminum layer in Ti3AlC2 using aqueous HF acid, in which the defects of Ti vacancy or vacancy clusters were concomitantly formed32 (Figure S1). Subsequent hydrothermal treatment in the presence of ethanediamine (EDA) at 80 oC for 8 h gave rise to FLPT. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that FLPT consists of micrometre-sized assemblies with a flower-like morphology (Figure 2a and Figure S2a). Closer examination indicates that each assembly is composed of curved nanosheets with the depth less than 100 nm (Figure 2b). Each nanosheet shows obvious distortion of (002) lattice over an extended area with the lattice fringe of 1.02 nm (Figure S2b). These primary 5 ACS Paragon Plus Environment

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nanosheets are oriented at different angles, the space-efficient packing secondary assemblies are generated through their interconnection, which not only increases structure stability during sulfur infiltration and charge/discharge cycling, but also results in high tap density. Notably, transparent and curved Ti3C2Tx nanosheets are also detected at the edge of FLPT, as denoted by red arrows in Figure 2c, further suggesting the flower-like assemblies are interconnected into a 3D architecture, which is favorable for the global electron transfer. Strikingly, in high-resolution TEM images of FLPT, the round-shaped nanopores with the size of 2-5 nm could be clearly detected in the (103) lattice plane of the Ti3C2Tx nanosheets (Figure 2d). The intralayered spacing of 0.24 nm is agreement with the lattice distance in the pattern of the selected area electron diffraction (SAED, inset in Figure 2d). The formation of the nanomeshes is probably attributed to the intralayered Ostwald ripening process (Figure 1b).33-35 The time-dependent TEM images have revealed the formation mechanism of the nanomeshes (Figure S3). When etched Ti3C2Tx is subject to hydrothermal treatment, EDA is intercalated into the interlayer of Ti3C2Tx. Strong electron-donor ability of amine groups induces the interaction between EDA and the thermodynamically active Ti sites, resulting in gradual dissolution of active Ti species on the periphery of Ti vacancies, the accumulation of Ti vacancy domains generates primary large pores, the dissolved Ti species are subsequently deposited in the fringes of large pores for decreasing the overall energy, the rearrangement of these nanopores generates the nanomeshes with uniform pore size (Figure 1b). It is believed that the nanomeshes not only could provide abundant active sites around the nanopores to entrap polysulfides, but also could serve as permeable channels for ion transport and electrolyte penetration, which are beneficial for the suppression of polysulfides shuttle effect and the facilitation of redox reactions. To shed light on the evolution course of FLPT during hydrothermal treatment, SEM images of 6 ACS Paragon Plus Environment

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hydrothermal products at different reaction temperatures were measured. As shown in Figure S4, the self-assembly of primary nanosheets occurs at 60 oC. As the rise of the temperatures from 60 to 70 and 80 oC, the assemblies become gradually compact by bridging neighboring Ti3C2Tx nanosheets, the corresponding tap densities increase from 3.02 to 3.33 and 3.89 g cm-3, respectively. However, when the reaction was performed at 90 oC, the flower-like architecture is apt to collapse owing to undesired dissociation of the assemblies at an elevated temperature, and the tap density is slightly decreased to 3.54 g cm-3. These results have demonstrated hydrothermal temperature and reaction time have exerted important effects on the formation of FLPT. To the best of our knowledge, the tap density of FLTP is the highest in reported MXene-based sulfur host materials. Of particular interest is that FLPT could be processed into flexible film despite such high tap density (inset in Figure 2a), which is favorable for the fabrication of the slurry-coated electrodes with high sulfur loading. The nitrogen adsorption and desorption isotherm of FLPT exhibits a hysteresis loop in the range of 0.5-1.0 P/P0, indicating the presence of mesopores (Figure S5a). The Brunauer-Emmett-Teller (BET) specific surface area of FLPT is 132.8 m2 g-1, which is roughly 26 and 8 times higher than those of commercial Ti3AlC2 (5.2 m2 g-1) and etched Ti3C2Tx (16.7 m2 g-1). The corresponding pore size distribution curve of FLPT reconfirms the presence of nanopores with diameter of 2-5 nm (Figure S5b), which is consistent with the high-resolution TEM results. FLPT possesses much larger pore volume (0.262 cm3 g-1) than commercial Ti3AlC2 (0.028 cm3 g-1) and etched Ti3C2Tx (0.005 cm3 g-1). In the XRD patterns of etched Ti3C2Tx and FLPT (Figure S6), the disappearance of typical diffraction peak at 2θ = 39.1o, corresponding to the (104) plane of Ti3AlC2, indicates complete removal of the aluminum layer from the raw material. Additionally, the main peak (002) is shifted to lower angle in etched Ti3C2Tx, and a further shift is observed in FLPT. Moreover, the diffraction 7 ACS Paragon Plus Environment

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peak gradually broadens, indicative of slight loss in crystallinity after the etching and subsequent thermal treatment. Notably, no typical peaks of carbon are detected in the XRD patterns, indicating the absence of carbon phase. The high-resolution C 1s X-ray photoelectron spectroscopy (XPS) spectrum also indicates the absence of carbon phase (Figure S7). The electrical conductivity of FLPT was measured via four-probes method, and it is as high as 1205 (± 50) S cm-1. High electrical conductivity together with the nanomesh structure in FLPT could efficiently facilitate electron transfer and ion transport, thus maximizing the utilization of sulfur in the case of high sulfur loading. To obtain well-contacted and densified FLPT-S composites, sulfur was infiltrated into FLPT by melt-diffusion process under appropriate pressure.37 Thermogravimetric analysis (TGA) curve under nitrogen shows that sulfur content is 61.5 wt% (Figure S8a). The weight loss temperature of sulfur in FLPT-S is much higher than that of element sulfur, indicating strong confinement effect of FLPT toward sulfur. The XRD patterns of FLPT-S show typical diffraction peaks of element sulfur (Figure S8b), which are indexed to the orthorhombic phase (JCPDS: 08-0247). In the SEM images of FLPT-S, the surface of FLPT is uniformly covered with sulfur to form a closely-packed composite, there are no appreciable dissociation of FLPT and random deposition of sulfur (Figure 2e). The element mapping images (Figure 2f) and energy-dispersive X-ray (EDX) analysis (Figure S9) further indicate uniform distribution of sulfur in FLPT. The electrochemical performance of the FLPT-S composite without the incorporation any carbon conductive additives was initially investigated by pairing it with a metallic Li foil. Three FLPT-S electrodes with ASL of 4.2, 6.8 and 10.5 mg cm-2 were fabricated by adjusting the thickness of the cathode piece, they are designated as FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5, respectively. 8 ACS Paragon Plus Environment

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Taking advantages of high tap density of FLPT-S composites, the electrolyte/sulfur ratio in this study is as low as 8:1 (uL/mg). The cyclic voltammetry (CV) profiles of the FLPT-S electrodes at a scan rate of 0.02 mV s-1 were performed in the voltage range from 1.6 to 2.8 V. As shown in Figure 3a and Figure S10, the FLPT-S-4.2 electrode shows two typical cathodic peaks at 2.31 and 2.03 V, which correspond to the reduction of element sulfur to soluble polysulfides and their further reduction to solid-state Li2S2/Li2S, respectively. The broad anodic peak at 2.41 V is associated with the reverse conversion of Li2S2/Li2S to polysulfides and ultimately to element sulfur.21 The response currents obviously decrease in the second cycle, which is ascribed to the rearrangement of active sulfur species from original positions to more energetically stable sites.38 However, the peak positions have no significant change (Figure S10a-c). As expected, the reduction/oxidation peaks in the CV curves of the FLPT-S-6.8 and FLPT-S-10.5 electrodes broaden, which probably results from increased electrode resistance and retardatory dynamics for electrolyte infiltration with the increment of sulfur loading (Figure S10d). The galvanostatic charge/discharge profiles of the FLPT-S electrodes at a current rate of 1/30 C consist of two well-defined discharge plateaus (Figure 3b), which are in well agreement with multi-step electrochemical reaction of sulfur in their CV curves. The collection coefficient (the area ratio of the peak associated with the formation of Li2S to that associated with the formation of polysulfides, QL/QH)6 in FLPT-S-4.2 is very close to the theoretical value of 3:1, clearly indicative of high sulfur utilization and effective suppression of the polysulfides shuttle effect. Despite high ASL in the FLPT-S-6.8 and FLPT-S-10.5 electrodes, their polarization remains at a low level, which indicates inherently favorable redox kinetics of polysulfides in these electrodes. To better understand the electrochemical process, the Li+ ion diffusion rate (DLi+) was evaluated 9 ACS Paragon Plus Environment

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using the Randles-Sevcik equation (see details in Figure S11). As shown in Figure 3c, despite high ASL, the DLi+ in the reduction/oxidation process of FLPT-S-10.5 only slightly decreases when compared with that in FLPT-S-4.2, implying fast ion transport in the FLPT-S electrodes, which is mainly attributed that the nanomeshes in FLPT facilitate ion transport and electrolyte penetration in the whole electrodes. The inherent advantage in term of mass transport is further displayed by charge/discharge profiles of the FLPT electrode. As shown in Figure S12a, FLPT shows high rate performance and negligible distortion of the charge/discharge curves even at ultrahigh rate up to 20 C, revealing high charge/ion transport in the absence of conductive additives. The electrochemical impedance spectroscopy (EIS) (Figure S12b) provides further insights into the charge transfer and ion transport in the FLPT electrode. For comparison, the electrode based on etched Ti3C2Tx was also investigated. The semicircles in the Nyquist plots show the charge transfer resistance in etched Ti3C2Tx and FLPT is low (approximately 30 Ohm) regardless of their different structures, but there is a clear disparity in ion transport. The Nyquist plot of the Ti3C2Tx electrode shows a 45o Warburg-type impedance in the mid-frequency region, while nearly vertical plot is observed in the FLPT electrode (Figure S12b), indicating FLPT is favorable for rapid ion transport, which is critical for the electrode systems with high ASL. The cycling performance of these electrode is shown in Figure S13. The initial capacities of the FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5 electrodes are 1178, 1120 and 957 mAh g-1, corresponding to the areal capacities of 4.95, 7.62 and 10.05 mAh cm-2, respectively. After 75 cycles, their discharge capacities are remained at 855 (3.60), 805 (5.48) and 746 mAh g-1 (7.83 mAh cm-2), respectively, with average Coulombic efficiencies more than 98%. The areal capacities in these electrodes are higher than commercial Li-ion batteries (4 mAh cm-2) (Figure 3d), and comparable 10 ACS Paragon Plus Environment

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with reported state-of-the-art carbon-based slurry-coating electrodes (Figure S14).14-20 Worthy of a special mention is the fact that the thickness of the electrodes will inevitably enhance when ASL is increased in order to achieve high areal capacity. However, the carbon materials usually possess low tap density, it is difficult to obtain thick carbon-based electrodes without the formation of pinholes and cracks in current collectors via traditional slurry-coating technique.20 The imbalance between ASL and electrode thickness makes high areal capacities accompanied by low volumetric energy densities in the carbon-based electrodes, which limits practical application of Li-S batteries in the confined space, such as electrical vehicles, portable electronics and miniature devices. Excitingly, the FLPT comprises the interconnected Ti3C2Tx nanosheets, and sulfur is homogeneously distributed on the surface of primary nanosheets, which guarantee high tap density in the FLPT-S composite and uniform coating with adjustable electrode thickness. The tap density of the FLPT-S composite is as high as 3.13 g cm-3, which is much higher than those in reported carbon-based sulfur composites (Table S1). Thus, FLPT-S is adaptable for the commercially accepted slurry-coating prototype to fabricate the electrodes with high areal and volumetric capacities. As shown in Figure S15, the cross-sectional SEM images show the total thicknesses of the FLPT-S cathode and aluminum foil in FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5 are approximately 32, 42 and 50 μm, respectively. The corresponding ratios of ASL to electrode thickness are 0.13, 0.16 and 0.21 mg cm-2 um-1, respectively. The sulfur contents per unit volume are significantly higher than those in most of reported carbon-based slurry-coating cathodes and even freestanding cathodes (Table S2).14-20 Considering above-mentioned areal capacities and electrode thickness, the volumetric capacities of FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5 are calculated to be as high as 1547, 1814 and 2009 mAh cm-3, respectively (Figure 3d). Their reversible volumetric 11 ACS Paragon Plus Environment

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capacities after 75 cycles are maintained at 1057, 1357 and 1360 mAh cm-3, respectively. The 3D projection map of ASL vs areal capacity vs volumetric capacity shows that the volumetric capacity of the FLPT-S electrode records the highest value in reported Li-S batteries (Figure 3e), indicating synchronous gains in high areal and volumetric capacities in the absence of conductive carbon hosts and carbon additives. Except for the structure engineering, the interfacial interactions between FLPT and polysulfides also play crucial roles in the solid-liquid-solid transformation of sulfur species. To better understand the interactions, the chemisorption of FLPT toward polysulfides was initially examined by immersing FLPT into the 15mM Li2S6 solution in DOL/DME. As shown in Figure 4a, brownish Li2S6 solution is decolored by FLPT, suggesting strong adsorption of FLPT toward Li2S6, which explains well the capacity retention in the charge/discharge cycles. To ascertain the nature of this chemisorption, the surface chemistry of FLPT after the adsorption experiment was examined by XPS analyses. In high-resolution Ti 2p XPS spectrum of pristine FLPT, the binding energy peaks at 458.3 and 454.4 eV are assigned to Ti-O and Ti-C bonds, respectively (Figure 4b).27,28 After adsorbing polysulfides, the typical peak of Ti-S bond occurs at 455.5 eV (Figure 4b), suggesting the Ti-S interaction in the FLPT-Li2S6 composite.27,28 In high-resolution S 2p XPS spectra (Figure 4c), FTLP-Li2S6 shows the characteristic peaks of thiosulfate (167.2 eV) and polythionate complexes (168.3 eV), together with residual polysulfides as indicated by bridging (SB-SB, 162.7 eV) and terminal (ST-ST, 160.4 eV) sulfur species.28 The formation of surface thiosulfate/polythionate species is probably attributed to the synergistic effect enabled by exposed active Ti atoms and terminal hydroxyl groups,27-29 which is also evidenced by the substantially decreased Ti-OH fraction (532.2 eV) in high-resolution O 1s XPS spectrum of FLPT-Li2S6 when compared with that of pristine FLPT 12 ACS Paragon Plus Environment

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(Figure S16). It is well known that the adsorbed polysulfide species will be accumulated sostenuto on the surface of host materials, and will finally detach from the surface unless the additional forces balance the accumulation and transformation of polysulfides. The existence of exposed active Ti atoms and abundant hydroxyl groups in FLPT not only provides active sites to entrap polysulfides in the cathode region, but also acts as the mediators to chemically regulate the conversion of sulfur species. To get better insight into the redox kinetics of polysulfides, the symmetrical cells were designed by sandwiching Li2S6-containing electrolytes between two identical FLPT electrodes. For comparison, the symmetrical cell based on carbon nanotubes (CNTs), one type of widely-used conductive carbon material as sulfur host and conductive additive,16 was also fabricated. It should be mentioned that the FLPT symmetrical cell only has the interaction between polar FLPT and polysulfides, the interfacial disturbance from carbon is thoroughly eliminated. As shown in Figure 4d, the current density in the FLPT electrode significantly increases when compared with that of CNTs, demonstrating that strong FLPT-polysulfides interaction not only statically exists, but also dynamically accelerates the electrochemical reaction of polysulfides. Benefiting from high conductivity, high polarity and strong chemisorption toward polysulfides, FLPT provides easy access for electric charges into the FLPT-polysulfide interfaces to initiate liquid-liquid redox reactions. Such substantially promoted charge transfer is verified by visible shrinkage of the semicircles in EIS of the symmetrical cells (Figure S17). Notably, the liquid-solid transformation could also be promoted through speeding up the reactions of liquid intermediates, which is conducive to uniform coverage of Li2S on the surface of primary Ti3C2Tx nanosheets in the FLPT architecture.23

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Besides facilitating liquid-solid transformation in the discharge process, FLPT also accelerates solid-liquid transformation in the reverse charge process, as confirmed by ex-situ electron paramagnetic resonance (EPR) spectra. Strikingly, the FLPT-based electrode system is of great significance to investigate the existence of polysulfide radicals via EPR technique because strong signal interference from sp2-hybridized carbon substrates could be largely deducted.41,42 As shown in Figure 5a and Figure S18, the electron spin signal at 3391 G with a peak-to-peak width of 7 G is detected in both FLPT and FLPT-S-4.2,43 suggesting that FLPT-S-4.2 inherits the Pauli paramagnetic character from FLPT.44 The corresponding EPR signal evolutions during charge/discharge processes are plotted in Figure 5a. A strong resonance peak around 3280 G, which is categorized to trisulfur radical (S3*−),43 could be detected in the charge and discharge states of the FLPT-S-4.2 electrode. The relatively weak peak at the final discharge state (1.6 V) indicates the presence of the radical-containing mixture in Li2S2/Li2S. In sharp contrast, the CNTs-S counterpart shows miscellaneous peaks and negligible peak of S3*− radical in the charge/discharge processes (Figure 5b). The S3*− radicals, generated from the disproportionation or dissociation of S62− anions,43,45 function as extra redox mediators to facilitate the electrochemical transformation of sulfur species, especially for the oxidation of Li2S to polysulfide intermediates and finally to sulfur in the charge process. These observations reveal that chemical and electrochemical reactions proceed harmoniously upon the interface of FLPT in the FLPT-S electrode, which validates the facilitated redox kinetics in the charge/discharge processes. Ex-situ UV-visible absorption spectra were also performed to investigate the sulfur redox process during cycling. As shown in Figure 5c, the characteristic peak of the S3*− radical at 619 nm45 dynamically exists during the cell operation, and the weakest peak occurs at the final discharge state 14 ACS Paragon Plus Environment

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(1.6 V), which is in accordance with the EPR results (Figure 5a). Interestingly, the wide absorbance band in the range of 250-350 nm, which corresponds to the characteristic peaks of S62−/S42−,6,45 almost keeps constant, suggesting fast dynamic equilibrium between S62− anion and S3*− radical. The strong absorption peak at 235 nm in the discharge state of 1.6 V could be assigned to S22− in the solid discharge product,45 echoing with the dynamic S3*− consumption process. In sharp contrast, CNTs-S shows strong absorption peaks for various polysulfides, the final solid charge/discharge species (S8 and S22−) could be clearly detected, while the S3*− radical is inappreciable except that in the discharge state of 2.0 V (Figure 5d). These results manifest that dynamic equilibrium between S62− and S3*− is sluggish during the cell operation of CNTs-S, leading to adverse disproportionation between polysulfides and slow electrochemical conversion among these species. As for the FLPT-S system, the nucleophilic S3*− radicals are susceptible to attack active Ti+ sites in FLPT framework based on Lewis acid-base interactions, resulting in an enrichment of S3*− radicals on reactive three-phase boundaries among conductive FLPT support, sulfur species and electrolyte. The enrichment of S3*− further promotes the redox conversion of polysulfides. As for the CNTs-S system, the S3*− radicals only attack the bridging S (a weak electrophilic site) in the polysulfides as supported by Liang’s report,46 resulting in sluggish dynamic equilibrium between S62− and S3*− in the CNTs-S cathode. On the basis of EPR and UV-visible absorption results together with the electrocatalytic analysis from symmetrical cells, it could be concluded that the structural and chemical superiorities of FLPT host material are responsible for the promising electrochemical performance in terms of high areal and volumetric capacities. As schematically illustrated in Figure 5e, the active materials are tightly adhered on the surface of the Ti3C2Tx nanosheets, the space-efficient arrangement of 15 ACS Paragon Plus Environment

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primary Ti3C2Tx nanosheets results in high tap density in the FLPT-S composite, which substantially improves the volumetric sulfur content and thus volumetric capacity. The abundant active sites in FLPT are preferable for strong chemisorption and catalytic conversion of polysulfides, which propels the redox reactions of sulfur species to occur on the surface of FLPT. Concurrently, fast dynamic equilibrium between S62− and S3*− radical further promotes the solid-liquid-solid transformation of sulfur species in the charge/discharge processes. The electrolyte penetration and ion transport between adjacent nanosheets are greatly accelerated by nanomesh structure in the Ti3C2Tx nanosheets. Meantime, high conductivity of FLPT allows for rapid charge transfer in the nanosheets and even whole FLPT. The robust architecture in FLPT could endure the volumetric fluctuation of the active materials in the charge/discharge processes. These merits have cooperatively improved electrochemical performance of FLPT-S despite of the electrodes with high ASL.

CONCLUSIONS The MXene-based FLPT-S cathode materials without the incorporation any carbon additives have been proposed for synchronous gains in high areal and volumetric capacities. The areal and volumetric capacities in FLPT-S can compete with the state-of-the-art those in high-energy-density Li-S batteries. The promising performance is mainly attributed to ingenious engineering of the micron-sized FLPT assemblies. FLTP is constructed through the space-efficient arrangement of primary Ti3C2Tx nanosheets, sulfur is tightly adhered on the surface of nanosheets, resulting in high tap density of FLPT-S when compared with traditional carbon-based cathode materials. Moreover, the dynamic equilibrium between S62− anion and S3*− radical during cycling has been proposed to get 16 ACS Paragon Plus Environment

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insight into the mechanism for the solid-liquid-solid transformation of sulfur species. In summary, this study provides inspiration for design of high-energy-density Li-S batteries based on MXene materials without the conductive carbon additives. We believe that this proposed methodology could be extended to other high-performance electrode materials, such as lithium ion batteries and supercapacitors, thus holding great promises for the development of various high-performance energy storage systems.

EXPERIMENTAL SECTION Synthesis of FLPT. Commercial Ti3AlC2 powder (2 g) was immersed in 32% HF (50 mL) and vigorously stirred under Ar bubbling at 50 oC for 48 h. The resultant powder was rinsed with deionized water until pH value of the solution reached 7. The powder was retrieved by centrifugation for 5 min, and the supernatant was discarded. The wet sediments were dried in vacuo at 80 oC for 24 h. In a N2-protected environment, the etched Ti3C2Tx powder was then dispersed in deionized water (30 mL) and EDA (2 mL), followed by sonication for 30 min. pH value of the mixture was adjusted around 10 using aqueous NaOH solution, the resultant mixture was transferred into 50 mL Teflon-lined stainless steel autoclave, and heated at 60, 70, 80 and 90 oC for appropriate time, respectively. After hydrothermal treatment, the cinereous supernatant was collected through centrifugating at 1000 rpm for 3 min, and the sediment was discarded. Then the obtained supernatant was retrieved by centrifugation at 5000 rpm for 5 min, and the supernatant was discarded. The FLPT assemblies are obtained after washed using a copious of deionized water and ethanol, followed by freeze drying for 24 h. 17 ACS Paragon Plus Environment

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Synthesis of FLPT-S composite. The as-prepared FLPT (300 mg) was ground with sulfur (500 mg, 99.999%, Sigma-Aldrich). The mixture was transferred in a glass vial in horizontal tube furnace and heated at 160 oC under nitrogen atmosphere with internal pressure of 110 KPa for 12 h, and further heated at 200 oC for 3 h. After cooling to room temperature, the FLPT-S composite was obtained. As a comparison, the CNTs-S composite was prepared through following the same procedures. Ex-situ EPR measurements. The cycled electrode materials (FLPT-S and CNTs-S) were extracted from coin cell in Ar-filled glove box and thoroughly washed by DOL/DME (1:1, v/v) mixture, followed by drying at 60 oC under vacuum for 24 h. The collected dry powder was quickly transferred into N2-filled paramagnetic tube before the EPR measurement. Ex-situ UV-visible adsorption measurements. The cycled electrode materials (FLPT-S and CNTs-S) were extracted from the coin cells in Ar-filled glove box and soaked in a mixture of DOL/DME (1:1, v/v). The obtained solutions were characterized by an UV1800 spectrophotometer (Shimadzu). A baseline correction was used to deduct the influence of the mixture of DOL/DME.

ASSOCIATED CONTENT Supporting Information Available: Additional SEM, TEM, XPS, EDX, TGA, and electrochemical performance is available free of charge on the ACS Publications website.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge National Natural Science Foundation of China (21600191, 21673241 ， 21471151 and 51572197). The Natural Science Foundation of Fujian Province (2018J01030). Natural

Science

Foundation

of

Zhejiang

Province

for

Distinguished

Young

Scholars

(LR18E020001). State Key Laboratory of Structural Chemistry (20170035). The Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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Figure 1. Schematic illustration for (a) the synthesis of FLPT and (b) the formation mechanism of the nanomeshes in the Ti3C2Tx nanosheet.


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Figure 2. (a, b) SEM, (c) TEM, and (d) high-resolution TEM images of FLPT. (e) SEM and (f) TEM image and corresponding elemental mapping of FLPT-S.


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Figure 3. (a) The second cycle of CV curves, (b) charge/discharge profiles and (c) Li+ diffusion rates for the FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5 electrodes. (d) Areal and volumetric capacities at 1/30 C for the FLPT-S-4.2, FLPT-S-6.8 and FLPT-S-10.5 electrodes. (e) The 3D projection map of ASL vs areal capacity vs volumetric capacity. The related ASL, areal capacity and volumetric capacity data are collected from Ref 6, 8-10 and 14-20.


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Figure 4. (a) Photographs for Li2S6 in DOL/DME before and after the addition of FLPT and CNTs adsorbents. High-resolution (b) Ti 2p and (c) S 2p XPS spectra of FLPT and FLPT-Li2S6. (d) CV curves of symmetrical Li2S6-Li2S6 cells.


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Figure 5. The ex-situ EPR spectra for (a) FLPT-S-4.2 and (b) CNTs-S at different charge/discharge states. UV-visible absorption spectra for (c) FLPT-S-4.2 and (d) CNTs-S at different charge/discharge states. (e) Schematic illustration for the interaction between FLPT and sulfur species during cycling.


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Synchronous Gains of Areal and Volumetric Capacities in Lithium

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