Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic

Subscriber access provided by Université de Strasbourg - Service Commun de la Documentation

Article

Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic Cathode Host with Low Surface Area for High Loading Li-S Batteries Rongrong Li, Xuejun Zhou, Hangjia Shen, Minghui Yang, and Chilin Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b02231 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic Cathode Host with Low Surface Area for High Loading Li-S Batteries Rongrong Li, a,c Xuejun Zhou,b Hangjia Shen,a Minghui Yang, a,c* and Chilin Li b, c* a

Solid-State Functional Materials Research Laboratory, Ningbo Institute of Materials

Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. b

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 585 He Shuo Road, Shanghai 201899, China. c

Center of Materials Science and Optoelectronics Engineering ， University of

Chinese Academy of Sciences, Beijing 100049, China.

Abstract: Li-S batteries have several advantages in terms of ultrahigh energy density and resource abundance. However, the insulating nature of S and Li2S, solubility and shuttle effect of lithium polysulfides (LiPSs), and slow interconversion between LiPSs and S/Li2S/Li2S2 are significant impediments to the commercialization of Li-S batteries. Exploration of advanced S host skeleton simultaneously with high conductivity, adsorbability and catalytic activity is highly desired. Herein, a heterojunction material with holey nanobelt morphology and low surface area (95 m2/g) is proposed as compact cathode host to enable a conformal deposition of S/Li2S with homogenous spatial distribution. The rich heterointerfaces between MoO2 and Mo3N2 nanodomains serve as job-synergistic trapping-conversion sites for polysulfides by combining the merits of conductive Mo3N2 and adsorptive MoO2. This non-carbon heterojunction substrate enables a high S loading of 75 wt% even under low surface area. The initial capacity of MoO2-Mo3N2@S reaches 1003 mAh/g

ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with a small decay rate of 0.024% per cycle during 1000 cycles at 0.5 C. The long-term cyclability is preserved even under a high loading of 3.2 mg/cm2 with a reversible capacity of 451 mAh/g after 1000 cycles. The Li-ion diffusion coefficient for MoO2-Mo3N2@S is extremely high (up to 2.7×10-7 cm2/s) benefiting from LiPSs conversion acceleration at heterojunctions. The affinity between LiPSs and heterojunction allows a dendrite-free Li plating at anode even after long-term cycling. Well-defined heterointerface design with job-sharing or job-synergic function appears to be a promising solution to high-performance Li-S batteries without the requirement of loose or high-surface-area carbon network structures. Keywords: MoO2-Mo3N2, heterojunction host, polysulfide adsorption-conversion, cathode material, lithium-sulfur batteries

The steady increase of the demands on portable electronic devices, electric vehicles and large-scale energy storage grids makes traditional lithium-ion batteries (LIBs) unsatisfactory because of limited charge-storage capacity and energy density.1 Lithium-sulfur (Li-S) battery is one of the most promising next-generation energy storage systems, relying on its high theoretical capacity (1675 mAh/g, ~5 times of those of LIBs) and energy density (up to 2500 kW/kg) as well as low cost and environmental friendliness.2-4 However, the commercialization of Li-S batteries is hindered by several obstacles: (a) intrinsic resistivity of S and its lithiated products Li2S2/Li2S, (b) obvious volume expansion (78%) upon cycling, (c) high solubility of intermediate product Li polysulfides (LiPSs) in organic electrolyte. LiPSs are prone to migrate from the cathode side to anode side through electrolyte, and are further


Page 2 of 40

Page 3 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

reduced to the deposit of insoluble Li2S2/Li2S at Li anode. This would result in the loss of active materials permanently during the charge/discharge process. The above problems are believed to be the main reasons for insufficient sulfur utilization, rapid capacity fade and low Coulombic efficiency in Li-S batteries.5-7 Till now, extensive efforts have been devoted to address these problems. The most popular method is to encapsulate sulfur into porous carbon structures of large specific surface area in order to increase the conductivity of sulfur cathode and mitigate the loss of LiPSs.8 But the non-polar carbon surface causes a weak physical interaction with polar LiPSs, leading to a limited effect on the confinement of polysulfides inside the pores of carbon.8,9 This physical confinement is insufficient to alleviate the shuttle effect of LiPSs over long-time cycling. To strengthen the binding energy between hosts and LiPSs, a number of research groups have resorted to polar inorganic compounds, such as TiO2, MnO2, VS2, VN, InN, Co4N and Mo2C,10-16 which are thought to enable electrostatic or chemical interaction with LiPSs. Among them, metal oxides and sulfides usually have poor electronic conductivity, which limits the conversion efficiency of LiPSs.17 In contrast, metal nitrides have higher electrical conductivity (e.g. 1.23×104 S/cm for VN and 46 S/cm for TiN),18-25 which can provide fast electron transfer pathways to the adsorption sites of LiPSs. However their adsorption ability towards LiPSs is much weaker than that of corresponding metal oxides, leading to a compromise of catalysis or confinement effect.2,17 Hence, it is especially important to design a job-sharing polar interface simultaneously with strong adsorption capacity for LiPSs and good electron conductivity facilitating


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

charge transport and mass conversion. The heterostructure of metal oxide and nitride can meet the above requirements, and recently two typical examples of TiO2-TiN and VO2-VN heterojunctions have proven their reliability.26,27 But these heterojunction materials still require the assistance of excess graphene conductive additive to reinforce the electrode conductivity and increase the specific surface area. Since the surface area of metal nitrides is usually not high as a consequence of high-temperature ammonothermal processing or nitridation, a large amount of carbon is required to mitigate the aggregation of nitride particles and enable sufficient exposure of catalysis sites.28 Therefore, it is highly desired to synthesize the nitride-oxide heterojunction with controllable morphology, microstructure and porosity. Metallic MoNx (MoN, Mo2N or Mo3N2) is thought to be an excellent electrocatalyst owing to its electronic structure, which is similar to that of noble metals.28,29 However its effect on maintaining the capacity retention of Li-S batteries is limited.30 Most recently, the incorporation of V is suggested as an approach to modulate the electronic structure of MoN, leading to the improvement of polysulfide adsorption capability and cycling stability.31 However the low surface area (even in two-dimensional form) can only guarantee a low S loading (e.g. < 60%). Similar to the case of Mo-based nitride, the Mo-based oxide (e.g. MoO2) as polysulfide immobilizer is also rarely reported; this is due to its relatively lower conductivity and associated capacity degradation effects.32 Herein, we propose a heterojunction of MoO2-Mo3N2 with well-defined holey nanobelt morphology to enable significant kinetics enhancement compared with its single-component counterparts (MoO2 or


Page 4 of 40

Page 5 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Mo3N2). The ammonification of MoO3 nanobelts ensures an intimate contact between MoO2 and Mo3N2 phases, which are the reductive products under ammonification. This reduction process not only promotes the roughness of nanobelts with the creation of porosity on grain surface, but also triggers the enrichment of lattice defects as excess adsorption and acceleration sites for LiPSs.33 As a sulfur cathode host, partially nitrided MoO2-Mo3N2 heterojunction synergizes the merits of adsorptive MoO2 and conductive/adsorptive Mo3N2 compared with the fully nitrided case. As a result, an initial specific capacity as high as 1003 mAh/g and a small decay rate of 0.024% per cycle at 0.5 C over 1000 cycles are achieved for Li-S batteries. Even under a high S loading of 75 wt% and 3.2 mg/cm2, the low surface area (95 m2/g) of MoO2-Mo3N2 still guarantees a high capacity reversibility for at least 1000 cycles due to the conformal deposition of S/Li2S with homogenous spatial distribution.

Results and discussion Synthesis and characterization of MoO2-Mo3N2 heterojunction The schematic of the synthesis of MoO2-Mo3N2 heterojunction and its S-impregnated composite (denoted as MoO2-Mo3N2@S) is illustrated in Figure 1a. Firstly, the α-MoO3 nanobelts are synthesized via a hydrothermal reaction.34 And then, these nanobelts react with NH3 at 550 oC to form MoO2-Mo3N2 heterostructure as a consequence of controllable reduction and nitridation of MoO3 phase. As comparison, pure MoO2 and Mo3N2 are also synthesized by adjusting the atmosphere to Ar/H2 mixture and prolonging the nitriding time respectively. Finally, nano-size sulfur is


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chemically deposited in MoO2-Mo3N2, Mo3N2, MoO2 or MoO3 host through the reaction between sodium thiosulfate and hydrochloric acid to obtain the following products: MoO2-Mo3N2@S, Mo3N2@S, MoO2@S or MoO3@S composite.35 The phase transition during the synthesis process is detected by X-ray diffraction (XRD) patterns of Figure 1b and c. After NH3 treatment, the raw α-MoO3 (PDF # 05-0508) is reductively converted into the mixture of MoO2 (PDF # 73-1249) and Mo3N2 (PDF # 89-3712). Note that the diffraction peaks are sharp for MoO3 precursor, while the peaks corresponding to Mo3N2 product become broad and those belonging to MoO2 phase still remain dominant. The nitridation-induced crystallinity degradation is more clearly observed when comparing the XRD patterns of single-component dominant products (MoO2 and Mo3N2, Figure S1). The crystallization of α-MoO3 nanobelts results in orientation along the b axis direction as indicated by the appearance of pronounced peaks of (020), (040) and (060). Such an orientation is disrupted during ammoniation and no orientation effect is observed for both MoO2 and Mo3N2. The nitridation reaction is caused by the adsorption of NH3 on the surface of α-MoO3, wherein nitrogen could be incorporated into the lattice of α-MoO3; in turn replacing oxygen atoms and reducing Mo cations in order to form Mo3N2 phase. Sufficient N injection enables the detachment of nitride nanodomains from the parent oxide phase due to lattice mismatch. In parallel, the hydrogen released from dissociated NH3 reacts with α-MoO3 and reduce it into MoO2.36 In MoO2-Mo3N2@S, the diffraction peaks corresponding to crystalline S (PDF # 78-1889) are evidently observed (Figure 1d), indicating a successful S deposition and


Page 6 of 40

Page 7 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

adsorption on the surface of MoO2-Mo3N2 nanobelts. The similar crystalline S signals are also observed in MoO3@S (Figure S2). Ammoniation enables a color darkening from white for MoO3 to black for MoO2-Mo3N2. Four-point probe method is used to obtain the resistance evolution during ammoniation. The resistances of MoO3 and MoO2 are too large to be detected. Partial nitridation to MoO2-Mo3N2 enables the successful detection of square resistance of 24.34103 Ohm/sq, which is drastically decreased to 339 Ohm/sq after full nitridation to Mo3N2. These results confirm the reinforcement of electron conductivity by nitridation. The morphology and microstructure evolution during ammoniation were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as shown in Figure 2. Figure 2a reveals that the as-synthesized MoO3 consists of uniform nanobelts with a width of 300-500 nm and a length of at least several micrometers. The post-treated sample of MoO2-Mo3N2 heterojunction roughly maintains the nanobelt morphology but with a higher aspect ratio (Figure 2b). It seems these nanobelts are dominantly split along the length direction with the appearance of more grain boundaries and surface roughening. Phase separation between nitride and oxide is most likely responsible for this morphology evolution. Images in Figure 2c and d more clearly disclose the morphology of the nanobelt, especially with the observation of porous texture in MoO2-Mo3N2. The precipitated Mo3N2 and MoO2 have much smaller cell volumes (72 and 131.5 Å3 respectively) than the MoO3 precursor (203 Å3). The lattice condensation is one of the main reasons for hole generation. The holey structure is also likely caused by the release of product


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gases (e.g. H2O) during ammoniation. In the high-resolution TEM (HRTEM) image of MoO3, the lattice fringes with d-spacing of 0.35 nm correspond to the (040) plane of orthorhombic α-MoO3. After nitriding, new lattice fringes with d-spacing of 0.20 and 0.24 nm appear, and they are assigned to the (200) plane of Mo3N2 and (-211) plane of MoO2 respectively. Their nanodomains are adjacent with discernable interface, verifying the formation of MoO2-Mo3N2 heterojunction. There are numerous such heterointerfaces existing in each nanobelt grain, which are expected to be the catalytically active centers to accelerate the charge transportation and conversion kinetics of LiPSs,26 especially those molecules trapped in surrounding holes. The porosity is further observed in magnified TEM image focusing on the edge region with the denotation of uniformly distributed holes of 2-3 nm based on the color contrast (Figure 2e and f). The grain splitting and roughening phenomena also exist when converting to single-component products (e.g. MoO2 and Mo3N2); however their grain texture and size appear to be less homogeneous than the heterojunction case (Figure S3). It is clear from both SEM and high-angle annular dark-field scanning TEM (HAADF-STEM) that even after S deposition, the profile of nanobelt is still well preserved. These are indications of a uniform distribution of S on MoO2-Mo3N2 host surface (Figure 3a and b). The surface enrichment of adsorption sites and chemical S infusion method (instead of physical thermal injection method) can effectively avoid the thick accumulation of active species in one region. The homogeneous attachment of S on nanobelt is demonstrated by HAADF-STEM elemental mapping in the region


Page 8 of 40

Page 9 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

marked by the red square (Figure 3c-f). The elements of Mo, O and N have homogeneous spatial distribution in MoO2-Mo3N2@S, implying that ammonization proceeds uniformly and the phase separation of oxide and nitride occurs in nanodomain scale as shown in HRTEM. The S distribution is almost overlapped with those of host elements, further confirming the thin adsorption of S species on host skeleton. Note that the aggregation regions appear to be more for MoO3@S in view of its interior adsorption effect towards S (Figure S4). The pore creation causes the increase of Brunauer-Emmett-Teller (BET) specific surface area from 24 m2/g for MoO3 to 95 m2/g for MoO2-Mo3N2 as shown in Figure 4a. The pore volume correspondingly increases from 0.1127 cm³/g to 0.1724 cm³/g. The pore size distribution mainly lies in the mesoporous range centered at 5 nm. This distribution is broader and more intensive for MoO2-Mo3N2 due to the creation of excess pores. The BET area value of MoO2-Mo3N2 is much larger than those of already reported VO2-VN (46 m2/g) and TiO2-TiN (75 m2/g) heterostructures, benefiting from the featured nano-morphology for the former.26,27 Although this specific surface area is not large enough compared with those of porous and loose carbon framework hosts, it still enables a substantial loading of S as high as ~75 wt% (by thermogravimetry (TG)) in view of the exitence of abundant adsorption/trapping sites or active pore edges for immobilizing S8 molecules (Figure 4b). This S loading amount is larger than that of VO2-VN (62 wt%).26 Thick accumulation of S does not seriously compromise the S loading (~70 wt%) in MoO3@S (Figure S5). The surface chemical composition and bonding states of as-synthesized MoO3


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and MoO2-Mo3N2 are identified by X-ray photoelectron spectroscopy (XPS, Figure S6). The Mo3d spectrum of MoO3 precursor shows typical Mo6+-O peak at 232.9 eV for Mo3d5/2.37 After nitridation, new peaks for Mo3d5/2 appear at 230.5 eV and 229.4 eV, corresponding to Mo-N and Mo4+-O bonds, respectively.32,38 The incorporation of N in Mo3N2 lattices is also observed from the evident N1s peak at 398.4 eV for MoO2-Mo3N2 sample.39 In N1s, two shoulder peaks at 401.9 eV and 395.2 eV are observed which correspond to the adsorption of N2 on sample surface as well as Mo-N bonding in other MoNx impurity.39 These results agree with the generation of MoO2 and Mo3N2 heterostructure as confirmed by XRD and TEM. Note that the Mo6+-O moieties in MoO2-Mo3N2 are still residual but with a decreased XPS intensity and with slight shift of Mo3d peaks towards smaller binding energy (BE) values (e.g. to 232.7 eV for Mo3d5/2). This displacement should be associated with the increase of surrounding electron cloud density and adjustment of coordination environment with the doping of N.

Adsorption performance of MoO2-Mo3N2 towards LiPSs Saturated lithium polysulfide solution is prepared by mixing lithium and element sulfur in electrolyte. It is then used to perform the adsorption experiment with the participation of Mo-oxide and nitride hosts. In order to verify the strong chemical anchoring advantage of MoO2-Mo3N2 towards LiPSs, the static adsorption capabilities of MoO2-Mo3N2, MoO2, Mo3N2 and MoO3 towards LiPSs are compared. As shown in Figure 5a, these powder samples with the same weight of 10 mg are added to


Page 10 of 40

Page 11 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

transparent glass vials containing proportionally diluted saturated LiPSs solution. The deep yellow Li2Sx solution becomes colorless after adding MoO2-Mo3N2 for 12 h standing, indicating a strong adsorption ability of MoO2-Mo3N2 for LiPSs. The addition of MoO3 also takes effect from the decoloration phenomenon. The adsorption ability of the solutions containing MoO2 and Mo3N2 appears to be relatively weak and the solutions still remain light yellow. From the color contrast, MoO2 shows relatively improved adsorption when compared to Mo3N2 as expected. This result clearly demonstrates the important role of MoO2-Mo3N2 heterojunction on promoting the adsorption of Li2Sx compared with the single-component Mo3N2 or MoO2. The concentration change of LiPSs in the electrolyte solution containing MoO2-Mo3N2, MoO2, Mo3N2, and MoO3 is further compared by ultraviolet-visible (UV) absorption (Figure 5b). The sharp peaks located at about 440 nm, 330 nm and 260 nm are attributed to S42-, S62-/S42- and S82-/S62- species, respectively.20,40-42 Clearly, compared with the pure LiPSs solution, the intensities of absorption peaks decrease drastically for these solutions containing MoO2-Mo3N2, MoO2, Mo3N2 and MoO3, especially for the cases of MoO2-Mo3N2 and MoO3. The S42- peaks almost disappear for the solutions with MoO2-Mo3N2 and MoO3, and MoO2-Mo3N2 enables the lowest absorbance intensity for S62-/S42- and S82-/S62- peaks. This phenomenon means MoO2-Mo3N2 exhibits the most effective adsorption for the broad composition range of Li2Sx. The ranking of adsorption degree is consistent with the result of visualization experiment (Figure 5a). To further illustrate the excellent adsorptive property of MoO2-Mo3N2 discussed above, an in-situ visual electrochemical


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experiment is conducted in an optically transparent vessel, as shown in Figure 5c and d. During the first discharge process from 3.0 V to 1.5 V, the color change of electrolyte in the MoO3@S based cell is from colorless to bright yellow gradually, meaning that lots of LiPS intermediates dissolve into electrolyte before converting to insoluble Li2S2 or Li2S. In contrast, the electrolyte color of MoO2-Mo3N2@S based cell undergoes a slight change, suggesting that the diffusion LiPSs out of MoO2-Mo3N2 host is greatly mitigated because of the strong absorption effect. The MoO2-MoO3N2 heterojunction after static LiPS adsorption was dried under vacuum for 24 h and then prepared for XPS measurement. The strong chemical interaction between MoO2-Mo3N2 and LiPSs is determined by comparing the XPS before and after adsorbing LiPSs, as shown in Figure 6 and S7. The presence of S element is obviously observed from the full spectrum of LiPS-adsorbed MoO2-Mo3N2, implying a substantial adsorption amount of LiPSs on the surface of MoO2-Mo3N2. After adsorption of LiPSs, a couple of new peaks corresponding to Mo-S bonding appear at 228.8 eV with a cost of weakening Mo-N and Mo4+-O signals as observed from Mo 3d5/2 spectrum (Figure 6a).43 It seems that the intensity of residual Mo6+-O signals is not greatly influenced. This result indicates that most of the polysulfide anions are likely adsorbed at the MoO2-Mo3N2 heterojunctions rather than near the residual MoO3 component. S 2p spectrum displays four couples of sub-peaks belonging to terminal (ST-1) and bridging (SB0) sulfur atoms, and thiosulfate and polythionate moieties at 161.2, 163.5, 167.1 and 168.4 eV for 2p3/2 respectively (Figure 6b).44 Note that the peak of SB0 is somewhat shifted to higher BE position


Page 12 of 40

Page 13 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

compared with as-prepared polysulfide (from ~163.1 to 163.9 eV).45 This is likely caused by the decrease of electron density along sulfur chain as a consequence of chemical interaction between Li and surface lattice O. However this interaction does not substantially change the BE position of ST-1 peak, since the electron density of terminal sulfur atoms is likely supplemented by surrounding Mo atoms. The surface redox reaction between LiPSs and MoOx is responsible for the formation of thiosulfate. The emergence of polythionate is associated with the subsequent reaction of polysulfides with the anchored thiosulfate.46 The interaction of Li-ion with surface O is indicated by the emergence of Li-O bonding (at 528.7 eV) in O 1s (Figure 6c).47 Other typical peaks of Mo-O, C=O and O-H at 530.6, 531.6 and 532.8 eV respectively are in accordance with the situation of MoOx with little exposure to air during sample transfer.38 The two-sided interactions of Li+-O and Sn2--Mo are expected to accelerate the adsorption-conversion process. The oxide-nitride heterojunction not only provides fast channels for charge and mass transport due to its good conductivity, but also catalyzes the redox kinetics of LiPS intermediates. Note that the conversion to polythionate is not pronounced based on the XPS peak intensity, since the reduction potential of MoO2-Mo3N2 is ~1.94 V versus Li/Li+ (Figure S8), which is below the redox potential (2.1-2.4 V) of polysulfides.46 It is also indicated by the negligible displacement of Mo-O XPS peaks (especially for Mo6+-O) towards lower BE values due to very limited reduction of Mo-oxides. Therefore the adsorption mechanism should be dominated by the strong polar surface interactions.46 The chemical anchoring capability of MoO2-Mo3N2 towards polysulfides is


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

responsible for the excellent cycling and rate stability of Li-S batteries as discussed later.

Electrochemical performance benefiting from MoO2-Mo3N2 nanobelts As shown in Figure 7a, the substantial facilitation of charge transfer from MoO2-Mo3N2 is verified by the electrochemical impedance spectroscopy (EIS) of pristine Li-S batteries. In Nyquist plots, the semicircle at high-frequency region corresponds to the interfacial resistance mainly containing the contribution of charge-transfer (Rct).19,48 The semicircle corresponding to MoO2-Mo3N2 electrode (60 ) shrinks in comparison with those for MoO2, Mo3N2 and MoO3 (100-120 ), implying improved charge transfer ability enabled by MoO2-Mo3N2 cathode. The comparison of typical cyclic voltammetry (CV) profiles of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S electrodes at 0.1 mV/s is shown in Figure 7b to further emphasize the kinetic advantage of MoO2-Mo3N2@S. Note that the first reduction peaks (C-I) for MoO2-Mo3N2 as well as Mo3N2 based cathodes appear at a higher potential (2.295 V) than those for MoO3 (2.26 V) and MoO2 (2.27 V) cathodes. The corresponding oxidation peak (A-I) for MoO2-Mo3N2 appears at a lower potential with the highest current intensity. The narrowing of peak potential positions and intensification of peak currents further confirm the kinetic advantage of MoO2-Mo3N2 at least for the conversion process associated with high-order LiPSs. This electrode also displays the strongest peak current for the cathodic process (C-II at 2.013 V) referring the conversion to low-order Li2S2/Li2S. Note that MoO3@S electrode


Page 14 of 40

Page 15 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

presents the broadest peak profiles with relatively lower peak current. The insufficient electron conductivity of MoO3 degrades the conversion kinetics to a certain degree, although its adsorption towards LiPSs (as seen from the visible adsorption experiment) does not lead to serious compromise of CV peak area. Deoxydation or nitridation substantially promotes the electron conductivity as shown in MoO2 and Mo3N2, resulting in the sharpening of CV peaks. However the weakening of adsorption capability or deactivation of catalysis for MoO2 still negatively influences the overpotential performance (for Peak A-II) and peak current intensity (for Peak C-I). The higher fraction of nitride in Mo3N2 sample enables a roughly overlap of CV profile with that of MoO2-Mo3N2 at least at the low scanning rate, stimulated by the potential existence of MoO2-Mo3N2 heterogeneous interfaces even in the former. Figure S9 further shows the comparison of CV curves between MoO2-Mo3N2@S and MoO3@S depending on cycle number at 0.1 mV/s in 1.8-3.0 V. In the first cathodic process, for the former two well-defined reduction peaks at 2.25 and 2.03 V are clearly observed, corresponding to the sequential conversion of elemental sulfur to soluble long-chain polysulfides (Li2Sn, 4 ≤ n ≤ 8) and then to short-chain solid lithium sulfides (Li2S2/Li2S) respectively.49 In the subsequent anodic process, the partially overlapped oxidation peaks located at 2.32 and 2.48 V are associated with the coupled conversion steps from Li2S2/Li2S to LiPSs and ultimately to elemental sulfur.50 However for the latter sample the first two cathodic peaks are poorly profiled due to the limitation of electronic conductivity. In the second cathodic process, both the samples undergo a shift of Peak C-I towards higher potential in view


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

of the activation of high-order polysulfides or electrochemically grinded S8 molecules after the first cycle. The highly reversible CV curves are observed during the following cycles, suggesting the effect of polar surface of oxide/nitride on suppressing polysulfide loss. The C-II peaks are much more intensive for MoO2-Mo3N2@S than for MoO3@S, indicating a sufficient utilization of LiPS conversion into Li2S2/Li2S modulated

by

quicker

charge

transfer

and

catalysis

along

MoO2-Mo3N2

heterojunctions. The rate performance of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S electrodes is assessed under the rates from 0.1 to 4 C per every 6 cycles and returning to 1.5 C and then 1 C (1 C = 1675 mA/g, Figure 7c). Visibly, the MoO2-Mo3N2@S cathode illustrates the superior rate capability compared with MoO2@S, Mo3N2@S and MoO3@S. Under 0.5, 1.5, 3 and 4 C, MoO2-Mo3N2@S electrode is able to deliver the discharge specific capacities of 900, 630, 448 and 340 mAh/g with high reversibility, respectively. When the rate is brought back to 1.5 C and 1 C, the reversible capacities are still preserved at 630 and 800 mAh/g respectively. This rate performance is excellent for this low-surface-area carbon-free cathode host, clearly benefitting from surface modulation rather than physical confinement. The corresponding charge and discharge curves at different rates are shown in Figure S10 with high Coulombic efficiency (CE) closing to 100%. The MoO2-Mo3N2@S sample exhibits a smaller voltage hysteresis (ΔV) than those of other single-component@S ones at 1 C, as well as a more than two-multiple longer low-voltage plateau (Figure S11), well agreeing with the impedance and CV results.


Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 7d compares the long cycling performance of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S electrodes at a high rate (1 C). After low-rate (0.1 C) activation, the discharge capacity of MoO2-Mo3N2@S is as high as 750 mAh/g at 1 C and is still kept at 511 mAh/g after 1000 cycles, with a small capacity degradation rate of 0.032% per cycle. However, for MoO2@S, Mo3N2@S and MoO3@S electrodes, the low sulfur utilization caused by electrode passivation or polysulfide dissolution leads to a lower discharge capacity of 357, 294 and 379 mAh/g after 1000 cycles respectively with a corresponding capacity degradation rate of 0.048%, 0.058% and 0.028% per cycle. The synergic effect in MoO2-Mo3N2 enables a lower degradation rate than its single-component counterparts. This degradation rate of MoO3@S is comparable to that of MoO2-Mo3N2@S because the adsorption effect is not bad for the former, although its initial capacity is lowest. Note that MoO3@S enables the better cycling and rate endurance, and its capacity can surpass those of other control samples after 1000 cycles or after undergoing higher-rates (Figure 7c and d). The above-mentioned cycling performance is obtained based on a sulfur loading of 1.3-1.6 mg/cm2. We further compare the cycling performance of MoO2-Mo3N2@S cathodes at 0.5 C depending on different sulfur loading from 1.2 to 3.2 mg/cm2 (Figure 7e). Under a sulfur loading of 1.2 mg/cm2, the initial capacity is as high as 1003 mA h/g with a CE above 99% and a low capacity degradation rate of only 0.024% per cycle during 1000 cycles. When the sulfur loadings are increased to 2.6 and 3.2 mg/cm2, the capacity reversibility is still well preserved with a capacity decay of 0.013%-0.018% per cycle and a CE above 98%


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

during 1000 cycles. After 1000 cycles, the discharge capacity of MoO2-Mo3N2@S cell still approaches to 451 mAh/g under a high loading of 3.2 mg/cm2. The good electron conductivity of MoO2-Mo3N2 nanobelts guarantees the achievement of high sulfur loading even without the assistance of in-situ conductive carbon network. The high CE can effectively retard the passivation of MoO2-Mo3N2 heterojunctions by “dead S/Li2S2/Li2S”. Therefore the synergetic or catalytic effect on trapping and then converting LiPSs can tolerate an extremely long-term cycling even under high rate or high loading.

Kinetic performance of MoO2-Mo3N2 for LiPSs conversion The polar surface affinity of MoO2-Mo3N2 towards polysulfides enables the acceleration of conversion kinetics, which is proven by the recently developed Li2S nucleation/deposition method by Chiang et al. (Figure 8a-d).51 The electrodes of MoO2-Mo3N2, MoO2, Mo3N2 and MoO3 loaded on carbon fiber paper (CP) are wetted by Li2S8 catholyte (i.e. Li2S8/tetraglyme solution). They are first discharged to 2.06 V galvanostatically at 0.112 mA and then kept at 2.05 V potentiostatically until the current is dropped to below 10-5 A. This voltage protocol is applied to consume most of the high-order polysulfides. A small overpotential of 10 mV is required to trigger the nucleation of Li2S.52 Based on this method, the different stages referring to Li2S8/Li2S6 reduction and following Li2S precipitation can be distinguished quantitatively by different-depth background colors.51 An evident current peak appears for all the electrodes, where an efficient precipitation of Li2S occurs. Note


Page 18 of 40

Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

that MoO2-Mo3N2 enables a much sharper peak current with an accumulated capacity as high as 247 mAh/g based on Li2S precipitation, whereas this capacity is much lower for MoO2, Mo3N2 and MoO3 electrodes (162, 197 and 136 mAh/g respectively) with broader peak currents. The much higher capacity for the former further confirms the accelerated conversion to Li2S benefiting from MoO2-Mo3N2 heterojunctions. MoO3 electrode has the lowest peak current and capacity as a consequence of insufficient conductivity. The diffusion coefficient of Li-ion (DLi) can also reflect the kinetics of LiPSs conversion reaction. CV analysis of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S cathodes at different scan rates from 0.1 to 0.5 mV/s is performed to evaluate the DLi values using Randles–Sevcik equation (Figure 8e-g and S12):53,54 0.5 𝐼𝑝𝑒𝑎𝑘 = 2.69 × 105𝑛1.5𝐴𝐷0.5 𝐿𝑖 𝑣 𝐶𝐿𝑖

therein, Ipeak is the peak current (A), n is the number of electrons in the reaction (n = 2), A is the electrode area (A = 1.13 cm2), v is the scanning rate (mV/s) and CLi is the Li-ion concentration in the electrolyte (CLi = 1.0×10-3 mol/cm3). Here the cathodic and anodic peaks at higher voltages are denoted as Peak C-I and A-I respectively, and the cathodic one at lower voltage denoted as Peak C-II as mentioned before. The linear fitting of the plots of peak current (Ipeak) versus the square root of scanning rate (ν) are used to calculate the DLi at the corresponding peak position. It is important to note that MoO2-Mo3N2@S displays the largest slope based on this linear fitting as well as the highest peak current intensity at corresponding scan rate, indicating the largest DLi values for both the Peak C-I and A-I. As shown in Figure 8g, its DLi values are as high


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

as 2.739×10-7, 7.516×10-8 and 7.113×10-8 cm2/s for A-I, C-I and C-II processes respectively, much higher than those of control samples (Table S1). This result further confirms the acceleration of conversion kinetics of LiPSs from the heterojunction sites. Accordingly, MoO3@S gives the lowest DLi values limited by the poor electron conductivity. The good cycle stability of MoO2-Mo3N2@S is also indicated from the SEM morphology evolution of cycled cathode and anode of Li-S battery after 100 cycles at 1.0 C (Figure 9). From the cathode side, the high-aspect-ratio morphology of MoO2-Mo3N2 is still preserved. This cycled MoO2-Mo3N2@S appears to be conformally covered by homogenous Li2S/Li2S2 species with the precipitation of tiny nano-particles after full discharge. The excellent chemisorption ability and electrochemical catalytic activity of MoO2-Mo3N2 avoid the thick and uneven accumulation of reaction products (Figure 9a-d). This desired spatial distribution of deposits mitigates the residual of dead Li2S/Li2S2 and boosts the CE value and capacity retention. The suppressed mass loss and migration of LiPSs would alleviate the corrosion toward Li anode and its roughening. From Figure 9e-f, the corresponding cycled Li anode exhibits the smooth and compact morphology even after long-term cycling under high rate. No evident dendrite protruding upward is found. The stabilization of Li anode morphology is in turn beneficial to the boost of capacity and rate performance. Here we wish to emphasize the importance of the extent of heterointerfaces on determining the superior LiPSs adsorption performance. Such well-defined


Page 20 of 40

Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

heterointerfaces can be only visualized from the high-resolution TEM imaging with the good discernability of different domains of crystallized phases.55 More such HRTEM images in different regions are shown in Figure S13. Therein the different assignment of lattice stripes is observed with the appearance of a distinct interface line between these lattice stripes (marked by a curved dotted line in white). These lattice stripes correspond to the coexistence of MoO2 (-211) or (011) and Mo3N2 (200) planes. XRD Rietveld refinement was performed to estimate the ratio of MoO2 and Mo3N2 (Figure S15). The molar ratio of MoO2 and Mo3N2 in the heterojunction is determined to be 1:2.3 (i.e. 14 wt% MoO2 and 86 wt% Mo3N2), further indicating a substantial existence of heterointerfaces. We also investigate the impact of introduction of Mo3N2 phase in different content on LiPS adsorption performance as shown in Figure S14 and S16. More heterojunction materials are synthesized via adjusting the nitridation time (e.g. for 1 h, 3 h or 6 h at 550 ºC) and nitridation temperature (e.g. at 550oC, 600 ºC or 700 ºC for 3 h), respectively. In order to obtain better quality of XRD patterns and more accurate estimation of component fraction, slower scan rate is used during XRD measurement (Figure S14). Note that the increase of nitridation time does not evidently weaken the diffraction peaks of MoO2 or decrease the ratio of MoO2 and Mo3N2, whereas the attenuation of MoO2 phase is evident when increasing the nitridation temperature. In order to magnify the contrast effect on LiPS adsorption, the concentration of LiPS solution is increased to obtain darker color (Figure S16). The nitridation process at 550oC with a shorter time enables a better decoloration effect. The inferior adsorption effect when higher


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nitridation temperatures are used stem from the decreased amount of heterointerfaces as a consequence of lower ratio of MoO2/Mo3N2. Figure S17 shows the Li-S cell performance depending on the MoO2/Mo3N2 ratio. The cathode based on a MoO2/Mo3N2 molar ratio of 1:2.3 (i.e. nitriding at 550 °C 3 h) enables the highest capacity release and retention under the similar sulfur loading of 1.2-1.6 mg/cm2. This ratio also enables the smallest interface resistance and narrowest overpotential of Li-S cell. These results indicate that the molar ratio of 1:2.3 for MoO2/Mo3N2 is close to the optimized ratio for this heterojunction system. Increased proportion of Mo3N2 degrades the interface conductivity and voltage polarization of Li-S cell, leading to a decrease of discharge capacity and its retention. Slight decrease of Mo3N2 fraction evidently lowers the utilization of active sulfur before 150 cycles. In the visible and spectral adsorption experiments, the MoO2-Mo3N2 heterojunction exhibits the stronger adsorption capability from the better decoloration effect as well as its bonding with both the cation and anion in LiPSs. All the electrochemical tools indicate that MoO2-Mo3N2 heterojunction has the evident catalysis-conversion kinetic advantage over the corresponding single components (MoOx and Mo3N2). In this work, we also confirm that the nitrided component has higher electron conductivity than oxides from the square resistance detection performed using four-point probe method. Furthermore nitride enables better conversion kinetics than oxides from higher accumulation capacity of Li2S nucleation and higher diffusion coefficient. In contrast, the job of oxide components mainly lies in the firmer adsorption towards LiPSs. Therefore the job-synergistic effect occurs at


Page 22 of 40

Page 23 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

the rich heterointerfaces between nitride and oxide nanodomains, where all the jobs of adsorption-catalysis-conversion are shared in a confined zone and therefore performed more effectively, leading to a desired synergic effect. The formation of MoO2-Mo3N2 heterojunction as a consequence of N doping into MoOx enables a further promotion of adsorption performance compared with MoO2. This advance is likely associated with the modulation of Mo electron states at the heterogeneous interface after doping of less electronegative N to displace O lattice. The optimizations of bonding orbital hybridization or interface defect/strain evolution would cause a stronger interaction of Mo atoms towards adsorbed polysulfide anions.31 We compare the electrochemical performance of this work with other already reported metal nitrides, oxides or nitride-oxide heterojunctions as sulfur host materials for Li-S batteries in Table S2.56,57 It shows that our MoO2-Mo3N2 material has better cycle stability for at least 1000 cycles even with higher sulfur loading (75 wt% and 3.2 mg/cm2) and without the aid of in-situ nano-carbon wiring.

Conclusion In summary, a heterojunction material composed of MoO2-Mo3N2 is shown to be an unusually low surface area (95 m2/g) cathode host. It enables highly stable operation of Li-S batteries even with high S loading (75 wt% and 3.2 mg/cm2). This conductive and holey heterojunction serves as job-synergistic polysulfide adsorption-conversion sites for conformal anchoring of active species with homogenous spatial distribution. Its superior effects on polysulfide adsorption and conversion acceleration are


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

confirmed using multiple characterization techniques when compared with the single-component counterparts (MoO2, Mo3N2 and MoO3). This non-carbon MoO2-Mo3N2@S cathode displays long-term cyclability with a reversible capacity of 511 mAh/g after 1000 cycles based on a low decay rate of 0.032% per cycle at 1 C. Its discharge capacity still approaches ~450 mAh/g after 1000 cycles at 0.5 C under a high loading of 3.2 mg/cm2 with a capacity decay of 0.018% per cycle. Well-defined heterointerface design with job-sharing or job-synergic function appears to be a promising solution to high-performance Li-S batteries without the requirement of loose or high-surface-area carbon network structures.

Experimental Section Preparation of MoO2-Mo3N2 nanobelts: α-MoO3 nanobelts are firstly synthesized by a hydrothermal route and then the nitridation process is carried out by thermal treatment of MoO3 under NH3 to obtain MoO2-Mo3N2 product. The synthesis of α-MoO3 nanobelts is carried out as follows. 1.2 mmol ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) is dissolved in 60 mL deionized water (DIW). Then 30 mL nitric acid is added to the solution under stirring. The resulting clear solution is transferred into a 150 mL Teflon-lined autoclave with a stainless-steel shell and is kept at 180 ℃ for 20 h. After it is cooled naturally to room temperature, the product is collected after filtration, thoroughly washed with deionized water and ethanol for several times and finally dried in an oven at 60 °C for overnight. Finally, the nitridation reaction is carried out in a tubular quartz reactor with the dried α-MoO3


Page 24 of 40

Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

nanobelts heat-treated in an ammonia gas stream. 1 g white α-MoO3 powder is heated at 550 °C for 3 h with a heating rate of 2.5 °C/min under NH3 atmosphere, and the black MoO2-Mo3N2 powder is obtained. MoO2 and Mo3N2 as control samples are also prepared.Pure MoO2 is synthesized under the atmosphere of Ar/H2 mixture, at 650 ℃ for 3 h, with a heating rate of 2.0 °C/min. Pure Mo3N2 sample is synthesized at 700 ℃ for 3 h with a heating rate of 2.0 °C/min under NH3 atmosphere. Synthesis of MoO2-Mo3N2@S cathode materials: In a typical synthesis, 0.02 g MoO2-Mo3N2 nanobelts are ultrasonically dispersed into 50 mL DIW. It is followed by the addition of 0.6771 g sodium thiosulfate pentahydrate (Na2S2O3·5H2O) with stirring for 30 min. Then, 4550 µL hydrochloric acid (0.12 M HCl) is dropwise added to the above solution under stirring. After 3 h, the stirring process is stopped, and the product is filtered, washed several times with deionized water and ethanol until the solution is neutral. The MoO2-Mo3N2@S composite is obtained by drying in an air-oven at 60 °C for 24 h. For comparison, the MoO3@S, MoO2@S, and Mo3N2@S cathodes are prepared via the same method. Physical characterization: The morphology and microstructure of samples are characterized by using a Hitachi S4800 at 4.0 KV scanning electron microscope (SEM) and a Tecnai F20 transmission electron microscope (TEM). The element distribution of composites is determined with an energy dispersive spectrometer (EDS) attached to the TEM instrument. Crystal structure characterization is carried out with a Rigaku MiniFlex 600 X-ray Diffractometer (XRD) with Cu Kα radiation between 5-85° at a scan rate of 1.0 degree/min. Surface component and bonding


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

characterizations are carried out on Axis Ultra DLD X-ray photoelectron spectroscope (XPS). Thermogravimetric (TG) analysis is used to determine the sulfur content of materials, on a Diamond TG/DTA employing a heating rate of 5 °C/min from 30 °C to 600 °C under N2 flow. The surface area measurement is performed by Brunauer-Emmett-Teller (BET) method on Accelerated Surface Area and Porosimetry System (Micromeritics ASAP 2020). The conductivity of samples is measured by four-probe method on CRESBOX, at room temperature. For the ex-situ SEM characterization of cycled samples, both the S and Li electrodes are taken out from the cycled cell in the Ar-filled glove box, and then carefully washed by 1,2-dimethoxyethane to remove the residual electrolyte. The washed electrodes are then dried in Ar-filled glove box before further characterization. Visualized adsorption test: Saturated polysulfide solution is prepared by mixing lithium and element sulfur into the electrolyte based on the mixture solvent of 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1:1 by volume). By diluting the saturated solution with the solvent by a volume ratio of 1: 9, the dark yellow polysulfide solution is obtained. Then, 10 mg MoO2-Mo3N2, MoO2, Mo3N2 and MoO3 powders are added into the above solution of 1000 µL, and the mixtures are vigorously ultrasonicated to realize thorough adsorption, in order to see the adsorption effect and decoloration process. 1000 µL polysulfide solution free of adsorber is also used as a comparison. All the experiments are performed in an Ar-filled glove box. After absorption for 12 h, the mixture is centrifuged at 10,000 rpm for 3 minutes. The supernatant solution is poured into cuvette for ultraviolet-visible (UV-vis) test (with a


Page 26 of 40

Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

comparison with the non-adsorbed polysulfide solution), and the polysulfide-adsorbed oxide/nitride precipitates are dried for XPS measurement. Visualization experiment during electrochemical process is performed in a sealed bottle, with MoO2-Mo3N2@S or MoO3@S as cathode and metal lithium as anode. Both the electrodes are connected with the external circuit and discharged under 0.5 C to see the color evolution in electrolyte. Nucleation and precipitation of Li2S on hosts: A Li2S8/tetraglyme solution with a concentration of 0.25 mol/L is used as a catholyte and is prepared by mixing sulfur and Li2S at a molar ratio of 7: 1 in tetraglyme with additional 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 2 wt% LiNO3 followed by vigorous mixing for 24 h. Commercial carbon papers (CP) are used as current collector to load the MoO2-Mo3N2 heterojunction host as well as other single-component hosts. CP-MoO3, CP-MoO2-Mo3N2, CP-MoO2 and CP-Mo3N2 are applied as cathodes and lithium foil is used as anode. They are assembled into coin-type (2025) cells with the employment of Celgard 2400 membrane as separator. 20 µL Li2S8/tetraglyme is first distributed into the cathode and then 20 µL blank electrolyte with 1.0 M LiTFSI and 2 wt % LiNO3 but without Li2S8 is dropped onto lithium anode. The cells are galvanostatically discharged to 2.06 V under a current of 0.112 mA, and is potentiostatically kept at 2.05 V until the current dropped below 10-5 A for deposition and growth of Li2S on various host surfaces. Electrochemical

characterization:

The

cathode

is

prepared

by

mixing

MoO2-Mo3N2@S nanocomposite or other single-component composite (80 wt.%),


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Super P (10 wt.%) and PVDF binder (10 wt.%) in N-methylpyrrolidone (NMP) solution. The well-mixed slurry is then cast onto a carbon paper current collector and the obtained film is dried in an oven at 60 °C. CR2032 coin cells consisting of a metallic lithium anode, Celgard separator and the above composite cathode are assembled in an argon-filled glovebox. The electrolyte is composed of 1 M LiTFSI with 1 wt% LiNO3 dissolved in a mixture of DOL and DME (1:1 by volume). The charge-discharge test is carried out on a Neware Battery Measurement System (Neware, China) at 1.8-2.8 V (vs. Li/Li+) at room temperature. The cyclic voltammetry (CV) data are obtained with a CHI660E Electrochemical Workstation at a scan rate of 0.1 mV/s in 1.8-2.8 V (or in 0.5-3.75 V). The electrochemical impedance spectrum (EIS) measurement is conducted at the open-circuit condition for pristine Li-S cells. The frequency ranges from 10-1 Hz to 106 Hz with an amplitude of 10 mV on a CHI660E Electrochemical Workstation. The diffusion coefficient of Li-ion is evaluated by using Randles–Sevcik equation based on the required CV curves of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S cathodes at different scan rates ranging from 0.1 to 0.5 mV/s.

Corresponding Author Email: [email protected] (C. L.) Email: [email protected] (M. Y.)

Supporting Information


Page 28 of 40

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

XRD of MoO2, Mo3N2 and MoO3@S composite, SEM of MoO2, Mo3N2 and MoO3@S composite, TGA of MoO3@S composite, XPS of MoO3, MoO2–Mo3N2 and MoO2-Mo3N2 with LiPSs adsorption, CV of MoO2–Mo3N2, MoO2-Mo3N2@S and MoO3@S cathodes, charge/discharge curves of MoO2-Mo3N2@S at different current rates, charge/discharge profile comparison of MoO2-Mo3N2@S and MoO3@S at 1.0 C, CV curves of MoO2@S, Mo3N2@S and MoO3@S at various scan rates, DLi values based on CV peaks, HRTEM of MoO2-Mo3N2 heterointerfaces, XRD, adsorption experiment, impedance and cycling performance of MoO2-Mo3N2 composites prepared under more nitridation time and temperature, Rietveld refinement of MoO2-Mo3N2, performance comparison of our heterojunction host with other reported nitride-based hosts. This material is available free of charge via the Internet at http:// pubs.acs.org.

Acknowledgments This

work

was

supported

by

National

Key

R&D

Program

of

China

(2016YFB0901600, 2016YFB0101205), National Natural Science Foundation of China (U1830113, 51772313, 51802334 and 21471147), Key Program of the Chinese Academy of Sciences (KFZD-SW-320), Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2017KF08M) and Shanghai Science and Technology Committee (16DZ2270100). M. Yang would like to thank for the Ningbo 3315 program. The authors would like to thank T. Thomas of IIT Madras and Q. P. Wu for careful reading and checking for the presentation of this work.

References 1. Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271-4302. 2. Liu, X.; Huang, J. Q.; Zhang, Q.; Mai, L. Nanostructured Metal Oxides and Sulfides for Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1601759. 3. Tao, T.; Lu, S.; Fan, Y.; Lei, W.; Huang, S.; Chen, Y. Anode Improvement in Rechargeable Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1700542.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4. Manthiram A.; Fu Y.; Chung, S. H. Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114, 11751-11787. 5. Ye, C.; Zhang, L.; Guo, C.; Li, D.; Vasileff, A.; Wang, H.; Qiao, S. Z. A 3D Hybrid of Chemically Coupled Nickel Sulfide and Hollow Carbon Spheres for High Performance Lithium-Sulfur Batteries. Adv. Funct. Mater. 2017, 27, 1702524. 6. Kaiser, M. R.; Chou, S.; Liu, H. K.; Dou, S. X.; Wang, C.; Wang, J. Structure-Property Relationships of Organic Electrolytes and Their Effects on Li/S Battery Performance. Adv. Mater. 2017, 29, 1700449. 7. Chen, W.; Lei, T.; Qian, T.; Lv, W.; He, W.; Wu, C.; Liu, X.; Liu, J.; Chen, B.; Yan, C.; Xiong, J. A New Hydrophilic Binder Enabling Strongly Anchoring Polysulfides for High-Performance Sulfur Electrodes in Lithium-Sulfur Battery. Adv. Energy Mater. 2018, 8, 1702889. 8. Liang, J.; Sun, Z. H.; Li, F.; Cheng, H. M. Carbon Materials for Li-S Batteries: Functional Evolution and Performance Improvement. Energy Storage Mater. 2016, 2, 76-106. 9. Wu, Q. P.; Zhou, X. J.; Xu, J.; Cao, F. H.; Li, C. L. Carbon-Based Derivatives from Metal-Organic Frameworks as Cathode Hosts for Li-S Batteries. J. Energy Chem. 2019, 38, 94-113. 10. Seh, Z. W.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 Yolk-Shell Nanoarchitecture with Internal Void Space for Long-Cycle Lithium-Sulphur Batteries. Nat. Commun. 2013, 4, 1331. 11. Li, Z.; Zhang, J.; Lou, X. W. Hollow Carbon Nanofibers Filled with MnO2 Nanosheets as Efficient Sulfur Hosts for Lithium-Sulfur Batteries. Angew. Chem. Int. Ed. 2015, 54, 12886-12890. 12. Cheng, Z.; Xiao, Z.; Pan, H.; Wang, S.; Wang, R. Elastic Sandwich-Type rGO– VS2/S Composites with High Tap Density: Structural and Chemical Cooperativity Enabling Lithium-Sulfur Batteries with High Energy Density. Adv. Energy Mater. 2018, 8, 1702337. 13. Sun, Z.; Zhang, J.; Yin, L.; Hu, G.; Fang, R.; Cheng, H. M.; Li, F. Conductive Porous Vanadium Nitride/Graphene Composite as Chemical Anchor of Polysulfides for Lithium-Sulfur Batteries. Nat. Commun. 2017, 8, 14627. 14. Zhang, L.; Chen, X.; Wan, F.; Niu, Z.; Wang, Y.; Zhang, Q.; Chen, J. Enhanced Electrochemical Kinetics and Polysulfide Traps of Indium Nitride for Highly Stable Lithium-Sulfur Batteries. ACS Nano 2018, 12, 9578-9586. 15. Deng, D. R.; Xue, F.; Jia, Y. J.; Ye, J. C.; Bai, C. D.; Zheng, M. S.; Dong, Q. F. Co4N Nanosheet Assembled Mesoporous Sphere as a Matrix for Ultrahigh Sulfur Content Lithium-Sulfur Batteries. ACS Nano 2017, 11, 6031-6039. 16. Shang, C.; Cao, L.; Yang, M.; Wang, Z.; Li, M.; Zhou, G.; Wang, X.; Lu, Z. Freestanding Mo2C-Decorating N-Doped Carbon Nanofibers as 3D Current Collector for Ultra-Stable Li-S Batteries. Energy Storage Mater. 2019, 18, 375-381. 17. Xu, J.; Lawson, T.; Fan, H.; Su, D.; Wang, G. Updated Metal Compounds (MOFs, S, OH, N, C) Used as Cathode Materials for Lithium-Sulfur Batteries. Adv.


Page 30 of 40

Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Energy Mater. 2018, 8, 1702607. 18. Cui, Z.; Li, C.; Yu, P.; Yang, M.; Guo, X.; Yin, C. Reaction Pathway and Wiring Network Dependent Li/Na Storage of Micro-Sized Conversion Anode with Mesoporosity and Metallic Conductivity. J. Mater. Chem. A 2015, 3, 509-514. 19. Cui, Z.; Zu, C.; Zhou, W.; Manthiram, A.; Goodenough, J. B. Mesoporous Titanium Nitride-Enabled Highly Stable Lithium-Sulfur Batteries Adv. Mater. 2016, 28, 6926-6931. 20. Hao, Z.; Yuan, L.; Chen, C.; Xiang, J.; Li, Y.; Huang, Z.; Hu, P.; Huang, Y. TiN as A Simple and Efficient Polysulfide Immobilizer for Lithium–Sulfur Batteries. J. Mater. Chem. A 2016, 4, 17711-17717. 21. Ma, L.; Yuan, H.; Zhang, W.; Zhu, G. Y.; Wang, Y. R.; Hu, Y.; Zhao, P. Y.; Chen, R. P.; Chen, T.; Liu, J.; Hu, Z.; Zhong, J. Porous-Shell Vanadium Nitride Nanobubbles with Ultrahigh Areal Sulfur Loading for High-Capacity and Long-Life Lithium-Sulfur Batteries. Nano Lett. 2017, 17, 7839-7846. 22. Li, X. X.; Ding, K.; Gao, B.; Li, Q. W.; Li, Y. Y.; Fu, J. J.; Zhang, X. M.; Chu, P. K.; Huo, K. F. Freestanding Carbon Encapsulated Mesoporous Vanadium Nitride Nanowires Enable Highly Stable Sulfur Cathodes for Lithium-Sulfur Batteries. Nano Energy 2017, 40, 655-662. 23. Li, Z. H.; He, Q.; Xu, X.; Zhao, Y.; Liu, X. W.; Zhou, C.; Ai, D.; Xia, L. X.; Mai, L. Q. A 3D Nitrogen-Doped Graphene/TiN Nanowires Composite as a Strong Polysulfide Anchor for Lithium-Sulfur Batteries with Enhanced Rate Performance and High Areal Capacity. Adv. Mater. 2018, 30, 1804089. 24. Wang, Y. K.; Zhang, R. F.; Pang, Y. C.; Chen, X.; Lang, J. X.; Xu, J.; Xiao, C. H.; Li, H. L.; Xi, K.; Ding, S. J. Carbon@Titanium Nitride Dual Shell Nanospheres as Multi-Functional Hosts for Lithium Sulfur Batteries. Energy Storage Mater. 2019, 16, 228-235. 25. Zhong, Y.; Chao, D. L.; Deng, S. J.; Zhan, J. Y.; Fang, R. Y.; Xia, Y.; Wang, Y. D.; Wang, X. L.; Xia, X. H.; Tu, J. P. Confining Sulfur in Integrated Composite Scaffold with Highly Porous Carbon Fibers/Vanadium Nitride Arrays for High-Performance Lithium-Sulfur Batteries. Adv. Funct. Mater. 2018, 28, 1706391. 26. Song, Y.; Zhao, W.; Kong, L.; Zhang, L.; Zhu, X.; Shao, Y.; Ding, F.; Zhang, Q.; Sun, J.; Liu, Z. Synchronous Immobilization and Conversion of Polysulfides on A VO2-VN Binary Host Targeting High Sulfur Load Li-S Batteries. Energy Environ. Sci. 2018, 11, 2620-2630. 27. Zhou, T.; Lv, W.; Li, J.; Zhou, G.; Zhao, Y.; Fan, S.; Liu, B.; Li, B.; Kang, F.; Yang, Q.-H. Twinborn TiO2–TiN Heterostructures Enabling Smooth Trapping-Diffusion-Conversion of Polysulfides Towards Ultralong Life Lithium-Sulfur Batteries. Energy Environ. Sci. 2017, 10, 1694-1703. 28. Tareen, A.K.; Priyanga, G.S.; Behara, S.; Thomas, T.; Yang, M. Mixed Ternary Transition Metal Nitrides: A Comprehensive Review of Synthesis, Electronic Structure, and Properties of Engineering Relevance. Prog. Solid State Chem. 2019, 53, 1-26. 29. Li, Y.; Xiao, K.; Li, J. J.; Jiang, P. P.; Jiang, Y. C.; Du, S. Y.; Leng, Y.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Molybdenum Nitride Nanocatalyst Derived from Melamine and Polyoxometalate-Based Hybrid for Oxidative Coupling of Amines to Imines with Air. ChemCatChem 2018, 10, 4317-4323. 30. Mosavati, N.; Salley, S. O.; Ng, K.Y. S. Characterization and Electrochemical Activities of Nanostructured Transition Metal Nitrides as Cathode Materials for Lithium Sulfur Batteries. J. Power Sources 2017, 340, 210-216. 31. Ye, C.; Jin, H.; Slattery, A.; Davey, K.; Haihui Wang; Jiao, Y.; Qiao, S. Z. 2D MoN-VN Heterostructure to Regulate Polysulfides for Highly Efficient Lithium-Sulfur Batteries. Angew. Chem. Int. Ed. 2018, 57, 16703-16707. 32. Qu, Q.; Gao, T.; Zheng, H.; Wang, Y.; Li, X.; Li, X.; Chen, J.; Han, Y.; Shao, J.; Zheng, H. Strong Surface-Bound Sulfur in Conductive MoO2 Matrix for Enhancing Li–S Battery Performance. Adv. Mater. Interfaces 2015, 2, 1500048. 33. Lin, H. B.; Zhang, S. L.; Zhang, T. R.; Ye, H. L.; Yao, Q. F.; Zheng, G. Y. W.; Lee, J. Y. Elucidating the Catalytic Activity of Oxygen Deficiency in the Polysulfide Conversion Reactions of Lithium-Sulfur Batteries. Adv. Energy Mater. 2018, 8, 1801868. 34. Lou, X. W.; Zeng, H. C. Hydrothermal Synthesis of α -MoO3 Nanorods via Acidification of Ammonium Heptamolybdate Tetrahydrate. Chem. Mater. 2002, 14, 4781-4789. 35. Su, Y. S.; Manthiram, A. A Facile In Situ Sulfur Deposition Route to Obtain Carbon-Wrapped Sulfur Composite Cathodes for Lithium-Sulfur Batteries. Electrochim. Acta 2012, 77, 272-278. 36. Dewangan, K.; Patil, S. S.; Joag, D. S.; More, M. A.; Gajbhiye, N. S. Topotactical Nitridation of α-MoO3 Fibers to γ-Mo2N Fibers and Its Field Emission Properties. J. Phys. Chem. C 2010, 114, 14710-14715. 37. Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Core-Shell MoO3-MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials. Nano Lett. 2011, 11, 4168-4175. 38. Yan, H.; Xie, Y.; Jiao, Y.; Wu, A.; Tian, C.; Zhang, X.; Wang, L.; Fu, H. Holey Reduced Graphene Oxide Coupled with an Mo2N-Mo2C Heterojunction for Efficient Hydrogen Evolution. Adv. Mater. 2018, 30, 1704156. 39. Zhu, L.; Sun, L.; Zhang, H.; Yu, D.; Aslan, H.; Zhao, J.; Li, Z.; Yu, M.; Besenbacher, F.; Sun, Y. Dual-Phase Molybdenum Nitride Nanorambutans for Solar Steam Generation Under One Sun Illumination. Nano Energy 2019, 57, 842-850. 40. Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M. e.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium–Sulfur Batteries. Adv. Mater 2015, 27, 2891-2898. 41. Liu, J.; Sun, M.; Zhang, Q.; Dong, F.; Kaghazchi, P.; Fang, Y.; Zhang, S.; Lin, Z. A Robust Network Binder with Dual Functions of Cu2+ Ions As Ionic Crosslinking and Chemical Binding Agents for Highly Stable Li-S Batteries. J.


Page 32 of 40

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Mater. Chem. A 2018, 6, 7382-7388. 42. Song, J.; Yu, Z.; Gordin, M. L.; Wang, D. Advanced Sulfur Cathode Enabled by Highly Crumpled Nitrogen-Doped Graphene Sheets for High-Energy-Density Lithium-Sulfur Batteries. Nano Lett. 2016, 16, 864-870. 43. Wang, P.; Tian, J.; Hu, J.; Zhou, X.; Li, C. Supernormal Conversion Anode Consisting of High-Density MoS2 Bubbles Wrapped in Thin Carbon Network by Self-Sulfuration of Polyoxometalate Complex. ACS Nano 2017, 11,7390-7400. 44. Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A Highly Efficient Polysulfide Mediator for Lithium-Sulfur Batteries. Nat. Commun. 2015, 6, 5682. 45. Li, G.; Lei, W.; Luo, D.; Deng, Y.-P.; Wang, D.; Chen, Z. 3D Porous Carbon Sheets with Multidirectional Ion Pathways for Fast and Durable Lithium-Sulfur Batteries. Adv. Energy Mater. 2018, 8, 1702381. 46. Liang, X.; Kwok, C. Y.; Lodi Marzano, F.; Pang, Q.; Cuisinier, M.; Huang, H.; Hart, C. J.; Houtarde, D.; Kaup, K.; Sommer, H.; Brezesinski, T.; Janek, J.; Nazar, L. F. Tuning Transition Metal Oxide-Sulfur Interactions for Long Life Lithium Sulfur Batteries: The “Goldilocks” Principle. Adv. Energy Mater. 2016, 6, 1501636. 47. Zou, P.; Chiang, S. W.; Li, J.; Wang, Y.; Wang, X.; Wu, D.; Nairan, A.; Kang, F.; Yang, C. Ni@Li2O Co-Axial Nanowire Based Reticular Anode: Tuning Electric Field Distribution for Homogeneous Lithium Deposition. Energy Storage Mater. 2019, 18, 155-164. 48. Chen, K.; Zhang, Y.; Li, C. High-Rate Nanostructured Pyrite Cathodes Enabled by Fluorinated Surface and Compact Grain Stacking via Sulfuration of Ionic Liquid Coated Fluorides. ACS Nano 2018, 12, 12444-12455. 49. Li, G.; Sun, J.; Hou, W.; Jiang, S.; Huang, Y.; Geng, J. Three-Dimensional Porous Carbon Composites Containing High Sulfur Nanoparticle Content for High-Performance Lithium-Sulfur Batteries. Nat. Commun. 2016, 7, 10601. 50. Liu, J.; Li, W.; Duan, L.; Li, X.; Ji, L.; Geng, Z.; Huang, K.; Lu, L.; Zhou, L.; Liu, Z.; Chen, W.; Liu, L.; Feng, S.; Zhang, Y. A Graphene-Like Oxygenated Carbon Nitride Material for Improved Cycle-Life Lithium/Sulfur Batteries. Nano Lett. 2015, 15, 5137-5142. 51. Fan, F. Y.; Carter, W. C.; Chiang, Y. M. Mechanism and Kinetics of Li2S Precipitation in Lithium-Sulfur Batteries. Adv. Mater. 2015, 27, 5203-5209. 52. Xu, J.; Zhang, W.; Fan, H.; Cheng, F.; Su, D.; Wang, G. Promoting Lithium Polysulfide/Sulfide Redox Kinetics by The Catalyzing of Zinc Sulfide for High Performance Lithium-Sulfur Battery. Nano Energy 2018, 51, 73-82. 53. Kim, H.; Lee, J.; Ahn, H.; Kim, O.; Park, M. J. Synthesis of Three-Dimensionally Interconnected Sulfur-Rich Polymers for Cathode Materials of High-Rate Lithium-Sulfur Batteries. Nat. Commun. 2015, 6, 7278. 54. Zhou, T.; Zhao, Y.; Zhou, G.; Lv, W.; Sun, P.; Kang, F.; Li, B.; Yang, Q.-H. An In-Plane Heterostructure of Graphene and Titanium Carbide for Efficient Polysulfide Confinement. Nano Energy 2017, 39, 291-296. 55. Shi, S.; Gao, J.; Liu, Y.; Zhao, Y.; Wu, Q.; Ju, W.; Ouyang, C.; Xiao, R.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Multi-Scale Computation Methods: Their Applications in Lithium-Ion Battery Research and Development. Chin. Phys. B 2016, 25, 018212. 56.Ren, W.; Xu, L.; Zhu, L.; Wang, X.; Ma, X.; Wang, D. Cobalt-Doped Vanadium N itride Yolk-Shell Nanospheres @ Carbon with Physical and Chemical Synergistic Effects for Advanced Li-S Batteries. ACS Appl. Mater. Interfaces 2018, 10, 11642 -11651. 57. Xiao, X.; Wang, H.; Bao, W. Z.; Wang, G. X.; Gogotsi, Y. Two-Dimensional Arrays of Transition Metal Nitride Nanocrystals. Adv. Mater. 2019, 31, 1902393.

Figure 1. (a) Schematic of fabrication process of MoO2-Mo3N2 holey nanobelts and MoO2-Mo3N2@S composite. XRD patterns of (b) MoO3, (c) MoO2-Mo3N2 and (d) MoO2-Mo3N2@S composite. The standard patterns of corresponding major phases are also listed as reference.


Page 34 of 40

Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 2. SEM images of (a) MoO3 and (b) MoO2-Mo3N2. HRTEM images of (c) MoO3 and (d) MoO2-Mo3N2 with discernable lattice stripes and heterojunction structure, insets: corresponding TEM images in lower magnification to disclose the morphology and texture of nanobelts. (e) TEM image of MoO2-Mo3N2 near the edge in higher magnification (f) with the denotation of holes in holey structure.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

Figure 3. (a) SEM image of MoO2-Mo3N2@S composite. (b) HAADF-STEM image of MoO2-Mo3N2@S with the red square indicating the element mapping area. Element mapping of (c) Mo, (d) O, (e) N, (f) S components and (g) element overlay of MoO2-Mo3N2@S.

Figure 4. (a) BET measurement curves of MoO2–Mo3N2 and MoO3, insets: pore size distribution

of

corresponding

samples.

(b)

TG

MoO2-Mo3N2@S composite.


measurement

curve

of

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 5. (a) Photographs of sealed vials containing LiPSs solution and ones after contact with MoO2-Mo3N2, MoO3, MoO2, and Mo3N2. (b) UV-vis absorption spectra of LiPSs solution and ones after adding MoO3, MoO2, Mo3N2, and MoO2-Mo3N2. Visual confirmation of LiPSs adsorption ability by (c) MoO2-Mo3N2 and (d) MoO3 as cathode hosts during discharging from 3.0 to 1.8 V at 0.5 C.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) Mo 3d XPS spectra of MoO2-Mo3N2 before and after absorbing LiPSs solution. (b) S 2p XPS spectrum of LiPSs after contacting with MoO2-Mo3N2. (c) O 1s XPS spectra of MoO2-Mo3N2 before and after absorbing LiPSs solution.


Page 38 of 40

Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 7. (a) Nyquist plots of pristine Li-S cells based on MoO3@S, MoO2@S, Mo3N2@S and MoO2-Mo3N2@S cathodes measured from 0.1 to 106 Hz at room temperature. (b) Typical CV profiles of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S and MoO3@S cathodes at a scan rate of 0.1mV/s in a potential window from 1.8 to 3.0 V. (c) Rate performance of MoO3@S, MoO2@S, Mo3N2@S and MoO2-Mo3N2@S cathodes at different rates. (d) Cycling performance and Coulombic efficiency of MoO3@S, MoO2@S, Mo3N2@S and MoO2-Mo3N2@S cathodes at 1.0 C for 1000 cycles. (e) Discharge capacity and Coulombic efficiency as a function of cycling number for MoO2-Mo3N2@S cathodes based on different sulfur loadings at 0.5 C for 1000 cycles.

Figure 8. Current profiles and accumulation capacities of Li2S nucleation and deposition on different cathodic surface for (a) MoO2-Mo3N2, (b) Mo3N2, (c) MoO2 and (d) MoO3 during potentiostatic discharge at 2.05 V. (e) CV curves of Li-S cell based on MoO2-Mo3N2@S cathode at various scan rates. Corresponding relationship between the square root of scan rate v0.5 and peak current Ipeak of different cathodes for (f) Peak C-I and (g) Peak A-I. (h) Diffusion coefficient (DLi) values of Li-ion of MoO2-Mo3N2@S, MoO2@S, Mo3N2@S, and MoO3@S electrodes for Peak C-I, A-I and C-II processes.


ACS Nano

Figure 9. (a-d) SEM images of cycled MoO2-Mo3N2@S cathode surface with different magnification after 100 cycles at 1.0 C. (e,f) SEM images of cycled Li metal anode surface with different magnification after 100 cycles at 1.0 C.

Table of Content (TOC)

0.2

247 mAh/g

0.1 0.0

MoO2

Li2S Precipitation

0

400 800 Time (min)

Conversion

100

2

1200

1.2 mg/cm

50

600 0

Adsorption

3.2 mg/cm2 2.6 mg/cm2 0.5 C

0

200

400 600 Cycle numbers

800

0 1000

Coulombic efficiency (%)

Current (mA)

Mo3N2

Capacity (mAh/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40


Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic

Recommend Documents