Hetero-structure Manipulation towards Ameliorating Electrodes for

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Hetero-structure Manipulation towards Ameliorating Electrodes for Better Lithium Storage Capability Jing Cuan, Fan Zhang, Hongyu Zhang, Jun Long, Shilin Zhang, Gemeng Liang, Qili Gao, Junnan Hao, Linxi Dong, Gaofeng Wang, and Xuebin Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04685 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Sustainable Chemistry & Engineering

Journal: ACS Sustainable Chemistry & Engineering Manuscript ID: sc-2018-046854

Hetero-structure Manipulation towards Ameliorating Electrodes for Better Lithium Storage Capability Jing Cuan,†, § Fan Zhang,‡ Hongyu Zhang,‡ Jun Long,§ Shilin Zhang,§ Gemeng Liang,§ Qili Gao,‡ Junnan Hao,§ Linxi Dong,† Gaofeng Wang,† Xuebin Yu†, ‡, ‖* †Key Laboratory of RF Circuits and Systems of Ministry of Education, Electronic and Information College

of Hangzhou Dianzi University, Hangzhou 310018, China ‡Department of Materials Science, Fudan University, Shanghai, China §Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, Australia ‖Shanghai Innovation Institute for Materials, Shanghai 200444, China Corresponding Author: *E-mail: [email protected] Full Mailing Address: Handan Road 220, Department of Materials Science, Fudan University, Shanghai, China Keywords: over-potential, hetero-structure manipulation, charge transfer kinetics, hybridity, lithium ion battery

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ABSTRACT As one of the major problems facing lithium ion batteries, sluggish charge transfer often induces undesirable large resistance, over-potential and round trip inefficiency of batteries during recharge. The need to improve charge transport kinetics is motivating research into directions that would rely on high quality hetero-structure designs, since it is reported that the synergistic effects and as-formed inbuilt electric fields of hetero-structures could facilitate charge transport across the hetero-structure, as well as enforce interactions between the active phases. Hetero-manipulation holds great promise for realizing

efficient interconnects between charge transport kinetics and hetero-structure designs. However, most previous studies delineate ensemble measurements of a given static hetero-electrode, which do not permit isolating and dissecting the effects of hetero-structural manipulation on electrochemical performances individually. Here, by choosing conversion type electrodes as example and comparing series samples which were collected in the evolution of hetero-structures, the effects of hetero-structure manipulation towards modifying over-potential and lithium storage capability have been systematically investigated. The results demonstrate that structural features (e.g. robust skeleton, smaller grain sizes, and high quality hybridity) play an important role in engendering faster charge transfer and narrowing overpotential than that at the level of micrometer scales.

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INTRODUCTION As the power system and cornerstones for energy storage techniques, charge carrier plays an important role in the well-operation of electrochemical devices, especially for the popular rechargeable lithium ion batteries (LIBs), which have wide applications in portable devices and electrification of transport. The ever-growing demands for high performance LIBs place requirements for electrode that could combine the high capacity of batteries and fast kinetics of capacitors.1-16 Conversion reaction-based transition metal compounds have been proposed as promising candidates, which can accommodate more than one Li+ in each molecular, with specific gravimetric capacities ~2-3 times larger than for electrodes that react through classical intercalation reactions.15 Upon a charging and discharging cycle, the active phase of transition metal compound will experience a reversible phase transformation between its counterpart metal element. However, despite their superiority in high theoretical capacity, grand challenges still remained. The low solid-state charge transport and sluggish mass/charge transfer kinetics across grain boundaries in most conversion type systems have been proved as the primary causes for giant voltage hysteresis and overpotential between a redox pair. Therefore, the resulted impaired roundtrip efficiency of conversion type systems can hardly approach the level of practical applications, which require further studies to address the associated problems.1, 17-21 Compared with homo-structures, materials made from hetero-structures show great potential to modify the dynamics and chemistry of charge carriers, which have been widely used to build devices that can be utilized for diode, capacitor, transistor, photocatalysis and non-volatile memory, etc.22,

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For

instance, Kim et al. has verified that after the construction of van der Waals hetero-interfaces between graphene and molybdenum dichalcogenide (MoX2, X = S, Se), nearly 0.5 V negative-shift of intercalation potential and 10-times accumulation of charges have been achieved in comparison with MoX2/MoX2 homo-interfaces.24 Loh et al. fabricated a graphene/black phosphorus hetero-structure, which induced a 3



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strong pseudo-magnetic fields (PMFs), leading to the formation of pseudo-Landau levels and valley Hall effect in graphene. Furthermore, as the relative orientation of graphene and black phosphorus changes, the spatial distribution of PMF and the intertwined moiré pattern could be tailored, which prominently influenced the transport properties of the composites.25 Pan et al. fabricated a WSe2/SnS2 composite, which had one order higher hole mobility than pristine WSe2 compound, showing that the charge transport capability of WSe2 has been improved greatly in the hetero-structured composite.26 Based on this, the chemically engineered hetero-structures are promising to reinforce the interactions between the active phases, as well as to tune the charge transport and thermodynamic landscapes of electrodes, which are anticipated to regulate the reversibility and over-potential of redox pairs. Nonetheless, these rational ameliorations of such electrochemical properties place demands on rigorous understanding of accurate hetero-structure manipulation, namely precise hetero-design, which is inaccessible in evaluating a given static hetero-structure. This work reported a dynamic hetero-structure evolution, in which an ensemble of electrodes with sequential hetero-structure-shifting have been compared to scrutinize the effects of hetero-structure manipulation on the electrochemical behaviors of conversion type electrodes. In this work, Mo2C-C⊂xMoO3 (x = 1, 3, 6, 14) hetero-electrodes have been taken as examples, considering the complementary advantages of Mo2C and MoO3 at diffusion barrier, electric conductivity, and lithium storage capacity, etc.2, 27-47 Here, four investigated compositions (x = 1, 3, 6, 14) were selected to check the associated hetero-architecture effects on electrochemical behaviors, mainly in view of their noticeable structural divergence, which could be reckoned as four representative stages during the hetero-morphology shifting. The inspection of physical properties (Rietveld refinement, transmission electron microscopy, field emission scanning electron microscopy, etc.) imply that as a result of varied component proportion and grain sizes, the structural integrity and hybridity (quantity and dispersibility of hetero-interfaces) in each 4


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composite has changed greatly. The evaluated electrochemical properties show that the electrochemical reactions of Mo2C-C⊂x-MoO3 with lithium are more facilitated at a lower value of x, yielding higher lithium storage capacity. Simultaneously, the over-potential and charge transfer resistance have a visible response to the variations of x. Among the four investigated compositions, Mo2C-C⊂x-MoO3 at x = 1, 3 show superior electrochemical performance to other counterparts. The examination of kinetic results revealed that this outperformance may mainly be attributed to the synergistic effects of robust fibrous construction, smaller grain sizes and high quality hybridity in their superior architecture, which could reduce the electron/lithium ion diffusion distance, boost the reaction kinetics, and provide rich interfacial electrochemical active sites at nanoscale. Such improvements signify the significance of hetero-structure manipulation in better ameliorating electrodes for advanced batteries and devices in other fields.

RESULTS AND DISCUSSION

Figure 1. Schematic illustration of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) nanofibers (NFs) prepared via an electrospinning process with a single-needle nozzle. :Mo-PVA nanofiber. :C nanofiber matrix. : Mo2C particle. : MoO3 particle.

Figure 1 delineates the synthesis procedure of the hetero-molybdenum composite series. The pristine Mo-PVA nanofiber (PVA=polyvinyl alcohol) was fabricated through a facile electrospinning method and calcined under Ar atmosphere to form Mo2C-C nanofiber. Subsequently, the collected Mo2C-C nanofiber were grouped into four batches, and were treated under flowing air for 1 h, 3 h, 6 h, and 14 h at the 5



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temperature of 300 oC. In this post-calcination treatment process, the phase transition from Mo2C to MoO3 happened,48 accompanying with gradient changes of hetero-structures. (The experimental details were shown in the Experimental Section in Supplementary Information.) The obtained samples were denoted as Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14), in which x represented the post-calcination time in air. Mo2C-C nanofiber also stands for Mo2C-C⊂0-MoO3. The phase purity and crystallographic nature of all the collected samples were examined by powder X-ray diffraction (XRD).

Figure 2. (a) XRD patterns of all the samples, involving Mo2C-C⊂x-MoO3 (x= 0, 1, 3, 6, 14) composites. The PDF card (JCPDS No.72-1683) corresponds to orthorhombic Mo2C (Pbcn(60)), while the other JCPDS card (No. 35-0609) is ascribed to orthorhombic MoO3 (Pbnm(62)). (b) The Rietveld refinement of the X-ray diffraction pattern for Mo2C-C⊂1MoO3. (c) Raman spectra of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) composites. (d-f) High resolution Mo3d, O1s, C1s XPS spectra for Mo2C-C⊂1-MoO3 composite.

Figure 2a demonstrated that two sets of diffraction peaks (at 2θ ≈ 12.8 °, 23.3 °, 25.7 ° and 27.3 °; 34.5 °, 38.1 °, and 39.5 °) were recorded in Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) composites, which could be readily indexed to orthorhombic phases of MoO3 (JCPDS No. 35-0609) and Mo2C (JCPDS No. 721683), respectively. The diffraction intensity of Mo2C component decreases gradually with the increase of x, while that of MoO3 phase increases simultaneously, which are mainly resulted from the continuous 6


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phase transition from Mo2C to MoO3 at 300 oC under flowing air.48 We note that the diffraction peaks of Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) nanofibers are much weaker and broader in comparison with those of bulk MoO3 and bulk Mo2C (Figure S1a, supporting information), implying that MoO3 and Mo2C particles are nanoconfined within the carbon matrix. The Rietveld refinements of XRD patterns are conducted to determine the mass ratio of two Mo species in Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) samples, which are shown in Figure 2b, Figure S1b-d. All the diffraction peaks of these samples can be indexed to those of orthorhombic MoO3 (space group: Pbnm) and orthorhombic Mo2C (space group: Pbcn). Furthermore, the carbon contents of Mo2C-C⊂x-MoO3 samples were calculated by the joint analysis of Rietveld refinement and thermo-gravimetric analysis (TGA) profiles (Figure S2) under flowing air, and detailed calculation methods have been provided in Supplemental Materials. Table 1 shows the component proportions of the Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) samples. Evidently, it is revealed that carbon content and the mass ratio of Mo2C to MoO3 decline simultaneously as the rise of x, manifesting the gradient evolution of hetero-structures. Note that the feeding ratio of molybdenum salts were identical in Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) composites, on this premise, the relative mole numbers of two individual molybdenum species largely determine the quantity of hybridity. It is easily understood that, if the mole ratio of two molybdenum species is close to the equimolar ratio (1:1), maximum number of hetero-interfaces will be approached, while once deviated from equimolar ratio, the number of heterointerfaces will decline. In this work, the molar ratio of two molybdenum species was proved to be more deviated from equimolar value at larger x (Table 1), which signifies the reduced quantity of corresponding hybridity. To identify the structural differences of the fibrous carbonaceous matrix in Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) samples, their Raman spectra are compared in Figure 2c. All tested samples show two strong and typical bands at 1358 and 1616 cm-1, which represent the disorder-induced D band (sp3 hybridized carbon atom) and in-plane vibrational G band (in-plane vibration of sp2 carbon atoms) of 7



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carbonaceous materials.49-51 The intensity ratios of D band to G band (ID/IG) for Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) were calculated as 1.4, 1.9, 2.1, 1.9, and 2.0, respectively, implying that amorphous carbon was the primary carbonaceous forms in Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) composites.52 The bonding configurations and chemical states of Mo2C-C⊂1-MoO3 sample were investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectrum in Figure S3 confirms the presence of all expected elements, Mo, O, and C. Three types of Mo valences are identified in the Mo 3d high resolution XPS spectrum (Figure 2d), which correspond to three different oxidation states of Mo. The initial two resolved peaks at lower binding energies of 231.6 eV and 228.5 eV could be ascribed to the typical Mo (II) 3d3/2 and Mo (II) 3d5/2 peaks of Mo2C nanoparticles.53 Another pair of split peaks with mid-range binding energies of 235.1 eV and 232 eV can be assigned to the Mo (V) 3d3/2 and Mo (V) 3d5/2 peaks in MoOx, which may be the partial reduction product of MoO3. The individual peaks located at 236.1 eV and 233 eV can be attributed to the characteristic response of Mo (VI) 3d3/2 and Mo (VI) 3d5/2 peaks in MoO3.54-56 The O 1s XPS spectrum (Figure 2e) can be resolved into three peaks centered at 530.4, 531.9, and 533.3 eV, corresponding to the Mo-O bonds, C-O bonds, and C=O functional groups in Mo2C-C⊂1-MoO3 composite, respectively.54, 57 As observed in the high resolution C 1s XPS spectrum (Figure 2f), three individual peaks of nonoxygenated C-C, C=O, and O=C-O groups are located at 285.4, 286.5, and 290.1 eV, respectively, which are mainly originated from the decomposition of polymer polyvinyl alcohol.54 Table 1. The obtained weight ratio of Mo2C to MoO3 by Rietveld refinement. Sample

Mo2C-C⊂0-MoO3 Mo2C-C⊂1-MoO3 Mo2C-C⊂3-MoO3 Mo2C-C⊂6-MoO3 Mo2C-C⊂14-MoO3

𝒎𝑴𝒐𝟐𝑪

𝒎𝑴𝒐𝑶𝟑

𝒎𝑴𝒐𝟐𝑪 + 𝒎𝑴𝒐𝑶𝟑

𝒎𝑴𝒐𝟐𝑪 + 𝒎𝑴𝒐𝑶𝟑

1 0.48303 0.25908 0.21729 0.2000

0 0.51697 0.74092 0.78271 0.8000

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𝐶𝑤𝑒𝑖𝑔ℎ𝑡 𝑟𝑎𝑡𝑖𝑜/ % 29.7 28.6 20.2 15.3 13.6

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Figure 3. Left frame (a, d, g, j, m): Zoomed-in schematic illustration of the interior morphology of a singular nanofiber in Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) within the same size of selected area. Grey color represents carbon phase, and pink red color stands for Mo2C phase. Light blue color represents MoO3 phase. Middle frame (b, e, h, k, n): TEM images of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14), respectively. Right frame (c, f, i, l, o): SEM images of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) composites, respectively.

The morphological feature and nanostructure of the as-prepared fibers were characterized by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM). In 9



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the panoramic views of TEM images (Figure 3b, e, h, k, n), we can see that Mo2C-C⊂0-MoO3 composite contains many integral and homogeneous fibers with lengths on micrometer scale. Mo2C-C⊂1-MoO3 composite showed a peapod-like fibrous structure, while Mo2C-C⊂3-MoO3 had an obvious carbon coating layer, which tightly encapsulated larger particles of the active phases. Mo2C-C⊂x-MoO3 (x = 0, 1) showed a large number of monodisperse nanoparticles uniformly distributed in the 1D fibrous carbon matrix (Figure 3b, 3e). No isolated nanoparticles of molybdenum species have been observed even after ultrasonic for 30 min to disperse the samples for TEM testing, implying the robust interactions between molybdenum species and fibrous carbon matrix. As post-calcination treatment went to greater depth, aggregated nano-grains appeared in the center of fibrous Mo2C-C⊂x-MoO3 (x = 6, 14) composites, and the particle sizes are found to increase prominently, probably due to heat induced aggregation of crystals.58, 59

As a result, it is easily understood that the dispersibility of hybridity will decline vastly due to the

aggregated and growing grains of molybdenum species. From the high resolution transmission electron microscopy (HRTEM) image of Mo2C-C⊂x-MoO3 (x = 1, 3) composites (Figure S4a, S4b), two types of inter-planar spacings (0.23 nm and 0.32 nm) have been identified, which correspond to the (1 2 1) and (0 2 1) crystal planes of orthorhombic Mo2C and MoO3 phase, manifesting the successful hybridization of molybdenum based species in carbon nanofiber, in contrast, only orthorhombic Mo2C phase has been identified from the HRTEM image of Mo2C-C⊂0-MoO3 (Figure S4c). More morphological details have been characterized by FE-SEM (Figure 3c, f, i, l, o, and Figure S5). The as-electrospun Mo-PVA nanofibers show smooth surfaces up to several micrometers in length with average diameters around 400 nm (Figure S5), which interlace in random orientations, as a result of the electrically driven bending fluctuations associated with the spinning nozzle.60,

61

In Mo2C-C⊂0-MoO3 composite, the fibrous

structures are well preserved with diameters of ~330 nm (Figure 3c). The contracted fibrous diameters are probably caused by the carbonization of organic species (polyvinyl alcohol) and thermal 10


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decomposition of the starting reactant (NH4)6Mo7O24·4H2O. Similar cases are found in prior reports on V2O5 and SrLi2Ti6O14 nanofibers.60,

62

Upon thermal treatment in air (T = 300 oC) for 1 to 6 h, the

morphology remains intact in the Mo2C-C⊂x-MoO3 (x = 1, 3, 6) composites, while plenty of short rods are observed at x = 14 (Figure 3f, i, l, o). The SEM images illustrate that as the post-calcination time increases, the steady contraction of fibrous diameters and transition of smooth surface to rough occur concurrently. In order to determine the process behind the change in carbon skeletons, isothermalgravimetric analysis of a blank pure PVA-C nanofiber was conducted at 300 oC under air atmosphere. The isothermal TG curve of pure PVA-C nanofiber (Figure S6) verified the occurrence of a continuous weight loss at constant temperature of 300 oC, which could be ascribed to the consumption of carbon by air (T = 300 oC) according to previous studies,48 and rationally interprets the rough surface and reduced diameters of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) nanofibers when x increases.

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Figure 4. (a) The initial discharge–charge profiles of all the samples for LIBs at the current density of 100 mA·g-1. (b) The corresponding coulombic efficiency of all as-collected electrodes. The first cycle was tested at 100 mA·g-1, while the subsequent cycles were tested at 1000 mA·g-1. (c) The comparisons of galvanostatic discharge profiles for all the electrodes and the corresponding coulombic efficiency of Mo2C-C⊂1-MoO3 at 1000 mA·g-1. (d) Rate performances (discharge capacities) of all the samples, at the current densities of 200, 500, 1000, 2000, and 200 mA·g-1, respectively. (e) The comparison of the second cyclic voltammetry profiles of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) samples within voltage range of 0.05-3 V vs. Li+/Li. (f) The Nyquist plots of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) in the frequency range of 1 kHz–10 mHz after 30 cycling tests at 200 mA·g-1. The inset is the equivalent circuit used to analyze the impedance plots.

To check out the effects of hetero-structure evolution towards modifying the lithium storage 12


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performance of MoO3, the electrochemical performances of hetero-structured samples were compared. Figure 4a shows the initial galvanostatic charge-discharge profiles of all the samples at 100 mA·g-1, which show similar voltage plateau between 1.1-0.4 V. This can be mainly ascribed to the formation of passivated solid electrolyte interphase (SEI) films in the first lithiation process.63, 64 The initial discharge capacities of Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) anodes are 893, 1331, 1158, 1123, and 879 mA·h·g-1, respectively, with corresponding coulombic efficiencies (CEs) of 59.9 %, 73.9 %, 73.7 %, 72.2 %, 65.5 %. Better lithium storage capacity and reversibility were achieved at x = 1, 3, as also reflected by the CE comparisons for the first four cycles (Figure 4b), showing faster CE increase in Mo2C-C⊂x-MoO3 (x = 1, 3). In the long-term cycling (Figure 4c), the discharge capacities of Mo2C-C⊂x-MoO3 (x = 0, 3, 6, 14) are 395, 712, 305 and 154 mA·h·g-1, respectively, while Mo2C-C⊂1-MoO3 anode maintains ~890 mA·h·g-1 over 300 cycles at 1000 mA·g-1. The lithium storage performances of Mo2C-C⊂x-MoO3 at x = 1, 3 appear better among the four investigated hetero-compositions, simultaneously, they were also superior to that of previously reported individual MoO3-C nanofiber (300 mA·h·g-1 at 800mA·g-1, 100th cycle).47 Moreover, the rate performance of Mo2C-C⊂x-MoO3 (x = 1, 3) are also noticeably ameliorated in comparison with those of Mo2C-C⊂x-MoO3 (x = 0, 6, 14) composites. Mo2C-C⊂1-MoO3 affords the discharge capacities of 928, 871, 815, 770 mA·h·g-1 at 200, 500, 1000, 2000 mA·g-1, respectively and retains ~1000 mA·h·g-1 in the subsequent cycles when the current density has been switched back to 200 mA·g-1, manifesting a good rate durability (Figure 4d). For Mo2C-C⊂3-MoO3, the rate performance resembles that of Mo2CC⊂1-MoO3 in the initial 40 cycles, and drops slightly at 200 mA·g-1 in the following cycles. With regard to Mo2C-C⊂6-MoO3, the discharge capacity drastically reduces to 647 mA·h·g-1 at the 110th cycle at 200 13



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mA·g-1, although it exceeds those of both the Mo2C-C⊂14-MoO3 and Mo2C-C⊂0-MoO3.65 The long-term cycling and rate performance of Mo2C-C⊂1-MoO3 nanofiber is superior to many reported MoO3-based materials (Table S1), including MoO3@FeOx nanobelts,66 MoO3 nanorods,2 MoO3/SnO2/CNTs,67 MoO3C nanofiber,45 which may originate from its advanced hybridity designs and structural stability with the more dispersive Mo2C-C coating layer to release the undesirable mechanical strain. This trend implies that high quality hetero-structural features at the early stage of reaction (x = 1, 3) may have a more positive role in modifying the reversibility of Li+ insertion/extraction process. It is reported that the hetero-structural characteristics in electrodes could facilitate and accelerate the ion/electron mobility during electrochemical reactions.68-70 To evaluate the effects of hetero-structure manipulation on Li+ transport behaviors, the cyclic voltammetry (CV) profiles of Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) composites were compared. Figure 4e compared the second cycle cyclic voltammetry profiles of Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) composites, in which the typical cathodic/anodic peaks of Mo2Cbased anodes (~1.26/1.36 V) and MoO3-based electrode (0.16/1.23V and 1.61/1.81 V) have been identified.2, 67 We note that, as x decreases, the anodic peaks (~1.23, 1.79 V) shift pronouncedly to cathodic peaks at ~1.55 V (marked by a star), which manifests that the voltage offset of the redox pairs declines significantly as x decreases. Furthermore, as shown in Figure S7, in comparison with Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) composites, bulk MoO3 has shown the largest voltage offset, manifesting that the asprepared Mo2C-C⊂x-MoO3 (x = 0, 1, 3, 6, 14) samples have lower over-potential than bulk MoO3. Among the four investigated hetero-compositions, the narrower over-potential of Mo2C-C⊂x-MoO3 (x = 1, 3) composites implies better mass/charge transfer capability to the other Mo2C-C⊂x-MoO3 (x = 6, 14) samples and bulk MoO3, confirming that the high quality hetero-structural features of robust interleaved conductive skeleton, substantial hetero-surficial sites, and high-quality hybridity have an important role 14


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in modulating the charge transport behaviors, and reduce the voltage offsets between reduction and reverse reaction, at which the redox event is genuinely occurred and ameliorated. The results of electrochemical impedance spectroscopy for Mo2C-C⊂x-MoO3 (x = 1, 3, 6, 14) samples (Figure 4f, Table 2) further verified that at x = 1, 3, the exchange current density together with the charge-transfer/Li+ diffusion conductivity increase prominently, evidencing that the electron/ion transfer kinetics and electrochemical activities of Mo2C-C⊂x-MoO3 (x = 1, 3) are better than that of Mo2C-C⊂x-MoO3 (x = 6, 14).71, 72 Table 2. Impedance parameters for all samples from the fitted equivalent circuits R1(R2CPE1)((R3Ws)CPE2). In the equivalent circuit, R1 represents the electrolyte and contact resistance, R2 and CPE1 represent the resistance of the asformed solid electrolyte interphase, and constant phase element, respectively. R3 and Ws represent charge-transfer resistance and Warburg diffusion impedance, respectively. CPE2 represents the constant phase element. j0 is calculated according to j0 = RT/nFRct, where j0 refers to the exchange current density. R is the gas constant, T is the absolute temperature, n is the number of transferred electrons, and F is the Faraday constant. Sample

R1 / Ω

R2 / Ω

R3 / Ω

j0 / A·cm-2

Mo2C-C⊂0-MoO3 Mo2C-C⊂1-MoO3 Mo2C-C⊂3-MoO3 Mo2C-C⊂6-MoO3 Mo2C-C⊂14-MoO3

1.591 2.164 2.882 2.138 1.826

90.02 220.6 249.8 226.7 398.8

115.7 267 380.1 713.4 2610

4.44 × 10-5 1.92 × 10-5 1.35 × 10-5 7.19 × 10-6 1.96 × 10-6

Figure 5. TEM images of (a, b) Mo2C-C⊂1-MoO3, (c, d) Mo2C-C⊂3-MoO3, (e) Mo2C-C⊂6-MoO3, (f) Mo2C-C⊂14MoO3 after 30 cycles at 200 mA·g-1. 15



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Furthermore, the hetero-structures of Mo2C-C⊂x-MoO3 (x = 1, 3, 6) electrodes have been well maintained after repeated cycle testing, showing intact fibrous structures (Figure 5a-e), while that of Mo2C-C⊂14-MoO3 aggregated severely after cycling tests, implying that Mo2C-C⊂x-MoO3 heteroelectrode at lower value of x is more robust to accommodate repeated volume changes during electrochemical reactions. Based on the above analysis, Mo2C-C⊂x-MoO3 designs at lower x (e.g. x = 1, 3) has more advantageous structural features such as robust conductive skeleton, high quality hybridity, and substantial hetero-surficial sites, which could give rise to high intrinsic electronic conductivity and facilitated lithium ion mobility in Mo2C-C⊂x-MoO3. The structural features are reminiscent of several pioneering work, which have shown that with the introduction of hybridity designs, lopsided charge distribution could be induced in the interior of electrodes. The unbalanced charge distribution could further engender the formation of inbuilt electric field at heterojunctions, and contributes to accelerating the ion diffusion/electron transfer among the active phases.24-26 Based on the above analysis and the as-verified high-rate capability/enhanced electrochemical kinetics of Mo2C-C⊂x-MoO3 at lower x (x = 1, 3), a possible mechanism has been proposed for interpreting the different lithium storage capability of Mo2CC⊂x-MoO3 (Figure 6a). The synergistic effects of high quality hybridity, smaller grain sizes and robust skeletons of Mo2C-C⊂x-MoO3 (x = 1, 3) may induce stronger inbuilt electric fields and interactions around the hetero-interfaces, which will enable the accumulation of charges around the active phases, and guarantee unimpeded charge migration paths inside the hetero-electrodes, as a result, both the charge transfer kinetics and cycling durability could be boosted. The deteriorating architectures (namely, reduced integrity and quality of hybridity) is probably one of the most important reasons for the decreasing lithium storage capability of Mo2C-C⊂x-MoO3 (x = 6, 14) series.

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Figure 6. (a) The schematic illustration of lithium diffusion process in Mo2C-C⊂x-MoO3 composites. (b) The estimated trend of hybridity in Mo2C-C⊂x-MoO3.

The above analysis reveals the significance of hetero-structure manipulation in regulating charge transport dynamics and modifying the associated lithium storage performances of electrodes. In this conversion type system, Mo2C-C⊂x-MoO3 compound at smaller x (e.g. x = 1, 3) appears to have more desirable hetero-structures. The associated structural benefits are summarized as: firstly, reduced Li+ diffusion distance and low electric resistance are achieved in such hetero-structures due to the small particle sizes and high electric conductive Mo2C-C hetero-matrix, which leads to accelerated electron/ion transport in the designed hetero-electrodes. In addition, the high quality hybridity of such hetero-structures further guarantee the accumulation of charges and provide substantial electrochemical active sites, leading to prominently improved Li+ insertion/extraction reversibility and reduced over-potential of redox pairs. Furthermore, their robust hetero-structures could render intact charge transport path, good rate durability 17



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and long-term cycling performance. This work suggests that, the carefully designed hetero-structures with the aforementioned integrated high quality features could contribute significantly to improving the charge transfer capability and further enhance the properties of devices.

CONCLUSIONS In summary, this work scrutinized the effects of hetero-structure manipulation on the electrochemical behaviors of conversion type electrode. Through the inspection of an ensemble of electrodes with gradient hetero-structure-shifting, it is demonstrated that the quality of hetero-structures has a significant role in regulating the charge transfer behavior, as verified by the results of lithium storage performances and kinetics. The results manifest that, among the four investigated compositions, Mo2C-C⊂x-MoO3 at the early stage of reactions (x = 1, 3) exhibit ameliorated electrochemical performances: small over-potential, high charge-discharge capacity and stable rate performance owing to their superior hetero-structures. Furthermore, Mo2C-C⊂1-MoO3 retained a capacity of ~890 mA·h·g-1 after 300 cycles testing at 1000 mA·g-1, and no capacity fading has been observed after various C-rates cycling. These improvements may mainly be attributed to the synergistic effects of high quality hybridity, small grain sizes and robust fibrous structure in Mo2C-C⊂x-MoO3 (x = 1, 3), which have significantly facilitated the electron and ion transfer among the active phase, as well as enhancing the cycling stability of electrodes. Compositions with such high quality hetero-structural features shed light on better ameliorating electrodes for advanced batteries and devices in other fields and further promoting the round-trip efficiency for batteries employing electrodes via conversion reactions.

ASSOCIATED CONTENT Supporting Information

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Experimental section, Rietveld refinement of X-ray diffraction patterns, TG profiles, X-ray photoelectron spectroscopy, Representative TEM image, SEM image, Table S1.

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

ACKNOWLEDGMENTS This work was partially supported by the National Key Research and Development Program of China (2017YFA0204600), the National Science Fund for Distinguished Young Scholars (51625102), the National Natural Science Foundation of China (51471053), the Key Research and Development Plan Project of Zhejiang Province (No.2018C01036), the Zhejiang Provincial Natural Science Foundation of China under grant LQ15F040006, and the Science and Technology Commission of Shanghai Municipality (17XD1400700). We thank Dr. Kun Rui for her valuable advices for the manuscript, and we also thank Dr. Tania Silver for her critical reading of the manuscript.

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TOC/Abstract Graphic

Synopsis Hetero-manipulation helps to ameliorate the reversibility of electrochemical reactions for sustainable energy storage, in particular for lithium ion batteries.

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Hetero-structure Manipulation towards Ameliorating Electrodes for

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