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Facile Synthesis of Large Area Two Dimensional Layers of Transition Metal Nitride and Their Use as Insertion Electrodes Siddharth Joshi, Qi Wang, Ajinkya Puntambekar, and Vidhya Chakrapani ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017

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ACS Energy Letters

Facile Synthesis of Large Area Two Dimensional Layers of Transition Metal Nitride and Their Use as Insertion Electrodes

Siddharth Joshi,a Qi Wang,a Ajinkya Puntambekar,a and Vidhya Chakrapania,$,# a

Howard P. Isermann Department of Chemical and Biological Engineering $

Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute, Troy, NY-12180 #

Email: [email protected]

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ABSTRACT A new versatile technique of synthesizing large area, vertically oriented two-dimensional (2D) layers of molybdenum nitride (MoN) is reported that involves synthesis of 2D nanosheets of MoO3 through hot-filament chemical vapor deposition process and subsequent phase transformation to δ-MoN. This simple two-step approach of phase transforming 2D oxide layers potentially enables easy synthesis of a wide variety of MXenes of nitrides, sulfides, and carbides of tunable composition. When used as electrodes in Li-ion batteries, the 2D layers of MoN show unusual Li+ storage mechanism. Using combined differential capacitance, X-ray diffraction, and X-ray photoemission spectroscopy, it is shown that unlike their bulk form, which behave as conversion electrodes, 2D layers of MoN behave as insertion electrodes that do not involve chemical transformation. As a result, the electrode shows stable capacity of 320 mAh/g for more than 200 cycles without any structural and electrochemical degradation.


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Two-dimensional (2D) layers of transition metal carbides, nitrides, carbonitrides, commonly referred to as MXenes,1 in the recent years have been increasingly explored for a variety of applications ranging from batteries,2-3 surpercapacitors,4 thermoelectrics,5 photothermal conversion.6 Although it is the MXenes of carbides that have been the most studied, MXenes of transition metal nitrides are fascinating and yet relatively unexplored class of material with some outstanding material properties, such as compositional and surface termination-dependent band gap,7 high electrical conductivity exceeding even graphene, high catalytic activity, high melting point (> 2000 K), exceptional hardness (> 10 GPa), chemical inertness in acidic and basic solutions, durability, and radiation resistance. Transition metal nitrides, in general, have long known to be effective heterogeneous catalysts for a number of reactions, such as hydrodesulphurization,8 NO dissociation9 and recently have been employed as electrode materials in Li-ion batteries and other electrochemical devices10-12 because of their high electrocatalytic activities, which is comparable to the activity of noble metals such as platinum. They are promising anode materials in Li-ion batteries because of their high specific capacity and lower redox potential for Li+ insertion than the corresponding oxides due to the lower polarization of M-N bond compared to M-O bond.13 Binary nitrides such as Co3N, WN, Fe3N, CrN, and VN, in both bulk and nanoparticle form, have shown promising performance as Li-ion battery electrodes.13-20 These studies have shown that nitrides, similar to oxides, store Li+ ions in their lattice by electrochemical conversion reactions that involve reaction of Li+ with the crystal lattice to form Li3N and corresponding metal nanoparticles that undergo further lithiation steps. Although, Li+ storage through conversion mechanism results in high gravimetric capacity in both oxides and nitrides, irreversible crystallographic phase transition of the host lattice and the massive volumetric changes associated with the transformation leads to the generation of severe


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mechanical stresses, loss of electrical contact with the conductive binder, and continuous consumption of the electrolyte for the solid-electrolyte interface (SEI) formation. These factors contribute to the low columbic efficiencies during initial cycles and rapid capacity fade during longer cycles.

Thus, despite their low gravimetric capacity, insertion electrodes, such as

graphene/graphite, that do not involve irreversible chemical reactions during Li+ insertion, are still preferred over conversion electrodes, which have higher capacity but poorer cycle life than the former. Interestingly, in a recent report on vanadium nitride nanoparticles, Kundu et al21 showed that nanoparticles can behave as insertion electrode unlike the bulk form that function as conversion electrodes.

In this report, we show that highly crystalline, 2D nanosheets of

transition metal nitride synthesized via a facile chemical vapor deposition technique also behave as insertion electrodes for Li+ storage, which shows superior cycling stability with cycle life in excess of 200 cycles without degradation with steady specific capacity of 320 mAh/g. Both bare and surface-terminated MXenes are typically synthesized from MAX phases, where M represents transition metal, A represents a Group A element, such as Al, Si, Sn, or In, and X represents C and/or N. MXenes are then produced by selectively etching the “A” metal from MAX phases using aqueous hydrofluoric acid (HF) at room temperature. MXenes of carbides with compositions of M2C, M3C2,22 and M4C323 have been produced by this technique. Recently, MXene of nitride, Ti4N3, was reported using synthesis procedure of mixing its corresponding MAX phase in a molten eutectic fluoride salt solution and treating it at a high temperature.24 In this work, we report a synthesis technique for obtaining micrometer-size 2D nanosheets of δ-MoN via a two step process involving first the synthesis of 2D nanosheets of MoO3 through a hot-filament chemical vapor deposition (HFCVD) process and subsequently transforming the oxide phase to the respective nitride phase through a simple reductive annealing


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process. Since phase transformation of metal oxide to respective nitride, sulfide or carbide can be easily achieved through reduction in NH3, H2S or CH4 environment respectively, this two-step approach of phase transforming 2D-oxide layers affords easy synthesis of potentially wide variety of MXenes of tunable composition. Highly-aligned, single-crystal 2D layers of MoO3 were synthesized using a HFCVD technique, which involves resistively heating a molybdenum wire close to its melting point in an O2 environment. The reaction of hot metal with O2 creates vapors of metal oxide, which then deposit on the cooler parts of the reactor. Nanosheets of 2D layer of oxides of various crystal sizes and a range of metal-to-oxygen ratios could be prepared using this technique with high purity without the introduction of extraneous reducing agents by simply tuning the filament power and partial pressure of oxygen and water vapor in the chamber. More details of the deposition technique are given in the Methods Section as well as in our previous works.25-27 Figure 1A shows a photograph of a hot filament during deposition. The technique also allows for the synthesis of gram-quantities of 2D oxide and respective nitrides, as also shown in the Fig.1A. In general, high oxygen pressures and high filament power favored the growth of 2D nanosheets of MoO3 with increasing crystallite size of several tens of microns in width. Phase transformation to respective nitride was carried out by annealing the oxide nanosheets in an ammonia atmosphere. Figure 1A also shows a schematic of the two-step synthesis process for 2D-MoN nanosheets.

Also shown are the photographs of MoO3 and MoN grown on a

fluorinated tin oxide- (FTO) coated glass substrate. The color of as-synthesized 2D nanosheets of MoO3 was bluish white in color, which turned black after nitridation to δ-MoN. Figures 1B-D and Fig.S1 in the Supplementary Information (SI) show the scanning electron micrograph (SEM) images of the as-synthesized MoN nanosheets. No change in the morphology was observed as a


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result of phase transformation. MoO3 grown on FTO substrates typically grew in verticallyoriented stacks with its basal plane perpendicular to the growth substrates. Crystallite size of these nanosheets was typically about 20-30 micron in width and had a thickness of 5-40 nm. These grew as stacks of nanosheets that had a thickness of several hundred nanometers, as shown in Fig.1C&D. Uniform deposition could be carried out in at least 1 inch substrates, which was dependent only on the filament size. In order to use these nanosheets in a battery electrode, material was scrapped from the FTO substrate, and then mixed with conductivity binder and glue to form a uniform coating onto a copper foil. Figure S2 in SI shows the SEM images of the asprepared electrodes of MoN. Electrochemical testing of as-prepared nanosheet electrodes was done by using them as an anode in a half-cell Li-ion battery setup, which is detailed in the Method section. The half-cell performance of MoN nanosheets during galvanostatic cycling of the battery at a constant current density of 35 µA/cm2, which corresponded to a C-rating of C/20, between 0.005 V and 3 V versus Li/Li+ is shown in the Fig.2A. The charge-discharge profiles of the electrode during the 1st, 30th, 60th, 90th and 120th cycle, as shown in Fig.2A, indicates a stable insertion and de-insertion process into the lattice even during long-term cycling. Figure 2B shows the summary of the battery performance under slow charging up to 200 cycles. The lithiation and delithiation capacities observed for the 1st cycle were 336 mAh/g and 302 mAh/g. The corresponding columbic efficiency was 90%, which is much larger than 30-60 % efficiency of the 1st cycle reported for bulk nitrides15-16 as well as for carbide-based MXene such as niobium and vanadium.3 The capacity then gradually increased for the first 15-20 cycles and eventually leveled of around 310-320 mAh/g. A significant change in the slope of the voltage profile can be seen at 1.5 V and 0.7 V during lithiation during the first few charge cycles, but the


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profile eventually becomes smooth around 40 cycles. After 60 cycles, all the charge cycles are almost identical to each other in shape. The columbic efficiency varies between 90-95% for the first 10-15 cycles, but is consistently above 99.4% after 20 cycles. The electrodes were also tested under rapid charging conditions at various C-ratings. The results are shown in the inset of Fig.2B. As can be seen, no significant drop in the capacity as a result of ohmic losses was observed when the cycling was switched from C/4 to 1C rating. The electrode showed a steady discharge capacity of 180 mAh/g at 1C-rating up to 100 cycles without significant degradation. In order to understand the reaction mechanism of Li+ in 2D-MoN layers, electronic and structural changes occurring during Li+ insertion were studied using cyclic voltammetry (CV), differential capacitance (DC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. These results are summarized in Fig.3. Cyclic voltammogram of δ-MoN during the first, third and the fourth cycle, as shown in Fig.3A, shows stable insertion and deinsertion of Li+ process. During the 1st cathodic scan, three distinct peaks are observed at 1.5, and 1.2, and 0.66 V versus Li/Li+. Prior studies on bulk transition metal nitride electrodes have shown that insertion of Li+ occurs via a two-step process, wherein the 1st step involves intercalation of Li+ ions into the bulk lattice, which during the second step undergoes conversion reaction to form Li3N and metal nanoparticles.14, 16 These reactions are typically written as:

MoN + xLi + + xe− → Lix MoN

Intercalation

Reaction-1

MoN + yLi + → Mo + Liy N

Conversion

Reaction -2

In thin film electrodes deposited through atomic layer deposition, the reduction potential for former step was reported to be ~1.0 versus Li/Li+, as ascertained from CV curves, while the later


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conversion reaction was seen to occur at 0.4 and 0.15 V versus Li/Li+.16 A broad peak in the anodic region at 0.4 versus Li/Li+ was observed that was attributed to the breaking of the solidelectrolyte-interface (SEI) layer. Cyclic voltammograms of 2D layers differ distinctly from the voltammograms of nanoparticles or thin films that have been reported in the literature. First, the 2D layers show reversible features that maintain their identities through repetitive cycles. This is also evident in the trend of differential capacitance values, which was calculated from the charge-discharge profiles shown in Fig.2A, that shown in Fig.3B. Both CV and DC curves exhibit sharp features at 1.55 and 1.27 V potentials during Li+ insertion that remain the same between the 1st and 4th cycle. This strongly suggests that no irreversible phase transformation occurs during the initial cycling of the electrode, as is typical with most transition metal nitride and oxides.

Further, the distinct peak in the CV curve at a potential of ~0.4 V that is

characteristic of the conversion reaction is absent in the present case. The absence of this feature along with the symmetry of the charge/discharge profile suggests that 2D nitride layers only undergo insertion step without involving a conversion reaction. The voltammogram of cycle 1 shows slight difference in area compared to the voltammogram corresponding to the subsequent cycles 3-4. This difference may arise due to the irreversible changes in the electrode surface during the first cycle such as formation of SEI layer, reduction of any surface functional groups. To further confirm the above conclusion, changes in the crystal structure during cycling were probed using ex-situ XRD measurements. XRD patterns were recorded on MoN electrodes in the pristine, lithiated (at 0.005 V), and delithiated electrode surfaces during the first cycle. The XRD pattern was also recorded for an electrode that had undergone 200 cycles of charge and discharge testing. The results, as summarized in Fig.3C, show no major shifts in the crystal structure during cycling. XRD pattern of pristine electrode shows peaks at 2θ values of 31º, 36º, 9 ACS Paragon Plus Environment

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49º and 65º degrees that are characteristic of (002), (200), (202) and (220) planes of hexagonal δMoN. The two most commonly occurring phases of molybdenum nitride are β-Mo2N and δMoN. β-Mo2N has a bcc structure, while δ-MoN has a hexagonal structure. The hexagonal structure of δ-MoN allows for easy intercalation and deintercalation of Li+ without imposing significant stress on the structure. Only a weak signal at 53º corresponding to the (211) plane of MoO2 was observed, which indicates almost complete phase transformation of MoO3 to MoN post nitridation. The electrode upon lithiation at 0.005 V versus Li/Li+ retains its well-defined crystal structure and shows no major shift in the diffraction pattern. This is in stark contrast to prior studies on transition metal nitrides anodes, where researchers have observed complete loss of diffraction signal from the host lattice upon lithiation,14 and in some instances an appearance of diffraction peak corresponding to the respective metal nanoparticles.17 Diffraction patterns were also recorded on electrode that were partially lithiated to potentials of 1.27 V and 1.55 V versus Li/Li+, which are potentials of characteristics peaks seen in CV and DC curves, and are shown in Fig.S3 in SI. XRD pattern of electrode charged to 1.55 V appears similar to the pattern recorded in the pristine state, thus indicating no crystallographic transformation. However, the XRD pattern of electrode charged to 1.27 V shows, in addition to the characteristic peaks of δMoN, peak signals at 38º, 41º, 41.8º and 44.6º that most likely originate from the products of solid-electrolyte-interface (SEI) layer. For instance, the peak signals at 38º and 44.6º may be arising from LiF, whose (111) and (200) planes gives rise to strong characteristic peaks at the same diffraction angles.

The characteristic peak of (111) at 38º can also be observed in

diffraction pattern of electrode that was fully lithiated at 0.005 V (Fig.3C). Most importantly, the diffraction pattern shown in Fig.3C of an electrode that had undergone 200 cycles of charge and discharge testing still shows well-defined crystal structure of MoN. Thus, ex-situ XRD 10 ACS Paragon Plus Environment

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results obtained during various stages of cycling as well as post-electrochemical cycling show conclusive evidence that no significant structural transformations occur during Li+ insertion/deinsertion in 2D layers. The invariance of the crystal structure of δ-MoN during lithiation can also lead to the invariance in the electronic structure of the δ-MoN, which was probed using ex-situ XPS technique. Samples for XPS measurements were prepared by charging under a constant current of −50 µA to a potential of 0.005 V versus Li/Li+. After charging, samples were emersed from the electrolyte, washed with propylene carbonate to remove superficial impurities, dried, and transferred immediately to the XPS system. Care was taken to avoid air-exposure. Figure 3D shows the higher-resolution Mo 3d XPS spectra of pristine and lithiated electrodes. The Mo 3d envelope shows twin peaks at the BEs of 229 eV and 232 eV that are characteristic of spin-orbit split peaks of Mo 3d5/2 and Mo 3d3/2 respectively. Because of the broad nature of the peak, no attempt was made to resolve the peaks into multiplet corresponding to the various oxidation states. Comparison of the Mo 3d spectrum before and after lithiation shows no significant shift in the BE of the 3d envelope. This indicates that Li+ insertion does not significantly affect the electronic structure of 2D-MoN, which would otherwise be expected if the charge storage mechanism involved conversion reaction to the metal nanoparticle and Li3N formation. Instead, XRD, CV, and XPS results strongly suggest that the Li+ are inserted probably at the interlayer spacing of the basal planes of MoN, similar to the Li+ insertion mechanism seen in other layered compounds such as graphene. Since the mechanism involves no conversion reaction and does not involve significant changes in the crystal structure, it enables the battery to maintain longer cycle life even at higher charging rates. In conclusion, we report here a new technique for the synthesis of 2D layers of transition 11 ACS Paragon Plus Environment

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metal nitrides that involves HFCVD of 2D layers of MoO3 and its subsequent phase transformation to δ-MoN through nitridation. This HF-free technique can potentially be used for synthesizing wide variety of MXenes of carbines, nitrides, and sulfides with tunable composition and grain size than the currently used etching technique from MAX phases. Two-dimensional layers of δ-MoN thus produced stable battery capacity of 320 mAh/g for Li+ storage with cycle life exceeding 200 cycles without degradation or structural transformation. CV, XRD and XPS measurements show that, unlike in nitride its bulk form, two dimensional layers exhibit reversible Li+ storage through an insertion mechanism without involving conversion reaction, which is the dominant cause of capacity fade in battery due to the large irreversible changes in the crystal structure that the material undergoes during continuous cycling. Further studies are currently underway to further probe the insertion mechanism through transmission electron microscopy studies. METHOD: Synthesis of Transition Metal Nitrides: Highly-aligned, single-crystal 2D layers of MoO3 were synthesized using a HFCVD technique, which involves resistively heating a molybdenum wire close to its melting point in an O2 environment at pressure of 700 mTorr. Nanosheets of 2D layer of oxides of various crystal sizes and a range of metal-to-oxygen ratios could be prepared using this technique with high purity without the introduction of extraneous reducing agents by simply tuning the filament power and partial pressure of oxygen and water vapor in the chamber. Deposition was carried out on various substrates such as fluorinated tin oxide (FTO), silicon, or quartz wafers at filament temperatures in the range of 1000-1950 K and substrate temperatures in the range of 550–823 K. Phase transformation to respective nitride was carried out by annealing the oxide nanosheets in an ammonia and argon (ratio of 1:4) atmosphere at 700 ºC for 8 hours at 12 ACS Paragon Plus Environment

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a total pressure of 2 torr. Although temperature as low as 500 ºC could be employed for phase transformation, higher temperature of 700 ºC was chosen to ensure complete transformation of the oxide phase to the nitride phase.

Electrochemical Testing:

Electrodes for battery testing were fabricated by mixing as-

synthesized molybdenum with carbon black at weight ratio of 8:1 using aqueous solution of styrene butadiene rubber (SBR) glue and carboxymethylcellulose (CMC). The slurry was then applied to a 50 micron thick copper foil and were then pressed and dried in vacuum. The mass of the electrode used for testing was 1.2 mg + 0.02 mg. Electrochemical half-cell testing was performed in a split, flat-cell (MTI Corp.) using metallic lithium foil as the counter and reference electrode, and glass microfiber as the separator. 1M Lithium hexafluorophosphate (LiPF6) in a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte for all experiments. XPS measurements: Ex-situ XPS measurements were conducted on the MoN electrodes before and after galvanostatic charge and discharge. The electrodes were washed in propylene carbonate to remove superficial impurities, dried under vacuum and subsequently transferred to the XPS chamber in order to minimize air exposure. XPS measurements were done with a Physical Electronics PHI 5000 Versa Probe system consisting of a monochromatic Al (Κα) (1486.7 eV) X-ray source and a 150 mm radius hemispherical electron energy analyzer. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support for this research provided by Rensselaer Polytechnic Institute through the Howard P. Isermann Fellowship.


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Supporting Information Available: Scanning electron micrographs of the MoN post CVD synthesis and post electrode fabrication, XRD patterns of MoN electrode during various stages of Li+ insertion.


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Figure 1 A) A schematic representation of the two-step procedure for synthesize of 2D layers of δ-MoN. Step 1 involves the synthesis of 2D layers of MoO3 by hot-filament chemical vapor deposition through the reaction of oxygen with a hot Mo filament. Step 2 consists of postsynthesis nitridation of 2D-MoO3 to 2D-MoN in the presence of ammonia at 800 ºC. Also shown are the photographs of the hot Mo filament, MoO3 and MoN nitride grown on FTO substrate, MoN in the bulk form;

B-D) Top down SEM images of vertically-aligned two-

dimensional layers of δ-MoN synthesized on FTO substrates. The inset in 1C is the magnified view of the synthesized nanosheets.


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A

B

Figure 2 A) Galvanostatic charge-discharge profiles of 2D-MoN anodes at a rate of C/20 in 1M LiPF6 in EC/DMC electrolyte obtained during the 1st, 30th, 60th, 90th and 120th cycle; B) Specific 16 ACS Paragon Plus Environment

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capacity versus cycle number of 2D-MoN anode in 1M LiPF6 in EC/DMC electrolyte cycled at a rate of C/20. The inset shows the battery performance during fast cycling at C/20, C/4, C/2 and 1C rating.


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Figure 3. A) Cyclic voltammogram of MoN anode measured with respect to a Li metal foil at a scan rate of 0.1 mV/s during the 1st, 3rd and the 4th cycle showing reversible Li+ intercalation and deintercalation; B) differential capacity curves obtained from the galvanostatic charge-discharge curves. The symmetry of the charge and discharge curve again points to the reversible cycling; C) Ex-situ X-ray diffraction patterns of δ-MoN anodes in the pristine, fully lithiated, and delithiated states measured during the first cycle. Also shown is the diffraction pattern of the electrode after the end of 200 cycles of charge and discharge. No observable shift in the diffraction patterns during battery cycling can be observed even after 200 cycles that would indicate structural transformation, which suggests that the mechanism of Li+ insertion does not involve conversion reaction; D) XPS spectra of the Mo 3d core level before and after Li+ 18 ACS Paragon Plus Environment

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insertion at potential of 0.005 V. The twin peaks at the binding energy of 229 eV and 232 eV are the characteristic spin orbit splitting of peaks of 3d5/2 and 3d3/2. Comparison of the curves shows no major shift in the shapes and binding energy, which indicates no major shift in the electronic structure of the MoN between the pristine and lithiated state. Thus, the lack of significant structural and electronic distortions in 2D-layers of MoN, which is in contrast to that seen in the bulk form, suggests that Li+ storage within the material occurs through insertion in the interlayer spacing without reaction with the host lattice.


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