Kinetic and Electrochemical Reaction Mechanism Investigations of

Subscriber access provided by University of South Dakota

Energy, Environmental, and Catalysis Applications

Kinetic and Electrochemical Reaction Mechanism Investigations of Rod-Like CoMoO4 Anode Material for Sodium-Ion Batteries Ghulam Ali, Mobinul Islam, Ji-Young Kim, Hun-Gi Jung, and Kyung Yoon Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16324 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 25, 2018

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 31 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 Applied Materials & Interfaces

Kinetic and Electrochemical Reaction Mechanism Investigations of Rod-Like CoMoO4 Anode Material for Sodium-Ion Batteries Ghulam Ali,† Mobinul Islam,†,‡ Ji Young Kim,§ Hun-Gi Jung,†,‡ Kyung Yoon Chung†,‡,* †Center

for Energy Storage Research, Korea Institute of Science and Technology, Hwarang-ro

14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea ‡Division

of Energy & Environment Technology, KIST School, Korea University of Science and

Technology, Seoul 02792, Republic of Korea §Advanced

Analysis Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5,

Seongbuk-gu, Seoul, 02792, Republic of Korea *Corresponding author: Email: [email protected]

Keywords: monoclinic-type, rod-like morphology, temperature-dependent, ex-situ XRD, X-ray absorption spectroscopy Abstract Sodium-ion batteries are considered the most promising power source for electrical energy storage system due to the abundance of sodium and its significant cost advantages. However, highperformance electrode materials are required for their successful application. Herein, we report a monoclinic-type CoMoO4 material which is synthesized by a simple solution method. An optimized calcination temperature with a high crystallinity and a rod-like morphology of the 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

material is selected after analyzing the as-synthesized powder by temperature-dependent timeresolved X-ray diffraction. The CoMoO4 rods exhibit initial discharge and charge capacities of 537 and 410 mAh g-1, respectively when used as an anode for sodium-ion batteries. Sodium diffusion coefficient in the bimetallic CoMoO4 anode is measured using galvanostatic intermittent titration technique and calculated in a range of 1.565 × 10-15 to 4.447 × 10-18 cm2 s-1 during the initial cycle. Further, the reaction mechanism is investigated using ex-situ X-ray diffraction and X-ray absorption spectroscopy and the obtained results suggest an amorphous-like structure and reduction/oxidation of Co and Mo during sodium insertion/extraction process. Ex-situ transmission electron microscopy and energy dispersive spectroscopy images of the CoMoO4 anode in fully discharged and recharged state reveal the rod-like morphology with homogenous element distribution.

Introduction Sodium-ion batteries (SIBs) have received great interest over the past decade because of low-cost and evenly distributed sodium resources on earth with high natural abundance. Although lithiumion batteries (LIBs) have played an important role in electrical energy storage and conquered the portable electronic industry with high energy and power densities. However, the adoption of LIBs is impeded for the use of large-scale applications such as grid storage where the consumption of lithium is high which raises enormous production cost. Thus, SIBs are considered a potential candidate for the use of large-scale applications such as stationary electrical energy storage (EES).1 Further, sodium is the next alkali metal to lithium with monovalent and possesses the similar intercalation electrochemistry to lithium in many electrode materials.1

2 ACS Paragon Plus Environment

Page 2 of 31

Page 3 of 31 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


Recent studies on cathode materials for SIBs have shown satisfactory performance when compared to lithium-ion batteries (LIBs).2 Some compositions with highly abundant and low-cost materials based on Fe and Mn (layered cathodes) are found electrochemically more active when tested in SIBs compare to LIBs.3 The major obstacle for the realization of SIBs is the development of the high-performance anode material. Graphite, which is commonly used anode material for commercial LIBs, do not intercalate much sodium ions to use for SIBs.3 Silicon is broadly investigated as a high capacity anode material in LIBs but it is found electrochemically inactive in SIBs.4 The use of sodium metal is also not favorable for the practical use because of its low melting temperature (98 0C) and dendrite formation.5 The anode materials for SIBs can be classified according to de/sodiation processes into three different types which are categorized as de/alloying, conversion and de/intercalation mechanism. Alloying based anodes exhibit high capacity such as NaxSn (295 mAh g-1),6 NaxSb (423 mAh g-1),7 Na15Pb4 (485 mAh g-1),8 Na15Ge4 (350 mAh g-1),9 etc., however, these materials face the problem of rapid capacity fade due to large volume expansion. Conversion reaction based materials also exhibit high capacity such as Fe2O3 (420 mAh g-1),10 Co3O4 (380 mAh g-1),11 etc. The conversion reaction based materials also suffer from limited cycle life because of irreversible Na2O phase. Intercalation based materials shows limited capacity such as TiO2 (130 mAh g-1),12 Li4Ti5O12 (120 mAh g-1),13 NaFeTiO4 (60 mAh g-1),14 NaTiO2 (152 mAh g-1),15 NaTi2(PO4)3 (201 mAh g-1),16 etc. Most of the above-mentioned intercalation based materials face the problem of rapid capacity fade over long cycling due to the larger ionic radius of sodium ions compare to lithium ions. Graphene and other carbon-based materials such as hard carbon show a capacity of 98%) were prepared separately with the stoichiometric ratio. Then both the solution were slowly mixed under vigorous stirring and kept on stirring for 2 h at room temperature to form the precipitates. The precipitates are collected by vacuum filtration and washed several times to remove impurities. The collected material is dried at 80 °C for overnight and hand ground using a mortar and pestle. Temperature-dependent time-resolved XRD was performed with a diffractometer (R–AXIS IV++, Rigaku) at Korea Institute of Science and Technology (KIST). The material was filled in a glass capillary tube and subjected to the measurement in a temperature range of 27 to 700 °C. Mo-Kα radiation (wavelength of 0.7107 Å) was used as an x-ray source and the data was recorded on an image plate detector. The 2θ values were later converted to Cu-Kα radiation (λ = 1.54 Å) for comparison with standard data. The XRD of the calcined powder was performed with the same procedure as above. The morphology of the material was observed with field emission scanning electron microscopy (FE–SEM; NOVA NanoSEM200, FEI, USA) and microstructure was evaluated by high-resolution transmission electron microscopy (TEM, Technai G2 F20 FEI). The porosity and specific area of CoMoO4 were measured by nitrogen adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method at a BEL instrument (BEL, Japan). Chemical composition investigations were acquired using X-ray photoelectron microscopy (XPS) with PHI 5000 5 ACS Paragon Plus Environment


VersaProbe (ULVAC-PHI). The measurements were taken using a monochromatic Al-Kα X-ray source (1486.6 eV) with a spot size of 100 × 100 µm under high vacuum conditions (6.8 × 10-8 pa). The C 1s peak (284.6 eV) was used for energy calibration. The electrochemical properties were measured using a coin-type cell (CR 2032). The working electrodes were prepared by mixing the CoMoO4 powder as an active material, Super P to increase electrical conductivity, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 7: 2: 1. N-methyl-2-pyrrolidinone was used as a solvent to make slurry and electrodes were prepared by casting the slurry onto a pure Cu foil. The working electrodes were prepared with an average mass loading of ~3 mg cm-2 and a thickness of ~35 µm. The cells were prepared using CoMoO4 electrode as a working, sodium metal foil as a counter electrode and a glass fiber as a separator. The electrolyte used to make cells was 1 M NaClO4 in propylene carbonate (PC) with 2 wt% of fluoroethylene carbonate (FEC). All the cell assembly was carried out in the argon-filled glove box (Mbraun Unilab, Germany) with a controlled environment (both H2O and O2 < 0.1 ppm). The cells were galvanostatically tested on a battery cycler (Maccor 4000) at room temperature. Ex-situ characterizations were carried out at fully discharged and recharged potential states during the 1st galvanostatic cycle and the cells were disassembled in the argon-filled glove box. The electrodes were collected and thoroughly washed with PC and dimethyl carbonate (DMC) solutions to remove the residue of the electrolyte. The washed electrodes with Cu foil were sealed in plastic bags for XRD and X-ray absorption spectroscopy (XAS) measurements while the material is gently scratched from the Cu foil for ex-situ TEM measurements. The obtained electrode materials were dispersed sonically for 30 minutes before conducting TEM.


Page 6 of 31

Page 7 of 31 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


Ex-situ XRD measurements were conducted using the same procedure as stated above for the powder sample. Ex-situ XAS was performed at 7D XAFS beamline of the Pohang light source (PLS-II) using a Si (111) double crystal monochromator and a bending magnetic source. The experiments were repeated at 8C Nano XAFS beamline of PLS-II. The data was collected in transmission mode and the energy was scanned from 200 eV to 1000 eV above the Mo and Co Kedges. Mo and Co pure metallic foils were simultaneously measured with the electrode samples and used as references to calibrate the spectra. The background removal and normalization of XAS data were performed using ATHENA package.26 Ex-situ TEM analysis of fully discharged and recharged electrode materials was carried out by applying a supersonic vibration to ensure fine dispersion of particles in ethanol. The solution was dropped onto carbon TEM grids and the measurements were taken on a high-resolution transmission electron microscopy (STEM, TalosTM F200X). Results and discussion The as-prepared purple colored material was subjected to temperature-dependent time-resolved Xray diffraction (TR-XRD) in order to identify the phase changes with temperature. Fig. 1 shows a stack of TR-XRD patterns of the as-prepared material in a temperature range of 27-700 °C. The XRD pattern of as-prepared material shows hydrated CoMoO4 phase which is stable up to a temperature of 341 °C. The diffraction peaks of CoMoO4•xH2O phase of as-prepared powder are matched with the previous report.27 The CoMoO4•xH2O phase suddenly vanishes when the temperature is raised to >342 °C and CoMoO4 phase is formed. However, the crystallinity improves with the rise in temperature but the diffraction peaks appear with less intensity when the temperature is further increased to 700 °C. The material has shown sharp diffraction peaks at ~600 °C. Overall, the TR-XRD results show two major phases in a temperature range of 27-700 °C. 7 ACS Paragon Plus Environment


Figure 1. Temperature-dependent time-resolved X-ray diffraction (TR-XRD) patterns of as prepared powder. The measurements were conducted at room temperature to 700 °C and the 2θ range is plotted between 17-60°.

After analyzing the TR-XRD results, it was observed that the sample shows sharp peaks at ~600 °C. Fig. 2a shows the XRD pattern of the material calcined at 600 °C for 2 h in air to obtain phase pure material with better crystallinity. All the diffraction peaks belong to monoclinic CoMoO4 phase with a space group of C2/m and the peaks are matched with PDF card # 021-0868. The lattice parameters of calcined CoMoO4 with monoclinic structure are calculated to be a = 11.07752 Å, b = 10.14019 Å, c = 6.73580 Å, β = 102.72027° and V = 738.0499 Å3. The crystallite size is calculated using the Scherrer formula from three major peaks of (001), (021), and (002). The average crystallite size was estimated to be 21 nm for calcined CoMoO4. Fig. 1b shows the SEM 8 ACS Paragon Plus Environment

Page 8 of 31

Page 9 of 31 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


images of the CoMoO4 powder where most of the particles are observed with rod-like morphology with a length of 90%. The coulombic efficiency significantly improved in the subsequent cycles and the electrode shows a coulombic efficiency of >98% at a 10th cycle where it delivers discharge and charge capacities of 351 and 346 mAh g-1, respectively. Moreover, the similar potential profiles during 2nd and 10th cycles indicate that the material repeatedly follows the reaction as indicated by Eq. 2. A high coulombic efficiency of >97% was retained by the electrode during long cycling and the electrode shows a coulombic efficiency of 99% at 100th cycle as shown in Fig. 5c. The CoMoO4 anode shows higher capacities compare to other Mo-based anodes for SIBs such as BaMoO4 delivered specific capacities of 121 and 50 mAh g-1 during the 1st and 100th cycle, respectively.37 Further, the CoMoO4 anode delivers capacities comparable or higher than single metal oxides such as MoO3 (298 mAh g-1 at 1st cycle),38 MoO3/C (335 mAh g-1 at 1st cycle),39 Co3O4 (327 mAh g-1 at 1st cycle),40 nano Co3O4 (168 mAh g-1 at 1st cycle),41 ZnSnO3 (315 mAh g-1 at 1st cycle),42 and NaVO3 (196 mAh g-1 at 1st cycle).43 The rate capability (Fig. 5d) of the CoMoO4 electrode was measured in a potential range of 0.0052.7 V at current densities of 49, 98, 245, 490, and 980 mA g-1 where the electrode delivered average specific capacities of 360, 317, 274, 221, and 165 mAh g-1, respectively. The electrode recovers a specific capacity of 266 mAh g-1 when current density reset to 49 mA g-1 after deep cycling. Hence, the electrode shows good rate capability.



Figure 5. (a) CV of CoMoO4 electrode at a scan rate of 0.5 mV s-1. (b) Galvanostatic chargedischarge profile of CoMoO4 electrode at 1st, 2nd, and 10th cycle. (c) Cycle test of the CoMoO4 electrode at a current density of 49 mA g-1. (d) Rate capability test of CoMoO4 electrode in a potential window of 0.005-2.7 V.

In order to follow the sodium diffusivity into CoMoO4, we have conducted a galvanostatic intermittent titration technique (GITT) during sodium insertion/extraction. GITT is the most reliable method to follow sodium kinetics into the electrode as it is based on chronopotentiometry at nearly thermodynamic equilibrium and to calculate the diffusion coefficient at various potential 14 ACS Paragon Plus Environment

Page 14 of 31

Page 15 of 31 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


points with highly resolved data.44,45 Fig. 6 shows the potential response profile of CoMoO4 anode material as a function of time with sodium insertion/extraction process. The cell was cycled at a rate of 49 mA g-1 in a potential range of 0.005-2.7 V and a current pulse was applied for 10 min with a subsequent relaxing time for 1 h to reach equilibrium potential state. This procedure was repeated for a complete discharge and recharge process. The electrode potential is influenced by each current pulse due to insertion/extraction of sodium ions into the CoMoO4 electrode. The inset of Fig. 6 shows a single step of applied current with an interpretation of parameters during sodium insertion process. The values of each parameter are recorded and used to calculate the sodium diffusion coefficient (DNa) for each step. DNa was calculated using Eq. (1) by assuming that sodium transport obeys Fick’s law.46

DNa+ =

4

𝑚 B𝑉 M 2

∆𝐸s

2

πτ ) 𝑀B𝑆 ( )∆𝐸τ(

(1)

Where τ is the pulse time during applied current density, mB (g) is the weight loading, VM (cm3 mol-1) is the molar volume, MB (g mol-1) is the molecular weight, S (cm2) is the effective surface area of the electrode, ΔEs is the change in steady state potential for the step, and ΔEτ is the total change in the cell potential during the current pulse for the time τ. The effective surface area was used as measured by the BET method and the value of VM is assumed to be constant during calculations. DNa was calculated to be 1.565 × 10-15 cm2 s-1 at the start and 4.447 × 10-18 cm2 s-1 at the end of discharge process and 4.37 × 10-16 cm2 s-1 at the start and 1.77 × 10-18 cm2 s-1 at the end of charge process. The calculated values of DNa show that sodium ions faced high polarization towards the depth of discharge and charge process which are related to the sluggish kinetics into the electrode. The reason for sluggish kinetics is mainly attributed to the formation of different phases during conversion reaction which creates high kinetic barriers due to structural 15 ACS Paragon Plus Environment


rearrangements.44 The calculated DNa values for CoMoO4 are comparable to the reported Na4Mn9O18 (10-17 cm2 s-1),47 and lower than the NaVO3 (10-13 cm2 s-1),43 and Na2Ti3O7 (10-12 cm2 s-1).48

Figure 6. Galvanostatic charge-discharge profile of GITT measurement in a potential range of 0.005 to 2.7 V along with the calculated sodium diffusion coefficient into the CoMoO4 electrode. The inset shows a magnified single step during the discharge process labeled with variables.

Probing the reaction mechanism is an important factor for electrode materials to observe and improve the electrochemical performance. Herein, we have investigated the sodium insertion/extraction mechanism using ex-situ techniques such as XRD, XAS, and TEM. Fig. 7a shows the XRD patterns of pristine, fully discharged and recharged electrodes. The XRD peaks in the pristine electrode are well-matched with the calcined material as shown in Figure 2a. No obvious diffraction peak was detected in both XRD patterns and the material shows transformation to the amorphous-like structure after sodium insertion. Further, X-ray absorption near edge 16 ACS Paragon Plus Environment

Page 16 of 31

Page 17 of 31 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


structure (XANES) spectroscopy was performed to observe the capacity contribution in the reaction. XANES is a powerful technique to probe the electronic structure of the material and the data is collected using the transmitted X-ray beam which passes through the electrode sample. Thus the information obtained from XANES is more reliable for ex-situ electrode materials compare to XPS and other surface techniques. Fig. 7b shows the Mo K-edge XANES spectra of pristine, fully discharged, full recharged CoMoO4 electrode and standard Mo metallic foil. The XANES spectrum of pristine shows a high pre-edge peak and its amplitude depends on the degree of Mo-O bond’s distortion and that of the 4d(Mo)-2p(O) hybridization.49 The XANES spectrum of fully discharged electrode shows a shift toward lower energy and almost overlaps to Mo metallic spectrum at lower intensity region, indicating a reduction in Mo oxidation state. However, the upper half part of the fully discharged XANES spectrum is away from the Mo metallic spectrum which is probably due to insufficient reduction of Mo ions. Upon fully recharged, XANES spectrum shows a reverse shift towards higher energy, indicating oxidation of Mo. Fig. 7c shows the XANES spectra of Co K-edge of pristine, fully discharged, fully recharged electrode, and standard Co metallic foil. The XANES spectrum of pristine electrode shows a less intense preedge peak which is an indication of less distorted Co-O octahedral symmetry. When the electrode is fully discharged, the XANES spectrum shows a significant shift to lower energy, indicating the reduction from Co2+ towards Co0 state. The fully discharged spectra of both Mo and Co do not overlap the metallic foils which are mainly due to incomplete reduction of both metals which was also evidenced by the low specific capacity of the material. The XANES spectrum of fully recharged electrode shows a shift to higher energy which is the indication of oxidation of Co. Hence, both metal species (Mo and Co) participate in the reversible electrochemical reaction.



Ex-situ XRD results of fully discharged and recharged electrodes show the amorphous-like structure of the materials and XANES spectra show incomplete reduction into Co and Mo metallic phases. Thus, in order to observe the local structure, extended X-ray absorption fine structure (EXAFS) is used to determine the short-range order in the materials. Fig. 7d and e show the EXAFS of pristine CoMoO4, fully discharged, and recharged along with references and metallic foils of Co and Mo edge, respectively. EXAFS of pristine CoMoO4 at Co edge shows clear peaks of Co-O interaction at 1.54 Å and Co-Co interaction peak at ~2.5 Å. In the fully discharged spectrum of Co edge, the first peak of Co-O at 1.54 Å could be observed with relatively low intensity whereas a new peak emerged at 2.16 Å (Co-Co interaction) which belongs to Co metallic foil. This confirms the conversion to metallic Co phase in the fully discharged electrode. The fully recharged spectrum at Co edge shows peaks at 1.54 Å and 2.5 Å which correspond to Co-O interaction. The peak at 2.5 Å in the fully recharged EXAFS spectrum matches to CoO phase as indicated in Fig. 7d. Mo K-edge EXAFS spectrum of pristine CoMoO4 shows a prominent peak at 1.25 Å which corresponds to the Mo-O interaction. The fully discharged EXAFS spectrum at Mo edge retains the first peak and an additional peak appears at ~2.5 Å, corresponding to Mo-Mo interaction of Mo metallic foil as indicated in Fig. 6e. The EXAFS spectrum at Mo edge of recharged electrode shows peaks at 1.2 and 1.7 Å, corresponding to Co-O-O interactions which are close to the peaks of reference MoO3 (1.15 and 1.63 Å). Thus, EXAFS results confirm the conversion reaction in CoMoO4 electrode. Fig. 7f shows Raman spectra of pristine, fully discharged and recharged electrode. The pristine spectrum shows three major peaks at 809, 876, and 930 cm-1 and can be assigned to Mo-O-Co vibrations in CoMoO4.50 The pristine spectrum also shows peaks from D and G bands of carbon in the electrodes. The vibrations from CoMoO4 is hardly detected in fully discharged and recharged 18 ACS Paragon Plus Environment

Page 18 of 31

Page 19 of 31 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


electrode. However, a new peak at 678 cm-1 was observed in both samples. This peak can be assigned to the Na-O stretching vibration.51 Thus, Raman spectroscopy results also confirm the formation of the amorphous-like behavior of the material after sodiation.

Figure 7. (a) Ex-situ XRD of pristine, fully discharged and recharged CoMoO4 electrode. XANES spectra of pristine, fully discharged and recharged electrodes along with metallic foils of (b) Mo K-edge and (c) Co K-edge. EXAFS spectra of pristine (CoMoO4), fully discharged and recharged electrodes along references and metallic foil of (d) Co and (e) Mo edge. (f) Raman spectra of pristine, fully discharged and recharged electrodes.

The ex-situ XRD results showed the amorphous-like phase of fully discharged and recharged electrode. In order to further gain insight into microstructure and element distribution, ex-situ TEM of the fully discharged and recharged electrode material was conducted. Fig. 8a shows the high19 ACS Paragon Plus Environment


resolution (HR) TEM image of the fully discharged electrode. The rod-like morphology of the material was retained after the sodiation process, however, no clear lattice fringes were observed in the HR-TEM images as shown in the inset of the Fig. 8a, indicating amorphous-like structure as also evidenced by ex-situ XRD results. Fig. 8b shows the high angle annular dark field (HAADF) image of rod and Fig. 8(c-f) show electron energy dispersive spectroscopy (EDS) images. The EDS images of Co, O, Mo, and Na reveal the homogenous distribution of respective elements.


Page 20 of 31

Page 21 of 31 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 8. (a) Ex-situ TEM image of the fully discharged electrode and the inset shows the highresolution image of the particle. (b) HAADF image of rod-like morphology and the corresponding EDS images of (c) Co, (d) O, (e) Mo, and (f) Na elements.

Fig. 9a shows ex-situ TEM image of a rod-like particle of the fully recharged electrode. The particle retains morphology but no sign of lattice fringes was observed as shown in the inset of the Fig. 9a. Fig. 9b shows the HAADF image of a rod and Fig. 9(c-f) show the corresponding EDS images of Co, O, Mo, and Na, respectively. All the images show the homogeneous distribution of each element, however, the signals of Na is either due to irreversible amorphous Na2O or formation of SEI layer and/or sodium salt. Overall, ex-situ TEM results reveal that the material retains its rod-like morphology, transforms into the amorphous-like structure and a homogeneous distribution of elements. The EDS images of discharged and recharged electrode show clear and larger Co-rich particles which is related to the higher electrochemical activity of cobalt in the sample. The unique rod-like morphology of the material which was retained after cycling shows stability and mechanical strength of the materials and is considered beneficial for electrochemical performance. The reaction products of CoO and MoO3 have shown the synergistic effect where they assure a suitable electrical conductivity, resulting in improved electrochemical performance.23



Figure 9. (a) Ex-situ TEM image of the fully recharged electrode and the inset shows the highresolution image of the particle. (b) HAADF image and the corresponding EDS images of (c) Co, (d) O, (e) Mo, and (f) Na elements.


Page 22 of 31

Page 23 of 31 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


Conclusion Single phase CoMoO4 with rod-like morphology was synthesized using a simple solution method. TR-XRD was conducted in order to find suitable calcination temperature and the material with high crystallinity was obtained at 600 °C as also evidenced by the HR-TEM image. The material delivers a specific capacity of 410 mAh g-1 at 1st cycle and retains a capacity of 229 mAh g-1 after 100th cycle. The average specific capacities of 274, 221, and 165 mAh g-1 were recorded when the material is tested under high current densities of 245, 490, and 980 mA g-1. Sodium diffusion coefficient was determined by GIIT method and the values are calculated to in a range of 1.565 × 10-15 to 4.447 × 10-18 cm2 s-1 during galvanostatic cycling. The reaction mechanism investigations conducted by ex-situ XRD, XAS and TEM show that the material transforms to amorphous-like structure, charge compensation is due to reversible redox of Mo and Co, and a homogenous distribution of elements in discharged and recharged products, respectively. Hence, the results presented in this study will pave the way for the development of mixed metal oxides as an anode for SIBs.

Acknowledgment This work was supported by the KIST Institutional Program (Project nos. 2E27090 and 2V05540). We acknowledge to Dr. Jee-Hwan Bae from advanced analysis center (KIST) for his technical support to measure HR-TEM images.



References

(1)

Su, D.; Dou, S.; Wang, G., Ultrathin MoS2 Nanosheets as Anode Materials for Sodium-

Ion Batteries with Superior Performance. Adv. Energy Mater. 2015, 5 (6), 1401205. (2)

Sun, J.; Lee, H.-W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y., A

Phosphorene–Graphene Hybrid Material as a High-Capacity Anode for Sodium-Ion Batteries. Nat. Nanotechnol. 2015, 10 (11), 980-985. (3)

Saurel, D.; Orayech, B.; Xiao, B.; Carriazo, D.; Li, X.; Rojo, T., From Charge Storage

Mechanism to Performance: A Roadmap toward High Specific Energy Sodium‐Ion Batteries through Carbon Anode Optimization. Adv. Energy Mater. 2018, 8, 1703268. (4)

Ali, G.; Lee, J.-H.; Oh, S. H.; Jung, H.-G.; Chung, K. Y., Elucidating the Reaction

Mechanism of SnF2@C Nanocomposite as a High-Capacity Anode Material for Na-Ion Batteries. Nano Energy 2017, 42, 106-114. (5)

Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T.,

Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5 (3), 5884-5901. (6)

Xu, Y.; Zhu, Y.; Liu, Y.; Wang, C., Electrochemical Performance of Porous Carbon/Tin

Composite Anodes for Sodium‐Ion and Lithium‐Ion Batteries. Adv. Energy Mater. 2013, 3 (1), 128-133. (7)

Wang, G.; Xiong, X.; Lin, Z.; Yang, C.; Lin, Z.; Liu, M., Sb/C Composite as a High-

Performance Anode for Sodium Ion Batteries. Electrochim. Acta 2017, 242, 159-164.


Page 24 of 31

Page 25 of 31 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


(8)

Ellis, L. D.; Wilkes, B. N.; Hatchard, T. D.; Obrovac, M. N., In Situ XRD Study of Silicon,

Lead and Bismuth Negative Electrodes in Nonaqueous Sodium Cells. J. Electrochem. Soc. 2014, 161 (3), A416-A421. (9)

Baggetto, L.; Keum, J. K.; Browning, J. F.; Veith, G. M., Germanium as Negative

Electrode Material for Sodium-Ion Batteries. Electrochem. Commun. 2013, 34, 41-44. (10)

Li, H.; Xu, L.; Sitinamaluwa, H.; Wasalathilake, K.; Yan, C., Coating Fe2O3 with

Graphene Oxide for High-Performance Sodium-Ion Battery Anode. Compos. Commun. 2016, 1, 48-53. (11)

Li, H.-H.; Li, Z.-Y.; Wu, X.-L.; Zhang, L.-L.; Fan, C.-Y.; Wang, H.-F.; Li, X.-Y.; Wang,

K.; Sun, H.-Z.; Zhang, J.-P., Shale-Like Co3O4 for High Performance Lithium/Sodium Ion Batteries. J. Mater. Chem. A 2016, 4 (21), 8242-8248. (12)

Ata Ur, R.; Ali, G.; Badshah, A.; Chung, K. Y.; Nam, K.-W.; Jawad, M.; Arshad, M.;

Abbas, S. M., Superior Shuttling of Lithium and Sodium Ions in Manganese-Doped Titania @ Functionalized Multiwall Carbon Nanotube Anodes. Nanoscale 2017, 9 (28), 9859-9871. (13)

Liu, Y.; Liu, J.; Hou, M.; Fan, L.; Wang, Y.; Xia, Y., Carbon-Coated Li4Ti5O12

Nanoparticles with High Electrochemical Performance as Anode Material in Sodium-Ion Batteries. J. Mater. Chem. A 2017, 5 (22), 10902-10908. (14)

Bhange, D. S.; Ali, G.; Kim, J.-Y.; Chung, K. Y.; Nam, K.-W., Improving the Sodium

Storage Capacity of Tunnel Structured NaxFexTi2-xO4 (x = 1, 0.9 & 0.8) Anode Materials by Tuning Sodium Deficiency. J. Power Sources 2017, 366, 115-122. (15)

Wu, D.; Li, X.; Xu, B.; Twu, N.; Liu, L.; Ceder, G., NaTiO2: A Layered Anode Material

for Sodium-Ion Batteries. Energy Environ. Sci. 2015, 8 (1), 195-202.



(16)

Liang, J.; Fan, K.; Wei, Z.; Gao, X.; Song, W.; Ma, J., Porous NaTi2(PO4)3@C Nanocubes

as Improved Anode for Sodium-Ion Batteries. Mater. Res. Bull. 2018, 99, 343-348. (17)

Balogun, M.-S.; Luo, Y.; Qiu, W.; Liu, P.; Tong, Y., A Review of Carbon Materials and

their Composites with Alloy Metals for Sodium Ion Battery Anodes. Carbon 2016, 98, 162-178. (18)

Yuvaraj, S.; Selvan, R. K.; Lee, Y. S., An Overview of AB2O4- and A2BO4-Structured

Negative Electrodes for Advanced Li-Ion Batteries. RSC Adv. 2016, 6 (26), 21448-21474. (19)

Yuan, C.; Wu, H. B.; Xie, Y.; Lou, X. W., Mixed Transition-Metal Oxides: Design,

Synthesis, and Energy-Related Applications. Angew. Chem. Int. Ed. 2014, 53 (6), 1488-1504. (20)

Wu, F.; Bai, J.; Feng, J.; Xiong, S., Porous Mixed Metal Oxides: Design, Formation

Mechanism, and Application in Lithium-Ion Batteries. Nanoscale 2015, 7 (41), 17211-17230. (21)

Islam, M.; Ali, G.; Jeong, M.-G.; Choi, W.; Chung, K. Y.; Jung, H.-G., Study on the

Electrochemical Reaction Mechanism of NiFe2O4 as a High-Performance Anode for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9 (17), 14833-14843. (22)

Islam, M.; Jeong, M.‐G.; Ali, G.; Oh, I.‐H.; Chung, K. Y.; Sun, Y.‐K.; Jung, H.‐G., A 4 V

Li-Ion Battery using All-Spinel-Based Electrodes. ChemSusChem 2018, 11 (13), 2165-2170. (23)

Cherian, C. T.; Reddy, M. V.; Haur, S. C.; Chowdari, B. V. R., Interconnected Network of

CoMoO4 Submicrometer Particles As High Capacity Anode Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5 (3), 918-923. (24)

Jiang, Y.; Zheng, X.; Yan, X.; Li, Y.; Zhao, X.; Zhang, Y., 3D Architecture of a

Graphene/CoMoO4 Composite for Asymmetric Supercapacitors Usable at Various Temperatures. J. Colloid Interface Sci. 2017, 493, 42-50. (25)

Wang, L.; Cui, X.; Gong, L.; Lyu, Z.; Zhou, Y.; Dong, W.; Liu, J.; Lai, M.; Huo, F.; Huang,

W.; Lin, M.; Chen, W., Synthesis of Porous CoMoO4 Nanorods As A Bifunctional Cathode


Page 26 of 31

Page 27 of 31 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


Catalyst for a Li–O2 Battery and Superior Anode for a Li-Ion Battery. Nanoscale, 2017, 9, 38983904. (26)

Ravel, B.; Newville, M., ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray

Absorption Spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12 (4), 537-541. (27)

Guo, D.; Zhang, H.; Yu, X.; Zhang, M.; Zhang, P.; Li, Q.; Wang, T., Facile Synthesis and

Excellent Electrochemical Properties of CoMoO4 Nanoplate Arrays as Supercapacitors. J. Mater. Chem. A 2013, 1 (24), 7247-7254. (28)

Yao, J.; Gong, Y.; Yang, S.; Xiao, P.; Zhang, Y.; Keyshar, K.; Ye, G.; Ozden, S.; Vajtai,

R.; Ajayan, P. M., CoMoO4 Nanoparticles Anchored on Reduced Graphene Oxide Nanocomposites as Anodes for Long-Life Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6 (22), 20414-20422. (29)

Yu, M. Q.; Jiang, L. X.; Yang, H. G., Ultrathin Nanosheets Constructed CoMoO4 Porous

Flowers with High Activity for Electrocatalytic Oxygen Evolution. Chem. Commun. 2015, 51 (76), 14361-14364. (30)

Yang, Q.; Liu, L.; Xiao, L.; Zhang, L.; Wang, M. J.; Li, J,; Wei, Z., Co9S8@N,S-Codoped

Carbon Core–Shell Structured Nanowires: Constructing a Fluffy Surface for High-Density Active Sites. J. Mater. Chem. A 2018, 6, 14752-14760. (31)

Chen, G.; Xu, Q.; Wang, Y.; Song, G.; Li, C.; Zhao, W.; Fan, W., Solubility Product

Difference-Guided Synthesis of Co3O4–CeO2 Core–Shell Catalysts for CO Oxidation. Catal. Sci. Technol. 2016, 6, 7273-7279. (32)

Sarma, D. D.; Rao, C. N. R., XPES Studies of Oxides of Second- and Third-Row Transition

Metals Including Rare Earths. J. Electron. Spectrosc. Relat. Phenom. 1980, 20 (1), 25-45.



(33)

Zou, G.; Hou, H.; Zhao, G.; Ge, P.; Yinb, D.; Jia, X., N-rich Carbon Coated CoSnO3

Derived from in Situ Construction of a Co–MOF with Enhanced Sodium Storage Performance. J. Mater. Chem. A 2018, 6, 4839-4847. (34)

Hou, H.; Banks, C. E.; Jing, M.; Zhang, Y.; Ji, X., Carbon Quantum Dots and Their

Derivative 3D Porous Carbon Frameworks for Sodium‐Ion Batteries with Ultralong Cycle Life. Adv. Mater. 2015, 27 (47), 7861-7866. (35)

Wang, W.; Qin, J.; Yin, Z.; Cao, M., Achieving Fully Reversible Conversion in MoO3 for

Lithium Ion Batteries by Rational Introduction of CoMoO4. ACS Nano 2016, 10 (11), 10106– 10116. (36)

Islam, M.; Jeong, M.-G.; Hwang, J.-Y.; Oh, I.-H.; Sun, Y.-K.; Jung, H.-G., Self-Assembled

Nickel-Cobalt Oxide Microspheres from Rods with Enhanced Electrochemical Performance for Sodium Ion Battery. Electrochim. Acta 2017, 258, 220-227. (37)

Ma, X.; Zhao, W.; Wu, J.; Jia, X., Preparation of Flower-Like BaMoO4 and Application

in Rechargeable Lithium and Sodium Ion Batteries. Mater. Lett. 2017, 188, 248-251. (38)

Zhang, X.; Fua, C.; Li, J.; Yao, C.; Lu, T.; Pan, L., MoO3/Reduced Graphene Oxide

Composites as Anode Material for Sodium Ion Batteries. Ceram. Int. 2017, 43 (4), 3769-3773. (39)

Chen, Z.; Liu, Y.; Zhang, H.; Ding, S.; Wang, T.; Zhang, M., In-Situ Phase Transition to

Form Porous h-MoO3@C Nanofibers with High Stability for Li+/Na+ Storage. Sci. China Mater. 2017, 60 (8), 755–765. (40)

Li, H.-H.; Li, Z.-Y.; Wu, X.-L.; Zhang, L.-L.; Fan, C.-Y.; Wang, H.-F.; Li, X.-Y.; Wang,

K.; Sun, H.-Z.; Zhang, J.-P., Shale-Like Co3O4 for High Performance Lithium/Sodium Ion Batteries. J. Mater. Chem. A 2016, 4, 8242-8248. 28 ACS Paragon Plus Environment

Page 28 of 31

Page 29 of 31 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


(41)

Subalakshmi, P.; Sivashanmugam, A., Nano Co3O4 as Anode Material for Li–Ion and

Na‐Ion Batteries: An Insight into Surface Morphology. ChemistrySelect 2018, 3 (18), 5040–5049. (42)

Wang, L. P.; Zhao, Y.; Wei, C.; Wong, C.; Srinivasan, M.; Xu, Z. J., Polycrystalline Zinc

Stannate as an Anode Material for Sodium-Ion Batteries. J. Mater. Chem. A 2015, 3 (26), 14033– 14038. (43)

Ali, G.; Islam, M.; Jung, H.-G.; Nam, K.-W.; Chung, K. Y., Probing the Sodium

Insertion/Extraction Mechanism in a Layered NaVO3 Anode Material. ACS Appl. Mater. Interfaces 2018, 10 (22), 18717–18725. (44)

Li, Z.; Du, F.; Bie, X.; Zhang, D.; Cai, Y.; Cui, X.; Wang, C.; Chen, G.; Wei, Y.,

Electrochemical Kinetics of the Li[Li0.23Co0.3Mn0.47]O2 Cathode Material Studied by GITT and EIS. J. Phys. Chem. C 2010, 114 (51), 22751-22757. (45)

Ali, G.; Lee, J. H.; Chang, W.; Cho, B.-W.; Jung, H.-G.; Nam, K.-W.; Chung, K. Y.,

Lithium Intercalation Mechanism into FeF3·0.5H2O as a Highly Stable Composite Cathode Material. Sci. Rep. 2017, 7, 42237. (46)

Weppner, W.; Huggins, R. A., Determination of the Kinetic Parameters of

Mixed‐Conducting Electrodes and Application to the System Li3Sb. J. Electrochem. Soc. 1977, 124 (10), 1569-1578. (47)

Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J.,

Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Adv. Mater. 2011, 23 (28), 3155–3160.



(48)

Rudola, A.; Saravanan, K.; Mason, C. W.; Balaya, P., Na2Ti3O7: An Intercalation Based

Anode for Sodium-Ion Battery Applications. J. Mater. Chem. A 2013, 1 (7), 2653-2662. (49)

Alexei, K.; Juris, P., Dehydration of the Molybdenum Trioxide Hydrates MoO3·nH2O: in

Situ X-Ray Absorption Spectroscopy Study at the Mo K Edge. J. Phys.: Condens. Matter 2000, 12 (9), 1959. (50)

Jinlong, L.; Meng, Y.; Suzuki, K.; Miura, H., Synthesis of CoMoO4 @RGO

Nanocomposites as High-Performance Supercapacitor Electrodes. Microporous Mesoporous Mater. 2017, 242, 264–270. (51)

Wang, L.; Zhang, C.; Cai, D.; Zhang, J.; Sasaki, Y.; Ostrovski, O., Effects of CaO/SiO2

Ratio and Na2O Content on Melting Properties and Viscosity of SiO2-CaO-Al2O3-B2O3-Na2O Mold Fluxes. Metall. Mater. Trans. B 2016, 48 (1), 516–526.


Page 30 of 31

Page 31 of 31 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


Table of Contents


Kinetic and Electrochemical Reaction Mechanism Investigations of

Recommend Documents