Molten Salt Synthesis, Characterization and Its Lithium-Storage Per

Jan 21, 2015 - Department of Physics, Solid State Ionics/Advanced Batteries Lab, National ... synthesized using molten salt method with KCl as the mol...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Mixed Oxides, (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1): Molten Salt Synthesis, Characterization and Its Lithium-Storage Performance for Lithium Ion Batteries M. V. Reddy,*,†,‡ Chu Yao Quan,†,§ Keefe Wayne Teo,†,§ Lim Ji Ho,†,§ and B. V. R. Chowdari† †

Department of Physics, Solid State Ionics/Advanced Batteries Lab, National University of Singapore, Singapore, Singapore 117542 Departments of Materials Science & Engineering, National University of Singapore, Singapore, Singapore 117546 § NUS High School of Math and Science, 20 Clementi Avenue 1, Singapore, Singapore 129957 ‡

S Supporting Information *

ABSTRACT: We prepared solid solutions based on Ni, Zn, and Fe oxides to be used as nanomaterials for anodes of Li-ion batteries. The materials were synthesized using molten salt method with KCl as the molten salt. The prepared nanomaterials (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) were subsequently characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), the Brunauer−Emmett−Teller surface and density methods. Cyclic voltammetry (CV) and galvanostatic cycling tests were then conducted to understand the lithium storage performance of the electrodes. Electrochemical impedance spectroscopy (EIS) was also performed to analyze the kinetics of our electrodes and other characteristics of the battery cell. The electrochemical properties of prepared compounds showed reversible capacities (mAh/g) of 706, 819, 603, 781, 637 for x = 0, 0.25, 0.5, 0.75, and 1 at the end of the 50th cycle.

1. INTRODUCTION Li-ion batteries are increasingly popular and constitute a vital part in today’s world.1 Not only do these batteries have higher capacity than traditional alkaline batteries, but they are also rechargeable and cheap to manufacture. This enables them to be a good choice in powering various gadgets like mobile phones and laptops. However, the current technology of conventional Li-ion batteries is unable to cater to the increasing demands of newer battery applications. First, conventional Liion batteries use LiCoO2 as the cathode material, and graphite as the anode material, which could result in the formation of potentially dangerous dendrites when cycled at high current rates.2 Not only does the commercial graphite anode material possess a lower theoretical capacity of 372 mAh/g but it also has high capacity fading at higher current rates, and is usually synthesized at high temperatures of ≥2000 °C. As increasing numbers of new devices require more power, especially in electric cars, it is important to explore alternatives for anode materials that could better address the disadvantages of the commercial Li-ion batteries, while being economical at the same time. Thus, we explored an array of compounds as an anode materials. After early work on conversion oxides by Guyomard et al.3 and Poizot et al.,4 there was great interest in oxide anodes based on conversion5,6 and alloying−dealloying/conversion reactions.6,7 Some of the known conversion and alloying-dealloying reaction mechanism anode materials are ZnCo 2 O 4 , 8,9 ZnFe2O4,10−13 and CdFe2O414 including other conversion © 2015 American Chemical Society

spinel oxides belonging to MFe2O4 (M = Fe, Ni, Co, Cu).13,15−20 To optimize the performance of the compounds, different preparative methods have been adopted. Such is the case for ZnFe2O4, where Fe undergoes the conversion reaction, while Zn undergoes the alloying−dealloying reaction, displaying its capability of undergoing two types of reaction, and thus, giving rise to higher capacities. On the other hand, NiFe2O4 undergoes only conversion (redox) reaction by virtue of Ni and Fe within the compound. Hence, it is important to understand the structure and electrochemical properties of (Ni1−xZnx)Fe2O4 solutions and its hybrid structure, since these materials are cheap and less toxic in nature, and its theoretical capacity (720−900 mAh/g) is much higher than graphite (372 mAh/g). Christie et al.21 reported anodic properties of (Ni1−xZnx)Fe2O4 using the sol−gel autocombustion method to synthesize solid solutions of nanocrystalline spinel oxides. The electrochemical results showed huge capacity fading (18−67%) and showed a reversible capacity of 200−570 mAh/g at the end of the 50th cycle.21 In this research, we adopted the molten salt method (MSM) to prepare our compounds. In molten media, reactions are controlled by chemical equilibria and proceed much quicker than diffusion controlled solid-state reactions.22,23 Present study we prepared five compounds which are Ni1−xZnxFe2O4 (x = 0, 0.25, 0.5, 0.75, 1), to study how the continuous variations in Received: January 8, 2015 Published: January 21, 2015 4709

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

separator (Sigma-Aldrich), followed by the prepared electrode (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1), and the coin cell cap. First, the Li metal was scraped to remove any possible oxide layer on its surface. The electrolyte used in our battery was 1 M LiPF6, which was dissolved in equal volumetric ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). Fabrication of our battery was carried out in a glovebox filled with argon gas. Once the components are in place, they were stapled together using the coin cell crimper (Hosen, Japan). Subsequently, the open-circuit voltage (OCV) test was employed to measure the voltage of the battery cells immediately after they were removed from the glovebox, and cells were tested for OCV again after 12h rest. 2.4. Characterization Techniques. XRD (Empyrean Pananlytical) was performed on all five compounds using CuKα radiation (wavelength 1.5418 Å), to investigate the lattice structure, lattice parameters and the purity of the compounds, which can be derived after using Rietveld refinement via TOPAS(v2.1). VISTA software was used to generate crystallographic representation of the compound. Scanning electron Microscopy (SEM) (JEOL) images of all five compounds were taken and studied to determine the morphology and the particle sizes of the anode surface, with all compounds being coated with a thin layer of platinum using a sputtering unit. The BET surface method (Micromeritics TriStar, USA) was used to investigate the surface area of our compounds, as well as to understand the pore diameter and density of the compound. Information about density was also collected using an AccuPyc 1330 (Micromeritics, USA) pyrometer at ambient temperature. The Bitrode multichannel tester (MCV16−05/001−5, Bitrode, USA) was used for testing the charge−discharge cycling of the batteries. Cyclic voltammetry (CV) was also performed at room temperature of 25 °C using a multipotentiostat (Macpile II, Biologic, France) at various potentials of 0.005 V − 3.0 V, which gives information about redox couples. It can be used to analyze electrochemical properties of the cell. Electrochemical impedance spectroscopy (EIS) was performed on the Solartron Impedance/gain-phase analyzer (SI 1255) with a potentiostat (SI 1268) at room temperature and it gives electrode kinetic values and variation of resistances with voltages in charged or discharged state. EIS studies were carried out in the frequency range 0.18 MHz to 3 mHz and ac amplitude of 10 mV. Galvanostatic charge−discharge cycling was conducted in the range of 0.005−3.0 V vs Li at a current rate of 60 mA g−1. To ensure good percolation of the electrolyte to the electrodes, the cells were left to rest for 12 h before measurements. Further details of instrumentation are given in our previous reports.24,25

relative proportions of Ni and Zn ions would have an effect on the performance of the battery.

2. EXPERIMENTAL METHODS 2.1. Preparation of Materials. Five compounds were synthesized via molten salt method, namely NiFe2 O4, (Ni0.75Zn0.25)Fe2O4, (Ni0.5Zn0.5)Fe2O4, (Ni0.25Zn0.75)Fe2O4, and ZnFe2O4. The starting materials used were of high purity (99%) from their suppliers, namely KCl (Merck), FeC2O4 (Aldrich), ZnSO4·7H2O (Merck), NiCO3·7H2O (Alfa Aesar). The compounds were prepared by mixing stoichiometric amounts of the raw materials, Fe, Zn, and Ni salts, with 10 M KCl and then mixed well in an Alumina crucible. KCl was used as the molten salt. In all five batches prepared, 30g of initial reactants were heated to 900 °C for 3 h (excludes heating and cooling time) in a box furnace (Carbolyte Uk), at heating rate of 3 °C/min. Eventually, all the brown or black powdered products were washed with distilled water to be removed and filtered off any excess potassium salts, before drying in a vacuum oven. 2.2. Electrode Fabrication. Electrodes were then fabricated using the synthesized compounds (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) as the active material, Super-P carbon (ENSACO, MMM Super P, surface area, 230 m2g−1) as a conductive additive, and polyvinylidene fluoride (PVDF) as a polymer binder (Kynar 2801) in the weight ratio of 70:15:15, and N-methyl-2-pyrrolidone (NMP) as an organic solvent(Aldrich). The anodes were then synthesized by grounding and mixing the Super-P carbon and the anode material until they were fine powders. Super P-carbon was used for its high purity and surface area, and it improves the electrical conductivity of the compound. Next, PVDF was added in the weight ratio stated above, together with the compounds into separate clean vials. The NMP solvates the mixture, and will disperse the (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) powders, super P carbon and binder so that a homogeneous mixture is created. A clean magnetic pellet was inserted in the vial and left on a magnetic stirring plate for 12 h, which further ensures that a uniform and consistent slurry is made, allowing the synthesized electrodes to be uniform as well. Subsequently, the slurry was used to coat an etched copper foil (Shenzhen Vanlead Tech. Co. Ltd., China), which served as a current collector. The doctor blade technique was used to coat the electrode uniformly with a slurry thickness of 20 μm. The foil was then placed in an oven at 80 °C for 12 h. A twin roller machine, applying 1500 kPa of pressure, was used to press the coated copper foil to increase the contact between composite material and the etched copper foil. Using an electrode cutter, the foil was then punched into discs with a 16 mm diameter (2.0 cm2 geometric area). Discs that were uniformly coated were selected and weighed. The electrodes were then placed in a vacuum oven for 12 h to allow complete drying. Once the copper foil with the compounds was dry, it was cut into discs of 16 mm diameter inside a glovebox (MV150B-G, MBRAUN). The steel discs were then crimped using a press (Schmidt, Germany) to make it spring-like, which compresses the contents of the completed cell, ensuring that the components are in contact with one another. The crimped disc was then spot-welded (Avio, Japan) to the cell casing. In accordance to coin cells (CR2016), the assembly of our battery cell is as follows, from top to bottom: the upper lid, a plastic ring, a metal spring, Li metal serving as our cathode, counter and reference electrode, glass microfiber (Whatman) paper as a

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. All synthesized compounds were either black or dark brown. The X-ray diffraction patterns of bare and solid solutions are shown in Figure 1a,b and the general crystallographic representation of select compounds are shown in Figure 1c. The lattice parameter values were derived via the Rietveld refinement using reported spinel structure space group (Fd3m) and these values of (Ni1−xZnx)Fe2O4 (x = 1, 0.75, 0.5, 0.25, 0) are shown in Table 1. NiFe2O4 shows a lower lattice parameter value of a = 8.3656 Å when compared to ZnFe2O4 at a = 8.4375 Å and (Zn0.5Ni0.5)Fe2O4 values lies between them at a = 8.3590 Å. The minor differences in the lattice parameter values were due to differences in the ionic radius of the metal ions (rZn2+ = 0.74 Å), rNi2+ = 0.69 Å), rFe3+ = 0.645 Å).26 A minor peaks ZnO are 4710

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

different scenarios: normal, inverse, or partially inverse structure. This is dependent on their site preference energy, the ionic radius of the metal ions, and the possibility of the different valence states. The morphology and porosity of the particles were found to have a profound and direct effect on the first cycle of discharge and charging profiles and long-term cyclability of the spinel oxides compounds. The morphology is vital as it must be able to allow Li-ions to flow in and out of the material easily so as to have a more efficient charge/discharge process, thus reducing capacity fading should the current be large. Therefore, aggregated primary nanoparticles, with a spherical or flaky morphology, may give rise to secondary particles with a macroporous structure. These structures are found to exhibit high and stable reversible capacities over a long period of cycling. Given that the compounds were able to have good capacity retention; this can also be attributed to the optimal macroporous structure of the phase and particle size, in comparison to those prepared at other temperatures. We noted that depending on the material, molten salt, and reaction conditions, like temperature and initial salts, different morphologies of products were obtained, namely polyhedral shapes,28,29 spherical particles,30,31 agglomerated nano particles,27,32−36 irregular cauliflower shape9,27 and needle type shape,37 rod type,38 and plate-like morphology.39 The experimental density of all compounds is given in Table 1. Depending on the composition, slight differences in density values were noted. Similarly, an ideal anode material should also have optimum surface area to density ratio. Thus, compounds with high BET surface area would be ideal. We note that BET surface area varies depending on materials,40 matrix elements (i.e., MCo2O4, where M = Co, Zn, and Cu),9,27,41 initial reactants41 and preparation conditions like preparation method and temperature40 and molten salts.31 Thus, from these two tests, we can see that NiFe2O4, alongside (Ni0.75Zn0.25)Fe2O4 and (Ni0.25Zn0.75)Fe2O4, is more suitable than the rest, as it has higher BET surface area and density values than other compounds. 3.2. Electrochemical Characteristics. 3.2.1. Galvanostatic Cycling. Galvanostatic charge−discharge cycling involves charging and discharging the cell at a constant current rate of 60 mA/g, and a voltage range (0.005−3.0 V) to analyze the capacity and Coulombic efficiency of the cell. Two cells were used in these tests to ensure consistent result. With the results obtained, the Li-storage and cyclability can be analyzed. The voltage vs capacity profiles are shown in Figure 3a,b. The cells underwent at least 50 cycles of testing and cycles 1 and 2 are chosen for clarity, as these cycles are essential in showing the structural phase transformation during charge and discharge cycles. The comparative cycling profiles of 5th, 10th, 20th, and 40th cycle are shown in Supporting Information, part 1. Figure 3a shows the first charge and discharge cycles, where the voltage is plotted against capacity. Upon its first discharge cycle, the voltage of the cell decreases sharply from the opencircuit potential (OCV) of about 2.8 V to 0.85 V. This commences the beginning of the single-phase Li-intercalation to the (Ni1−xZnx)Fe2O4 structure, as shown in reaction 1. At this point, the capacity observed is at ∼60 mAh/g. The discharge profile then shows a voltage plateau at about 0.85 V for all compounds, except for A = Zn0.75Ni0.25Fe2O4 and NiFe2O4, which had two abrupt voltage plateaus at 0.75 and 0.7 V. At about 0.75 V, the voltage starts to slope downward gently for the compounds apart from A, spanning over a capacity of about 650 mAh/g. On the other hand, A experiences a gentle

Figure 1. (a) Powder X-ray diffraction patterns of (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) and (b) Rietveld refined X-ray diffraction pattern of ZnFe2O4 The red line is fitted data, black symbols are experimental pattern, difference patterns are shown at center, vertical lines are (hkl) lines. (c) General crystallographic representation of ZnFe2O4 (zinc = yellow color, oxygen = red color, iron = blue color).

Table 1. Lattice Parameter, Particle Size, Density, and BET Surface Area Values of ((Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1)

compound

lattice parameters (Å)

average crystalline size (nm)

experimental density (g/cm3) (±0.00005)

NiFe2O4 (Zn0.25Ni0.75)Fe2O4 (Zn0.5Ni0.5)Fe2O4 (Zn0.75Ni0.25)Fe2O4 ZnFe2O4

8.3656(4) 8.3605(6) 8.3590(6) 8.3663(2) 8.4375(0)

25.7 28.2 29.1 31.2 25.8

5.7141 6.4397 8.0005 7.0315 6.8338

BET surface area (±0.1) m2/g 6.88 4.93 2.25 4.26 0.79

also seen also seen in the XRD patterns of x = 0.75, 0.25, which is not clear at present, since minor phases are electrochemically active, which will contribute to electrochemical properties. We note hybrid composites will also give improved capacity, when compare to bare material.27 The values are in between NiFe2O4 (JCPDS 86−2267) and ZnFe2O4 (JCPDS 82−1042), and a slight difference in lattice parameter value was noted depending on the preparation method.21 From the scanning electron microscopy (SEM) images shown in Figure 2, we can see that all the 5 compounds have fairly similar morphology, which were all agglomerated and porous. The metal nanoparticles (Figure 2) crystallize in three 4711

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

Figure 2. Scanning electron microscopy (SEM) images of (a) ZnFe2O4 at magnification of 40000x; (b) Zn0.75Ni0.25Fe2O4 at magnification of 20000×; (c) Zn0.75Ni0.25Fe2O4 at magnification of 40000×; (d) Zn0.5Ni0.5Fe2O4 at magnification of 30000×; (e) Zn0.25Ni0.75Fe2O4 at magnification of 30000×; (f) NiFe2O4 at magnification of 40000×.

subsequently the formation of metal nanoparticles embedded in an amorphous matrix of Li2O.6 All compounds at the end of the first deep discharge capacity are in the range 1400−1515 (±10) mAh/g. The overall capacity at the end of the first deep charge cycle is 958 mAh/g, which houses an irreversible capacity loss (ICL) of 557 mAh/g. This loss is attributed to the formation of the

downward slope that spans over 550 mAh/g at 0.65 V, which shows evidence of the Li-intercalation undergoing a two-step reaction mechanism. Reaction 2 represents this portion of the graph. As the discharge voltage decreases to 0.005 V, an extended reaction of Li with the intercalated phase of our (Ni1−xZnx)Fe2O4 cells occurs, resulting in the destruction of the crystalline structure, the reduction of metal ions and 4712

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

necessary for the cells to be subjected at above 2.0 V in order to attain a stable capacity. This is due to the fact that the dissolution of the polymeric layer (which is formed during discharge) occurs above 2.0 V. The capacity vs cycle number plots of all compounds are shown in Figure 4a−e and capacity values and capacity fading values are given in Table 2. All cells displayed good capacity retention and columbic efficiency over a large number of cycles, as shown in the table below. The present capacity values are greater than other reported values via the sol−gel auto combustion method, which are 2−3 times greater at the 50th cycle. The capacity value of ZnFe2O4 prepared by sol−gel assisted combustion method is about 330 mAh/g (sample prepared at 600 °C) and 570 mAh/g (sample prepared at 800 °C) at the end of 50th cycle at current rate of 60 mA/g.21 On the other hand, MgxCu0.2Zn0.82‑xFe1.98O4 (x = 0.20, 0.25, 0.30, 0.35 and 0.40) prepared sol−gel assisted combustion method at 600 °C showed a reversible capacity in the range 203−335 mAh/g after the 50th cycle and the same compounds reheated at 800 °C showed a small improvement in capacity from 430 to 540 mAh/g.42 It clearly indicates that the MSM-prepared spinel solid solutions showed a better reversible capacity values and retention, mainly due to the fact that defect free uniform oxides can be obtained from MSM. 3.2.2. Cyclic Voltammetry (CV). Cyclic voltammetry yields results on both conversion reaction and redox potentials, which is an effective form of analysis that complements the galvanostatic tests, a scan rate of 0.058 mV/s at a voltage range of 0.005−3.0 V and Li as the counter electrode was used. The CV curves of 1st, 2nd, 5th, and 50th cycles of all compounds are shown in Figure 5a−e. The first cathodic scan differs from the subsequent cathodic scans in all five compounds. Not only do they exhibit a cathodic peak of 0.6(±0.1) V vs Li, but there is also a continuous drop in potential from the OCV ranges from about 2.8 V to 0.8 V is observed (Figure 5a−e), which is similar to that of the galvanostatic scans. Such cathodic peaks occur due to crystal structure destruction, which is caused by the reduction of the Ni(II), Zn(II), and Fe(III) to corresponding metals. Formation of SEI also contributed to the peak, which resulted in the reduction of solvents in the electrolyte. In all compounds except B = ZnFe2O4 (Figure 5e) and Zn0.5Ni0.5Fe2O4 (Figure 5c), there are two cathodic peaks, with the smaller peak appearing before a larger peak. This can be explained by the Liintercalation of the spinel structure of Ni1−xZnxFe2O4 just before the destruction of the crystalline structure and reduction of the metals. However, the second cathodic scan does not experience a huge dip in its scans. These dips occur at about 1.0(±0.1) V. The first anodic scan generally occurs at 1.7(±0.1) V. In subsequent cycles, the anodic peak becomes flat and shifts toward a higher voltage value of 2.0(±0.2) V. This effect may account for the oxidation of Fe, Ni, and Zn nanoparticles. This decreasing trend of potentials is consistent throughout the subsequent cycles, as the peaks began to become flatter and gentler. CV studies clearly showed the good reversibility of electrodes. CVs of cathodic and anodic curves after 50 cycles (Supporting Information. Figure 2a−e) showed minor differences in the peak potentials. The main peak potentials observed are similar to those obtained in literature studies.21 3.2.3. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) studies on (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) was carried out at select

Figure 3. Galvanostatic cycling plots of (Ni1−xZnx)Fe2O4: (a) first cycle and (b) second cycle. Voltage rang: 0.005−3.0 V vs Li. Current rate: 60 mA/g.

solid electrolyte interface (SEI) and partially to the reduction of the solvent in the electrolytes when the electrode potential dips to 0.005 V. Similar ICL is commonly seen in many other similar metal oxides and reports on (Ni1−xZnx)Fe2O45. In the charging process, lithium extraction occurs, and the initial spinel ferrite phase was not recovered, but simpler oxides were formed as reaction 4 takes place. The first charge cycle has no voltage plateau as observed on the graphs, but has a slope between 1.5 and 2.0 V spanning a capacity of about 550 mAh/ g. During these charging cycles, the Li-intercalated (Ni1−xZnx)Fe2O4 cells gradually reform through reaction 5. The proposed reaction mechanism is as follows:6,21 + − (Ni1 − xZnx)Fe2O4 + b Li + be → Lib(Ni1 − xZnx)Fe2O4

(1)

Lib(Ni1 − xZnx)Fe2O4 + (8 − b)Li+ + (8 − b)e− → 2Fe + (1 − x)Ni + x Zn + 4Li 2O

xZn + Li+ + e− ⇌ LixZn

(2) (3)

1 − x Ni + x Zn + Li 2O ⇌ (1 − x)NiO + x ZnO + 2Li+ + 2e−

(4)

2Fe + 3Li 2O ⇌ Fe2O3 + 6Li+ + 6e (where x = 0, 0.25, 0.5, 0.75, 1), (0.4 ≤ b ≤ 0.6)

(5)

The second discharge cycle has a different profile from the first discharge cycle. The voltage plateau at 0.85 V is no longer observed. Instead, the sloping profile of the discharge cycle changes at 1.75 V, and varies until 0.005 V. This results in a final discharge capacity of 944 mAh/g, which differs from the first cycle. Despite this, the second charge cycle is analogous to the first charge cycle, suggesting that similar electrochemical reactions are taking place in both cycles. However, it is 4713

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

Figure 4. Capacity vs cycle number plots: (a) NiFe2O4; (b) Ni0.75Zn0.25Fe2O4; (c) Ni0.5Zn0.5Fe2O4; (d) Ni0.25Zn0.75Fe2O4; (e) ZnFe2O4. Voltage cycling range = 0.005−3.0 V; current rate = 60 mA/g.

Table 2. Discharge and Charge Capacities of (Ni1−xZnx)Fe2O4 at 1st, 2nd, and 50th Cycles compound NiFe2O4 Ni0.75Zn0.25Fe2O4 Ni0.5Zn0.5Fe2O4 Ni0.25Zn0.75Fe2O4 ZnFe2O4

1st discharge capacity (charge capacity) (±10 mAh/g) 1416 1469 1509 1499 1515

(923) (958) (942) (929) (863)

2nd discharge capacity (charge capacity) (±10 mAh/g) 895 944 905 904 817

(871) (908) (846) (874) (797)

50th discharge capacity (charge capacity) (±10 mAh/g) 706 819 603 781 637

(687) (802) (595) (767) (629)

% capacity fading (2nd to 50th cycle) 21.1 13.2 33.4 13.6 22.0

(21.1) (11.7) (29.7) (12.2) (21.1)

amplitude of 10 mV. An equivalent circuit model was created. (Supporting Information, part 3) This circuit comprises of Re (electrolyte resistance), Rct (resistance during charge transfer) which arises due to electrode and electrolyte interface, Rsf (surface film resistance due to solid electrolyte resistance), CPEsf,dl (surface film, double layer capacitance), W1 (Warburg impedance), and Ci (capacitance). Constant phase elements (CPE) represent the behavior of an imperfect capacitor due to

voltages during discharge−charge cycle to understand the electrode kinetics. EIS is one of the well-adopted Electroanalytical techniques to give information on surface film resistance, charge transfer resistance, bulk resistance values and corresponding capacitance due to surface film, double layer and bulk capacitance. A wide variety of cathode 43−46 and anode47−50materials was evaluated by EIS technique. EIS was performed in the frequency range 0.18 to 3 MHz, with an ac 4714

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

Figure 5. Graphs showing the cyclic voltammetry: (a) NiFe2O4; (b) Zn0.25Ni0.75Fe2O4; (c) Zn0.5Ni0.5Fe2O4; (d) Zn0.75Ni0.25Fe2O4; (e) ZnFe2O4. Voltage range = 0.005−3.0 V; scan rate = 0.058 mV/s.

porosity of electrode.19 Some selected voltages of surface and charge transfer film resistance were used. The Nyquist plots of (Ni1−xZnx)Fe2O4 (x = 0, 0.25, 0.5, 0.75, 1) at select discharge−charge voltages which are shown in Figure 6a−j. During cycling, all compounds showed a surface film and charge transfer resistance values which are on the order of 10−120 ohm, the corresponding double layer capacitance value is on the order of 10−50 μF, and the values are given in the Supporting Information (part 3). The apparent diffusion coefficient values are also calculated using51 DLi = πf lL2, where f l is limiting frequency, and L is the diameter of the particles. The observed DLi value was found to be in the range of 1 × 10−12 − 5.9 × 10−14 cm2/s during the first discharge− charge cycle (Supporting Information, part 3) and these values are close to other anode materials.

4. CONCLUSION Nanosized materials, (Ni1−xZnx)Fe2O4 (where x = 0, 0.25, 0.50, 0.75, 1) were synthesized via MSM and were characterized through XRD, SEM, BET, and density methods. Galvanostatic cycling, CV, and EIS were carried out to understand physical and chemical−electrochemical properties. Ni0.75Zn0.25Fe2O4 and Ni0.25Zn0.75Fe2O4 showed the best performance among all of our compounds. With a current rate of 60 mA/g, Ni0.75Zn0.25Fe2O4 retained a discharge capacity of 944 mAh/g at the end of the second cycle and 819 mAh/g at the end of the 50th cycle, and capacity fading was 13.2%, while Ni0.25Zn0.75Fe2O4 retained a discharge capacity of 904 mAh/g at the end of the second cycle and 781 mAh/g at the end of the 50th cycle and capacity fading was 13.6%. Other compounds like Ni0.5Zn0.5Fe2O4 showed greater capacity fading (33.4% at the end of the 50th cycle), possibly due to differences in surface area, morphology, and crystalline structure. The reason for higher capacity fading would be an area to be further studied. 4715

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

Figure 6. Nyquist plots (Z′ vs Z″ plots) during discharge−charge cycles NiFe2O4 (a) discharge and (b) charge, Zn0.25Ni0.75Fe2O4 (c) discharge and (d) charge, Zn0.5Ni0.5Fe2O4 (e) discharge and (f) charge, Zn0.75Ni0.25Fe2O4 (g) discharge and (h) charge, and ZnFe2O4 (i) discharge and (j) charge. Equivalent electrical circuit and fitted impedance resistance and capacitance values are shown in the Supporting Information (part 3).

five of our compounds showed higher reversible capacity than the commercial graphite anode, and they have nearly 2 to 3 times its capacity on average on the second discharge cycle.

The other two compounds, ZnFe2O4 and NiFe2O4, showed battery performances (in capacity) which were almost as high as Ni0.25Zn0.75Fe2O4 and Ni0.75Zn0.25Fe2O4. In conclusion, all 4716

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

(8) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. Nanophase ZnCo2O4 as a High Performance Anode Material for Liion Batteries. Adv. Funct. Mater. 2007, 17, 2855−2861. (9) Reddy, M. V.; Kenrick, K. Y. H.; Wei, T. Y.; Chong, G. Y.; Leong, G. H.; Chowdari, B. V. R. Nano-ZnCo2O4 Material Preparation by Molten Salt Method and Its Electrochemical Properties for Lithium Batteries. J. Electrochem. Soc. 2011, 158, A1423−A1430. (10) Sharma, Y.; Sharma, N.; Rao, G. V. S.; Chowdari, B. V. R. Listorage and Cyclability of Urea Combustion Derived ZnFe2O4 as Anode for Li-ion Batteries. Electrochim. Acta 2008, 53, 2380−2385. (11) Guo, X. W.; Lu, X.; Fang, X. P.; Mao, Y.; Wang, Z. X.; Chen, L. Q.; Xu, X. X.; Yang, H.; Liu, Y. N. Lithium Storage in Hollow Spherical ZnFe2O4 as Anode Materials for Lithium ion Batteries. Electrochem. Commun. 2010, 12, 847−850. (12) Yao, L.; Hou, X.; Hu, S.; Wang, J.; Li, M.; Su, C.; Tade, M. O.; Shao, Z.; Liu, X. Green Synthesis of Mesoporous ZnFe2O4/C Composite Microspheres as Superior Anode Materials for Lithiumion Batteries. J. Power Sources 2014, 258, 305−313. (13) Wang, N.; Xu, H.; Chen, L.; Gu, X.; Yang, J.; Qian, Y. A General Approach for MFe2O4 (M = Zn, Co, Ni) Nanorods and Their High Performance as Anode Materials for Lithium Ion Batteries. J. Power Sources 2014, 247, 163−169. (14) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. Li-storage and Cycling Properties of Spinel, CdFe2O4, as an Anode for Lithium ion Batteries. Bull. Mater. Sci. 2009, 32, 295−304. (15) Lavela, P.; Tirado, J. L. CoFe2O4 and NiFe2O4 Synthesized by Sol-gel Procedures for their use as Anode Materials for Li ion Batteries. J. Power Sources 2007, 172, 379−387. (16) Bomio, M.; Lavela, P.; Tirado, J. L. Electrochemical Evaluation of CuFe2O4 Samples Obtained by Sol-gel methods used as Anodes in Lithium Batteries. J. Solid State Electrochem. 2008, 12, 729−737. (17) Ding, Y.; Yang, Y.; Shao, H. One-pot Synthesis of NiFe2O4/C Composite as an Anode Material for Lithium-ion Batteries. J. Power Sources 2013, 244, 610−613. (18) Liu, H.; Zhu, H.; Yang, H. A low Temperature Synthesis of Nanocrystalline Spinel NiFe2O4 and its Electrochemical Performance as Anode of Lithium-ion Batteries. Mater. Res. Bull. 2013, 48, 1587− 1592. (19) Cherian, C. T.; Sundaramurthy, J.; Reddy, M. V.; Suresh Kumar, P.; Mani, K.; Pliszka, D.; Sow, C. H.; Ramakrishna, S.; Chowdari, B. V. R. Morphologically Robust NiFe2O4 Nanofibers as High Capacity LiIon Battery Anode Material. ACS Appl. Mater. Interfaces 2013, 5, 9957−9963. (20) Hameed, A. S.; Reddy, M. V.; Chowdari, B. V. R.; Vittal, J. J. Preparation of rGO-wrapped Magnetite Nanocomposites and their Energy Storage Properties. RSC Adv. 2014, 4, 64142−64150. (21) Cherian, C. T.; Reddy, M. V.; Subba Rao, G. V.; Sow, C. H.; Chowdari, B. V. R. Li-cycling Properties of Nano-crystalline (Ni1‑xZnx)Fe2O4 (0 ≤ x ≤ 1). J. Solid State Electrochem. 2012, 16, 1823−1832. (22) Afanasiev, P.; Geantet, C. Synthesis of Solid Materials in Molten Nitrates. Coord. Chem. Rev. 1998, 178, 1725−1752. (23) Liu, X.; Fechler, N.; Antonietti, M. Salt Melt Synthesis of Ceramics, Semiconductors and Carbon nanostructures. Chem. Soc. Rev. 2013, 42, 8237−8265. (24) Cherian, C. T.; Zheng, M. R.; Reddy, M. V.; Chowdari, B. V. R.; Sow, C. H. Zn2SnO4 Nanowires versus Nanoplates: Electrochemical Performance and Morphological Evolution During Li-Cycling. ACS Appl. Mater. Interfaces 2013, 5, 6054−6060. (25) Reddy, M. V.; Cherian, C. T.; Ramanathan, K.; Jie, K. C. W.; Daryl, T. Y. W.; Hao, T. Y.; Adams, S.; Loh, K. P.; Chowdari, B. V. R. Molten Synthesis of ZnO·Fe3O4 and Fe2O3 and its Electrochemical Performance. Electrochim. Acta 2014, 118, 75−80. (26) Shannon, R. D. Revised effective Ionic radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. Sect. A: Found. Crystallogr. 1976, A32, 751−767. (27) Reddy, M. V.; Yu, C.; Jiahuan, F.; Loh, K. P.; Chowdari, B. V. R. Molten Salt Synthesis and Energy Storage Studies on CuCo2O4 and CuO·Co3O4. RSC Adv. 2012, 2, 9619−9625.

Furthermore, all our compounds have lower capacity fading, which then allow them to have an even greater (estimated ∼5 times on average) 50th discharge capacity. Above compounds are cheap and eco-friendly and use a fast and versatile method of preparation, MSM. Furthermore, one could produce anode materials in bulk, and it is a simple process. Thus, our research product has very high potential for industrial applications by reducing the discharge−charge voltages.

■

ASSOCIATED CONTENT

S Supporting Information *

Additional information on the galvanostatic cycling graphs of 5th, 10th, 20th, and 40th cycles and fitted impedance values and apparent diffusion coeffcient values of (Ni1−xZnx)Fe2O4 at selected voltage values (parts 1−3). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(M.V.R.) E-mail: [email protected]; [email protected]. sg; [email protected]. Telephone: +65-65162607. Fax: +65-67776126. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Authors would like to thank the Gifted Education Branch, Ministry of Education, Singapore and Faculty of Science, National University of Singapore for the opportunity to participate in the Science Mentorship Program (2013 SMP) and Dr. Teh Yun Ling from NUSHS for her help. M.V.R. would like to thank associate Prof. Stefan Adams, Department of Materials Science & Engineering for his support and the National Research Foundation (NRF) Singapore for Research Grant No. WBS:R-284-000-115-281. We applied the “sequence determines credit” (SDC) approach for the sequence of authors, with majority of the paper and experimentation done by Dr. M. V. Reddy and Mr. Chu Yao Quan.

■

REFERENCES

(1) Alifantis, K. E.; Hackney, S. A.; Vasant Kumar, R. High Energy Density Lithium Batteries: Materials, Engineering, Applications; High Energy density Lithium batteries: Materials, Engineering, Applications Wiley VCH: Weinheim, Germany, 2010. (2) Wakihara, M. Recent Developments in Lithium Ion Batteries. Mater. Sci. Eng., R 2001, 33, 109−134. (3) Guyomard, D.; Sigala, C.; La Salle, A. L. G.; Piffard, S. Y. New Amorphous Oxides as High Capacity Negative Electrodes for Lithium Batteries: the LixMVO4 (M = Ni, Co, Cd, Zn; 1 < x ≤ 8) Series. J. Power Sources 1997, 68, 692−697. (4) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-sized Transition-Metal oxides as Negative-Electrode Materials for Lithium-ion Batteries. Nature 2000, 407, 496−499. (5) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions. Adv. Mater. 2010, 22, E170−E192. (6) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364−5457. (7) Park, C. M.; Kim, J. H.; Kim, H.; Sohn, H. J. Li-alloy Based Anode Materials for Li Secondary Batteries. Chem. Soc. Rev. 2010, 39, 3115−3141. 4717

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718

Article

The Journal of Physical Chemistry C

(46) Reddy, M. V.; Sakunthala, A.; SelvashekaraPandian, S.; Chowdari, B. V. R. Preparation, Comparative Energy Storage Properties, and Impedance Spectroscopy Studies of Environmentally Friendly Cathode, Li(MMn 1 1 / 6 )O 4 (M = Mn 1 / 6 , Co 1 / 6 , (Co1/12Cr1/12)). J. Phys. Chem. C 2013, 117, 9056−9064. (47) Levi, M. D.; Aurbach, D. Simultaneous Measurements and Modeling of the Electrochemical Impedance and the Cyclic Voltammetric Characteristics of Graphite Electrodes Doped with Lithium. J. Phys. Chem. B 1997, 101, 4630−4640. (48) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Nano(V1/2Sb1/2Sn)O4: a High Capacity, High Rate Anode Material for Liion Batteries. J. Mater. Chem. 2011, 21, 10003−10011. (49) Reddy, M. V.; Wen, B. L. W.; Loh, K. P.; Chowdari, B. V. R. Energy Storage Studies on InVO4 as High Performance Anode Material for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5, 7777−7785. (50) Reddy, M. V.; Prithvi, G.; Loh, K. P.; Chowdari, B. V. R. Li Storage and Impedance Spectroscopy Studies on Co3O4, CoO, and CoN for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 680− 690. (51) Das, B.; Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Nano-phase Tin Hollandites, K2(M2Sn6)O16 (M = Co, In) as Anodes for Li-ion Batteries. J. Mater. Chem. 2011, 21, 1171−1180.

(28) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Preparation and Characterization of LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 by Molten Salt Synthesis for Li ion Batteries. J. Phys. Chem. C 2007, 111, 11712−11720. (29) Kim, Y. Lithium Nickel Cobalt Manganese Oxide Synthesized Using Alkali Chloride Flux: Morphology and Performance As a Cathode Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 2329−2333. (30) Zhao, X.; Reddy, M. V.; Liu, H.; Ramakrishna, S.; Subba Rao, G. V.; Chowdari, B. V. R. Nano LiMn2O4 with Spherical Morphology Synthesized by a Molten Salt Method as Cathodes for Lithium ion Batteries. RSC Adv. 2012, 2, 7462−7469. (31) Reddy, M. V.; Zhang, B. C.; Loh, K. P.; Chowdari, B. V. R. Facile Synthesis of Co3O4 by Molten salt Method and its Li-storage performance. CrystEngComm 2013, 15, 3568−3574. (32) Reddy, M. V.; Jose, R.; Teng, T. H.; Chowdari, B. V. R.; Ramakrishna, S. Preparation and Electrochemical Studies of Electrospun TiO2 Nanofibers and Molten Salt Method Nanoparticles. Electrochim. Acta 2010, 55, 3109−3117. (33) Peining Zhu, M. V. R.; Wu, Y.; Peng, S.; Yang, S.; Nair, A. S.; Loh, K. P.; Chowdari, B. V. R.; Ramakrishna, S. Mesoporous SnO2 Agglomerates with Hierarchical Structures as an Efficient Dualfunctional Material for Dye-sensitized Solar Cells. Chem. Commun. 2012, 48, 10865−10867. (34) Reddy, M. V.; Yu Tse, L.; Bruce, W. K. Z.; Chowdari, B. V. R. Low Temperature Molten Salt Preparation of Nano-SnO2 as Anode for Lithium-ion Batteries. Mater. Lett. 2015, 138, 231−234. (35) Reddy, M. V.; Khai, V. H.; Chowdari, B. V. R. Facile One Pot Molten Salt Synthesis of Nano (M1/2Sb1/2Sn)O4 (M = V, Fe, In). Mater. Lett. 2015, 140, 115−118. (36) Reddy, M. V.; Tse, L. Y.; Bruce, W. K. Z.; Chowdari, B. V. R. Low Temperature Molten Salt Preparation of Nano-SnO2 as Anode for Lithium-ion Batteries. Mater. Lett. 2015, 138, 231−234. (37) Reddy, M. V.; Yu, C.; Fan, J. H.; Loh, K. P.; Chowdari, B. V. R. Li-Cycling Properties of Molten Salt Method Prepared Nano/ Submicrometer and Micrometer-Sized CuO for Lithium Batteries. ACS Appl. Mater. Interfaces 2013, 5, 4361−4366. (38) Reddy, M. V.; Adams, S.; Liang, G. T. J.; Mingze, I. F.; Van Tu An, H.; Chowdari, B. V. R. Low Temperature Molten Salt Synthesis of Anatase TiO2 and its Electrochemical Properties. Solid State Ionics 2014, 262, 120−123. (39) Tan, K. S.; Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. High-performance LiCoO2 by molten salt (LiNO3:LiCl) Synthesis for Li-ion Batteries. J. Power Sources 2005, 147, 241−248. (40) Reddy, M. V.; Teoh, X. W. V.; Nguyen, T. B.; Lim, Y. Y. M.; Chowdari, B. V. R. Effect of 0.5 M NaNO3:0.5 M KNO3 and 0.88 M LiNO3:0.12 M LiCl Molten Salts, and Heat Treatment on Electrochemical Properties of TiO2. J. Electrochem. Soc. 2012, 159, A762− A769. (41) Reddy, M. V.; Beichen, Z.; Nicholette, L. J.; Kaimeng, Z.; Chowdari, B. V. R. Molten Salt Synthesis and Its Electrochemical Characterization of Co3O4 for Lithium Batteries. Electrochem. Solid State Lett. 2011, 14, A79−A82. (42) Hameed, A. S.; Bahiraei, H.; Reddy, M. V.; Shoushtari, M. Z.; Vittal, J. J.; Ong, C. K.; Chowdari, B. V. R. Lithium Storage Properties of Pristine and (Mg, Cu) Codoped ZnFe2O4 Nanoparticles. ACS Appl. Mater. Interfaces 2014, 6, 10744−10753. (43) Levi, M. D.; Gamolsky, K.; Aurbach, D.; Heider, U.; Oesten, R. On Electrochemical Impedance Measurements of LixCo0.2Ni0.8O2 and LixNiO2 Intercalation Electrodes. Electrochim. Acta 2000, 45, 1781− 1789. (44) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Long-term Cycling studies on 4V-cathode, Lithium Vanadium Fluorophosphate. J. Power Sources 2010, 195, 5768−5774. (45) Prabu, M.; Reddy, M. V.; Selvasekarapandian, S.; Subba Rao, G. V.; Chowdari, B. V. R. Synthesis, Impedance and Electrochemical Studies of Lithium Iron Fluorophosphate, LiFePO4F Cathode. Electrochim. Acta 2012, 85, 572−578. 4718

DOI: 10.1021/jp5121178 J. Phys. Chem. C 2015, 119, 4709−4718