Accepted Manuscript Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries P. Sivakumar, Prasant Kumar Nayak, Boris Markovsky, Doron Aurbach, Aharon Gedanken PII: DOI: Reference:

S1350-4177(15)00039-5 http://dx.doi.org/10.1016/j.ultsonch.2015.02.007 ULTSON 2795

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

13 August 2014 18 January 2015 18 February 2015

Please cite this article as: P. Sivakumar, P.K. Nayak, B. Markovsky, D. Aurbach, A. Gedanken, Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries, Ultrasonics Sonochemistry (2015), doi: http://dx.doi.org/10.1016/j.ultsonch.2015.02.007

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Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries P. Sivakumara, Prasant Kumar Nayaka, Boris Markovskya, Doron Aurbacha, Aharon Gedankena, b a

Department of Chemistry, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel. b

National Cheng Kung University, Department of Materials Science & Engineering, Tainan 70101, Taiwan

Corresponding author: Aharon Gedanken Tel.: +972-3-5318-315; Fax: + 972-3-7384053 E-mail address: [email protected] (Aharon Gedanken)

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Abstract LiNi0.5Mn1.5O4 was synthesized as a cathode material for Li-ion batteries by a sonochemical reaction followed by annealing, and was characterized by XRD, SEM, HRTEM and Raman spectroscopy in conjunction with electrochemical measurements. Two samples were prepared by a sonochemical process, one without using glucose (sample-S1) and another with glucose (sample-S2). An initial discharge specific capacity of 130 mAh g−1 is obtained for LiNi0.5Mn1.5O4 at a relatively slow rate of C/10 in galvanostatic charge-discharge cycling. The capacity retention upon 50 cycles at this rate was around 95.4 and 98.9 % for sample-S1 and sample-S2, respectively, at 30 °C. Key words: Sonochemical synthesis, LiNi0.5Mn1.5O4, Li-ion batteries, 5 V cathodes, Spinel, Electrochemical properties.

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1.

Introduction Li-ion batteries have become the major power source for portable electronic devices and

these are now promoted for use in electric vehicles because of their high energy density and long cycle life [1-5]. A large variety of materials such as layered (LiCoO2, LiMn1/3Ni1/3Co1/3O2, LiNi0.5Mn0.5O2), spinel (LiMn2O4, LiNi0.5Mn1.5O4), and olivines (LiFePO4, LiMnPO4, LiMn0.8Fe0.2PO4) have been tested as cathode materials for Li-ion batteries. Spinel cathodes are important class of cathode materials applicable for high power Li-ion batteries, because of their suitable 3D tunnel structure for intercalation/de-intercalation of Li+ ions. Among these two spinel cathodes, there is a great interest in LiNi0.5Mn1.5O4, because of its high redox potential (4.7 V) and better cyclability as compared to LiMn2O4 (4.0 V) [6-21]. While the charge storage in LiMn2O4 is based on Mn3+/Mn4+ redox reaction, it is the Ni2+/Ni4+ redox reaction that determines the maximum charge that can be stored in LiNi0.5Mn1.5O4. However, research issues related to this cathode material that need to be addressed are as follows. First, the reaction of the cathode surface with electrolyte at the high operating voltage. Second, ordering of Ni2+ and Mn4+ in the 16d octahedral sites of the spinel lattice results in the capacity fading [6, 7]. Third, the synthesis of LiNi0.5Mn1.5O4 is accompanied by the formation of LixNi1-xO impurity phase, which results in a capacity fading. Finally, synthesis of disordered phase (Space Group: Fd3തm) is always a challenging issue for these cathode materials [8-10]. The electrochemical properties of the electrode materials are influenced by their particle size, morphology, and structure, which in turn depend on the preparation methodology. Numerous synthesis methods such as organic co-precipitation method [11], microwave assisted solid-state method [12], solid-state method [13], hydrothermal method [14], co-precipitation method [15], sol-gel method [16], and the electro-spinning method [17] have been reported to 3

prepare LiNi0.5Mn1.5O4 cathode material. In most of these reports, the delivered capacity is well below the theoretical capacity because of the undesirable ordered phase [21]. To achieve best possible electrochemical performances Fd3തm phase is desirable. So, there is a need to explore new synthetic methodologies to synthesise highly electrochemically active LiNi0.5Mn1.5O4 cathode materials (Fd3തm). It is obvious that different preparation processes have robust impacts on the morphology, structure and electrochemical performance of obtained cathode materials. Sonochemistry has been proven to be green and cost-effective method [22] for the synthesis and coating of nano-sized metal-oxide, composites, and alloys [22-28]. This technique involves usage of acoustic cavitation phenomenon for the synthesis of different nano/micro-sized materials due to the continuous formation, growth and drastic collapse of bubbles in a solution. The cavitation bubbles collapse rapidly and violently, and in doing so, generate extremely high temperature (>5000 K), pressure (>200 Mpa), and cooling rate (>107 K s−1) [29]. By taking an advantage of these extreme conditions, efforts were taken to synthesize nano-sized LiNi0.5Mn1.5O4 cathode material. In the present work, to the best of our knowledge, sonochemical synthesis of high voltage spinel LiNi0.5Mn1.5O4 is reported for the first time. The product, LiNi0.5Mn1.5O4, showed a specific capacity of about 130 mAh g−1 with good cyclability, indicating their potential as cathode materials for lithium-ion batteries. The synthesis of LiNi0.5Mn1.5O4 involves an annealing stage following the ultrasonic irradiation. Glucose is added to the sonochemical mixture of precursors in an attempt to coat the material with a carbon layer. The characterization of the products was performed by XRD, SEM, TEM and Raman spectroscopy along with electrochemical studies. The sample prepared by using glucose in the sonochemical synthesis possesses better electrochemical performance as compared to that synthesized without glucose. 4

2.

Experimental

2.1.

Materials Analytical grade chemicals, namely, Lithium hydroxide monohydrate (LiOH·H2O;

LiOH, 56%; Acros Organics), Manganese (II) acetate tetrahydtrate (Mn(CH3COO)2·4H2O, 99+%; Sigma-aldrich) and Nickel (II) acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%; Sigmaaldrich), D-glucose (Aldrich), Poly(vinylidene fluoride) (PVDF) (Aldrich), and 1-methyl-2pyrrolidinone (NMP) (Aldrich) were used as received. Double distilled (DD) water was used to dissolve the metal acetates, LiOH and D-glucose. Commercially available, Li grade, EC-DMC / LiPF6 solutions were used as received (could contain trace water, HF and PF5 at the ppm level). 2.2.

Sonochemical synthesis of LiNi0.5Mn1.5O4 The precursors mixture for the synthesis of LiNi0.5Mn1.5O4 included of nickel (II) acetate,

manganese (II) acetate and Lithium hydroxide. In a typical procedure, 0.015 mol of lithium hydroxide monohydrate (0.629 g), 0.0225 mol of manganese (II) acetate tetrahydtrate (5.515 g), and 0.0075 mol of nickel (II) acetate tetrahydrate (1.866 g) were dissolved in 100 ml DD water. The sonochemical irradiation was employed by using a high intensity ultrasonic Ti-horn (20 kHz, 750 W at 60% efficiency, Sonics & Materials VCX600 Sonofier). After 1 h of ultrasonic irradiation (temperature: 0 min = ∼ 25 °C and 60 min = ∼ 94 °C), the obtained samples were kept on a hot plate at 70–80 °C to evaporate the excess water until a gel was obtained. Then the gel was additionally dried at 150 °C in a hot air oven for 12 h to get the as-prepared product. The as-prepared product was grinded uniformly in an agate mortar. The sample was annealed at 400 °C for 10 h and re-annealed at 800 °C for 10 h after heating the

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sample at the rate of 10 °C min−1 in ambient atmosphere using electric furnace to produce black color nano-crystalline LiN0.5M1.5O4 powder. After cooling, the samples were re-grinded uniformly in an agate mortar into fine powder to carry out the electrochemical measurements. The samples were modified with 0.0 moles and 0.01 moles of D-glucose (1.8 g) were labeled as sample-S1 and sample-S2, respectively. To compensate for Li loss at high temperature, an excess of 5% of the lithium compound was used in all the starting materials. Schematic illustration of synthesis processes of LiNi0.5Mn1.5O4 by sonochemical method is shown in Fig. 1. Fig. 2 shows photographs of sonochemical apparatus from two different angles. 2.3.

Materials characterization The crystalline structures and phase composition of the obtained powders were identified

by powder X-ray diffraction taken with a Bruker D8 Advance X-ray diffractometer using Cu Kα (λ=1.5406 Å) as the source. The chemical analysis of the materials was carried out using the inductive coupled plasma technique (ICP-AES, spectrometer Ultima-2 from JobinYvon Horiba). For ICP measurement the solid samples were first dissolved in aqua regia (concentrated HCl : HNO3 = 3:1) and then diluted with DD water. The thermal stability of the products were determined with a TGA/SDTA851e thermogravimetric analysis (TGA) instrument in the range of 25–800 °C at a heating rate of 10 °C min−1conducted under a flow of air . Elemental analysis was carried out to measure the amount of carbon content in LiNi0.5Mn1.5O4 with and without glucose by a Thermo CHNS-O elemental analysis device, model EA 1110 CHN analyzer. The particle size and morphology of the samples were measured using an environmental scanning electron microscope (E-SEM, Inspect FEI microscope). TEM characterization was carried out with a 6

JEOL-JEM 2100 electron microscope with LaB6 emitter operating at 200 kV. Samples for the TEM studies were prepared by dispersing and sonicating the powdered samples in isopropyl alcohol and adding a few drops of the resulting suspension to a TEM copper grid. The powder specific surface area data of the samples were calculated by the Brunauer-Emmett-Teller (BET) method by measuring under liquid nitrogen adsorption-desorption isotherm obtained with a Nova 3200e Quantachrome analyzer. The surface area was calculated from the linear part of the BET plot. Micro-Raman spectroscopy studies of the spinels were performed using a micro-Raman spectrometer from Renishaw Via (UK) equipped with a 514 nm laser, a CCD camera, and an optical Leica microscope. A 50x objective lens to focus the incident beam and an 1800 lines/mm grating were used. We recorded spectra from 5-10 locations on each sample. 2.4.

Electrodes preparation The electrodes for the electrochemical studies were prepared by making slurry of 80 wt

% active material of LiNi0.5Mn1.5O4, 10 wt % of conductive super P carbon, and 10 wt % of PVDF binder in N-methyl-2-pyrrolidinone (NMP) as the solvent. The slurry was uniformly coated by using a doctor-blade onto Al foils current collectors, dried at 80 °C for 12 h in an oven. The coated Al foil was then pressed uniformly and then cut into circular electrodes of 14 mm diameter. The electrodes were finally dried at 110 °C for 12 h under vacuum. 2.5.

Electrochemical measurements LiNi0.5Mn1.5O4 electrodes were tested using coin-type cells 2032 (NRC, Canada)

assembled in an argon-filled dry glove box (MBroun). Li metal foil was used as the counter and reference electrodes. Typical loading of the active mass was 5-6 mg cm−2. A commercial battery electrolyte solution LP 30 (Merck) consisting of 1 M LiPF6 in ethylene carbonate/dimethyl 7

carbonate (EC/DMC) (1:1 w/w) was used. A porous polypropylene based membrane (Celgard) was used as the separator. The cells were stored for 12 h at room temperature after assembling, at OCV, in order to ensure the complete impregnation of the electrodes and the separators with the electrolyte solution. The cells were measured by galvanostatic charge-discharge cycling in the potential range 3.5-4.9 V vs. Li/Li+ using computerized multi-channel battery testing instruments from Arbin Inc. Electrochemical impedance spectra (EIS) were recorded at open circuit potential with an amplitude of 5 mV around equilibrium in the frequency range of 100 kHz-0.01 Hz, using ECO CHEMIE potentiostat/galvanostat Autolab PGSTAT302N with Frequency Response Analyzer (FRA). The electrochemical measurements were performed at 30 ºC in thermostats. 3.

Results and discussion

3.1

ICP analysis ICP-AES data analysis reveals that the metal molar ratio in the solid of Li/Ni/Mn for

sample-S1 and sample-S2 are 1.0:0.46:1.46 and 1.0:0.46:1.49, respectively. The experimental Li/Ni/Mn molar ratios are in good agreement with the designed nominal values in the spinel oxides. Thus, the compositions of the obtained active materials are suggested to be LiNi0.5Mn1.5O4 for both samples -S1 and -S2. The ICP results of the molar ratios of Li:Ni:Mn were close to 2:1:3. This result is consistent with metal ion concentration (Li:Ni:Mn 2:1:3) in the starting solution and also with the energy dispersive X-ray spectroscopy (EDX) (Ni:Mn 1:3) results. 3.2.

Powder XRD analysis

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The powder XRD patterns of the sample-S1 and sample-S2 powders are illustrated in Fig. 3. The obtained samples are identified as single phase spinel having a face centered cubic (FCC) structure and Fd3തm space group. The final products have no diffraction lines assigned to impurities. The diffraction peaks are very strong and sharp, which suggests that the obtained samples are highly crystalline in which lithium atoms occupy the tetrahedral (8a) sites, the transition metals atoms (Ni and Mn) reside at the octahedral (16d) sites and the oxygen atoms are located in the Wyckoff position of 32e sites. The lattice parameters are found to be a = 8.178 Å and 8.173 Å for sample-S1 and sample-S2, which are highly consistent with standard JCPDF data (PDF 80-2162) reported values for this spinel [12-13]. 3.3.

Thermal analysis Fig. 4 shows the thermogravimetric analysis (TGA) curves of the sample-S1 and sample-

S2 of these as-synthesized precursors carried out through continuous weight loss in the temperature range from 25 °C to 800 °C. The weight losses occurs in three steps corresponding to (i) the removal of surface water incorporated in the lattice of the material, (ii) crystal water and the decomposition of precursors left in the solid product, and (iii) combustion nature of inorganic and organic constituent of the precursor like acetate and glucose. The weight loss is nearly steady when the temperature is above 380 °C and 420 °C for sample-S1 and sample-S2, respectively. It reveals that the decomposition of precursors is completed above this temperature. Besides, the total mass loss throughout the entire temperature range is 56.72 % for sample-S1 and 64.75% for sample-S2. The observed difference between sample-S1 and sample-S2 may be explained as due to the percentage of the metal ions mass in sample-S1 which is larger than in sample-S2, due to addition of glucose in sample-S2. The decomposition of glucose leaves a

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residue of carbon in sample-S2. Upon further increasing the temperature, the residual carbon in sample-S2 is oxidized. 3.4.

CHN analysis Further, CHN analysis is used to trace the carbon concentration in the obtained cathode

materials. The elemental analysis of the samples with and without glucose annealed at 800 °C clearly indicates the very less concentration of carbon presence in the both materials. However, the measured carbon concentrations are 0.0013 % and 0.0390 % for sample-S1 and sample-S2, respectively indicate a higher concentration of carbon in sample-S2. The carbon presence in sample-S2 leads to better electrochemical performance (capacity fade and retention) of electrodes comprising of sample-S2. 3.5.

Morphological analysis The morphology of samples prepared with and without glucose is analyzed by SEM

measurements and presented in Fig. 5. An aggregated cluster of small size particles of 70 nm and 80 nm for the LiNi0.5Mn1.5O4 nanoparticles are observed for sample-S1 and sample-S2, respectively after annealing at 400 °C. A clear sintering of the nanoparticles is observed for sample annealed at 800 °C (Fig. 5 (b) and (d)). The particle size is increased upon raising the annealing temperature from 400 °C to 800 °C for both LiNi0.5Mn1.5O4 synthesized samples. The average particle size is measured from the TEM images and found to be 170 nm and 150 nm for samples -S1 and -S2, annealed at 800 °C, respectively. It is noted that while a semi-crystalline product is observed from TEM images of LiNi0.5Mn1.5O4 nanoparticles synthesized without glucose, highly crystalline faceted structure is observed for the LiNi0.5Mn1.5O4 nanoparticles synthesized with glucose. The HRTEM images of sample annealed at 800 °C revealed a high 10

crystallinity and an apparent absence of lattice defects (Fig. 6 (c) and (d)). The analysis of the fringes detected in the HRTEM images is showing that the d-spacing between two atomic planes are calculated to be 0.45 nm, which corresponds to the atomic planes. This result is consistent with the standard JCPDF data (PDF 80-2162). The compositions of synthesized LiNi0.5Mn1.5O4 nanoparticles were also analyzed by energy dispersive X-ray spectroscopy (EDX) measurement. The characteristic peaks of the Ni, Mn and O are present at their respective position in the EDX spectrum. The atomic ratio of nickel to manganese detected from the EDX spectra is approximately 1:3, which is in good agreement with compositional results measured by ICP-AES. 3.6.

BET analysis The Brunauer−Emmett−Teller (BET) surface areas of the spinels are measured from

Nitrogen sorption isotherms measurements. The obtained surface area values are 1.5 and 2.5 m2 g−1 for sample-S1 and sample-S2, respectively. The surface area of sample-S2 is slightly larger than that of the spinel prepared without glucose. The increase in the surface area of sample-S2 is attributed to first to the smaller size of the S2 sample and also to the release of large volumes of gases (e.g., H2O and CO2) upon the decomposition of glucose leaving behind a more porous product. 3.7.

Raman analysis Fig. 8 shows the Raman spectra of sample-S1 and sample-S2. Raman spectroscopy is a

valuable tool to verify the cations ordering. The strong band at around 636 cm−1 corresponds to the symmetric Mn–O stretching vibration of MnO6 octahedra (A1g), and the peaks at 395 (F2g) and 495 cm−1(F2g) correspond to the Ni2+–O stretching mode in the spinel structure [14]. The 11

obtained spectra of the samples are characteristic of a typical cubic spinel structure and disordered Fd3തm (F phase) space group. The Raman spectra of both sample-S1 and sample-S2 are assigned to cubic spinel structure and Fd3തm space group, due to the absence of a distinguishable peak split in the 588–623 cm−1 region, which is characteristic of the ordered structure (P4332) of the spinel [14]. There is no evidence for any other peaks (Fig. 8 inset) in the Raman analysis, namely, the expected carbon D and G bands are not detected. 3.8.

Electrochemical analysis Fig. 9 presents the charge-discharge curves of LiNi0.5Mn1.5O4 recorded at 14 mA g−1

(C/10) in the potential range of 3.5-4.9 V for several cycle numbers. A long plateau is observed above 4.6 V while charging, which corresponds to the de-intercalation of Li+ ions from the spinel and Ni2+/Ni3+/Ni4+ oxidation. During discharge, the plateau observed corresponding to the reverse process, i.e., intercalation of Li+ ions into the active mass at the same potential and Ni4+/Ni3+/Ni2+ reduction. The initial discharge capacities are found to be 131 and 127 mAh g−1 for sample-S1 and sample-S2, respectively. The Coulombic efficiency recorded was nearly 87% in the 1st cycle. However, the Coulombic efficiency was found to be about 95 % from the 2nd cycle. Feng et al. reported a specific capacity of about 130 mAh g−1 for LiNi0.5Mn1.5O4 synthesized by an organic co-precipitation method [11]. In this work 8-hydroxyquinoline was used as a precipitant in the solid-liquid dispersion then the solid-liquid dispersion was vigorously stirred for 2 h. The precursor was pressed into pellets at 15 MPa and subsequently sintered in air at 800 °C for 24 h. Porous LiNi0.5Mn1.5O4 microspheres synthesized in a co-precipitation method exhibited a specific capacity of 137 mAh g−1 at 0.1 C rate [15]. Here Na2CO3 solution

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(aq.) and NH4OH solution (aq.) were used as precipitating agent. The precursor was dried inside a vacuum oven set at 100 °C over 24 h. Thereafter, the obtained (Ni0.25Mn0.75)CO3 precursor was mixed with a stoichiometric amount of Li2CO3, and calcined in air at 500 °C for 5 h and then calcined at 850 °C for 15 h. LiNi0.5Mn1.5O4 synthesized by sol-gel method exhibited a specific capacity of 128 mAh g−1 at 0.5 C rate [16]. In this synthesis method NH3·H2O was used to obtain homogeneous solution then the solution was heated to 80 °C under normal stirring conditions and the resulting precursor was dried at 90 °C for 5 h and then heated at 750 °C for 15 h in the air. Hollow nanowires of LiNi0.5Mn1.5O4 synthesized by electrospinning exhibited a specific capacity of 120 mAh g−1 at 0.01 A g−1 [17]. In this preparation method polyvinyl alcohol were dissolved by aging at 90 °C for 1 h in the starting mixed solution. Here polyvinyl alcohol was used as a gelling agent. Then the precursor solution was poured into a syringe with a metal needle. A high voltage of 20 kV was then applied across the aluminum foil collector and metal needle. The precursor wires were dried at 100 °C for 1 h under vacuum conditions. The dried wires were then peeled off and heated at 800 °C for 3 h under air flow conditions. Thus, the specific capacity of 130 mAh g−1 obtained in the present study for LiNi0.5Mn1.5O4 synthesized by sonochemical method is comparable with those obtained for LiNi0.5Mn1.5O4 synthesized by other synthesis routes. These results emphasize the advantages of the sonochemical method which requires shorter fabrication time, and does not entail the addition of precipitating and gelling agent. Fig. 10 shows the differential capacity plot obtained from the differentiation of the charge-discharge curves. Two pairs of sharp peaks are clearly observed in the differential capacity plot of both samples, which is a typical characteristic of spinel cathodes. The peaks

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corresponding to the de-intercalation in the charge at 4.69 and 4.75 V and the reverse, i.e., intercalation during discharge at 4.72 and 4.66 V are clearly observed. The peak potential separation of 0.03 V between the first pair (4.69/4.66 V) and second pair (4.75/4.72 V) clearly indicates that the charge storage involves a two electron transfer process (Ni2+/Ni4+). The cells were subjected to repeated galvanostatic cycling at 14 mA g−1 (C/10) in the voltage domain of 3.5-4.9 V. Fig. 11 shows typical plots of specific capacity vs. cycle number. The specific capacity of LiNi0.5Mn1.5O4 electrodes decreased during 50 cycles from around 131 mAh g−1 to values around 125 mAh g−1 for sample-S1 and from 127 to 125 mAh g−1 for sample-S2, thus retaining about 95.4 and 98.9 % of initial capacity for sample-S1 and sample-S2, respectively. Thus, sample-S2 is found to possess better cyclability as compared to sample-S1. The rate capabilities of LiNi0.5Mn1.5O4 electrodes of sample-S1 and sample-S2 were tested at different current densities (C-rates). In typical experiments, cells were measured at increasing current densities (rates) each 5 cycles, in the voltage range of 3.5-4.9 V. At the end of these experiments, the cells are returned to low rate (e.g. C/10) in order to demonstrate that the low rate specific capacity does not change due to the operation at high rates. Fig. 12 provides typical results of such experiments: upon doubling the rate from 14 mA g−1 to 28 mA g−1, the discharge capacity of LiNi0.5Mn1.5O4 electrodes decreased from around 130 mAh g−1 to around 110 mAh g−1. A discharge capacity around 40 mAh g−1 is obtained at 2C rate. As clearly demonstrated in Fig. 12, after cycling these electrodes at high rates, returning to low rates retains the specific capacity to their initial high values. The sample synthesized by using glucose (sample-S2) exhibited a little better rate capability as compared to that synthesized without glucose (sample-S1).

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Electrochemical impedance spectra (EIS) of LiNi0.5Mn1.5O4 samples were recorded at open circuit potentials with amplitude of 5 mV in the frequency range of 100 kHz-0.01 Hz. The cells were subjected to three galvanostatic charge-discharge cycles for activating and stabilizing the electrodes before the impedance measurements. The Nyquist plots are shown in Fig. 13. The Nyquist plots contain (i) a high to medium frequency semicircle which corresponds to the parallel combination of surface films resistance to Li+ ions migration (Rf) and surface films’ capacitance (Cf), (ii) a medium to low frequency semicircle which we assign to interfacial charge-transfer resistance (Rct) coupled with an interfacial capacitance (Cdl), followed by (iii) a linear portion at low frequency corresponding to solid state diffusion of Li+ ions into the active mass. It can be clearly seen that the impedance due to migration of Li+ through surface film (Rf) and Rct are very less for sample-S2 as compared to sample-S1. Also, the linear portion corresponding to solid state diffusion of Li+ ions is clearly visible at low frequency for sampleS2, indicating that the Li+ ion diffusion is more facilitated in sample-S2 as compared to sampleS1. 4.

Conclusions Phase pure LiNi0.5Mn1.5O4 spinel samples were synthesized as a high voltage cathode

material by green, simple, low-cost, and very effective sonochemical method. Powder XRD and Raman spectroscopy results confirm the preparation of impurity free highly crystalline LiNi0.5Mn1.5O4 spinel material having a cubic structure (Fd3തm space group). We established that better electrochemical cyclability of electrodes comprising LiNi0.5Mn1.5O4 synthesized in the presence of glucose, demonstrated better electrochemical cyclability comparison to those synthesized without glucose (sample-S1). In addition, glucose aids to enhance the crystallinity of the LiNi0.5Mn1.5O4 spinel material (sample-S2) thus providing better electrochemical stability of 15

sample-S2. In Li-cells both samples exhibited a reversible capacity of about 130 mAh g−1. ICPAES experimental results reveal Li/Ni/Mn molar ratios which are in good agreement with the designed nominal values of the spinel oxides. The TGA results exhibit the obtained samples having good thermal stability. The elemental study of the samples synthesized with and without glucose annealed at 800 °C clearly indicates the presence of minor content of carbon in both materials. Semi-crystalline product is observed from TEM images of LiNi0.5Mn1.5O4 nanoparticles synthesized without glucose, highly crystalline faceted structure is observed for the LiNi0.5Mn1.5O4 nanoparticles synthesized with glucose. The obtained LiNi0.5Mn1.5O4 spinel with high crystalline structure and good electrochemical stability properties is expected to be attractive material for practical application. A simple synthesis approach of sonochemical method used in the present work is expected to be a favorable for synthesis of different kinds of electro-active materials for energy storage application. Acknowledgements P. Sivakumar thanks the Council for Higher Education, State of Israel for the PBC scholarship for outstanding postdoctoral researchers from China and India. References [1]

M.S. Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 104 (2004) 4271–4302.

[2]

M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657.

16

[3]

V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243– 3262

[4]

A. Manthiram, Materials Challenges and Opportunities of Lithium Ion Batteries, J. Phys. Chem. Lett. 2 (2011) 176–184.

[5]

J.B. Goodenough, K.-S. Park, The Li-Ion Rechargeable Battery: A Perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176.

[6]

R. Santhanam, B. Rambabu, Research progress in high voltage spinel LiNi0.5Mn1.5O4 material, J. Power Sour. 195 (2010) 5442–5451.

[7]

T. Noguchi, I. Yamazaki, T. Numata, M. Shirakata, Effect of Bi oxide surface treatment on 5 V spinel LiNi0.5Mn1.5−xTixO4, J. Power Sour. 174 (2007) 359–365.

[8]

J. Liu, A. Manthiram, Understanding the Improved Electrochemical Performances of FeSubstituted 5 V Spinel Cathode LiMn1.5Ni0.5O4, J. Phys. Chem. C 113 (2009) 15073– 15079.

[9]

K.-J.

Hong,

Y.-K.

Sun,

Synthesis

and

electrochemical

characteristics

of

LiCrxNi0.5−xMn1.5O4 spinel as 5 V cathode materials for lithium secondary batteries, J. Power Sour., 109 (2002) 427–430. [10]

Q. Zhong, A. Bonakclarpour, M. Zhang, Y. Gao, J.R. Dahn, Synthesis and Electrochemistry of LiNixMn2−xO4, J. Electrochem. Soc. 144 (1997) 205–213.

17

[11]

J. Feng, Z. Huang, C. Guo, N.A. Chernova, S. Upreti, M.S. Whittingham, An Organic Co-precipitation Route to Synthesize High Voltage LiNi0.5Mn1.5O4, ACS Appl. Mater. Inter. 5 (2013) 10227−10232.

[12]

P. Gao, L. Wang, L. Chen, X. Jiang, J. Pinto, G. Yang, Microwave rapid preparation of LiNi0.5Mn1.5O4 and the improved high rate performance for lithium-ion batteries, Electrochim. Acta 100 (2013) 125–132.

[13]

J. Chong, S. Xun, X. Song, G. Liu, V.S. Battaglia, Surface stabilized LiNi0.5Mn1.5O4 cathode materials with high-rate capability and long cycle life for lithium ion batteries, Nano Energy 2 (2013) 283–293.

[14]

Y. Xue, Z. Wang, F. Yu, Y. Zhang, G. Yin, Ethanol-assisted hydrothermal synthesis of LiNi0.5Mn1.5O4 with excellent long-term cyclability at high rate for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 4185–4191.

[15]

Y. Yao, H. Liu, G. Li, H. Peng, K. Chen, Multi-shelled porous LiNi0.5Mn1.5O4 microspheres as a 5 V cathode material for lithium-ion batteries, Mater. Chem. Phy. 143 (2014) 867–872.

[16]

X.-L. Cui, Y.-L. Li, S.-Y. Li, L.-X. Li, J.-L. Liu, Nanosized LiNi0.5Mn1.5O4 spinels synthesized by a high-oxidation-state manganese sol-gel method, Ionics 19 (2013) 1489– 1494.

[17]

E. Hosono, T. Saito, J. Hoshino, Y. Mizuno, M. Okubo, D. Asakura, K. Kagesawa, D.N. Hamane, T. Kudo, H. Zhou, Synthesis of LiNi0.5Mn1.5O4 and 0.5Li2MnO3– 0.5LiNi1/3Co1/3Mn1/3O2 hollow nanowires by electrospinning, Cryst. Eng. Comm. 15 (2013) 2592–2597.

18

[18]

N. Zhang, T. Yang, Y. Lang, K. Sun, A facile method to prepare hybrid LiNi0.5Mn1.5O4/C with enhanced rate performance, J. Alloy. Comp. 509 (2011) 3783–3786.

[19]

D.W. Shin, A. Manthiram, Surface-segregated, high-voltage spinel LiMn1.5Ni0.42Ga0.08O4 cathodes

with

superior

high-temperature

cyclability for

lithium-ion

batteries,

Electrochem. Commun. 13 (2011) 1213–1216. [20]

C. Zhu, T. Akiyama, Designed synthesis of LiNi0.5Mn1.5O4 hollow microspheres with superior electrochemical properties as high-voltage cathode materials for lithium-ion batteries, RSC Adv. 4 (2014) 10151–10156.

[21]

A. Manthiram, K. Chemelewski, E-S. Lee, A perspective on the high-voltage LiMn1.5Ni0.5O4 spinel cathode for lithium-ion batteries, Energy Environ. Sci. 7 (2014) 1339–1350.

[22]

K. Ghule, A.V. Ghule, B-J. Chen, Y-C. Ling, Preparation and characterization of ZnO nanoparticles coated paper and its antibacterial activity study, Green Chem. 8 (2006) 1034–1041.

[23]

K.G. Leea, J.-M. Jeonga, S.J. Leea, B. Yeomb, M.-K. Leea, B.G. Choi, Sonochemicalassisted synthesis of 3D graphene/nanoparticle foams and their application in supercapacitor,

Ultrason.

Sonochem.

(2014),

http://dx.doi.org/10.1016/j.ultsonch.2014.04.014. [24]

E. Malka , I. Perelshtein , A. Lipovsky , Y. Shalom , L. Naparstek, N. Perkas , T. Patick , R. Lubart , Y. Nitzan , E. Banin, A. Gedanken, Eradication of Multi-Drug Resistant Bacteria by a Novel Zn-doped CuO Nanocomposite, small 9 (2013) 4069–4076.

[25]

C. Cau, Y. Guari, T. Chave, J. Larionova, S.I. Nikitenko, Thermal and sonochemical synthesis of porous (Ce,Zr)O2 mixed oxides from metal b-diketonate precursors and their

19

catalytic activity in wet air oxidation process of formic acid, Ultrason. Sonochem.21 (2014) 1366–1373. [26]

B.G.S. Raj, A.M. Asiri, A.H. Qusti, J.J. Wu, S. Anandan, Sonochemically synthesized MnO2 nanoparticles as electrode material for supercapacitors, Ultrason. Sonochem.21 (2014) 1933–1938.

[27]

S. Khanjani, A. Morsali, Ultrasound-assisted coating of silk yarn with sphere-like Mn3O4 nanoparticles, Ultrason. Sonochem.20 (2013) 413–417.

[28]

V.D. Nithya, B. Hanitha, S. Surendran, D. Kalpana, R. KalaiSelvan, Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors,

Ultrason.

Sonochem.

(2014),

doi:

http://dx.doi.org/10.1016/j.ultsonch.2014.06.014. [29]

G. Applerot, A. Lipovsky, R. Dror, N. Perkas, Y. Nitzan, R. Lubart, A. Gedanken, Enhanced Antibacterial Activity of Nanocrystalline ZnO Due to Increased ROSMediated Cell Injury, Adv. Funct. Mater. 19 (2009) 842–852.

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Figure captions: Fig. 1. Schematic illustration of synthesis processes of LiNi0.5Mn1.5O4 by sonochemical method. Fig. 2. Photographs of sonochemical apparatus from two different angles. Fig. 3. Powder XRD patterns of sample-S1 and sample-S2. Fig. 4. TGA curves of sample-S1 and sample-S2. Fig. 5. SEM images of LiNi0.5Mn1.5O4 nanoparticles annealed at (a) 400 °C and (b) 800 °C without glucose SEM images of LiNi0.5Mn1.5O4 nanoparticles annealed at (c) 400 °C and (d) 800 °C with glucose. Fig. 6. TEM images of (a) sample-S1 and (b) sample-S2; HRTEM images of (c) sample-S1 and (d) sample-S2. Fig. 7. EDX spectra of (a) sample-S1 and (b) sample-S2. Fig. 8. Raman spectra of sample-S1 and sample-S2. Fig. 9. Charge discharge curves at mA g−1 (C/10) in the potential range of 3.5 - 4.9 V for LiNi0.5Mn1.5O4 synthesized (a) sample-S1 and (b) sample-S2. Fig. 10. Differential capacity plots measured in the potential range of 3.5 - 4.9 V for LiNi0.5Mn1.5O4 synthesized (a) sample-S1 and (b) sample-S2. Fig. 11. Cycle-life test by galvanostatic cycling at 14 mA g−1 (C/10) in the potential range of 3.5 - 4.9 V for LiNi0.5Mn1.5O4 electrodes in Li-cells.

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Fig. 12. Rate capability tests by galvanostatic cycling at different current densities (C/rates) in the potential range of 3.5 - 4.9 V for LiNi0.5Mn1.5O4 electrodes in Li-cells. Fig. 13. Nyquist plots of LiNi0.5Mn1.5O4 (i) sample-S1 and (ii) sample-S2 recorded at the ocp (OCV potential - 3.8 V) with amplitude of 5 mV in the frequency range 0.01 Hz - 100 kHz.

22

Figures:

Fig. 1

23

Fig. 2

24

Fig. 3

25

Fig. 4

26

Fig. 5

27

Fig. 6

28

Fig. 7

29

Fig. 8

30

+

Potential / V vs. Li/Li

5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0

(a) 1st 25th 50th

Potential / V vs. Li/Li

+

0

20

40

60

80

100 120 140 160 -1

Specific capacity / mAh g

5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0

(b) 1st 25th 50th

0

20

40

60

80

100

120 -1

Specific capacity / mAh g

Fig. 9

31

140

160

-1 -1

(dQ/dV) / mAh g V

-1 -1

(dQ/dV) / mAh g V

3000 (a) 2500 2000 1500 1000 500 0 -500 -1000 -1500 -2000 3.4 3.6 3000 (b) 2500 2000 1500 1000 500 0 -500 -1000 -1500 3.4 3.6

1 st 25 th 50 th

3.8 4.0 4.2 4.4 4.6 + Potential / V vs. Li/Li

4.8

5.0

4.8

5.0

1 st 25 th 50 th

3.8

4.0

4.2

4.4

4.6 +

Potential / V vs. Li/Li

Fig. 10

32

180 charge (0 mole glucose) discharge (0 mole glucose) charge (0.01 mole glucose) discharge (0.01 mole glucose)

Specific capacity / mAh g

-1

170 160 150 140 130 120 110 100 90 80

0

4

8

12

16

20 24 28 32 Cycle number

Fig. 11

33

36

40

44

48

52

160 charge (0 mole glucose) discharge (0 mole glucose) charge (0.01 mole glucose) discharge (0.01 mole glucose)

140 Specific capacity / mAh g

-1

C/10 120

C/10

C/5

100

C/2 C

80 60

2C 40 20

0

4

8

12 16 20 Cycle number

Fig. 12

34

24

28

32

500

0.0 mole glucose 0.01 mole glucose

400

0.01

-Z'' / Ω

300 0.027

200 0.01

0.19

100

1.0 3 3 5 5.18x10 2.68x10 1x10 0.05

0 71.9

0

100

37.2

1.0

200

Z' / Ω

Fig. 13

35

300

400

500

Highlights:


Sonochemical method is used to prepare high voltage spinel LiNi0.5Mn1.5O4
Sonochemistry has been proven to be green and cost-effective method
The ICP results of the molar ratios of Li:Ni:Mn are close to 2:1:3
LiNi0.5Mn1.5O4 is a promising cathode material for high voltage Li-ion batteries
Addition of glucose helps to enhance crystallinity and electrochemical activity

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