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In the present work, by keeping in view of the above, we present a systematic investigation on the structural, optical, electrical and electrochemical properties of spinel LiMn2O4 cathode materials synthesized via the sol-gel technique under optimal conditions at different pH values (3, 6 and 9) without any surfactant for the application of rechargeable lithium-ion batteries.
Journal of Science: Advanced Materials and Devices (2019) 245e251 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Effect of pH values on structural, optical, electrical and electrochemical properties of spinel LiMn2O4 cathode materials Prakash Chand a, *, Vivek Bansal a, Sukriti a, Vishal Singh b a b Department of Physics, National Institute of Technology, Kurukshetra, 136119, India Centre for Materials Science & Engineering, National Institute of Technology, Hamirpur, 177005, India a r t i c l e i n f o a b s t r a c t Article history: Received 15 October 2018 Received in revised form 16 April 2019 Accepted 21 April 2019 Available online 25 April 2019 In the present work, we have synthesized the spinel LiMn2O4 cathode materials via a sol-gel method at 750 C for h under optimal conditions at different pH values (3, and 9) and studied the effect of different pH values on the structural, optical, electrical and electrochemical properties X-ray diffraction (XRD) analysis identified the synthesized materials as crystallized in the cubic spinel structure (Fd3m) with slight decrease in the lattice parameters SEM exhibits the formation of a spongy and fragile network structure in the synthesized samples An enhancement in the optical energy band (Eg) leads to the blue shift in the synthesized samples with reduced crystallite size Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) investigations show that the LiMn2O4 nanostructures synthesized at pH exhibit the long-term cycle constancy and a superior electrochemical reproducibility as compared to those synthesized at pH values of and The results revealed that pH plays a significant role in tuning the structural, optical and electrochemical properties of the LiMn2O4 cathode material, which is considered a promising substitute of cathode materials for the novel lithium-ion battery applications © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Nanostructures LiMn2O4 Cathode materials Cyclic voltammetry Introduction In recent years, there is a swift development of digital technologies in diverse fields of technology and production, electric vehicles and, space applications as well as of portable user electronic devices, for instance, laptops, cell phones, digital cameras etc Even the majority of the electronic devices have become instrumental in managing the everyday activities The prompt augmentation of such electronic devices obviously stimulates immense attention on cheap, light-weight, eco-friendly, safe and, high energy density battery materials, for both economic and environmental benefits [1e4] To congregate the increasing energy demands of the modern society and the potential ecological anxiety, Li-ion battery technology has the potential to meet the requirements of high energy compactness and high dominance density applications Recently, researchers and the scientific community are interested in the LiMn2O4 cathode material with a three-dimensional framework for the application of rechargeable Li-ion batteries It has numerous advantages, such as abundant resources, non-toxic in nature, low cost, simple * Corresponding author E-mail address: kk_pc2006@yahoo.com (P Chand) Peer review under responsibility of Vietnam National University, Hanoi preparation, environmental friendliness and superior safety in comparison to some layered oxides, for instance, LiCoO2 and LiNiO2 [2,3] Spinel LiMn2O4 with the 3D tunnel structure (space group Fd3m) consists of a cubic close-packed array in which the oxygen ions are positioned at the 32e sites and the Li ions in the tetrahedral 8a sites, whereas, the Mn3ỵ and Mn4ỵ ions are placed at the octahedral 16d sites [1,5] At present, the prime challenges for the development of Li-ion batteries for the mass market are price, safety, energy and, power densities, charging and discharging rate and, service life Thus, the development and investigation of LiMn2O4 nanostructured cathode materials are very important, in view of the future progress in the battery industry To meet up, for such global relevance, it has become abundantly apparent that the design and fabrication of electrode materials of Li-ion batteries (LIBs) play an important role to adapt the increasing worldwide demand for energy Various properties such as the crystallite size, the stoichiometry and the homogeneity govern the electrochemical properties of electrode materials The small particle size will improve the recycleability and the rate capability of the cathode materials [6] Arof et al observed that the variation in the synthesis process of LiMn2O4 using tartaric acid introduced impurities that affect the specific capacity of the cell [7] Santiago et al reported two reversible cyclic voltammograms for spinel LiMn2O4, synthesized by the combustion https://doi.org/10.1016/j.jsamd.2019.04.005 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 246 P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 process [8] However, for the large scale power and energy storage application of LIBs, the price, safety, environmental friendliness and, long stability of the electrode materials are of major concerns The performance of LIBs depends upon a number of factors, including the properties of the anode, the cathode, and the electrolyte Hence, an improvement in the capacity of the cathode material has a larger upshot on the volume, and consequently, the energy density of a lithium-ion battery Further, for subsequent applications, the chemio-physical properties of the material are strongly dependent on its dimension, morphologies, surface area and occasionally, on the synthesis process and conditions throughout the processing procedures The augmented convention of materials is significantly affected by the peripheral conditions, such as temperature, precursor concentration and pH value of the precursor The literature investigation stipulates that the pH of the precursor solution appears to be a significant constraint for the crystal structure development, particles dimension and morphology of the final product through the sol-gel synthesis [9,10] Therefore, in the present work, by keeping in view of the above, we present a systematic investigation on the structural, optical, electrical and electrochemical properties of spinel LiMn2O4 cathode materials synthesized via the sol-gel technique under optimal conditions at different pH values (3, and 9) without any surfactant for the application of rechargeable lithium-ion batteries Photoluminescence (PL), Fourier Transform Infra Red (FTIR) and UV-Visible spectra were recorded at room temperature to explore the optical properties of LiMn2O4 nanostructures, synthesized via the sol-gel method at different pH values The current-voltage (IeV) measurements were carried out to examine the electrical property of the LiMn2O4 nanostructures on Ag-layered pellets through a setup of Keithley (two-probe Model) In order to assess the cycling behavior of the synthesized cathode materials, the cyclic voltammetry measurements were done using the Biologic SP-240 Potentiostat considering three electrode configurations For the electrochemical performance of LiMn2O4 nanostructures, the working electrode was prepared by a combination of 80 wt.% of the prepared LiMn2O4, 10 wt.% of acetylene black as a conductor, and 10 wt.% of polyvinylidene difluoride (PVDF) as a binder in Nmethyl-2-pyrrolidone (NMP) The obtained slurry was extended on a Ni foil and dehydrated at 120 C for 12 h The cells consisted of the LiMn2O4 composites which act as the positive electrode, while a Pt electrode as the negative electrode and an electrolyte composed of 2M KOH solution in DI water Results and discussion 3.1 X-ray diffraction studies Experimental 2.1 Chemicals All the chemical reagents used in the present work for the synthesis of LiMn2O4 nanostructures were of analytical grade and were utilized without further purification For the synthesis of LiMn2O4 at different pH values, CH3COOLi, Mn(CH3COO)2, C6H8O7 (citric acid) and Zn(CH3COO)2, NaOH, Ammonia solution and deionized H2O were used as precursor materials 2.2 Synthesis LiMn2O4 spinel nanostructured cathode materials were synthesized via the sol-gel technique C6H8O7 was used as a chelating mediator in the synthesis process For synthesizing LiMn2O4, lithium acetate (1 mol), manganese acetate (2 mol), and citric acid (3 mol) were independently liquified in 50 mL deionized (DI) water Further, such solutions were mixed collectively to make a final solution of 150 mL The molar ratio of C6H8O7 to metal ions was Initially, due to the presence of citric acid, the pH value of the solution was low (4e5) To maintain the pH at 3, citric acid was gently mixed to this solution with constant stirring using a magnetic stirrer In order to maintain pH at and 9, NH3 solution was gradually mixed to this solution with constant stirring The resulting solution was heated at 60 C, stirred with a magnetic stirrer for h until a gel was formed The gel precursor achieved was dried in an electric oven for 12 h at 120 C to get rid of the moisture and thus, obtain the dry powder The obtained dry fine particles were then calcined at 450 C for h in a tubular furnace in air and then, at 750 C for h to get a fine black colored powder of LiMn2O4 nanoparticles XRD study has been carried out on LiMn2O4 nanostructures prepared at different values of pH to find out the effect of pH on the structural properties of these materials Fig illustrates the XRD patterns of the LiMn2O4 nanostructures synthesized by the sol-gel technique The recorded X-ray diffraction peaks could be indexed as (111), (311), (222), (400), (331), (511), (440), (531), (533) and (622) Miller planes which validate the configuration of the single-phase cubic spinel crystal structure having the space group Fd3m (JCPDS card no 35-782) [3] The observed broadening of the XRD peaks is an indication of the grain size of the synthesized samples in the nano range The average crystallite size of the LiMn2O4 nanostructures was determined from the broadening of XRD peaks via the Scherrer's formula [11]: 2.3 Characterization The crystal structure and phase purity analysis of the synthesized samples was performed by X-ray diffraction measurement (Rigaku diffractometer) with Cu (Ka) radiation source of the wavelength 1.54 Å The surface morphology investigation was carried out by Scanning Electron Microscopy (SEM) Fig Room temperature XRD patterns of LiMn2O4 nanostructures synthesized at different pH values P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 D¼ kl b cosq (1) where, the shape factor (k), is approximately 0.89, the crystallite size is denoted as D, and l is the wavelength of the X-ray radiation (Cu-Ka) used ¼ 1.542 Å, b is the full width at half maxima (FWHM) and q is the Bragg's angle in radians The average crystallite size, as determined from the (111) high intensity reflections, came out to be 46, 38 and 32 nm, respectively, for LiMn2O4 nanostructures synthesized at different pH values of 3, and 9, respectively It has been found that the crystallite size decreases as we increase pH values from to The lattice constant (a) and volume (V) for the LiMn2O4 nanostructures prepared at different pH values were estimated by using the following equations [12e14]: l aẳ 2sinq p h2 ỵ k2 ỵ l2 (2) and V ¼ a3 (3) here, l is the wavelength of the X-ray radiation (Cu-Ka) used ¼ 1.542 Å, q is the Bragg's angle in radians and, h, k, l are the Miller indices The changes in the crystallite size and lattice parameters of the LiMn2O4 nanostructures prepared at different pH values are depicted in Fig It is observed that the size decreases with an increase in the pH values and the lattice constants also vary with the pH values It is well known that the specific surface area (S), the X-ray density (dx) and the bulk density (dB) play an extensive role in the alteration of the structural properties of the cubic spinel structure The specific surface area (S) and the X-ray density (dx) of the LiMn2O4 nanostructures prepared at different pH values was calculated by using the relation given below [13]: S¼ Â 103 Ddx dx ¼ 8M 6:022 Â 10 23 Â a3 (4) (5) here, M is the molecular mass of the sample The specific surface area (S) is observed to be increasing with the reduction in the crystallite size As the crystallite size decreases, the surface to 247 volume ratio increases and consequently, the specific surface area (S) increases The surface area of the electrode material is a significant characteristic constraint that establishes the energy and power density of a particular battery system In the present study the surface area as obtained is higher for the LiMn2O4 cathode material synthesized at pH because of its smaller crystallite size The bulk density (dB) of LiMn2O4 nanostructures was estimated from the following equation [13]: dB ¼ m pr2 h (6) The porosity (P) of the LiMn2O4 nanostructures was calculated by using the formula as below: p¼1À dB dX (7) The results of measurements of the crystallite size, the lattice parameters, the cell volume and the variation in all calculated parameters, i.e dx, S, dB, and P for the LiMn2O4 nanostructures are presented in Table 3.2 Scanning electron microscopy study The surface morphologies of the spinel LiMn2O4 nanostructures were observed by SEM Fig 3(aec) shows the SEM images of the LiMn2O4 nanostructures prepared at the different pH values of 3, and 9, respectively, which clearly show significant changes in the nanostructures and in the porosity The development of the spongy and fragile network structure is easily visible The sample consists of round-shaped particles, since these particles were prepared by the sol-gel technique The voids and pores, as manifested in the synthesized nanomaterials, are endorsed which may be due to the liberation of a huge volume of gases through the combustion process 3.3 Photoluminescence (PL) spectroscopy analysis To study the optical properties of the spinel LiMn2O4 nanostructures, photoluminescence (PL) spectra at room temperature were recorded by using the Xenon lamp light as the irradiation source for all the samples prepared at different pH values of 3, and Fig depicts the PL spectra of the LiMn2O4 spinel nanostructures prepared at different pH values For the excitation wavelength of 320 nm, the emission spectrum gives two peaks, one around 376 and the other around 473 nm The broad peak in the UV emission region appeared around 376 nm may be endorsed due to the near band edge (NBE) emission which originates through the free exciton recombination from the conduction band (CB) to the valence band (VB) This indicates that the LiMn2O4 nanostructures have a weak photoluminescence property due to the forbidden spin of Mn2ỵ (3d5) [15] A visible emission peak observed around 473 nm is related to the structural imperfections, present in the LiMn2O4 nanostructures as well as to the recombination of holes and electrons in the VB and CB The PL intensity is the highest for the samples synthesized at pH ¼ and lowest for that at pH also indicating the variation in the surface defects with the change in the crystallinity of the synthesized samples 3.4 Fourier transform Infra-red (FTIR) studies Fig Variation of lattice constant (a) and crystallite size (D) of LiMn2O4 nanostructures with pH values In order to investigate the vibrational and functional groups present in the as synthesized samples, FTIR spectra of the LiMn2O4 nanostructures were recorded at room temperature Fig shows the FTIR spectra of the LiMn2O4 nanostructures prepared at 248 P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 Table Variation of the crystallite size (D), the Lattice parameters (a), Unit cell volume (V), X-ray density (dx), Specific surface area (S), Bulk density (d), Porosity (P) and the optical energy band gap (Eg) of LiMn2O4 nanostructures synthesized at different pH values pH values (hkl) Average Crystallite Size (D) (nm) Lattice Constant (a) (Å) Volume of unit cell (V) (Å)3 X-ray density (dx) (g/cm3) Specific surface area (S) (m2/g) Bulk density (dB) (g/cm3) Porosity (P) Optical energy band gap (Eg) (eV) (111) (111) (111) 46 38 32 8.27 8.26 8.25 566 563 561 4.24 4.23 4.27 30.70 37.22 44.18 0.57 0.69 0.71 0.86 0.83 0.83 3.86 3.95 4.06 Fig (aec): SEM images of LiMn2O4 nanostructures synthesized at different pH values (a) pH ¼ (b) pH ¼ (c) pH ¼ Fig Room-temperature photoluminescence (PL) spectra of LiMn2O4 nanostructures synthesized at different pH values Fig Room-temperature FTIR spectra of the LiMn2O4 nanostructures synthesized at different pH values P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 different pH values of 3, and The spectra were recorded in the range between 500 and 1200 cmÀ1 It is evident from the FTIR spectra that two broad infrared spectral bands are observed One lies around 565 cmÀ1 and the other around 617 cmÀ1 that can be assigned to the LieO bending and the LieMneO stretching vibration band, respectively [16] In the FTIR spectra, the characteristic peaks appearing below 1500 cmÀ1 confirm the presence of the metal-oxygen vibration band Hence, the FTIR analysis confirms the phase formation and the functional groups present in the LiMn2O4 nanostructures, which is in accordance with the XRD results 3.5 UV-visible absorption studies To investigate the optical properties of the spinel lithium manganese oxide nanostructures, room temperature UVeVisible absorption spectroscopy was employed It is well-known that the absorbance of nanomaterials relates to the energy band gap and depends on the defects of the surface The electronic structure of the material governs its optical properties, which in turn determine the material's light absorption The absorption data, therefore, play a vital role in the evaluation of the energy gap The energy gap of the as prepared LiMn2O4 nanostructures were estimated through the absorbance versus wavelength data The as-prepared nanomaterial was extensively diluted in distilled H2O and then, its UVeVisible absorbance spectra were recorded Various models were proposed to study the optical properties of the synthesized samples, although, the most familiar was the Tauc's model that allows to derive the energy gap (Eg) from the (ahn)2 versus (hn) plot [17] Fig depicts the Tauc plot of the absorbance spectrum of LiMn2O4 spinel nanostructures recorded in the range 200e800 nm From this, the energy gap of the LiMn2O4 nanostructures, prepared at different pH values of 3, and 9, were found as 3.86, 3.95 and 4.06 eV, respectively The estimated band gap values are found to be increased with the increasing pH values The enhancement in the energy gap (Eg) of the LiMn2O4 nanostructures with the increase in the pH values may be related to the decrease in the crystallite size Fig Plot of (a∙h∙n) versus photon energy (h∙n) for the LiMn2O4 nanostructures synthesized at different pH values The insets shows the plot of absorption versus wavelength spectra of as-synthesized LiMn2O4 nanostructures 249 3.6 IeV characteristics To determine the electrical properties of the as synthesized LiMn O spinel nanostructures, the current-voltage (IeV) characteristics were performed on the Ag-layered pellets using a Keithley two-probe set-up Fig shows the IeV curves of the LiMn O samples prepared at different pH values It is seen that the synthesized samples obey the Ohm's law and show the conducting nature From the slope of the IeV graph, we can determine the resistance of the synthesized samples The estimated values of the resistance are 207, 143 and 26 k U for LiMn O nanostructures synthesized at different pH values of 3, and 9, respectively These results show that with the increasing pH values in the synthesis process, the resistance of the corresponding LiMn O nanostructures is decreased The decrease in the sample's resistance is correlating with the crystallite size also 3.7 Electrochemical impedance spectrum studies In order to investigate the effect of pH values on the electrochemical cycling performance of the LiMn2O4 nanostructures, the electrochemical behavior of as-synthesized LiMn2O4 nanostructures was studied by the Electrochemical Impedance Spectroscopy (EIS) using a potentiostat and the recorded spectrum is depicted in Fig EIS was carried out to examine the electrode resistance and impedance change in the as synthesized LiMn2O4 nanostructures The Nyquist plots of LiMn2O4 nanostructures prepared at different pH values of 3, and are shown in Fig The recorded impedance spectra reveal a depressed and a spike arc in the high-frequency and the low-frequency region, respectively The intercept at the real impedance axis corresponds to the ohmic resistance, whereas, the arc corresponds to a charge transfer resistance and a binary layer capacitance of a parallel combination The charge transfer resistance value is premeditated through the real axis by the diameter of the arc A spike provides information about the Warburg impedance as attained in the low-frequency section, that is linked to the diffusion in lithium-ion particles The impedance was found to be 298, 225, and 210 U-1, for the LiMn2O4 nanostructures synthesized at the different pH values of 3, and 9, respectively, reaving clearly that the impedance of the LiMn2O4 Fig I-V characteristics of LiMn2O4 nanostructures synthesized at different pH values 250 P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 Fig Nyquist plots for LiMn2O4 nanostructures synthesized at different pH values nanostructures decreases with the increasing pH values in the synthesis procedure 3.8 Cyclic voltammetry studies A study of the electrochemical behavior of the as-synthesized LiMn2O4 nanostructure was performed by the measurements using a potentiostat and the results are shown in Fig 9(aec) Cyclic voltammograms were measured at a scan rate of mV/s in 2M KOH for the potential window from V to 0.5 V The anodic peaks observed in the cyclic voltammograms of the as-synthesized LiMn2O4 nanostructures correspond to the lithium extraction whereas the cathodic peaks observed correspond to the lithium insertion The anodic peak is the evident for the elimination of the Li ions from the tetrahedral sites, where the LieLi interactions have occurred The possible inconsistency between the oxidation and reduction peaks may be seen in the values of 80, 90 and 71 mV, respectively, for the LiMn2O4 spinel nanostructures synthesized at different pH values of 3, and The LiMn2O4 sample prepared with pH ¼ shows a smaller potential difference between the anodic and the cathodic peak as compared to those observed in the LiMn2O4 samples with pH ¼ and 6, indicating that the reversibility of LiMn2O4 synthesized at pH ¼ is much better than that of the other samples, as it is shown in Fig It is clearly to see in this figure that the reversibility of the synthesized materials increases with the increasing pH value used in the synthesis procedure The peak current values are seen as 77, 90 and 110 mA, respectively, for the LiMn2O4 samples prepared with different pH values of 3, and 9, indicating that the peak current in the LiMn2O4 nanostructures increases with the increase in the pH values 3.9 Efficiency study The cycling lifetime of the as-synthesized LiMn2O4 electrodes materials was examined via a galvanic charge/discharge measurement at AgÀ1 in a M KOH electrolyte Fig 10 depictes the plots showing the efficiencies versus the cycling numbers for all the electrodes in the study (up to 300 cycles) The recorded efficiency for the LiMn2O4 nanostructures prepared with different pH values Fig (aec): Cyclic voltammetry studies of LiMn2O4 nanostructures synthesized at different pH values P Chand et al / Journal of Science: Advanced Materials and Devices (2019) 245e251 251 and Hence, our the present study has revealed that the pH plays an important role in tuning the structural, optical, electrical and electrochemical properties of the spinel LiMn2O4 cathode material This material also is considered as a potential alternative of cathode materials for novel lithium-ion battery applications Acknowledgments The authors would like to thank the Director of the NIT Kurukshetra for providing the facilities in the Physics Department for this study References Fig 10 Efficiency studies of LiMn2O4 nanostructures synthesized at different pH values of 3, and at the 50th cycle was 65, 70 and 78%, respectively The efficiency as a function of cycle number was estimated using the following relation [18] Efficiency %ị ẳ Td 100 Tc (8) here, Td and Tc are discharge and charge temperatures As it is seen in Fig 10, there is an increase in the efficiency recorded over up to 300 cycles observed for the LiMn2O4 nanostructures synthesized at different pH values of 3, and 9, from 70, 76, 83% respectively This imlplies that the LiMn2O4 nanostructures synthesized at pH exhibit the long-term cycle constancy and also superior electrochemical reproducibility as compared to the ones synthesized at pH values and Conclusion In summary, the spinel LiMn2O4 cathode materials were successfully prepared via the sol-gel technique XRD analysis has revealed that all the samples synthesized at different pH values were identified as the spinel structure of LiMn2O4 with space group Fd3m The lattice parameters have been observed to slightly decrease with the increasing pH values from to SEM studies have shown the spongy and fragile network type morphology of the nanostructures PL and FTIR spectra also confirm the phase formation of LiMn2O4 An enhancement in the optical energy band gap (Eg) from 3.86 eV to 4.06 eV has been observed for the asprepared LiMn2O4 nanostructures with the increase in pH values This exhibits the blue shift in the synthesized samples with the reduction in the crystallite size The EIS and CV examination studies have revealed the long-term cycle constancy and superior electrochemical reproducibility of the LiMn2O4 nanostructures synthesized at pH as compared to those samples synthesized at pH [1] Q Liu, S Wang, H Tan, Z Yang, J Zeng, Preparation and doping mode of doped LiMn2O4 for Li-ion batteries, Energies (2013) 1718e1730 [2] A Iturrondobeitia, A Goni, V Palomares, I Gil de Muro, L Lezama, T Rojo, Effect of doping LiMn2O4 spinel with a tetravalent species such as Si(IV) versus with a trivalent species such as 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and use as a cathode in lithium ion batteries, Nano Res (2010) 733e742 ... strongly dependent on its dimension, morphologies, surface area and occasionally, on the synthesis process and conditions throughout the processing procedures The augmented convention of materials. .. work, by keeping in view of the above, we present a systematic investigation on the structural, optical, electrical and electrochemical properties of spinel LiMn2O4 cathode materials synthesized... powder of LiMn2O4 nanoparticles XRD study has been carried out on LiMn2O4 nanostructures prepared at different values of pH to find out the effect of pH on the structural properties of these materials