Combustion synthesis and characterization of Ni-doped LiMn2O4 cathode nanoparticles for lithium ion battery applications

In this research work, fine powders of spinel-type LiMn2-xNixO4-δ (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) as cathode materials for lithium ion batteries were synthesized by combustion synthesis using urea as fuel and metal nitrates as oxidizers at a temperature of 600°C. The physiochemical properties of the prepared cathode materials were investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), particle size analysis, energy dispersive analysis (EDAX) and scanning electron microscopy (SEM). The electrochemical characteristics were studied by impedance spectroscopy. It was found that the physical charactetertistics were moderately influenced because of different dopant (Ni) concentration. Among the samples studied, LiMn1.9Ni0.1O4-δ resulted in better electrical conductivity (6.49 x 10 -5 Scm -1 ) at room temperature and hence it may be suitable for lithium ion battery applications.


INTRODUCTION
Electrochemical devices, i.e, batteries can convert chemical energy into electrical energy. With increase in the consumption of electronic energy storages in our daily life usage such as portable electronic toys such as laptops, digital cameras, cellular phones and other usages like electric vehicles, hybrid vehicles, military and aerospace which have been developed and explored. The lithium-ion batteries have received much attention as the most viable and eco-friendly power source [1][2][3][4][5]. Furthermore it is an extreme challenging for renewable energy sources like wind and solar energy. The world market has valued billions of dollars for lithium-ion batteries in large scale storage [6][7][8][9]. The three dimensional crystal structure of LiMn 2 O 4 is one of the most promising cathode materials for its high abundance, low toxicity, high energy density and high excellent voltage characteristics [10][11][12]. The main usage of Li-ion battery technology reveals that itnot only possess high energy density also it is the most electropositive metal [13]. However, LiMn 2 O 4 faces some disadvantages by fading in capacity on storage and charge-discharge cycling at certain temperature [14]. Researchers noticed the drawback in loss of capacity of LiMn 2 O 4 is because of LiMn 2 O 4 include dissolution of a disproportionation of Mn 3+ into the electrolyte [15], 2Mn 3+ → Mn 2+ + Mn 4+ at high electrode potential, electrolyte decomposition [16]. In order to improve the performance of LiMn 2 O 4 needs a further improvement by doping divalent or trivalent LiM x Mn 2-x O 4 spinel phase (m = Co, Ni, Fe, Cr) including by various synthesis such as sol-gel method [17], one-step precipitation method [18], solid-state reaction method [19] and combustion method [20]. Among the above methods, combustion method is a promising technique, which is based on a highly exothermic, self-sustaining reaction generated by heating solution mixture of aqueous metal salts with fuels, such as, urea, glucose, glycine and citric acid. This method has been used efficiently to prepare a variety of oxide materials for the application of energy storage devices such as fuel cells, super capacitors and batteries. It not only yields nanomaterials with very high surface areas but also enables uni-form (homogenous) doping of trace amounts of various elements in a single step [21][22][23].
Yu and Zhou have studied the effect of sintering temperature on structure and electrochemical properties of LiMn 2 O 4 [24]. It was reported that LiMn 2 O 4 suffers from the surface dissolution of manganese in the electrolyte at elevated temperature, especially above 60 °C, which leads to a severe capacity fading. To overcome this barrier, LEE et al. [25] have developed an imaginative material design; a novel heterostructure. LiMn 2 O 4 with epitaxially grown layered (R3 m) surface phase. Mg 2+ and Ti 4+ co-doped spinel LiMn 2 O 4 lithium-ion cathode material was prepared via a simple high-temperature solid-state route, presenting the high specific capacity, upgraded cyclability, and enhanced rate capability contemporaneously [26]. Among the various dopants reported literature, Ni doping in LiMn 2 O 4 is regarded as a typical method to enhance the structural stability and increase in the electrochemical performance of the LiMn 2 O 4 material [27]. The importance of doping Ni and also other metals in the LiMn 2 O 4 was to weaken the Jahn-Teller effect which results in the improvement of stability of the material and also in the cyclic performance even after 500 cycles [28].
In this research paper, we describe the urea-nitrate based combustion synthesis of Ni doped LiMn 2 O 4 fine particles, LiMn 2-x Ni x O 4-δ (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) and their physio-chemical / electrochemical characterization for use as cathode material in lithium-ion batteries.

Synthesis of LiMn2-xNixO4-δ nanoparticles
In the typical experiment, stoichiometric amounts of lithium nitrate, manganese nitrate, nickel nitrate were calculated based on propellant chemistry calculations [29] and dissolved in a minimum quantity of distilled water (approximately, 20 ml) along with appropriate quantity of urea as organic fuel. The mixed solution was heated in a mantle at 50 -70 o C and the volume was reduced to half. Afterwards, the solution was introduced into a muffle furnace maintained at 600 o C where it boiled, frothed, ignited and caught fire (temperature rise up to 1100 ± 100 o C). At these high temperatures, the metal nitrates decomposed to metal oxides of nitrogen and hence acted as oxidizer for further combustion which led to voluminous foamy combustion residue within 5 -10 minutes.The flame persisted for about 1 minute. The foam was them lightly ground in glass mortar with pestle to obtain fine nanoparticles. The stoichiometric proportion of precursor materials used for the synthesis of Ni doped LiMn 2 O 4 oxide nanoparticles is indicated in Table 1.
The mechanism for the above reaction is reported as follows in the literature [30]. The metal nitratefuel mixture reaction involve dehydration, decomposition, swelling and burn. When urea (CO(NH 2 ) 2 ) is used as a fuel, the probable mechanism involves melting and dehydration in the first few minutes, then the mixture decomposes, with frothing that may be due to the formation of metal-(OH)(NO 3 ) 2 gel alongwith other products urea nitrate (CH 5 N 3 O 4 ), (H 2 N-CO-NH-CO-NH 2 ) (biuret), HNCO, and NH 3 . It then foams, due to the gaseous decomposition products of the intermediates, causing enormous swelling of the reaction product. The gaseous decomposition product is a mixture of N 2 , NH 3 and HNCO, which are combustible.
Finally, the accumulation of the combustible mixture of gases causes the foam to burst into flame and burn into incandescence, with further swelling, producing a doped oxide powder.

Physical characterization
The powder XRD study was carried out using a Shimadzu XRD6000 X-ray diffractometer at a scan speed of 5 deg/min using CuK radiation. The crystallite sizes of the ceramic powders were calculated by Scherrer"s formula. FTIR spectra of all the samples were studied by Shimadzu IR Prestige -21 model FTIR spectrometer. The particle size of the powder was measured using Malvern particle size analyzer (Malvern Instruments, Worcestershire, UK) using triple distilled water as medium. The morphology of the particles and percentage of elements present in the samples (EDAX) was studied by means of JEOL Model JSM-6360 scanning electron microscope (JEOL Ltd., Tokyo, Japan).

Electrochemical characterization
The resultant Ni doped LiMn 2 O 4 materials was ground into a fine powder with addition of PVA binder solution, mixed well, dried and placed in a die. A pressure of around 4000 kg/cm 2 was been applied to form the pellet with a thickness in the range of 1.4-1.5 cm and a diameter of 1 cm. After this process, the pellets were sintered at 600°C for 6 hrs before subjecting them for conductivity studies. The electrochemical impedance studies were conducted using an electrochemical work station with two electrode system under aluminium foil substrate in the frequency range of 40 Hz -1 MHz at room temperature.

K λ D = ------------
(2) β cos θ where "D" is the crystalllite size, "k" is the numerical constant (~0.9), "λ" is the wavelength of x-rays (for CuKα radiation, λ = 1.5418 Å), "β" is the effective broadening taken as a full width at half maximum (FWHM) (in radians), "θ" is the diffraction angle for the peak. The calculated average crystallite size is found to be 10.6 to 24.7 nm respectively. Crystallography parameters obtained on Ni doped LiMn 2 O 4 nanoparticles are given in Table.2. The XRD data of LiMn 2-x Ni x O 4-δ is in line with the reported data [31]. From this, we could understand that when high concentration of dopants is added the intensity of peaks gets increased, however, pure LiMn 2 O 4 resulted with low intensity peaks. Therefore, the crystalline behavior of the materials is highly dependent on dopant concentration. LiMn 2 O 4 with high dopant concentration will have high crystalline characteristics than others.

Particle size measurements
The particle size patterns of the Ni doped LiMn 2 O 4 nanoparticles prepared by combustion technique are shown in Figure 3.For all the measurements, 0.01g of sample was sonicated in 30 ml triple distilled water for about 10 minutes and after that the sample was subjected for particle size analysis. The particle size distribution data is indicated in Table.3. The particle are present in the range of 234 -329 nm. The presence of bigger particles (> 200 nm) in the sample may be due to high temperature treatment.

SEM studies
The SEM photographs of the Ni doped LiMn 2 O 4 nanoparticles prepared by combustion technique are displayed in Figure 4.    The pellets were kept in between the conducting aluminium foils like a sandwich and the measurements were made in air. From the impedance data conductivity values were calculated by using Equation.  Figure 7.

Electrochemical impedance studies
The impedance spectra was fitted to the conventional equivalent electronic circuit containing three Resistance-Constant Phase Element (R-CPE) sub circuits in series, which generates two semicircles on the Nyquist plots at the room temperature. The electrical conduction of Ni doped LiMn 2 O 4 based materials results from impurity and intrinsic factors. At room temperatures, its conduction is dominated by the dissociated electron concentration from the energy gap of the impurity, whose activation energy of electrical conduction is much lower than that of the intrinsic conduction [34][35][36]. The conductivity decreases is predominantly due to the addition of nickel. The electrons from the energy gap of the impurity are all dissociated and activated.   (Where, the symbol referred as capacitor (constant phase element, CPE) and the symbol referred as resistor) The bulk conductivity was calculated from the impedance plot and reported in the