LIQUID-COMBUSTION SYNTHESIS OF SPINEL LiMn 2 O 4 CATHODE MATERIAL FOR ENHANCED ELECTROCHEMICAL PERFORMANCE LITHIUM-ION BATTERIES

. A cubic spinel LiMn 2 O 4 cathode material with nanoscale was synthesized via a facile liquid-combustion method at 600, 750 and 850 ℃. The crystal structure and surface morphology of LiMn 2 O 4 electrodes have been characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technique. The samples have an average diameter of 200 nm with an octahedral structure with high crystallinity. Benefiting from the excellent structure, the sample prepared at 750 ℃ delivers the highest first discharge capacity of 115.3 mAh·g −1 with remaining 101.5 mAh·g −1 after 400 cycles at 1 C. Moreover, it exhibits a good cycling performance at 55 ℃, still had the capacity of 105.4 mAh·g −1 with a capacity retention of 71.2% following 282 cycles. The facilely synthesized octahedral LiMn 2 O 4 provides a practical application strategy for lithium-ion batteries.


INTRODUCTION
Nowadays, rechargeable lithium-ion batteries (LIBs) have promoted renewable energy development and achieved impressive success in application fields such as essential energy storage devices as well as electric vehicles (EVs) and hybrid electric vehicles (HEVs).The electrochemical performance of LIBs is mainly determined by the cathode materials [1,2].Considering the high abundance, environmental friendliness, low cost, fine safety and abundant manganese resources, spinel LiMn2O4 has been intensively studied as one of the most promising LIBs cathode materials.In particular, spinel LiMn2O4 has high operating voltage and a robust structure with a three-dimensional network, which can provide excellent Li + /electron transport channels during Li + insert/extraction and high operation potential [3][4][5].However, it still suffers from structural transformation and capacity fading caused by Mn dissolution, the Jahn-Teller effect and electrolyte decomposition [4,5].These problems also severally limit its large-scale application.
The conventional solid-state reaction is widely used to prepare LiMn2O4 materials.However, the drawbacks of this method, such as long calcination time, slow reaction kinetics and impure phase product have constrained its practical application [6][7][8][9][10].Therefore, various researches and approaches have been studied to overcome above drawbacks and enhance performance of LiMn2O4 materials, such as liquid state reactions, sol-gel method, Pechini method, spray drying method and combustion synthesis [11][12][13][14][15][16][17][18][19][20][21][22][23].Zhao et al. used the sol-gel technology to prepare the Er-doped LiMn2O4 cathode materials.This sample delivered a reversible capacity of 130.2 mAh•g −1 with an excellent retention of 95.2% after 100 cycles at 0.5 C [11].Yang and co-workers synthesized a two-stage calcination Co-doped LiMn2O4 sample which revealed an initial specific discharge capacity of 121.9 mAh•g −1 at 0.2 C and a capacity retention of 80.6% after 60 cycles [19].Palaniyandy and co-workers successfully prepared the renovated spinel LiMn2O4 material, and it exhibited a better cycling stability with the discharge capacity of 56 mAh•g −1 after 500 cycles at 1 C [20].Huang and co-workers adopted a carbohydrate-assisted combustion method synthesized LiMn2O4 spinel that showed a good cycling stability of 92% capacity retention after 100 charge/discharge cycles [21].Yao and co-workers prepared the LiMn2O4 cathode material directly from the leaching solution of waste LIBs, which avoided the complex separation process [22].Nakajima and co-workers used calorimetry to measure the interfacial energies of LiMn2O4 nanoparticles for the first time, and they found that Sc-doped LiMn2O4 has lower surface energy significantly than undoped LiMn2O4 likely because of the dopant's segregation to grain boundaries affecting the distribution of Li + across the material [23].
In this work, we demonstrate a liquid-state combustion synthesis to prepare spinel LiMn2O4 cathode material, in which a solution mixture of aqueous metal salts and nitric acid used as the fuel generated a self-combustion reaction at temperature from 600 to 850 ℃ in air atmosphere.Based on this method, the LiMn2O4 samples showed not only pure single-phase spinel product with high crystallinity but also high cell-cycling and electrochemical performance.Specially, it provides an efficient method with facile process, time-consuming and uniform distribution particles for spinel LiMn2O4 cathode material in lithium-ion batteries.

Spinel LiMn2O4 preparation
The spinel LiMn2O4 cathode material was prepared by using a liquid-state combustion method.In a typical synthesis required for preparing 3 g LiMn2O4 target product, the reactor consisting of LiNO3 (AR, Aladdin, 99.0%) and Mn(CH3COO)24H2O (AR, Aladdin, 99.0%) were accurately weighted according to a molar ratio of Li:Mn = 1:2 and dissolved in the nitric acid with a concentration of 9 mol/L in a crucible.The mixture was heated in an oven at 100 ℃ to form a homogenous solution.To obtain spinel LiMn2O4, the as-prepared solutions were calcined at 600 ℃ (LMO-T600), 750 ℃ (LMO-T750) and 850 ℃ (LMO-T850) for 3 h, respectively.Through an autoxidative combustion reaction, the products were cooled off to room temperature and grounded to form spinel LiMn2O4 samples.

Characterization of materials
X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer with Cu K radiation, and 2 data range collected from 10° to 70°.The morphology and lattice structure of the samples were characterized by field emission scanning electron microscope (FE-SEM, NOVA NANOSEM 450, America FEI) and transmission electron microscope (TEM, JEM-2100).

Electrochemical measurements
The electrochemical properties of the samples were carried out using CR2025 coin-type cells assembled in a glove box (MBraun, Germany) filled with high purity argon gas.The cathode slurry was fabricated by ball-mill mixing the active material, polyvinylidene fluoride (PVDF) and carbon black with a weight ratio of 8:1:1 dissolved in N-methyl-2-pyrrolidone (NMP) solvent, and the weight of active material in each electrode was about 2 mg.Then, the slurry was homogeneously coated onto an aluminum foil current collector using doctor-blade technique and dried in an oven, and then the spinel LiMn2O4 cathode material with the mass loading of about 1.0 mg/cm 2 was obtained.Finally, all as-prepared cathode disks were stored a vacuum drying oven at 120 ℃ for 8-12 h. 1 M LiPF6 in EC/DMC (1:1 in volume) and Celgard 2320-type membrane were used as anode, electrolyte and separator, respectively.The cells were cycled by a galvanostatical charge-discharge tests were measured by the Land electric test system CT2001A in the voltage from 3.0 to 4.5 V (versus Li + /Li).Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested by an electrochemical workstation of CHI660D between 3.6 and 4.5 V.

XRD studies
XRD patterns of LMO-T600, LMO-T750 and LMO-T850 samples are shown in Figure 1.The XRD spectra of all samples could be indexed to the standard PDF card No. 35-0782.The diffraction peaks of (111), (311), ( 222), (400), ( 331), ( 511), ( 441) and (531) correspond to a single-phase of spinel LiMn2O4 with Fd-3m space group.As shown in this result, the LMO-T600 electrode shows impurities peaks of Mn3O4, which may be due to the lower calcination temperature resulting in poor crystallization.Furthermore, compared to the T850 sample, the LMO-T750 electrode shows sharper peaks and larger peak area, indicating a better crystallinity.Considering of the crystallinity property, the optimal temperature has been determined as 750-850 ℃.

SEM analysis
The morphology images and size distribution of the LiMn2O4 samples calcined at different temperature are shown in Figure 2. The LMO-T600 sample has random particle morphology with the arrangement of particles in aggregates/agglomerations, which will greatly decrease specific capacity.With an increase in the calcination temperature at 750 ℃, the LMO-T750 samples became more uniform and maintain a narrow distribution with particles diameter of about 200 nm.When the calcination temperature was as high as 850 ℃, The LMO-T850 samples showed heterogeneous particle size dispersion with diameters of larger particles of about 300 nm and less than 100 nm, which could be due to severe sintering effect [1,13].On the basis of SEM images, the LMO-T750 samples showed nanosized particles with high surface area and high crystallinity, which may exhibit better electrochemical properties and rate performance.To obtain more insights in the microstructure, the LMO-T750 material was carefully examined by high-resolution transmission electron microscope (HRTEM), shown in Figure 2(d) and (e).As seen, the sample showed as octahedrons and a well-defined lattice fringe of 0.48 nm, which is in accordance with the (111) plane of the spinel LiMn2O4 [3,4,28].

Electrochemical performance
To evaluate the electrochemical performance of the LMO-T600, LMO-T750 and LMO-T850 samples, a galvanostatic charge/discharge test was conducted in a voltage range of 3.0-4.5V.All cycling performances of the samples at 1 C are depicted in Figure 3.The initial and 400th discharge specific capacities and capacity retentions of the samples are summarized in Table 1.
As shown Figure 3(a), the first discharge capacity is 101.7,112.1 mAh•g -1 at 1 C with a capacity retention of 72.2% and 82.3% after 400 cycles for the LMO-T600 and LMO-T850 samples, respectively.The LMO-T750 electrode delivers a first cycle discharge specific capacity of is 115.3 mAh•g -1 at 1 C and 101.5 mAh•g -1 after 400 cycles, corresponding to a capacity retention of 88.0%, which probably is due to the reduced Mn dissolution in the truncated octahedron and decrease of Li + loss inside the lattice [5]. Figure 3(b) shows the first cycle of discharge curves for the samples at current density of 1 C. Obviously, the samples exhibit two plateaus at around 3.9 and 4.1 V, corresponding to the two-step oxidation/reduction reaction process for the spinel LiMn2O4 [24].
In addition, we compared the cycling performance of the LiMn2O4 and Co-doped LiMn2O4 samples, as shown in Table 2.It can be found that the LMO-750 displays good cycling performance.This result suggests that the facile liquid-combustion method can effectively improve cycle stability of lithium-ion batteries.

This work
Figure 4(a) displays the rate performance of the LMO-T600 and LMO-T750 cathodes at charge/discharge current densities ranging from 0.5 C to 10 C. Apparently, the discharge specific capacities of both of samples gradually decrease with increasing of charge/discharge rate, which is due to the electrochemical polarization in the Li + diffusion process [25].The LMO-T750 electrode delivers an average specific capacity of 110.1, 104.4,96.5, 89.6 and 72.1 mAh•g -1 , respectively, in contrast to 102.7, 95.8, 85.1, 68.8 and 48.3 mAh•g -1 for the LMO-T600 electrode.While the current density returns back to 0.5 C again, the reversible capacity of 97.7% of the original capacity for the LMO-T750 electrode, proving a superior reversibility.The good highrate capability is governed not only by the size of crystallites but also by the perfectness of crystals, which is well consistent with the analytical results of XRD patterns and SEM analysis.The high-temperature cycle performance of the LMO-T600 and LMO-T750 samples at 55 ℃ is shown in Figure 4(b).The LMO-T750 electrode delivers an initial discharge capacity of 105.4 mAh•g -1 , which is higher than that of 92.3 mAh•g -1 for the LMO-T600 sample.Therefore, the results indicate the optimized calcination temperature could strongly enhance the cycle performance and rate performance.

Cyclic voltammogram
To analyze the phase transformation process, cyclic voltammetry curves (CV) of the samples in voltage range of 3.6-4.5V with a scanning rate of 0.05 mV/s are provided in Figure 5(a) and (b).
As seen, all of the samples display a couple of distinct redox peaks, indicating the two-stage Li + insertion/extraction process of the spinel LiMn2O4 cathode material, which is consistent with the two-plateaus in the charge/discharge profiles [27,29].

Electrochemical impedance spectroscopy
To further investigate the electrochemical mechanism and inner impedance for the samples, the electrochemical impedance spectroscopy (EIS) and the fitting equivalent circuit diagram is studied, as shown in Figure 6.Where, the ohmic resistance (Rs) represents the resistance of electrolyte and solution inner the cell, corresponding to one semicircle in the high frequency region.Besides, the charge transfer resistance (Rct) is related to the electrochemical performance of the cells, which is located in the medium region.In addition, the CPE responds the doublelayer capacitor and W is the Warburg impedance in the EIS [27].Table 3

CONCLUSION
We have prepared the spinel LiMn2O4 cathode materials by a facile liquid-state combustion synthesis and then qualitatively and quantitatively invested the property and electrochemical performance in the lithium-ion batteries, including the changes in the crystal structure, Li + ions diffusion kinetics, phase transformation process, cycling performances, etc.The truncated morphology of the LMO-T750 sample caused by the solution combustion method and then resulted in an increase of the discharge capacity.As a result, the LMO-T750 electrode showed 88.0% capacity retention with an initial discharge specific capacity of is 115.3 mAh•g -1 , which was much higher than that of the LMO-T600 and LMO-T850 samples.We also observed that the minimal Mn dissolution in the LMO-T750 electrode induced a high current rate and hightemperature cycle performance.Moreover, the lower electrochemical impedance of the LMO-T750 electrode accelerated the Li + insertion/extraction process, indicating a better Li + diffusion capability in the lithium-ion batteries.Our study suggested that this strategy may be an effective novel approach for improving the electrochemical performance and properties of lithium-ion batteries.

Figure 5 (
a) shows that the area of the LMO-T600 cathode is obviously smaller than that of the other two electrodes with similar peak areas, corresponding to the current densities of the cycle curves.After 100 cycles, the LMO-T750 electrode shows a large peak area and sharp peak shown in Figure5(b), indicating a higher discharge capacity and a faster insertion/extraction kinetics of Li + ions than the other samples[26].

Table 1 .
Discharge specific capacity and capacity retention rate of LMO-T600, LMO-T750 and LMO-T850 samples at room temperature.

Table 2 .
Comparison of the LiMn2O4 and Co-doped LiMn2O4 samples described in the references including this work.

Table 3 .
The fitting valves of the equivalent circuit.