Single-crystalline LiMn2O4 nanoparticles by the gel combustion method assisted by microwave for high-performance lithium-ion battery

Low-cost and efficient electrode materials play a key role in improving the performance of lithium-ion battery. In this paper, the single-crystalline LiMn2O4 nanoparticles were synthesized through the gel combustion method assisted by microwave followed by calcination treatment. High-quality single-crystallinity characteristics of the LiMn2O4 precursor powder could be well retained after a high-temperature (990 °C) solid-state reaction. When valued as the working electrode materials for lithium-ion battery, they presented the exceptional electrochemical performances including the high specific capacity for an initial discharge of 120.53 mAh/g, the good rate capability with the retention of 65.52% at 5 C of the capacity at 0.2 C, and better cyclic performance with a capacity retention ratio of 90.01% after 300 cycles. The outstanding electrochemical performance of our prepared single-crystalline LiMn2O4 nanoparticles was perceived as the hopeful electrode materials for high-power lithium-ion battery.


Introduction
Nowadays, the concerns over depletion of fossil fuels and environmental pollution have galvanized the endeavors to develop a new energy vehicle.Power batteries used in electric vehicles need cathode materials with good safety performance, excellent rate performance and cycling, and low price [1,2].Nowadays, the commercial cathode materials of lithium-ion power batteries mainly include lithium cobalt oxide, lithium manganate, and lithium iron phosphate.Among them, LiMn 2 O 4 cathode material has been widely studied to use in power battery due to their highervoltage platform, higher safety performance, lower cost, and nontoxicity (environmentally friendly) [3,4].However, several drawbacks of LiMn 2 O 4 are crystalline structural changes taking place, manganese leaching, poor high-temperature cycle performance, and relatively low practical achievable capacity [5,6].These drawbacks will result in the capacity decay and poor cycling stability.
Many scientists all over the world have made great efforts to solve the drawbacks of LiMn 2 O 4 .A variety of methods have been employed to enhance the comprehensive performance of LiMn 2 O 4 , such as new preparation process [7], cationic doping [8], anionic doping, co-doping [9], and surface synergistic modification [10].There are many relevant literatures which reported that LiMn 2 O 4 is used as the commercial cathode materials of lithium-ion power batteries.For example, Kim et al. [11] reported the hydrothermal synthesis of single-crystalline β-MnO 2 nanorods and their chemical conversion into free-standing single-crystalline LiMn 2 O 4 nanorods using a simple solid-state reaction.The LiMn 2 O 4 nanorods are able to deliver 100 mAh/g at a high current density of 148 mA/g with high reversibility and good capacity retention after 100 cycles [11].Jiang et al. [12] synthesized phase-pure spinel LiMn 2 O 4 nanoparticles by one-step hydrothermal reaction of γ-MnO 2 with LiOH in an initial Li/Mn ratio of 1 at 200 °C.Huang et al. [13] reported that LiMn 2 O 4 spinels with diverse achievable morphologies were realized using solution processing techniques including sol-gel and cofuel-assisted combustion synthesis.The as-prepared LiMn 2 O 4 cathode displayed high capacity (~ 120 mAh/g), good cycling stability (~ 95% capacity retention after 100 charge/discharge cycles), and high charge/discharge rates (up to 86 mAh/g at 10 C) [13].Cai et al. [14] reported that the Al-doped LiMn 2 − x Al x O 4 (x = 0.05, 0.10, and 0.16) was synthesized using a simple combustion method with degreased cotton fiber as the carrier.A doping content of 16 at.%to LiMn 2 O 4 showed excellent electrochemical performance, with a first charge specific capacity of 100.7 mAh/g and a capacity retention rate of 93.9% after 400 cycles at a current rate of 0.5 C [14].Hou et al. [15] provide an effective strategy for drastically improving the cyclic behavior and rate capability of a LiMn 2 O 4 electrode at elevated temperature through P element doping.The electrochemical results reveal that at a high 10 C rate of current density and 55 °C, the optimized LiMn 2 O 4 (1.5 wt% P) electrode showed the highest initial specific capacity of 101.2 mAh/g, which became 74.4 mAh/g after 500 cycles, corresponding to 73.5% capacity retention [15].Yu et al. reported that the nickel (Ni) and magnesium (Mg) co-doping strategy was employed to synthesize the LiNi 0.03 Mg 0.05 Mn 1.92 O 4 cathode material via a facile solid-state combustion approach.When cycled at 1 C, the LiNi 0.03 Mg 0.05 Mn 1.92 O 4 delivered a first capacity of 112.3 mAh/g and capacity retention of 73.7% after 1000 cycles, while the LiMn 2 O 4 is only 50.3% [16].Wen et al. [17] synthesized the spinel LiMg x Mn 2 − x O 4 − 2x F 2x (x = 0.05, 0.1, 0.2) samples via a solid-state reaction route with the MnF 2 as the dopant.The LiMg 0.1 Mn 1.9 O 3.8 F 0.2 showed the highest initial discharge capacity of 121.1 mAh/g, and the capacity retention is still as high as 89.2% even after 400 cycles at a rate of 1 C under an elevated temperature of 55 °C [17].Wang et al. [18] reported that spinel LiMn 2 O 4 modified with perovskite LaCoO 3 was prepared using a novel molten salt method.The LiMn 2 O 4 modified with 4% LaCoO 3 showed the best rate performance at a larger current at 2 C and 5 C [18].Michalska et al. [19] reported a facile, wet chemical synthesis of surface modification of LiMn 2 O 4 grains with graphene oxide flakes.The LiMn 2 O 4 material modified with 5 wt% of graphene oxide flakes retained more than 91% of its initial specific capacity, as compared with 86% measured for pristine LiMn 2 O 4 material [19].Wang et al. [20] developed a novel and facile modification method to improve the performance of LiMn 2 O 4 electrodes for lithium-ion batteries, which was used an aluminum-zirconium coupling agent to treat LiMn 2 O 4 cathodes via a simple pyrolysis method at 450 °C.Although lithium-ion power batteries based on LiMn 2 O 4 cathodes have achieved great development, the design and synthesis of LiMn 2 O 4 with innovative structure, good cycling stability, and superior elevated-temperature performance remain a daunting task.
At present, the cathode material of single-crystal lithiumion battery is the research hotspot and development direction, and there is no grain boundary which is supplied electrolyte osmosis.At the same time, the density of single crystal is high, so it not only has no gap, but also can bear greater force during the calendaring process without cracking.Therefore, single-crystal electrode materials have good cycle stability, good process ability, and high temperature performance.
Herein, in the present work, the single-crystalline LiMn 2 O 4 nanoparticles were synthesized through the gel combustion method assisted by microwave followed by calcination treatment.As demonstrated by experimental results, the asprepared LiMn 2 O 4 electrode exhibited the exceptional electrochemical performances including the high specific capacity for an initial discharge of 120.53 mAh/g, the good rate capability with the retention of 65.52% at 5 C of the capacity at 0.2 C, and better cyclic performance with a capacity retention ratio of 90.01%after 300 cycles.The outstanding electrochemical performance of our prepared single-crystalline LiMn 2 O 4 nanoparticles was perceived as the hopeful electrode materials for high-power lithium-ion battery.
The single-crystalline LiMn 2 O 4 nanoparticles were synthesized via the gel combustion method assisted by microwave followed by calcination treatment.In a typical procedure, as shown in Fig. 1, first, 1 mmol (68.95 mg) LiNO 3 and 2 mmol (502.08 mg) Mn(NO 3 ) 2 •4H 2 O were accurately weighed, dissolved in 20 mL of deionized water in the 50-mL beaker, and stirred under high-power ultrasonications for 15 min to form a clear solution.Then, 1 mmol (180.16mg) C 6 H 12 O 6 was added and further stirred under high-power ultrasonications for 15 min to form a homogeneous mixed solution.Second, the obtained mixed solution was transferred into a 50-mL ceramic crucible.The ceramic crucible was heated in the microwave oven for 5 min.The mixed solution was transformed into gel.Third, the gel was heated on the resistance furnace to make it burn in the fume hood.The process of gel combustion produced a large amount of gas.The loose powders were obtained after complete combustion, which were called LiMn 2 O 4 precursor powder.Finally, the precursors were calcined under air atmosphere at 990 °C for 24 h, resulting in the single-crystalline LiMn 2 O 4 nanoparticles.

Characterization of materials
The crystal structure of LiMn 2 O 4 was examined by X-ray diffraction (XRD) using a DX-2700 diffractometer (China) with Cu Kα radiation (λ = 1.5418Å) operating at 35 kV and 30 mA.The patterns were recorded in the 2θ range of 10-80° at a scanning rate of 0.03°/s.The morphology of LiMn 2 O 4 was characterized by the scanning electron microscopy (S-3400) with an accelerated voltage of 5 kV.The Brunauer-Emmett-Teller (BET) specific surface area was obtained from the N 2 adsorption/desorption isotherm recorded at 77 K.

Electrochemical characterization
The electrochemical properties of LiMn 2 O 4 were evaluated using coin-type cells with lithium metal as the counter electrode.The working electrode was fabricated by mixing the active material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio 80:10:10.The electrolyte was a 1 mol/L solution of LiPF 6 in a 1:1 mixture of ethylene carbonate (EC) diethyl carbonate (DEC).The separator was polypropylene microporous membranes (Celgard 2320).Cyclic voltammetry measurements were performed on a CHI660D electrochemical workstation.The galvanostatic charge-discharge tests were performed on a Neware battery tester.

Complexation and LiMn 2 O 4 formation
The combustion is based on the mixing of metal nitrate-oxidizing agents and an organic fuel, acting as a reducing agent.
In this case, external heat is needed to initiate the ignition resulting in self-sustainment of an exothermic redox reaction.Figure 1 illustrates a representative fabrication process of the LiMn 2 O 4 nanoparticles.We selected the glucose as a complexing agent and organic fuel.Glucose was easily soluble in water, and its molecule contained multiple hydroxyl groups, which can be effectively bound metal ions (Li + , Mn 2+ ) by Van der Waals force and hydrogen bonds.In our synthesis process, the complexing reaction was achieved in a microwave oven, where heat was generated within the sample volume itself by the interaction of microwaves with the reactants.Unlike the conventional heating systems that heat the material from outer surface to interior resulting in steep thermal gradients, the microwave heating can provide a uniform heating environment for the complexing reactions, which was conducive to improving the structure and performance of synthetic materials.The mixed solution was transformed into gel by the microwave heating reaction for 5 min.Then, the gel was heated on the resistance furnace to make it burn.The process of gel combustion produced a large amount of gas (H 2 O, CO 2 , N 2 ).The typical total combustion reaction can be written as follows: The loose powders were obtained after complete combustion, which were called LiMn 2 O 4 precursor powder.Finally, the precursors were calcined under air atmosphere at 990 °C for 24 h, resulting in the single-crystalline LiMn 2 O 4 nanoparticles.

Structure and surface morphology characterization
The phase composition and crystal structure information were investigated by X-ray diffraction.The XRD pattern was identified by comparison with the JCPD standard.Figure 2 shows XRD patterns of the as-prepared LiMn 2 O 4 .Obviously, the 2θ values for the diffraction peaks at 18  The resultant diffraction peaks corroborated well with the standard pattern of spinel LiMn 2 O 4 (PDF, Card No. 35-0782, cubic crystal system), and no peaks of other phases were detectable, revealing the high purity of the as-synthesized samples.In addition, the crystal lattice parameter (a = b = c = 8.2472 Å) calculated from the XRD results of LiMn 2 O 4 is highly similar with the standard data (a = b = c = 8.2476 Å) of LiMn 2 O 4 .The narrow and high-intensity peaks confirmed the high crystallinity and small dimensions of the synthesized nanoparticles.According to the Scherrer equation, Dc = 0.89λ/(βcosθ), where λ is the corrected wavelength of the X-radiation, β is the full width at half maximum corrected for instrumental broadening, and θ is the Bragg angle of the diffraction peak, and the crystalline LiMn 2 O 4 nanoparticles were calculated as measuring 75~98 nm.
In general, high-temperature calcination is beneficial to the single-crystalline materials.In this paper, the LiMn 2 O 4 precursor powder was calcined at 990 °C for 24 h, the crystal cell of the material shrank, the grains became larger, and the crystal form became more complete, resulting in the singlecrystalline LiMn 2 O 4 nanoparticles.
Detailed structural information was further obtained using SEM. Figure 3 shows SEM images of the LiMn 2 O 4 at 5000, 10,000, 20,000, and 30,000 magnifications.The results showed randomly distributed smaller sizes with spherical-shaped globule-like particles, where some were in elongated form.In addition, it can be seen from the figure that the obtained material was composed of single-crystal particles, with a complete crystal form, and a smooth and flat surface for the primary particles.The complete single-crystal particles can provide a smooth three-dimensional channel for the migration and diffusion of lithium ions during the charging and discharging process, improving the migration and diffusion rate of lithium ions in the material.Therefore, it improves the rate performance of the material.The more complete the crystal form of a single crystal, the larger the grain size, and the smaller the crystal defect, the capacity attenuation caused by the crystal defect will be suppressed, and the cyclic performance of the material will be improved.The specific surface area of material is a very critical parameter.The N 2 adsorption-desorption measurement was conducted.The N 2 adsorption-desorption measurement was conducted.Figure 4 shows the N 2 adsorptiondesorption isotherms of the as-prepared single-crystalline LiMn 2 O 4 nanoparticles.As shown in Fig. 4, the N 2 adsorption-desorption isotherms of LiMn 2 O 4 belong to a typical IV type with an obvious hysteresis loop according to the IUPAC classification.BET surface area data was calculated to be about 56.18 m 2 /g of the LiMn 2 O 4 sample.The high surface area can enhance the utilization of active material.

Electrochemical measurements
The electrochemical intercalation/deintercalation behaviors of the as-prepared LiMn 2 O 4 were valued by the galvanostatic charge/discharge cycling test between 3 and 4.3 V. Figure 5 shows the initial charge-discharge curve of LiMn 2 O 4 at 0.2 C. Obviously, it can be seen that the initial charge-discharge curve of LiMn 2 O 4 has two distinguished charge/discharge plateaus.At 0.2 C, initial charge plateaus of the single-crystalline LiMn 2 O 4 were 4.01 V and 4.12 V, and initial discharge plateaus were 3.97 V and 4.08 V.In the charge curve, the first peak located at 4.01 V (Li/Li + ) means an extraction of Li ions from half of the tetrahedral sites with Li-Li interaction, and the second peak at 4.12 V (Li/Li + ) means an extraction of Li ions from the other half of the tetrahedral sites without Li-Li interaction.The potential drop with about 110 mV between these two extraction processes is from the repulsion between Li ions.So, in the discharge curve, two peaks also correspond to the two insertion processes, respectively.The lithium ion's insertion and extraction can be expressed as The specific capacity for the initial charge was 126.78 mAh/g, and the specific capacity for the initial discharge was 120.53 mAh/g, so the coulombic efficiency reached up to 95.07%, which showed that the single-crystalline LiMn 2 O 4 nanoparticles have very good initial charge and discharge performance.
In order to investigate the fast ion transportation ability of the single-crystalline LiMn 2 O 4 , the rate capability experiments were tested at rates ranging from 0.2 to 5 C. Figure 6 shows the discharge capacity with 5 cycling numbers under different current rates (0.2-5-0.2C).With the increasing current rate (0.2-5 C), the discharge capacities of the asprepared LiMn 2 O 4 decreased gradually, and this was due to the limited migration and diffusion rates of lithium ions and electrons in the material.The result was consistent with many reports [17][18][19].At 0.2 C, the specific capacity was 120.43 mAh/g, and at 5 C, the specific capacity was 78.9 mAh/g.Therefore, the as-prepared LiMn 2 O 4 exhibited the good rate capability with the retention of 65.52% at 5 C of the capacity at 0.2 C. The possible reason for this phenomenon was attributed to the material's single crystallization.The LiMn 2 O 4 which was calcined at 990 °C for 24 h could generate single-crystalline LiMn 2 O 4 with larger particles.The single-crystalline LiMn 2 O 4 can provide an unobstructed three-dimensional channel for lithium ion migration and diffusion and improve migration and diffusion rates in the charge discharge process, thus improving the rate performance of the material.
Long-term cyclic stability is an important requirement for lithium-ion battery to be used practically.The cyclic stability of the LiMn 2 O 4 electrode was tested for 100 continuous charge-discharge cycles at 0.2 C rate, as presented in Fig. 7. Obviously, we can see that the specific capacity of  as-prepared LiMn 2 O 4 was 120.80 mAh/g at the first cycle, and after 300 cycles, the specific capacity was 108.73 mAh/g, cycle capacity retention rate was 90.01%, and the average attenuation rate per cycle was about 0.033%, indicating excellent long-term cycle stability performance.We compared the capacity retention reported in some of the recent studies.For example, Zhao et al. [21] reported that octahedral LiMn 2 O 4 with exposed {111} crystal facets was prepared using MnCO 3 as a precursor via solid-phase reactions, and the capacity retentions of 87% for 200 cycles at 0.1 A/g were achieved.Li et al. [22] prepared LiMn 2 O 4 with Mn 3 O 4 as a manganese source, and the initial charge specific capacity of the LiMn 2 O 4 is 115.7 mAh/g at 0.1 C. Additionally, the charge specific capacity of the LiMn 2 O 4 is still 78.64 mAh/g after 100 cycles at 1 C [22].Tron et al. [23] introduce AlF 3 -coated LiMn 2 O 4 cathodes, which the optimized LiMn 2 O 4 , having a 2 wt% coating of AlF 3 , exhibited a long cycle life having a capacity retention of 90% after 100 cycles.Chen et al. [24] synthesized the single-crystalline LiMn 2 O 4 nanorods by a simple solid-state reaction, which showed better cyclic performance with a capacity retention ratio of 86.2% after 100 cycles.Zhou et al. [25] synthesized the spinel LiMn 2 O 4 by solid-state combustion synthesis, which showed a long cycle life having a capacity retention of 87.2% after 50 cycles at 0.2 C. Zhang et al. [26] have fabricated Sm 3+ and Mo 6+ dual-doped LiMn 2 O 4 -positive materials using a facile solid-phase strategy, which showed a long cycle life with a capacity retention ratio of 88.08% after 200 cycles.Obviously, it can be seen that the as-prepared LiMn 2 O 4 fabricated in our study showed to be comparable with or higher than previously reported cycling performance.This was attributed to larger crystalline grain, which provided a smoother channel for lithium ion migration and diffusion, allowing for rapid migration and diffusion, reducing the concentration difference between lithium ions on the surface and those inside the material, reducing the Jahn-Teller effect.Moreover, larger crystalline grain had a smaller specific surface area, which reduced the contact between the material and the electrolyte and reduced side reactions, which was beneficial for the material's cycling performance.In addition, the crystal form of the single crystal was more complete and the structural stability was stronger, which was also the reason for the better cycling performance of materials.Therefore, the single-crystalline LiMn 2 O 4 displayed excellent cycling performance.

Conclusions
In summary, we have successfully synthesized the singlecrystalline LiMn 2 O 4 nanoparticles through the gel combustion method assisted by microwave followed by calcination treatment.High-quality single-crystallinity characteristics of the LiMn 2 O 4 precursor powder could be well retained after a high-temperature (990 °C) solid-state reaction.The test results showed that the as-prepared LiMn 2 O 4 was composed of single-crystal nanoparticles, and reached to good accordance with the reported peak value from the standard JCPDS Card No. 35-0782.BET surface area data were calculated to be about 56.18 m 2 /g.When valued as the working electrode materials for lithium-ion battery, they presented the exceptional electrochemical performances including the high specific capacity for an initial discharge of 120.53 mAh/g, the good rate capability with the retention of 65.52% at 5 C of the capacity at 0.2 C, and better cyclic performance with a capacity retention ratio of 90.01%after 300 cycles.Considering the remarkably improved performance and facile fabrication method, the single-crystalline LiMn 2 O 4 nanoparticles could be a promising electrode material for high-power lithium-ion battery.

Fig. 1
Fig. 1 Schematic representation of the gel combustion method assisted by microwave of LiMn 2 O 4