Microwave Solvothermal Controlling MnC₂O₄ Morphology And Its Electrochemical Properties

Recently,transition metal oxalate anode materials based on conversion reactions have been considered as potential anode materials for lithium-ion batteries due to their high specific capacity.Among these transition metal oxalates,manganese oxalate has many advantages such as wide sources,low cost,and good safety.However,the electrochemical performance of manganese oxalate is poor.The poor performance can be attributed to the difficulty of ion/electron migration related to its low conductivity and the structural collapse and pulverization caused by the volume change during charging and discharging.Synthesis of nanostructured manganese oxalate with specific morphologies is an effective way to improve its electrochemical performance.Compared with other methods for synthesizing transition metal oxalates,hydrothermal method/solvothermal method is easier to control the morphology,and microwave heating technology can effectively improve the heating efficiency and shorten the reaction time.Therefore,a microwave-assisted solvothermal method was developed to nanostructured manganese oxalate using Mn（CH₃COO）₂·4H₂O and H₂C₂O₄·2H₂O as raw materials and ethylene glycol as solvent.The effects of reaction time,reaction temperature,solvent ratio and surfactant on the crystal structure,particle morphology and electrochemical properties of the prepared manganese oxalate were investigated,and the formation process of manganese oxalate under different reaction conditions was briefly analyzed.The hydrated precursors prepared under different conditions are monoclinic MnC₂O₄·2H₂O,with space group C2/c（PDF#25-0544）.The hydrated precursors are transformed into orthogonal structure MnC₂O₄ with space group Pmna（PDF#32-0646）by removing crystal water under vacuum,and the morphology of particles remains stable in the process of removing crystal water in vacuum.When the reaction temperature is lower than the boiling point of ethylene glycol,the growth process of manganese oxalate crystal maintains its initial cubic shape,but its size decrease.When the reaction temperature is higher than the boiling point of ethylene glycol,the initial cubes gradually change into rods through dissolution-recrystallization-oriented growth processes under the high-temperature/pressure environment.With the extension of microwave reaction time,the morphology evolution of rods follows the Ostwald ripening mechanism,and the aspect ratio,specific surface area,and pore volume first increase and then decrease.The solvent plays a key role in determining the crystal growth environment and its interaction with microwaves.When the solvent is changed from water to water-ethylene glycol mixed solution and pure ethylene glycol,the microenvironment of crystal growth changes from water molecules to ethylene glycol molecules,and the selective absorption of ethylene glycol on the crystal surface is gradually enhanced.Therefore,the particles gradually evolve from the flake structure to the block structure and rold-like structure,respectively.The polyethylene glycol 4000（PEG）and citric acid（CA）surfactant added in the preparation process can effectively limit crystal growth and reduce particle size.The nonionic surfactant PEG is wrapped on the surface of the crystal nucleus,which makes the particles grow into cubic particles anisotropically.The selective adsorption of anionic surfactant CA on the crystal surface makes the crystal grow into fine rod-shaped particles.Manganese oxalate bundles can be prepared by adding 0.02 g of citric acid surfactant at 220°C for 10 min,which is composed of nanorods and abundant mesoporous structure.After 500 cycles at the current of 2 A/g,5 A/g and 8 A/g,the prepared bundles can still obtain the specific discharge capacities of 1016,925 and 767 m Ah/g,respectively,exhibiting higher capacity,better cyclic stability and rate performance.The enhanced performance can be ascribed to the unique bundle-like structure with a higher specific surface area,larger pore volume,and nanosize rod,which can reduce polarization,enhanced Li⁺absorption,accelerate lithium-ion diffusion kinetics and tolerate the volume changes during the charge/discharge processes.In summary,the microwave solvothermal method is an effective method to rapidly prepare manganese oxalate with specific morphology.The findings on the microstructure evolution provide insights into the controllable synthesis of manganese oxalate and other oxalates,ultimately enhancing their performance.