Controllable Synthesis Of Micro-/Nano-Structured LiMn₂O₄ And Electrochemical Performance For Lithium Ion Batteries

Spinel LiMn2O4 has been considered as one of the most promising cathode materials on a large-scale application for lithium ion batteries. The limitation is the rate capabilities when spinel LiMn2O4 is used for a high-power application, such as, electric vehicles. At present, the electrode material with micro-/nano-structure has been considered as one of the most practical structures because it can take both the advantages of nanometer-sized building blocks and micro- or submicrometer-sized assemblies. The former is beneficial to enhance Li+ ions diffusion kinetics and high capacity, the latter can guarantee good integral structural stability.The main goal of the present work is to develop a simple and effect self-template method to synthesize micro-/nano-structured LiMn2O4 with different sizes and controllable shape in order to improve high-rate capability and cycling performance for lithium ion batteries. Micro-/nano-structured LiMn2O4 including single-crystalline nanotube, metal-doped single-crystalline nanotube and double-shelled hollow microsphere has been synthesized by controlling the structure and morphology of self-template and conditions of thermal treatments. The relation is also investigated between structure and morphology of LiMn2O4 and electrochemical performance. Additionally, one dimensional coaxial nanotubes of MnO/C was synthesized based on a self-template and used as anode materials for lithium ion batteries. The main contents and results are as follows:1. The fabrication of single-crystalline nanotubes of LiMn2O4 with high rate capability and cycling performanceFor the first time, single-crystalline nanotubes of LiMn2O4 with about 600nm in diameter, about 200 nm in wall thickness and 1-4μm in length were synthesized usingβ-MnO2 nanotube as self-templates. The results show that single-crystalline nanotubes of LiMn2O4 deliver superior rate capability, cycling performance and structural stability after cycling. About 80 mAh g-1 of discharge capacity at 10 C rate and 70% of capacity retention after 1500 cycles at 5 C rate were obtained. Single-crystalline nanotube of LiMn2O4 has been considered as a kind of micro-/nano-structure because it not only holds a nanometer-sized building block with about 200nm in radial direction and a micrometer-sized assembly with 1-4μm in axial direction. Therefore, the micro-/nano-structured LiMn2O4 can supply fast Li+ ion diffusion and guarantee the structural integrity. In addition, one dimensional tubular single-crystalline structure is also beneficial to improve the high-rate capabilities.2. The fabrication of single-crystalline nanotubes of Al-doped LiAlxMn2-xO4 with improved high-temperature performance and Ni-doped LiNi0.2Mn1.8O4 with enhanced energy densityTo further improve high-temperature performance and specific energy of LiMn2O4 nanotube, the single-crystalline nanotubes of LiAlxMn2-xO4 (x=0.1,0.2, 0.3) and LiNi0.2Mn1.8O4 were synthesized usingβ-MnO2 as a self-template. The results show that LiAlo.1Mn1.9O4 delivers the highest rate capabilities and high-temperature performance among LiAlxMn2-xO4. About 80 mAh g-1 (25℃) and 70 mAh g-1 (55℃) of discharge capacity were obtained at 10 C rate, respectively. 80% of capacity retention was obtained after 200 cycle with 5 C rate at 55℃. Additionally, single-crystalline nanotubes of LiNi0.2Mn1.8O4 deliver higher specific energy than those of LiMn2O4 and LiAlxMn2-xO4.3. The fabrication of micro-/nano-structured LiMn2O4 with double-shelled hollow space and its superior rate capabilities and cycling performanceThe fabrication of double-shelled hollow LiMn2O4 is synthesized using MnCO3 microspheres obtained by a precipitation method as self-template. Actually, MnCO3 microsphere was considered as a core-shelled structure because it delivered different nanobuilding units in size in the inner and outer surface of the whole microsphere. By utilizing the different reaction activity for different nanobuilding units and controlling the temperature and time of pre-calcinaiton, a multilevel core-shelled MnCO3@MnO2@MnCO3@MnO2 with different size in the shell thickness would be obtained. After HCl washing and calcinaiton at high temperature, double-shelled hollow LiMn2O4 with different shell thickness and solid LiMn2O4 were obtained, respectively. This micro-/nano-structured LiMn2O4 with about 4μm in diameter is composed of nanoparticles with 100-400nm. The results show that double-shelled hollow LiMn2O4 obtained by pre-calcining MnCO3 self-tempate for 4 h at 350℃delivers the best electrochemical performance.94 mAh g-1 of discharge capacity at 10C and about 80% of capacity retention after 800 cycles at 5 C rate were achieved, respectively. The superior electrochemical performance is attributed to its micro-/nano-structured LiMn2O4, which holds the nanometer effect for improving Li+ ions diffusion and the micrometer structure for enhancing the integrity stability during charge/discharge cycling. In addition, the double-shelled hollow structure is beneficial to the penetration of electrolyte and Li+i on diffusion, also accommodating the volume changes resulted from repeated cycling, leading to superior rate capabilities and cycling performance.4. The fabrication of MnO/C coaxial nanotubes and its application as anode for lithium ion batteriesCoaxial nanotubes of MnO/C were synthesized in an in situ reduction and carbonization route usingβ-MnO2 as self-templates under C2H2 atmosphere. The thickness of carbon layer on the MnO/C was controlled by adjusting C2H2 flux. The results show that the coaxial nanotubes of MnO/C delivered better electrochemical performance than MnO nanotubes and MnO nanoparticles.