Synthesis Of Micro/Nano-structured LiMn₂O₄and Their Electrochemical Performance

Abstract:It has always been the main research direction of lithium ion battery to improve the electrochemical performance and reduce the cost of electrode materials. Spinel LiMn2O4owns the advantages of material abundance, low cost, high safety and environmental friendliness, which make it one of the most promising candidates of cathode material for lithium ion battery. However, it still suffers from the bottlenecks of poor cycle performance and low specific capacity. Hence, the key to research on spinel LiMn2O4is to develop new preparation and modification methods. Micro/nano structure has both the primary particles with nano size and second structure with micro size and maintains the advantage of nano materials with excellent structural stability, so its application for lithium ion battery cathode has obvious advantages. This thesis attempts to prepare spinel LiMn2O4with micro/nano structure and studies its electrochemical performance.MnO2was prepared by hydrothermal method with MnSO4and (NH4)2S2O8as raw material and AgNO3as auxiliary reagent. The crystallinity, morphology and particle size of product were controlled through adjusting the content of AgNO3and reaction time and characterized by XRD、SEM and TEM. The results state that the optimal synthesis conditions were0.01mmol AgNO3and10h reaction time. MnO2prepared under the above conditions were microspheres with sea urchin morphology and hollow structure and has high phase purity, uniform distribution and good crystallinity. It was used as precursor and calcined with LiOH·H2O to obtain LiMn2O4. By controlling the calcination temperature, double shell hollow LiMn2O4microspheres were obtained. The obtained LiMn2O4material has high crystallinity and nano-sized primary particles and shows excellent rate capability and cyclic stability as cathode material for lithium ion battery. The discharge capacity at0.2C reaches125mAhg-1and maintains119mAhg-’after100cycles,95%of the initial capacity, which is much better than the LiMn2O4prepared by commercialized MnO2under the same conditions.y-MnOOH was prepared by hydrothermal method with MnSO4and NaClO3as raw material. The experimental results state that PVP could control the nucleation and growth of crystal, which has significant effect on the structure and morphology of the product. By controlling the reaction time and adding appropriate amount of PVP (15mmol), y-MnOOH nanotube with multi-slot wall was synthesized successfully for the first time. Tubular shaped LiMn2O4was obtained by calcination taking the obtained y-MnOOH as template. Its tube wall was formed by loose adhesion of nano-sized particles. The discharge capacity at0.1C reaches128.8mAhg-1and remains106.9mAhg-1at2C, showing high discharge capacity and good rate capability.The results state that LiMn2O4materials with hollow micro/nano structure own unique advantage in electrochemical performance. Its nano-sized primary particles could shorten the path of lithium ion diffusion, hence obtaining higher discharge capacity. The higher specific area of nano structure could enhance the current density of cathode materials. The gap between primary particles could buffer the volume change during the intercalation/deintercalation process effectively, resulting in better cycle performance. Hence the LiMn2O4material with special micro/nano structure is feasible for lithium ion battery industry.