Mechanochemical Preparation Of Nano-α-Al₂O₃ And Its Application In Lithium Ion Battery Anodes

Nano α-Al₂O₃ is widely used in aerospace,luminescent ceramics and medical applications due to its excellent properties such as high melting point,high hardness,controllable structure,high mechanical strength and high electrical/chemical stability.Particularly in the field of lithium-ion battery applications,α-Al₂O₃ could be utilized as a coating material to prohibit direct contact between the active material and the electrolyte,so as to reduce surface/interface side reactions and thereby considerably to improve material cycling performance.However,the thermodynamic instability ofα-Al₂O₃ and the high activation energy during preparation results in a great challenge to realize the green and economical preparation of nanoα-Al₂O₃ with high specific surface area.This thesis first proposes an effective method for producing nanoα-Al₂O₃by mechnochemistry induced toplogical thin alumina,and then using nano α-Al₂O₃ modifies graphite and silicon-carbon anode materials.This thesis takes the modification of lithium-ion battery anode materials by nanoα-Al₂O₃ as an orientation.In-depth research has been paid on the improvement mechanism of the electrochemical performance of graphite and silicon-carbon anodes owing to nanoα-Al₂O₃introduction.The achievments made in this work are summarized as follows:（1）To solve the issues of high energy barrier in phase transition and thermodynamic instability in the conventional preparation of nanoα-Al₂O₃,mechnochemical route was employed to eanble their formation of nanoα-Al₂O₃ with high specific surface area and samll size by using pseudoboehmite and aluminium hydroxide as precursors.To investigate the effects of crystallined water existing in precursor lattice,the process control agents and grinding media were optimized to examine the size and crystalline phase of nanoα-Al₂O₃.The optimized nanoα-Al₂O₃has an average grain size of 16.5 nm and a specific surface area of 101.7 m² g^-1.By monitoring the microstructural evolution of the crystalline phase and grain size ofα-Al₂O₃ at different grinding times,it was confirmed that the mechnochemistry induced topological conversion of the pseudoboehmite and aluminium hydroxide to α-Al₂O₃ acts as a decrystallization-recrystallization process.（2）To solve the issue of high rate performance due to the low ion transport rate of commercial graphite anode materials,theα-Al₂O₃/graphite composite was prepared by mechnochemical means for the purpose of enhancing the fast charging performance of graphite anode.The optimizedα-Al₂O₃/graphite anode has a reversible capacity of200.4 m Ah g^-1(graphite:59.6 m Ah g^-1)after 500 cycles at 8 C.Meanwhile,edge folding and layer spacing diffusion in graphite caused by mechanical forces can accelerate Li⁺transport;on the other hand,α-Al₂O₃ nanoparticles can reduce electrolyte/graphite interface side reactions and effectively improve cycling stability under high currents.（3）To solve the issue of cycling stability caused by high silicon volume expansion,particle fragmentation-agglomeration and electrode pulverization in silicon-carbon anode,two methods of both high-energy ball milling and plasma ball milling were used to prepareα-Al₂O₃/Si-C composites and investigate the lithium storage performance respectively.In comparison,the plasma ball mill preparedα-Al₂O₃/Si-C had the better cycling stability;the reversible capacity maintained at 811.21 m Ah g^-1 after 100 cycles at 0.2 A g^-1 with a capacity retention of 62.1%（Si:35.3%）.The transformation of graphite into amorphous carbon by shear forces effectively disperses the silicon particles,the alumina effectively reduces volume expansion while avoiding surface/interface side reactions.The in-situ formation of the Al_3.21Si_0.47 alloy phase due to C/CO reduction during ball milling substantially improves the electrical conductivity of the material.