SOLVOTHERMAL SYNTHESIS OF LiFePO4/C NANOPLATE WITH NEST-LIKE MICCROSTRUCTURES FOR LITHIUM ION BATTERY

摘要

Olivine-type LiFePO4 is now receiving attention as cathode material for Li-ion batteries in hybrid electric vehicles (HEVs) and electric vehicles (EVs), because of its high theoretical specific capacity (170 mAh g~(-1)), thermal stability, nontoxicity, safety, and potentially low cost. [1-4] Although LiFePO4 possesses many advantages, the inherent poor electronic conductivity (~10~(-9) S cm~(-1)) and Li-ion diffusion coefficient (~1.8 X10~(-14) cm~2 s~(-1)) at room temperature bring difficulties for high-rate battery applications.[5,6] Modifications of LiFePO4 particles by minimizing the particle size and coating them with an electron-conducting carbon layer are considered to be effective in surmounting electronic and ionic transport limitations. By combining both of these approaches, many synthesis methods, including solid state methods, [7] sol-gel methods, [8, 9] hydrothermal/solvothermal methods, [10] etc., have been developed to prepare nano-sized LiFePO4/C composite materials. However, the interfacial energy of nano-sized LiFePCVC particles is very large and the particles tend to aggregate easily, which resulted in poor battery cycling performance. Hierarchical micro/nanostructured materials, which are composed of microsized objects with nanostructures assembled by nano-sized building blocks, could overcome such disadvantages. [11] The simplest synthetic route to hierarchical nanostructures is probably self-assembly, in which ordered aggregates are formed in a spontaneous process. Recently, the hydrotbermal/sorvothermal methods as typical solution-based approaches have been proven effective and convenient processes in preparing various inorganic materials with diverse controllable morphologies and architectures in terms of reproducibility and potential for large-scale production. However, they are not doing well in LiFePO4 systems. This situation has been attributed to the fact that the different precursor chemicals generally have different reactivities, which is not favorable for crystal growth and formation of pure products. Although .some significant efforts have been made to synthesize LiFePO4 architectures,[12-14] it is still a challenging and urgent task for us to prepare hierarchical LiFePO4 nanostructures with high-quality crystalline and tailored morphology under soft reaction conditions, consequently achieving a better understanding of the observed complex phenomena of crystal growth and revealing the underlying fundamental theories and principles.