High Performance Cathode Materials for Lithium Ion Batteries

Abstract

With increasing interest in environmental and energy issues, the application fields of lithium ion batteries have been used for electric vehicle as well as portable devices. However, lithium ion batteries with present technology can’t fulfill the requirement of full-range electric vehicles due to insufficient energy density and power density. The promising candidates of cathode materials for those drawbacks are Li-rich (Li2MnO3-LiMO2, M=Ni, Mn and Co) and spinel (LiM2O4, M=Al, Li, Mg, etc.) cathode materials. However, those materials still has problem which should be overcome for application of electric vehicles. For examples, the Li-rich materials have very high gravimetric energy density but suffer from low initial coulombic efficiency, voltage decay upon cycling, large side reaction at elevated temperature and poor rate capability. The spinel cathode materials have high rate capability and thermal stability but suffer from Mn dissolution at elevated temperature. Also it delivers low capacity at high C rate if secondary particle size is large (>10㎛). To overcome this barrier, newly developed material modification methods for Li-rich and spinel cathode materials are proposed in this dissertation. 1) The chemical activation is frequently used to improve the first cycle efficiency for Li-rich material. However, it causes the formation of lithium impurities. Also the surface coating is carried out to reduce side reaction on the surface, but this method can solve the coulombic efficiency at 1st cycle and rate capability. To overcome these barriers, we here report an efficient and effective surface modification method. The chemical activation (acid treatment) and LiCoPO4 coating were carried out simultaneously. During synthesis process, the lithium ions were extracted from the lattice leading to the improved columbic efficiency and these ions were used for the formation of LiCoPO4. The Ni and Co doped spinel phase was formed at the surface of host material, which gives rise to the facile pathway of lithium ions. The LiCoPO4 and highly doped spinel on the surface acted as the double protection layers that effectively prevented side reactions on the surface at 60oC. 2) The surface coating is widely used to protect surface of spinel cathode material from acidic present electrolyte. However, metal compound coatings cause surface resistance because usually coating material is electrically and electrochemically inactive. Also even though the coating material is electrode material, it still has resistance issues due to structural mismatch (grain boundary). To overcome this barrier, we here report an imaginative material design; a novel hetero-structure LiMn2O4 with epitaxially grown layered (R3 ?m) surface phase. No defect was observed at the interface between the host spinel and layered surface phase, which provides an efficient path for the ionic and electronic mobility, leading to the improved rate capability. In addition, the layered surface phase protects the host spinel from being directly exposed to the highly active electrolyte at 60 oC. 3) Many researchers have investigated nanosized spinel cathode materials to increase their rate capability because the lithium diffusion pathway is reduced and the surface where the electrochemical reaction can occur increase. However, the nanosized materials can’t fulfill the requirement of electrode density for the application of electric vehicles. Also the carbon coating is not able to be conducted on the spinel cathode materials due to the formation of oxygen deficiency. To overcome these problems, we report composites with super-p and nanosized spinel material via spray drying process. The acid-treated super-p was used for better distribution of super-p in secondary particle. The developed material showed outstanding rate capabilities at -10 oC as well as 24 oC.