Study On Preparation And Characterization Of Advanced Lithium Ion Cathode Materials

Lithium batteries have a wide range of application in digital camera, mobile phone, mobile notebook, electric measuring device and further electric vehicle systems for their high voltage, light weight, excellent electrochemical performance, low self discharge, long cycle life, no memory effect, safety and non-toxic property. How to find new materials for cathodes or modify present cathodes has been hot spots in lithium battery field.In this thesis, a series of lithium compounds were prepared by solid state synthesis. X-ray diffraction(XRD) was used to identify the phase composition, secondary battery tester was used to test the capacities of the cathodes, and the prior cathodes were chosen for modification. The results showed that lithium iron oxide and lithium iron phosphate exhibited high discharge capacity, which could be modified by applying new synthesis methods.Spray drying and freeze drying technology were used for the preparation of lithium iron oxide. The results showed that capacites of the cathodes calcined at 300℃by freeze drying and spray drying were 260.8mAh/g and 232.4mAh/g respectively, larger than 180.6mAh/g by solid state synthesis. The discharge capacities decreased with calcination temperature increasing. After 10 cycles,the capacity of the cathode prepared by freeze drying was 198.2mAh/g,but the capacity of the cathode prepared by spary drying kept only 113.0 mAh/g. The cathodes prepared by freeze drying had better electrochemical performance than those prepared by spray drying. When the precusor solutions were frozen, the ions could not move, but water was sublimated. During spray drying process, water was evaporated through a high temperature, which could lead to the growth and aggregation of particle size. The size distribution results showed the particle size of the powder prepared by freeze drying was 127.0nm at 300℃and 220.5nm 700℃, respectively. while the powder prepared by spray drying was 340.4nm at 300℃and 481.6nm at 700℃, so the cathodes obtaind by freeze drying possessed better discharge performance and higher capacity retention.Lithium iron phosphate was prepared by freeze drying due to its advangtage, and lithium iron phosphate was modified through adding carbon by using citric acid as carbon resouce. The results showed both LiFePO₄ and LiFePO₄/C had olivine crystal structure. The particles of LiFePO₄ and LiFePO₄/C had homogeneous size distribution and mean sizes of 234.6nm and 174.5nm, respectively. The electrochemical results showed that LiFePO₄ gave 136.0mAh/g at the discharge rate of 0.1C, while LiFePO₄/C gave 145.0mAh/g, both were higher than the cathodes by solid state synthsis of 120.1mAh/g. After 10 cycles, the discharge retention value of LiFePO₄ and were 97.5%,96.6%,95.3% and 94.3% at 0.1C,0.5C,1C and 2C, while the value of LiFePO₄/C were 98.0%,98.9%,99.3% and 98.6%. Capacity retention value were improved by freeze drying technology, especially at 1C and 2C. After adding carbon, the cathodes particle size reduced, so lithium ions travelled shorter and more convenient route, so more lithium ions can insert through carthodes and anodes, which can lead to better electrochemical performance.