Research On Synthesis And Performance Of Li₃V₂(PO₄)₃ Cathode Materials For Lithium Ion Battery

The development of lithium ion batteries and the survey of cathode materials have been described in this dissertation. The technics condition, the preparation methods, and some existing problems of Li₃V₂(PO₄)₃ have been introduced with emphasis. A study on the synthesis, structure and electrochemical performance of Li₃V₂(PO₄)₃-based compounds as cathode materials for lithium ion batteries by the research techniques of XRD, SEM and the electrochemistry test, was carried out systemically and in detail.In this work, Li₃V₂(PO₄)₃ cathode materials were successfully synthesized by carbon-thermal reduction method in the mixed gas of hydrogen and argon(1:9), their preparation and properties were deeply and systematically explored. Effects on the structure and properities of samples were studied from different Li resources, different carbon resources, calcining heat condition and calcining time, the best synthesis technics were optimized. The Li₃V₂(PO₄)₃ sintered for 24 h at 700℃from lithium fluoride as Li resources, carbon nanometer tube as carbon resources are best. In the voltage range of 3.0⁴.2 V and at a specific current of 0.1 C, the prepared Li₃V₂(PO₄)₃ has good electrochemical performances with the initial charge and discharge capacity of 130 mAh/g and 125 mAh/g respectively, the efficiency of charge and discharge is 96%. After 50 cycles, the sample retaines a discharge specific capacity of 111 mAh/g, the capacity fading rate was 11%.Tap density test results indicated, The tap density was biggish when saccharose as carbon resource. Carbon nanometer tube granule was loose, so the tap density of Li₃V₂(PO₄)₃ for Carbon nanometer tube as carbon resource was lesser. The XRD text results showed that the sample synthesized at 700℃for 24 h from lithium fluoride as Li resources, carbon nanometer tube as carbon resources has distinct diffraction peaks and a pure crystal structure. The SEM image showed symmetrical and lesser granule.CV test results indicated that in the voltage range of 3.0⁴.2 V, there were three couples of redox peaks at 3.64 V/3.55 V, 3.71 V/3.61 V and 4.11 V/3.92 V respectively; There appeared a fourth oxide peak at 4.55 V in the voltage range of 3.0⁴.8 V. The first Charge-Discharge test results indicatedthat the charge-discharge flats of cathode material Li₃V₂(PO₄)₃ were obvious, corresponded to the redox peaks in CV curves. the charge voltage of Li₃V₂(PO₄)₃ was increased from 4.2 V to 4.8 V, while its cycle performance was good and the discharge capability had been increased about 28%, but along with the augment of voltage range, the attenuation extent of discharge capacity was also augment. The Charge-Discharge test at different current indicated: along with the augment of current, the discharge capacity of Li₃V₂(PO₄)₃ was reduced, when the current was reduce again, the discharge capacity of Li₃V₂(PO₄)₃ was increscent again. EIS test results indicated that the extraction for Li⁺ was the best difficulty at 3.7 V. The interface double eletricity layered of Li₃V₂(PO₄)₃ was infulenced by charged voltage slightly.