Surface Modifications And Applications Of Li₃V2（PO₄）₃ Cathode Materials For Lithium Ion Batteries

Due to the high energy density, high power density, long cycle life and a wide operating temperature range, etc, lithium-ion battery（LIB） has been widely applied in mobile phones, notebook computers, electric vehicles, aerospace and weapons and so on, since its inception. Moreover, the eletrochemical performance of the LIB largely depends on the cathode materials. So, it is very important to study cathode materials. Owing to the natural abundance, safety, high voltage platform and excellent thermal stability, lithium vanadium phosphate（Li₃V₂（PO₄）₃） is considered to be a very promising cathode material for LIB. However, the poor intrinsic electronic conductivity and fast charge-discharge ability limit the application of Li₃V₂（PO₄）₃ in smart power grid electric vehicles and other large-scale energy storage. Currently, carbon-coating is the simplest and most widely used method to improve the electronic conductivity of Li₃V₂（PO₄）₃. But, the carbon layer is an ion insulator and improves the electronic conductivity limitedly. That is, carbon-coating cannot improve the electrochemical performance of Li₃V₂（PO₄）₃ greatly to meet the needs of high-power battery.In this paper, Li₃V₂（PO₄）₃/C is prepared via a solid-state reaction, and the surface structure are modified by different methods, which improve the kinetics property of the materials thus leading to better electrochemical performance. Moreover, in order to comprehend the application value of Li₃V₂（PO₄）₃/C cathode material deeply, we study Li4Ti5O12/Li₃V₂（PO₄）₃ full cell and Li₃V₂（PO₄）₃/C as cathode for aqueous lithium-ion battery comprehensively and systematically. The results are as follows:First, we prepare the Li₃V₂（PO₄）₃/C by a solid-state reaction, and followed by precipitation method to introduce Ru O2 nanoparticles as second phase into the surface of carbon layer. In the hybrid surface coating layer, amorphous carbon atcs as the main body, which improves the electronic conductivity of material, and a nanosized secondary phase impregnated into the carbon coating will not only protect vanadium from dissolved in the electrolyte, but also act a channel for Li-ion. As a consequence, the Li₃V₂（PO₄）₃ cathode material coated with the C+Ru O2 hybrid surface layer shows good electrochemical performance, it deliver a discharge capacity of 102.5 m Ah/g at 10 C rate with a capacity retention of 98.4% after 100 cycles, which is nearly 30 m Ah/g higher than Li₃V₂（PO₄）₃/C.In addition, carbon nanofibers（CNFs） with high electronic conductivity are in-situ prepared on carbon coated Li₃V₂（PO₄）₃ by chemical vapor deposition method, the CNFs connect the isolated Li₃V₂（PO₄）₃ regions thus constructing an efficient conductive network for electrons transportation. In addition, they also restrain the volume expansion during cycling, which improves the electrochemical performance of Li₃V₂（PO₄）₃/C. The Li₃V₂（PO₄）₃/C+CNFs shows a discharge capacity of 113.7 m Ah/g after 300 cycles and the voltage plateaus keep rather well during long term cycling.After that, we combine Li₃V₂（PO₄）₃/C cathode and Li4Ti5O12 anode to form a new and safe full cell system-Li4Ti5O12/Li₃V₂（PO₄）₃, the electrochemical performance of the full cell is adjusted by changing kinds of factors which includes the ratios of capacity, the range of voltage and so on. The full cell delivers 124.1 m Ah/g and 154.9 m Ah/g in the range of 1.5-2.8 V and 1.5-3.3 V, respectly, and the cycle retention is 72.3% and 71.7% after 50 cycles. What is more, we also try to explore the reason of capacity fading and the stability of thermal by EIS and DSC tests.At last, we study the feasibility of Li₃V₂（PO₄）₃/C as a cathode material for aqueous lithium-ion battery（ALIB）, which Li₃V₂（PO₄）₃/C is used as cathode and active carbon（AC） as anode. The electrochemical performance of the full cell is adjusted by change various kinds of factors which includes the kind and concentration of solutions, the range of voltage and the oxygen/oxygen-free of the solutions and so on. The full cell delivers 82.2 m Ah/g and the cycle retention is 93.3% after 100 cycles.In conclusion, this work provides a comprehensive understanding on the surface modifications and application of Li₃V₂（PO₄）₃ cathode materials, and offeres a necessary theoretical and technical guidance for the research of Li₃V₂（PO₄）₃ cathode.