The Preparation And Modification Of Phosphate Cathode For Lithium Ion Batteries

Monoclinic Li₃V₂?PO₄?₃ has attracted considerable interest as a promising lithium ion battery cathode candidate because of its robust structure,high cell-voltage,thermal stability and high safety.However,Li₃V₂?PO₄?₃ suffers poor electronic conductivity due to the nature of its separated V06 octahedral arrangement,which significantly limits its rate performance and the further commercialization.Many efforts have been done to enhance its electrochemical performance such as carbon coating,ion doping and morphology control.In this work,by tuning the Li deficiency and V excess,nonstoichiometric Li_3-3xV_2+x?PO₄?₃composites were prepared by various methods,such as sol-gel method,high energy ball milling assisted sol gel?HEBM-SG?,high energy ball milling and so on.In addition,Li₃V₂?PO₄?₃/C nanofiber was prepared through electrospinning.This paper presents the detailed studies on the effect of nonstoichiometric by different preparation methods and controlling morphology on the structure and electrochemical performance.First,nonstoichiometric Li_3-3xV_2+x?PO₄?₃ composites?x up to 0.12?were prepared by sol-gel method.There is no change of structure by tuning the Li deficiency and V excess.All diffraction peaks can be indexed to the monoclinic Li₃V₂?PO₄?₃ phase,and no secondary phase exists.Without considering the possible O deficiency,Rietveld refinement results revealed that nonstoichiometry resulted in the expansion of lattice,which may make the Li-ions diffusion into and out of Li_3-3xV_2+x?PO₄?₃ particles easier,leading to the increase of ionic mobility and diffusion coefficient,and improvement of high-rate charge-discharge characteristics.Among them,Li_2.7V_2.1?PO₄?₃ exhibits the most excellent performance.The initial discharge capacity is 131 mAh g^-1,which is close to the theoretical capacity(133 mA h g^-1 in 3.0-4.3 V),when the cell was cycled at a rate of 0.5 C in the range of 3.0-4.3 V,and121.6 mAh g^-1 at 10 C.Even at 20 C,it still obtains an initial discharge capacity of 92.5 mAh g^-1,and a notable capacity of 85.1 mAh g^-1 after 1000 cycles,with capacity retention of 92%.Second,in order to optimize the preparation method,we further prepared nonstoichiometric Li_3-3xV_2+x?PO₄?₃ composites by HEBM-SG.Based on the best results of sol-gel method,and compared the performance of cell using Li_2.7V_2.1?PO₄?₃ prepared by sol-gel method and HEBM-SG.Li_2.7V_2.1?PO₄?₃?CSLVP?nanocomposites show core-shell structure with Li₃V₂?PO₄?₃ core and LiVOPO₄ shell by using nonstoichiometric design and HEBM-SG treatment,which is different from LVP?carbon shell and Li₃V₂?PO₄?₃ core?by sol-sel method.The CSLVP nanocomposites have better electrochemical performance.Such kind of electrode can present a high reversible capacity of 131.5 mA h g^-1 at 0.5 C,while thecapacity of LVP is 129.2 mA h g"1.Even at 20 C,it still delivers an excellent dischargecapacity of 116.3 mA h g^-1,higher than that of LVP(92.5 mA h g^-1).Third,based on the results obtained from HEBM-SG,we synthesized nonstoichiometric Li_3-3xV_2+x?PO₄?₃ composites by high energy ball milling,with the purpose of meeting the demand of industrialization.The samples were identified to be the monoclinic Li₃V₂?PO₄?₃and orthorhombic LiVOPO₄,which is agreement with the results using HEBM-SG.The mass concentration of each phase in composites is obtained through Rietveld refinement.With increasing the content of V,the mass concentration of the shell LiVOPO₄ increases.Among them,Li_3-3xV_2+x?PO₄?₃ nanocomposites with x = 0.10 show the best performance,especially remarkable for its high-rate performance.The discharge capacity of 124.3 mA h g^-1 isdelivered at 20 C,close to 1.5 times that of stoichiometric Li₃V₂?PO₄?₃ composites(85 mA h g^-1).Long life cycling test at a high rate of 20 C is also explored,retaining 98.3% of itsoriginal discharge capacity(124.3 mA h g^-1)after 1000 cycles.The calculated diffusioncoffcient D of Li2 7V2.1?PO₄?3 is 1.84 × 10-9 cm2 s-1,more than three times of Li₃V₂?PO₄?₃?5.37 × 10-10 cm2 s-1?.LiVOPO₄ is high capacity cathode during the charge/discharge processto improve the capacity of the composites,with the synergistic contribution from Li₃V₂?PO₄?₃to improve the capacity of the composites.But too thick shell may hinder the Li+/e-transport in the charge/discharge process caused by the poor electronic conductivity of LiVOPO₄.Last,by controlling morphology,carbon-coated Li₃V₂?PO₄?₃ nanofibers?C-LVP/CNFs?were made through electrospining combined with plasma enhanced chemical vapor deposition?PECVD?.The LVP/CNFs consist of long and continuous composite nanofibers with an evenly-distributed diameter of 220 nm.It was observed that the LVP nanoparticles with an average size of ⁵0 nm were embedded in the nanofibers uniformly.And there was no change about morphology after PECVD.C-LVP/CNFs have more stable charge/discharge property,especially in high rate and cycle performance.At 0.5 C in the range of 3.0-4.3V,the composite C-LVP/CNFs showed an initial discharge capacity of 128 mA h g^-1?96.2% of the theoretical capacity?.The discharge capacity of 122 mA h g^-1 was obtained,and a capacity retention of 98.9% was retained after 500 cycles at 5 C,indicating excellent high-rate capability and capacity retention.