The Interface Behavior Of Lithium Ion Battery Cathode Lithium Vanadium Phosphate During Cycling

With more and more applications for lithium ion batteries in the electric car and the national grid,high energy density,high power density and long cycle stability are becoming permanent pursuits of the new generation of lithium-ion batteries.Li₃V₂?PO₄?₃?LVP?has emerged as an appealling cathode material for its thermodynamically stable structure,high oxidation-reduction potential,large theoretical specific capacity and abundance in natural.However the practical application is largely obstructed for LVP because its poor electronic conductivity,poor cycle stability and capacity fading,which is much severer at high voltage.Exploring the attenuation mechanism for the electrochemical performance of lithium battery electrode materials has drawn long-term interest in the battery industry research,which also helps to better design the battery material.Here,the amorphous carbon layer is coated on the surface of Li₃V₂?PO₄?₃ nanoparticles to improve the conductivity and cycling stability of Li₃V₂?PO₄?₃.By means of transmission electron microscopy?TEM?,we can carefully characterization the the structure of LVP after cycling and focus on probing the influence of LVP structure on the electrochemical performace to imporve the structure and promote the the diffusion of lithium ions with the ultimate goal that is enhancing the overall performance of the battery.The Li₃V₂?PO₄?₃ was prepared via a convention solid state reaction with a-tennanometer amorphous carbon layer coated at the surface to improve the intrinsic conductivity of vanadium phosphate.The size of as-synthesised LVP particle ranges from 500 to 1000 nm with a 10.92 wt.% carbon content.The electrochemical properties were tested at 3.0 4.3V and 3.0 4.8V respectively using coin-type cells.When cycled at a rate of 0.5C,1C,2C,5C,10C?1C=197mAh/g?in 3.0⁴.3V,LVP/C shows a capacity of 124mAh/g,115.3mAh/g,107mAh/g,102mAh/g,82mAh/g respectively and almost no fading after 500 cycles,which stands for an outstanding cycling performance and also reflects the good effect of carbon coating and nanocrystallization.However,when cycled at 0.2C in 3.0⁴.8V,the capacity fast fades to 110.7 mAh/g from the initial 165.7 mAh/g,displaying a serious attenuation problem.In order to investigate the interface behavior between cathode and the electrolyte and the origin of the difference in electrochemical performance of LVP/C in two electrochemical windows,we carried out the electrochemical impedance measurements and TEM analysis.We fouce on the solid electrolyte interface?SEI?formed on the surface of LVP/C,because the formation of SEI can cause irreversible capacity and the capacity fading and display a large effect on the performance of LVP/C.SEI film can be generated in both electrochemical windows and consisted of C,O,P,F.The thickness of SEI in 3.0⁴.3V after 500 cycles is uniform and about 20 nm,which is formed and tends to be stable in the first ten cycles.As a result,the excellent electronchemical performance during long term cycling can be acquired in 3.0⁴.3V.While SEI film becomes much thicker in 3.0⁴.8V and its thickness ranges from 40 to 113 nm.Such a thick SEI film should lead to more irreversible capacity,thus the electronchemical performance becomes poor.What is worse,the Al on cathod electrode current collector foil would be dissolved so that the contact between the active materials becomes poor,resulting in an increase in the transport resistance of lithium ions.The thicker SEI film and the dissolution of the aluminum foil make the impedance after 500 cycle grow about 5 times bigger than that of pristine impedanc in 3.0⁴.8V.But,the impedance after 500 cycle decreases to half of the pristine impedance in 3.0⁴.3V.So the electronchemical performance is much worse in 3.0⁴.8V than that in 3.0⁴.3V.Besides,the V element would dissolve during cycling in 3.0⁴.8V.On the one hand,the dissolvation of V into the electrolyte can decrease the the conductive group of VO6 octahedron so that weakens the electronic conductivity.On the other hand,the repulsive force between the negative groups is increased,which can induce the increase of interplanar spacing in both directions [020] and [112] up to 3.31% and 3.97% respectively.What is more,the extration of the third lithium in LVP along [020] happens in every second layer,resulting in a set of supperlattice.