Preparation And Electrochemical Properties Of MnO₂ Coated Li₃V₂?PO₄?₃/C Cathode Materials

Li₃V₂?PO₄?₃/C was prepared by an improved sol-gel method?PH=4?and then modified with various amounts of MnO₂?0,2,3 and 4 wt.%?by a suspension coating process assisted by polyvinyl alcohol?PVA?.The composition and microstructure of these composites were analyzed by XRD,XPS,BET,SEM,HRTEM and so on.The cathode films were made by coating method with MnO₂coated Li₃V₂?PO₄?₃/C powder and then assembled into coin batteries.The electrochemical properties of the samples at different temperature?room temperature,0°C and-20°C?were characterized by galvanostatic charge/discharge test,cycle voltammetry?CV?and electrochemical impedance spectroscopy?EIS?.Galvanostatic electrochemical measurements indicated that the surface modification with MnO₂ was an effective way to improve the first discharge capacity and the cycling stability of Li₃V₂?PO₄?₃/C cathode materials,especially at high rates and low operation temperatures.3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C showed the best electrochemical performance in the voltage range of 3.0-4.8 V.At room temperature,the discharge specific capacity of 3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C was 94.7 mAh g^-1?56.7%capacity retention?after 60 cycles at 0.1-5C rate.When recycled after 10th cycle at 0.2 C,the capacity retention of 3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C was still as high as 75.3%.Therefore,with the increase of the rate and cycle times,the effect of Mn O₂ modification became more obvious.At 1 C rate,the first discharge specific capacity of 3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C sample at room temperature,0°C and-20°C was 144.4,132.4 and100.4 mAh g^-1,respectively,and the growth rate of capacity was 10.9%,13.4%and72.8%respectively,compared with the pristine sample.It was significant that,with the decrease of temperature,the effect of MnO₂ modification was more remarkable.EIS test showed that the surface modification of MnO₂ for Li₃V₂?PO₄?₃/C could significantly reduce the charge transfer resistance(R_ct).The R_ct value of 3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C increased from 18.88?at room temperature to 135.12?at-20°C,while the R_ct value of the pristine sample increased 47.04?at room temperature to 410.31?at-20°C.It was clearly seen that the growth rate of the R_ct value for 3 wt.%MnO₂ coated Li₃V₂?PO₄?₃/C was far below as the temperature decreased that for the pristine sample,indicating that the MnO₂ modification could remarkably improve the charge transfer processes at low temperatures.