Doping Of Layered Orthormbic Li MnO₂ Cathode Materials And Electrochemical Properties

The layered LiMnO₂ with an orthormbic phase（o-LiMnO₂）,as a cathode material for lithium-ion batteries（LIBs）,has a theoretical specific capacity as high as 286 mAh/g when carrying out one electron reaction.However,the low initial coulombic efficiency,poor cycling performance and the Jahn-Teller effect from high-spin Mn³⁺that making the material unstable during battery operation,which hinder the commercial application of the o-LiMnO₂ cathode in LIBs.In order to overcome these shortcomings,the layered o-LiMnO₂ can be modified via a lattice-doping way.In this work,layered o-LiMnO₂cathode materials were prepared by three methods.Of which,the optimized sample was doped with fluoride ion and vanadium ion,respectively.The effects of element-doping amount on the electrochemical performances of the as-prepared cathode materials were investigated in detail.The main contents include:Firstly,o-LiMnO₂ materials were prepared by high-temperature solid-phase method and hydrothermal method using lithium hydroxide and lithium carbonate as lithium sources,respectively.SEM images show that all samples display obvious sheet-like morphology.XRD patterns suggest that the LMO-1 sample,which synthesized by the high-temperature solid-phase method using Li OH salt,possessed the highest crystallinity and purity among three samples.The results of electrochemical tests show that the LMO-1 material has the best electrochemical performance.It can deliver the maximum specific capacity of 125.6 mAh/g at 0.1 C rate.After 100 cycles,the specific capacity still retained 82.5 mAh/g with a capacity retention of 65.7%.The results indicate that the specific capacity and cycle performance of the LMO-1 material are unsatisfied when used as cathode material in LIBs,implying that it needs further investigated.Secondly,the LMO-1 were prepared by doping of various amounts of fluoride ions（molar ratios changed from 0,2%,5%to 8%）.SEM images show that the as-prepared materials have obvious flake-like morphology.The crystallites seem to grow along the stacking direction of the flakes.Meanwhile,the LMOF-5 sample has an uniform particle size and displays smooth surface.XRD patterns suggest that fluoride ions have been successfully doped into the crystals.Among the F-doped samples,the LMOF-5sample that doped with 5%fluoride ions presents the best electrochemical performance.It can deliver the maximum specific capacity of 198.2 mAh/g at 0.1 C rate,and the capacity retention is 82.0%after 200 cycles at 0.2 C rate.Besides,it can also deliver a specific capacity of 120.5 mAh/g at 5 C rate.The results indicate that appropriate amount of fluoride ions doping can effectively suppress the phase transition of o-LiMnO₂ cathode,thus improving the structural stability and electrochemical performance.Finally,the LMO-1 material was prepared by doping various amounts of vanadium ions（0,2%,5%）.From the SEM images,the V-doped materials were found to still present sheet-like morphology,and the crystal planes seem to grow along the stacking direction of the sheets.XRD patterns suggest that vanadium ions have been successfully doped into the material.Among the samples,the sample doped with 2%vanadium ions displays the best electrochemical performance.It can deliver the maximum specific capacity up to 185.4 mAh/g at 0.1 C rate,and retains 79.8%of the maximum capacity after 200 cycles at 0.2 C rate.Besides,it can also deliver a specific capacity of 110.8 mAh/g at 5 C rate.The results demonstrate that a proper amount of vanadium ion doping can improve the charge-discharge specific capacity and cycle stability.