Study On Preparation And Modification Of LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials For Lithium-Ion Batteries

The Ni-rich(LiNi_0.8Co_0.1Mn_0.1O₂)cathode materials have attracted widespread on account of their high discharge specific capacity.Unfortunately,poor cycle stability and low capacity retention of Ni-rich cathode materials are recognized as the major obstacles for large-scale commercialization,especially under high temperature and high voltage.Therefore,this paper proposes new strategies to improve the cycle and structural stability of NCM811 by doping Gd³⁺and coating Pr F₃.The main research results are as follows:（1）The LiNi_0.8Co_0.1Mn_0.1O₂ and modified Li(Ni_0.8Co_0.1Mn_0.1)_1-xGd_xO₂（x=0,0.005,0.01,0.03）is synthesized by sol-gel method.The XRD results show that Gd³⁺doping hardly change the crystal structure and 0.01 mol of Gd doping shows the smallest of Li⁺/Ni²⁺mixing.XPS results indicate that Gd³⁺doping inhibits the escape of lattice oxygen in the material due to the high Gd-O bond energy.Owing to the stable structure,0.01 mol of Gd doping exhibits a 91.5%capacity retention at 1 C over 2.8～4.3 V after 100 cycles,while the bare NCM811only delivers the retention of 73.2%.At a high rate of 5 C,the Gd-doped sample can still maintain a superior capacity of 147.3 m Ah g^-1,which is much higher than the bare NCM811(68.5 m Ah g^-1).The CV and EIS tests show that Gd doping can reduce polarization and alleviate the charge transfer impedance.In addition,GITT calculated that the Li⁺diffusion rate increased significantly after Gd doping.All this is mainly due to the large ionic radius of Gd,which supports the crystal structure and makes the doped material more stable during the charge and discharge process.Therefore,the above results signify that moderate Gd doping is an effective strategy to develop high-performance of Ni-rich cathode materials.（2）The Pr F₃ layer is employed to protect LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）via a simple wet chemical process.It is confirmed by XRD,HR-SEM,TEM,EDS,and XPS tests that Pr F₃ is evenly covered throughout the surface of NCM811 without affecting the particle size and surface morphology.In particular,1 wt.%Pr F₃ coated NCM811 exhibits excellent stability and rate capability with the capacity retention of 86.3%after 100 cycles at 1 C under a cut-off potential of 4.3 V,while the retention of pristine one is only 73.8%.Moreover,the capacity retention of 1 wt.%Pr F₃ coated samples enhances from74.5%to 88.5%after 50 cycles at 1 C under higher cut-off voltage of 4.6 V.The CV and EIS tests also reveal that the coated samples have lower polarization and charge transfer impedance.The superior performance for coated samples can be attributed to the fact that Pr F₃ can effectively isolate the active material and the electrolyte from HF corrosion,and at the same time,reduce the generation of micro-cracks on the surface during prolonged cycles.Furthermore,as a physical barrier,Pr F₃alleviates the dissolution of transition metals in the electrolyte largely.These results suggest that the stability of NCM811 can be greatly upgraded at high voltage by Pr F₃ coating.