Synthesis And Modification Of Nickel-rich LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material For Lithium-ion Batteries

As a green renewable energy,lithium-ion batteries（LIBs）have become one of the feasible alternatives for power sources in portable electronic devices and electrical vehicles,because of their high specific energy,high voltage and long cycle life.LIBs with high electrochemical performance is critically need for power batteries.Among various issues,the cathode material is one of the important factors for LIBs.Therefore,developing alternative cathode material with high performance is imperative.Recently,nickel-rich layered Li[Ni_xCo_yMn_1-x-y]O₂（x≥0.6）cathode material have attracted much attention due to their high specific capacity and low cost.In this work,the synthesis of high performance nickel-rich layered LiNi_0.6Co_0.2Mn_0.2O₂ were studied.A spherical-like precursor was controllably tuned to synthesize the high performance LiNi_0.6Co_0.2Mn_0.2O₂ cathode material.A one-step method was used to synthesize the tungsten compound coated LiNi_0.6Co_0.2Mn_0.2O₂ sample.The electrochemical performance of the as-prepared LiNi_0.6Co_0.2Mn_0.2O₂ sample was also tested under different cut-off voltages.The as-prepared material was characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscope（TEM）,inductively coupled plasma（ICP）,Xray Photoelectron Spectroscopy（XPS）,laser particle size analysis,charge-discharge test,cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）measurements.The contents are as follows:（1）A spherical-like Ni_0.6Co_0.2Mn_0.2（OH）₂ precursor was controllably synthesized via co-precipitation method to tune the properties of LiNi_0.6Co_0.2Mn_0.2O₂ as a cathode material.The morphology and structure of the precursor is important for the high performance LiNi_0.6Co_0.2Mn_0.2O₂ material.The effects of calcination temperature on the structure,morphology and electrochemical performance of the material were investigated in detail.The results show that the spherical-like LiNi_0.6Co_0.2Mn_0.2O₂ material obtained by calcination at 900°C displayed the most obvious layered structure among samples calcined at various temperatures,with a particle size of approximately 10μm.It delivered an initial discharge capacity of 189.2 mAh g^-1 at 0.2C(1C=180 mA g^-1)with a capacity retention of94.0%after 100 cycles between 2.7 and 4.3 V.The as-prepared cathode material also exhibited good rate performance,with a discharge capacity of 119.6 mAh g^-1 at 5C.（2）A tungsten compound was successfully coated on the nickel-rich layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode material by one-step method.A uniform distribution of Li₂WO₄ was obtained on the LiNi_0.6Co_0.2Mn_0.2O₂.The effects and content of the coating layer were investigated.The Li₂WO₄ coating layer not only was benefit for the insertion and extraction of lithium ions,but also restrained the side reaction between LiNi_0.6Co_0.2Mn_0.2O₂ and the electrolyte and the transition metal dissolution effictively.These all helped to improve the electrochemical performance of the nickel-rich layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode material.The best electrochemical performance was achieved with a content of 2wt%.It delivered an initial discharge capacity of 191.1 and 177.8 mAh g^-1,respectively,at 0.2C and 1C.The corresponding capacity retention was 95.6%and94.4%,respectively,after 100 cycles between 2.7 and 4.3 V.It also yielded high initial discharge capacity of 146.8 and 126.5 mAh g^-1 at 2C and 5C rate,respectively,after 100cycles,the corresponding capacity retention was 87.9%and 81.3%.（3）The electrochemical performance of the as-prepared LiNi_0.6Co_0.2Mn_0.2O₂ cathode material was also examined within the voltage range from 2.7 to 4.3,4.4,and 4.5 V,respectively.With the increasing of charge cut-off voltage,the discharge capacity of the as-prepared material increased,but the cycling stability became worse.The uniform spherical-like shape is beneficial to maintaining the structural stability of the material and to reducing Co dissolution,thus the as-prepared LiNi_0.6Co_0.2Mn_0.2O₂ exhibited good cycle performance.Within the cut-off voltage ranges from 2.7 to 4.3,4.4,and 4.5 V,the initial discharge capacity of the calcined samples was 170.7,180.9,and 192.8 mAh g^-1,respectively,at a rate of 1C.The corresponding retention was 86.8%,80.3%,and 74.4%after 200 cycles,respectively.2wt%tungsten compound coated material exhibited bettter electrochemical performance than the bare LiNi_0.6Co_0.2Mn_0.2O₂ cathode sample,especially the long cycle stability at higher voltages.Its initial discharge capacity was 177.8,183.4and 194.6 mAh g^-1,respectively.The corresponding retention was 88.6%,82.7%,and 76.7%after 200 cycles,respectively.