Synthesis And Electrochemical Performance Of Li-rich Manganese-based Li_1.2 Mn_0.54Ni_0.13Co_0.13O₂ Cathode Materials For Lithium Ion Batteries

As an all-important energy storing device,lithium-ion batteries?LIBs?have been widely applied in many fields,such as transport system,communication,digital products and aerospace.As known,the performance of LIBs is highly depended on the cathode materials.The current commercial cathodes of LIBs?e.g.,layered LiCoO2,olivine LiFeO4 and spinel LiMn2O4,etc.?have inferior specific capacities?120¹70 mAh g-1?and thus cannot meet the requirements of high energy density for new generation power LIBs.In recent years,Li-rich layered oxide of Li_1.2Mn_0.54Ni_0.13Co_0.13O2 has attracted considerable attention because of its large specific capacity.Li_1.2Mn_0.54Ni_0.13Co_0.13O2 cathode has a large specific capacity?> 250 mAh g-1?,wide working potential?4.6⁴.8 V vs.Li/Li+?and good cycle stability and is expected to be a new generation cathode material for LIBs.However,this cathode material also suffer from several serious problems,for instance,large initial irreversible capacity loss,poor rate performance and fast discharge potential and capacity fading.In this work,La2O3 and Bi2O3,which are easily prepared and stable at high-voltage,were used as coating materials,while the electrochemical inactive Sn4+was choosed as a dopant for Li_1.2Mn_0.54Ni_0.13Co_0.13O2 cathode material.La2O3@Li_1.2Mn_0.54Ni_0.13Co_0.13O2,Bi2O3@Li_1.2Mn_0.54Ni_0.13Co_0.13O2 and Li_1.2Mn_0.54-xNi_0.13Co_0.13SnxO2 cathode materials were fabricated and the effects of La2O3-coating,Bi2O3-coating and Sn4+-doping on the structure,morphology and electrochemical properties of the Li_1.2Mn_0.54Ni_0.13Co_0.13O2 were studied.The main results are listed as following:?1?La₂O₃@Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials were fabricated via acombined method of sol-gel and wet chemical processes and the structure,morphology and electrochemical performance of the materials were investigated.The structural and morphological characterizations of the materials demonstrate that a thin layer of La2O3 is uniformly covered on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O2 particles,and the coating of La2O3 has no obvious effect on the crystal structure of Li-rich oxide.All materials prossess a layered a-NaFeO2 structure.The electrochemical performance of La2O3-coated Li-rich cathodes including specific capacity,cycling stability and rate capability has been significantly improved after the coating of La2O3.The Li_1.2Mn_0.54Ni_0.13Co_0.13O2 coated with 2.5 wt% La2O3 exhibits the highest discharge capacity,delivering a large discharge capacity of 276.9 mAh g-1in the 1st cycle and a high capacity retention of 71%?201.4 mAh g-1?after 100 cycles.The optimal rate capability of the materials is observed at the coating level of 1.5 wt% La2O3 such that the material exhibits the highest discharge capacity of 90.2 mAh g-1at 5 C.The surface coating of La2O3 can effectively facilitate Li+ interfacial diffusion,reduce the structural change and secondary reactions between cathode materials and electrolyte during the charge-discharge process,and thus induce the great enhancement in the electrochemical properties of the Li_1.2Mn_0.54Ni_0.13Co_0.13O2 materials.?2?Li-rich layered oxide of Bi₂O₃@Li_1.2Mn_0.54Ni_0.13Co_0.13O2 was fabricated via a combined method of sol-gel and wet chemical coating processes and the structure,morphology and electrochemical performance of the material were investigated.Bi2O3 is homogenously coated on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O2 particles.The crystal structure of Li_1.2Mn_0.54Ni_0.13Co_0.13O2 has no obvious change after the coating of2 wt% Bi2O3,which still prossess a layered a-NaFeO2 structure.After the coating of 2wt% Bi2O3,the initial coulombic efficiency of the material has been improved to76.9% compared to 72.6% for the bare Li_1.2Mn_0.54Ni_0.13Co_0.13O2.After 100 cycles at 0.1C,Bi2O3-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O2 electrode retains a discharge capacity of182.9 mAh g-1 with capacity retention of 73.5% which are much higher than that of Li_1.2Mn_0.54Ni_0.13Co_0.13O2?146.9 mAh g-1and 58.8%?.In addition,Bi2O3-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O2 electrode exhibits better rate capability with discharge capacities of 155.8,124.8 and 89.2 mAh g-1 at 1,2 and 5 C,respectively,superior to that of Li_1.2Mn_0.54Ni_0.13Co_0.13O2.The enhanced cyclic stability and rate capability should be ascribed to the existence of Bi2O3 on the surface of active material,which can efficiently restrain the side reactions between electrode and electrolyte,stabilize the structure of Li_1.2Mn_0.54Ni_0.13Co_0.13O2 and improve the lithium ion diffusion.?3?Sn⁴⁺-doped Li-rich layered oxides of Li_1.2Mn_0.54-xNi_0.13Co_0.13SnxO2 have been synthesized via a sol-gel method and the structure,morphology and electrochemical performance of the materials have been studied.The addition of Sn4+ ions has no distinct influence on the crystal structure of the materials.After doping with appropriate amount of Sn4+,the electrochemical performance of Li_1.2Mn_0.54-xNi_0.13Co_0.13SnxO2 cathode materials is significantly enhanced.The optimum electrochemical performance is obtained at x = 0.01.The Li_1.2Mn0.53Ni_0.13Co_0.13Sn0.01O2 electrode delivers a high initial discharge capacity of 268.9 mAh g-1 with initial coulombic efficiency of 76.5% and a reversible capacity of 199.8 m Ah g-1 at 0.1 C with capacity retention of 75.2% after 100 cycles.In addition,the Li_1.2Mn0.53Ni_0.13Co_0.13Sn0.01O2 electrode exhibits the superior rate capability with discharge capacities of 239.8,198.6,164.4,133.4 and 88.8 mAh g-1 at 0.2,0.5,1,2 and5 C,respectively,which are much higher than those of Li_1.2Mn_0.54Ni_0.13Co_0.13O2.The substitution of Sn4+ for Mn4+ enlarges the Li+ diffusion channels due to its larger ionic radius in comparison with Mn4+ and enhances the structural stability of Li-rich oxides,leading to the improved electrochemical performance in Sn-doped Li_1.2Mn_0.54Ni_0.13Co_0.13O2 cathode materials.