Perparation And Modification Of High-Capacity LiNi_0.6Co_0.2Mn_0.2O₂ Cathode For Lithium Ion Batteries

As energy shortage and environment problem are increasingly outstanding,the new green lithium ion batteries has become a research hotspot.To meet the widening demand for lithium ion batteries in the field of electric vehicles and communications,it is a persistent challenge in recent years to develop long life batteries.The high-nickel LiNi_0.6Co_0.2Mn_0.2O₂ cathode material is gradually entering the high-energy-density battery market due to its excellent comprehensive performance.It is an effective way to increase the working voltage to increase the energy density of Li-ion batteries,however,it may bring adverse effects to the LiNi_0.6Co_0.2Mn_0.2O₂ material,such as poor cycling and rate performances.The material design,attenuation study and the modification of this material has been done to improve the above problems.The main results are listed below:The sub-micron level nickel-high material LiNi_0.6Co_0.2Mn_0.2O₂,to be object in the subsequent degradation mechanisms study,has been synthesized by using a versatile hydrothermal/solid phase method.The sub-micron level ensures the material has a short transmission channel,which is in favor of rapid lithium-ion insertion/deinsertion rate,and enables it to reduce side reactions between the cathode and the electrolyte.The material displays a high capacity of 183 mAh·g^-1 at 0.2C and 153 mAh·g^-1 at a rate of 10C-rate under the voltage of 2.8 V⁴.3 V.Interestingly,it has also performed well under a high operating voltage?4.6 V?,which has 202 mAh·g^-1 at 0.2C.The effect of different current density on the structure transformation of LiNi_0.6Co_0.2Mn_0.2O₂ material is studied under a definite voltage?4.6 V?.It shows that the capacity fading is accelerated by the high discharge current density.It is mainly caused by the structure instability of the material.When the current density increase,the loss of lithium of the surface materialbecomes sever,and the oxidation state becomes higher.At the same time,the unit cell parameters change,that is the a becomes bigger,while the c becomes smaller.Meanwhile the high discharge current density plays a catalytic effect on the structural transformation that the surface structure changes to the rock salt phase structure under high voltage.The cathode material is covered by a 3-nm-thick and discontinuous rock salt phase after cycling 50 times at the current density of 100 mA·g^-1.However,when the material cycled at the current density of 1000 mA·g^-1,the rock salt phase becomes thicker?5⁷-nm-thick?and more continuous,which may lead to the fast capacity fading.The high valence and large ionic radius Mo⁶⁺is selected as a dopant to modified the LiNi_0.6Co_0.2Mn_0.2O₂ material though the molecular level mixed calcination method.The Mo⁶⁺substitutes proportionately the transition metal atoms in the material,which has no influence on the original atom ratio of the transition metal,and can expand the Li⁺diffusion pathways.The strong bond energy of Mo-O benefits the stability of the material.Meanwhile the electrochemical reaction of Mo⁶⁺/Mo⁴⁺in the process of charging and discharging could increase the capacity of the material.The electrochemical impedance spectroscopy has clearly demonstrated that the Mo-doping reduces the charge transfer impedance and enhances the electrochemical reaction activity of the lithium ion.When x=0.01,the material shows an excellent cycling and rate performance.The discharge specific capacity can reach up to 208 mAh·g^-1 at 0.2 C under 2.8V⁴.6V,and remains 75%of the first discharge capacity after 101 cycles,which is 10%higher than the un-doped material.Li₂SiO₃-coated LiNi_0.6Co_0.2Mn_0.2O₂ cathode material has been synthesized by the simultaneously synthetic method.The coating layer of Li2SiO3 has a good interface compatibility with the bulk material.The coated material displays an excellent rate and cycling performance under high voltage due to the excellent Li⁺-ion and electron conductor of the coating layer.The LiNi_0.6Co_0.2Mn_0.2O₂ coated with 3 mol.%Li₂SiO₃ exhibits an excellent electrochemical performance.It remains 85%of the first discharge capacity after100 cycles at 0.2C and reaches 158 mAh·g^-1 at 10C rate.The improved properties may be attributed to the coating layer which suppresses the side reactions at electrode/electrolyte interface,and enhances the structure stability of the bulk materials with high Li⁺-ion conductivity,and smaller polarization.Meanwhile Li2SiO3 in the positive electrode can improve the electrolyte performance.So it can bring new visions to the coating modification fot the cathode materials.