Synthesis And Electrochemical Performance Improvement Of Layered Nickel-Based Cathode Material For Lithium-Ion Batteries

Energy plays a vital role in social and economic development.At present,the problem of depletion of traditional fossil fuels has become increasingly significant,and the environmental problems caused have to be solved urgently.Therefore,the development of clean energy,energy conservation and environmental protection technology is an important way to achieve human sustainable development.Lithium ion batteries are widely used as highly efficient and environmentally friendly energy storage and conversion devices.However,the lack of high-performance lithium-ion battery cathode material is an important bottleneck restricting its further development.As a kind of cathode materials used in high-energy lithium batteries,layered LiNi_xCo_yM_zO₂（x≥0.5,M=Mn/Al）oxides,which are realizing its wide application in new energy vehicles,own high specific capacity.But their cycling stability,rate capability and safety performance still need to be further improved.In this study,the performance of nickel-rich materials of Ni-Co-Mn system has been improved.The following research results are mainly achieved:（1）A wet process has been adopted to remove the Li₂CO₃/LiOH residue of LiNi_0.5Co_0.2Mn_0.3O₂ and form MoO₃/Li₂MoO₄ coating layer on its particle surface.The coating layer enhances the stabilization of particle surface effectively,suppressing the erosion of the electrolyte and enhancing the structural stability of the material.Therefore,the MoO₃/Li₂MoO₄ coating layer enhances the cycling stability and rate capability of LiNi_0.5Co_0.2Mn_0.3O₂.The capacity retention of 3 wt%-MoO₃/Li₂MoO₄ coated LiNi_0.5Co_0.2Mn_0.3O₂ is 90.9%after 100 cycles at 0.2 C rate charge/discharge in the voltage range of 2.7⁴.4 V.As a comparison,the pristine LiNi_0.5Co_0.2Mn_0.3O₂ exhibits the capacity retention of only 82.8%.The discharge capacity of the pristine LiNi_0.5Co_0.2Mn_0.3O₂ at 5 C is less than 90 mAh g^-1.However,the discharge capacity of 3 wt%-MoO₃/Li₂MoO₄ coated LiNi_0.5Co_0.2Mn_0.3O₂ at 5 C increases to 124.4 mAh g^-1.（2）The effect of the added Li source amount during the sintering process on the cation disorder of LiNi_0.8Co_0.1Mn_0.1O₂ has been studied.The increase of Li amount in the Ni-rich material can increase Ni²⁺content by the reduction of Ni³⁺,which contributes to the suppression of Li⁺/Ni²⁺cation disorder,thus enhancing the structural stability and improving the electrochemical performance.The pristine and 10%,20%Li-excess of the material show discharge capacity of 120.6 mAh g^-1,125.0 mAh g^-1,151.2 mAh g^-1（55.5%,60.1%,73.4%capacity retention）after 100 cycles at 0.2 C between 2.7⁴.6 V,respectively.The appropriate increase in the Li amount during the calcination process also improves the rate capability of Ni-rich material.The pristine and 10%,20% Li-excess of the material reveal discharge capacity of 22.5 mAh g^-1,35.8 mAh g^-1,83.8 mAh g^-1 after 100 cycles at10 C between 2.7⁴.6 V,respectively.（3）Nickel-rich LiNi_0.7Co_0.15Mn_0.15O₂ material with the dominant growth of（010）-system crystal plane has been prepared by PVP-directed method.The dominant growth of（010）-system crystal plane provides smooth channels for the diffusion of lithium ions in Ni-rich material.Therefore,the material exhibits excellent rate performance,which shows stable discharge capacity of 196.0 mAh g^-1,185.6 mAh g^-1,177.9 mAh g^-1,168.6mAh g^-1,155.6 mAh g^-1 and 143.0 mAh g^-1 between 2.7⁴.5 V at 0.2 C,0.5 C,1 C,2 C,5C and 10 C,respectively.The LiNi_0.7Co_0.15Mn_0.15O₂ material also shows good cycling performance during the charge/discharge performance.（4）The surface rock-salt Fm3m phase of nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂ material,which is detrimental to the structural stability and cycling performance,has been successfully removed in this work.The obtained LiNi_0.71Co_0.09Mn_0.1O₂ material containing a uniform layered Fm3m phase with concentration-gradient Mn-rich shell improves the structural stability,contributing to better cycling performance as well as the thermal stability.The pristine LiNi_0.8Co_0.1Mn_0.1O₂ shows discharge capacity of 145.9 mAh g^-1 with capacity retention of 74.7%at 0.1 C between 2.8⁴.5 V.However,the modified material reveals discharge capacity of 164.4 mAh g^-1 with capacity retention of 92.2%.The thermal stability of Ni-rich material is also improved by this modification method.