The Modification Of Li-rich Li_1.2Ni_0.17Co_0.07Mn_0.56O₂ Cathode Materials For Lithium-ion Batteries

With the rapid development of energy storage systems such as smart grids and electric vehicles,exploiting high-energy density lithium-ion batteries has become more and more urgent,which is restricted by cathode materials.Lithium-rich manganese-based layered oxide is one of the candidates for advanced lithium-ion battery cathode materials due to its high capacity,low cost and less pollution.However,there are still several tough problems to be resolved,which block their commercialization,such as unsatisfied cycle performance,poor rate capability,low initial coulombic efficiency and severe voltage decay.In this paper,Li_1.2Ni_0.17Co_0.07Mn_0.56O₂?LLMO?cathode materials were successfully synthesized via the method of co-precipitation and solid-state reaction and the surface modification was studied.LLMO was modified with aluminum-based materials to study the effects of Al³⁺doping,LiAlO₂ coating and Al³⁺doping and LiAlO₂ coating co-modification on LLMO performance.The cycle retention of the Li_1.2(Ni_0.17Co_0.07-x.07-x Mn_0.56Al_x)O₂increases with the increase of the Al³⁺doping amount,which is attributed to the fact that the high Al-O bond energy can reduce ion rearrangement resulting from the extraction and insertion of Li⁺and maintain layered frame structure.The best performance is obtained when x is 0.05,and the capacity retention after 50 cycles is 81%,which is much higher than 67%of LLMO.The fast ion conductor LiAlO₂acting as a coating can improve the Li⁺migration rate of the material.The LiAlO₂modified LLMO has excellent electrochemical performance,especially the 5wt.%LiAlO₂ modified sample has a retention rate of 91%after 50 cycles,which is higher than 73%of the uncoated LLMO.The Al³⁺doped and LiAlO₂ coated co-modified LLMO was obtained by a simple method.The carbonate precursor is treated with different amounts of C₉H₂₁O₃Al,and then calcined with a stoichiometric ratio of Li₂CO₃.XRD and electrochemical performance analysis show that the sample has a better layered structure and property when the amount of C₉H₂₁O₃Al is 5wt.%.The first discharge specific capacity at a current density of250mA·g^-1and the retention rate after 100 cycles of this simple are 222mAh·g^-1and 89%,which are higher than 218mAh·g^-11 and 74%of the LLMO at the same conditions.The method not only stabilizes the layered structure,but also prevents the electrolyte from attacking the base material,thereby improving the cycle life of the material.The effect of V₂O₅,LiV₃O₈,Li₃V₂?PO₄?₃ and Li₃V_2-x-x Al_x?PO₄?₃ on the performance of LLMO was studied.After the introduction of V₂O₅,LiV₃O₈ and Li₃V₂?PO₄?₃,the electrochemical performance of LLMO is enhanced to some extent,especially the sample treated by Li₃V₂?PO₄?₃,which not only has the best capacity retention rate,the voltage decay is also suppressed.The average discharge voltage dropped from 3.25 to 3.22V after 50 cycles with a decrease of only 0.03V,while LLMO decreased from 3.39 to 3.01V,and the voltage drop was as high as 0.38V.Li₃V₂?PO₄?₃ doped with Al³⁺has more stable properties,so LLMO is treated with Li_x V_2-x-x Al_x?PO₄?₃ with different values of x.The results show that the performance is optimal when x=0.5,the first coulomb efficiency is92%,the capacity retention is 95%after 50 cycles,and the reversible capacity is up to 159mAh·g^-1at the high current density 1250mA·g^-1.However,the first coulomb efficiency,retention rate and discharge specific capacity of LLMO are85%,76%and 100mAh·g^-1,respectively.Considering the positive effect of Li₃V_1.5Al_0.5?PO₄?₃,LLMO was modified with different coating amounts of Li₃V_1.5Al_0.5?PO₄?₃.Physical and chemical analysis show that a spinel phase which contributes to structural stability and lithium ion diffusion is formed after the introduction of Li₃V_1.5Al_0.5?PO₄?₃.In addition,the similar structural defects as the post-cycle LLMO sample is observed in modified sample.At the same time,the electrochemical properties of the modified samples were greatly improved.Among them,5wt.%Li₃V_1.5Al_0.5?PO₄?₃-LLMO has the best performance,its first coulomb efficiency is 93%,the capacity remains 96%after 100th charge and discharge,and the reversible specific capacity reaches 176mAh·g^-11 under 5.0C,while LLMO only releases 100mAh·g^-1capacity.In addition,compared to the rapid voltage decay trend of LLMO?from 3.36 to 2.89V?,the average discharge voltage of the modified sample decreased from 3.24 to 3.07V,and the voltage drop was only0.17V.Therefore,Li₃V_1.5Al_0.5?PO₄?₃ can not only improve the discharge specific capacity,cycle performance and first charge-discharge efficiency of lithium-rich materials,but also greatly suppress the voltage decay.