Controllable Preparation And Surface Modification Of Lithium-Rich Manganese-Based Cathode And Its Full-Cell Performance Analysis

Recently,with the rise and development of electric vehicles and smart grids,the demand for a new generation of high-performance lithium-ion batteries with high safety,high energy density and long cycle life has become increasingly urgent.At present,the low discharge capacity of commercial cathode materials is a key factor that restricts the energy density of lithium ion batteries.Lithium-rich manganese-based cathode material has become one of the most promising cathode materials for lithium-ion batteries due to its large specific capacity(>250 mA h g-1),high operating voltage,long cycle life,low cost and environmental friendliness.However,the low initial Coulombic efficiency,severe capacity and voltage decay,and poor rate performance of lithium-rich manganese-based cathode material have severely hampered its commercial application.Therefore,the aim of this paper is to improve the comprehensive electrochemical performances of lithium-rich manganese-based cathode materials,which can be achieved by preparing the hierarchical porous lithium-rich manganese-based oxides via the solvothermal method and two-step calcination treatments.The effects of synthesis conditions on the morphology,structure and electrochemical performance of the material were systematically investigated.On this basis,the technical route of engineering surface oxygen vacancy defects and spinel hetero structured layers is proposed.And the effects of oxygen vacancies and spinel layer on accelerating ion diffusion rate and enhancing structure stability were investigated.Finally,the foll cell systems with lithium-rich manganese-based oxides as cathode were assembled and investigated to promote its application.Firstly,the hierarchically porous lithium-rich manganese-based materials were synthesized through a solvothermal route,followed by calcination treatment.The effects of PEG600,lithium salt amounts,calcination temperature and composition on the morphology,structure and electrochemical performance of materials were systematically investigated.In summary,the hierarchically porous spherical lithium-rich manganese-based cathode material Li1.2Mn0.54Ni0.13Co0.13O2 with the most excellent electrochemical performance could only be obtained until controlling the following conditions:(1)PEG600 is added as a chelating agent;(2)the ratio of Mn,Co and Ni is 4:1:1;(3)the amount of LiOH·H2O is 1.5 times of transition metal;(4)the calcination temperature is 800?.Specifically,the discharge specific capacity is 294.9 mA h g-1 with a high initial Coulombic efficiency of 85.5%at 0.1 C.After 200 cycles at 1.0 C,the reversible capacity remains 211.7 mA h g-1,corresponding to a capacity retention of 81.1%.When cycled at 5.0 C for 200 cycles,the capacity is stable at 178.7 mA h g-1.Even at 10.0 C,it can still deliver a discharge capacity of 1 70.9 mA h g-1,showing excellent discharge performance at high rate.Then,engineering oxygen vacancies in hierarchically porous Li1.2Mn0.54Ni0.13Co0.13O2 microspheres in situ encapsulated by spinel surface layer(L@S)is carried out to significantly improve the Li+ diffusion kinetics and prevent structure collapse from the Li+ channel blocking.As a result,the prepared L@S cathode delivers a high initial Coulombic efficiency of 92.3%.In addition,the discharge specific capacity reaches to 290 mA h g-1 at 0.2 C.After 100 cycles at 10 C?a large reversible capacity of 222 mA h g-i with a capacity retention of 95.7%is obtained.Even at 20 C,L@S can still maintain a capacity of 153 mA h g-1after cycling 100 times.Moreover,the full cell using L@S as cathode and Li4Ti5012 as anode exhibits a reversible capacity of 141 mA h g-1 with a very high voltage retention of 97%after 400 cycles at a large current density of 3 C.Finally,L@S-AG,L@S-Si/C and L@S-Li4Ti5O12 full cell systems were assembled and investigated.It was found that the side reactions on the surface of the AG and Si/C anode materials continued to consume Li+in the whole battery system,resulting in severe capacity decay.By contrast,the L@S-Li4TisO12 full cell has an initial discharge specific capacity of 196.6 mA h g-1 at 0.2 C.After 400 cycles at 3 C,it still exhibits a reversible capacity of 141.0 mA h g-1 with a capacity retention of 80%,and the voltage retention is as high as 97%with almost no voltage decay.