| The lithium-rich oxide x Li2Mn O3·?1-x?Li MO2?M=Mn,Ni,Co?have attracted tremendous attention due to their long cycle life,high operating voltage and high discharge capacity.In this paper,Li-rich oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2 was prepared by coprecipitation method and sol-gel method,respectively,and the material was surface coating and element doping.The material's microscopic crystal structure,morphology,elemental valence,cycle performance and discharge capacity,and the behavior of lithium ion insertion and extraction were investigated by means of XRD,SEM,XPS,galvanostatic charge and discharge tests,CV,and EIS.First of all.Lithium source,ammonia concentration,calcination method and calcining temperature for the synthesis of lithium-rich materials by coprecipitation were studied in detail.The optimum process conditions were as follows:the concentration of ammonia was controlled at about 0.56 mol/L when preparing the carbonate precursor,lithium carbonate was used as the lithium source,and the temperature was directly increased to 850°C for 12 h after preheating at 500°C for5 h.The target product can be obtained.The first discharge specific capacity of the materials prepared by this process condition at 0.1C(1C=250 m Ah·g-1)is 258.38 m Ah·g-1,and the first charge-discharge coulombic efficiency is 79.99%.The discharge specific capacity of the materials after completion of 100 cycles at 1C rate was 117.50 m Ah·g-1,and the capacity retention rate was 77.60%.Li2Zr O3 is coated on the material prepared by the co-precipitation method in two different ways.The results showed that when the Li2Zr O3 was coated on the surface of the carbonate precursor,the discharge specific capacities of the materials before and after 100 cycles of 1C were 149.74 m Ah·g-1and 117.50 m Ah·g-1,respectively,and the capacity retention rate was 78.47%.At the same time,the material's rate performance has also been effectively improved.The stepped pre-cycle mode was used to study the activation system of the material.It was found that the cycle performance of the material after the stepped pre-cycle has a significant increase,and the improvement of the cycle performance at the large rate is most obvious.Secondly,V-doped Li-rich materials are coated with Li2Zr O3,Li Nb O3,and Li1.13Mn0.47Ni0.2Co0.2O2,respectively.The results show that when the coating amount of Li2ZrO3 is 3%,the capacity retention rate of the materials after cycling 100 times at 1C rate is 89.82%,and the rate performance also has obvious improvement.When the coating amount of Li Nb O3 is 1%,the discharge specific capacity of the material at the high rate of 5C is 123.31 m Ah·g-1,which is much higher than 87.51 m Ah·g-1of the doped V materials.When the coating amount of Li1.13Mn0.47Ni0.2Co0.2O2 is 1%,the discharge specific capacity of the materials before and after 100 cycles at 5C rate is 122.9181 m Ah·g-1,which is higher than the initial discharge capacity of 120.23 m Ah·g-1.It shows that the lithium-containing oxide coating on the surface of lithium-rich materials can prevent direct contact between the materials and the electrolyte,reduce the possible side reactions during charging and discharging,and prevent the excessive elution of oxygen ions in the materials.At the same time,during charging and discharging provides a channel for lithium ions to improve the performance of the materials.Finally,Ta-doped and W-doped were carried out on the lithium-rich materials.The results show that when the doping amount of Ta is 0.03,the discharge capacity of the material at 5C current is 45.75%of the discharge capacity at 0.1C,which is higher than 40.10%of the undoped material.The material's rate performance is obviously improved.When the doping amount of W is 0.02,the discharge capacities of the materials after 100 cycles at 1C and 5C are 154.67 m Ah·g-1and 135.09 m Ah·g-1,respectively.The material's cycle performance and rate performance have improved significantly. |
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