Systhesis, Characterization And Electrochemical Performance Of Li-rich Mn-Based Cathode Materials For Lithium-ion Batteries

Li-rich Mn-based cathode material xLi₂MnO₃·?1-x?LiMO₂(M=Co?Fe and Ni_1/2Mn_1/2) has been taken the people's attention due to the high capacity(200-300mAh·g^-1), high working voltage?3.8V?, good thermal stability, low cost and environmentally friendly pollution-free advantages. But limiting its commercial application because the first charge and discharge efficiency low and the cycle performance and rate performance. In order to solve these shortcomings, this paper from the synthesis process, surface coating and bulk doping to investigate and modification, and have made some progress.In this paper, SEM, XRD, XPS, HRTEM, Raman and a series of electrochemical tests were used to analyze the physical morphology, crystal structure and electrochemical properties. In order to find out the the synthesis process and modification methods to preparation of high performance Li-rich cathode. In this paper, the lithium-rich materials were synthesized by sol-gel method using acetate as raw material. The effects of calcination temperature,calcination time, complexing agent selection and composition on the morphology and electrochemical properties of the composites were investigated.In order to find the optimum synthesis conditions. The results show that when citric acid as complexing agent, x=0.5 component, calcined at 450 ? for 12h and calcined at 900 ? for 12h, the crystal morphology and the chemical performance of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ are the best. At a current density of 1C(200 mAh·g^-1), the reversible discharge capacity reached 176.4 mAh·g^-1 after 100 cycles. At 5C, there are still 60.1 mAh·g^-1.Firstly, the lithium-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was modified by surface coating. Comparing the structure and electrochemical properties of FePO₄,Mg3?PO₄?2 and AIPO₄ coatings, the results showed that structure of AIPPO₄ coating will not change. When the amount of AIPO₄ coating is 5%, the electrochemical performance is the best, and the reversible discharge specific capacity after cycling 100 times at 1C current density is 212.3 mAh ·-1, and the capacity retention is as high as 91%. At 5C, there are still 99.1 mAh·g^-1.Indicating that the AIPO₄ coating hinders the contact between the bulk material and the electrolyte, suppresses the occurrence of side reactions with the electrolyte.Secondly, the graphene and the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ materials were compounded by the liquid phase method. When the compound amount is 4%,after 50 times cycles at 1C, the reversible discharge capacity is 209.2 mAh·g^-1 and the capacity retention rate is 85.8%, the specific capacities of the discharge at 0.1C, 0.2C, 1C, 2C and 5C are 274.9 mAh·g^-1, 260.3 mAh·g^-1, 237.6 mAh·g^-1,164.6 mAh·g^-1 and 110.7 mAh·g^-1. Indicating that the high electrical conductivity of graphene improves the rate performance of the material.Finally, the Al is substituted to the bulk phase to reduce the cationic mixture in the material. When the doping amount is 0.05, at 1C the first discharge specific capacity is 230.0 mAh·g^-1, and after cycle 100 times capacity is 204.3 mAh·g^-1, the capacity retention rate is 88.8%. At 5C, there are still 90.9 mAh·g^-1. As the introduction of Al³⁺ can reduce the metal cation mixing,improve the stability of the material structure, thereby enhancing the electrochemical stability.