Research On Synthesis Of High Voltage LiNi_0.5Mn_1.5O₄ Material,Surface Coated And Electrochemical Performance

LIBs has been widely used in portable electronic devices such as mobile communication tools,laptop and digital cameras,owing to its high operating voltage,large specific energy,environmental friendliness and excellent cycling performance and so on.In addition with the development of the traditional fuel vehicles caused the series of problems such as energy crisis,environmental pollution and climate warming have been increasingly attention,the development of new energy electric vehicles is the fundamental measures to solve these problems.Therefore LIBs as a new type of power battery are applied in the field of new energy electric vehicles has is inevitable trend.However,the commercialization of LIBs still hardly fulfill the requirement of low costs and high energy density of battery for electric vehicles.The cathode materials is an important part for the LIBs and the energy/power density of LIBs is mainly determined by the cathode materials.Thus the researches about the developments of cathode material with high energy density and low cost,which promote the development of LIBs and electric vehicle in large-scale commercial has very important significance.Spinel LiNi0.5Mn1.5O4 material with three-dimensional lithium ion diffusion channel,relatively high theoretical capacity(147 mAh/g)and high operating voltage(～4,7 V),which raise the energy density and power density of the device,therefore it is considered to be one of the most promising candidate for cathode materials in LIBs.However,the most critical barrier for the successful commercialization of LiNi0.5Mn1.5O4 material is severe capacity fading,especially at elevated temperatures.In order to improve the cycle performance of LiNi0.5Mn1.5O4,this paper reports on the preparation of LiNi0.5Mn1.5O4 material with the optimization of conditions and based on its surface coated.The main research contents and results are as follows:(1)LiNi0.5Mn1.5O4 material was synthesized by solid-state method and sol-gel method respectively,the SEM and electrochemical performance tests show that the LiNi0.5Mn1.5O4 product by a sol-gel method had more uniform particle size distribution and a superior cycle stability.On this basis,the effects of calcining temperature and calcining time of sol-gel method on the structure and morphology,performance of the products were analyzed.The XRD,SEM and constant current charge-discharge tests show that the material has the excellent electrochemical properties when calcining temperature is 900℃ and calcining time for 6h.Its discharge capacity is 120 mAh/g at 1C,the capacity retention after 200 cycles is still high up to 94%.showing the excellent cycle performance.At the same time,the material also reveals excellent rate performance(0.1C:130 mAh/g,0.5C:125 mAh/g,1C:120 mAh/g,3C:115 mAh/g,5C:110 mAh/g).(2)With the LiNi0.5Mn1.5O4 as base material.we coated fast ionic conductor La0.7Sr0.3MnO3 on LiNi0.5Mn1.5O4 particles and investigated the influence of different coating amounts(from 1 wt%to 3 wt%)on the electrochemical performance of LiNi0.5Mn1.5O4.The results of electrochemical experiments demonstrate that 2%LNMO material displays an obviously electrochemical performance.At 25℃，the first discharge capacity is 127 mAh/g at 0.1C,and the discharge capacity is 126 mAh/g at 1C,maintaining a capacity retention of 98%,96%and 93%after 100,200 and 300 cycles,respectively;However,the first discharge capacity of pristine LNMO is 117 mAh/g at 0.1C,the discharge capacity is 116 mAh/g at 1C,maintaining a capacity retention of 95%.87.5%and 85%after 100，200 and 300 cycles,respectively.Here 2%LNMO also show excellent rate performance,it deliver a discharge capacity of 134 mAh/g(0.1C),128 mAh/g(0.5C).125 mAh/g(1C),122 mAh/g(3C).118 mAh/g(5C),113 mAh/g(10C).When cycling at 55℃,the 2%LNMO and pristine LNMO deliver discharge capacity of 125 mAh/g and 120 mAh/g at 1C and capacity retention of 94%and 91%after 100 cycles,respectively.The 2%LNMO maintained a capacity retention of 92%after 200 cycles at 1C,but the pristine LNMO cell has gone bad,showing that 2%LNMO has better high temperature performance.(3)For LiNi0.5Mn1.5O4 material about the worse high temperature performance,we propose to use NiFe2O4 with good thermal stability coating on the LiNi0.5Mn1.5O4 particles surface.The experimental results show that:the NiFe2O4 coating LNMO material with excellent electrochemical performance.At 25℃,the pristine LNMO and NiFe2O4@LNMO material cycling under the 1C rate:after 100,250 and 300 cycles,maintaining a capacity retention of 95%,90%,85%and 96%,95%,90%,respectively.When cycling at 55 ℃:the pristine LNMO and NiFe2O4@LNMO material show the capacity retention of 93%and 96%at 1C after 100 cycles respectively.The NiFe2O4@LNMO material exhibits a capacity retention 92.4%after 250 cycles under 1C rate,but the pristine LNMO cell has gone bad after 100 cycles.We considered that it is attributed to the NiFe2O4-coated-layer suppress the dissolution of Mn from the spinel material,reduce the reaction between active electrode materials and electrolyte solution and the loss of active material,compared with the pristine LNMO material,the NiFe2O4@LNMO material also shows the better rate performance,it is attributed the NiFe2O4 conductive layer to reduce the charge-transfer resistances.