Research On The Preparation And Modification Of High Voltage LiNi_0.5Mn_1.5O₄ As Cathode Material For Lithium Ion Batteries

Due to its high energy density,lithium ion batteries have attracted much attention as a power source for electronic products and electric vehicles.LiNi_0.5Mn_1.5O₄,as a key material for lithium-ion batteries,has become a research hotspot due to its low cost,simple preparation process and high platform voltage（4.7V）.In this paper,Ni_0.25Mn_0.75（OH）₂precursor was synthesized by co-precipitation method,and LiNi_0.5Mn_1.5O₄material was prepared and modified by solid-phase method.The physical and electrochemical properties of the materials were studied by methods of ICP,SEM,XRD,TEM and constant current charge-discharge tests.In this paper,Ni_0.25Mn_0.75（OH）₂precursors was synthesized with MnSO₄·4H₂O and NiSO₄·6H₂O as raw materials.Thermodynamic analysis shows that ammonia concentration control in the range of 0.1～1.0 mol/L,and p H is 10～12.The paper systematically investigated the effects of metal mixture solution concentration,feeding time,stirring speed,p H value and reaction time on the properties of the precursor.The experimental results show that when the metal mixture solution is 2 mol/L,the feeding time is6 h,the stirring speed is 750 r/min,the p H value is 11.5,and the reaction time is 24 h,the spherical precursor Ni_0.25Mn_0.75（OH）₂with a solid density of 1.84 g/cm³ can be prepared.The molar ratio of Mn to Ni is（2.9972）close to the theoretical design value.LiNi_0.5Mn_1.5O₄ material was prepared by high temperature solid phase sintering method,which used Ni_0.25Mn_0.75（OH）₂ as precursor and Li₂CO₃as lithium source.We study the effects of reaction condition on the properties of LiNi_0.5Mn_1.5O₄ powder materials,including calcination temperature,holding time and the amount of mixed lithium.The experimental results show that when Li/（2Me）=1.08,the sintering temperature is 870℃and the sintering time is 24 h,the prepared LiNi_0.5Mn_1.5O₄ electrode material has the best properties.XRD results show that LiNi_0.5Mn_1.5O₄ electrode material has the best spinel structure,sharp peak shape and good crystallinity.The electrochemical results reveal that he first discharged specific capacity of 0.1 C LiNi_0.5Mn_1.5O₄ material is 138.38 m Ah/g at room temperature,when the voltage range is 3.0-5.0 V;the discharge specific capacity of 3 C is 116.7 m Ah/g,and the capacity retention rate is 97.94%after 20 cycles at 0.5 C.Using Al（OH）₃ as doping raw material,Al³⁺doped LiNi_0.5Mn_1.5O₄material was synthesized by controlled crystallization high temperature solid phase method.XRD test shows that the characteristic peaks of the doped material and the undoped material were consistent,indicating that Al³⁺entered into the crystal lattice of the material well,and successfully replaced part of Mn and Ni,which improves the balance of electricity price and structural stability of the material,and enhances the performance of material circulation and large discharge.Electrochemical test results show that when the Al doping 0.05 mol,the 0.1 C first discharge specific capacity of LiNi_0.475Mn_1.475Al_0.05O₄ material is 131.88 m Ah/g,after room temperature 100 times charging and discharging at 0.5 C,the capacity retention is 96.75%.The capacity retention rate is 93.24%at high temperature（55℃）after 100 cycles.Under the condition of 5 C discharge,the specific discharge capacity of LiNi_0.475Mn_1.475Al_0.05O₄ is 128.18 m Ah/g.The conductivity of lithium ion is 2.5731?10^-16 cm²/s,which is 2.1 times that of the undoped material.LiNi_0.5Mn_1.5O₄ was coated with Al₂O₃ by high temperature solid phase method.TEM results show that after Al₂O₃ coating,the surface of LiNi_0.5Mn_1.5O₄ is coated with a dense Al₂O₃ coating layer,which reduces the side reaction between the materials and electrolyte,improves the dissolution of Mn³⁺and enhances the cyclic stability of the material,improves the cycle and large rate discharge properties of the material.When the coating amount of Al₂O₃ is 8 wt%,the electrochemical test results show that under the normal temperature,the 5 C discharge capacity is 118.54 m Ah/g;the 0.5 C initial discharge capacity of C-LNMA-8 is124.16 m Ah/g at 55℃,and the capacity retention rate is 97.22%after100 cycles,the charge transfer resistance of C-LNMA-8 material increases by 140.91Ωand that of C-LNMA-0 by 292.44Ω.