Study On Solid State Systhesis And Doping Modification Of High-voltage LiNi_0.5Mn_1.5O₄ Cathode Material

High-voltage LiNi_0.5Mn_1.5O₄ material has two different crystral structures:ordered and disordered structures.The ordered structure contains only Mn⁴⁺ions,while disordered structure contains a little Mn³⁺ions.The Mn³⁺amount shows significant influence on the structure and electrochemical properties of LiNi_0.5Mn_1.5O₄ spinel.In this thesis,LiNi_0.5Mn_1.5O₄ was synthesized via soild-state method by using Li₂CO₃,NiO and Mn₃O₄ as raw materials.The influence of calcination temperature,rare-earth element doping,and extra lithium addition on the crystral structure,morphology and electrochemical properties was systematically investigated.The influence of different calcination temperature?750,800,850,900^oC?on the crystral structure,morphology and electrochemical properties was investigated.The results show that with the calcination temperature increasing,the crystallinity and particle distribution uniformity firstly increase and then decrease,and the particle size gradually increases.Among which,the sample prepared at 850^oC exhibits better crystallinity,moderate and uniform particle size,thus leading to its better electrochemical performance.The effects of rare-earth element(Ce⁴⁺,Y³⁺)doping and Y³⁺doping content?0,0.005,0.01,0.02?on the crystral structure,morphology and electrochemical properties were investigated.The results show that Ce⁴⁺or Y³⁺doping has little influence on the octahedral particle morphology,but the substitution of Ni by Ce⁴⁺or Y³⁺both increases the Ni/Mn disordering degree.The higher the valence of doping ions is,the higher Ni/Mn disordering degree and Mn³⁺content are.The Ni/Mn disordering degree and Mn³⁺content firstly increase and then decrease with Y³⁺doping content increasing.Among which,the Y³⁺doped sample LiY_0.01Ni_0.49Mn_1.5O₄ exhibits highest Mn³⁺content,thus leading to its better electrochemical performance.The discharge capacity at 10 C rate can reach 113.2 mAh/g,and the capacity retention rate after 100 cycles at 1 C rate can reach 99.27%.In addition,the introduction of Y-O bond is also beneficial to its better structural stability.The influence of lithium excess?0,2,6,10%?on the crystral structure,morphology and electrochemical properties was investigated.The results show that lithium excess has little influence on the octahedral particle morphology.the impurity Li_xNi_1-xO amount increases with the lithium excess increasing,whereas the Mn³⁺content firstly increases and then decreases.Among which,LiNi_0.5Mn_1.5O₄ with 6%lithium excess the exhibits the highest Mn³⁺content and lowest Li/Ni confusion degree,thus leading to its better electrochemical performance.The discharge capacity at 10 C rate can reach 119.8 mAh/g,and the capacity retention rate after 100 cycles at 1 C rate can reach 96.49%.It can be concluded that calcination temperature has a great influence on the morphology,and the electrochemical performance of the sample is mainly affected by the morphology.However,rare-earth element doping and lithium excess have little influence on the morphology,and the electrochemical properties of the samples are mainly affected by the crystal structure.