Research On Electrochemistry Performance Of High Voltage Lithium Nickel Manganese Oxide Material Prepared By Carbonate Precipitation Methode

The high-voltage spinel Li Ni_0.5Mn_1.5O₄?LNMO?has the theoretical specific capacity of 147 m Ah g^-1,the operating voltage as high as 4.7 V with high energy density.It is one of the most promising cathode materials for next generation lithium ion batteries.To overcome the problems of poor long cycle performance,the effects of micromorphology,calcinating temperature,lithium content and nickel-manganese ratio on the performance of LNMO are studied.The electrochemical performance of the material has been improved by surface modification of LNMO.The s torage performance and full battery performance of LNMO are studied,too.Manganese carbonate or manganese oxide is used as a precursor to control the morphology of LNMO by changing the morphology of the precursor.Comparing the properties of LNMO with different morphology,it is found that porous LNMO material has both excellent rate performance and outstanding long cycle performance.After 1000 cycles of 5 C,the capacity retention rate is as high as91.4%.The non-uniform distribution of Ni and Mn leads to an increase in the content of Mn³⁺ and impurities in LNMO.The experimental results show that the particle size of LNMO has a great influence on its performance.LNMO with particle size of 2?m has the best rate and cycle performance.It is found that the hollow structure of LNMO is originated from the core-shell structure of manganese carbonate.The core of manganese carbonate is formed by the accumulation of nuclei and the density is low.And after the volume shrinkage during calcination the core became more sparse.In addition,due to Ostwald ripening,the smaller grains in the core will transfer to the shell during the calcination process,forming a hollow structure.LNMO material is prepared by sodium bicarbonate co-precipitation method,and Ni/Mn elements is uniformly mixed.The influences of various factors on the properties of the materials are studied.The results show that the morphology of the LNMO particles changes from spherical to polyhedral with the increasing of calcination temperature.And LNMO materials prepared at higher calcining temperatures have better rate performance.When the amount of lithium is relatively low,part of the nickel-manganese oxide remains in the product after calcination,resulting in a decrease in specific capacity and rate performance.And a small amount of lithium-rich oxidation occurs when the amount of lithium is too high,which reduces the first coulombic efficiency of the material.As the Ni/Mn ratio decreases,the specific capacity of LNMO decreases,the Mn³⁺content and the rate performance increase,but its cycle performance decreases.Ni/Mn ratio decreased from 0.5/1.5 to 0.4/1.6 will change the lithium deintercalation reaction path,from the two-phase reaction to a solid solution reaction.LiNi_0.4Mn_1.6O₄ with single-phase solid solution reaction has higher lithium ion diffusion rate and significantly improved the rate performance.The low Ni/Mn ratio material with good rate performance is used as the core,and the high Ni/Mn ratio material with good cycle performance is served as the shell.The core-shell material exerts advantages of both core and shell and thus good electrochemical performance is obtained.LNMO is prepared by sodium carbonate co-precipitation method and modified by cobalt coating.The results show that the rate performance and cycle performance of the material can be improved as the increased coating amount,but the excess coating amount will reduce the specific capacity.Given the above results,5%cobalt coating is the best.When the modified temperature is higher than 700?,the coating can enter the LNMO surface layer,and cobalt doping is formed on the LNMO surface layer.Surface cobalt doping can increase the disordered phase and Mn³⁺content of LNMO,thus increasing rate performance of the material.Surface cobalt doping reduces the content of Ni and Mn³⁺on the surface and helps to improve cycle performance of the material.After 2000 cycles at 5 C,the discharge specific capacity remains 93 m Ah g^-1,and the capacity retention rate reaches 80.5%.When stored in charged state,self-discharge happens to LNMO.With voltage decreases,the self-discharge rate becomes slow.The self-discharge at high temperature is significantly accelerated.After shelved at discharged state at 25?,the cycle performance of LNMO is poorer than that of shelved at charged state.Contrary to the case at 25?,the cycle performance of LNMO after shelved at discharged state is much better at 55?,which indicates that high voltage at high temperatures can cause more severe capacity fade.The results of the full cell test show that the rate performances and cycle performances of the LNMO/LTO batteries are better than the LNMO/graphite batteries.In LNMO/LTO batteries,the discharge capacity of LNMO at 2 C and 5 C is about 112 and 92 m Ah g^-1,respectively.After2000 cycles at 5 C,the capacity retention is as high as 87.2%.In the full battery,the SEI film is generated on the surface of the graphite,which severly reduce the capacity.The dissolved manganese migrates to the anode electrode and deposits on the surface of the anode electrode,which promotes the growth of the SEI film on the graphite surface.Therefore,LNMO with a high Mn³⁺content has more severe capacity fade in the LNMO/graphite full battery.