Study On The Synthesis And Properties Of LiNi_0.5Mn_1.5O₄ Cathode Material

Spinel LiNi_0.5Mn_1.5O₄ has been considered as one of the most promising cathode materials for application in lithium-ion batteries due to its high working voltage and high energy density.However,the poor cycling performance at high temperature limits its industrialization.A secondary micro-sized particles composed of primary nanoparticles?hierarchical micro/nano structure?can improve both the rate and cycling performances.Therefore,in this thesis,Ni_0.25Mn_0.75CO₃ precursor was firstly synthesized by hydrothermal method using Mn?CH₃COO?₂·4H₂O and Ni?CH₃COO?₂·4H₂O as raw materials and urea as a precipitator,which was pre-heated to obtain(Ni_0.25Mn_0.75)₃O₄ oxide.Finally,LiNi_0.5Mn_1.5O₄ with porous"peanut-like"hierarchical micro/nano structure was obtained by high-temperature lithiation process.The influence of hydrothermal temperature,lithiation temperature and EG:H₂O volume ratio on the morphology and electrochemical properties of Ni_0.25Mn_0.75CO₃ precursor and LiNi_0.5Mn_1.5O₄ cathode material was systematically investigated.And the formation process of"peanut-like"Ni_0.25Mn_0.75CO₃ precursor and porous LiNi_0.5Mn_1.5O₄ was investigated.The main contents are as follows:?1?The influence of hydrothermal temperature and lithiation temperature on the morphology and electrochemical properties of the Ni_0.25Mn_0.75CO₃ precursor and LiNi_0.5Mn_1.5O₄ cathode material was investigated.The results showed that all the samples were with porous"peanut-like"hierarchical micro/nano structure.Among them,the sample synthesized at hydrothermal temperature 190? and lithiation temperature 750? exhibited the smallest and most homogeneous particle size,as well as proper Mn³⁺content,thus leading to its better rate and cycling performances.?2?The effects of different EG:H₂O volume ratio on the morphology and electrochemical properties of the Ni_0.25Mn_0.75CO₃ precursor and LiNi_0.5Mn_1.5O₄ cathode material were studied.SEM observation illustrated that EG could obviously reduce the precursor particle size.With the increase of EG:H2O volume ratio,the precursor particle size first decreased and then increased.The LiNi0.5Mn1.5O4 cathode material synthesized in the presence of EG showed higher crystallinity,smaller particle size with homogenous particle size distribution,higher cation ordering degree and better electrochemical performance.Among which,the LiNi0.5Mn1.5O4 cahtode material synthesized in the mixed solution with EG:H₂O volume ratio of 1:6 exhibited the optimal electrochemical performance,whose discharge capacity at 10 C rate could reach 99.20%of that at 0.2 C rate.?3?In order to disclose the formation process of"peanut-like"Ni_0.25Mn_0.75CO₃precursor,the precursor synthesized under the conditions of hydrothermal temperature190? and EG:H₂O volume ratio 1:6 was chosen as study object.The samples prepared at different hydrothermal times were characterized by SEM.The results showed that lots of homogenous nanosized spherical particles were formed at the initial 2 h hydrothermal time.When the hydrothermal time was 4 h,some larger spherical particles were formed via the Ostwald ripening mechanism.As the hydrothermal time was further extended,two larger spherical particles were combined with each other to form"peanut-like"structure to reduce the overall surface energy.During the high-temperature lithiation process,the LiNi_0.5Mn_1.5O₄ with porous hierarchical micro/nano structure was obtained by the Kirkendall effect.