Study On Surface Structure And Electrochemical Performance Of Spinel LiNi_0.5Mn_1.5O₄ High Voltage Cathode Material

In order to overcome the mileage problem of large-scale promotion of electric vehicles,the next generation of lithium-ion batteries need higher energy density.Spinel LiNi0.5Mn1.5O4(LNMO)is widely considered as a potential candidate material for lithium-ion batteries in the future,because its high working voltage of 4.7 V can bring more than 500 Wh/kg energy density.At the same time,the increase of working voltage can reduce the number of batteries in series and improve the safety of the battery pack.In addition,due to the abundant reserves of raw materials and environmental friendliness,LNMO also shows certain advantages in cost control.At present,the main factors restricting the promotion of LNMO are the low initial coulomb efficiency and the poor cycle stability of low temperature full cell,which are due to a number of side reactions between LNMO and electrolyte,including the dissolution of transition metals in spinel materials and the anodic decomposition of electrolyte.Previous studies have shown that the interfacial reaction of LNMO is closely related to its surface structure.Therefore,it is important to study the crystal face effect,interfacial chemistry and surface modification of lnmo for improving its electrochemical performance.In this paper,we focus on the diversity of space group structure and micro morphology of spinel LNMO high-voltage electrode materials,optimize and modify the synthesis process and interface structure,establish the relationship between the microstructure and electrochemical performance,and deeply study the electrochemical dynamic process.In the aspect of the crystal plane effect,the grain structures with different crystal planes exposed were synthesized by different heat treatment methods.It is proved that the crystal plane orientation of the grain surface can be easily and effectively controlled by heat treatment.The results show that the sub micron particles with quasi octahedral structure have excellent cycling and rate performance.Polyethylene glycol(PEG)as a soft template and dispersant makes the grain of spinel LNMO electrode material closer to the standard octahedron,and the micro-nano-structure formed by secondary agglomeration inhibits the dissolution of transition metal ions and electrolyte decomposition side reactions of high voltage electrode material to a certain extent,and reduces the interface resistance.Polyethylene glycol with appropriate molecular chain length can inhibit the formation of heterophase in the synthesis process,and the battery shows better performance in long cycle and multiple rate charge discharge tests.The surface of LNMO electrode material was coated with amorphous carbon as passivation layer using glucose as carbon source,and the carbon coated electrode material had a more stable interface structure.Moreover,the electrochemical performance of Li ion battery was improved with different carbonization degree,and the coulomb efficiency of the first cycle of electrode was further improved.This paper can provide theoretical and experimental reference for the accurate synthesis,structure control,surface modification and battery performance improvement of LNMO cathode materials.