Microstructure Control And Surface Modification Of High Voltage LiNi_0.5Mn_1.5O₄ Cathode Material

Spinel LiNi_0.5Mn_1.5O₄（LNMO）cathode material has attracted the attention of both academia and industry due to its advantages of high operating voltage（4.7 V vs.Li/Li⁺）,high energy density(650 Wh kg^-1),cobalt-free nature,and low environmental impact.However,it suffers from severe structural degradation,dissolved Mn³⁺ions,surface-interface side reactions,and rapid capacity fading,especially under high-voltage and high-temperature operations,limiting its practical application and commercialization.In view of the above problems,a series of studies on LNMO cathode materials have been carried out in this paper from three aspects:crystal structure regulation,microscopic morphology design and surface modification treatment,in order to improve various properties of LNMO cathode materials.The specific research contents are as follows:Disordered single-crystalline LNMO cathode materials with different Mn³⁺contents were prepared by a simple temperature control strategy of a solid-state reaction.The effects of the mutual modulation of the Mn³⁺content and the bulk microstructure on the crystal structure stability and electrochemical properties of LNMO were systematically studied.Results showed that a suitable Mn³⁺content can enhance the structural stability and alleviate structural degradation and capacity fading.The excellent performance originates from the fact that the submicron particles can simultaneously suppress the formation of microcracks and interfacial side reactions,ensuring the rapid diffusion of Li⁺during extraction and insertion process.Consequently,the LNMO-800 sample delivers an excellent cycling stability at both high and low temperature,as well as remarkable rate capability(10C,115 m Ah g^-1)and fast Li⁺diffusion coefficient(4.43×10^-9cm²s^-1).Polycrystalline LNMO cathode materials with radial grain distribution microstructure were prepared by chemical and microstructural regulation starting from the precursor,and the mechanism of enhanced electrochemical performance of this unique microstructure was studied.The excellent rate performance of the R-LNMO electrode proves that the microstructure of the radial grain distribution contributes to the rapid diffusion path and efficient transmission of Li⁺from the inside of the particle to the surface.The orientation of the grain helps to enter the electrolyte and reduce the polarization effect,thus giving the LNMO excellent rate performance.Compared with the conventional polycrystalline cathode,the R-LNMO electrode still delivers a high specific discharge capacity of 101.5 m Ah g^-1when discharged at a large current density of 10 C,with a retention rate of 86.9%at 10C/0.2 C.Even after 1000 high-rate long cycles at 10 C,the capacity retention rate can still reach 77.2%,with excellent cycling stability.The surface of LNMO cathode was modified with urea to obtain a highly stable cathode/electrolyte interface,which effectively reduced the interface impedance and significantly improved the electrochemical performance.The results show that urea decomposition can induce the release of reactive oxygen species on the surface and the reduction of manganese ions on the surface,and the surface structure of LMNO cathode is adjusted to the disorderly phase structure of spinel containing oxygen vacancy.At the same time,nano carbon nitride organic layer generated by urea decomposition was coated on the surface of LNMO material particles.Therefore,the thermally modified sample（U-LMNO）with bifunctional urea exhibits a initial discharge capacity of 130.1 m Ah g^-1at 0.2 C and a capacity retention rate of 95.1%after 200 cycles at 1 C.The excellent properties are mainly attributed to the adjustment of the surface phase structure of the active particle material,and the effective improvement of charge transfer impedance and the formation of a highly stable electrode/electrolyte interface by the carbon-nitrogen nanocoating.