Study On Crystal Structure Regulation And Electrochemical Properties Of LiNi_0.5Mn_1.5O₄

Spinel LiNi_0.5Mn_1.5O₄ has two types of crystal structure,that is,ordered and disordered structure,which have their virtues and their faults on the electrochemical performance.Therefore,it is essential to optimize their proportion to achieve better electrochemical properties.In this thesis,LiNi_0.5Mn_1.5O₄ cathode material was synthesized by high temperature solid-state method using NiO,Li₂CO₃,Mn₃O₄ as raw materials.The ions doping and annealing treatment were applied to regulate the crystal structure.The effects of ions doping and annealing treatment on the crystal structure and electrochemical properties were systematically investigated,and the one-step solid-state method and two-step solid-state method were compared.The effects of different valence ions doping(Na⁺,Zn²⁺,Cr³⁺,Ti⁴⁺,V⁵⁺,F^-)on crystal structure and electrochemical performance were systemically investigated.The results show that ions doping increases the Ni/Mn disordering degree to different extent.Under the same doping amount conditions,the greater ion valence difference between doping ion and doping position ion often leads to higher Ni/Mn disordering degree and Mn³⁺content.With the increase of Mn³⁺content,the electrochemical properties increase firstly and then decrease.Among which,the Cr³⁺and Ti⁴⁺co-doped sample LiNi_0.45Cr_0.025Ti_0.025Mn_1.5O₄exhibits the optimal electrochemical performance,which can be attributed to the appropriate Ni/Mn disordering degree(Mn³⁺content)under the conditions of octahedral morphology before and after ions doping.The annealing treatment was applied to regulate the crystal structure(Mn³⁺content)of the materials,and the orthogonal experiment was carried out using Mn³⁺content and 5C discharge specific capacity as examining index.The results show that the annealing time is the most significant factors influencing the annealing process.With the decrease of Mn³⁺content,the 5C discharge capacity of the material decreases.The sample with the most obvious annealing effect was compared with the un-annealed sample.The results show that the annealing treatment decreases the Ni/Mn disordering degree.And the rate capability of the material is reduced after annealing treatment,which is mainly caused by the lower Mn³⁺content.This further confirms that the Mn³⁺content,namely Ni/Mn disordering degree in LiNi_0.5Mn_1.5O₄,should be appropriate.The effects of one-step and two-step solid-state methods on the structure,morphology and electrochemical performance of LiNi0.5Mn1.5O4 cathode material were studied.SEM results show that the primary particle of two-step sample is in truncated octahedral morphology,which is different from the octahedral morphology of one-step sample.The electrochemical results show that the rate and cycling performance of two-step sample are much better than that of one-step sample.However,FT-IR results show that two-step sample has lower Ni/Mn disordering degree,namely lower Mn³⁺content.Therefore,the better electrochemical performance of two-step sample is mainly attributed to the existence of some new crystal orientations besides?111?of the truncated octahedral morphology.These new crystal planes can effectively inhibit the Mn dissolution and accelerate the Li⁺ions diffusion.Therefore,based on the initial two chapters,it can be concluded as follows.For LiNi0.5Mn1.5O4 spinel with different particle morphology,the particle morphology is the foremost influence factor of electrochemical properties.However,when the particle morphology is similar,the electrochemical properties are mainly influenced by crystal structure,namely Ni/Mn disordering degree.Therefore,it is very essential to regulate the crystal structure for LiNi_0.5Mn_1.5O₄ cathode material.