Study On Preparation And Modification Of Li₂MnSiO₄ Cathode Materials For Lithium Ion Battery

Li₂MnSiO₄ as the cathode material of lithium ion battery has the features of high theoretical capacity?333.0 mAh g-1?,good thermal stability,low cost and environmental friendliness.However,this material has many disadvantages,such as the low electronic conductivity,the collapsing of the structure and Mn dissolved in the electrolyte during the cycle process,resulting in the poor cycle performance.In this work,the preparation and modification of Li₂MnSiO₄ materials have been studied systematically.The synthesized samples are physically characterized by XRD,SEM,EDS,TEM and other methods,while galvanostatic charge-discharge tests,cyclic voltammetry and electrochemical impedance spectroscopy have been carried out to investigate the electrochemical performance.Li₂MnSiO₄/C is synthesized by fast sol-gel method with TEOS as silicon source.Effects of synthetic process on the structure and electrochemical performance of products have been studied.The optimal synthesis process of Li₂MnSiO₄/C is obtained with the amount of acetic acid for 10 g,calcination temperature of 650 ?,calcining time for 6 h,and the carbon content of 20 wt%.But the cycle performance of the Li₂MnSiO₄/C sample prepared under the optimal conditions is still not satisfactory due to the essential defects of structural collapse and Mn dissolution in the electrolyte during the process of charge and discharge.In order to improve the electrochemical performance of Li₂MnSiO₄/C prepared by sol-gel method,metal ions doped in Mn or Si site are conducted.Our results indicate covalent ions(Zn²⁺ or Cu²⁺)and aliovalent ions(Ag⁺ or Cr³⁺)doped in the Mn site can improve the electrochemical properties of Li₂MnSiO₄/C to varying degrees.And Al³⁺ ion substitution of Si can increase the lithium content in the sample.Li_2.02MnSi_0.98Al_0.02O₄/C presents the optimum electrochemical performance among the samples doped with different content of Al³⁺ ion.The oxide and carbon co-coating is proposed to improve the performance of Li₂MnSiO₄ material for the first time.This paper studies the influence of three different oxide materials?MoO₂,ZnO and TiO₂?on the structure and properties of Li₂MnSiO₄/C material.1 wt% MoO₂ coated Li₂MnSiO₄/C exhibits the best electrochemical performance among the samples coated with different content of MoO2 which is prepared by one step sol-gel method.And the sample can deliver an initial discharge capacity of 184.9 mAh g-1 at 0.1 C rate.And the discharge capacity of 182.8 mAh g-1 is obtained by the optimal sample coated with ZnO at a rate of 0.1 C,while 186.4 mAh g-1 for 3 wt% TiO₂ coated sample.In addition,all the coated samples present the enhanced cycle performance.The results of ex-situ XRD show that coating oxide can suppress the collapse of the structure to some extent.ICP tests indicate that the oxide coated on the surface of the active material can reduce the concentration of Mn dissolved in the electrolyte during the cycle process.Li₂MnSiO₄/C is prepraed for the first time by hydrothermal method using inorganic silicate instead of organic TEOS as silicon source.And the effects of different synthesis conditions on the structure and electrochemical performance of the final sample have been investigated.The optimal conditions of preparing Li₂MnSiO₄/C in this method are: hydrothermal reaction temperature of 180 ?,hydrothermal reaction time for 36 h,calcination temperature of 600 ?,solvent with 20 ml of ethylene glycol and calcination time for 10 h.The optimal sample delivers the discharge specific capacities of 171.7 and 159.4 mAh g-1 at the rate of 0.1 C and 0.5 C,respectively.