Studies On Surface Modification Of High-Voltage Cathode Material LiNi_0.5Mn_1.5O₄ By Two Kind Of Solid Electrolyte Materials

With the rapid growth of automobiles,human beings are facing hugechallenges on severe environmental pollution and serious energyshortage.To solve these problems,many countries have tried todecrease the amounts of fossil-fuel cars.Therefore,developing newenergy vehicles has become more and more important.Due to thehigh energy density,excellent cycle stability,and goodenvironmental benign,lithium-ion batteries?LIBs?are believed to beone of the most possible candidates for future automobiles,whichhave been widely used in portable electronic devices and large-scaleenergy storage applications.In order to meet the growingrequirements in power and energy densities,exploration and designof cathode materials with high energy densities and long cyclelifespan have attracted great attention in the past two decades for LIBs.LiNi_0.5Mn_1.5O₄?LNMO?spinel has drawn increasing attention due to its high voltage?⁴.7 V?,high energy density(⁶50 Wh kg^-1)and low cost as cathode for lithium-ion batteries.However,there are several drawbacks that hinder its commercial application.It suffers from the surface structural instability,especially at elevated temperature?55 ^oC?,and the compatibility with the commercial electrolyte,etc.Compared with traditional liquid electrolyte,solid electrolyte has superior electrochemical stability,high safety and wide electrochemical window,so it has attracted extensive research.However,the contact between solid electrolyte and electrode material is poor,so the research and exploration of all-solid battery is still in the stage.To this end,in order to solve some defects of high voltage LNMO materials and take advantage of the solid electrolyte materials,we use a solid electrolyte materials to modify the surface of the material.A stable and compatible interface for LNMO and electrolyte taking advantage of appropriate solid electrolyte material could open up a new way for the application of LNMO in next generation all-solid batteries.In this paper,my research work mainly includes two parts:Firstly,we investigated the effect of surface modification using garnet-type oxides Li_6.4La₃Al_0.2Zr₂O₁₂?LLAZO?to modify the surface of LNMO on the structure and electrochemical performance of LNMO via a combination of techniques,e.g.XRD,SEM,TEM etc.During the treating process at high temperature,generate a coating layer on the surface and accompanied with the penetration of external ions into LNMO.Additionally,the coating layer restricts the growth of LNMO particles.Such surface modification obviously improved the electrochemical cycling performance of LNMO.However,the modified material?LNMO-1LLAZO?exhibits much bettr capacity retention(at 25 ^oC,72.6%after600^th;at 55^oC,77.2%after 100^th,respectively)compared with the Pristine-LNMO sample(at25 ^oC,45.8%after 600^th).This significant improvement is attributed to the multi-functional effects of the method of precursor coating and co-crystallization,which effectively protect the spinel to avoide some undesired side reactions with electrolyte and improve the Li⁺diffusion kinetics.Our work explains the mechanism of the improvement of electrochemical cycling performance for the LLAZO surface modified LNMO.Secondly,we investigated the effect of surface modification using NASICON-type Li_1.4Al_0.4Ti_1.6?PO₄?₃?LATP?to modify the surface of LNMO on the structure and electrochemical performance of LNMO via a combination of techniques,e.g.Depth-XPS,SEM,TEM etc.During the treating process at high temperature,generate a modification shell on the surface of LNMO and accompanied with the penetration of external ions into LNMO.Additionally,the modification shell restricts the growth of LNMO particles.Such surface modification obviously improved the electrochemical cycling performance of LNMO.However,the modified material?LNMO-2LATP?exhibits much bettr capacity retention(at 25^oC,84.8%after 500^th)compared with the Pristine-LNMO sample(at 25 ^oC,50.3%after500^th).This significant improvement is attributed to the multi-functional effects of the method of precursor coating and co-crystallization,which effectively protect the spinel to avoide some undesired side reactions with electrolyte and improve the Li⁺diffusion kinetics.Our work explains the mechanism of the improvement of electrochemical cycling performance for the LATP surface modified LNMO.