Performance Studies Of Rare Earth Elements(M = Y,La,Ce) Doped Lithium Cobalt Oxide Cathode Materials With High Working Potentials

Lithium-ion batteries have been rapidly developed in recent decades due to their high energy density,portability and relative safety,and have been widely used in electronics products,automobiles,and industrial fields.In recent years,lithium-ion battery cathode materials are moving toward high energy density and power density.LiCoO₂?LCO?is the first commercialization cathode materials of lithium-ion battery since proposed by Goodenough`s group in 1980.Then,several kinds of lithium-ion battery cathode materials such as LiMn₂O₄,LiFePO₄,LiNi_xCo_yMn_1-x-yO₂ successive entered into application market over the past 40 years.Nevertheless,LCO is still one of the most popular cathode materials for its high volumetric energy density,high tap density and high theoretical specific capacity(274 mAh g^-1).However,the actual cutoff voltage of LCO used in commercial LIBs was 4.2V?vs.Li/Li⁺?and the reversible capacity was 140 mAh g^-1.When LCO electrodes charging at higher cutoff voltages,more lithium ions can be extracted from the lattices.However,at higher operation voltages than 4.2 V,there are many problems need to overcome.Such as the increases side reaction between the LCO cathode and the electrolyte,and the instable structure of LCO at the highly delithiated states,,which causes irreversible phase transition,and introduces huge volume changes,which lead to structural degradation,accompanied with the Co metal ion will easily dissolve into the electrolyte solution,which further exacerbates side reactions on the electrode surface.In particularly,LCO exhibits phase transitions from the O3 to the H1-3 and O1 at charge voltages upper 4.5 V,which was considered to be a terrible phase transition causes sever electrodes bulk structure collapse and lead to fast capacity fading.In order to break through the LCO capacity limitation,different approaches have been researched to improve the performances of LCO at high-voltages,the main ways are surface coating to hinder the electrolyte corrosion the cathode materials and bulk doping to stabilize the bulk structure.With the deepening of research,it is more challenging to solve multiple problems by dual modification on high-voltage LCO.In this work,the layered LCO was prepared by sol-gel method,and carried out the bulk phase doping modification.The rare earth elements Y,La and Ce with larger ionic radius than Co³⁺were selected,so it is easy to achieve saturated doping,and does not change the main structure of LCO,while after saturated doping forming a coating in-situ on the surface of the active material,obtaining a dual modified cathode material.The main works of the paper are summarized as follows:1.A series of Y-doped LCO?Y@LCO?samples with in-situ Y₂O₃ coatings were prepared by sol-gel method.Comparing the electrochemical performance at high cut-off voltages of 4.3V,4.4 V,4.5 V and 4.6 V,it was found that samples with 0.5 at.%content showed excellent cycle stability and significantly improved rate capability.With every 1 V increase in cut-off voltage,the pristine has a considerable increase in capacity,with consequent problems such as faster capacity decay,increased material polarization during cycling,and a rapid increase in impedance.However,after Y modification,these problems were obviously suppressed.In order to explore the mechanism,we analyzed the surface morphology and physical structure of LCO before and after doping,and the aging experiment designed and cyclic voltammetry?CV?tests,galvanostatic intermittent titration technique?GITT?and Fourier infrared spectroscopy?FTIR?chemistry and electrochemical environment for further testing,explored its modification mechanism.2.Designed contrast experiment of La³⁺and Ce⁴⁺rare earth elements doped by sol-gel method in LCO.The optimum doping contents were found through constant current charge/discharge cycles.The changes of surface morphology and physical structure before and after modification were investigated by XRD,FESEM and XPS.The results shown that the La and Ce doping does not change the bulk structure of LCO,and 0.5 at.%doping content not only achieved doping in the bulk,but also formed an in-situ coating layer LaCoO₃ on the surface of LCO after saturated doping,which obtained a dual modification structure.However,due to the higher valence than Co³⁺,Ce⁴⁺aggregated on surface to form a coating of CeO₂.The electrochemical performance of La³⁺and Ce⁴⁺were compared under different high cut-off voltages.The modification structure effects were compared by analyzing dQ/dV,EIS and GITT tests.