Study On Preparation And Electrochemical Performance Of High-Voltage LiCoO₂ Cathode Materials

With the blossom of science and technology,as well as the gradual improvement of living standard,Lithium cobaltate（LiCoO₂,LCO）,the traditional cathode material,has been pursued the higher energy density when it was firstly utilized in 1980 to cater the increasing market demands.The most direct way to achieve the higher specific capacity is to adopt the higher voltage,leading more Li⁺released from the cathode.Unfortunately,the capacity is extremely unstable,and would decay obviously after several cycles and the attendant problem of safety at the higher cut-off voltage.Coating strategy is the most efficient method to improve the cycling life.In this work,we provide a traditional and easy-to-commercial approach,co-precipitation method,to improve the cycling and rate performance of LCO at high cut-off voltage.We developed various kinds of protective layers on the LCO surface,studied the microstructure and electrochemical performance of LCO@LiF（FLCO）,LCO@Li₃PO₄（LP）and FLCO@PAN（PFLCO）composite cathode materials.By analyzing the mechanism and impact of the technological parameter such as coating thickness,heat treatment temperature,and heat treatment time on the micromorphology and electrochemical properties of the cathode materials,to prepare the cathode material with superior electrochemical properties.This preparation method is easily operated,safety and conductive to large-scale production.The contents are as follows:（1）Nanosized LiF particles were prepared on the LCO surface by traditional co-precipitation method with heat treatment.By adjusting the technological parameters,such as the coating content and heat treatment temperature,achieving the uniform coating with the optional electrochemical performance.The result indicated that the 1.0 wt%LiF@LCO with heat treatment at 550℃shows the best cycling and rate performance,the initial capacity can achieve 192 m Ah g^-1,the capacity retention can achieve 86%after100 cycles,and the discharge capacity can achieve 127 m Ah g^-1 at 4 C as well as the capacity retention can reach 71.4%.（2）A crystalline Li₃PO₄coating was successfully prepared on the LCO surface by traditional co-precipitation method.We compared the micromorphology and electrochemical performance of LCO@Li₃PO₄ cathode materials with different heat treatment time and Li₃PO₄ content,achieving the preparation of the LCO@LP cathode material with reasonable crystallinity coating and has the most excellent electrochemical properties.The result indicated that 1.0 wt%Li₃PO₄ coating with heat treatment for 5 h can show the best cycling and rate performance.It has a well initial discharge capacity of181 m Ah g^-1 with the retention of 90%after 100 cycles at 0.5 C as well as at 3 C the capacity can achieve 123 m Ah g^-1 after 100 cycles.（3）Nanosized LiF and conductive c PAN double coating was prepared on the LCO surface by the wet chemical method.LiF has the significant chemical inertness,cyclized PAN has the ionic conductivity,that can make up for the insulation properties of lithium fluoride as well as form a physical barrier to hamper the direct contact with electrolyte.With the synergistic effect of both characteristics,this double coating structure can performance a great initial capacity of 173 m Ah g^-1 with the retention of 79%after 100cycles at 0.5 C as well as at 1、2、3 and 4 C the capacity can achieve 166、150、144and 129 m Ah g^-1,respectively,and all the retentions are over 90%after 100 cycles.