Research On Performance Optimization Of High Voltage Lithium Cobalt Oxide Cathode Material

LiCoO₂ cathode material has the benefits of high energy density,high operating voltage,high tap density,high safety performance,and long cycle life,etc,being one of the most widely used and most promising key materials in the field of lithium-ion batteries.LiCoO₂ has a high theoretical specific capacity of 274 m Ah?g^-1,but in practical commercial applications,it can only release a capacity of 174 m Ah?g^-1 at a higher operating voltage of 4.45 V（vs.Li⁺/Li）.Theoretically speaking,LiCoO₂operating at a higher cut-off voltage will have more Li⁺in its bulk structure to undergo reversible deintercalation movement,so as to realize the performance optimization that takes into account the simultaneous improvement of energy density and power density.However,due to the limitation of the material structure,the high working voltage brings about the irreversible and harmful phase transition of the LiCoO₂ crystal structure.In addition,the ionic conductivity(10^-8 S?cm^-1)of LiCoO₂ is much lower than the electronic conductivity(10^-3 S?cm^-1),while the most ideal cathode material is a mixed conductor of electrons and ions whose electronic conductivity and ionic conductivity are relatively balanced and with high values.Therefore,in this paper,the energy barrier for Li⁺diffusion in LiCoO₂ is reduced to increase its ionic conductivity under the premise of ensuring the stable operation of LiCoO₂ at high cut-off voltage,by means of dual co-doping,and realizing the effective improvements of LiCoO₂ cathode material in terms of discharge specific capacity,rate performance and cycle stability under 4.5 V high voltage.First of all,the performance optimization of high-voltage LiCoO₂ cathode materials was realized through the study of La-Al dual-doped LiCoO₂,and the mechanism of La element playing the"Pillar Effect"and the lattice stability of Al element were deeply analyzed in this paper.At the same time,a new LiCoO₂ dual doping scheme was designed on this basis,that is,one of the doping elements（X）has a large ionic radius and weak electronegativity,and the second doping element（Y）has a slightly larger ionic radius than Co,a slightly weaker electronegativity than Co,and a high binding energy with O.To be specific,Ba or La was selected as the doping element X,and Ga or Al was the doping element Y,and the doping effects of Ba and La,and Ga and Al were respectively compared.In view of the weaker electronegativity and larger ion radius of Ba,its ability to weaken the attraction of O^2-and its extranuclear electrons to Li⁺is better than that of La,namely,Ba is superior to La in the“Pillar Effect”.At the same time,Ga has a greater lattice stability effect than Al,which is mainly due to the higher binding energy between Ga and O.Finally,we also found that because of the smaller ionic radius difference between Ba and Ga,the synergistic doping effect between Ga and Ba is better than that between Al and Ba.Specifically,four LiCoO₂ samples were synthesized by high-temperature solid-state method in this paper,namely undoped LiCoO₂（PLCO）,La-Al co-doped LiCoO₂（LA-LCO）,Ba-Al co-doped LiCoO₂（BA-LCO）and Ba-Ga co-doped LiCoO₂（BG-LCO）.The order of the electrochemical performance of the four samples is as follows:BG-LCO>BA-LCO>LA-LCO>PLCO,Ba-Ga binary doping shows the best performance optimization effect.The Li⁺diffusion coefficient of BG-LCO is improved by an order of magnitude relative to PLCO,and it maintains a much higher specific capacity(114.3 m Ah?g^-1)than PLCO(1.6 m Ah?g^-1)at the high rate of 10 C in the rate performance test.In addition,BG-LCO also exhibits a high specific capacity of 199.3m Ah?g^-1 when charged and discharged at the rate of 0.5 C in the voltage range of 3 to4.5 V,and the capacity retention rate is still as high as 85%after 100 cycles of charge and discharge.