Investigation Of The Capacity Degradation And Optimization Mechanism For High Voltage Lithium Cobalt Oxides

In recent years,the rise of electric vehicles and the advancement of base stations and intelligent terminals for a new generation of mobile communication networks have put forward higher requirements for the energy density and cycle life of lithium-ion batteries.With its superiority of high energy density,long cycle life,high compacted density and so on,lithium cobalt oxides?LiCoO₂?is one of the most promising cathode materials for the next generation of lithium batteries with high energy density and long cycle life.However,although the theoretical specific capacity of lithium cobalt oxide is up to 274 mAh/g,the reversible charge and discharge capacity of 175 mAh/g can only be achieved at a cut-off voltage of 4.45 V actually.When the cut-off voltage is further increased,its specific capacity will rapidly decay due to the severe collapse of the crystal structure.The purpose of this study is to explore the modification scheme of lithium cobalt oxides and improve its cycle stability at high cut-off voltage by deeply understanding the capacity degradation mechanism.This study will develop the lithium batteries based on lithium cobalt oxides material with high energy density and long cycle life.In this thesis,the contents and innovations are as follows:Firstly,the capacity degradation mechanism of LiCoO₂ at high voltage was analyzed.The evolution of phase transition and electronic structure for LiCoO₂ during the progress of delithiation were uncovered at high voltage,and the electrochemical cycling curves at different cut-off voltage from 4.2 V to 4.8 V were showed,which is clarified that the charge compensation behavior of oxygen accompanied by the irreversible phase transition is related to the capacity degradation behavior.It is revealed that preventing the charge compensation behavior of oxygen to stabilize the crystal structure of LiCoO₂ at cut-off voltage higher than 4.5 V is the key breakthrough for further optimizing the electrochemical performance for high voltage LiCoO₂,which points out the subsequent research direction.Secondly,considering the microstructure and service performance of material will be affected by its synthetic processes,we compared and studied the capacity degradation behavior of LiCoO₂ at high voltage with three commonly used synthetic processes:solid-phase sintering method,coprecipitation method and sol-gel method.The nucleation and growth of LiCoO₂ are more controllable by the sol-gel synthesis process.The LiCoO₂ synthesized by sol-gel method has higher crystallinity and more stable layered structure.The uniform size and regular shape of the material particles contribute to the process of lithiation/delithiation.Compared with LiCoO₂ synthesized by solid-phase sintering or co-precipitation,the LiCoO₂ synthesized by sol-gel method shows better cyclic stability during the charging and discharging process at high cut-off voltage of 4.6V,and possesses a higher discharge specific capacity and capacity retention rate.Thirdly,through the combination of theoretical and experimental studies,an optimization mechanism for cycling stability with high voltage LiCoO₂ based on trace Ru doping is established.The first-principles calculations revealed that the top level of the valence band of O was moved down by Ru doping,reducing the overlap area of Co 3d and O 2p orbital energy levels.To some extent,the charge transfer process of oxygen was prevented at high voltage.Furthermore,the Ru-doped LiCo_1-xRu_xO₂cathode materials?x=0,0.1,0.01,0.001?were synthesized by sol-gel method.Trace Ru-doped LiCo_0.999Ru_0.001O₂ cathode material with x=0.001 exhibits the initial discharge specific capacity of 208 mAh/g?0.2C?and the capacity retention rate of60%after 100 cycles,higher than the undoped LiCoO₂'s capacity retention rate?54%?at 4.6 V.Finally,in order to further suppress the crystal structure distortion and collapse of LiCoO₂ due to the Jahn-Teller effect in the deep delithiated state,we further introduced Al³⁺with similar ionic radius and the same chemical valence as Co³⁺based on Ru doped LiCoO₂.With constant valence,Al³⁺is contributed to stabilizing the layered structure of LiCoO₂ during the process of lithiation/delithiation.Through the combined effect of Ru and Al binary co-doping,the electronic structure and crystal structure of LiCoO₂ are coordinately regulated,which significantly improves its discharge specific capacity and cycle stability.The LiCo_0.998Ru_0.001Al_0.001O₂ cathode material synthesized by the sol-gel method exhibited higher discharge capacity and better cycling stability than Al or Ru single element doped LiCoO₂ at the cut-off voltages of 4.2 V,4.5 V and 4.6 V.In summary,we optimized the synthetic method and explored a modification strategy for high voltage LiCoO₂ by elemental doping based on a deep understanding of the capacity degradation mechanism of LiCoO₂ at high cut-off voltage.Combining research methods of simulation and experiment,the effectiveness of preventing the charge compensation behavior by Ru doping is verified.Then with stable crystal structure of Al³⁺,the Ru and Al co-doped LiCoO₂ cathode material(LiCo_0.998Ru_0.001Al_0.001O₂)was designed and synthesized.The long-term cycle stability for LiCo_0.998Ru_0.001Al_0.001O₂ during the process of charging and discharging at a high cut-off voltage above 4.5 V was achieved with the synergistic effect of Ru and Al binary co-doping.The conclusion of this paper lays a foundation for the study and development of lithium cobalt oxides with high voltage and high capacity,which is expected to support the technological progress of high specific energy lithium batteries.