The Study On Modification And Working Mechanism Of High Voltage Lithium Cobalt Oxide Cathode Material

Lithium cobalt oxide(LiCoO2,LCO)is the most widely used cathode material for lithium-ion batteries(LIBs)in consumer electronics.In recent years,the rising climbing demand for high energy density drives the development of high-voltage LiCoO2 cathode material.Nevertheless,the high voltage will induce the structural and interfacial instability issues for LiCoO2,resulting in deterioration of electrochemical performance.Element doping is one of the most commonly used methods to improve the electrochemical performance of LiCoO2.However,the corresponding modification mechanism,especially the influences of the long-and short-range structural evolution at high-voltage on electrochemical performance,is still lacking and needs further exploration.In this paper,LiCo1-xAlxO2 materials were synthesized with one-time sintering.And LiCo0.99Al0.01O2 presents the best electrochemical performance.In the voltage range of 3-4.5 V with a current density of 1 C,the capacity retention of LiCo0.99Al0.01O2 is increased from 20%to 50%over 150 cycles.Subsequently,on this basis,the sintering conditions were improved and the synthesis details were optimized(secondary sintering),which significantly improved the cycle performance.Under the same cycle conditions,the capacity retention of LiCoo.99Al0.01O2 was improved to 72%over 200 cycles,while that of LiCoO2 was only 58%.The modification mechanism of element doping for high-voltage LiCoO2 has been studied in detail via electrochemical characterization for LiCoO2,Al-doped LiCoO2,La+Al-doped LiCoO2 in combination with in-situ XRD,ss-NMR,and XPS techniques.La and Al co-doping can postpone the appearance of H1-3 phase,thus decreasing the transition degree of H1-3 phase at the same upper cutoff voltage of 4.6 V.The H1-3 phase transition in LiCoO2 is often accompanied by severe structure variation.And La+Al co-doping can lower the content of H1-3 phase and enhance the structural reversibility of LiCoO2,thereby significantly improving the cycle performance of LiCoO2 at high voltage.Otherwise,ss-NMR results infer that La+Al-LCO also presents a delayed H1-3 phase at the local scale.And the introduction of La and Al causes more local structural phase as compared to pure LCO,which can help to alleviate the volume change in the(de)lithiation process.XPS results indicate that the co-doping of La and Al results in valence band moving towards Fermi energy.Removing lithium ions from host crystal causes the transformation of local electrons to delocalized electrons.Such a comprehensive characterization including long-range,local,and electronic structure is beneficial to gain a deeper understanding of properties for LiCoO2,which can provide an important theoretical and experimental basis for developing high-voltage LiCoO2based lithium-ion batteries.