Design Of Lattice Oxygen Activity And Local Structural For Lithium-rich Layered Oxide Cathode Material

As a high-energy-density,long-cycling-life,and environmentally friendly energy storage device,lithium-ion batteries are widely employed in portable electronic devices,electric vehicles,and aerospace.However,current lithium-ion batteries are unable to meet the growing demand from the whole society,especially on long battery life and high specific energy.It is very urgent and important to research and develop next-generation lithium-ion batteries with higher energy density.The specific capacity of cathode materials used in commercial lithium-ion batteries at this stage is much lower than that of anode materials,resulting to hardly exceed 400 Wh kg^-1 in the existing battery system design.Lithium-rich layered oxides have become an important candidate cathode material for Li-ion batteries to break through the energy-density limitation due to their ultra-high specific capacity(>300 m Ah g^-1),which is demonstrated to be inseparable from the joint contributions from transition metal redox and lattice oxygen redox.Though the activity of lattice oxygen is helpful to elevate the specific capacity,it will inevitably trigger irreversible lattice oxygen release,which further deteriorates the performance of Li-ion batteries.Although there is an inseparable relationship between lattice oxygen activity and structures in Li-rich layered oxide cathode materials,the intrinsic connection between them has not been fully revealed due to a lack of in-depth evidence from advanced methods and characterizations.Specifically,a sole characterization and analytical method cannot obtain accurate information once studying the local structures which plays an important role on the activity of lattice oxygen.Meantime,there are no fundamental solutions on oxygen release in view of their structural characteristics while the inhibition of irreversible lattice oxygen release remains at modification.Furthermore,the consistency and stability of the samples in the study also affect their results,which is likely to cause inconsistent conclusions.Thus,based on a series of engineered lithium-rich layered oxide cathode materials with high consistency,the relationship between their local structure and lattice oxygen redox activity was studied from the crystal structural characteristics.The specific research contents are as follows:First,based on synchrotron X-ray diffraction（SXRD）,time-of-flight neutron powder diffraction（TOF-NPD）and X-ray near-edge absorption spectroscopy（XANES）,the activation of Li-rich layered oxide cathode materials is susceptible to local Co coordination.The Co ions can invade into the Li₂Mn O₃-like domains and tune related electronic structures.The activation of lattice oxygen activity is further promoted during the first charge,resulting to higher reversible capacity of Li-rich layered oxide cathode materials.However,Ni ions hardly join the formation of Li₂Mn O₃-like domains with limited contribution on the activation.Simultaneously,the optimal composition design of such materials was explored,verifying the slight Co/Mn exchange in the Li₂Mn O₃-like domains promotes the activation of lattice oxygen activity.These findings reveal the important role of Co ions on the activation of lattice oxygen activity in Li-rich layered cathode materials,which also provide new insights on the route of achieving high-capacity Li-rich layered cathode materials.Second,a structural regulation strategy was proposed to eliminate irreversible lattice oxygen release by regulating the distribution of Li₂Mn O₃-like domains.The original structure of tested material was characterized by differential electrochemical mass spectrometry（DEMS）,EXAFS,SXRD and fitted via FAULTS program.The distribution Li₂Mn O₃-like domains in the transition metal layer is more dispersed in the Li-rich layered oxide cathode material with less oxygen release,which is further controlled by the intrusion of Co ions on Li Mn₆ configuration.Via applying in-situ X-ray diffraction（in-situ XRD）,it was found that the dispersed Li₂Mn O₃-like domains accommodate more structural distortion stress induced by the activation of oxygen activity.A 10.5 Ah pouch battery prepared with this no-oxygen-release materials（lithium metals as anode）exhibits a high energy density of 504 Wh kg^-1,providing a promising way to suppress lattice oxygen release for achieving high-energy-density Li-ion batteries.Finally,based on DEMS,Li-rich layered oxide cathode material system has a limitation on its reversible lattice oxygen activity during increasing its Mn content.SXRD,TOF-NPD and high-resolution scanning transmission electron microscopy（HR-STEM）were used to detailly analyze the change of lattice oxygen site and local structures of a series of charged materials.The lattice oxygen stacking has been evolved from O3 to O1 triggered by releasing abundant oxygen after the activation of lattice oxygen activity,which leads to a significant loss on the discharge capacity.In addition,the atomic configuration of the O1/O3 stacking interface was investigated by geometric phase analysis（GPA）.The atomic mismatch along the c-axis of O1 and O3 stacking was found to cause the formation of dislocations,and the continuous sheared transition region between O1 and O3 stacking along a-b plane was also found.The above research revealed the structural change characteristics of the Li-rich layered oxide cathode material after fully activation of the lattice oxygen activity,and provided a guidance for the targeted modification on cathode materials with high energy density.