Neutron Total Scattering Study Of Lattice Oxygen Charge Compensation Mechanisms In High-capacity Li-rich Cathode Materials

Due to the ultrahigh specific capacity,Li-rich oxides have been considered as the candidate cathode materials for next-generation high-energy-density Li-ion batteries.The ultrahigh capacity in Li-rich oxides originates from the cumulative redox charge compensation of transition metals and lattice oxygen.However,the irreversible lattice oxygen redox?e.g.,O₂ gas loss?usually cause inferior electrochemical performance,such as voltage fade,voltage hysteresis,etc.,which seriously restrict the commercial application of Li-rich cathode materials.In-depth understanding of the charge compensation mechanism provided by the lattice oxygen redox is of great significance for the modification of its stability and the achievement of excellent electrochemical properties in Li-rich cathode materials.This work mainly studies the lattice oxygen redox mechanisms from the perspective of crystal structure,aiming at providing guidance for the optimization of high-performance electrode materials in the future.In order to detect both the average and local structure of materials,neutron pair distribution function analysis was employed in this work to study the oxygen lattice structure evolution.A local distorted oxygen lattice?short O-O pair?accompanying the lattice oxygen redox was for the first time detected through neutron pair distribution function in the Li-rich layered oxide material Li_1.2Ni_0.13Mn_0.54Co_0.13O₂.Combined with theoretical calculation,it was found that?-type overlap is very likely to dominate the interaction in this short O-O pair,which can lower the energy of the local distorted oxygen lattice and preserve the global layered structure.Meanwhile,it was revealed that the oxygen lattice distortion is highly dependent on the transition metal species?i.e.,the covalency of the transition metal-oxygen bond?.Specifically,compared with the transition metal with stronger covalency,the oxygen lattice is more likely to distort around the transition metal with stronger ionicity.Based on these findings,it can be observed that the stable lattice oxygen redox can be achieved by tuning the transition metal species and their atomic ratio.In addition,it is worth noting that the distorted oxygen lattice occurred in the transition metal interlayer rather than intralayer in Li-rich layered oxides,the phenomenon of which is associated with the unique structure dimensionality in the material.However,till now,how the structure dimensionality affects the lattice oxygen redox activity and how to employ such an effect to tune the stability of lattice oxygen redox remain open questions.Therefore,it is necessary to further study the lattice oxygen redox in Li-rich oxide material with different structure dimensionality.For such a comparative study,the cation-disordered Li-rich oxides with a 3D-disordered cationic framework and high lattice oxygen redox activity is a suitable candidate research model.Accordingly,in this work,a new cation-disordered Li-rich oxide Li_1.2Ti_0.35Ni_0.35Nb_0.1O_1.8F_0.2 with the reversible capacity of about 280 mA h/g was firstly prepared by a simple solid-state calcination method.Then the coupling relationship between the oxygen lattice evolution and lattice oxygen redox was studied in detail.Ex situ X-ray absorption spectroscopy and resonant inelastic X-ray scattering provide the direct experiment evidence for the existence of lattice oxygen redox charge compensation.Crucially,differ from the distorted oxygen lattice in Li-rich layered oxides,a relative stable oxygen lattice structure was observed in this system materials upon the lattice oxygen redox process.It was indicated that this discrepant structure response originates from the different spatial-distribution of unhybridized O 2p orbitals in the different structure dimensionality.These findings provide an important theoretical basis for the stability adjustment of lattice oxygen redox using the structure dimensionality in Li-rich cathode materials.