Study On Voltage Decay Mechanism And Suppression Strategy Of Li-Rich Layered Oxide Cathode Materials

As an energy storage device,lithium-ion batteries enable efficient storage and transport of energy.After years of commercial development,lithium-ion batteries have been widely used in mobile electronic devices.Nowadays,in order to further meet the demand for electric vehicle range,ultra-high energy density lithium-ion batteries need to be developed,and the energy density of lithium-ion batteries depends crucially on the capacity provided by the cathode material.Li-rich oxide cathode materials are considered to be the prime candidates for the next generation of Li-ion battery cathode materials due to their ultra-high capacity(>250 m Ah g^-1).Recent studies reveal that the lattice oxygen anions participate in electrochemical redox,generating additional charge compensation during the intercalation process of the material and boost capacity.The discovery of lattice oxygen anion redox breaks the perception that only cations can be used as redox centers to provide capacity,opening up new pathways for the design of high specific energy cathode materials.At the same time,the use of anionic redox is also regarded as a"double-edged sword",increasing capacity while also creating a unique electrochemical problem for Li-rich oxide cathode materials,namely voltage decay.A proper understanding of the intrinsic link between anion redox and structural evolution is the key to solve this problem.In this dissertation,the relationship between the evolution of the oxygen lattice and the voltage decay is first investigated via a typical Li-rich layered oxide Li_1.2Ni_0.13Co_0.13Mn_0.54O₂.Due to the complex charge compensation mechanism and structural evolution of Li-rich layered oxide cathode materials,the origin of the voltage decay has yet not been clarified.In this work,we first suppressed oxygen release by reducing the upper voltage,and confirmed the absence of oxygen release and oxygen vacancy generation during cycling by gas production analysis and electron energy loss spectroscopy（STEM-EELS）,thus ruling out the effect of oxygen vacancy diffusion on the bulk phase structure.To investigate the evolution of the long-range ordered structure during cycling,neutron diffraction experiments（TOF-NPD）were carried out using a high flux pulsed neutron source on samples with different number of cycling and were analyzed in combination with X-ray diffraction mapping（XPD）.The results show that as cycles increases,the lattice undergoes continuous expansion due to the accumulation of distortions,and the average position of the lattice oxygen shows a regular change.Further temperature-dependent in situ X-ray diffraction characterization on the cycled samples shows that the lattice distortion is removed during the heating process,accompanied with the shrinkage of the expanded lattice and the return of the position of the displaced lattice oxygen to its initial position.In conjunction with previous studies,the above results suggest that the further disordered transformation of the activated lattice oxygen during cycling is directly related to the voltage decay,which is manifested by an increase in lattice distortion and a shift in the average position of the oxygen.The continued disorderly transformation of lattice oxygen leads to changes in the local chemical environment,which in turn shifts the oxygen reduction potential and ultimately exhibits voltage decay.The continuous transition of activated lattice oxygen to disorder can be attributed to a mismatch between the disordered structure and the rigid crystal structure.Therefore,it is possible to increase the flexibility of the crystal structure to cooperate with the evolution of the lattice oxygen and thus inhibit its further transition.In this work,a high Li/O ratio component Li_1.26Ni_0.0741Co_0.0741Mn_0.593O₂ is designed to activate the lattice oxygen sufficiently to generate a large number of disordered structures within the bulk phase to increase the flexibility for a lithium-rich layered oxide cathode material with potential applications.At the same time,the activation of a large amount of lattice oxygen enables the construction of high energy density lithium-ion batteries with high capacity.Characterization of the synthesized pristine samples shows that the composition oxide cathode material has a typical lithium-rich layered oxide crystal structure.Electrochemical tests show that the first charge lattice oxygen activation contributes 80.5%of the overall capacity and subsequent discharges can reach 280 m Ah g^-1.The voltage retention of 96.2%and the capacity retention of 92%were tested for 100 cycles.By temperature-dependent in situ X-ray diffraction,we found that the lattice shrinks dramatically during the reversion process,indicating that the material activated by the first full lattice oxygen cycle has a large lattice distortion,indicating the presence of a large amount of disordered structure in the bulk phase structure.By ex in situ X-ray diffraction,it was found that the lattice distortion no longer increased further during the subsequent cycles,inhibiting further transformation of the lattice oxygen during the subsequent cycles.These results suggest that the construction of disordered structures within the bulk phase is a very effective strategy for suppressing voltage decay.In lithium-rich layered oxide cathode materials,we have recognized that the lattice oxygen undergoing redox undergoes a transition to disorder.Is this evolution prevalent in Li-rich oxide cathode materials of different compositions and structures,and what are the differences between the redox of transition metal cations and the redox of lattice oxygen anions?To answer these questions,the present work investigates the structural evolution of Li_1.2Ni_0.13Co_0.13Mn0.54O₂,Li_1.3Nb_0.3Mn_0.4O₂ and the conventional cathode materials Li Ni_0.333Co_0.333Mn_0.333O₂,Li Ni_0.5Mn_1.5O₄ with different compositions and structures of Li-rich oxide cathode materials during heating after the first cycle,and the effect of heating on their electrochemical properties.By heating in situ X-ray diffraction,it was revealed that the post-cycle Li-rich oxide cathode materials all undergo negative thermal expansion during heating,accompanied by the elimination of lattice distortion,whereas the conventional cathode materials exhibit normal thermal expansion.Electrochemical testing of the heat-treated materials revealed that the electrochemical properties of the Li-rich oxide cathode materials returned to their initial state upon heating,while the electrochemical properties of the conventional cathode materials remained unchanged.The above results suggest that redox of lattice oxygen always couples disordered transitions,while the oxidation of transition metal cations also maintains structural order,which can be attributed to the inherent differences in the nature of oxygen and transition metals.This finding provides an important reference for better understanding and utilization of anionic redox.