Stabilising Nickle-rich Layered Cathodes By Introducing Oxygen Vacancies

The nickel-rich layered oxide cathode has been considered as one of the advanced research topics in the field of lithium-ion batteries,due to its high energy density and low raw material cost.Nevertheless,the capacity of layer-structured nickel-rich materials fades fairly,and this fading can be attributed to the unstable oxygen ions at the highly delithiated state,which inhibits its practical application in the long-range passenger electric vehicles.This dissertation targeted at the several main scientific issues concerning thermal stability,chemo-mechanical stability and interfacial stability,and launched a series of studies on the material design and structural optimization of the electronic structure of oxygen ions on the surface of the nickel-rich ternary materials by introducing an oxygen ion conductor,which can provide stable oxygen vacancies.Based on that,this dissertation studied the mechanism of oxygen ion conductors in improving the thermal and chemical stability of materials.The main conclusions of this dissertation are drawn as follows:(1)The thermal stability was improved by introducing an oxygen ion conductor(Ce0.8Dy0.2O1.9)with the fluorite structure on the surface of primary particles used in-situ co-precipitation method.The results demonstrated that the modified electrode showed the higher onset temperature of oxygen evolution and lower heat generation compared to pristine.It was revealed that the ceria-based solid electrolyte with abundant oxygen vacancies is able to absorb/suppress the activated lattice oxygen ions(O-,O22-,etc)originating from surface irreversible reactions,thus slowing down its continued oxidation kinetics or irreversible processes.In addition,it also promoted the reversible and gas-free redox process of surface oxygen ions and enhanced the safety performance of the battery.(2)Research was conducted on improving the chemo-mechanical stability of nickel-rich ternary materials by introducing oxygen ion conductors on the surface of secondary particles by the ex-situ method.Consolidation points were constructed at the grain boundaries of the secondary particles to strengthen grain-to-grain bonding to prevent the secondary particles from being broken and pulverized.The electrochemical test confirmed that the phase transition of hexagonal to hexagon(H2?H3)for the modified electrode was not prominent,and there was little change in the position of the redox peak,which effectively inhibited the generation and propagation of microcracks.Meanwhile,the capacity retention of pristine NCM811 electrode after 100 cycles at 1 C/1 C was only 81.93%,while that of NW3 electrode showed the highest capacity retention of 96.27%.Moreover,the pristine NCM811 material suffered from rapid capacity decay(84.7 mAh/g),displaying a capacity retention of 62.65%after 200 cycles at 1 C/5 C.In contrast,the NW3 material remained 87.49%with 125.2 mAh/g after the same cycle,which showed a remarkable cycling stability enhancement compared to pristine NCM811 material.Based on detailed electrochemical tests and multiple characterization techniques,it is confirmed that an appropriate amount of oxygen ion conductor has a positive effect on inhibiting the precipitation of oxygen between the layered structure and the rock salt phase,thereby stabilizing the crystal structure.(3)Research was conducted on improving the interfacial stability of nickel-rich ternary materials by synergistically introducing the structural design of lithium-ion conductor and oxygen ion conductor.By tailoring the surface structure,the electrochemical performance and thermal stability of the optimized electrode have been significantly improved even at the high cut-off voltage.The obtained binary solid electrolyte coating layer composited by Ce0.8Dy0.2O 1.9 and Li8CeO6 effectively enhances the surface stability and improves the interface kinetic behavior.In detail,the oxygen ion conductor Ce0.8Dy0.2O1.9 can successfully capture high active oxygen-containing substance escaping from the cathode surface,and thus intercept O2 gas release and improve the safety performance of the battery.In addition,the lithium ion conductor was introduced synergistically to minimize capacity sacrifice by facilitating lithium-ion diffusion.More critically,at the high cut-off voltage,the NCM-LCD3 electrode exhibited remarkably high discharge capacity of 165.8 mAh/g at a high rate of 10 C,whereas the pristine delivered a specific capacity of only 133.8 mAh/g.Meawhile,the calculation of the lithium-ion diffusion coefficient also confirmed that the modified material had the fast Li+insertion/extraction kinetics.(4)Research on the influence of perovskite-type fast ion conductors was studied aiming at improving the stability of nickel-rich ternary cathode materials.By introducing a dual-ion conductor,the electronic and ionic conductivity of the nickel-rich ternary material can be increased.Meanwhile,the introduction of stable oxygen vacancies suppresses the formation of lattice oxygen vacancies on the surface of the material during charging and discharging and high-temperature phase transition,thereby effectively inhibiting the release of oxygen.More importantly,at the high cut-off voltage,the capacity retention of NCM9-LNLO3 electrode was still 82.19%after 100 cycles at 1 C/1 C.Moreover,at a high rate of 10 C,it still showed a higher discharge specific capacity of 141.6 mAh/g,while the pristine electrode had a specific capacity of only 87.9 mAh/g.The electrochemical results fully confirmed that the in-situ La4NiLiO8 coating enabled faster electron and Li+ion surface/interface transport of NCM9,effectively improved the lithium-ion diffusion kinetics of the electrode,and it could greatly avoid the direct contact between the active substance and the electrolyte.In addition,a uniform La4NiLiO8 coating layer and the strong Zr-O covalent bonds doped to the surface of nickel-rich ternary materials can act as a buffer layer,which reduce mechanical microstrain and stabilize surface chemistry and structure.