Research On Electrochemical Performance And Mechanism Of Ni-rich Cathode Materials Doped And Modified By Group-ⅢA Elements

Lithium-ion batteries are widely used in various consumer electronics fields today.When lithium-ion batteries were used as power batteries in the new energy automobile industry,electric vehicles have more stringent requirements for comprehensive performance such as battery life,fast charging,storage and safety.Therefore,it is necessary to develop high-performance lithium-ion power batteries with higher energy density,excellent high rate performance,long service life and strong stability,especially cathode materials.The Ni-rich ternary cathode material is considered to be an ideal cathode material for lithium-ion power batteries due to its high energy density,working voltage,and reasonable cost.However,the high nickel content of the Ni-rich ternary cathode material brings high capacity,but also causes serious lithium-nickel cation mixing and electrode-electrolyte interface side reactions during the charge and discharge process,which in turn causes the material to change from the layered phase to the rock salt phase.And the battery impedance increases.In order to effectively solve the problems of cation mixing and interfacial side reactions in the current application of Ni-rich cathode materials,this paper takes the Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）ternary cathode material as the research object,and carries out In³⁺ion doping modification.Through bulk doping with large ion radius In³⁺ions,the purpose of reducing the mixing of lithium and nickel cations and inhibiting irreversible phase transition is achieved.At the same time,given that Li In O₂is a lithium ion conductor,as a positive electrode material coating layer,it can not only suppress the occurrence of side reactions at the interface,but also facilitate the conduction of lithium ions at the interface.Therefore,this paper intends to use a simple and controllable preparation process to spontaneously form the synergistic effect of bulk doping and lithium ion conductor coating,and to improve the structural stability,interface stability and lithium ion conduction of the Ni-rich cathode material in an all-round way.The main research contents are as follows:Various characterization tests show that the original sample after In doping forms a synergistic modification of In³⁺ionic bulk phase doping and in-situ coating of Li In O₂.In³⁺ions are doped into the layered lattice structure of the material,occupying lithium sites to expand the interlayer spacing,and effectively inhibit cation mixing through the strong electrostatic repulsion between the divalent Ni²⁺ions.At the same time,the Li In O₂ion conductor coating layer is formed in situ on the surface of the material after In doping,which effectively reduces the existence of residual lithium on the surface.The NCM-In 1%sample battery has the best performance,and the 100th capacity retention rate is increased from 71.90%to 81.5%at 4.5 V and 1 C,and the battery impedance and polarization are also significantly reduced.It is proved that the lithium-site doping of In³⁺ionic bulk has the pillar effect,which effectively reduces the repulsive force between oxygen layers,inhibits irreversible phase transition,and maintains the stability of the layered structure.The Li In O₂protective layer coated on the surface not only improves the lithium ion interface transmission performance,but also effectively isolates the material from being corroded by air and electrolyte,inhibits the generation of particle microcracks,and significantly improves the interface stability of the electrode.In order to take full advantage of the high nickel content of the Ni-rich cathode material and provide a higher actual capacity,it is usually necessary to serve under a high cut-off voltage to increase the Li⁺ions for the deintercalation reaction of lithium.This high-de-lithium state will cause the layered structure to collapse sharply,causing a rapid decline in capacity.In order to effectively improve the service performance of Ni-rich cathode materials under high voltage,this paper selects Al³⁺ions and In³⁺ions for co-doping modification.Through the synergistic modification of Al³⁺ionic bulk transition metal site doping and In³⁺ionic bulk lithium site doping,the capacity attenuation problem caused by structural instability of Ni-rich materials in the state of high delithiation can be effectively improved.The main research contents are as follows:The experimental results prove that the transition metal site doping of Al³⁺ions and the lithium site doping of In³⁺ions significantly increase the layer spacing of the sample layered structure.At the same time,because Al³⁺ions prevent the overside migration of Ni²⁺ions and the strong electrostatic repulsion of In³⁺ions work together,they jointly hinder the migration of Ni²⁺ions to the lithium site,and cation mixing is significantly suppressed.Because Al³⁺ions form an Al-O bond that is stronger than the M-O bond energy in the transition metal layer,it improves the lattice energy of the material and plays a role of framework support.In³⁺ions play a pillar effect in the lithium layer,and the two work together to reduce the stress and strain of the crystal structure caused by the expansion and contraction of the material lattice during the high-voltage cycle.And this effect enhances the stability of oxygen in the structure and effectively inhibits the interlayer expansion and irreversible phase transition caused by the repulsive force between oxygen layers.The battery performance of the NCM-Al 0.5%+In 1%sample is the best.The 100th capacity retention rate of the sample at 4.5 V and 1 C is as high as88.58%,which is a significant improvement over the 71.90%of the original sample.The cycle performance,rate performance and high voltage stability have been significantly improved.