Study On Lanthanides Doping And Electrochemical Properties Of High Voltage Lithium Cobalt Oxide Cathode Materials

Lithium cobalt oxide（Li Co O₂）,the first-generation lithium-ion battery cathode material,has been dominating the market in consumer electronic products since its commercial application in 1991.To meet the ever-increasing demand for endurance of lithium-ion battery,researchers enhanced the energy density of Li Co O₂ batteries by increasing their charge cut-off voltage.However,as the charge voltage increases,Li Co O₂ suffers a series of phase transitions,causing severe structural deterioration and rapid capacity fading.Herein,we systematacially studied the relationship between lanthanide contraction and structural stability of 4.6 V high-voltage Li Co O₂ by doping with seven lanthanide elements,and screened out which doping element has the best electrochemical performance.Then,taking the Lu-doped Li Co O₂ with the best electrochemical performance as the main research object,the deep-level improvement mechanism of Lu-doping on Li Co O₂ was revealed.Finally,towards better electrochemical performance,Er and Mg co-doped Li Co O₂ with ultra-long cycle life is synthesized,and the performance modulation mechanism of Er and Mg in Li Co O₂ is revealed by combining various experimental characterization techniques and theoretical calculations.The detailed research contents are as follows:（1）In the first part of this paper,we propose an electrochemical delithiation/high-temperature solid-state reaction method to synthesize seven lanthanide-doped Li Co O₂,where lanthanides were doped in the Li layer of Li Co O₂.Electrochemical characterization shows that the electrochemical performance of lanthanide-doped Li Co O₂ was improved with the increase of lanthanides atomic number.Herein the reasons for the regular change of electrochemical performance with lanthanide contraction were studied by combining various characterization techniques and theoretical calculations.The research results showed that the ionic radius of the doped element was the key factor affecting the performance of Li Co O₂,that is,doping oversized lanthanide ions into the lattice of Li Co O₂ will aggravate the lattice distortion around the doping elements.As the lanthanide contraction,the ionic radius of the lanthanides gradually decreases.Therefore,the lattice distortion of Li Co O₂ gradually decreases with the lanthanide contraction,and the electrochemical performance was improved accordingly.（2）The second part of this paper was a further work based on the results of the first part,we taking Lu-doped Li Co O₂ with the best electrochemical performance as the main research object,and revealed the improvement mechanism of Lu-doping on Li Co O₂ performance through various characterization techniques.The results indicated that Lu plays three important roles in the performance improvement of Li Co O₂:1)Lu atoms in the Li layers,supporting the layered structure of Li Co O₂,reducing the variation range of layer spacing.2)The formation of H1-3 phase was inhibited by the strong interaction between Lu and O.3)Lu atoms provide abundant electrons for the O atoms,thus inhibiting the oxidation of lattice oxygen.Due to the similar chemical properties of lanthanides,the performance improvement mechanism of Lu doping also applies to other lanthanide-doped Li Co O₂.（3）In the first two sections,we found that lanthanide elements are only effective for the high-voltage problems of Li Co O₂ but almost ineffective for low-voltage phase transitions.It has been reported that the Co site substitution doping of Mg can suppress the low-voltage phase transitions,so lanthanide elements can be used for co-doping with Mg to further improve the electrochemical performance of Li Co O₂.Therefore,in this section,Er and Mg co-doped Li Co O₂ were prepared by using the low-cost lanthanide Er and Mg as dopants.This Er,Mg co-doped Li Co O₂ not only achieved a long cycle life over 500 cycles in a 4.6 V half-cell,but also provided a high energy density of 811.9 Wh/kg in a pouch full-cell,cyclability with 91.3%energy density retention and stable median voltage of 3.87 V after 100 cycles.Herein,the performance improvement mechanism of Er and Mg co-doping on Li Co O₂ was revealed by combining various in-situ characterization techniques and theoretical calculations.The results show that Er-doping mainly plays the role of suppressing the formation of H1-3 phase at high voltages,while Mg-doping mainly plays the role of inhibiting the formation of monoclinic phase at low voltages.Thus,the synergistic effect of Er and Mg promotes the long-cycle stability of Er and Mg co-doped Li Co O₂.