Study On Redox In Li_1.17Ni_0.17Fe_0.17Mn_0.49O₂ Cathode Materials Of Li-ion Batteries

Nowadays one of the main research directions on cathode materials of lithium ion batteries is focusing on exploring high specific capacity and long cycle life electrodes.Li-rich layered cathode materials have been recognized as an ideal candidate for the next generation lithium ion batteries due to theirs high capacity and environment friendly.Therefore,redox processes during the charging of Li_1.17Ni_0.17M_0.17Mn_0.49O₂?M=Fe or Co?cathode materials with the O3 and T2 phase structures during the charging are studied using a GGA-PBE method in the work.Based on density functional theory,the internal mechanism of the anionic and cationic participating into the oxidation process is explained at atomic level.For Li_1.17Ni_0.17M_0.17Mn_0.49O₂?M=Fe or Co??LNFMO or LNCMO?systems with the O3 phase,the calculated results show that the Ni²⁺/Ni³⁺/Ni⁴⁺redox couple first occurs,then the Fe³⁺/Fe⁴⁺couple,and finally O^2-/O^-in the LNFMO system.O ions with a Li-O-Li configuration easily participate into oxidization and the most active O ions are coordinated by Mn⁴⁺and Li⁺.LNCMO system has a similar redox process except that the O redox is ahead of schedule along with Co oxidization,and the O with a Fe-O-Fe configuration in LNFMO system is activated to participate into charge compensation.Fe-dopings delays oxygen oxidation and avoids the oxygen release and voltage decay caused by the excessive oxidation of O ions.In addition,the calculation results show that there are no O-O bonds formed,which ensures the cyclic stability of the material.These results are consistent with our experiment.XPS confirms the oxidations of Fe,Ni and O and the inhibition of Fe-dopings on voltage decay.Based on the results that the Fe-doping inhibits O oxidation,one Li_1.17Ni_0.17Fe_0.17Mn_0.49O₂ system with the T2 phase structure is designed to study the effect of O coordination unsaturation degree on the redox process.The calculated results show that Ni³⁺/Ni⁴⁺and Mn³⁺/Mn⁴⁺redox couples first occur,then Fe³⁺/Fe⁴⁺and O^2-/O^-redox couples.Different from the Li_1.17Ni_0.17Fe_0.17Mn_0.49O₂ system with the O3 phase structure,the oxidation of O ions in the T2 phase system triggers ahead of schedule due to the O unsaturation degree in the Li-O-Li configuration.The larger the O unsaturation degree is,the higher the activity of O is,leading to the worse oxygen stability and the poor cyclic of materials.In all,Fe-dopings achieve a balance between high capacity and high stability,which provides a theoretical basis for the design of a low-cost and high-energy density cathode material for lithium ion batteries.However,the material with the T2 phase structure should be avoided in use.These studies provide a new scope for the future design of materials.