Study On The Effect Of Li-rich Distribution On Energy Storage Mechanism Of Cobalt-free Layered Cathode Materials Li-Fe-Ni-Mn-O

Along with the rapid development of green and clean energy technologies,the social requirements for energy storage and conversion devices are increasing,and Li-ion batteries have attracted much attention for their universal applicability.The cathode material of Li-ion batteries is the bottleneck of development.Although the ternary layered cathode material containing Co has occupied a pivotal position in the application markets,the reserves and the high cost of Co restrict its wide application.Therefore,other transition metals in Li-rich materials with low cost and environmental protection are ideal choices.The Li-rich layered cathode Li-Fe-Ni-Mn-O is one of the hot materials in current research because of its high energy density and low cost.In this thesis,the effects of transition metal oxidation processes,oxygen ion oxidation processes,and Li-rich distribution on the energy storage mechanism of layered cathode materials Li-Fe-Ni-Mn-O during charging are studied using a first principle,which provides a feasible research direction for the experimental design of Li-ion battery cathode materials with excellent performance.First,this work builds the structure models of layered cathode material Li1.21Fe0.13Ni0.12Mn0.54O2 and Li1.17Fe0.25Ni0.12Mn0.46O2.Then,the two most stable structures with Li-rich localized distribution（LD）and delocalized distribution（DD）at the transition metal layer are found by the first principle.Finally,the delithiation of the stable structure is carried out to simulate the first-cycle charging process of Li-ion battery.For the redox process and mechanism of Li_1.21Fe_0.13Ni_0.12Mn_0.54O₂ and Li_1.17Fe_0.25Ni_0.12Mn_0.46O₂ systems,the results show that there is a redox competition between the metal cation and oxygen anion.The general order of the oxidation process is that Ni²⁺/Ni³⁺/Ni⁴⁺couple firstly,then Fe³⁺/Fe⁴⁺couple,and finally O^2-/O^n-（0<n<2）couple.Furthermore,the oxygen involved in oxidation is mainly the O coordinated with Fe and the one with linear Li-O-Li configuration,among which the one with linear Li-O-Li configuration is the most active and easy to participate in oxidation.The oxygen release enthalpyΔH_O and the migration energy barriers of Mn⁴⁺ions for Li-rich LD and DD systems have been studied.The results show that the structure of Li-rich DD is stable and redox reversible of oxygen ions,which can explain the reason that the materials have the same composition but different electrochemical properties.Whereafter,comparing the electrochemical properties of DD systems under two different Li-rich degrees,it can be found that lower Li-rich content Li_1.17Fe_0.25Ni_0.12Mn_0.46O₂ system has a stable structure,good oxygen redox,and strong cycling stability of the battery.Less Li-rich means more delocalized distribution of lithium at the transition metal layer,which is consistent with the previous conclusion.Therefore,materials with less Li-rich content and delocalized distribution of lithium at the transition metal layer are expected to be ideal cathode materials for Li-ion batteries.