Research On Li-excess Cation-disordered Rock-salt Cathode Material With High Specific Energy In Lithium-ion Batteries

The rapid development of portable electronic devices and electric vehicles has driven the evolution of lithium-ion batteries(LIBs)with higher performance.Currently,the classical layered oxides cathode materials,LiMeO2(Me=Co,Ni,Mn,etc.)and olivine-structured LiFePO4 have been extensively investigated and widely used in commercial LIBs.However,their capacities are limited by the number of reversible(de)intercalation lithium ions during(dis)charge,and cannot meet the growing demand for the energy density.Thus,it is crucial to explore new types of cathode materials with much higher energy density.Over the past decade,Li-excess cation-disordered rocksalt oxides(DRXs)have attracted many interests,which can deliver high capacities of higher than 250 mAh g-1 on account of both cationic and anionic redox.Recently,Lirich cation-disordered rock-salt materials,eg.Li1.3Nb0.3Mn0.4O2,Li1.2Mn0.4Ti0.4O2,Li1.2Ni1/3Ti1/3Mo2/15O2,Li1.3Ta0.3Mn0.4O2,have got many attentions,which are capable of delivering reversible capacities of higher than 250 mAh g-1.We firstly research the capacity degradation mechanism and surface modification method of Li1.2Ti0.4Mn0.4O2.Combining in-situ X-ray diffraction(XRD),differential electrochemical mass spectrometer(DEMS),X-ray absorption near edge spectroscopy(XANES),etc.,we analyzed the redox process of the carbon-coated Li1.2Ti0.4Mn0.4O2 material in the(dis)charge.That is,the redox reaction of the Mn2+/Mn4+ at low voltage is relatively stable,while the redox reaction of O2-/O2n-in the high voltage range is unstable.In addition,this work further clarifies that the anionic redox process is the main reason for the sluggish kinetics,polarization,and capacity fading of Li1.2Ti0.4Mn0.4O2 during cycling.It is reveal that a certain amount of surface carbon coating can suppress the irreversible lattice oxygen loss of the material and result in its cycling stability,providing a new surface modification method for better design of DRXs.We explores the use of Mn4+ substitution to obtain a series of DRXs cathodes of Li1.2Mn0.43+Mnx4+Ti0.4-xO2(LMMxTO,0≤x≤0.4)oxides.The XRD results show that when the x range increases from 0 to 0.2,they are belong to the disordered rock-salt structure.After Mn4+ substitution,LMM0.2TO delivers a high specific capacity of 322 mAh g-1 at a current density of 30 mA g-1 and room temperature,and can even reach 352 mAh g-1(30 mA g-1,at 45℃),the energy density is 1041 Wh kg-1.According to dQ/dV curve,XANES,DFT calculation and in situ XRD results,it can be seen that the redox activity of cation(Mn)and anion(O)of LMM0.2TO is significantly increased,thus making LMM0.2TO released high charge-discharge capacity.In addition,the roles of Mn3+ and Ti4+ in LMM0.2TO are discussed in detail,and the ternary phase diagram for the formation of DRXs cathode materials is established,which can further optimize the Mn3+-Mn4+-Ti4+system.This work presents an innovative strategy to improve energy density and broadens the idea of designing DRXs with better performance.We also systematically studies the effect of fluorine(F)substitution on cationic and anionic redox in DRXs.We have successfully synthesized a series of DRXs(Li1.2Mn0.4+xTi0.4-xO2-xFx)(0≤x≤0.2)with different fluorine(F)contents.The electrochemical performance results show that the Li1.2Mn0.55Ti0.25O1.85F0.15(LMTOFO.15)exhibits the highest reversible capacity(275 mAh g-1,under 30 mA g-1),better cyclability,and voltage retentions.The mapping of resonant inelastic X-ray scattering(mRIXS)and DEMS results reveal that the fluorination enhances the reversible lattice oxygen redox reaction while suppressing irreversible gas release and surface reactions.The Mn K-edg XAS during the initial two cycles shows that F-substitution alleviates the reduction of the Mn valence state during the whole(dis)charge processes in the bulk and at the surface of the material,results in higher average discharge voltage.In addition,the introduction of F improves the structural stability and suppresses local lattice distortion of the material.Therefore,LMTOFO.15 is able to cycle with smaller polarization,less interfacial side reaction and Mn dissolution,and therefore results in enhanced cyclability.This work provides a comprehensive understanding of the fluorination effect on the cationic and anionic redox activities in DRXs.Finally,this paper investigate the improvement of the cycling stability of DRXs by tailoring the redox-active transition metal(Mn)content.We investigate a series of xLi2TiO3-(1-x)LiMnO2(0≤x≤1)materials and find that only Li1.2Mn0.4Ti0.4O2(x=0.4)and Li1.1Mn0.7Ti0.2O2(x=0.2)can form phase-pure DRXs,which both deliver high capacity(>250 mAh g-1).The newly discovered Li1.1Mn0.7Ti0.2O2 DRX exhibits remarkably high capacity retention of 84.4%after 20 cycles compared to only 60.8%for L11.2Mn0.4Ti0.4O2.Our result indicates that the irreversible oxygen loss is reduced by raising the Mn content.Theoretical calculations further reveal that increasing the redoxactive Mn content from Li1.2Mn0.4Ti0.4O2 to Li1.1Mn0.7Ti0.2O2 causes the orbitals near the Fermi level to change from O 2p non-bonding(Li-O-Li unhybridized orbitals)to(Mn-O)*antibonding bands,exhibiting a high O-O aggregation barrier,preventing O2 release and resulting in sustained capacity retention.Hence,these new findings demonstrate that regulating oxygen redox by tailoring the redox-active transition metal content is an effective strategy to enhance the cycling stability of DRXs.