| High-energy-density cathodes used for lithium-ion batteries are urgently needed to meet the harsh demands of portable electrics and electric vehicles.One of the most promising candidates is Li-rich layered oxides(LLOs)cathodes because of their extraordinary capacity,high voltage,and low cost.Their high capacity comes from unique anionic redox and conventional cationic redox.However,excessive oxidization of lattice oxygen leads to oxygen release and a series of associated problems,such as low initial Coulombic efficiency,rapid capacity fading,and voltage decay.Fortunately,various impressive and effective methods have been put forward to solve the above problems in the past decades including doping,and surface coating.On the base of these modifications,researches on three aspects were conducted in our work:a guideline for tailoring lattice oxygen by strains,lithium deficiencies engineering,and energy energydensity stability during fast charging.Lattice oxygen activity plays a dominant role in balancing discharge capacity and performance decay in lithium-rich layered oxide(LLOs)cathodes.Based on density functional theory(DFT),the activity of lattice oxygen can be improved by tensile strain,while suppressed by compressive strain.To verify the conclusion,LLOs with large lattice parameters(L-LLOs)were synthesized taking advantage of the lattice expansion effect in nanomaterials.Compared with conventional LLOs with small lattice parameters(S-LLOs),primary particles in L-LLOs are imposed by tensile strain.Thus,L-LLOs show a larger initial discharge capacity while decaying faster in the prolonged cycles than S-LLOs.Most of the modified methods in LLOs can come down to straininduced changes in lattice parameters.We believe this conclusion is a useful guideline to understand and tailor the lattice oxygen activity and may be generalized to other layered oxide cathodes involving anionic redox.A long-neglected Li-deficient method is demonstrated to address problems of LLOs including poor kinetics,severe voltage decay,and capacity fading,which are closely related to anionic redox,by simply reducing the lithium content.Appropriate lithium vacancies can improve dynamics features,induce in-situ surface spinel coating,and nickel doping in the bulk.Therefore,the elaborately designed Li1.098Mn0.533Ni0.113Co0.138O2 cathode possesses improved initial Coulombic efficiency,excellent rate capability,largely suppressed voltage decay,and outstanding long-term cycling stability.Specifically,it shows superior capacity retention of 93.1%after 500 cycles at 1 C(250 mA g-1)to the initial discharge capacity(193.9 mA h g-1)and the average voltage still exceeds 3.1 V.Besides,the discharge capacity at 10 C can be as high as 132.9 mA h g-1.More importantly,Li-deficient cathode can also serve as a prototype for further performance enhancement as there are plenty of vacancies.Although great improvements have been achieved on electrochemical performances of LLOs recently,little attention is paid on the energy-density stability during fast charging,Indeed,LLOs have severe capacity fading and voltage decay especially at a high state of charge(SOC),disabling the application of frequently-used constant-current-constant-voltage mode for fast charging.Herein,we address this problem by manipulating the external electric field and tensile strain induced by lattice expansion effect in nano-materials under the guideline of theoretical calculations which indicates that LLOs at high SOC have almost a zero bandgap and a low oxygen formation energy.This strategy will weaken polarization,stabilize lattice oxygen,and suppress phase transition simultaneously.Thus,the energy density can be highly stabilized during fast charging.Therefore,it may be of great value for the practical application of LLOs. |
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