| Li-rich maganese-based layered oxides have become one of the promising candidates for next-generation lithium-ion battery cathode materials due to their ultra-high specific capacity over 280 mAh g-1 and low cost.The unique anionic charge compensation mechanism in lithium-rich materials contributes more than half of the capacity,but brings serious problems such as initial irreversible capacity loss,voltage hysteresis and decay,capacity attenuation,and poor rate performance,which reduces the advantages of high specific capacity and energy density of the material to some extent.In this paper,multi-dimensional structural control such as electrochemical regulation,surface functionalization modification,and superlattice structure regulation of Li-rich manganese-based layered oxides is applied to adjust the redox activity of highly active anions,maintain good crystal structure of bulk materials and improve electrochemical performance.At the same time,the correlation between the anion behavior mechanism and the local structure is deeply studied,and the high-capacity advantage of anionic charge compensationis fully utilized.First,Li1.2Ni0.13Co0.13Mn0.54O2 material was regulated by electrochemical means,and the structure and electrochemical properties of the material under different cut-off voltages were studied.Pre-cycling(especially at 4.5 V)can effectively regulate the activation behavior of anions,and improve the Li+active sites and diffusivity in the activated material;low-voltage operation can suppress the irreversible release of oxygen and the decomposition of electrolyte,thereby stabilizing bulk phase structure and electrode/electrolyte interface.Under the synergistic effect of pre-cycling and low-voltage operation,Li-rich material exhibits excellent electrochemical performance with a capacity retention of 92.77%and a voltage decay of only 0.3731 V after 200 cycles.Precisely regulating the redox behavior of anions can effectively optimize the structure and electrochemical performance of Li-rich materials.Second,oxygen-vacancy-rich and low-valence MoOx was deposited in situ on the surface of Li-rich material Li1.2Ni0.13CoO0.13Mn0.54O2 to improve the irreversible oxygen loss problem on the surface layer.The study found that the surface coating layer can effectively accommodate the free oxygen released from the near-surface layer of the material,avoiding the side reaction between free oxygen and the electrolyte and improving the reversibility of reduction;the spinel phase with threedimensional Li+ diffusion channels induced between the bulk material and the coating layer,and low-valence MoOx with good electrical conductivity can significantly improve the electrode process kinetics.Coated with 3 wt%MoOx,the material remains the specific capacity of 224.2 mAh g-1 after 100 cycles at 0.5C(125 mAh g-1),with the retention rate of 85.8%,and the specific capacity reaches 192.0 mAh g-1 at 5C.Thirdly,for the bulk material structure,Li1.2Ni0.13Co0.13Mn0.54O2 was surface doped with Os element to regulate the spatial coordination environment of surface oxygen ions and anchor the lattice oxygen.Surface doping induces surface heterostructures with cationic disorder,which promotes the three-dimensional disordered arrangement of O 2p unhybridized orbitals and effectively suppresses the generation of surface O-O dimers.2 at%Os-doping significantly improves the voltage decay and hysteresis of Li-rich materials.The average per-cycle voltage decay decreases from 2.17 mV to 1.01 mV during 300 cycles,and the voltage hysteresis between charging and dischargeing shrinks from 1.2278 V to 0.7326 V and at the 300th cycle.Furthermore,control the superlattice structure of materials,and fundamentally solve the problem of poor anionic redox reversibility caused by the unstable of traditional honeycomb superlattice.O2-type manganese-based cathode material Lix[Li0.2Mn0.8]O2 with a novel ribbon-like superlattice structure was successfully prepared by electrochemical ion exchange.During the electrochemical ion exchange process,the P2-type precursor transformed into the O2-type Lix[Li0.2Mn0.8]O2 by the slip and shrinkage of the adjacent slabs,and the Li-Mn ordered ribbon-like superlattice structure in the transition metal layer still maintain stably.This ribbon-like superstructure eliminates the generation of O-O dimers and enables highly reversible anionic redox and excellent cycling stability.The specific capacity of the material can reach 227.3 mAh g-1 through low-voltage pre-cycling treatment,and there is no obvious voltage decay during cycling.Finally,the anionic charge compensation mechanism in a series of manganesebased cathode materials with Li-Mn(Li:Mn=1:2,1:5,1:6)ordered superstructure types is systematically investigated.These Li-Mn ordered superstructures can be prepared by directly controlling the Li/Mn ratio in the precursor material,where the material with a Li/Mn ratio of 1:3 exhibits a honeycomb(?)superstructure as the material with a Li/Mn ratio of 1:2.With the decrease of Li/Mn ratio,the LiMn6 superstructure unit in the material is gradually delocalized,which inhibits the aggregation of Li vacancies caused by the migration of Mn elements in the interlayer,enhances the reversibility of anionic redox and improves the voltage hysteresis problem.Accordingly,the number of reactive oxygen species and their capacity contribution decreases.In addition,the slow electrochemical reaction kinetics of the sluggish anion can fully unleash its high-capacity advantage in a lowrate conditions.Therefore,it is necessary to comprehensively consider the mechanism of different superstructures on the electrochemical performance of materials,and reasonably take advantage of the unique characteristic brought by anionic charge compensation. |
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