Prepration And Modification Of Li-Rich Mn-Based Cathode Materials For Liithium-Ion Batteries

Lithium-ion batteries are the main power source of portable electronic products and electric vehicles.The ubiquitous battery life/mileage anxiety constantly puts forward new requirements for the energy density and safety performance of lithium-ion batteries.Li-rich Mn-based cathode material x Li₂Mn O₃·（1-x）Li MO₂（0<x<1,M=Ni,Co,Mn etc.）,due to its higher operating voltage,more lithium content,and anion redox excess characteristics are powerful candidates for the new generation of cathode materials.However,some key issues before real commercialization,including low coulombic efficiency in the first cycle,poor rate performance,severe capacity loss,serious voltage attenuation and the obvious voltage hysteresis needs to be solved urgently.This paper conducts the following research on the characteristics and defects of Li-rich Mn-based cathode materials:1.This work proposes a new low-voltage thermal activation（LVTA）strategy.Aiming at the irreversible oxygen loss problem during the first charging process of Li-rich Mn-based cathode materials,the Pyr₁₄TFSI-based ionic liquid electrolyte is used to match it,avoiding the problems caused by the decomposition and oxidation of the electrolyte.Meantime,by regulating the redox behavior of anionic,using a higher temperature（55℃）to overcome the slow reaction kinetics limited by low voltage,the full activation of the anion capacity was achieved at a quasi-equilibrium potential（4.5 V）,effectively eliminating the lattice oxygen loss by O₂and CO₂pathways caused by conventional high voltage（HVA）activation,and achieved a high reversible capacity of295.5 m Ah g^-1.2.The LVTA strategy suppresses the oxygen loss in the near-surface region due to severe phase transition and electrolyte side reactions,but the activation process of the Li₂Mn O₃component in the Li-rich Mn-based cathode material still generates oxygen vacancies and lithium vacancies in the transition metal to form a defect-rich structure.The structural reconstruction of stable rock-salt facies is realized by utilizing the self-decomposition characteristics of the overcharged near-surface region.This surface structure remains stable in the subsequent high-voltage cycle,inhibits the loss of active oxygen anions,and enhances the cycle reversibility of the material,and the capacity retention rate reaches98.58%after 150 cycles at 55°C,4.8-2.0 V and 0.2 C(1C=250 m A g^-1),which is far better than the 39.85%retention rate of the unmodified sample.3.Near-surface structural evolution mechanism of Li-rich and Mn-based cathode material after anion activation reaction was investigated by first-principles calculation.TM ion migration energy barrier results indicate that the structural reconstruction process is mainly dominated by the migration of Ni,while lattice oxygen vacancy defects lead to the difference of structural reconstruction results.Few oxygen vacancies promote the transformation of surface structure to rock salt structure,and serious lattice oxygen loss leads to phase transition from layered to spinel structure,so suppressing the lattice oxygen loss in near-surface region is the key to control the surface structure reconstruction of Li-rich and Mn-based cathode materials.Calculation of the decomposition reaction energy shows that the surface rock salt cladding layer has better stability compared with the spinel structure;the electrostatic potential and work function calculations also show that the rock salt structure has lower electrostatic potential,and the rock salt phase cladding can effectively improve the stability of the surface structure and thus inhibit the electron escape of the high potential driving anionic.