Structure Regulation Of Lithium-rich Manganese-based Oxide Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries have provided energy guarantees for the development of mobile electronic devices,and have gradually become an important support power source for new fields such as electric vehicles and energy storage.Developing higher energy density lithium-ion batteries is the unremitting goal of researchers.However,the energy density of lithium-ion batteries is mainly restricted by cathode materials.The development of cathode materials with high capacity and high voltage is the key to break through the energy density of lithium-ion batteries.Lithium-rich manganese-based oxides（LRMO）with the formula denoted as x Li₂Mn O₃·（1-x）LiMO₂,（M=Ni,Co,Mn,etc.）are considered as the most promising next generation cathode material due to its high capacity（>250 m Ah/g）,high safety and low cost.However,the LRMO still have drawbacks such as fast capacity degradation,poor rate performance,and voltage decay during cycling,which cannot meet the needs of practical applications.To solve these problems,we used different methods to regulate the structure（oxygen vacancy,surface protonation,O2 crystal structure and composite phase structure）of LRMO and showed improvement of the electrochemical performance.The main contents and results are as follows:1.The construction of dual modification layer of LiF and oxygen vacancy layers.Using NH₄F combined with H₂（5%H₂/95%Ar）atmosphere heat treatment,a dual modification layer of LiF and oxygen vacancies is constructed on the surface of LRMO Li_1.2Ni_0.13Co_0.13Mn_0.54O₂,which improves the cycle stability and rate performance of the material.It’s found that pre-protonation of the material can induce changes in the electron cloud on O atoms and transition metals,which makes it easier to produce oxygen vacancy when the material is heat-treated in H₂ atmosphere,thus improves the structural stability of the material.The modified material has a specific discharge capacity of～255 m Ah/g at a current density of 100 m A/g,and maintain a capacity retention of 91%after 200 cycles.2.Surface protonation.Aiming at the drawbacks of voltage fading of LRMO Li_1.2Ni_0.13Co_0.13Mn_0.54O₂,we designed a dual stabilization strategy combined with surface coating and protonation to stabilize the surface lattice structure of LRMO.Polyacrylic acid（PAA）is used as a binder to form a uniform coating layer on the surface of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ material.TEM and elemental analysis indicates that the surface coating effectively alleviates the erosion of the material surface by acidic species in the electrolyte,and the dissolution of transition metals in the electrolyte is significantly reduced.Moreover,the H⁺in-COOH group inserted into the crystal lattice through the H⁺/Li⁺exchange reaction.DFT calculations show that H⁺can inhibit the migration of transition metal ions and stabilize the lattice structure,which is an important reason for the improvement of material stability.The LRMO-PAA electrode exhibits high stability with a capacity retention of 88%and an average voltage drop of0.88 m V/cycle at a current density of 200 m Ah/g for 500 cycles.This protonation strategy may provide a new route to building a stable Li-rich oxide cathode with high capacity retention and low voltage fading for practical Li-ion battery applications.3.The construction of a stable O2 layered structure.It’s considered that the layered oxides with O2 structure cannot transform into the spinel structure due to its different O atom stacking sequence from the spinel structure,thereby suppressing the voltage fading.However,the O2-structure LRMO prepared by ordinary ion exchange methods have low Licontent,which limits their electrochemical performance.By adding the reducing agent LiI in the ion exchange process,more Li⁺intercalation in the O2-structure is achieved during the ion exchange process,and improved the electrochemical performance of the O2-structure LRMO.Electrochemical tests show that the material has a specific capacity of～255 m Ah/g,and maintains a high capacity retention of 85%with a minor voltage decay of 0.90 m V/cycle after 300 cycles,showing excellent electrochemical performance.In addition,it’s found that the redox potential of the LRMO with O2 structure is slightly lower than that of the LRMO with O3 structure,which may be beneficial to the stability of anion redox.4.Considering the limited resources and high price of Co and Ni,and the Jahn-Teller effect of Mn,an all-manganese-based Co and Ni-free cathode material was synthesized through ion exchange reaction.XRD and Raman tests show that the material is composed of spinel and layered structure,and the chemical formula can be expressed as 0.57Li₂Mn O₃·0.43Li₄Mn₅O₁₂（denoted as LS-L-Mn-O）.This material delivers a high specific discharge capacity of～290 m Ah/g.XPS indicates that the high specific capacity of LS-Li-Mn-O is an accumulation of both cationic and anionic redox:anion redox occurs mainly at 4.8-3.25 V,while Mn³⁺/Mn⁴⁺cationic redox located at 3.25-2.0V.In addition,LS-Li-Mn-O shows good rate capability which is benefit from the layered and spinel composite structure:the specific discharge capacity reaches 245,218,and 193 m Ah/g at 0.5C,1C,and 2C rates（1C=200 m Ah/g）,respectively.This new composite structure of all-manganese-based cathode material provides an optional system for the development of low-cost,high-performance lithium-ion batteries.