Study On Constructing Surface Heterostructure To Stabilize Lattice Oxygen In Li-Rich Mn-Based Cathode Materials

Li-Rich Mn-based cathode materials with anionic and cationic redox contributing higher discharge specific capacity than Li Fe PO₄,Li Co O₂ and ternary materials are considered as ideal choice for the next generation of cathode materials for lithium-ion batteries.Unfortunately,oxygen anionic redox isn’t completely reversible,resulting in continuous lattice oxygen loss and bringing structural attenuation and degradation and making materials face many problems including low initial coulombic efficiency,capacity reduction,voltage decay and so on.This thesis focuses on surface heterostructure constructing to solve the problem of lattice oxygen instability,inhibit oxygen loss and improve electrochemical performance.The main results are as follows:（1）Li-Rich Mn-based cathode materials with abundant Li vacancies were synthesized by regulating the amount of lithium salt.Lithium vacancies induce surface spinel phase structure and rich oxygen vacancies on the surface,which inhibit oxygen evolution and improve the reversibility of anionic redox.Li vacancies makes Li₂Mn O₃ transform to Li Me O₂ phase,and cationic mixing is found to a certain extent in the bulk phase,which changes the oxygen environment and stabilizes the lattice oxygen.And the lithium layers are enlarged,reducing the lithium ion migration energy barrier and increasing the migration rate and solid phase diffusion coefficient of lithium ion.Li_1.11 Ni_0.18 Co_0.18 Mn_0.53 O₂ with rich lithium vacancies showed a high discharge specific capacity(293.9 m A h g^–1 at 0.1 C),cycling stability(199.7 m A h g^-1 after 100 cycles at 1 C)and excellent rate performance(106 m A h g^-1 at 10 C).（2）The composite structure wiyh Li₃PO₄ and spinel phase double coating layers and more oxygen vacancy was constructed on the surface by NH₄H₂PO₄ pretreatment for Li-Rich Mn-based cathode materials.The surface composite structure obviously inhibits the release of oxygen and the dissolution of transition metal,and accelerate the transfer rate of Li⁺,making the pretreated material exhibit higher anionic redox reversibility,lattice oxygen stability and better kinetic performance.The initial coulombic efficiency of the pretreated materials increased from 80.47%to95.30%,the capacity retention rate reached 84.4%after 100 cycles,and the discharge specific capacity of 10 C was 132.2m A h g^-1.After 100 cycles at3 C/1 C fast charging cycling,the discharge specific capacity is still up to205.1 m A h g^-1.（3）By pretreatment with（NH₄）₂B₄O₇,a composite surface structure of spinel phase layer and borate layer was constructed to inhibit the surface side reaction and oxygen loss and stabilize the material structure.Meanwhile,the strong B-O bond in the bulk stabilizes the lattice oxygen and improves the reversibility of the anionic redox.The initial coulombic efficiency of Li-Rich Mn-based cathode materials after pretreatment was improved from 84.9%to 89.1%,and the capacity retention rate reached88.6%after 100 cycles,and the discharge specific capacity even at 10 C is still up to 125.7 m A h g^-1.After 100 cycles at 3 C/1 C fast charging cycling,the discharge specific capacity is still up to 213.5 m A h g^-1.