Study On Modification And Mechanism Of Co-free Mn-based And Ni-based Cathode Materials For Lithium-ion Batteries

In recent years,lithium-ion batteries have been widely used and developed rapidly due to their high energy density and long life.However,the current commercial Co-rich cathode materials,such as lithium cobalt oxide and lithium nickel cobalt manganese oxide,are facing the problems of increasing prices and lacking Co resources.Therefore,it is an important aspect to improve the comprehensive properties of Co-free Mn-based materials（such as spinel Li Mn₂O₄）and Ni-based materials(such as Ni-rich layered Li Ni_0.75Mn_0.25O₂)for lithium-ion batteries.However,problems such as Mn dissolution and side reaction of electrolyte of Li Mn₂O₄ affect its cycling performance at high temperature.In addition,Li Ni_0.75Mn_0.25O₂has problems of ion mixing,phase transformation,surface/interface,and stress accumulation,which make its mechanical stability and electrochemical performance rapidly decay.To solve the above problems,the electrochemical performance of Li Mn₂O₄ was improved by controlling the morphology of single crystal and constructing chemically stable surface,and the mechanism and modification of the rapid deterioration of the structure,mechanical and electrochemical performance for single crystal Li Ni_0.75Mn_0.25O₂ under high voltage（4.6 V）were studied.The main study of this paper is as follows:（1）Single crystal Li Mn₂O₄ cathode materials were prepared by three routes:high temperature pretreatment for precursor followed by low temperature sintering method,double-sinter method,and rusting method to prepare in-situ doped precursor followed by mixed lithium salts for sintering method.The differences of sintering temperature,doping element type and doping amount on the electrochemistry properties and morphology of Li Mn₂O₄ were analyzed.Additionally,in comparison with the traditional single-sinter high temperature solid phase method,the three routes adopted show obvious improvement effect,and the single crystal Li Mn₂O₄ prepared by the third route has the best high temperature cycle performance(the specific capacity of initial discharge at 1 C is 115.7 m Ah g^-1,after 100 cycles,the capacity retention rate is 95.0%).（2）Herein,Sr-modified Li Mn₂O₄ is designed and prepared.A basic understanding of the existing form and position of large radius Sr ions,as well as their influence on the structural stability of the bulk are expounded.Results reveal that the Sr Mn O₃enriches at grain boundaries of Li Mn₂O₄and Mn-O-Sr bonds at the Sr Mn O₃/Li Mn₂O₄ interface.Furthermore,stable SMO alleviates the migration of Mn ions in the Li Mn₂O₄associated with structure integrity,suppresses side reactions between electrode and electrolyte.Compared with the fresh Li Mn₂O₄,the Sr-modified material shows excellent cycling and rate properties due to more stable structure and faster Li⁺/electron conductivity.The capacity of Sr-modified Li Mn₂O₄maintains 94.8%at 25°C and 79.6%at 55°C after 500 cycles.Moreover,the capacity retention of that at 5 C is 99.1%in the rate performance test.Consequently,designing crystal boundary enrichment of strontium presents to be a promising way.（3）Degradation mechanism of single crystal Li Ni_0.75Mn_0.25O₂ and its Mg Modification.Co-free Ni-rich cathodes suffer from electrochemical performance degradation,mechanical and structural instability.Although the design of single-crystal structure can mitigate these drawbacks,the cracking can still exist upon prolonged cycling and failure mechanism also remains elusive.Herein,multi-directional planar gliding and cracking along the（003）and（100）plane in Li Ni_0.75Mn_0.25O₂ are observed.Moreover,relationship between slipping and cleavage barrier energy,particle internal stress and crystal slipping and cracking by theory calculation and simulation of cathodes are elucidated.The results affirmed that Mg as pillar in the Li layer of Li_0.99Mg_0.01Ni_0.75Mn_0.25O₂ can play a role in improving slipping and cleavage barrier energy and reducing internal stress,thus effectively alleviating multi-directional planar gliding and cracking.Finally,the electrochemical performance of it is improved（after 100 cycles at 4.3 V,4.4 V and 4.6 V,the capacity retentions of Mg-modified cathode are increased by 31.6%,41.6%and 38.0%,respectively）.This work provides an in-depth understanding of structural stability of Co-free Ni-rich cathodes,also offer a new path for improving the structural stability for developing low-cost and high-energy density cathode materials.