Modification And Redox Chemistry Mechanism Of Layered Oxide Cathode Materials For Lithium/Sodium Ion Batteries

With the development of the intelligent industry,the demands of energy density,safety and cost for power batteries are increasing,and it is of great significance to develop higher performance lithium/sodium ion battery materials.Cathode materials are an important part of rechargeable batteries.Therefore,the development and improvement of cathode materials with high energy density,high safety and long cycle life is an important research direction to promote the industrial application of lithium/sodium ion batteries.Among them,the layered oxide cathode material stands out among the cathode materials of lithium/sodium ion batteries because of its simple synthesis steps and high specific capacity,and become one of the most widely studied cathode materials.The dissertation aims to solve the key problems currently existing in layered oxide cathode materials,such as increased impedance under high voltage,oxygen loss,structural distortion during cycling,and irreversible phase transition,by an arsenal of ss NMR,EPR and synchrotron radiation and so on.We carried out a series of basic research on Li Co O₂,lithium-rich and sodium-lithium-manganese layered oxides,focusing on the influence of synthesis modification,doping and surface optimization on electrochemical performance,and expounding its charge-discharge mechanism.The results of this study provide an experimental basis for the future industrial applications of layered oxide cathode materials.The main work is summarized as follows:（1）we employed a binary Ba and Ti-based hybrid surface treatment on LCO by facile wet chemical routes to get LCO@BT material.This strategy integrates the advantages of both interface Ti⁴⁺doping and Ba Ti O₃ particles coating.A combination of in-situ NMR and electrochemical techniques was conducted to detect the detailed structure evolution during Li⁺intercalation/de-intercalation.The results indicat that by introducing Ba Ti O₃ particles,the diffusion of Li⁺in the vicinty of interface is enhanced,and meanwhile,the interface doping layer prohibits the side reactions with electrolyte and improves structural integrity,thus retarding the irreversible phase transition at high working voltage.These advantages result in an ultrahigh capacity and excellent cycling stability of the obtained LCO@BT material when operated at a high cut-off potential of4.5 V.Capacity retention is as high as 90.29%at 0.2 C after 100 cycles,and the discharge capacity remains 180.4 m Ah g^-1 at 0.1 C after 90 cycles,which is superior to most of Li Co O₂ operated at high cut-off voltage of 4.5 V.（2）we propose a multifunctional Mg bulk doping and binary Ba,Ti-based surface modification strategy,involving trace Mg doping,surface gradient Ti doping layer and Ba Ti O₃ dot coating,to alleviate irreversible lattice oxygen and structure reconstruction of LCO.Within the voltage range of 3.0-4.6 V,the as-designed Li Co O₂（LCMO@BT）delivers 82.3%capacity retention under a current density of 100 m A g^-1 after 100 cycles,which is more than twice that of bare LCO(36.7%m Ah g^-1).Moreover,a reversible capacity of 150.7 m Ah g^-1 after 300 cycles under 200 m A g^-1 can be remained for LCMO@BT（75.0%capacity retention）,which is one of the best one among the ever reported 4.6 V-Li Co O₂.We propose a multi-functionality mechanism of LCMO@BT cathode in a 4.6 V-class Li Co O₂/Li cell,consisting of:1.Mg atoms successfully alleviate the oxygen stacking fault at deeply delithiated state,thus altering the O3 to H1-3 phase transition behavior;2.Multifunctional modification inhibits the increased impedance resulting from two-phase isolation and electrode-electrolyte side reaction at high working voltage;3.the doping of Ti on the grain interface not only reduces the irreversible O^2-→O^（2-α）-or V_O·reaction,to stop the further losses of O and Li sites,but also suppresses the reduction of Co³⁺to Co²⁺,thus preventing the layered structure collapsing into densified Co₃O₄ spinel upon prolonged cycling;4.k All of these advantages synergistically add up to significantly promote an ultrastable high-voltage LCMO@BT cycling to 4.6 V.（3）we prepared two nano-particle Li-rich layered oxides by very similar sol-gel precursor method,just adjusting the reacting time.They have similar chemical composition,particle size,and crystal configuration etc.,and the difference is mainly concentrated in internal structure.The irregular interlayer glide is clearly observed in LNCMO-s along the[110]orientation,while an ideal layered structure is observed in LNCMO-s.In electrochemical evaluation,The LNCMO-p exhibits less capacity decay and voltage fade,maintaining a capacity of 200 m Ah g^-1 at the current density of 100m A g^-1 after 150 cycles.Offering enough reacting time before adjusting the p H value leads to not only enhanced electrochemical performance but also higher structural stability.The s XAS study after prolonged cycles illustrate that less Mn³⁺is produced in the surface of LNCMO-p material,which proves that the LNCMO-p with a neat structure has less phase transition.This is because the order of the microstructure maintains the stability of the material during the cycles and reduces the glide between layers.Meanwhile,we found that the transition metal migration of LNCMO-p material is unobvious,indicating that the transition from the layered structure to the spinel structure is inhibited,which further explains its smaller voltage drop during the charge and discharge processes.（4）we successfully ameliorate the cyclic performance of the P2/O3-Na_0.8Li_0.27Mn_0.73O₂ cathode material by a facile partial Ti-substitution strategy.The comparison of electrochemical performance between Ti-substituted P2/O3-Na_0.8Li_0.27Mn_0.68Ti_0.05O₂ and non-substituted counterpart clearly proves that the incorporation of Ti-substitution improves the initial discharge specific capacity(from120.2 m Ah g^-1to 143.2 m Ah g^-1)and also the capacity retention after long cycles.By increasing the electron density around O ions in Ti-O bonding upon charge,as well as inhibiting the problematic Jahn-Teller effect of Mn³⁺during discharge,Ti-incorporation effectively promotes the oxygen electron contribution to the redox reaction without sacrificing O-redox reversibility.Especially,analyses based on ex-situ HRTEM and ⁷Li solid-state NMR characterizations reveal that multiple factors are important to achieve the structure stability for Ti-substituted materials:（1）the irreversible surface crack is eliminated within a voltage range of 2.0-4.3 V;（2）lithium loss in the lattice is effectively alleviated and lithium ions in the TM layer is well-maintained.