Research On The Structure/Interface Regulation And Electrochemical Performance Of Li-Rich Mn-Based Cathode Materials

In recent years,the electric vehicle and energy storage industries are developing rapidly,putting higher requirements on the cost and energy density of lithium-ion batteries（LIBs）.Li-rich Mn-based layered oxides（LRs）are rich in low-cost Mn,and their specific capacity is contributed by the redox of anions and cations,which result in the advantages of low cost and high specific capacity（>250 m Ah/g）.Therefore,LRs have the potential to become the cathode materials for next-generation LIBs.However,although the oxygen anionic redox improves the specific capacity of LRs,it also causes some challenges such as large voltage hysteresis,low initial coulombic efficiency,poor rate performance and serious capacity/voltage decay,which greatly limit the commercial application of LRs.In this thesis,aiming at suppresing the capacity and voltage decay of LRs,structure and interface regulation were performed to improve the cycle performance.The influence of material composition on crystal/electronic structure was studied,and the formation mechanism of cathode-electrolyte interphase was revealed.The structure function relationship between structure/interface and electrochemical performance is established,which will provide theoretical guidance and technical solutions for the structure and interface design of high-performance LRs.The specific research contents are as follows:（1）As the cycle progresses,the oxygen release of LRs gradually spreads from the particle surface to the inside lattice,continuously causing structure degradation,which results in capacity and voltage decay.To address this problem,a bulk oxygen vacancy strategy was proposed to simultaneously reduce the redox activity of oxygen on the surface and inside of the particles,thereby inhibiting oxygen release and structure degradation.Gd³⁺doping was used to reduce the formation energy of oxygen vacancies in LRs,and Gd-doped LRs rich in bulk oxygen vacancies were synthesized.The strong Gd-O bond and bulk oxygen vacancies stabilize the oxygen framework and crystal structure of LRs,reducing the energy level and density of states（DOS）of unhybridized O2p(u O_2p)states,which suppress the electron transfer from O^2-and the formation of O₂molecules during charge.As a result,oxygen release,lattice volume change,transition metal（TM）ion reduction and layered-to-spinel phase transition are reduced,suppressing the capacity and voltage decay of LRs.（2）To further improve the stability of LRs-electrolyte interface at high voltage,the electrolyte optimization method was used to construct a stable interface structure.According to the advantages and frontier molecular orbital energy levels of different fluorinated lithium salts,a ternary fluorinated lithium salts electrolyte（TLE）was designed.TLE established a robust inorganic-rich cathode-electrolyte interphase（CEI）with inorganic/organic/inorganic-rich architecture from inside to outside,which stabilizes the cathode-electrolyte interface at high voltage,reduces the interfacial side reactions,oxygen release,TM dissolution and structure degradation,and further suppresses the capacity and voltage decay.In addition,a robust inorganic-rich anode-electrolyte interphase（AEI）was also constructed on the surface of graphite,stabilizing the anode-electrolyte interface.The AEI and CEI together inhibit the crossover effect between the cathode and anode in the full cells and improve the cycle performance.（3）To further simplify the method of structure regulation,reduce the cost of materials and improve cycle performance,the element component adjustment method was adopted to regulate the crystal structure and electronic structure.Four LRs with different Mn,Ni and Co content were prepared,and the relationships between composition,structure,and electrochemical performance were studied.The substitution of Co for Mn reduces the content of C2/m phase in LRs,while the substitution of Co for Ni increases the content of C2/m phase.The decrease of C2/m content reduces the energy levels of TM 3d states and u O_2p states,increases the DOS of TM 3d states and reduces the DOS of u O_2p states,thus increasing the initial coulombic efficiency,the proportion of slope capacity and the oxygen redox equilibrium potential.The increase of Ni content expands the interlayer spacing and increases the Li/Ni mixing.The Ni in the Li layer stabilizes the layered structure,reduces the volume change and the TM ions migration during cycle,inhibits structure degradation,which improve the cycle and rate performance.Among the four LRs,Li_1.221Mn_0.581Ni_0.198O₂ with the highest Ni content exhibits the best cycle performance,and it still exhibits excellent cycle performance when the synthetic volume is improved to 2 kg.Finally,the TLE was used to improve the interface stability and further improve the capacity and voltage retention of LRs.