Surface Modification Of High Nickel Ternary Cathode And Electrochemical Performance In Solid State Batteries

High nickel ternary materials(LiNi_xMn_yCo_1-x-yO₂)are ideal cathode materials for lithium-ion batteries due to their advantages of high discharge specific capacity,high voltage plateau,and environmental friend liness.However,the high nickel ternary cathode material has poor interfacial compatibility with the electrolyte during charging and discharging,serious side reactions and short cycle life.To address the above issues,two different surface coating methods are proposed to modify the interface between high nickel ternary cathode NMC811(LiNi_0.8Mn_0.1Co_0.1O₂)and electrolyte,and the electrochemical performance of NMC811 with different coatings in solid-state lithium batteries is investigated in this thesis.The specific experiments are as follows:（1）A layer of LiNbO₃ ionic conductor was coated on the surface of NMC811 by the sol-gel method,and the electrochemical performance of the high nickel ternary cathode material was significantly improved by the LiNbO₃ coating as corroborated by the characterization means of scanning electron microscopy and transmission electron microscopy.The initial discharge specific capacity was 176 mAh/g at a current density of 0.5 C in the voltage range of2.8-4.3 V at 25℃,and the capacity retention rate reached 86%after 150 cycles.The discharge specific capacity was 70 mAh/g at a current density of 10 C.The reason for the obvious modification effect is that the LiNbO₃ ionic conductor artificially coated on the surface of the high nickel ternary particles stabilizes the structure of the material,enhances its multiplicative properties,and effectively suppresses the interfacial side reactions of the high nickel ternary particles.The encapsulated MNC811 particles were coated on a porous aluminum foam collector with high specific surface area,and the initial discharge specific capacity was 175 mAh/g at a high active substance loading of 42 mg/cm²,and the capacity retention rate was 85%after 100cycles.The active substance loading was substantially increased,and the better cycling performance at high active substance loading was attributed to the high porosity and high specific surface area collector fluid attached to more cathode material particles and assembled into a full cell with electrochemically pre-lithiated silicon-carbon cathode at this loading,with a specific capacity of 201 mAh/g for the first cycle discharge and 213 mAh/g for the discharge after 10 cycles.Based on two types of cathode collectors,porous aluminum foam and aluminum foil,LiNbO₃-coated NMC811 and garnet LLZTO solid-state electrolytes were assembled into solid-state batteries with first cycle discharge specific capacities of 155.2 mAh/g and 156.2 mAh/g at 0.2 C current density,and capacity retention rates of 53.4%and 8%after 50 and 20 cycles,respectively.（2）An in situ electrochemical method was used to coat the surface of NMC811 with a LiF-rich solid electrolyte interfacial layer.The prepared solid electrolyte was first tested for ionic conductivity,ultimate current density,and Li||Li symmetry cell,and the prepared solid electrolyte was assembled with NMC811 to form a solid-state cell with a pyrrole-like PYR₁₃TFSI ionic liquid combined with a commercial carbonate-based electrolyte（EC:EMC:DMC=1:1:1 1 1M LiPF₆）as a hybrid electrolyte to modify the electrode/electrolyte interface,compared with the carbonate-based electrolyte and PYR₁₃TFSI ionic liquid alone to modify the solid-solid interface,the battery with the hybrid electrolyte modification had a discharge specific capacity of 161 mAh/g after 150 cycles at a current density of 0.2 C and a capacity retention rate of 80%,which was significantly better than the electrochemical performance when the two electrolytes were modified separately.The composition and morphological evolution of the cathode/electrolyte interface after cycling were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy,respectively,and it was found that a LiF-rich solid electrolyte interface film was formed on the surface of NMC811particles after cycling,which also proved that a hybrid electrolyte was used in this paper to in situ coat a layer of LiF on the surface of NMC811 and improve its electrochemical properties.