Study On Synthesis And Modification Of Lithium-Rich Layered Oxide Cathode Materials And The Compatibility With Electrolytes

Lithium-ion batteries?LIBs?have been widely used in the fields of portable electronic devices,electric vehicles etc..Nevertheless,with the development of technology,the traditional cathode materials with lower specific capacity can never meet demand for high energy of lithium-ion batteries in energy storge equipment at this stage.Therefore,the development of cathode materials with high specific capacity and high voltage has become a hot topic.Layered lithium-rich manganese-basedcathodematerialsofxLi₂MnO₃·?1-x?LiMO₂?0<x<1,M=Ni,Co,Mn,etc.??0<x<1,M=Ni,Co,Mn,etc.?have become the most potential new generation of power battery cathode materials due to the advantages of high capacity,high operating voltage and low cost.This paper starts with the preparation process of materials,element doping modification and compatibility of materials and electrolytes.The crystal structure and particlesizeofLi-richmanganese-basedcathodematerial Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,the interface reaction and electrochemical performance are discussed.The major research contents and results are as follows:?1?Synthesis of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ by sol-gel method.The effects of different lithium salts,calcination temperature and calcination atmosphere on the structure,morphology and electrochemical properties of the materials are studied,and the synthesis process of the materials is optimized.The experimental results shown that the materials had complete crystal structure,uniformly dispersed particles,uniform size and excellent electrochemical performance when lithium salt is lithium acetate,and calcines in oxygen atmosphere under 900?C.The first discharge specific capacity is 266.5 mAh g^-1 at the 0.1 C rate,and the Coulomb efficiency is 71.9%.The specific discharge capacity is 207.8 mAh g^-1 after 100 cycles,and the capacity retention rate is 77.97%.?2?Mg doping modification.Aiming at overcoming the defects such as poor rate performance of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ material,the effects of Mg doping on the structure,morphologyandelectrochemicalpropertiesof Li_1.2Mn_0.54-x.54-x Ni_0.13Co_0.13Mg_x O₂?x=0,0.01,0.02 and 0.05?are investigated.The research showed that the doping of Mg can improve rate performance and stabilize structure for the materials that is because Mg²⁺owns a relatively large radius which is favorable to enlarge the path for Li⁺and promotes the rate performance.In addition,the radius of Mg²⁺?0.72??is similar to Li⁺?0.76??that will migrate into lithium layer to lessen the mixed arrangement of transition metal ion.When the doping amount of Mg at x=0.02(Li_1.2Mn_0.52Ni_0.13Co_0.13Mg_0.02O₂),the material shows the best electrochemical properties.?3?Mg-Al co-dopedmodification.Althoughthemodified material Li_1.2Mn_0.52Ni_0.13Co_0.13Mg_0.02O₂ with Mg element has good electrochemical performance,it still has the problem of serious first irreversible capacity loss and poor cycle performance.Therefore,the effects of Mg-Al co-doping on the structure,morphology and electrochemical properties of Li_1.2Mn_0.54-x-y.54-x-y Ni_0.13Co_0.13Mg_x Al_y O₂?x=0.02;y=0,0.01,0.02 and 0.05?are investigated in this chapter.The research showed Mg-Al co-doping can reduce the degree of initial irreversible capacity loss,improve the structural stability and promote cyclic stability and rate performance,which is attributed to the formation of some compounds containing Al?such as LiAlO₂and AlF₃ etc.?on the surface because of doping Al,which can enhance interfacial stability of materials,inhibitthe oxidation of O^2-and the formation of oxygen vacancies,thereby effectively reducing the migration of transition metal elements,cation mixing and continuous decline of valence state.And Mg doping synergistically inhibits the phase transition of the lithium-rich manganese-based material during the cycle and improves the electrochemical properties of the material.When doping amount is y=0.02(Li_1.2Mn_0.50Ni_0.13Co_0.13Mg_0.02Al_0.02O₂),Lithium-rich layered oxide owns the best electrochemical conductivity.?4?The traditional LiPF₆-based electrolyte's drawbacks are taken into account which has narrow electrochemical window and is sensitive to moisture,and easy to create HF to corrode materials on the surface.In this paper,the novel electrolyte lithium salt difluorobis oxalate lithium borate?LiODFB?-based electrolyte is chosen to study the compatibility with Li_1.2Mn_0.50Ni_0.13Co_0.13Mg_0.02Al_0.02O₂.The results have shown that due to the good film-forming property of LiODFB,a dense and smooth solid electrolyte interface film is formed on the surface of the positive electrode material.So that he occurrence of side reaction of the material surface and the dissolution of the transition metal ions are reduced,the phase transition of the electrode surface is suppressed,and the cycle life and rate performance of the battery are improved as well.In summary,the preparation of layered lithium-rich manganese-based cathode materials is prepared by sol-gel method,through the study of two-element co-doping modification and the compatibility of material and electrolyte,the comprehensive performance of the materials are significantly improved,which has very important practical significance and will provide a new idea for the further development of layered lithium-rich manganese-based cathode materials.