Structural Evolution And Interface Engineering Of Nickel-rich Ternary Cathode Materials For Lithium-Ion Batteries

Since the concept of lithium-ion battery was proposed in the last century,lithiumion battery technology has been rapidly developed and achieved fast commercialization,which is widely used in various consumer electronic products and electric vehicles.With the development of society and the increasing demand of consumers,the traditional lithium-ion battery can no longer meet the actual needs,mainly in terms of energy density,power density,cycle life and safety performance.The cathode material plays a limiting role in the whole lithium-ion battery system,both in terms of cost and capacity,so it is necessary to vigorously develop high-performance cathode materials.Traditional cathode materials,lithium cobalt oxide,have low specific capacity and poor safety performance,followed by lithium iron phosphate with inferior kinetic performance.Ni-rich ternary materials have shown great advantages regarding specific capacity and low cost,exhibiting nearly 190 mAh g-1 specific capacity and possessing excellent rate capability,thus attracting the attention of researchers and business people recently.However,this material still suffers from some key issues that need to be addressed,such as harsh storage conditions,poor cycling performance,and safety performance that needs to be improved.In this thesis,based on the above scientific questions,Ni-rich cathode materials with different transition metal ratios were synthesized,combined with a variety of characterization methods at different scales.Through some in situ characterization measurements,the structural degradation mechanism is explored,and later optimizing strategies such as improving the morphology and interfacial properties are proposed,which effectively enhance the electrochemical performance and safety performance of Ni-rich ternary materials.The main research contents are as follows.1.Ni-rich cathode materials with different ratios were synthesized by coprecipitation method,whose morphology are made of primary particles agglomerated into secondary spherical particles.In situ X-ray diffraction(in situ XRD)and in situ Xray absorption fine structure spectroscopy(in situ XAFS)techniques were applied to analyze in detail the changes in material bulk structure,elemental valence and coordination environment during high-voltage charging and discharging.Subsequently,the behavior and morphological changes of the material in the high-voltage state of lithium surroundings were analyzed by solid-state nuclear magnetic techniques(ssNMR)and scanning electron microscopy(SEM)characterization.Finally,the variable temperature XRD technique was used to compare the thermal stability of them.In summary,the specific functions of Ni,Co and Mn during the electrochemical process are explored,and the important roles of cobalt and manganese are demonstrated,i.e.,cobalt can provide electrochemical activity and ensure the kinetic performance with many lithium ions extracted at high voltages,while manganese plays a pillar part to maintain the stability of the structural skeleton.2.The failure mechanisms of Ni50(LiNi0.5Co0.25Mn0.25O2)and Ni80(LiNi0.8Co0.1Mn0.1O2)cathode materials under high-rate overcharge conditions were analyzed to investigate the safety performance of the batteries.The gas evolution behavior under overcharge conditions was evaluated by online electrochemical mass spectrometry(OEMS),and the 18O isotope labeling method was used to determine the gas evolution region.The subsequent combination of various in situ and ex situ spectroscopy techniques,such as in situ X-ray diffraction(in situ XRD),in situ Raman scattering spectroscopy(in situ Raman),solid-state nuclear magnetic techniques(ssNMR)and X-ray absorption fine structure spectroscopy(XAFS),ruled out the material decay from the inner bulk in high voltage.The subsequent combination of scanning electron microscopy(SEM)and transmission electron microscopy(TEM)techniques revealed that the root cause of the material decay is the destruction of the morphology,with a severe intergranular and intragranular cracks generating and propagating at the grain boundaries.Much exposed fresh surface exacerbates side reactions at the interphase and the gas evolution phenomenon deteriorates dramatically,posing a great safety hazard.In view of this,single-crystal Ni-rich materials with larger particle size and fewer grain boundaries was introduced,which can significantly reduce the gas evolution under overcharge conditions and effectively improve the electrochemical properties and enhance the safety performance.3.Succinic anhydride(SA)was introduced as a functional additive in Li/LiNi0.8Co0.1Mn0.1O2(NCM811)cells,which is structurally simple,cost effective and environmental.By comparison,it was found that by adding 3%mass fraction of SA to the conventional carbonate electrolyte,the cyclic retention could be increased to 93.8%after 400 cycles at 1 C and the capacity ratio of 5 C/0.5 C was increased from 34.5%to 82.4%.By transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS)techniques,a uniformly distributed Cathode electrolyte interphase(CEI)film formed on the surface of the cathode material can be observed,which was mainly composed of organic polymers.This benign Li+ conductor helps to improved kinetic performance of NCM811.Electrochemical impedance spectroscopy(EIS)and TEM results also indicate that the formed film with SA additives at the interphase can reduce the surface activity of NCM811 and suppress the degree of side reactions.As the surface of NCM811 particles can be protected to reduce the crystal defects of the material,the microcracks and irreversible phase transition inside the secondary particles,the migration and dissolution of Ni and the excess consumption of electrolyte solvent are avoided,therefore the electrochemical properties of NCM811 are improved in this comprehensive way.In summary,this thesis utilizes multi-scale in situ and ex situ characterization measurements to provide a combined analysis of the variation in bulk,surface,local structure and morphology of Ni-rich ternary materials,and proposes strategies such as improving the morphology and interfacial properties,which can effectively promote the electrochemical and safety properties of the materials.