Transmission Electron Microscopy Study On The Microstructure And Thermal Stability Of High Nickel Ternary Cathode Materials

In order to deal with the increasing energy shortage and environmental degradation,it is extremely valuable to convert the human energy structure from traditional fossil fuels to novel renewable energy sources.Li-ion batteries（LIBs）with high energy density,are widely used in the field of portable electronics and electric vehicles.LIBs have an irreplaceable role in building new energy structures due to their high voltage,high energy density and environmental friendliness.Li Ni1-x-y Cox Mny O2（NMC）layered oxide is currently one of the most widespread lithium-ion battery cathode materials.In an effort to obtain high energy density coupled with a lower cost,a high Ni content is normally employed.However,the increase of Ni content seriously reduces the lattice structure stability of cathode materials,resulting in severe capacity degradation,during charge/discharge cycles and thermal treatment.Nevertheless,the mechanism of thermal degradation at the microscopic level is still unclear,which seriously hinders the safe design,and further application of high nickel ternary layered lithium-ion batteries cathode materials.In this paper,we take the high nickel LiNi_0.6Mn_0.2Co_0.2O₂cathode as the model.The microscopic mechanism of electrolyte modification to improve the cycling stability of NMC622 and the microstructural evolution of NMC622 during in-situ heating,are systematically investigated by means of spherical aberration-corrected transmission electron microscopy in combination with in situ heating characterization.The main findings are as follows:（1）The stable cycling of NMC622 at a high voltage has been achieved through component modulation to increase the local electrolyte concentration,and enable it to exhibit high multiplicity performance in Li metal-based Li||Li Ni0.6Mn0.2Co0.2O2（NMC622）batteries.Advanced electron microscopy characterization reveals that a stable in-situ Al-rich cathode electrolyte interphase（CEI）is formed on the cathode surface.This CEI inhibits surface side reactions and improves the cycling stability of the battery.Benefiting from a uniform and stable CEI on the cathode surface,the battery exhibits an excellent capacity retention of～85%after 300 cycles at a charge voltage of4.4 V（C/3 charge and 1 C discharge）.This work offers a promising strategy for developing practical secondary batteries based on Li metal with outstanding high voltage and rate performance.（2）Traditional ex-situ experiments are difficult to realize the study on thermal stability of cathode materials at the nanoscale,and the intrinsic mechanism of structural evolution in NMC622 during heating,is not fully understood due to the lack of direct and real-time observations.The thermal degradation of NMC622 has been observed in real time at the microscopic level by in-situ heating up to 1000°C in a spherical aberration corrected transmission electron microscope.The evolutionary mechanisms of structural degradation and internal nucleation as well as growth of nanopores are revealed.The evolution of the elemental composition of NMC622 after high-temperature thermal disruption,and the differences in the electronic structure of the split phases occurring at grain boundaries,are investigated in conjunction with electron energy loss spectroscopy.An in-depth understanding of the influence of the structural evolution of cathode materials on their thermal stability at high temperatures from a microscopic perspective,is of great significance for the subsequent thermally safe design of high nickel cathode materials.（3）Based on the inevitable electron beam irradiation effect during in-situ TEM characterization,we have observed the evolution from a layered oxide phase to a nickel-cobalt alloy phase,on an atomic scale by in-situ electron beam irradiation of specific primary grains of NMC622 samples,during heat treatment under transmission electron microscopy.During in-situ heating,oxygen atoms in the crystals are removed from the pristine NMC622 in response to high temperatures.When the temperature exceeds500°C,electron beam irradiation induces collapse of the crystal structure and evaporation of a large number of Mn atoms from the crystal,allowing precipitation of Ni₃Co alloy particles from the matrix.