High-resolution TEM Characterizations Of Surfaces And Interfaces Of Cathode Materials For Layered Manganese-based Secondary Batteries

The large-scale consumption of fossil fuels has caused a lot of greenhouse gases such as carbon dioxide and methane to be emitted into the atmosphere and caused severe damage to the environment.The development of sustainable energy systems is imperative.Among them,secondary batteries have been considered as one of the most reliable electrochemical energy storage systems.Although it has been widely used in portable electronic equipment,electric vehicles,and stationary energy storage,the stability of battery materials and the phenomenon of structural degradation have severely limited the capacity and cycle life of secondary batteries,preventing their further application and development.The main difficulty is the study and analysis of the material surface interface properties at the nano and atomic scales.Therefore,It is necessary to study and analyze the material’s lattice distortion,composition changes and electronic structure changes at the atomic scale.In recent years,with the development of spherical aberration correction technology,the resolution of transmission electron microscopy（TEM）has reached the subatomic scale,which provides a powerful means of characterization for materials research.The combination of scanning transmission electron microscopy（STEM）and related analytical techniques X-ray spectroscopy（EDS）and electron energy loss spectroscopy（EELS）enables us to better understand the relationship between structure evolution,chemical composition and electronic structure changes at the atomic scale.This article will be based on the above-mentioned powerful comprehensive experimental characterization platform,taking lithium-rich manganese-based and sodium electro-manganese-based materials as research subjects.In-depth analysis of the failure problems of different material systems and the design guidance of material modification was given and explain the mechanism of modification strategy.The study mainly includes the following two parts.In the first part,The characterization utilizing atomic resolution spherical aberration corrected STEM shows that lithium-rich manganese-based material LMNO(Li_1.5Mn_0.667Ni_0.333O₂)undergos severe structural degradation after electrochemical cycles,such as the release of oxygen and stress accumulation,which ca cause voids,cracks,phase transitions and the oxidation decomposition products of electrolyte to the corrosion of material surfaces.However,our studies show that the structure of materials modified with Ce O₂remained intact after electrochemical cycling.Utilizing the STEM technology,we found that the modified primary particles have three different phase structures from surface and subsurface to bulk phase:the Ce O_2-xphase（thickness about 6nm）,the rock salt phase and layered structure in order.The layer spacing of the modified sample was apparently enlarged in comparison with that of the pristine sample,which facilitated the insertion/extraction of lithium ions during the cycle.In the case of the surface areas without the Ce O₂coating,a rock salt/layered two-phase structure was formed on the surface through Ce ion doping.This structure can effectively suppress side reactions at the electrode/electrolyte interface.Furthermore,the electronic structure changes of Ce O_2-xcoating layer were analyzed by STEM-EELS.The results showed that Ce valence state of Ce O_2-xcoating layer increased and oxygen vacancies decreased after cycling.It is proved that it has a good inhibitory effect on lattice oxygen release.In short,the synergy effect of among the surface modification,element doping and rock salt phase can also inhibit the release of oxygen and the positive electrode/electrolyte interface reaction and hance stabilize the crystal structure and accelerate the conduction rate of lithium ion during cycling,resulting in an excellent electrochemical performance of LMNO.In the second part,using STEM and its related analytical techniques,we characterizedthesodiumbatteriesmanganese-basedmaterials NMCNO(Na_0.65Mn_0.67Co_0.17Ni_0.17O₂)after cycling 100 cycles at 1.5-4.1V and show that there was no significant decrease in crystallinity and electrochemical performance.When the voltage was increased to 1.5-4.5V,we found that the P2 layered phase underwent a reversible transition to the OP4 phase by combining STEM,in situ X-ray diffraction（XRD）,and first-principles.Such transition resulted in significant expansion and contraction of the crystal lattice,generating inhomogeneous internal stress that caused severe lattice distortion and intragranular cracks.Furthermore,our analaysis showed that the epitaxial spinel-like nanolayer can effectively alleviate the strain of P2 type sodium ion battery materials under high voltage,and suppress the crack nucleation and propagation.As a result of that,there were no cracks in the primary particles of surface modification.The crack-free grains ensured a tight connection of transition metal layers（TMO₂）,alleviating the excessive exposure of active surface to electrolyte as well as material degradation and improving the structural durability and cycling stability.