title: Understanding fundamental migration processes during battery cycling and material synthesis using advanced transmission electron microscopy methods
Creators: name: Wang, Kai
Publisher: name: UNSPECIFIED
Year: 2022

description: To satisfy the increasing need for efficient energy storage systems resulting from the worldwide rapid development of renewable energy sources, rechargeable ion batteries are getting unprecedented attention. Anode materials, as essential components of rechargeable ion batteries, have been widely investigated to meet the increasing demands for higher energy density. Metal oxides (MOs) and transition-metal dichalcogenides (TMDs) are regarded as promising future anode materials with their potential to provide high capacity, cycling stability and volumetric energy density. However, these materials undergo complex reaction processes and generate large amounts of amorphous products, making their characterization difficult. Nevertheless, understanding the reaction mechanisms during synthesis and during electrochemical cycling is essential to guide the design of novel electrode materials. Transmission electron microscopy (TEM) is commonly considered to be a powerful technique for morphological characterization, structure determination, elemental mapping and valence state analysis, both for amorphous and crystalline materials. In this work, conventional TEM methods and a number of advanced TEM-characterization techniques, such as in-situ heating in defined gas environment, pair distribution function (PDF) analysis and 4D-STEM, have been carried out to understand the fundamental aspects of elemental migration, the local chemical environment and the structural evolution during material synthesis and battery cycling. Moreover, additional characterization techniques including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and electrochemical testing have been applied to obtain complementary bulk information about the battery systems. Due to their outstanding electrochemical performance high entropy oxides (HEOs) have attracted intensive attention for application in the area of energy storage. However, the fundamental understanding of the elemental processes during material synthesis and of the reactions during electrochemical cycling is still limited. This knowledge is essential to design and optimize materials, particularly for advanced materials based on the high entropy concept. First, the processes during HEO (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O synthesis by calcination of the corresponding oxides have been studied, mainly by in-situ heating XRD and TEM techniques. The results of this work reveal the structural evolution and morphological from the separated raw metal oxides with increasing temperature to the final HEO. The interpretation of elemental diffusion parameters indicates that some intermediate oxide phases are formed prior to the HEO generation, thus implying that the formation of the HEO proceeds in multiple steps. The results further demonstrate that Co3O4 is an alternative to CoO as Co source for the HEO (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O synthesis. Furthermore, the de/lithiation mechanism of the HEO (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O was studied by correlative TEM and XAS measurements at various battery states. The results indicate that during the 1st IV discharge, Ni, Cu, Co and Zn are reduced. Ni, Cu and partially Co do not further participate in the electrochemical reactions in the following cycles as they form an inert alloy at grain boundaries and fine dispersed inside the grain. In the entire particle, the alloy builds up a 3D network that enables the fast electron transfer during the following electrochemical cycles. Zn, and partially Co provide the capacity of HEO through conversion and alloying reactions. As Mg cannot be reduced by Li during battery cycling it remains in a 2+ oxidation state during all cycling stages and forms a continuous oxide network that provides a 3D network for lithium ion transfer. In addition, MgO also stabilizes the overall structure of the material inducing an epitaxial relationship with the phases generated during battery cycling. Thus, this study revealed the synergy of the constituent cations critical for the excellent electrochemical performance of this HEO. These findings can be used in the quest of obtaining high performance anode materials. Molybdenum disulfide (MoS2) as another promising anode material for both, lithium ion batteries (LIBs) and sodium ion batteries (SIBs), has a high theoretical capacity based on a multi-electron conversion reaction. Although various novel sample designs have improved its electrochemical performance significantly, the reaction mechanism of MoS2 in sodium ion batteries is still equivocal due to the strongly reduced crystallinity upon deep sodiation. Using advanced TEM techniques effective for analyzing amorphous materials, in the present work the de-/sodiation reaction mechanism of MoS2-based materials was revisited. The results from the electron based PDF analysis and X-ray absorption spectroscopy show that the initially long range ordered MoS2 is breaking apart into Mo-Sx clusters during sodiation, rather than the originally expected fully conversion into metallic Mo and Na2S. It was further found that, although MoS2/carbon composites show a significant enhancement of the electrochemical performance compared to bare MoS2, the carbon in the MoS2/carbon composite does not directly influence the electrochemical reaction of MoS2. S(TEM) images and 4D-STEM analysis demonstrated that the carbon matrix prevents leaching of Mo-Sx clusters into electrolyte and is therefore beneficial during battery cycling. This work adds a new perspective to the understanding of the MoS2 reaction mechanism in SIBs.