Advanced experimental techniques play a crucial role in the development of materials by studying processes where they are actively involved. Of particular significance are techniques that enable obtaining data at high spatial, spectral, or temporal resolution. This allows accessing detailed information on morphology and structure, measuring electronic configurations, and studying time evolution during processing/operation, respectively. While each of these capabilities is very important, the synergy and tremendous additional benefits arise from bringing them together. This is the best exemplified in various in situ methods where some form of active stimulus is applied to a sample and its response is probed at high spatial and/or spectral resolution and monitored in real time. By recognizing the significance of this type of approach, the Conn Center has been steadily expanding its in situ experimental infrastructure. In parallel, there have also been steady efforts to develop capabilities for fundamental studies of materials, with particular emphasis on measuring surfaces and interfaces.
In situ Studies of Energy Materials
One of our focuses in the Materials Characterization Theme is to expand Conn Center’s capabilities for in situ characterization of materials. In situ capabilities are invaluable as they provide a direct insight into processes that are critical for the synthesis of materials, their modifications, as well as operation in actual device structures. Our current capabilities include in situ TEM heating stage that allows heating experiments in temperatures up to 1300 oC and simultaneous nanoscale imaging, diffraction and chemical analysis. Dynamical studies at the nanoscale of thermally-driven phase transitions and processes are therefore possible, which is particularly useful for studding synthesis of materials as well as high temperature processes such as deactivation of heterogenous catalysts under high temperature conditions. Thermally-driven synthesis of materials as well as phase and structure evolution can also be studied in situ by X-ray diffraction in our Discovery D8 Bruker HR-XRD system using 1100 Dome Heating Stage with gas injection system. The system allows for heating experiments in temperatures up to 1100 oC and in either, inert gas atmosphere, air, or vacuum. Recently, we have also modified our Raman system to allow in situ Raman measurements of the samples inside high temperature diamond anvil cell (DAC), where pressures of ~20 GPa and temperatures of ~700 oC are accessible. In the near future, we are also planning to acquire electrochemical cells for in situ Raman and XRD measurements so the structure evolution and phase transformations of electrodes as well as electrode-electrolyte reactions at interfaces can be studied in real time during charging-discharging cycling of the electrode in electrochemical energy storage or conversion systems.
High Pressure Synthesis and Transformation of Materials
High pressure methods are commonly being used in solid state physics and materials science to modify the atomic and electronic structure of materials and tune different materials properties, including electronic and heat transport, optical response, or magnetic ordering. Furthermore, they are also being used in solid-state chemistry to modify existing materials and to synthesize non-equilibrium phases or novel compounds. The effect of high pressure is the reduction of inter-atomic distances and consequently densification effects or enhancement of the reactivity. One of the advantages of using high-pressure in solid state chemistry is the fact that these are in general cleaner and faster methods than purely chemical routs, which typically involve the formation of by-products, or lead to undesired phase separation or disorder. Moreover, a high pressure approach can be useful in the case of reaction when under atmospheric pressure condition (i.e., at 1 atm.) one of the precursors undergoes decomposition at temperatures lower than that needed for the reaction to occur. In this case, the role of high pressure is to stabilize the precursor by increasing its decomposition temperature. The high pressure research at the Conn Center is based on the use of a diamond anvil cell and focuses on high pressure synthesis and transformation of materials. Raman spectroscopy is used for in situ monitoring Our recent example studies include the solid state synthesis of delafossite rhombohedral AgGaO2 as well as high pressure optical studies of phosphorene, a 2D layered form of black phosphorous.
Surface and Interfacial Evolution in Energy Materials
These research efforts aim at understanding structural evolution at surfaces and interfaces of active energy materials, especially during their operation in energy conversion and storage processes. The in-depth knowledge on such changes helps to define and model the mechanisms responsible for the materials performance, including their efficiency and durability. A recent example in this area is our research on the electrochemical capacity loss in manganese-rich nickel cobalt oxides (NMC), which are some of the most promising high capacity (∼280 mAh/g) cathode materials for the next generation of Li-ion batteries (LIBs). Despite significant benefits, there are still problems associated with NMC operation and electrochemical cycling that need to be addressed for their large-scale deployment and commercialization as practical high-energy cathodes. This includes voltage fade as well as voltage hysteresis during charging/discharging cycles. It is believed that interfacial reactions at the electrode/electrolyte interface during lithiation-delithiation may be responsible for these unwanted phenomena. Indeed, our recent study, using direct analytical methods, such as high-resolution elemental mapping, suggests that the charging cycling causes the electrode degradation due to the migration of transition metal (TM) cations and their preferential segregation at grain boundaries and particles surfaces. Using such measurements, we are obtaining the nanoscale data insight into the TM migration and surface degradation in these NCM cathodes. Similar projects are focused on a number of other important aspects of energy materials, including the formation mechanism of the solid-electrolyte interphase (SEI) at the electrode/electrolyte interface in electrochemical battery systems, the catalytic activity and deactivation mechanism in various catalysis and electrocatalysis systems. Similar studies are also conducted for in-depth analysis of corrosion and electrochemical oxidation. This allows to correlate electrochemical parameters with nanoscale structural changes and reveal the mechanisms critical to the processing and stability of nanostructures. In order to expand our capabilities dedicated to the interface/surface analysis, we have recently established a ultramicrotomy-based cross-sectional sample preparation methodology that allows to produce samples sections with the nominal thickness of the order of 100 nm. Cross-sectional samples of battery electrodes or catalysts can therefore be prepared for electron microscopy another analytical analysis.