Structural studies of Li1.2Mn0.55Ni0.15Co0.1O2 electrode material

Domain Formation in Lithium-Rich Manganese-Nickel-Cobalt-Oxide Epitaxial Thin Films and Implications for Interpretation of Electrochemical Behavior

Due to the directional dependence of physical properties, it is advantageous to grow and then study materials in specific orientations. Films of battery materials grown in epitaxy offers the possibility to gain new insight into the role of physical structure on electrochemical behaviors. Here we demonstrate the growth, testing, and characterization of monoclinic-phase (space group C2/m) Li-Mn-Ni-Co-O epitaxial films. The monoclinic phase is a layered structure and as such lithium diffusion is favored along specific crystallographic directions. Films were grown by pulsed laser deposition onto SrRuO₃/SrTiO₃ substrates with (001) and (111) orientations. Cyclic voltammetry measured the response of these positive electrode materials, while the film structure was characterized using scanning transmission electron microscopy. A combination of imaging and diffraction identifies the presence of orientational variants. Variants disrupt the orientation anisotropy expected of these layered materials when grown in epitaxy, thereby masking differences in electrochemical behavior as a function of substrate orientation. Learning to control the domain structure now presents itself as a challenge to realize the potential of low symmetry battery materials grown in epitaxy on high symmetry substrates.

Local degradation pathways in lithium-rich manganese-nickel-cobalt-oxide epitaxial thin films

The electrochemical performance and microstructure of positive electrodes are intimately linked. As such, developing batteries resistance to capacity and voltage fade requires understanding these underlying structure-properties relationships and their evolution with operation. Epitaxial films of a Li-rich manganese-nickel- cobalt oxide cathode material were deposited on (100) and (111) orientated SrRuO3/SrTiO3 substrates. Cyclic voltammetry and impedance spectroscopy tracked the response of these positive electrode materials, while the microstructure of the pristine and cycled films was characterized using transmission electron microscopy. Energy-dispersive X-ray spectroscopy identifies compositional fluctuations in as-deposited films. Phase transformations and dissolution were observed after electrochemical testing. There is a correlation between both local composition and substrate orientation (i.e., surface faceting) and what degradation pathways are active. Regions with comparatively higher concentrations of Ni and Co were more resistant to dissolution and unfavorable phase transformations than those with relatively more Mn. As such, a global composition metric may not be an accurate predictor of degradation and performance. Rather possessing the synthetic ability to engineer the chemical profile as well as characterizing it, pose a challenge and opportunity.