Examining the Evolution of Heterogeneities in Li- and Mn-rich Cathodes

The reduction of greenhouse gas emissions within the transportation industry relies on the ability to provide cheap, efficient, and reliable energy storage devices for electric vehicle (EV) applications. Moreover, the rapid expansion of the EV industry will put a large burden on already strained supply chains for critical materials, like cobalt. To mediate this strain, reduce material costs, and increase energy density, we can utilize a new family of cathode materials that introduces a Li-rich (Li₂MnO₃) component to the typical layered oxide cathode (LiMO₂ where M is a transition metal) that is widely used in commercial Li-ion batteries. With this blend, we can achieve zero-Co chemistries with higher theoretical energy densities at cheaper costs (after commercialization) than the state-of-the-art layered oxide chemistry. This chemistry, a blend of Li₂MnO₃ with LiMn_0.5Ni_0.5O₂, takes advantage of Earth-abundant elements, like Mn, to help manufacturers move towards energy independence. Many hurdles exist to realizing this chemistry as a viable cathode material. We focus on the issue of significant capacity and voltage fade with extended cycling, relating dynamic heterogeneities in morphology, crystal structure, and local chemistry/composition to capacity and voltage losses in the cathodes. To investigate morphological changes in these electrodes, we utilize X-ray absorption computed tomography (XCT). XCT allows for the 3D visualization of X-ray absorption differences in samples. With this, we can resolve sub-micron features in cathode particles, including particle cracking and internal pore features. In these cathodes, we reveal high porosity near the outer surface of most particles, a direct result of gas generation during synthesis. Moreover, the average absorption in this area increases with extended cycling, indicating some level of pore filling from interfacial layer build up. We take this further with single-particle X-ray absorption near-edge structure computed tomography (XANES-CT) across both the Mn and Ni k-edges. This technique allows for the 3D visualization of element-specific trends in composition and oxidation state within single cathode particles. We used the absolute X-ray absorption at the element edge as a proxy for the amount of that element present at each location in the particle. We found that both Ni and Mn absorption decreases monotonically from near the center all the way to the particle edge. Moreover, the gradient of this decrease flattens with cycling, indicating that both Mn and Ni are mobile within the particle, trending towards some equilibrium. We can also fit the peak of the X-ray absorption spectra at each point in space to roughly map oxidation state. This yields a general trend of decreasing average oxidation state for both Mn and Ni towards the particle surfaces along with isolated pockets of significant heterogeneity within particles. This heterogeneity in oxidation state also decreases with cycling for both Mn and Ni, indicating both elements participate in charge compensation. Meanwhile, the average oxidation state for both increases, as expected, due to Li loss. Finally, we probe crystallographic heterogeneities with X-ray diffraction computed tomography (XRD-CT) which yields spatially resolved powder diffraction patterns. Using the c-axis lattice parameter as a proxy for Li inventory, we can map Li loss throughout a volume. This supports the Li loss theorized from XANES-CT but reveals little overall trend in either radial or depth direction. We do observe, however, significant heterogeneity in lattice parameter and crystal structure throughout the cathode. Each of these techniques reveals critical information for understanding the nature of these cathodes throughout their cycle lifetimes. Together, they have helped inform future iterations of cathode particle synthesis and electrode design, furthering progress towards commercialization of this higher energy, Co-free cathode alternative.