Nanoimaging of solid solution domains during phase transitions inside LiNi0.5Mn1.5O4

Abstract

Lithiation dynamics in many battery cathode materials remain poorly understood, because of the difficulty in differentiating inter and intra-particle heterogeneity. In this work, operando X-ray nanodiffraction microscopy is used to directly image the structural evolution inside Li1-xMn1.5Ni0.5O4 single crystals during electrochemical delithiation. Metastable domains of solid-solution intermediates are detected, but are not associated with the reaction front between the lithiated and delithiated phases as predicted by current theory. Unusually persistent strain gradients inside the single crystals suggest that the shape and size of solid solution domains is instead templated by lattice defects, which guide the entire (de)lithation process. Morphology, strain distributions, and tilt boundaries reveal that the (Ni2+/Ni3+) and (Ni3+/Ni4+) phase transitions proceed through different mechanisms, offering solutions for reducing local strain and structural degradation in high voltage spinel active materials.

Plastic deformation of LiNi0.5Mn1.5O4 single crystals due to domain orientation dynamics

“…Figure 6 summarizes the delithiation mechanisms and tilt grain dynamics of the measured particle. The onset of both transition supports the mechanisms proposed in our previous work [12] where we observed different mechanisms for the two phase transitions. The first Li 1 -Li 0.5 phase transition follows a "phase-field" mechanism, growing from multiple points on the surface.…”

“…The location of these regions also correlates with the most delithiated regions of the crystal (red regions of the strain map in Supplementary Video 1). This also confirms the results of our previous work [12], where we suggested that the regions with sharp tilt boundaries act as nucleation points for the delithiation process due to high concentration of defects.…”

“…During the first plateau on the electrochemical curve (Figure 6) gradual change of d-spacing (Figure 5A) is also accompanied by a gradual reorientation of different regions of the particle (Supplementary Video 1 (scans 1-18)). This behaviour is consistent with the lattice orientation dynamics that was observed in our previous work [12]. In addition, while approaching the phase transition, sharp wedgeshaped features can be observed on the polar plot (teal, blue and pink -scan 5-18 in Supplementary Video 1).…”

“…The maximum altitude deviation between two pixels of the real-space maps does not exceed 0.1 • . Similar degrees of deviation were observed for pristine LMNO particles measured ex situ [12], while particles with higher values of tilt misorientation were found in LCO cathode materials using Laue microdiffraction [21].…”

“…The detailed procedure of single particle scanning X-ray imaging with a Fresnel zone-plate focused beam is described in Ref. [12]. Here, the X-ray beam has been focused down to ∼ 80 nm.…”