Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy

Abstract

Understanding the reaction pathway and kinetics of solid-state phase transformation is critical in designing advanced electrode materials with better performance and stability. Despite the first-order phase transition with a large lattice mismatch between the involved phases, spinel LiMn_1.5Ni_0.5O₄ is capable of fast rate even at large particle size, presenting an enigma yet to be understood. The present study uses advanced two-dimensional and three-dimensional nano-tomography on a series of well-formed Li_xMn_1.5Ni_0.5O₄ (0≤x≤1) crystals to visualize the mesoscale phase distribution, as a function of Li content at the sub-particle level. Inhomogeneity along with the coexistence of Li-rich and Li-poor phases are broadly observed on partially delithiated crystals, providing direct evidence for a concurrent nucleation and growth process instead of a shrinking-core or a particle-by-particle process. Superior kinetics of (100) facets at the vertices of truncated octahedral particles promote preferential delithiation, whereas the observation of strain-induced cracking suggests mechanical degradation in the material.

Figure 1. XANES measurement of the crystal samples.

(a) Experimentally measured Ni K-edge XANES spectra of chemically delithiated Li_xMn_1.5Ni_0.5O₄ (Li_xMNO) crystals, (b) Ni edge energy as a function of the state of charge in Li_xMNO samples, (c) Ni K-edge XANES spectra of phase-pure LiMn_1.5Ni_0.5O₄ (red), Li_0.5Mn_1.5Ni_0.5O₄ (green) and Mn_1.5Ni_0.5O₄ (blue) phases. The experimentally measured spectrum of Li_0.51Mn_1.5Ni_0.5O₄ (black) is also shown for comparison, d,e enlarged views of the two highlighted regions in c. Horizontal error bars in b were obtained by quantifying the SOC multiple times. Vertical error bars in b were obtained by evaluating the energy resolution of the beamline used for the XANES measurements.

Figure 2. Two-dimensional chemical mapping of Li_xMn_1.5Ni_0.5O₄ crystals.

(a) x=0.82, (b) x=0.71, (c) x=0.51 and (d) x=0.25. (e–h) the variation of local XANES spectrum across each particle. (i–l) the relative concentration of the three chemical phases, as well as the variation in the local oxidation state of Ni within the same particle. Scale bar, 0.5 μm.

Figure 3. Three-dimensional map of Ni oxidation state at sub-particle scale.

The shape of the particle is presented as the transparent grey surface with the internal oxidation state heterogeneity illustrated using the diagonal slices (a–d) surfaces of the 3D Ni oxidation state map (e–j). All the panels are colour coded in order to show the state of charge heterogeneity at the sub-particle level in a quantitative manner.

Figure 4. Surface facets of LiMn_1.5Ni_0.5O₄ crystals.

(a) Scanning electron microscopy images showing the truncation at some corners of the crystals and (b) atomic models of the (111) and (100) planes in the ordered spinel with a P4₃32 space group: green balls (Ni cations), magenta balls (Mn cations) and yellow balls (O anions). Scale bars, 10 and 1 μm in a, left and right, respectively.

Figure 5. Schematics of possible phase transformation mechanisms in LiMn_1.5Ni_0.5O₄.

Figure 6. Particle fracturing in chemically delithaiated Li_xMn_1.5Ni_0.5O₄ crystals.

(a) 3D surface renderings of two selected Mn_1.5Ni_0.5O₄ crystals (the left one at ∼4 μm and the right one at 2 μm) show the formation of irregularly shaped cracks on the particle surface, (b,c) SEM images of pristine LiMn_1.5Ni_0.5O₄ and delithiated Mn_1.5Ni_0.5O₄ crystals, respectively. Scale bars, 1 μm (a–c).