Influence of Transition-Metal Order on the Reaction Mechanism of LNMO Cathode Spinel: An Operando X-ray Absorption Spectroscopy Study

Abstract

An operando dual-edge X-ray absorption spectroscopy on both transition-metal ordered and disordered LiNi_0.5Mn_1.5O₄ during electrochemical delithiation and lithiation was carried out. The large data set was analyzed via a chemometric approach to gain reliable insights into the redox activity and the local structural changes of Ni and Mn throughout the electrochemical charge and discharge reaction. Our findings confirm that redox activity relies predominantly on the Ni^2+/4+ redox couple involving a transient Ni³⁺ phase. Interestingly, a reversible minority contribution of Mn^3+/4+ is also evinced in both LNMO materials. While the reaction steps and involved reactants of both ordered and disordered LNMO materials generally coincide, we highlight differences in terms of reaction dynamics as well as in local structural evolution induced by the TM ordering.

Figure 1

Structural models of the (a) TM-disordered spinel LNMO, described in a Fd3̅m unit cell, and (b) TM-ordered spinel LNMO, described in a P4₃32 unit cell. Orange balls represent the oxygen atoms; Li are located at the center of the green tetrahedron; in (b), Ni and Mn occupy the center of the blue and purple octahedra, respectively, while in (a) they are randomly distributed in the center of the violet octahedra. (c) Local environment of Li (green ball), surrounded by 4 oxygen atoms (orange balls), 3 Ni (blue balls), and 9 Mn (purple balls) in the perfectly TM ordered spinel.

Figure 2

(a) Normalized Raman spectra and (b) ⁷Li MAS NMR spectra of the samples LNMO-O and LNMO-D. NMR shifts are indicated as dashed lines at the approximate center of mass of each resonance. Rotational sidebands appear as echoes of the center lines at a distance in the spectrum of 50 kHz (MAS frequency) and are denoted by (+). Minor signals are observed at 0 ppm (∗) which are assigned to lithium-containing diamagnetic salts.

Figure 3

Rietveld refinement pattern of LNMO-D (top) and LNMO-O (bottom). A minority rock-salt phase was used besides the main spinel phase to refine the LNMO-D diffraction pattern. A single spinel phase including an antiphase model was used for LNMO-O pattern refinement.

Figure 4

Evolution of XAS on Ni and Mn K-edge during 1.5 electrochemical cycles vs Li of LNMO-D. No spectra were acquired for the dark red region around spectra #125 due to a beam loss.

Figure 5

Concentration profile for Mn (upper) and Ni (lower) components upon 1.5 electrochemical cycles vs Li⁺/Li for LNMO=D. Gap in the concentration profile around spectra #125 is due to a beam loss. Vertical dashed and dashed-dotted lines indicate EOC1 and EOD, respectively.

Figure 6

XANES K-edge region of MCR-ALS derived pure spectral components for Ni (left) and Mn (right). Spectral components for disordered phase LNMO-D are marked in solid lines and ordered LNMO-O in dash-dotted lines. A Ni₂O₃ reference spectra is traced with a dashed light blue line. The inset depicts the Mn pre-edge region.

Figure 7

Evolution of path length (top) and Debye–Waller factor (middle) of two closest Ni–next neighbor shells (Ni–O1 and Ni–TM) upon electrochemical cycling of LNMO-D (left) and LNMO-O (right). The solid and dashed lines without markers depict the path length and Debye–Waller factor of Mn shells (Mn–O1 and Mn–TM). Error bars fall within the width of markers and have therefore been omitted. The gaps are due to beam loss. Vertical dashed and dashed-dotted lines indicate EOC1 and EOD, respectively. The shaded area indicates nonconformity of cell behavior during the second charge of LNMO-O.