Understanding the spectroscopic signatures of Mn valence changes in the valence energy loss spectra of Li-Mn-Ni-O spinel oxides

The valence energy loss spectrum which is characterized by valence-to-conduction band and plasmon excitations is rarely used in spectroscopy of Li-ion battery materials. One reason being the large number of different excitations observed in this region as well as the difficulty in interpreting their nature and origin. We have determined the nature and origin of spectral features observed in Li-Mn-Ni-O spinel oxides with respect to Mn valency changes during the insertion of lithium ion. The lithiation process is accompanied by a Mn valency change from Mn 4+ in ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ to Mn 3+ in lithium rich ${\mathrm{Li}}_2{\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$. The valence energy loss spectrum of ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ is characterized by sharp peaks in the 7–10 eV energy loss range whose intensity decrease with lithiation to ${\mathrm{Li}}_2{\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$. Using electronic structure calculations and molecular orbital considerations we show that the intense peaks in the valence loss spectra of ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ have a large contribution from ligand-metal charge transfer transitions. These transitions arise from the mainly O $2p$ nonbonding $t_{2u}$ and bonding $t_{1u}$ orbitals to the mainly Mn $3d$ antibonding $t_{2g}^{*}$ and $e_g^{*}$ orbitals. We discuss the origins of the observed valence spectra differences between the two phases in relation to peaks shift, variations in occupancy, and variations in covalency as a result of Mn valency changes occurring during lithiation.

Figure 1

Experimental valence loss electron energy loss spectra (VEELS) obtained from lithium rich ${\mathrm{Li}}_2{\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ (thin dotted curve) and ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ (thick solid curve).

Figure 2

(a) Calculated energy loss functions (ELF) for cubic spinel ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ (Mn 3.5+, thick solid curve) and ${\mathrm{Li}}_2{\mathrm{Mn}}_2{\mathrm{O}}_4$ (Mn 3+, thin dotted curve). (b) Layered ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ (Mn 4+, thick solid curve) and ${\mathrm{LiMnO}}_2$ (Mn3+, thin dotted curve).

Figure 3

Symmetry projected partial density of states (PDOS) for (a) Mn $3d$, (b) Mn $4s/4p$, (c) O $2s$, and (d) O $2p$ for the ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ phase.

Figure 4

${\mathrm{Li}}_2{\mathrm{MnO}}_3$ band structure for majority spin (solid thin line) and minority spin (dashed thin line). Bands 50, 57, 65, and 75 below the Fermi level and bands 80, 85, and 90 above the Fermi level are also indicated (thick dashed line) The radius of the circles represents the Mn $3d$ character of each band with a large circle showing more Mn $3d$ character and vice versa.

Figure 5

Calculated partial energy loss functions (ELF) for transitions between group of occupied bands (a) $50-78$, (b) $65-78$, and (c) $75-78$ to the group of unoccupied bands $79-80,79-85,79-90$, and $79-100$ above the Fermi level, respectively. Bands 78 and 79 are the highest occupied and lowest empty band, respectively, and (d) full ELF for ${\mathrm{Li}}_2{\mathrm{MnO}}_3$.

Figure 6

A schematic molecular orbital diagram from a $\mathrm{Mn}{{\mathrm{O}}_6}^{9-}$ cluster with Mn 3+ after Ref. [37]. The dark squares represent filled states and the unfilled rectangles show the unfilled states. The solid arrows show the occupying spin of the electron, while dashed arrows show the possible interband transitions.