Insight into the Role of Metal–Oxygen Bond and O 2p Hole in High-Voltage Cathode LiNi_xMn_2–xO₄

Abstract

The role of transition-metal d and ligand p hybridization continues to be of immense interest in Li-ion battery cathode, and yet it is still poorly understood. Using combined experimental and theoretical soft X-ray absorption spectroscopic study and density functional theory calculation, we investigated the fundamental electronic structure of the high-voltage spinel LiNi x Mn2–x O4. An oxygen-participating charge rebalance between manganese and nickel ions was found; that is, the content of O 2p holes close to the Fermi level increases along with the increasing Ni content. Moreover, these unstable oxygen holes primarily congregate around the redox active dopants. The underlying mechanism accounting for this charge-compensated occurrence is the reverse of two bonding levels when manganese ions are oxidized from +3 to +4 states. On the basis of these new findings, we further exposed the role of oxygen in electrochemical performance. First, oxygen ions afford the charge variation together with the cations during Li insertion/deinsertion process. Second, the O 2p holes can largely screen the strong electrostatic repulsion between Mn4+ and Li+ ions to effectively enhance the rate capacity. Lastly, the excessive amount of O 2p holes is disadvantageous to the thermal stability associated with the O2 evolution. Also, we point out that O 2p holes concentration can be modified by the metal–oxygen bonding character, and the “charge-transfer energy” is a crucial point for designing high-capacity positive electrodes for Li-ion battery.

An Efficient Interfacial Synthesis of Two‐Dimensional Metal–Organic Framework Nanosheets for Electrochemical Hydrogen Peroxide Production

“…The Fourier transforms of EXAFS (FT‐EXAFS) for Ni MOF NSs (Figure 4 d) showed a dominant peak appeared at ca. 1.6 Å belongs to the first shell coordination of Ni–O, and the typical Ni–Ni bond in the Ni foil was absent in Ni MOF NSs samples, [31, 32] The short Ni–O distance in Ni MOF NSs as shown in the Supporting Information, Table S4 as compared with that in NiO indicates high oxidation state of Ni ions in the former in line with the results from XANES and XPS. To further understand the origin of the different activities between the Ni MOF NSs samples, the coordination nature of their surface metal atoms was investigated.…”

Fast operando spectroscopy tracking in situ generation of rich defects in silver nanocrystals for highly selective electrochemical CO2 reduction

“…Fast operando XAS analysis. Briefly, XANES provides reliable clues on an element's oxidation state 33,47 , while EXAFS conveys local structural information surrounding the absorbing atom 28,35,48 . Although finding out a novel catalyst is an interesting task, it is much more important to understand the mechanism and the formation process of active sites in catalysts during ECR.…”

An Efficient Interfacial Synthesis of Two‐Dimensional Metal–Organic Framework Nanosheets for Electrochemical Hydrogen Peroxide Production

“…[29,30] The XANES spectra indicated that the average valence state of Ni in the samples are larger than NiO reference (Figure 4 S3). The EXAFS wavelet transform (WT) analysis gives an intuitive way of further ascertaining the dispersion state of Ni atoms in the Ni MOF NSs through the comparison with reference samples (Figure 4 foil was absent in Ni MOF NSs samples, [31,32] The short Ni-O distance in Ni MOF NSs as shown in the Supporting Information, Table S4 as compared with that in NiO indicates high oxidation state of Ni ions in the former in line with the results from XANES and XPS. To further understand the origin of the different activities between the Ni MOF NSs samples, the coordination nature of their surface metal atoms was investigated.…”