Orbital-Specific observation of O2p and Ni3d electrons in LiNi0.5Mn0.5O2, a cathode material for lithium-ion batteries

Yoshinori Satou,Shigeki Komine,Sumera Shimizu

doi:10.1063/1.4996705

Abstract

Cathode materials for lithium-ion batteries containing Ni2+ have attracted much interest because of their high theoretical capacity. However, the precise electronic structures of these cathode materials have not yet been clearly observed, especially the energy positions of the O2p and Ni3d orbitals and the shape of the density of states. The aim of this study was to investigate the relative energy positions and shape of the density of states of O2p and Ni3d for LiNi0.5Mn0.5O2 experimentally. We cleaved a LiNi0.5Mn0.5O2 pellet in an Ar-filled glove box and performed synchrotron ultraviolet photoelectron spectroscopy for different photon energies, which enabled us to investigate the relative cross-section intensity of O2p and Ni3d. As a result, the valence-band structure was determined. We found that O2p electrons are itinerant and exist in the vicinity of the Fermi energy more than Ni3d electrons. Ni3d electrons are more localized and spread mainly from 1.2–1.5 eV below the Fermi energy. To validate the electronic structure, we measured the synchrotron O K-edge X-ray absorption fine structure of electrochemically lithium-extracted LiNi0.5Mn0.5O2. The electronic structure demonstrated that ligand holes in the oxygen atoms form below the Fermi level during the initial stage of Li extraction and that the formation rate of the holes decreases with Li extraction.

Highlights

Cathode materials containing Ni2+ have attracted much interest because of their high theoretical capacity
We clarified the contributions of the O2p and Ni3d orbitals in the vicinity of the Fermi level by using this relation of the cross-section to consider the order and tendency of electron use, which are strongly related to the cathode performance
It shows a broad weak tail toward the onset ( 0.8 eV). This means that there are fewer Ni3d electrons at the valence-band top than O2p electrons. These results indicate that, in this layered cathode, Ni3d electrons are more localized than O2p electrons and spread mainly from 1.2–1.5 eV below EF to the higher binding energy side and that the valence-band top is mainly composed of O2p electrons

Summary

Introduction

Cathode materials containing Ni2+ have attracted much interest because of their high theoretical capacity. Ex situ X-ray absorption fine structure (XAFS) measurements and very low C-rate in situ measurements have made it clear that the Ni ion mainly acts as a redox center between Ni2+ and Ni4+, e.g., in LiNi0.5Mn0.5O2, LiNi0.33Co0.33Mn0.33O2, and LiNi0.5Mn1.5O4.1–4 Cathodes generally include a variety of ions besides Ni, which causes oxygen bonding with various covalencies. Ray photoelectron spectroscopy (XPS) have shown that oxygen redox reactions compensate for lithium extraction from the cathode materials.[5,6,7] Further, the compositional ratio of the cathodes containing Ni2+ affects the amount of O2p charge compensation.[8,9,10] It suggests the differences of the energy positions of the O2p and Ni3d orbitals and the shape of the density of states

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