Electronic structure of Li$_{1+x}$[Mn$_{0.5}$Ni$_{0.5}$]$_{1-x}$O$_2$ studied by photoemission and x-ray absorption spectroscopy

TL;DR

This study investigates the electronic structure of a lithium transition metal oxide used in batteries, revealing how lithium excess influences manganese valence states and site exchange effects through spectroscopic analysis.

Contribution

It provides new insights into how lithium content affects the valence states and site exchange phenomena in Li-rich layered oxides via spectroscopic techniques.

Findings

Mn is mainly 4+ and Ni is 2+ in the samples.

Mn$^{3+}$ increases with bulk sensitivity in x=0.00 sample.

Excess Li suppresses Mn$^{3+}$ formation and site exchange.

Abstract

We have studied the electronic structure of Li$_{1+x}$[Mn$_{0.5}$Ni$_{0.5}$]$_{1-x}$O$_2$ ($x$ = 0.00 and 0.05), one of the promising cathode materials for Li ion battery, by means of x-ray photoemission and absorption spectroscopy. The results show that the valences of Mn and Ni are basically 4+ and 2+, respectively. However, the Mn$^{3+}$ component in the $x$ = 0.00 sample gradually increases with the bulk sensitivity of the experiment, indicating that the Jahn-Teller active Mn$^{3+}$ ions are introduced in the bulk due to the site exchange between Li and Ni. The Mn$^{3+}$ component gets negligibly small in the $x$ = 0.05 sample, which indicates that the excess Li suppresses the site exchange and removes the Jahn-Teller active Mn$^{3+}$.