Depth-dependent oxygen redox activity in lithium-rich layered oxide cathodes

Andrew J Naylor,Adam Sobkowiak,Julia Maibach,M Saiful Islam,Kristina Edström,Peter G Bruce,Matthew R Roberts,Niccolò Guerrini,Reza Younesi,Erik Björklund,Juan G Lozano,Ashok S Menon,Eszter Makkos

doi:10.1039/c9ta09019c

Abstract

Energy-tuned photoelectron spectroscopy demonstrates the surface preferential oxidation of oxygen for the Li-rich cathode material Li1.2Ni0.2Mn0.6O2 upon charge.

Highlights

The lithium ion battery utilising a LiCoO2 cathode, with a capacity of 160–190 mA h gÀ1, was the rst commercially successful chemistry released to the marketplace
Capacities exceeding 250 mA h gÀ1 are possible, making these materials candidates for future Li-ion batteries. This process of O-redox has been extensively characterised using X-ray absorption near edge structure (XANES), so X-ray absorption spectroscopy (SXAS), online electrochemical mass spectrometry (OEMS), X-ray photoelectron spectroscopy (XPS), resonant inelastic X-ray scattering (RIXS) and several models exist for the nature of the oxygen redox process.[7,8,9,10]
As the excitation energy is increased from 1.09 to 2.35 and to 7.05 keV, the signal for the surface-bound species reduces signi cantly, con rming their presence only at the very surface of the particle as terminating species or a sparse coverage. The position of these peaks is constant throughout the series of spectra, as there is no indication to show that these species undergo any chemical change; for example, if there was signi cant electrolyte oxidation at the surface during charge we would expect to see changes in the C 1s spectra, which we do not

Summary

Introduction

The lithium ion battery utilising a LiCoO2 cathode, with a capacity of 160–190 mA h gÀ1, was the rst commercially successful chemistry released to the marketplace. In recent years it has been shown that so called “Li-rich” cathode materials, e.g. Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.134Co0.134Mn0.54O2, can exceed the limit of TM-redox by invoking O-redox, where electrons are stored on oxygen as well as the transition metal ions.[1,2,3,4,5,6] Capacities exceeding 250 mA h gÀ1 are possible, making these materials candidates for future Li-ion batteries This process of O-redox has been extensively characterised using X-ray absorption near edge structure (XANES), so X-ray absorption spectroscopy (SXAS), online electrochemical mass spectrometry (OEMS), X-ray photoelectron spectroscopy (XPS), resonant inelastic X-ray scattering (RIXS) and several models exist for the nature of the oxygen redox process.[7,8,9,10] Typically, whilst the oxygen oxidation and reduction processes are reversible, a signi cant oxygen loss component is seen. It has been proposed that O-loss from Li-rich materials on charging, inherently a surface process, results in densi cation forming rocksalt/spinel structures at the surface.[12,13,14,18,19]

Results

Discussion

Conclusion