Abstract
Lithium-rich materials, such as Li1.2Ni0.2Mn0.6O2, exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, On- and O2-, where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi0.5Mn1.5O4, for which no On- species is detected. The depth profile results demonstrate a concentration gradient exists for On- from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core-shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for On-.
Original language | English |
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Pages (from-to) | 25355-25368 |
Number of pages | 14 |
Journal | Journal of Materials Chemistry A |
Volume | 7 |
Issue number | 44 |
Early online date | 27 Sept 2019 |
DOIs | |
Publication status | Published - 28 Nov 2019 |
Funding
P. G. B. is indebted to the Engineering and Physical Sciences Research Council (EPSRC), including the SUPERGEN Energy Storage Hub [EP/L019469/1], Enabling Next Generation Lithium Batteries [EP/M009521/1], Henry Royce Institute for Advanced Materials [EP/R00661X/1, EP/S019367/1, EP/R010145/1] and the Faraday Institution All-Solid-State Batteries with Li and Na Anodes [FIRG007, FIRG008] for nancial support. The authors gratefully acknowledge the MCC/Archer consortium (EP/ L000202/1) and the Swedish National Graduate School in Neutron Scattering (SwedNess), part of the Swedish Foundation for Strategic Research (SSF). We thank Diamond Light Source for access to beamline I09 (proposal numbers NT14733-1 and SI16629-1) that contributed to the results presented here. We are grateful to Dr Tien-Lin Lee, Dr Christoph Schlueter and Dr Pardeep Thakur Kumar for their assistance.
ASJC Scopus subject areas
- General Chemistry
- Renewable Energy, Sustainability and the Environment
- General Materials Science