Abstract
LiNiO2 remains a critical archetypal material for high energy density Li-ion batteries, forming the basis of Ni-rich cathodes in use today. Nevertheless, there are still uncertainties surrounding the charging mechanism at high states of charge and the potential role of oxygen redox. We show that oxidation of O2− across the 4.2 V vs. Li+/Li plateau forms O2 trapped in the particles and is accompanied by the formation of 8% Ni vacancies on the transition metal sites of previously fully dense transition metal layers. Such Ni vacancy formation on charging activates O-redox by generating non-bonding O 2p orbitals and is necessary to form vacancy clusters to accommodate O2 in the particles. Ni accumulates at and near the surface of the particles on charging, forming a Ni-rich shell approximately 5 nm thick; enhanced by loss of O2 from the surface, the resulting shell composition is Ni2.3+1.75O2. The overall Ni oxidation state of the particles measured by XAS in fluorescence yield mode after charging across the plateau to 4.3 V vs. Li+/Li is approximately +3.8; however, taking account of the shell thickness and the shell Ni oxidation state of +2.3, this indicates a Ni oxidation state in the core closer to +4 for compositions beyond the plateau.
Original language | English |
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Pages (from-to) | 2530-2540 |
Number of pages | 11 |
Journal | Energy and Environmental Science |
Volume | 17 |
Issue number | 7 |
DOIs | |
State | Published - Feb 27 2024 |
Funding
We are indebted to the Engineering and Physical Sciences Research Council (EPSRC), the Henry Royce Institute for Advanced Materials (EP/R00661X/1, EP/S019367/1, EP/R010145/1, EP/L019469/1) and the Faraday Institution (FIRG007, FIRG008, FIRG016) for financial support. We acknowledge Diamond Light Source for time on Beamlines I15-1 XPDF and I21 under Proposals MM27764-1 and MM29028-1. We thank the Spallation Neutron Source, a Department of Energy Office of Science User Facility operated by the Oak Ridge National Laboratory, for providing neutron powder diffraction measurements. We acknowledge the support of the Australian Centre for Neutron Scattering, ANSTO and the Australian Government through the National Collaborative Research Infrastructure Strategy in supporting the neutron research infrastructure used in this work via ACNS proposal MI13571. This project was supported by the Royal Academy of Engineering under the Research Fellowship scheme. We also acknowledge the resources provided by the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service ( https://www.hpc.cam.ac.uk ) funded by EPSRC Tier-2 capital grant EP/P020259/1, via the BATTSurface, NextCATHODE and SOLEL projects. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.