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Electrolyte Oxidation Pathways in Lithium-Ion Batteries

preprint
submitted on 11.06.2020 and posted on 18.06.2020 by Bernardine L. D. Rinkel, David S. Hall, Israel Temprano, Clare P. Grey

The mitigation of decomposition reactions of lithium-ion battery electrolyte solutions is of critical importance in controlling device lifetime and performance. However, due to the complexity of the system, exacerbated by the diverse set of electrolyte compositions, electrode materials, and operating parameters, a clear understanding of the key chemical mechanisms remains elusive. In this work, operando pressure measurements, solution NMR, and electrochemical methods were combined to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment LiCoO2/Li cells were cycled with a lithium-ion conducting glass-ceramic separator so that the species formed at each electrode could be identified separately and further reactions of these species at the opposite electrode prevented. One principal finding is that chemical oxidation (with an onset voltage of ~4.7 V vs Li/Li+ for LiCoO2), rather than electrochemical reaction, is the dominant decomposition process at the positive electrode surface in this system. This is ascribed to the well-known release of reactive oxygen at higher states-of-charge, indicating that reactions of the electrolyte at the positive electrode are intrinsically linked to surface reactivity of the active material. Soluble electrolyte decomposition products formed at both electrodes are characterised, and a detailed reaction scheme is constructed to rationalise the formation of the observed species. The insights on electrolyte decomposition through reactions with reactive oxygen species identified through this work have direct impact on understanding and mitigating degradation in high voltage/higher energy density LiCoO2-based cells, and more generally for cells containing nickel-containing cathode materials (e.g. LiNixMnyCozO2; NMCs), as they lose oxygen at lower operating voltages.

History

Email Address of Submitting Author

bldr2@cam.ac.uk

Institution

University of Cambridge

Country

United Kingdom

ORCID For Submitting Author

0000-0003-4455-7313

Declaration of Conflict of Interest

No conflict of interest

Version Notes

References to supporting figures corrected

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