Understanding Transition Metal Dissolution from Battery Materials with Solution NMR Methods

Rechargeable batteries are a critical modern technology, with widespread and growing use in consumer electronics, transport, and grid energy storage. Lithium-ion batteries commonly use lithium transition metal oxides as cathode materials, many of which can undergo dissolution of the transition metal(s) into the electrolyte solution. Transition metal dissolution can lead to cathode restructuring, electrolyte degradation, thickening of the anode solid electrolyte interphase, and loss of lithium from the active lithium inventory, ultimately causing battery capacity loss. It is generally accepted that dissolved transition metals (e.g., Mn²⁺, Ni²⁺, Co²⁺) are paramagnetic. In NMR measurements, paramagnetic solutes can significantly affect the peak positions and relaxation rates of nearby chemical species in solution. This work therefore explores the use of solution NMR to characterise and quantify transition metal dissolution in battery electrolytes. ¹H, ¹⁹F, ³¹P, and ⁷Li NMR measurements are performed on LiPF₆ solutions containing model transition metal compounds or metals dissolved from cathode materials. NMR of transition metal-contaminated battery electrolyte solutions is used to quantify dissolved metals at micromolar concentrations; determine their oxidation states, spin states, and coordination numbers; and understand the solvation shell in pristine and degraded electrolyte solutions. Specifically, it is shown that Mn²⁺ and Ni²⁺ coordinate primarily to ethylene carbonate in pristine electrolyte solutions, and to difluorophosphate (or other fluorophosphate species) in degraded electrolyte solutions. Sufficient transition metal coordination to fluorophosphate degradation products can induce severe signal broadening, rendering these species undetectable by ¹⁹F and ³¹P NMR, but this issue can be mitigated with the use of suitable coordinating solvents or precipitation agents. This work demonstrates the broad capabilities of easily accessible solution NMR measurements towards elucidating transition metal dissolution-migration-deposition mechanisms. The methods explored in this work may further be applied to any battery chemistry with dissolved paramagnetic species, including sodium-ion, potassium-ion, multivalent, and redox flow chemistries.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Allen, Jennifer

Advisor dc:contributor.advisor

• Grey, Clare

• lithium-ion batteries
• transition metal dissolution
• solution NMR
• paramagnetic NMR

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0002-9800-9382

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/344422