Performance improvements in electric vehicle batteries are needed in order to reduce their cost and encourage greater use.¹ This improvement is dependent on the properties (e.g. specific capacity and stability) of the cathode active material in the electric vehicle’s lithium ion battery.¹ Lithium Nickel Cobalt Manganese Oxide (NMC) is a layered transition metal oxide that shows great promise as an electrode in lithium ion batteries for electric vehicles, with a high theoretical specific capacity and good stability in the layered structure.² As the nickel content of the cathode material increases, so does the discharge capacity, however this increase comes at the cost of significantly decreased capacity retention.¹ The mechanisms that contribute to this degradation are complex and interlinking and many of them are accompanied by some form of gas evolution. For example, the solid electrolyte interphase (SEI) formation at the cathode evolves CO₂, which is able to crossover and contribute to solid electrolyte interphase formation at the anode.¹ Similarly, upon electrochemical cycling a reactive form of oxygen can be evolved from the NMC lattice, resulting in a cascade of parasitic reactions within a lithium-ion battery.²

Herein, we probe these gas evolving degradation mechanisms through the development and use of a novel type of electrochemistry mass spectrometry (EC-MS) with unprecedented time resolution and sensitivity.^3,4 The new technique, known as on-chip EC-MS, employs a microfabricated membrane chip to precisely control the transfer of volatile species from an electrochemical cell to a mass spectrometer. Its design also allows instantaneous gas exchange, particularly useful to simulate cross talk phenomena. This work demonstrates the first application of this technique to the study of lithium-ion batteries; this study uses a new cell design to facilitate operando measurements of gas evolution in lithium-ion batteries to provide important insight into these complex mechanisms.⁴ More specifically, the effect of transition metal dissolution on the stability of the anodic SEI is investigated by monitoring ethylene evolution. Isotopic labelling studies are also performed to probe the evolution and consumption of CO₂ that is evolved from the cathode. Complementary and correlative ex situ surface sensitive analysis (such as x-ray photoelectron spectroscopy and secondary ion mass spectrometry) are carried out in order to develop a holistic understanding of the complex reactivity and chemistry evolution of lithium-ion batteries during operation.

References

(1) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Chemical versus Electrochemical Electrolyte Oxidation on NMC111, NMC622, NMC811, LNMO, and Conductive Carbon. J. Phys. Chem. Lett. 2017, 8 (19), 4820–4825.

(2) Wandt, J.; Freiberg, A. T. S.; Ogrodnik, A.; Gasteiger, H. A. Singlet Oxygen Evolution from Layered Transition Metal Oxide Cathode Materials and Its Implications for Lithium-Ion Batteries. Mater. Today 2018, 21 (8), 825-833.

(3) Trimarco, D. B.; Scott, S. B.; Thilsted, A. H.; Pan, J. Y.; Pedersen, T.; Hansen, O.; Chorkendorff, I.; Vesborg, P. C. K; Stephens, I. E. L. Enabling Real-Time Detection of Electrochemical Desorption Phenomena with Sub-Monolayer Sensitivity. Electrochim. Acta 2018, 268, 520–530.

(4) Thornton D. B.; Cavalca F.; Aguadero A.; Ryan M.; Stephens I.E.; UK Patent filed 17 September 2021