Singlet oxygen is not the source of ethylene carbonate degradation in nickel-rich Li-ion cells

Abstract

Nickel-rich intercalation electrodes (i.e. Ni_0.8Mn_0.1Co_0.1O₂) are seeing widespread adoption in high-performance lithium-ion batteries due to their high energy density and reduced need for cobalt. However, as nickel content increases, so too does the rate of cell capacity fade, which in part has been assigned to reactions between ethylene carbonate (EC) and reactive oxygenic species formed at the surface of nickel-rich electrodes. In particular, singlet oxygen (¹O₂) has long been suspected as a primary source of ethylene carbonate degradation and has been proposed to drive its conversion to either vinylene carbonate, a graphite stabilising additive, or to complete oxidation products such as CO₂, and protic species (i.e. H₂O, H₂O₂) that accelerate cell failure. Contrary to this understanding, we show using online mass spectrometry and quantitative ¹H NMR spectroscopic analysis that ethylene carbonate is stable in the presence of photocatalytically generated ¹O₂. Furthermore, this study indicates the use of rose bengal as a photocatalyst to study ethylene carbonate reactivity with ¹O₂ may lead to unexpected side-reactions under operationally-relevant conditions, producing misleading results. We conclude that the choice of photocatalyst is critical when assessing degradation with ¹O₂ for battery applications. Despite eliminating the direct reaction of ¹O₂ with EC as a source of degradation, ethylene carbonate to vinylene carbonate conversion is still found to occur in cells. We demonstrate that vinylene carbonate production begins before gas release from the positive electrode. These findings show that degradation driven by ¹O₂ reaction with EC is unlikely to be an important factor at nickel-rich intercalation electrodes highlighting the need for the community to explore alternative degradation pathways in nickel-rich lithium-ion batteries.