Revisiting the Mechanism Behind Transition-Metal Dissolution from Delithiated LiNi <sub>x</sub> Mn <sub>y</sub> Co <sub>z</sub> O <sub>2</sub> (NMC) Cathodes
Ritu Sahore, Daniel C. O’Hanlon, Adam Tornheim, Chang‐Wook Lee, Juan C. Garcia, Hakim Iddir, Mahalingam Balasubramanian, Ira Bloom
Abstract
Dissolution of transition metals (TMs) from lithium-ion battery cathodes under high-voltage conditions is a major issue affecting battery performance that is not well understood mechanistically. Here, this phenomenon is studied by chemically aging pristine and charged LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) cathodes in the presence of different solutions. The solution composition was varied by 1) adding water to a standard electrolyte, 2) replacing LiPF 6 salt with lithium acetylacetonate (Li-acac), 3) and/or adding oxidatively unstable tris(2,2,2-trifluoroethyl) phosphite (TTFP) as an electrolyte additive. Our results demonstrate that while TM dissolution from pristine NMC532 cathodes is dominated by HF-attack, TM dissolution from charged NMC532 cathodes is affected by many other factors apart from HF-attack. We suggest that reduction of TMs due to chemical/electrochemical oxidation of the electrolyte at cathode/electrolyte interface, followed by formation of soluble TM-complexes with concomitant Li + intercalation into the cathode, is the dominant mechanism of TM-dissolution at high voltage.