Nanoscale complex metal oxides have transformed how technology is used around the globe. One of the most widespread examples are the electroactive components of Li-ion batteries found in portable electronic devices and electric cars. Due to the lack of a recycling infrastructure, these battery materials end up in landfills, and finally, into aqueous environments. In this work, we focus on the materials found in the Li-ion battery cathode, Li(Ni1/3Mn1/3Co1/3)O2 (NMC). Previous work has shown that exposure of model organisms to nanoscale NMC can have adverse effects on survival owing to dissolution of metal ions. This dissolution process has unequal, or incongruent, amounts of Ni, Mn, and Co ions in solution, and an Mn enriched lattice is left over after 72 hours of exposure. To understand the dissolution mechanism, we have developed an analysis, which combines density functional theory (DFT)-computed total energies and experimentally tunable reaction conditions to compute the surface dissolution of complex metal oxides. Using this method, we captured the observed incongruent dissolution of metals from the lattice, Li+&gt;&gt;Ni2+&gt;Co3+&gt;Mn4+. However, to understand the persistence of Mn in the lattice, we further explore the thermodynamics of metal release from a defect surface. We also considered the effects of surface healing (reprotonation), oxidation state and site proximity to vacancies on metal release. We find that it is more favorable to release metals that are closest to the vacancy site after at least one reprotonation event on the surface. Using electronic structure calculations, we find that Ni and Co have the greatest fluctuation in oxidation states and are thermodynamically favorable to be removed after an initial Ni-OH of Co-OH unit it removed. Alternatively, releasing Mn from a defect surface is predicted to be the least favorable. By modeling the pathways of secondary cation release, the persistence of Mn in the lattice is further explained and advances molecular-level understanding of how nanosheets of NMC transform in aqueous environments.

Mechanistic study of the secondary cation release from Li(Ni1/3Mn1/3Co1/3)O2

ACS Spring Meeting 2021

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