A Critical Analysis of Chemical and Electrochemical Oxidation Mechanisms in Li-Ion Batteries

Abstract

Software availability, data availability, computational methods, calculation of the solubility of O 2 in EC, and cited refs 61−72 (PDF)

Hybrid Density Functional Theory Comparison of Oxygen Release and Solvent Decomposition Kinetics on LixNiO2 Surfaces

“…We focus on a “locally stable surface” perspective, rather than pursuing the global energy minimum, which would be challenging for anion-redox-active systems like Li x NiO 2 in slab supercell configurations. Our overarching hypothesis is that EC must react with reactive O 2 species while weakly tethered on the oxide surface to make oxygen-EC reaction relevant, because 1 O 2 species have been predicted to react slowly with EC in the bulk electrolyte region. ,, Our computational findings do not support this surface O 2 -mediated EC oxidation mechanism; this conclusion regarding 1 O 2 is consistent with recent computational and experimental work. As discussed in the Introduction, the quantum yield in the bulk electrolyte region would be too small to explain formation of a nanometer layer of CEI.…”

“…This still appears to rule out EC reaction with 1 O 2 as a major contribution to the CEI on kinetics grounds. All this suggests non- 1 O 2 -mediated CEI formation mechanisms are dominantunless surface-mediated reactions, not the liquid-phase reaction routes examined in previous modeling work, ,, can accelerate the degradation. Predicting the 1 O 2 (or any kind of O 2 ) release rate is therefore imperative.…”

“…The primary reaction barrier for EC reaction with 1 O 2 is predicted to be ΔE * = 0.92 eV via a widely used hybrid DFT functional . Multireference (CASTP2) methods yield even higher barriers, at least ΔE *=1.27 eV. ,, The M11 DFT functional also predicts a high ΔE *=1.38 eV, while corrected, range-separated hybrid generalized gradient approximation DFT predicts ΔE * > 1 eV in the most favorable pathway . As before, , we estimate unimolecular reaction rates k from ΔE * using k = k 0 exp ( − β Δ E * ) where k 0 is the kinetic prefactor we have chosen to be 10 12 /s, , and β is the inverse thermal energy 1/ k B T at T = 300 K. 1 O 2 -EC reactions are bimolecular; in a pure EC solvent, the EC concentration would be ∼15 M, and to a first approximation, we adopt an effective k 0 = 10 13 /s here.…”

“…High-nickel-content layered oxides − have emerged as leading candidates for cathode materials in electrical vehicles. The higher voltages associated with these materials and the exposure of Ni cations on layered oxide cathode surfaces are however known to enhance degradation reactions, including oxidation of liquid electrolytes − accelerated at high voltages; ,− oxygen release (including reactive O 2 like spin-singlet 1 O 2 , superoxide anions O 2 – , and peroxide anions O 2 2– ), − transition metal ion dissolution, surface layer phase transformation, − and particle cracking. , These mechanisms are likely interrelated; for example, surface oxygen loss mostly likely contributes to rock salt transformation. , Elucidating and mitigating these degradation mechanisms will yield improvements in battery cycle life and safety and allow cost reduction.…”

“…Our focus in this work is to compare direct EC interfacial reaction and 1 O 2 release rates because 1 O 2 has been a recent literature focus. − However, O 2 – and O 2 2– have been predicted to react with EC at rates much faster than 1 O 2 . Comparing the release rate of such oxygen species with an EC direct reaction is also relevant but is complicated by several factors.…”