Image courtesy of Sandia National Laboratories
(Left) An electrolyte molecule (ethylene carbonate: C3H4O3) weakly binds (dashed line) to the positive electrode surface (Li0.6Mn2O4); the resulting interaction (oval) between the carbonyl carbon (C) and surface oxygen (O) atom destabilizes the molecule and leads to its unexpected decomposition. (Right) After decomposition, a fragment of the electrolyte molecule is bound to the electrode surface through manganese atoms and a hydrogen atom is bound to an adjacent oxygen. (color key: red = oxygen; green = carbon; purple = manganese; blue = lithium; grey = hydrogen).
Computational studies revealed that the manganese spinel (LixMn2O4) surface of positive electrodes in lithium ion batteries is reactive enough to destabilize an electrolyte molecule in proximity to the electrode surface and accelerate its subsequent decomposition.
Scientific understanding of how both electrolytes and electrodes react and degrade after many battery cycles of charging and discharging provides insight into new materials and battery designs that can greatly improve battery lifetimes.
Ethylene carbonate (EC) electrolytes and manganese spinel (LixMn2O4) positive electrodes are commonly used in lithium ion batteries. A comparison of the electrochemical potentials of EC and bulk LixMn2O4 suggests that decomposition of the electrolyte would not occur directly by electrons being transferred from EC to the electrode material, but the surface of a solid can have very different properties than its interior bulk. Researchers at Sandia National Laboratories, as part of the Nanostructures for Electrical Energy Storage (NEES) EFRC, have completed detailed coupled simulations of the molecules of the electrolyte and the surface of the positive electrode showing that the oxygen atoms on a Li0.6Mn2O4 surface can deform and weakly bind the EC molecule when it is near the electrode surface. This initial interaction does not involve the transfer of electrons (i.e., oxidation) but does enable breaking of the carbon-oxygen bond and subsequent molecular rearrangements that result in two electrons and a proton being transferred to the electrode surface. Therefore, a predicted series of five steps breaks down the electrolyte molecule, leaving the oxidized EC fragment still bound to the now acidified electrode surface. Acidification of positive electrodes is widely believed to initiate corrosion of the electrode surface and possible dissolution of manganese atoms. The proposed acidification mechanism illustrates the importance of modeling the electrolyte and the electrode surface together.
Sandia National Laboratories
Director, Nanostructures for Electrical Energy Storage (NEES) EFRC
DOE Office of Science, Basic Energy Sciences, Energy Frontier Research Centers (EFRC) Program.
K. Leung, “First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces” J. Phys. Chem. C, 2012, 116 (18), 9852. [DOI: 10.1021/jp212415x]
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