Role of Disproportionation in the Dissolution of Mn from Lithium Manganate Spinel

Journal Article · Mon Sep 18 00:00:00 EDT 2017 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.7b05940· OSTI ID:1416181

Benedek, Roy  ^[1]

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1. Argonne National Lab. (ANL), Argonne, IL (United States)

Dissolution of Mn from lithium-manganese spinel has hindered its commercialization as a cathode material in Li-ion batteries. Disproportionation of near-surface Mn(III), in the presence of acid, has been widely thought to result in dissolved divalent Mn. To what extent stray acidic water in the cell (as opposed to the organic electrolyte) acts as the solvent for Mn ions has not been established. Simulations by Leung show that a small displacement of trivalent Mn from its equilibrium site at an LiMn₂O₄ (001)/ ethylene carbonate interface leads to its reduction to Mn(II). In the present work, Thermodynamic Integration is performed, based on first-principles molecular dynamics simulations within the Blue-Moon ensemble, for the detachment of Mn(III) ions at the LiMn₂O₄ (001)/water interface. The results show that reduction of Mn(III) to Mn(II) occurs also in the case of an aqueous interface. The simulations were performed for both neutral and acidic water (in the presence of HF), with the coordination number of the dissolving Mn ion with substrate oxygen ions taken as the reaction coordinate. The simulations indicate that an F^- ion strongly binds to a surface Mn(III) ion, and weakens its adhesion to the substrate. Owing to this weakening, a surface Mn-F complex traverses regions of phase space at room temperature where disproportionation becomes energetically favorable. Although this disproportionation occurs close to the substrate, where the Mn coordination number is only slightly lowered from its equilibrium value, we argue that the likelihood of reattachment after disproportionation is small (Leung arrived at a similar interpretation in the case of the LiMn₂O₄ (001)/ EC interface). We suggest that the critical role of F^- in promoting dissolution is to weaken the Mn binding to the substrate so as to enable disproportionation. The partially detached MnF complex may then undergo additional interaction with the solvent to form, e.g., MnF₂, which would enable transport away from the substrate. In conclusion, the EPR measurements by Shilina et al. which appear to show Mn(III) as the predominant solvated species are discussed.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1416181
• Journal Information: Journal of Physical Chemistry. C, Vol. 121, Issue 40; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
25 ENERGY STORAGE
Li-ion battery
acid
cathode
disproportionation
dissolution
first principles
spinel