歧化作用在锰酸锂尖晶石中溶解锰中的作用,The Journal of Physical Chemistry C

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歧化作用在锰酸锂尖晶石中溶解锰中的作用
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2017-09-28 00:00:00 , DOI: 10.1021/acs.jpcc.7b05940
Roy Benedek ₁

Affiliation

从锰酸锂尖晶石中溶解锰阻碍了其作为锂离子电池正极材料的商业化。这种溶解被广泛地归因于在酸存在下近表面Mn（III）的歧化，所生成的二价Mn易于溶解。尚未确定电池中的水分（与有机电解质相对）在多大程度上用作Mn离子的溶剂。Leung的模拟表明，三价Mn在其LiMn ₂ O _4的平衡位点处有少量位移（001）/碳酸亚乙酯界面导致其还原为Mn（II）。在当前的工作中，基于蓝月亮系中的第一性原理分子动力学模拟执行了热力学积分，以研究Mn离子在LiMn ₂ O ₄（001）/水界面处的分离。结果表明，在水界面的情况下，也会发生Mn（III）还原为Mn（II）。对于中性和酸性水（在HF存在下）均以溶解Mn离子（相对于基质氧离子）的配位数作为反应坐标进行了模拟。仿真表明，F ^–离子与表面Mn（III）离子牢固结合，并削弱其与基材的附着力。由于这种弱化，表面MnF配合物在室温下穿过相空间区域，其中歧化在能量上变得有利。尽管这种歧化发生在靠近基体的位置，那里的Mn配位数仅比其平衡值略低一些，但我们认为歧化后重新附着的可能性很小（对于LiMn ₂ O _4， Leung得出了类似的解释（ 001）/ EC接口）。我们建议F的关键作用^–促进溶解的“作用”是削弱与底物的Mn结合以使歧化。然后，部分分离的MnF配合物可与溶剂进行另外的相互作用以形成例如MnF ₂，这将使之能够转运离开基底。Shilina等人的电子顺磁共振测量。讨论了似乎将Mn（III）列为主要的溶剂化物质。

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

Dissolution of Mn from lithium manganate spinel has hindered its commercialization as a cathode material in Li-ion batteries. This dissolution has been widely attributed to disproportionation of near-surface Mn(III), in the presence of acid, with the resultant divalent Mn being vulnerable to dissolution. To what extent moisture 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 to investigate the detachment of Mn 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 (to 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 MnF 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 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. The electron paramagnetic resonance measurements of Shilina et al. that appear to show Mn(III) as the predominant solvated species are discussed.

更新日期：2017-09-28

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