First-Principles Investigation of the Li-Fe-F Phase Diagram and Equilibrium and Nonequilibrium Conversion Reactions of Iron Fluorides with Lithium
First-Principles Investigation of the Li-Fe-F Phase Diagram and Equilibrium and Nonequilibrium Conversion Reactions of Iron Fluorides with Lithium
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DOI：
10.1021/cm801105p
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发表时间：
2008-08-26
影响因子：
8.6
通讯作者：
Ceder, Gerbrand
中科院分区：
文献类型：
--
作者：
Doe, Robert E.;Persson, Kristin A.;Ceder, Gerbrand
我们已经使用密度功能理论（DFT）来研究LI-FE-F系统的三元相图以及LI与氟化铁的反应。预测几种以前未在LI-FE-F系统中鉴定出的新型化合物是稳定的。来自LI化学电位在计算的相图中的演化而得出的电化学电压曲线与实验趋势是合理吻合的。还研究了LixFef3与LI反应时粒径对Fe的影响。我们发现，当形成1 nm的Fe颗粒时，该反应的潜力大大降低了其整体值，并将其与实验观察结果相关联。此外，我们为在FEF3的锂化和界定中观察到了重要的滞后模型。通过假设LI的扩散比Fe得出的非平衡路径与实验曲线合理一致。我们的动力学模型预测，氟化铁反应在转化率（放电）和重新交换过程中遵循不同的路径，这导致了在实验过程中观察到的电压剖面滞后。提出的动力学模型还解释了为什么从Lif和Fe A的3/1混合物中提取Li时，即使应通过提取三个Li+每Fe的三个Li+，也会形成金红石Fef2样结构。
We have used density functional theory (DFT) to investigate the ternary phase diagram of the Li-Fe-F system and the reactions of Li with iron fluorides. Several novel compounds, not previously identified in the Li-Fe-F system, are predicted to be stable. Electrochemical voltage profiles, derived from the evolution of the Li chemical potential in the calculated phase diagram, are in reasonable agreement with experimental trends. The effect of particle size on the Fe that precipitates when LixFeF3 reacts with Li is also investigated. We find that when 1 nm Fe particles form, the potential for this reaction is considerably reduced from its bulk value and relate this to the experimental observations. Furthermore, we formulate a model for the significant hysteresis that is observed in the lithiation and delithiation of FeF3. Nonequilibrium paths derived by assuming much faster diffusion of Li than Fe are in reasonable agreement with experimental profiles. Our kinetic model predicts that the iron fluoride reaction follows a different path through the phase diagram during conversion (discharge) and reconversion (charge), which results in the voltage profile hysteresis observed during experiment. The proposed kinetic model also explains why upon extraction of Li from a 3/1 mixture of LiF and Fe a rutile FeF2-like structure can form, even when iron should be oxidized to Fe3+ by extraction of three Li+ per Fe.