Transition-Metal Mixing and Redox Potentials in Lix(M1‿i>yM‿sub>y)PO4 (M, M‿= Mn, Fe, Ni) Olivine Materials from First-Principles Calculations

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• 作者：David H. Snydacker ; Chris Wolverton
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：March 24, 2016
• 年：2016
• 卷：120
• 期：11
• 页码：5932-5939
• 全文大小：339K
• ISSN：1932-7455

文摘

The performance of olivine cathode materials can be improved using core/shell structures such as LiMnPO₄/LiFePO₄ and LiMnPO₄/LiNiPO₄. We use density functional theory to calculate the energetics, phase stability, and voltages of transition-metal mixing for a series of olivine phosphate materials. For LiMn_1–yFe_yPO₄, LiFe_1–yNi_yPO₄, and LiMn_1–yNi_yPO₄, we find phase-separating tendencies with (mean-field) maximum miscibility gap temperatures of 120, 320, and 760 K respectively. At room temperature, we find that Mn is completely miscible in LiFePO₄, whereas Mn solubility in LiNiPO₄ is just 0.3%. Therefore, we suggest that core/shell LiMnPO₄/LiNiPO₄ particles could be more effective at containing Mn in the particle core and limiting Mn dissolution into the electrolyte relative to LiMnPO₄/LiFePO₄ particles. We calculate shifts in redox potentials for dilute transition metals, M, substituted into Li_xM′PO₄ host materials. Unmixed Li_xMnPO₄ exhibits a redox potential of 4.0 V, but we find that dilute Mn in a LiNiPO₄ shell exhibits a redox potential of 4.3 V and therefore remains redox inactive at lower cathode potentials. We find that strain plays a large role in the redox potentials of some mixed systems (Li_xMn_1–yFe_yPO₄) but not others (Li_xMn_1–yNi_yPO₄).