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首页> 外文期刊>Physical review. B, Condensed Matter And Materials Physics >First-principles study of surface properties of LiFePO_4: Surface energy, structure, Wulff shape, and surface redox potential
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First-principles study of surface properties of LiFePO_4: Surface energy, structure, Wulff shape, and surface redox potential

机译：LiFePO_4表面性质的第一性原理研究：表面能，结构，Wulff形状和表面氧化还原电势

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Using first-principles calculations within the generalized gradient approximation (GGA) + U framework, we investigate several surface properties of olivine structure LiFePO_4. Calculated surface energies and surface redox potentials are found to be very anisotropic. Low-energy surfaces are in the [1 0 0], [0 1 0], [0 1 1], [1 0 1], and [2 0 1] orientations of the orthorhombic structure. We find that the coordination loss of Fe atoms on the surface is energetically more unfavorable than for Li, and generally a low-energy surface has fewer Fe-O bonds affected by the surface cut. Conversely, undercoordinated Li on the surface are somehow beneficial to reduce the energy of a surface except for the twofold coordinated Li. With the calculated surface energies, we provide the thermodynamic equilibrium shape of the LiFePO_4 crystal through a Wulff construction. The two low-energy surfaces (0 1 0) and (2 0 1) dominate in the Wulff shape and make up almost 85% of the surface area. Similar calculations for FePO_4 indicate a very low energy for the (0 1 0) surface of FePO_4. This result suggests that surface chemistry can induce a change in the aspect ratio of the Wulff shape. Surface redox potentials for the extraction and insertion of Li from various surfaces are also investigated in this work. The Li redox potential for the (0 1 0) surface is calculated to be 2.95 V, which is significantly lower than the bulk value of 3.55 V. For several other surfaces the Li extraction potential is above the bulk potential. We also develop a simple model that can be used to predict surface energies based on the change in the coordination of Fe and Li.
机译：使用广义梯度近似（GGA）+ U框架内的第一性原理计算，我们研究了橄榄石结构LiFePO_4的几种表面性质。发现计算出的表面能和表面氧化还原电势非常各向异性。低能表面处于正交结构的[1 0 0]，[0 1 0]，[0 1 1]，[1 0 1]和[2 0 1]方向。我们发现，与Li相比，Fe原子在表面上的配位损失在能量上更加不利，并且通常，低能表面具有较少的受表面切割影响的Fe-O键。相反，除双重配位的Li外，表面上配位不足的Li在某种程度上有利于降低表面的能量。利用计算出的表面能，我们通过Wulff结构提供了LiFePO_4晶体的热力学平衡形状。两个低能表面（0 1 0）和（2 0 1）以Wulff形状为主，几乎占表面积的85％。 FePO_4的类似计算表明，FePO_4的（0 1 0）表面的能量非常低。该结果表明表面化学可以引起Wulff形状的长宽比的变化。在这项工作中，还研究了从各种表面提取和插入Li的表面氧化还原电势。计算出（0 1 0）表面的Li氧化还原电势为2.95 V，大大低于3.55 V的体积值。对于其他几个表面，Li提取电势高于体积电势。我们还开发了一个简单的模型，该模型可根据铁和锂的配位变化来预测表面能。

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