Koçer Can P, Griffith Kent J, Grey Clare P, Morris Andrew J
Theory of Condensed Matter, Cavendish Laboratory , University of Cambridge , J. J. Thomson Avenue , Cambridge CB3 0HE , U.K.
Department of Materials Science and Engineering , Northwestern University , Evanston , Illinois 60208 , United States.
J Am Chem Soc. 2019 Sep 25;141(38):15121-15134. doi: 10.1021/jacs.9b06316. Epub 2019 Sep 12.
Wadsley-Roth crystallographic shear phases form a family of compounds that have attracted attention due to their excellent performance as lithium-ion battery electrodes. The complex crystallographic structure of these materials poses a challenge for first-principles computational modeling and hinders the understanding of their structural, electronic and dynamic properties. In this article, we study three different niobium-tungsten oxide crystallographic shear phases (NbWO, NbWO, NbWO) using an enumeration-based approach and first-principles density-functional theory calculations. We report common principles governing the cation disorder, lithium insertion mechanism, and electronic structure of these materials. Tungsten preferentially occupies tetrahedral and block-central sites within the block-type crystal structures, and the local structure of the materials depends on the cation configuration. The lithium insertion proceeds via a three-step mechanism, associated with an anisotropic evolution of the host lattice. Our calculations reveal an important connection between long-range and local structural changes: in the second step of the mechanism, the removal of local structural distortions leads to the contraction of the lattice along specific crystallographic directions, buffering the volume expansion of the material. Niobium-tungsten oxide shear structures host small amounts of localized electrons during initial lithium insertion due to the confining effect of the blocks, but quickly become metallic upon further lithiation. We argue that the combination of local, long-range, and electronic structural evolution over the course of lithiation is beneficial to the performance of these materials as battery electrodes. The mechanistic principles we establish arise from the compound-independent crystallographic shear structure and are therefore likely to apply to niobium-titanium oxide or pure niobium oxide crystallographic shear phases.
沃兹利-罗斯晶体学剪切相形成了一类化合物，因其作为锂离子电池电极的优异性能而备受关注。这些材料复杂的晶体结构对第一性原理计算建模提出了挑战，并阻碍了对其结构、电子和动态性质的理解。在本文中，我们使用基于枚举的方法和第一性原理密度泛函理论计算，研究了三种不同的铌钨氧化物晶体学剪切相（NbWO、NbWO、NbWO）。我们报告了支配这些材料阳离子无序、锂嵌入机制和电子结构的共同原理。钨优先占据块状晶体结构内的四面体和块体中心位置，材料的局部结构取决于阳离子构型。锂嵌入通过三步机制进行，这与主体晶格的各向异性演化相关。我们的计算揭示了长程和局部结构变化之间的重要联系：在该机制的第二步中，局部结构畸变的消除导致晶格沿特定晶体学方向收缩，缓冲了材料的体积膨胀。由于块体的限制作用，铌钨氧化物剪切结构在初始锂嵌入过程中容纳少量局域电子，但在进一步锂化后迅速变为金属态。我们认为，锂化过程中局部、长程和电子结构演化的结合有利于这些材料作为电池电极的性能。我们确立的机理原理源于与化合物无关的晶体学剪切结构，因此可能适用于铌钛氧化物或纯铌氧化物晶体学剪切相。
