Ab Initio Investigation of Li and Na Migration in Guest-Free, Type I Clathrates
Ab Initio Investigation of Li and Na Migration in Guest-Free, Type I Clathrates
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DOI：
10.1021/acs.jpcc.9b06424
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发表时间：
2019-09-19
影响因子：
3.7
通讯作者：
Chan, Candace K.
中科院分区：
文献类型：
--
作者：
Dopilka, Andrew;Peng, Xihong;Chan, Candace K.
式为TT(46)(TT=Si，Ge，Sn)的无客体I类笼状化合物由开放的笼状骨架组成，具有容易传导Li或Na的潜力。本文采用从头算密度泛函理论(DFT)，通过笼状晶体结构计算了Li和Na的离子迁移率。确定了笼状结构中Li和Na的有利位置，并用微动弹性带(NEB)方法评估了迁移路径和势垒。结果表明，Li原子在小的TT笼中占据中心位置在能量上是有利的，而在较大的TT(24)笼中则倾向于偏心位置。Si-46、Ge-46和Sn-46的最低Li迁移势垒分别为0.35、0.13和0.37 eV，扩散路径主要沿TT(24)面六角面连接的槽道进行。在稀释区，由于高能垒(2.0 eV)，Li在Si-46中对Si-20笼子的可及性似乎受到限制，除了Li原子存在于相邻笼子中的情况外；这通过一种Si-Si键暂时断裂的机制将迁移势垒降低到0.77 eV。相反，Na原子更倾向于笼状中心，表现出比Li更高的迁移势垒。总体而言，在无客体的I型笼状结构中，TT(24)通道大小是快速Li扩散的理想选择，而Na太大，不能在笼子之间有效地迁移。第I类包合物中Li的能量分布与钻石立方结构中的完全不同，导致Li迁移的能量势垒显著降低。这些结果表明，金属间化合物的开放骨架可以方便地实现锂的迁移，并有可能作为锂离子电池的负极。
Guest-free, type I clathrates with formula Tt(46) (Tt = Si, Ge, Sn) are comprised of open, cage-like frameworks with the potential for facile Li or Na conduction. Herein, ab initio density functional theory (DFT) is used to evaluate the ionic mobility of Li and Na through the clathrate crystal structures. The favorable Li and Na positions inside the clathrate structures are determined, and the migration pathways and barriers are evaluated using the nudged elastic band (NEB) method. The results show that it is energetically favorable for a Li atom to occupy the center position inside the small Tt cages while preferring the off-center positions in the larger Tt(24) cages. The lowest Li migration barriers are found to be 0.35, 0.13 and 0.37 eV for Si-46, Ge-46, and Sn-46, respectively, with the dominant diffusion pathway along channels of Tt(24) cages connected by hexagonal faces. Li accessibility to the Si-20 cage in Si-46 appears to be restricted in the dilute regime due to a high energy barrier (2.0 eV) except for the case in which Li atoms are present in adjacent cages; this lowers the migration barrier to 0.77 eV via a mechanism where a Si-Si bond is temporarily broken. In contrast, Na atoms show preference for the cage centers and display higher migration barriers than Li. Overall, the Tt(24) channel sizes in the guest-free, type I clathrates are ideal for fast Li diffusion, while Na is too large to migrate effectively between cages. The energy landscape for Li inside the type I clathrates is uniquely different than that in diamond cubic structures, leading to significantly lower energy barriers for Li migration. These results suggest that open frameworks of intermetallic elements may enable facile Li migration and have potential use as Li-ion battery anodes.