Structural Origin of Reversible Li Insertion in Guest-Free, Type-II Silicon Clathrates
Structural Origin of Reversible Li Insertion in Guest-Free, Type-II Silicon Clathrates
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DOI：
10.1002/aesr.202000114
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发表时间：
2021-05-01
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影响因子：
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通讯作者：
Chan, Candace K.
中科院分区：
文献类型：
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作者：
Dopilka, Andrew;Weller, J. Mark;Chan, Candace K.
无客体的ii型硅包合物（Si-136）是一种开放的硅笼多晶体，具有适合电化学锂存储的结构特征。然而，Li在Si-136的空笼内可逆插入和迁移的详细机制尚未建立。本文采用x射线表征和密度泛函理论（DFT）计算来了解电化学Li插入ii型包合物结构的结构起源。在低Li含量时，与Li/Li+相比，在大约0.3V的电压下，拓扑定向Li插入到空笼中，容量约为231mAhg(-1)（对应于成分Li32Si136）。锂化后电极的同步加速器粉末x射线衍射分析表明，锂在Si-20和Si-28笼内占据，体积膨胀0.22%，DFT计算证实了这一点。轻推弹性带计算表明，锂离子通过互连Si-28笼迁移的势垒较低（0.2eV），而锂离子迁移到Si-20笼的势垒较高（2.0eV）。然而，如果Li存在于相邻的笼中，则可能出现一个具有0.65eV势垒的协同迁移途径。结果表明，ii型硅包合物具有独特的电化学性能，具有作为锂离子电池阳极的潜力。
The guest-free, type-II Si clathrate (Si-136) is an open cage polymorph of Si with structural features amenable to electrochemical Li storage. However, the detailed mechanism for reversible Li insertion and migration within the vacant cages of Si-136 is not established. Herein, X-ray characterization and density functional theory (DFT) calculations are used to understand the structural origin of electrochemical Li insertion into the type-II clathrate structure. At low Li content, instead of alloying with Si, topotactic Li insertion into the empty cages occurs at approximate to 0.3V versus Li/Li+ with a capacity of approximate to 231mAhg(-1) (corresponding to composition Li32Si136). A synchrotron powder X-ray diffraction analysis of electrodes after lithiation shows evidence of Li occupation within the Si-20 and Si-28 cages and a volume expansion of 0.22%, which is corroborated by DFT calculations. Nudged elastic band calculations suggest a low barrier (0.2eV) for Li migration through interconnected Si-28 cages, whereas there is a higher barrier for Li migration into Si-20 cages (2.0eV). However, if Li is present in a neighboring cage, a cooperative migration pathway with a barrier of 0.65eV is possible. The results show that the type-II Si clathrate displays unique electrochemical properties for potential applications as Li-ion battery anodes.