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Type II germanium clathrates have recently been investigated for potential applications as anodes in batteries due to their cage-like structures that can accommodate electrochemical insertion of guest ions. To synthesize type II Ge clathrates (Ge136), several experimental routes use thermal or electrochemical desodiation of the Zintl phase compound Na4Ge4. However, the mechanism by which Na atoms are removed from the precursor to form clathrates is not well understood. Herein, we use first-principles density functional theory and nudged elastic band calculations to understand the reaction mechanism and formation energies of the products typically observed in the synthesis, namely, NaδGe136 (0 < δ < 24) type II clathrates and hexagonal phase Na1–xGe3+z. Specifically, we confirm the energetic feasibility of Na vacancy formation in Na4Ge4 and find that the barrier for Na vacancy migration is only 0.37 eV. This relatively low energy barrier is consistent with the ease with which Na4Ge4 can be desodiated to form the products. We also discuss the energetics, sodium migration pathways, and potential electrochemical performance of Ge136 as anode material for Na-ion batteries. Overall, this study highlights how first-principles calculations can be used to understand the synthesis mechanism and desodiation processes in clathrate materials and will help guide researchers in the design and evaluation of new open framework compounds as viable materials for energy storage applications. 

The guest‐free, type‐II Si clathrate (Si₁₃₆) 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 Si₁₃₆is 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 ≈0.3 V versus Li/Li⁺with a capacity of ≈231 mAh g⁻¹(corresponding to composition Li₃₂Si₁₃₆). A synchrotron powder X‐ray diffraction analysis of electrodes after lithiation shows evidence of Li occupation within the Si₂₀and Si₂₈cages and a volume expansion of 0.22%, which is corroborated by DFT calculations. Nudged elastic band calculations suggest a low barrier (0.2 eV) for Li migration through interconnected Si₂₈cages, whereas there is a higher barrier for Li migration into Si₂₀cages (2.0 eV). However, if Li is present in a neighboring cage, a cooperative migration pathway with a barrier of 0.65 eV is possible. The results show that the type‐II Si clathrate displays unique electrochemical properties for potential applications as Li‐ion battery anodes. 

Clathrates of Tetrel elements (Si, Ge, Sn) have attracted interest for their potential use in batteries and other applications. Sodium-filled silicon clathrates are conventionally synthesized through thermal decomposition of the Zintl precursor Na₄Si₄, but phase selectivity of the product is often difficult to achieve. Herein, we report the selective formation of the type I clathrate Na₈Si₄₆using electrochemical oxidation at 450 °C and 550 °C. A two-electrode cell design inspired by high-temperature sodium-sulfur batteries is employed, using Na₄Si₄as working electrode, Naβ″-alumina solid electrolyte, and counter electrode consisting of molten Na or Sn. Galvanostatic intermittent titration is implemented to observe the oxidation characteristics and reveals a relatively constant cell potential under quasi-equilibrium conditions, indicating a two-phase reaction between Na₄Si₄and Na₈Si₄₆. We further demonstrate that the product selection and morphology can be altered by tuning the reaction temperature and Na vapor pressure. Room temperature lithiation of the synthesized Na₈Si₄₆is evaluated for the first time, showing similar electrochemical characteristics to those in the type II clathrate Na₂₄Si₁₃₆. The results show that solid-state electrochemical oxidation of Zintl phases at high temperatures can lead to opportunities for more controlled crystal growth and a deeper understanding of the formation processes of intermetallic clathrates. 

Recently, a phosphorus isomer named green phosphorus was theoretically predicted with a similar interlayer interaction compared to that of black phosphorus, thus indicating that individual layers can be mechanically exfoliated to form two-dimensional (2D) layers known as green phosphorene. In this work, we investigated the properties of green phosphorene nanoribbons along both armchair and zigzag directions with ribbon widths up to 57 Å using density functional theory. Effects of ribbon width and strain on the mechanical and electronic properties of the ribbons were studied. The Young’s modulus, effect of quantum confinement on the band gap, and effect of strain on the band structures of the ribbons were investigated. The green phosphorene ribbons were found to exhibit prominent anisotropic properties, with the Young’s modulus in the range of 10-35 GPa for the armchair green phosphorene nanoribbons (AGPNR) and 160-170 GPa for the zigzag green phosphorene nanoribbons (ZGPNR), which are the same order of magnitude as those of the 2D sheets. The work function was found to be between 5 eV ∼ 5.7 eV for the range of widths studied. Both size and strain trigger direct-indirect band gap transitions in the ribbons and their transition mechanisms were discussed.