Tetrel (Tt ¼ Si, Ge, Sn) clathrates are host-guest materials comprising cage frameworks of Tt elements that encapsulate alkali metal and alkaline earth metal guest atoms. Well known as promising candidates for thermoelectric materials, [1] clathrates also have interesting properties for optoelectronics [2][3][4] and superconducting [5][6][7] applications. Due to the large interest in Tt elements as high-capacity Li-ion battery anodes, the electrochemical properties of Tt clathrates have also been investigated in recent years, revealing properties distinct from those of diamond cubic structured analogues. [8][9][10][11][12][13][14][15][16][17][18] For instance, the reaction of Li with the type-I clathrate Ba 8 Al 16 Si 30 is dominated by surface rather than bulk reactions, [15] whereas the Ba 8 Al y Ge 46-y (0 < y < 16) clathrate undergoes bulk phase transitions to form amorphous Li-Ba-Ge phases with local structures similar to those in Li-Ge crystalline phases. [10,16] For the type-II clathrate Na 24 Si 136 , the lithiation profile is similar to that for diamond cubic Si, [12] whereas Na 1.6 Si 136 displays one more similar to that of amorphous Si. [8] Due to the wide range of possible clathrate structures and compositions, [1] we are interested in establishing a better understanding of the structure-property relationships of clathrates within the context of Li-ion battery applications.Clathrates crystallize in a variety of structural types where different face-sharing polyhedra are built from tetrahedral bonding

Tetrel (Tt = Si, Ge, and Sn) clathrates have highly tunable host–guest structures and have been investigated as novel electrode materials for Li-ion batteries. However, there is little understanding of how the clathrate structure affects the lithiation processes and phase evolution. Herein, the electrochemical lithiation pathway of type I clathrate Ba8Ge43 is investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to the lithiation of germanium with a diamond cubic structure (α-Ge). The results confirm previous laboratory XRD studies showing that Ba8Ge43 goes through a solely amorphous phase transformation, which contrasts with the crystalline phase transformations that take place during lithiation of micrometer-sized α-Ge particles. The local structure of framework-substituted clathrate Ba8Al16Ge30 after lithiation is found to proceed through an amorphous phase transformation similar to that in Ba8Ge43. In situ PDF and XRD during heating show that the amorphous phases derived from lithiation of Ba8Ge43 are structurally related to various Li–Ge phases and crystallize at low temperatures (350–420 K). We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li–Ge clusters and kinetically prevent the nucleation and growth of bulk crystalline phases. The amorphous phase evolution of the clathrate structure during lithiation results in electrochemical properties distinct from those in α-Ge, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as anodes in Li-ion batteries.

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 Na8Si46 using electrochemical oxidation at 450 °C and 550 °C. A two-electrode cell design inspired by high-temperature sodium-sulfur batteries is employed, using Na4Si4 as working electrode, Na β″-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 Na4Si4 and 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 Na8Si46 is 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.

Germanium clathrates with the type II structure are openframework materials that show promise for various applications, but the difficulty of achieving phase-pure products via traditional synthesis routes has hindered their development. Herein, we demonstrate the synthesis of type II Ge clathrates in a two-electrode electrochemical cell using Na 4 Ge 4−y Si y (y = 0, 1) Zintl phase precursors as the working electrode, Na metal as the counter/reference electrode, and Na-ion conducting β″-alumina as the solid electrolyte. The galvanostatic oxidation of Na 4 Ge 4 resulted in voltage plateaus around 0.34−0.40 V vs Na/Na + with the formation of different products depending on the reaction temperature. When using Na 4 Ge 3 Si as a precursor, nearly phase-pure, alloyed type II Ge−Si clathrate was obtained at 350 °C. The Na atoms in the large (Ge,Si) 28 cages of the clathrate occupied off-centered positions according to Rietveld refinement and density functional theory calculations. The results indicate that electrochemical oxidation of Zintl phase precursors is a promising pathway for synthesizing Ge clathrates with type II structure and that Si alloying of the Zintl phase precursor can promote selective clathrate product formation over other phases.

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their...