Graphite, as the most common anode for commercial Li-ion batteries, has been reported to have a very low capacity when used as a Na-ion battery anode. It is well known that electrochemical insertion of Na(+) into graphite is significantly hindered by the insufficient interlayer spacing. Here we report expanded graphite as a Na-ion battery anode. Prepared through a process of oxidation and partial reduction on graphite, expanded graphite has an enlarged interlayer lattice distance of 4.3 A yet retains an analogous long-range-ordered layered structure to graphite. In situ transmission electron microscopy has demonstrated that the Na-ion can be reversibly inserted into and extracted from expanded graphite. Galvanostatic studies show that expanded graphite can deliver a high reversible capacity of 284 mAh g(-1) at a current density of 20 mA g(-1), maintain a capacity of 184 mAh g(-1) at 100 mA g(-1), and retain 73.92% of its capacity after 2,000 cycles.

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Expanded graphite as superior anode for sodium-ion batteries

The anomalous properties of cold and supercooled water, such as the fact that at sufficiently low temperatures it becomes more compressible and less dense when cooled, and more fluid when compressed, have attracted the attention of physical scientists for a long time. The discovery in the 1970s that several thermodynamic and transport properties of supercooled water exhibit a pronounced temperature dependence and appear to diverge slightly below the homogeneous nucleation temperature inspired a large number of experimental and theoretical studies. Likewise, an important body of work on glassy water has been stimulated by experiments, starting in the mid-1980s and continuing to this date, which suggest that vitreous water can exist in at least two apparently distinct forms. A coherent theory of the thermodynamic and transport properties of supercooled and glassy water does not yet exist. Nevertheless, significant progress towards this goal has been made during the past 20 years. This article summarizes the known experimental facts and reviews critically theoretical and computational work aimed at interpreting the observations and providing a unified viewpoint on cold, non-crystalline, metastable states of water.

https://iopscience.iop.org/article/10.1088/0953-8984/15/45/R01/pdf

Supercooled and glassy water

Compressed under ambient temperature, graphite undergoes a transition at ∼17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the π-bonds between graphite layers convert to σ-bonds, whereas the other half remain as π-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form σ-bonds, whereas the nonbridging carbon atoms remain unpaired with π-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

https://science.sciencemag.org/content/sci/302/5644/425.full.pdf

Bonding Changes in Compressed Superhard Graphite

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4–4, 3–5 and 2–6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions1,2. On the other hand, its liquid form has much in common, thermodynamically, with water and other ‘tetrahedral network’ liquids that show density maxima3,4,5,6,7. Proper study of the ‘liquid–amorphous transition’, documented for non-crystalline silicon by both experimental and computer simulation studies8,9,10,11,12,13,14,15,16,17, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger–Weber potential18, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar10. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction3 that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.

Liquid-liquid phase transition in supercooled silicon.

We report a novel phase of carbon possessing a monoclinic $C2/m$ structure ($8\text{ }\text{ }\mathrm{\text{atoms}}/\mathrm{\text{cell}}$) identified using an ab initio evolutionary structural search. This polymorph, which we call $M$-carbon, is related to the ($2\ifmmode\times\else\texttimes\fi{}1$) reconstruction of the (111) surface of diamond and can also be viewed as a distorted (through sliding and buckling of the sheets) form of graphite. It is stable over cold-compressed graphite above 13.4 GPa. The simulated x-ray diffraction pattern and near $K$-edge spectroscopy are in satisfactory agreement with the experimental data [W. L. Mao et al., Science 302, 425 (2003)] on overcompressed graphite. The hardness and bulk modulus of this new carbon polymorph are calculated to be 83.1 and 431.2 GPa, respectively, which are comparable to those of diamond.

Superhard Monoclinic Polymorph of Carbon

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 002 GPa During the transformation, the two forms of liquid coexist These features are strongly suggestive of a first-order liquid-liquid phase transition

A first-order liquid–liquid phase transition in phosphorus

High-pressure in situ x-ray diffraction was carried out to clarify the nature of the pressure-induced phase transformation in graphite at room temperature. The combined use of a Drickamer-type high-pressure apparatus with sintered diamond as an anvil material and very intense x rays from synchrotron radiation made it possible to obtain high-quality x-ray-diffraction data, as well as information on the orientation relation, for this phase transformation. It was found that the transition starts at 14 GPa at room temperature, although this onset pressure is sensitive to the nature of the sample and of the applied pressure. X-ray-diffraction profiles obtained on the high-pressure phase are well explained by the hexagonal diamond structure, but the observed c/a ratio is slightly larger than that of ideal packing. The observed orientation relation satisfies the previously proposed martensitic transition mechanism from graphite to hexagonal diamond. But this hexagonal diamond formed at room temperature is unquenchable upon the release of pressure, and how it differs from the quenched phase formed under high pressure and temperature remains to be clarified.

High-pressure in situ x-ray-diffraction study of the phase transformation from graphite to hexagonal diamond at room temperature.

Structural transformation between a dense molecular fluid and a polymeric liquid of phosphorus that occurred at about 1 gigapascal and 1000°C was investigated by in situ x-ray radiography. When the low-pressure fluid was compressed, dark and round objects appeared in the radiograph. X-ray diffraction measurements confirmed that these objects were the highpressure liquid. The drops grew and eventually filled the sample space. Decompressing caused the reverse process. The macroscopic phase separation supported the existence of a first-order phase transition between two stable disordered phases besides the liquid-gas transition. X-ray absorption measurements revealed that the change in density at the transition corresponds to about 40% of the density of the high-pressure liquid.

https://science.sciencemag.org/content/sci/306/5697/848.full.pdf

Macroscopic Separation of Dense Fluid Phase and Liquid Phase of Phosphorus

Since first opening its doors to public research in 1997, SPring-8 has seen the accomplishment of many important studies in a wide variety of fields through its stable operation and cutting edge technology. High-pressure experiments have been carried out on a number of beamlines using a diamond anvil cell or a multi-anvil press. Here, we review the multi-anvil presses installed on the SPring-8 beamlines and a few research projects currently utilizing this technology. The significant difference in post-spinel boundary between multi-anvil experiments and diamond anvil studies will also be discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/14/44/322/pdf

High-pressure science with a multi-anvil apparatus at SPring-8

An apparatus to load gases to the sample chamber of the diamond‐anvil cell has been devised. The apparatus is driven by a conventional 50 ton hydraulic press and no gas compressor is required. The gas from a commercial gas bomb is compressed to 150 MPa and loaded into the diamond‐anvil cell sample chamber. After loading, the pressure of the diamond‐anvil cell is increased further using the lever and spring mechanism. This kind of gas loading apparatus will become indispensable not only for studying gaseous materials themselves, but also for making precision measurements at high pressures and high temperatures under hydrostatic conditions.

Masaaki Yamakata

Papers

A first-order liquid–liquid phase transition in phosphorus

High-pressure in situ x-ray-diffraction study of the phase transformation from graphite to hexagonal diamond at room temperature.

Macroscopic Separation of Dense Fluid Phase and Liquid Phase of Phosphorus

High-pressure science with a multi-anvil apparatus at SPring-8

An apparatus to load gaseous materials to the diamond‐anvil cell