Kui Du

Abstract Conductive polymer/sulfur composite materials were prepared by heating the mixture of polyacrylonitrile (PAN) and sublimed sulfur. During the heating process, PAN was dehydrogenated by sulfur, forming a conductive main chain similar to polyacetylene. At the same time, the high‐polarity functional group –CN cyclized at the melt state, forming a thermally stable heterocyclic compound in which sulfur was embedded. The nanodispersed composites showed excellent electrochemical properties. Tested as cathode material in a non‐aqueous lithium cell based on poly(vinylidene fluoride) (PVDF) gel electrolyte at room temperature, the composite exhibited a specific capacity up to 850 mA h g –1 in the initial cycle. Its specific capacity remained above 600 mA h g –1 after 50 cycles, about five times that of LiCoO 2 , and recovered partly after replacement of the anode with a fresh lithium sheet. The utilization of the electrochemically active sulfur was about 90 % assuming a complete reaction to the product, Li 2 S.

Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years. Deformations in nanocrystals smaller than 10 nm are not well understood. The authors perform compression high-resolution transmission electron microscopy studies of gold nanoparticles, and determine that the nanoparticles deform through the emission of partial dislocations from free surfaces.

Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12-37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals.

Using an aberration-corrected transmission electron microscope, we observed the oxygen vacancies, profiled the concentration in the SnO2−δ nanocrystals on an atomic scale, and estimated the amount of oxygen vacancies to be ca. 3.3 atom%. The SnO2−δ nanocrystals show much improved initial Coulombic efficiency, rate capability and specific capacity compared with stoichiometric SnO2 when used as an anode material for lithium ion batteries.