Atomic structure of Li_2-xMnO₃ studied by neutron diffraction and STEM

Li2MnO3 is an important cathode material with extra high capacity (>300mAh g-1 for the first charge process). The exact charge-discharge mechanism and the structure evolution still remain controversial. Here the atomic structures of Li2MnO3 after partial delithiation and lithiation are investigated by neutron powder diffraction and spherical aberration-corrected scanning transmission electron microscopy (STEM). Neutron diffraction experiments are performed on Li2-xMnO3 (x=0, 0.25) in a bulk level. It can be found that the volume of the unit cell almost keeps constant, while the lattice constants in the a, b direction increases after the chemical delithiation, but the c direction decreases. For the delithiated compound Li1.75MnO3, the Li occupancies are 0.7(+-0.3), 0.9(+-0.1) for the 2c and 4h sites, respectively, resulting in the Li-concentration of 1.75(+-0.27), while the 2b sites are fully occupied. Furhtermore, the isotropic thermal vibration factors of the 2c and 4h Li atoms are considerably larger than that of the 2b Li atoms, also seemingly implying the feasible delithiation of Li atoms at the 2c and 4h sites in the Li-O layer. It is interesting to note that the thermal factor of Mn atoms is slightly larger than O atoms, which probably means that the Mn atoms are more mobile than O atoms. The STEM results suggest that the Li ions can be extracted both from the LiMn2 planes and Li planes, and the Mn ions can move reversibly in the (110) plane during delithiation and lithiation.

Exploiting weak supramolecular interactions to assemble organic cage materials

Intensive research into microporous materials has been driven by potential applications in areas such as catalysis, gas separation, storage, and sensing. Recently, a new class of purely organic molecular cage materials has emerged, which can exhibit significant porosity arising from the internal molecular cavity as well as extrinsic porosity from packing in the crystal structure [1]. Unlike extended frameworks, porous molecular materials lack strongly directional interactions to drive their assembly, complicating the crystal engineering possible for isoreticular metal-organic frameworks [2], for example. Our work has focused on covalent imine-linked cages, which exhibit diverse crystal chemistry. The connectivity of the pore network is derived from the cage packing: Therefore, the crystal structure directly affects the observed porosity. The imine cages synthesised so far lack strongly hydrogen bonding groups. Thus, the solid state supramolecular assembly of cage molecules is governed by the aggregate of weak interactions, such as van der Waals forces. By identifying robust `tectons', that is, regularly occurring supramolecular motifs, progress toward designing the crystal structure and therefore controlling the physical properties of organic cage materials becomes possible. Here, we report exploiting robust supramolecular motifs, comprising either cage modules or host and guest molecules to gain control over the porosity of the bulk material. We demonstrate how formation of a desired void network topology can be driven by hosting a specific guest in preferred sites which maximise weak host-guest interactions [3]. Subsequent guest removal can produce stable polymorphs, one of which exhibited double the Brunauer-Emmett-Teller surface area with respect to the originally observed polymorph. We also examine how the interaction between gas phase guests and cage host is important in the application of porous organic cages in rare gas separation.