Staging effects on the structural and energetic properties of fast ionic conductor LixTiS2

Abstract

A series of cell models for stage-2 ordered LixTiS2(x=1/8，1/6，1/4，1/3，3/8，1/2)system were constructed according to concentration-wave theory. Geometry optimizations and total energy computations were carried out for the stage-2 ordered systems and stage-1 ordered LixTiS2(x=0，1/4，1/3，1/2，2/3，3/4，1)system by plane-wave pseudopotential method. The results of stage-2 ordered systems were compared with that of stage-1 ordered systems. It turns out that there is a similar increment tendency in lattice parameters c(d) of stage-2 cell and 2c0(d) of stage-1 cell. Furthermore，there are extra expansions of c1(d)，i.e. the space between two sandwiches with intercalated ions in stage-2 system，compared with c0(d)，i.e. the space between two sandwiches in stage-1 system，and the corresponding c2(d)，i.e. the space between two sandwiches without intercalated ions in stage-2 system, is compressed. The extrc expansion of c1(d) is complementary to the compression of c2(d) to a certain extent. Staging has remarkable effect on the atomic structure of TiS2 sandwiches. The formation energy of stage-2 ordered Li1/2TiS2 is distinctly higher than that of stage-1 ordered Li1/2TiS2，indicating that the latter is more stable than the former，which is well in accordance with the experimental results reported. The lower formation energy of stage-2 ordered Li1/4TiS2，compared with stage-1 ordered Li1/4TiS2，predicts a new ordered structure.