Twinning and Crack Detection in a Layered Cathode Battery Material with High Resolution FESEM and Low Voltage STEM

One of the most important planar defects that can be present in polycrystalline materials are twins. Three kinds of twinning can occur in a polycrystalline solid: I) growth twinning, II) deformation twinning, and III) annealing twinning. Deformation and annealing twins can be formed during mechanical deformation and heat treatment of the crystalline material, respectively. While crystal or growth twinning occurs upon crystal growth from melt or vapor and it either is favored by defects in the growth face or is due to the growth mistakes, which are attributed to the atomic mobility when the atoms do not get to the correct positions [1]. It is believed that twinning can affect the transition metal layered oxide cathode materials during the electrochemical cycling of the battery. Cracks are frequently observed mechanical failures after electrochemical cycling. Most evidence suggests that the twin boundaries are free of voids and cracks in the pristine samples [2, 3]. However, in the present study with the use of low voltage scanning transmission electron microscope (LV-STEM), which can be an effective technique for studying beam sensitive Li-ion battery cathode materials [4, 5], we observed that voids and cracks can be formed before cycling. This crack formation might be attributed to I) the growth twins and their interactions with each other at their intersections and II) the Focused Ion Beam (FIB) related artifacts and therefore, more results and analyses are needed to confirm these observations. Figure 1 shows the morphology of a cathode material powder particle acquired with the Hitachi SU8230 FESEM at 5 kV. As can be seen, the powder particles are almost irregular in shape (Figures 1a and b) which can be related to the presence of twins. Also, in figure 1c, the growth twinning can be seen at relatively low magnification. The most important difference in morphology of growth twin in comparison to deformation and annealing twins is that the growth twin is mostly propagated across the whole surface of the bulk sample. Figure 2 presents the obtained bright-field (BF) and dark-field (DF) images of the powder particle (for which the TEM lamella prepared by the NX-5000 Triple Beam FIB) with the Hitachi SU-9000 dedicated STEM at low voltage of 30 kV and different magnifications. Twin boundaries with different orientations can be observed in this figure. The intersection of these boundaries can be expected and hence, these twin boundaries are able to interact with each other, as illustrated. The twin boundaries/twin boundaries interaction may cause the local strain and therefore, voids and wedge-shaped cracks can be frequently seen in the pristine powder material. These observations might show that the crack formation does not necessarily need the electrochemical cycling-induced strain and twinning interaction can be possibly another mechanism for crack formation. a-c) SE images showing the morphology of a pristine cathode material powder particle at different magnifications. LV-STEM images of a pristine cathode material powder particle illustrating growth twins (yellow arrows), cracks (red arrows), and voids (orange arrows): a) BF mode, b and c) DF and BF modes of the green section at higher magnification, respectively.