Supporting Information. Rock-salt Growth Induced (003) Cracking in Layered Positive Electrode for Li-ion Batteries

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1 Supporting Information Rock-salt Growth Induced (003) Cracking in Layered Positive Electrode for Li-ion Batteries Hanlei Zhang 1, 2, Fredric Omenya 2, Pengfei Yan 3, Langli Luo 3, M. Stanley Whittingham 2, Chongmin Wang 3, Guangwen Zhou 1 1. Materials Science & Engineering Program and Mechanical Department, State University of New York, Binghamton, New York 13902, USA 2. NorthEast Center for Chemical Energy Storage, State University of New York, Binghamton, New York 13902, USA 3. Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA Methods Sample preparation. Pristine NCA material was obtained from TODA America Inc., which has a nominated formula of LiNi 0.80 Co 0.15 Al 0.05 O 2. The NCA material was prepared into positive electrode by mixing the active material, carbon black and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1, using the N-methyl-2- pyrrolidone solvent. The positive electrode with 3-5 mg of the active material was assembled in 2325 type coin cells in a glovebox filled with helium. The electrolyte was 1 M LiPF 6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), with a volume ratio of 1:1. The coin cells were cycled with a MPG2 multichannel potentiostat (Biologic). Galvanostatic cycling was performed at a rate of C/10 with a current density of 15 ma/g. The samples were electrochemically cycled between 3.0 V and 4.3 V: 1-cycle and 30-cycle samples were prepared. TEM specimen preparation and characterization. TEM specimens were prepared using a Reichert-Jung Ultracut E ultra-microtome machine. The NCA positive electrode S1

2 (pristine and cycled) were mixed with white resin, pulled into a bar-shaped model and settled at 60 C for 24 h to solidify. Then the resin bars were cut into slices with a nominal thickness of ~ 50 nm using the ultra-microtome machine. The slices are put on TEM copper grids coated with a lacey carbon film for TEM observation. TEM observation was performed using an FEI Titan environmental microscope with a field emission gun (FEG) and an image aberration corrector, operated at an acceleration voltage of 300 kv. High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) observation was performed using an FEI Titan microscope with an FEG and a probe aberration corrector, operated at an acceleration voltage of 300 kv. To exclude possible artifacts from the mechanical damage effect caused by the microtomy in the TEM specimens, the TEM results obtained from the microtomysectioned specimen were confirmed with the specimen prepared with the focus ion beam (FIB) method, as shown in Fig. S8. To exclude possible artifacts from the damaging effect of the electron beam, layered phase was kept in the electron beam at a magnification of over 300k, which shows no structural change in 1 minute (Fig. S11 and supplementary Movie_1), confirming that short-term e-beam illumination does not induce structural damage. In our experiments, the exposure time of sample to e-beam was carefully minimized by adjusting the imaging condition in one area and then moving to neighboring, fresh areas for HRTEM imaging, which takes only a few seconds. S2

3 Fig. S1: (a) Low magnification STEM view showing agglomerated pristine primary particles. (b) Magnified STEM view of a pristine primary particle intact, with smooth surfaces. (c) Selected area electron diffraction (SAED) pattern of the primary particle shown in (b). Only one set of the [100] diffraction of the layered phase is observed without any other diffractions, proving that the particle in (b) is a single crystal of the layered phase. Fig. S1(a) is an STEM view of agglomerated pristine NCA particles. The size of the primary particles varies in a large range (~100 nm to ~5 µm), and the morphologies of the particles are basically intact. Fig. S1(b) is a magnified STEM image showing a representative primary particle of ~1.5 µm. Fig. S1(c) presents the electron diffraction pattern from the primary particle in Fig. S1(b), confirming that it is a single crystal with the R3 m layered structure. No defects or cracking are observed in Fig. S1(b) and the surface remains smooth and faceted, confirming that the primary particle has an intact structure. S3

4 Fig. S2: (a) Simulated electron diffraction pattern of the rock-salt phase (using the structure of NiO 1 ) at the [12 1] zone axis, which is present in the experimental diffraction patterns in Figs. 1(d, e, f, h). (b) Simulated diffraction pattern of the layered phase (based on the layered structure of NCA with the nominated composition of LiNi 0.80 Co 0.15 Al 0.05 O 2 2 ) at the [22 1] zone axis, which is present in the experimental diffraction patterns in Figs. 1(e, f, h). Compared with the [12 1] diffraction pattern of the rock-salt phase in (a), this pattern is very similar in shape but with the presence of extra diffraction spots (highlighted with an orange arrow) which do not show up in (a). The extra spots are hereby useful indicators for the layered rock-salt phase transformation. (c) Simulated diffraction pattern of the rock-salt phase (based on the structure of NiO 1 ) at the [12 0] zone axis, which matches with the experimental diffraction pattern in Fig. 1(g). Fig. 1(d) matches with Fig. S2(a), proving that region 1 in Fig. 1(a) is the rock-salt phase. Fig. 1(g) matches with Fig. S2(c), proving that region 4 in Fig. 1(a) it is also the rock-salt phase. Fig. 1(e, f, h) are combinations of the rock-salt diffraction (Fig. S2(a)) and the layered diffraction (Fig. S2(b)), so regions 2, 3 and 5 contain both the layered and the rock-salt phases. The only difference between Figs. S2(a, b) is the presence of the extra spots highlighted with an orange arrow in Fig. S2(b), while the rest spots of Fig. S4

5 S2(a, b) are overlapping. Therefore, the extra spots in Fig. S2(b) are useful indicators for the presence of the layered phase. S5

6 Fig. S3: (a) Diffractogram and (b) the corresponding diffraction simulation of the unfractured rocksalt platelet in Fig. 2(b), indicating that (a) is the [110] diffraction of the rock-salt phase. The platelet structure in Fig. 2(b) is thus confirmed to be the rock-salt phase and the layered/rock-salt interface corresponds to the [1 11] spot in (b), meaning that the interface is along the (111)-type plane of the rock-salt phase, and thereby also parallel to the (003) plane of the layered phase 3. (c) Diffractogram and (d) the corresponding diffraction simulation from the region above the rock-salt platelet in Fig. 2(b), proving that the region next to the rock-salt platelet remains as the layered phase. S6

7 Fig. S4: (a) TEM view of a (003) plate-shape fragment formed on the surface of a 1-cycle particle. The crack has only partially developed so the fragment is still dangling on the bulk of the particle. (b) Zoom-in view showing the tip of the fragment in (a) (region 1), which is fully off the bulk. (c) Zoom-in view showing the mid-part of the fragment (region 2), which is partially off the bulk. (d) Zoom-in view showing the end of the fragment (region 3), which is closely attached to the bulk. The boundary between the fragment and the bulk is highlighted with a red arrow, which is determined to be a rocksalt platelet in Figs. 2 and 3. Fig. S4(a) presents the TEM view of a platelet-like (003) fragment (highlighted by a red arrow) which is peeling off the surface of a 1-cycle particle. The crack has only partially developed so the fragment is still dangling on the particle. Fig. S4(b) is a zoomin view from the tip of the fragment (position 1 in Fig. S4(a)), showing that this part of the fragment is completely off the bulk. Fig. S4(c) is a magnified view from the mid-part of the fragment (position 2). Since the crack has partially propagated to this point, the fragment is partially attached to and partially off the bulk. Fig. S4(d) is a zoom-in view showing the end of the fragment (position 3). The crack has not developed to this site so the fragment is still attached to the parent particle. A planar structure is present between the fragment and the bulk (highlighted with a red arrow), which is proved to be S7

8 a rock-salt platelet in Figs. 2 and 3. Therefore, we can conclude that a (003) rock-salt platelet forms in the primary particle and progressively develops into a (003) crack. The (003) crack, nucleating in the (003) rock-salt platelet, initiates at one end, progressively develops along the (003) platelet, and finally peels the fragment off the bulk particle. S8

9 Fig. S5: (a) Magnified HRTEM view of the rock-salt phase in Fig. 3(c). (b) Zoom-out HRTEM view of the unfractured region of the developing crack in Fig. 3(a), which is a long, coherent rocksalt/layered interface, proving the presence of a long (003) rock-salt platelet with the length of over 500 nm in front of the crack tip. (c, d) HRTEM views of two other rock-salt/layered interfaces in front of the crack tips of developing (003) cracks in 1-cycle particles, other than the one analyzed in Fig. 3(a-c). The presence of the rock-salt/layered interfaces in (c, d) confirms that a rock-salt platelet is present in front of a developing (003)-type crack, severing as the nucleus for the cracking. This also confirms that the rock-salt induced (003) cracking is a popular mechanism rather than an uncommon case. (e) HRTEM view of a stepped structure formed on the (003) surface of a 1-cycle primary particle, similar to the stepped structure shown in Fig. 3(g). (f) Zoom-out STEM view of another (003) stepped structure on a 30-cycle particle. The presence of (e, f) proves that the stepped structure is a universal phenomenon rather than a random case. S9

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11 Fig. S6: Schematics showing the concentration of tensile strain in the surface region of the rock-salt platelet upon delithiation. (a) Rock-salt platelet formed on the (003) surface of the primary particle with the layered structure, which is bent upon delithiation. (b) schematic showing the formation of the rock-salt platelet in the bulk of the primary particle via the combination of two rock-salt platelets formed on the (003) surface. (c) Schematic showing the gap of in rock-salt platelet formed upon the combination of two half rock-salt platelets. (d) Schematic showing the concentrated tensile strain in the surface region of the rock-salt platelet generated due to the closing of the gap shown in (c). S11

12 According to the (111) R //(003) L rock-salt/layered interfacial relationship 3, the (110) d-spacing of the layered phase corresponds to the (220) d-spacing of the rock-salt phase. The (220) d-spacing of the rock-salt phase is about 1.45 Å 1, 4, similar to the (110) d-spacing of the lithiated layered phase (1.41 Å) 2. Upon delithiation, layered phase undergoes a retraction of a-axis of ~ 0.5%, applying in-plane strain onto the coherent rock-salt phase. To explain the concentrated tensile strain in the surface region of the rock-salt platelet as mentioned in Fig. 5 (a), firstly we consider the strain condition in a rock-salt platelet formed on the surface (Fig. S6(a)). The layered phase goes through lattice retraction upon delithiation. The rock-salt platelet is electrochemically inactive, so its lattice parameter remains unchanged during this process. Accordingly, the layered phase closer to the rock-salt platelet undergoes less retraction due to the interfacial coherency, while the layered phase away from the rock-salt platelet goes through more retraction, leading to the bending of the rock-salt platelet, as schematically shown in Fig. S6(a). The rock-salt platelet in the bulk of the particle can be considered as the combination of two surface rock-salt platelets, as schematically shown in Fig. S6(b). Since the bending in the surface region of the rock-salt platelet is much more significant compared with the deeper region, upon the combination of the two symmetric parts a gap will be present in the surface region of the rock-salt platelet, as shown in Fig. S6(c). To form a complete rock-salt platelet, this gap has to be forced together by dragging the surface regions of the two half rock-salt platelets together, as schematically demonstrated in Fig. S6(d). The layered phase in contact with the rock-salt platelet will S12

13 drag the rock-salt platelet to return to the gapped structure in Fig. S6(c), meaning a tensile strain field is generated in the rock-salt platelet. Since the gaping is the largest at the surface region of the rock-salt platelet, the tensile strain is also the largest at this point, namely a concentration of tensile strain in the surface region of the rock-salt platelet. Upon lithiation, the bending of the layered phase is restored to the flat status. The dragging force on the rock-salt platelet thus disappears and so does the concentrated tensile stress in Fig. S6(d). Electrochemical cycling leads to cyclic lithiation and delithiation of the particle, applying cyclic tensile strain onto the rock-salt platelet. S13

14 Fig. S7: (a, b) Diffractogram of Fig. 6(c) and the corresponding electron diffraction simulation, indicating that the diffractogram is the [110] diffraction of the rock-salt phase. The [11 1] spot corresponds to the facet at the crack tip in Fig. 6(c), demonstrating that the facet is along a {111} - type plane. The [002 ] spot corresponds to the etching facet in Fig. 6(c), demonstrating that the facet is along a {002} -type plane. (c) Diffractogram from the orange twin as marked in Fig. 6(e), demonstrating that the twin is the rock-salt phase at the [110] orientation. (d) Diffractogram across the twin boundary in Fig. 6(e), in which two symmetric diffraction patterns are present, confirming the twining structure in Fig. 6(e). The twin boundary is confirmed to be the (111)-type plane. S14

15 Fig. S8: 1-cycle material which is prepared into a TEM sample using the focus ion beam (FIB) method, showing the same (003) cracking phenomenon as the samples prepared using the microtomy method, confirming that the (003) cracking in this work is not generated by the mechanical effect of microtomy. (a) Primary particle with a (003) crack across the particle. (b) Another primary particle with a (003) crack. (c) Magnified view of the (003) crack in (b). (d) Primary particle with a step structure on the (003) surface. (e) Magnified view of the step structure in (d). S15

16 Fig. S9: Representatives of the zoom-out STEM images used to calculate the percentage of cracked particles in the 1-cycle sample. Analysis of the cycled samples prepared by microtome indicates that 3.0% of the 1- cycle material have developed (003) cracking (Fig. S9). In the FIBed 1-cycle sample (Fig. S8), a similar percentage of particles have developed (003) cracking. Apparently, the number of cracked particles is not increased by the microtome method. The absence of microtoming-induced (003) cracking is also supported from the comparison with the microtome-sliced pristine material. TEM analysis of pristine particles prepared by the same microtome process indicates they have the typical layered phase without any cracking and rock-salt platelets in the bulk. S16

17 Fig. S10: Representatives of the zoom-out STEM images used to calculate the percentage of cracked particles in the 30-cycle sample. S17

18 Fig. S11: (a, c) In-situ HRTEM images and (b, d) the corresponding diffractograms showing the stability of the layered phase under electron beam for one minute. In (b, d), the diffractions from the layered phase are marked out with orange rectangles, which does not change during the one-minute beam shower, suggesting that there is no significant phase transformation during the one-minute beam shower. S18

19 Supporting References (1) Sasaki, S.; Fujino, K.; Takéuchi, Y., X-ray Determination of Electron-density Distributions in Oxides, MgO, MnO, CoO, and NiO, and Atomic Scattering Factors of Their Constituent atoms. Proc. Jpn. Acad., Ser. B 1979, 55, (2) Mori, D.; Kobayashi, H.; Shikano, M.; Nitani, H.; Kageyama, H.; Koike, S.; Sakaebe, H.; Tatsumi, K. Bulk and Surface Structure Investigation for the Positive Electrodes of Degraded Lithium-ion Cell after Storage Test Using X-ray Absorption Near-edge Structure Measurement. J. Power Sources 2009, 189, (3) Jung, S. K.; Gwon, H.; Hong, J.; Park, K. Y.; Seo, D. H.; Kim, H.; Hyun, J.; Yang, W.; Kang, K. Understanding the Degradation Mechanisms of LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Material in Lithium Ion Batteries. Adv. Energy Mater. 2014, 4, (4) Goodenough, J.; Wickham, D.; Croft, W. Some Magnetic and Crystallographic Properties of the System Li + xni xNi +++ xo. J. Phys. Chem. Solids 1958, 5, S19