Supporting Information Inter-granular cracking as a major cause of longterm capacity fading of layered cathodes Hao Liu, Mark Wolf, Khim Karki, Young-Sang Yu,#, Eric A. Stach, Jordi Cabana, Karena W. Chapman *, Peter J. Chupas * X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL, 60439, United States. Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States. Center for Function Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973-5000, United States. # Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. Science Directorate, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL, 60439, United States. correspondence to: chapmank@aps.anl.gov, chupas@aps.anl.gov 1
Experimental Electrode preparation The NCA composite electrode was prepared from a mixture of 60 wt% NCA (Toda), 20 wt% PTFE (Sigma Aldrich), 10 wt% graphite (Alfa Aesar) and 10 wt% carbon black (Vulcan). The mixture was mixed and ground in a mortar and pestle. To make an electrode pellet, ~20 mg of the mixture was loaded in a 10 mm pellet die and compressed at ¼ Ton. Battery preparation and electrochemical cycling The design of the operando battery is described in reference 1. The flat gasket was replaced with a PTFE o-ring to ensure long-term cycling durability. The operando battery cell was assembled in an Ar-filled glovebox with the NCA composite electrode as the cathode, lithium metal foil as the anode, glass fiber as the separator and LiPF 6 in ethylene carbonate and dimethyl carbonate solution as the electrolyte. Electrochemical cycling of the operando battery cell was performed on a Maccor 4300 battery cycler. For the operando battery examined during the 92 nd and the 93 rd cycles, a rest period was imposed between cycle 63 and 64, cycle 71 and 72 and cycle 91 and 92. The battery was cycled at a rate of 14 mah/g (~C/20) between 2.7 and 4.5 V. A second battery was used for operando studies during the 1 st and the 2 nd cycles. The battery was cycled at a rate of 18.6 mah/g (~C/15) between 2.7 and 4.5 V while X-ray diffraction patterns were collected. To decouple the mechanical and chemical contributions to the capacity loss of NCA, coin cells prepared following the same protocol as described above were cycled to the same 2
upper cut-off voltage at 4.5V but to different lower cut-off voltages at 4.25V, 4.05V and 2.5V. An initial formation cycle was performed at C/20 (14 mah/g) between 2.5-4.05V, followed by cycling within specific voltage windows. Cycling within each voltage subrange (2.5-4.05V, 4.05-4.25, and 4.25-4.5V) induces approximately the same amount of volume change ( Vol/Vol = 2%). The current rates were dependent on the voltage range: C/10 (28 ma/g) for 2.5 4.05 V, C/50 (5.6 ma/g) for 4.05 4.25 V, and C/100 (2.8 ma/g) for 4.25 4.5 V. This ensures that the rate of volume change of NCA is constant throughout the entire voltage window (2.5 4.5 V). A full charge and discharge between 2.5 4.5V was conducted to check the full discharge capacity at specific cycles as described in the text; each full cycle was followed by a rest (open circuit) period of at least 24 hours before continuing cycling in limited voltage ranges. Operando XRD X-ray diffraction data were collected at the Advanced Photon Source at Argonne National Laboratory using beamline 11-ID-B (λ = 0.2114 Å), equipped with an amorphous silicon-based area detector from Perkin-Elmer. The detector was positioned at a distance of 95 cm from the sample. CeO 2 powder was used to calibrate the detector tilt and the sample-to-detector distance. Data reduction was performed with FIT2D software 2. Rietveld refinement Rietveld refinement of NCA was performed in TOPAS v5 software package. The sum of the site occupancy factors Ni in the transition metal layer and the Li layer was constrained to 0.8. Isotropic atomic displacement parameters (ADPs) of Li and O are 3
fixed to 1 Å 2 ; identical anisotropic ADPs were used for Ni, Co and Al and refined. Anisotropic peak broadening profile 3 was used. Scanning Electron Microscopy Scanning electron microscopy (SEM) images of cycled NCA meatballs were collected at Brookhaven National Laboratory using a field emission SEM (JEOL 7600F) at 15 kv. Transmission X-ray Microscopy Full-field transmission X-ray tomography was performed at beamline 6-2c at SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA (USA). The incoming beam energy was set well above the white line of absorption at the Ni K-edge; specific values were 8355 ev and 8383 ev for cycled and pristine samples, respectively. Reliable contrast can be achieved with any energy above an absorption edge, so this difference bore no consequence on the morphological analysis performed with the images. The cathode material was recovered from the NCA electrode charged to 4.5V in the 94th cycle (2.7-4.5V, C/20), packed into a glass capillary and mounted in the X-ray microscope. Projection micrographs were then collected between -90 and +90 in 0.5 increments with an exposure time of 0.5 seconds. Five particles were capture in the field of view for both pristine and cycle samples. Iterative alignment and reconstruction via filtered back projection with the Shepp-Logan filter were performed using the TXM- Wizard application 4. Virtual slices of the tomogram were subsequently produced to observe the internal microstructure of the particles. Single peak analysis 4
The experimentally measured intensity profile of a Bragg peak, I(2θ), is determined by the convolution of the instrument profile, G(2θ), and the sample s broadening function, W(2θ): 2 = 2 2. (1) The sample s broadening function is a convolution of a symmetrically broadened profile due to crystal size, f size (2θ), and a profile due to lattice parameter, i.e. d-spacing, distribution, f d (2θ). 2 = 2 2 (2) Accordingly, the population density distribution of the d-spacing of the corresponding Bragg peak can be obtained by deconvolution. The convolution of the instrument profile and profile due to the NCA crystal size, i.e. 2 2, is given by the profile at the pristine state, which does not exhibit substantial broadening due to macrostrain or lattice parameter distribution. This profile is used in the decovolution to obtain f d (2θ) of the (113) reflection during extended cycling. Deconvolution was performed using the Richardson-Lucy algorithm 5, 6 as implemented in Python. f d (2θ) can be converted to a function of Li composition, x, via the relationship between 2θ and x given in Figure S9a. Because the (113) peak intensity changes with 2θ, hence x, the Li composition distribution is given by = (3) where I 113 (x) is the integrated intensity of (113) at different Li composition, i.e. 2θ, given in Figure S9b. 5
The average Li composition, X, is thus given by = (4) 6
Determination of the nature of the asymmetric peak shape and peak splitting during 92 nd and 93 rd cycles In addition to compositional variation, stacking faults can also lead to asymmetric peak profile. However, stacking faults only affect certain class of reflections, whereas peak asymmetry is observed for all peaks in this work, and cannot explain the peak splitting. Continuous phase transition from a high to a low symmetry phase can occur with peak splitting. For example, transition from the rhombohedral to the monoclinic phase could explain the splitting of the (hkl) reflections, where h and k cannot be 0 simultaneously, but cannot account for the splitting of the (00l) reflection. Therefore, the deviation from the symmetric peak profile can only be attributed to compositional variation of the same phase. The length-scale of the phase segregation Phase segregation can occur (i) within primary particles, (ii) between secondary particles and (iii) within secondary particles, which are schematically illustrated in Figure S10. The (113) reflection splits into two peaks during the 92 nd charge and can be modelled by two separate peaks between 3.79 V and 4.26 V. The two separate peaks represent two NCA populations with different lattice parameters hence the Li composition. The peak having a consistently higher intensity is denoted as Peak A, and the other as Peak B. When phase segregation occurs within primary particles, the domain corresponding to each phase will become smaller than the size of the primary particle. Reduction in the domain size will contribute to additional broadening to the peak width according to the 7
Scherrer equation. As shown in Figure S11a, the width of Peak A evolves in the same way as a single peak during the 2 nd charge, where no phase segregation occurs and serves as a baseline for the peak width evolution during single-phase reaction. Because no additional broadening is observed for Peak A, the phase segregation could not have occurred between primary particles. The increase in the width of Peak B is attributed to the development of a wider distribution of Li composition for the NCA population modelled by Peak B. Phase segregation between secondary particles can be induced as a result of a reaction gradient in the electrode. The reaction gradient changes during cycling and will lead to changes in the relative intensity of Peak A and Peak B. As shown in Figure S11b, the integrated intensity of Peak A and Peak B remains effectively constant when charging from 3.79 V to 4.26 V, during which period the average Li composition changes by ~0.3. This indicates that a stagnant boundary exists between the two NCA populations, which is inconsistent with the reaction gradient hypothesis. A reaction gradient will also lead to an even distribution of Li composition, which is inconsistent with the observation of a bimodal distribution of Li composition manifested by the peak splitting. Hence, phase segregation cannot occur between secondary particles. By eliminating phase segregation within primary particles and between secondary particles, we conclude that phase segregation can occur only within secondary particles. 8
Figure S1. Rietveld refinement profiles for operando XRD pattern collected at the start of (a) the 1 st, (b) 2 nd and (c) 92 nd cycles. Reflections corresponding to PTFE and graphite are indicated by * and #, respectively. 2θ ranges of 4.80 4.92, 6.85 7.00, 8.42 8.52, 9.8 9.9 and 10.4 10.55 were excluded from refinement as they correspond to positions of the Li metal peaks, which can no longer be completely masked before data reduction. 9
Figure S2. The normalized phase fraction of Li x Ni 0.8 Co 0.15 Al 0.05 O 2 (NCA) at the start of different charge-discharge cycles (2.7 4.5 V). 10
Figure S3. The scanning electron micrograph of a secondary NCA particle charged to 4.5V in cycle 94. No apparent fracture in the secondary or the primary particle is observed. 11
Figure S4. Stack plot of select reflections during the 1 st and the 2 nd cycles. 12
Figure S5. Stack plot of select reflections during the 92 nd and the 93 rd cycles. 13
Figure S6. Peak position of select reflections as a function of time during the first 2 cycles at a current rate of 19 ma/g (C/15). 14
Figure S7. Stack plot of the population density distribution of Li composition during the 92 nd and the 93 rd cycles. The bimodal distribution indicates two groups of NCA population with different reaction kinetics. 15
Figure S8. Select reflections at the top of charge (4.5V) during the 92 nd cycle (blue curve) and after >10 hours of relaxation after charged to 4.5V on the 94 th cycle (red curve). 16
Figure S9. Relationship between Li composition and the position and intensity of the (113) reflection. (a) The Li composition and (b) the integrated peak intensity of (113) reflection as a function of scattering angle (2θ). 17
Figure S10. Schematic drawing of three different phase segregation scenarios. (a) Phase segregation within primary particles. (b) Phase segregation between secondary particles. (c) Phase segregation within secondary particles. 18
Figure S11. Evolution of the full width at half maximum (FWHM) and the intensity of the (113) reflection. (a) The FWHM of the two peaks used to model the splitting of the (113) reflection as a function of the scattering angle during the 92 nd charge between 3.79 V and 4.26 V. The FWHM of the (113) reflection during the 2 nd charge is shown as a reference for single phase evolution. Fitting profiles at 3.79 V and 4.26 V on the 92 nd charge are shown on the right. (b) The integrated intensity of Peak A (blue dots) and Peak B (red dots) as a function of time during the 92 nd charge. The voltage profile is shown as red line. The ratio between the areas of Peak A and B indicates the sluggish population constitutes nearly 40% of the entire population. 19
Figure S12. Voltage (vs. Li) and specific charge capacity of NCA as a function of the volume change relative to pristine NCA. Data obtained from an operando XRD study of NCA during the 2nd charge at C/20. 20
Table S1. The amount of Li-Ni anti-site mixing after different number of cycles. Numbers in parentheses indicate errors in last digits. Cycle number Pristine Start of 2 nd charge Start of 92 nd charge Li-Ni mixing 0.014(2) 0.012(2) 0.012(2) References 1. Borkiewicz, O. J.; Shyam, B.; Wiaderek, K. M.; Kurtz, C.; Chupas, P. J.; Chapman, K. W. J. Appl. Crystallogr. 2012, 45, 1261-1269. 2. Hammersley, A. P. J. Appl. Crystallogr. 2016, 49, 646-652. 3. Stephens, P. W. J. Appl. Crystallogr. 1999, 32, 281-289. 4. Liu, Y.; Meirer, F.; Williams, P. A.; Wang, J.; Andrews, J. C.; Pianetta, P. Journal of Synchrotron Radiation 2012, 19, 281-287. 5. Lucy, L. B. Astron. J. 1974, 79, 745. 6. Richardson, W. H. J. Opt. Soc. Am. 1972, 62, 55. 21