Insights into Mg 2+ Intercalation in a Zero-Strain Material: Thiospinel Mg x Zr 2 S 4

Transcription

1 Supporting Information for Publication Insights into Mg 2+ Intercalation in a Zero-Strain Material: Thiospinel Mg x Zr 2 S 4 Patrick Bonnick, a Lauren Blanc, a Shahrzad Hosseini Vajargah, a Chang-Wook Lee, b Xiaoqi Sun, a Mahalingam Balasubramanian b and Linda F. Nazar a * a) Department of Chemistry and the Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada b) Argonne National Laboratory, 9700 S Cass Ave, Lemont, Illinois, 60439, United States - These authors contributed equally to this work. * lfnazar@uwaterloo.ca 1

2 Extended discussion of the formation of two-phases above x = 0.5 in MgxZr2S4 As discussed previously, 1 it is unclear if slow diffusion is caused by tetrahedral site occupation at higher Mg 2+ concentrations, or vice versa. Regardless, tetrahedral occupation and slow diffusion are coincident at about x ~ 0.5 in MgxZr2S4. Although the Mg 2+ occupation of tetrahedral sites has been observed in thiospinel Ti2S4, 1,2 the stark appearance of separate phases in MgxZr2S4 is novel. A larger XRD peak separation is achieved in this system because Phase 1 is zero-strain and thus its XRD reflections are invariant, while Phase 2 grows in size, so that its XRD reflections are shifted far enough from the Phase 1 peaks to resolve both sets. In the case of MgxTi2S4, the lattice expanded upon octahedral or tetrahedral site occupation, although tetrahedral site occupation caused a larger lattice expansion than octahedral site occupation (Figure S8). If two phases do exist in MgxTi2S4, then the lattice parameters of the two phases do not separate enough to resolve the two sets of XRD peaks. Alternatively, it is possible that the distribution of Mg[tet] is moderately inhomogeneous in MgxTi2S4, preventing a 2 nd phase from forming and having a large enough domain to yield strong XRD reflections. A combination of thermodynamics and kinetics controls the ratio of Phase 1 to Phase 2, as well as their occupancies. Figure 7c depicts the effects of discharging a cell very slowly or resting a cell after discharge. Table S3d contains the Rietveld refinement results from those experiments while Figure 8 depicts those results pictorially. By comparing the non-equilibrated to the equilibrated materials (Figure 8a to b), it is evident that when allowed to approach equilibrium, the Mg 2+ ions in Phase 2 do not redistribute themselves into octahedral sites, suggesting that the ~ Mg[oct]0.54Mg[tet]0.40 distribution is thermodynamically stable. Note that since about 0.06 Cu remains in tetrahedral sites in the structure, this brings the total occupancy to about 1.00 (i.e.[oct]0.54[tet]0.46). As such, any further increase in the concentration of Mg 2+ in Phase 2 would require Zr 3+ to reduce to Zr 2+, which is highly unlikely. Intriguingly, during the rest period, Phase 1 expanded despite containing a Mg 2+ concentration only about half that of Phase 2, which requires the Mg 2+ concentration in Phase 2 to increase from = 0.90 to = Although the uncertainties in the non-equilibrated and equilibrated [oct] and [tet] occupations of Phase 2 are large enough to imply that the occupations did not change during the rest period, the amount of Phase 1 increased beyond the uncertainty range and a striking 2

3 difference between the non-equilibrated and equilibrated samples is evident in Figure 7c. Since the total amount of Mg 2+ in the whole system is constant in this experiment, an increase in the volume of Phase 1 with no accompanying increase in Mg 2+ concentration requires a simultaneous increase in the Mg 2+ concentration in Phase 2. Such behavior suggests that the occupancy of 1.00 in Phase 2 (i.e. Mg[oct]0.54Mg[tet]0.40) is more stable and Mg 2+ ions concentrate into Phase 2 to reach that maximum, expanding Phase 1 as a consequence. The stability of Phase 2 might be further enhanced by its freedom to expand, in comparison with Phase 1 which is zero-strain. The internal dynamics can be elucidated further by examining the behavior of the two phases upon slow discharge, as shown in Figure 8c and d. The rate of discharge determines the maximum occupancy of Phase 1. We hypothesize that a lower discharge current creates a shallower Mg 2+ concentration gradient in the MgxZr2S4 particle (from surface to core), which allows the core to reach a higher Mg 2+ concentration before the surface begins populating tetrahedral sites. Hence, the C/100 discharge rate produces a higher Mg 2+ concentration than C/40 in the Phase 1 core (i.e. 0.9(2) vs 0.75(8), respectively). Similarly, Phase 2 can penetrate deeper into the particle at slower discharge rates because it has more time to do so. Furthermore, these observations suggest that the MgxZr2S4 particles adopt a core-shell distribution for approximately 0.5 < x < 1, with Phase 1 forming the core and Phase 2 the shell. If a total occupancy of 1.00 is energetically favored in the Phase 2 shell, then the surface of the particles will become full above x ~ 0.5 and remain so even after equilibrating the material using a rest period (such as is done in GITT experiments). This allows us to ascribe the sharp drop in Mg 2+ diffusivity in Figure 6 to the surface being full, despite the lightly concentrated core. Experimental Section Rietveld refinement procedure After preparation of the active material (through lithiation and chemical oxidation), 3 secondary agglomerate particle sizes were maintained around 10 μm and the removal of Cu from the thiospinel lattice is indicated by the disappearance of Cu Kα, Kβ, and Lα peaks at 8.0, 8.9, and 0.9 kev respectively in the EDX spectra (Figure S1). Rietveld refinement 4 revealed that the Fd3 m 3

4 lattice size shrank significantly from (1) Å to (2) Å as the amount of Cu decreased from nearly full occupancy to 0.063(6) (Table S3a). We note that EDX, while not highly quantitative, indicated no significant change in Cu content in any of the (dis)charged samples, and hence the Cu occupancy in all subsequent refinements was fixed to to account for any Cu remaining in the lattice. Given the small Cu content, the oxidized compound is referred to as Zr2S4 for simplicity. The refined occupancies for both Zr and S in synthesized CuZr2S4 and oxidized CuZr2S4 (i.e., Zr2S4) were very close to full occupancy. Refined occupancies for Zr were 1.014(9) and 0.99(1) in CuZr2S4 and Zr2S4, respectively, and these refined values were 0.989(8) and 1.01(1) for S. Therefore, the occupancies for Zr and S were fixed at 1.0 for these refinements, as well as for all subsequent refinements once the values were checked for significant deviation from full occupancy. Zr and S full occupancy was verified for each material to ensure that the zero-strain behavior was not due to site mixing within the Zr2S4 lattice. Partially discharged materials were initially refined using fixed values for occupancies and atomic displacement parameters (ADPs) of Cu, Zr, and S taken from the refined values for Zr2S4. After this initial refinement, Fourier mapping was used to locate electron density in order to identify Mg 2+ intercalation sites. In the single phase Mg0.3Zr2S4, electron density was present on 16c sites only, and in the two phase composition Mg0.6Zr2S4, electron density was found on both 16c and 8a sites in the Fourier difference map. Mg 2+ was introduced onto these respective sites, and its occupancy was refined. In the two phase composition Mg0.6Zr2S4, Mg 2+ was initially introduced to 16c and 8a sites in both phases; however, in phase 1, the Mg 2+ occupancy on 8a dropped to zero upon further refinement and was therefore removed from that site (Table S3c). Once these intercalation sites were identified, they were used to refine the Mg 2+ occupancies in fully discharged materials (Table S3d and S3e). In materials that had very low occupancy on a given site, it was often difficult to simultaneously refine the ADP and occupancy (as is well known). In that case, that site s ADP was fixed to a series of reasonable values based on the value from the parent phase and the refinement was repeated to ensure no significant deviation of the refined occupancy and/or the 4

5 goodness of fit. Examples of this procedure are shown for the ADP of Cu (8a) in Zr2S4 (Table S3b) and for Mg (16c) in charged Mg0.1Zr2S4 (Table S3g). XANES analysis of Zr K-edge Table S2 compares the measured difference in XANES edge energies (ΔEXANES, at absorption 0.50) between various x and y values of MgxCuyZr2S4. The Mg and Cu contents were also measured by EDX. By assuming Mg and S have a +2 and -2 oxidation state, respectively, ΔEXANES for the discharge of Zr2S4 to Mg0.76Zr2S4 (0.73 ev) was used to generate a conversion factor of 0.48 ev/[δzr oxidation state]. If the discharged and charged states are used instead, this factor is nearly identical (0.49), while using the oxidized and charged states generates The conversion factor of 0.48 was in turn used to generate ΔEEDX values from the EDX composition measurements for the two possible Cu oxidation states: +1 or +2. Under the assumption that S is not redox active, it is evident from Table S2 that the XANES and EDX ΔE measurements are only self-consistent if Cu exists in a +2 oxidation state. EXAFS analysis of Zr-S and Zr-Zr distances EXAFS fits in Figure S7: For these fits, the Zr S and Zr-Zr coordination numbers were fixed at 6, as required by the spinel structure. An amplitude reduction factor (S0 2 ) of 1.1 for Zr was determined by fitting the EXAFS of synthesized CuZr2S4. Detailed fits show that the mean squared relative displacement (σ 2 ) of the Zr-Zr correlation is smaller in the oxidized state, as expected. Contribution from potential Zr-Cu correlations is not evident in these room temperature studies, suggesting these correlations are highly disordered. The analysis of the Cu EXAFS of CuZr2S4 (Figure S6) is consistent with tetrahedral site occupancy, as shown in Table S3. 5

6 List of Supplementary Tables Table S1a. Refined parameters for synthesized CuZr2S4 and the oxidized material (Zr2S4) space group = Fd3 m Atom Wyckoff x y z Occ. Biso (Å 2 ) CuZr2S4: Cu = 0.950(2), a = (1) Å, χ 2 = 1.58, Bragg R-factor = 2.29 Cu 8a (2) 0.81(7) Zr 16d (4) S 32e (2) (2) (2) (6) Zr2S4: Cu = 0.063(6), a = (2) Å, χ 2 = 2.78, Bragg R-factor = 1.44 Cu 8a (6) 0.81 Zr 16d (5) S 32e (2) (2) (2) (7) Table S1b. Residual Cu occupancies in Zr2S4 refined using different fixed ADPs Biso (Å 2 ) Cu (8a) χ 2 RBragg (5) (6) (6) (6) (6)

7 Table S1c. Refined parameters for Zr2S4 partially discharged at a C/20 rate at 60 C to Mg0.3Zr2S4 (single phase) or Mg0.6Zr2S4 (two separate phases), space group = Fd3 m Atom Wyck. x y z Occ. Biso (Å 2 ) Mg0.3Zr2S4: MgOh = 0.31(5), a = (3) Å, χ 2 = 3.69, Bragg R-factor = 3.03 MgOh 16c (3) 1(2) Cu 8a Zr 16d (4) S 32e (4) (4) (4) (6) Mg0.6Zr2S4: Mgtotal = 0.59(12), χ 2 = 2.04 Phase 1: 57(12)%, MgOh = 0.41(7), a1 = (5) Å, Bragg R-factor = 1.91 MgOh 16c (4) 0.3(8) Cu 8a (1) Zr 16d (6) S 32e (7) (7) (7) (9) Phase 2: 43(11)%, MgOh = 0.5(1), MgTd = 0.28(8), a2 = (2) Å, Bragg R-factor = 2.32 MgOh 16c (5) 0.3(8) MgTd 8a (8) 0(1) Cu 8a (1) Zr 16d (6) S 32e 0.249(2) 0.249(2) 0.249(2) (9) 7

8 Table S1d. Refined parameters for Zr2S4 fully discharged at a C/20 rate at 60 C to Mg0.74(9)Zr2S4 (non-equilibrated) or Mg0.74(4)Zr2S4 (equilibrated), space group = Fd3 m Atom Wyck. x y z Occ. Biso (Å 2 ) Non-equilibrated material: Mgtotal = 0.74(4), χ 2 = 5.35 Phase 1: 43(2)%, MgOh = 0.54(5), a1 = (4) Å, Bragg R-factor = MgOh 16c (3) 1.0(9) Cu 8a (7) Zr 16d (7) S 32e (5) (5) (5) 1 0.6(1) Phase 2, 57(2)%, MgOh = 0.53(4), MgTd = 0.37(4), a2 = (4) Å, Bragg R-factor = 1.19 MgOh 16c (2) 1.0(9) MgTd 8a (4) 0.0(7) Cu 8a (7) Zr 16d (7) S 32e (5) (5) (5) 1 0.6(1) Equilibrated material: Mgtotal = 0.74(9), χ 2 = 3.94 Phase 1: 55(7)%, MgOh = 0.53(6), a1 = (5) Å, Bragg R-factor = MgOh 16c (3) 2.4(1.1) Cu 8a (9) Zr 16d (8) S 32e (6) (6) (6) 1 1.0(2) Phase 2: 45(6)%, MgOh = 0.59(7), MgTd = 0.41(6), a2 = (8) Å, Bragg R-factor = 1.42 MgOh 16c (4) 2.4(1.1) MgTd 8a (6) 1.2(9) Cu 8a (9) Zr 16d (8) S 32e (8) (8) (8) 1 1.0(2) 8

9 Table S1e. Refined parameters for Zr2S4 fully discharged at slow rates at 60 C to Mg0.89(6)Zr2S4 (C/40) or Mg0.93(8)Zr2S4 (C/100), space group = Fd3 m Atom Wyck. x y z Occ. Biso (Å 2 ) C/40: Mgtotal = 0.89(6), χ 2 = 3.60 Phase 1: 32(3)%, MgOh = 0.75(8), a1 = (7) Å, Bragg R-factor = MgOh 16c (4) 1.9(9) Cu 8a (7) Zr 16d (7) S 32e (8) (8) (8) 1 1.2(2) Phase 2: 68(3)%, MgOh = 0.54(4), MgTd = 0.41(4), a2 = (4) Å, Bragg R-factor = 1.14 MgOh 16c (2) 1.9(9) MgTd 8a (4) 1.2(7) Cu 8a (7) Zr 16d (7) S 32e (5) (5) (5) 1 1.2(2) C/100: Mgtotal = 0.93(8), χ 2 = 4.33 Phase 1: 25(3)%, MgOh = 0.9(2), a1 = (2) Å, Bragg R-factor = 1.03 MgOh 16c (8) 2(3) Cu 8a (8) Zr 16d (9) S 32e 0.249(2) 0.249(2) 0.249(2) 1 1.2(2) Phase 2: 75(4)%, MgOh = 0.55(6), MgTd = 0.40(4), a2 = (5) Å, Bragg R-factor = 1.27 MgOh 16c (3) 2(3) MgTd 8a (4) 1.3(8) Cu 8a (8) Zr 16d (9) S 32e (5) (5) (5) 1 1.2(2) 9

10 Table S1f. Refined parameters for the charged material (Mg0.1Zr2S4) after a single cycle; charged at C/20 at 60 C, space group = Fd3 m Atom Wyck. x y z Occ. Biso (Å 2 ) Mg0.1Zr2S4: MgOh = 0.08(6), a = (5) Å, χ 2 = 5.19, Bragg R-factor = 1.73 MgOh 16c (3) 1(2) Cu 8a Zr 16d (8) S 32e (7) (7) (7) 1 0.2(2) Table S1g. MgOh occupancies in the charged material (Mg0.1Zr2S4) refined using different fixed ADPs Biso (Å 2 ) MgOh (16c) χ 2 RBragg (59) (59) (59) (59) (62) Table S2. Analysis of the Zr K-edge XANES data compared to the expected energy differences (ΔE) from EDX composition measurements. a Mg and S are assumed to be in the +2 and -2 oxidation states, respectively, while Cu is compared in both the +1 and +2 states. If Cu is +1 If Cu is +2 Comparison (compositions determined by EDX) ΔE XANES (ev) ΔZr ox. state ΔE EDX (ev) ΔZr ox. state ΔE EDX (ev) CuZr 2S 4 Cu 0.08Zr 2S Cu 0.08Zr 2S 4 Mg 0.76Cu 0.08Zr 2S Mg 0.76Cu 0.08Zr 2S 4 Mg 0.11Cu 0.08Zr 2S Mg 0.11Cu 0.08Zr 2S 4 Cu 0.08Zr 2S Mg 0.76Cu 0.08Zr 2S 4 CuZr 2S a. While EDX is only semi-quantitative, the average residual Cu content of 0.08(1) determined by EDX was considered reliable for this analysis due to its good agreement with 0.063(6) Cu determined by Rietveld refinement of Cu0.063Zr2S4 (see Table S1a). A conversion factor of 0.48 ev/[δzr oxidation state] was determined using the discharge (Cu0.08Zr2S4 Mg0.76Cu0.08Zr2S4) comparison, and was used to calculate ΔEEDX from the ΔZr oxidation state. 10

11 Table S3. Structural parameters extracted from the Cu K-Edge EXAFS analysis; the coordination number (CN), distance (R), and mean-squared relative displacement ( ) are reported. Sample Correlation CN (set) R (Å) R eff (Å) 2 (10-4 Å 2 ) CuZr 2 S 4 (pristine) Cu S ± ± 3.2 Cu S ± ± 12.7 Table S4. Comparison between structural parameters extracted from Zr K-Edge EXAFS analysis and those obtained from Rietveld refinement of synchrotron XRD data. The ratio between Phase 1 and Phase 2 in the discharged material is 43(2):57(2). Sample Lattice parameter, a (Å, Rietveld) RZr-Zr (Å, XRD) RZr-Zr (Å, EXAFS) RZr-S (Å, XRD) RZr-S (Å, EXAFS) Synthesized, CuZr2S (1) (1) 3.71(2) 2.573(2) 2.584(6) Oxidized, Zr2S (2) (5) 3.678(9) 2.564(2) 2.569(6) Ph 1: (4) (2) 2.575(5) Discharged, Ph 2: (4) (2) 2.583(5) Mg0.7Zr2S4 Avg: 10.40(8) 3.7(1) 3.67(2) 2.58(3) 2.583(5) Charged, Mg0.1Zr2S (5) (2) 3.68(1) 2.565(7) 2.570(6) 11

12 List of Supplementary Figures Figure S1: EDX spectra of (a) CuZr2S4 as synthesized and (b) Zr2S4 after oxidation of CuZr2S4; only a small trace of Cu remains in the lattice and this material is referred to as Zr2S4 for brevity. Each spectrum shows three small impurities: hafnium was present as a trace metal in the zirconium used during synthesis, oxygen appears due to air exposure during sample transfer, and carbon tape was used to prepare samples. The insets in each plot show SEM images of the respective material showing an agglomerate particle size of approximately 10 μm, with primary crystallites < 0.5 μm in dimension. 12

13 1.5 Typical soft shortcircuits due to dendrites Cell Potential (V) Time (h) Figure S2. Demonstration of the electrochemical signs normally associated with soft short circuits due to dendrite growth. Soft shorts can be destroyed by the heat from the current passing through them (i.e. they burn out) or through reversing the cell current to dissolve them. The final 3 charge half-cycles are sometimes referred to as resistive shorts, since they are hard (permanent) short-circuits, but have a notable resistance. Mg dendrites are not common, but it is suspected that some methods of creating electrolytes (such as those used here) make them more prone to support dendrite growth. 13

14 Figure S3: XRD Rietveld refinements of materials formed by discharge at C/20 (a) Mg0.3Zr2S4, (b) Mg0.6Zr2S4, (c) nonequilibrated Mg0.7Zr2S4, (d) equilibrated Mg0.7Zr2S4, as well as (e) C/40 MgZr2S4, and (f) C/100 MgZr2S4 that shows experimental data (black crosses), fitted profile (red line), difference map between observed and calculated data (blue line), as well as Bragg positions of phase 1 (green ticks) and phase 2 (purple ticks) 14

15 Figure S4: XRD Rietveld refinement of Mg0.1Zr2S4 formed by charging the fully discharged material at C/20, showing experimental data (black crosses), fitted profile (red line), difference map between observed and calculated data (blue line), and Bragg positions of Fd3 m spinel phase (green ticks) 15

16 Discharged XPS: S 2p Results appear identical Charged S 2p S 2p 34 Name S 2p S 2p Pos FWHM L.Sh. GL(30) GL(30) Area %Area Name S 2p S 2p Pos FWHM L.Sh. GL(30) GL(30) Area %Area CPS x CPS x Binding Energy (ev) Binding Energy (ev) Figure S5. Sulfur 2p XPS spectra of discharged (Mg0.7Zr2S4) and charged (Mg0.1Zr2S4) active material. Since the spectra are identical, sulfur does not experience redox during Mg 2+ intercalation. The following constraints were applied in the fits: Area of 1/2 spin = Area of 3/2 * (Thermo Sci.). Position of 1/2 spin = Position of 3/2 spin ev (Thermo Sci.). a b Figure S6. (a) Copper K-edge XANES comparing CuZr2S4 to CuS and Cu2S. Due to the differing ligands in each compound, the oxidation state of Cu in CuZr2S4 could not be confirmed. (b) Real part of Fourier transformed data and corresponding fit at Cu K-edge EXAFS for pristine CuZr2S4. The dotted lines indicate the range of the fit. 16

17 Figure S7. (a) Raw, isolated EXAFS oscillations (k 2 -weighted) and (b) Magnitude of Fourier transform data for MgxZr2S4. (c) to (f) Real part of Fourier transformed data and corresponding fit for (c) pristine CuZr2S4, (d) oxidized Zr2S4, (e) discharged Mg0.76Zr2S4, and (f) charged Mg0.11Zr2S4. The dotted line indicates the range of the fit. 17

18 Figure S8: Raw GITT data used to generate the DMg coefficients shown in Figure 6 in the main text. The methods for converting from GITT to D are covered in our previous publications. 1,5 18

19 Figure S9. Fd3 m lattice parameter, a, of Mg xti 2S 4 as x increases. Mg 2+ only occupies octahedral sites in the lattice up to about x = 0.6. Above that, Mg 2+ occupies both octahedral and tetrahedral sites, and the lattice grows in size at an increased rate compared to x < References (1) Bonnick, P.; Sun, X.; Lau, K.-C.; Liao, C.; Nazar, L. F. Monovalent versus Divalent Cation Diffusion in Thiospinel Ti2S4. J. Phys. Chem. Lett. 2017, 8, (2) Sun, X.; Bonnick, P.; Duffort, V.; Liu, M.; Rong, Z.; Persson, K. A.; Ceder, G.; Nazar, L. F. A High Capacity Thiospinel Cathode for Mg Batteries. Energy Environ. Sci. 2016, 9, (3) James, A. C. W. P.; Ellis, B.; Goodenough, J. B. Lithium Insertion into the Normal Thiospinel CuZr2S4 and the Defect Thiospinel Cu0.05Zr2S4. Solid State Ion. 1988, 27, (4) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, (5) Talaie, E.; Bonnick, P.; Sun, X.; Pang, Q.; Liang, X.; Nazar, L. F. Methods and Protocols for Electrochemical Energy Storage Materials Research. Chem. Mater. 2017, 29,