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Operando Nano-beam Diffraction to Follow the Decomposition of Individual Li2O2 Grains in a Non-aqueous Li-O2 Battery Swapna Ganapathy, Jouke R. Heringa, Maria S. Anastasaki, Brian D. Adams, Martijn van Hulzen, Shibabrata Basak, Zhaolong Li, Jonathan Paul Wright, Linda F. Nazar, Niels H. van Dijk, and Marnix Wagemaker J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01368 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016

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The Journal of Physical Chemistry Letters

Operando Nano-Beam Diffraction to Follow the Decomposition of Individual Li2O2 Grains in a Non-Aqueous Li-O2 Battery Swapna Ganapathy1, Jouke R. Heringa1, Maria S. Anastasaki1, Brian D. Adams2, Martijn van Hulzen1, Shibabrata Basak3, Zhaolong Li1, Jonathan P. Wright4, Linda F. Nazar2, Niels H. van Dijk1 and Marnix Wagemaker1*

1

Department of Radiation Science and Technology, Delft University of Technology, Mekelweg

15, 2629JB, Delft, The Netherlands 2

Department of Chemistry and the Waterloo Institute for Nanotechnology, University of

Waterloo, Waterloo, Ontario, N2L 3G1, Canada 3

Kavli Institute of Nanoscience Delft, Department of Quantum Nanoscience, Delft University of

Technology, Lorentzweg 1, 2628CJ, Delft, The Netherlands 4

European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex,

France *

[email protected]

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Abstract The intense interest in the Li-O2 battery system over the past five years has led to a much better understanding of the various chemical processes involved in the functioning of this battery system. However, the detailed decomposition of the nano-structured Li2O2 product, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. Here, we report operando nano-beam X-ray diffraction experiments that enables to monitor the decomposition of individual Li2O2 grains in a working Li-O2 battery. Platelet-shaped crystallites with aspect ratio’s between 2.2 and 5.5 decompose preferentially via the more reactive (001) facets. The slow and concurrent decomposition of individual Li2O2 crystallites indicates that the Li2O2 decomposition rate limits the charge time of these Li-O2 batteries, highlighting the importance of using redox mediators in solution to charge Li-O2 batteries. TOC graphic


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With the growing segment in the automotive market occupied by electric vehicles and hybrid electric vehicles, the demand for better energy storage devices capable of delivering higher energy densities is ever increasing. The Li-air - or more accurately the Li-O2 battery shows potential for vehicular application on account of its high theoretical specific energy discharge

 → Li2O2 delivers a theoretical energy density. The overall battery reaction 2Li + O2 ←  charge density of 3500 Wh/kg.1 Initially studied decades ago,2 a renewed interest in this battery system over the last five years1,3,4 has led to huge strides forward in understanding its complex chemistry,5-12 which increasingly leads to strategies that can mitigate3,4,13-15 the issues that impede the functioning of this system.

One involves the reactivity of the porous carbon matrix,

commonly used as a gas diffusion electrode in this system, both with the electrolyte and the Li2O2 discharge product. This reactivity can be eliminated by virtue of using non-carbonaceous gas diffusion electrodes including gold,3 titanium carbide,4 metallic Ti4O7,14 or by coating the carbon substrate with a thin layer of non-carbonaceous material.15 Another important step forward has been to establish the relationship between the electrolyte solvent and the morphology of the formed peroxide.7 The formation and decomposition of Li2O2 has been studied by a host of operando techniques including spectroscopy,16-20 diffraction,21-24 and microscopy techniques,25-28 each providing valuable information about the transformation process. Although operando transmission electron microscopy (TEM) measurements25,26 are an extremely elegant and informative method to study the transformation of individual grains of Li2O2, the artificial nature of the setup involved and the high resistances associated with the use of micro batteries25 do not accurately reflect a representative environment within a realistic Li-O2 battery electrode. On the other hand most spectroscopy and diffraction techniques are relatively easy to perform under operando conditions but often only give average information that hides the behavior of individual 3 ACS Paragon Plus Environment


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electrode grains. Ganapathy et al. used operando X-ray diffraction (XRD) to show that a Li deficient component (i.e., Li2-xO2) is initially formed during the charging process, as a result of Li+ deinsertion.21 This mechanism was first proposed by theoretical studies which showed that topotactic delithiation based on Li2-xO2 is rendered accessible at relatively small overpotentials of 0.3 – 0.4 V.29 Experimental data on individual Li2O2 grain decomposition mechanism and decomposition times in realistic Li-O2 batteries may help understand the high overpotentials and sluggish kinetics restricting Li-O2 battery performance. Here, we report the use of operando nano-beam X-ray diffraction to monitor the decomposition of individual Li2O2 crystallites in a working non-aqueous Li-O2 battery. This technique was recently successfully applied to study the (dis)charge rate dependence of the firstorder phase transformation of individual LiFePO4 particles, a well-known positive Li-ion electrode material.30 When a bright X-ray beam with a sub-micron beam size is used, the diffraction rings break up into spots, each representing an individual nano-sized Li2O2 crystallite. This allows one to monitor the individual crystallite transformation of many crystallites in parallel in a realistic electrochemical environment, thereby providing both local as well as statistical information over many crystallites. Two types of Li2O2 have been studied; a) electrochemically generated Li2O2 toroids comprised of Li2O2 platelets31 formed at a discharge current density of 65 µA/cm2 (E-Li2O2, supporting information Figure S1) and b) commercial Li2O2 that was incorporated into a carbon electrode32 (C-Li2O2). The most obvious difference between the two types of Li2O2 is the shape and size of the primary crystallites as discussed below. The time-resolved information obtained by nano-beam diffraction on the transformation of a number of individual Li2O2 crystallites in both E-Li2O2 and C-Li2O2, makes it possible to study the difference in decomposition mechanism and decomposition rate. A schematic picture 4 ACS Paragon Plus Environment

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of the experimental setup is given in Figure 1, showing the high energy X-ray beam passing through the battery stack consisting of a Li-metal negative electrode, electrolyte soaked separator, and carbon cathode on which the E-Li2O2 (C-Li2O2) is embedded. The cell is continuously rotated over an angular range of 7.5° in a direction normal to the X-ray beam in order to satisfy the Bragg condition for diffraction for a large number of illuminated grains. Only when the incoming and diffracted beams both correspond to the Bragg angle of a specific crystal plane an individual reflection will appear on the 2D detector.



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Figure 1. Top: Schematic picture of the operando synchrotron X-ray diffraction experiment on the non-aqueous Li-O2 battery. During the exposure time, the working battery was continuously rotated around its vertical axis. Bottom: Voltage curve corresponding to the Oxygen Evolution Reaction (OER) recorded during operando synchrotron X-ray diffraction using a current density of 100 µA/cm2, and the corresponding evolution of a 2D (100) reflection of a Li2O2 crystallite during its decomposition. The pictures on the 2D detector cover a range in reciprocal space of

∆Q = 0.14 Å-1 in the horizontal and vertical direction around the reflection. To determine the total transformation time, as well as the phase fractions, a powder average over all the angular steps in a total ω scan covering 7.5° was made and then normalized to account for variations in beam flux. A contour plot of the average intensity of the {101} and {110} powder reflections as a function of charge time is shown in Supporting Figure S3. It is observed that for the E-Li2O2 sample the Li2O2 phase is completely decomposed in about 10.5 h, whereas for the C-Li2O2 sample the Li2O2 phase is not completely decomposed at the end of the measurement. Even so, for the C-Li2O2 sample a linear decrease in diffracted intensity is only observed during the first 7 h, after which the average peak intensity remains more or less constant. This implies that only a fraction of the crystallites illuminated by the X-ray beam transforms/decomposes, while a significant fraction of the crystallites remains untransformed or partially transformed. The presence of these crystallites also indicates that there is no measurable influence of the X-ray beam on the decomposition process. Concurrent with the disappearance of the {101} and {110} powder peaks observed for E-Li2O2, an increase in charge voltage to values exceeding 4 V is observed in Figure S3. This was also observed previously using operando lab source diffraction experiments21 suggesting that at this stage electrolyte oxidation reactions are introduced.5,33,34 Another interesting feature, also previously reported21 for E-Li2O2, is the 6 ACS Paragon Plus Environment

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delayed onset of the decrease in peak intensity, indicating an initial decomposition of an amorphous lithium component. This phase may be comprised of both Li2O2 and side products arising from electrolyte degradation, such as formate, that can be oxidized at relatively low potentials without a catalyst.35 Consistently, this is not observed for C-Li2O2, which does not contain that component.

Figure 2. Examples of the evolution of (a), (b) peak intensity and (c), (d) grain volume as a function of charge time for various reflections of E-Li2O2 and C-Li2O2, along with the corresponding galvanostatic voltage curves recorded during operando synchrotron X-ray diffraction. Current densities of 100 and 135 µA/cm2 were used for E-Li2O2 and C-Li2O2, respectively.



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In the lower panel in Figure 1 the decomposition of the (100) reflection of an individual Li2O2 crystallite is depicted for E-Li2O2. In Figure 2 the decomposition time of the individual crystallites, the evolution of the peak intensities (Figure 2a,b) and the corresponding crystallite volumes (Figure 2c,d) were monitored as a function of the charge time (see supporting information for details). This gives unique experimental insight into the average crystallite decomposition time , yielding 8.9 h for E-Li2O2 and 9.7 h for C-Li2O2, which correspond to average decomposition rates of 8.0 × 10-5 and 3.9 × 10-5 µm3/mAhcm-2 respectively. Considering the average crystallite dimensions, this results in an average local current density per crystallite of about 200 nA/cm2. These values are relatively large compared to the macroscopically measured exchange current density of 2 nA/cm2.36 This indicates that the overpotential is largely due to crystallite decomposition. Assuming that the capacities in Figure 2 above 4.2 V are mainly due to side reactions,5,33,34 the average Li2O2 crystallite decomposition time covers approximately the full charging time which implies that all grains are actively decomposing during charge. Generally a small overpotential, at a current density smaller than the exchange current density, initiates the nucleation of the phase transition in a relatively small fraction of crystallites, resulting in a small active particle fraction over which the total current is distributed.30,37 Increasing the current density increases the overpotential, resulting in an increasing active particle fraction, as observed in LiFePO4.30,37 When all particles are actively transforming, as observed at present for the Li2O2 decomposition, the overpotential is apparently sufficient to initiate decomposition in each crystallite. This behavior reflects the condition that the phase transition rate (the Li2O2 decomposition rate in this case) limits the overall charge rate, in contrast to for instance the O2, Li-ion or electron transport. Hence we conclude that the decomposition rates of the individual grains limit the charging rate of these Li-O2 batteries. In contrast to C-


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Li2O2, a delay in the decrease in E-Li2O2 crystallite volume is observed (Figures 2c and 2d). Comparing the time evolution of the maximum peak intensity with the crystallite volume, the average decay time of the maximum peak intensity for E-Li2O2 (Figure 2a) is 6.6 h, which is significantly shorter than the 8.9 h for the average crystallite decomposition time. This difference can be attributed to broadening of the reflections, a consequence of the shrinking grain volume upon decomposition during battery charging. To investigate this phenomenon in more detail, the dimensions of individual crystallites were determined from the peak profile I(Q) of the scattered intensity, originating from a single particle at the Bragg condition Q = Qhkl. The peak profile of the scattered intensity I(Q) = ∝ |F(q)|2 can be described by the form factor of the particle F(q), where the wave vector is expanded as Q = Qhkl + q. This analysis is performed by assuming that the shape of the toroidal particles can be satisfactorily approximated by an oblate ellipsoid of revolution for the Li2O2 crystallites (details given in the supporting information).38,39 The dimensions of the individual crystallites are determined from the full width at half maximum (FWHM) of the peak profile I(Q). The evolution of the crystallite dimensions as a function of the charge time were determined by fitting the peak shape to a two dimensional Gaussian (an example is shown in the supporting information). From the fitted parameters we determine the short and long dimension of the crystallite, correspond to the thickness and diameter of the ellipsoid of revolution.



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Figure 3. Time evolution of the (110) peak intensity and aspect ratio of the long and short dimensions of two individual Li2O2 crystallites during the initial charge stages for (a)/(c) E-Li2O2 and (b)/(d) C-Li2O2 as a function of the charge time. The pictures on the 2D detector cover a range in reciprocal space of ∆Q = 0.14 Å-1 in the horizontal and vertical direction around the reflection. The evolution of the aspect ratio, defined as the average long dimension divided by the average short dimension, of the individual crystallites of E-Li2O2 and C-Li2O2 are shown in Figure 3c and 3d. The short dimension tends to decrease faster than the long dimension, as can be observed from the increase in the aspect ratio of the grain as a function of charge time. This is directly observed in Figure 3a where during the initial stages of charging the elongated (110) 10 ACS Paragon Plus Environment

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reflection broadens significantly, directly reflecting that the Li2O2 platelet thickness decreases as a function of charging time. The simultaneous increase in aspect ratio (Figure 3c) reflects that the short dimension decreases faster than the long dimension. This implies that as a function of charge time the particles preferentially decompose from the (001) facets, which have been determined to be the predominant31 interfacial facets between the platelet stack in the toroidal particle. This is in agreement with first-principle studies, which have determined that the (001) facet of Li2O2 is dominant in the Wulff construction of this material.40

In addition DFT

calculations have predicted that topotactic delithiation of Li2O2 to form off-stoichiometric Li2xO2,

experimentally observed,21 is most facile at this facet29 explaining the higher reactivity.

Although Li-ion transport through the off-stoichiometric Li2-xO2 may be fast29 we suggest that the subsequent O2 evolution associated with the observed two-electron reactions33,41 is responsible for the slow decomposition rate of individual grains observed at present. Similar preferential crystallite decomposition is observed for C-Li2O2, especially during the initial stages of charge, albeit to a lesser extent, as can be seen from Figures 3b and 3d. This indicates that the behavior is a general phenomenon seen for Li2O2, irrespective of the way it was produced. The diffraction pattern of individual E-Li2O2 crystallites is found to show different peak shapes, corresponding to different crystallite dimensions, in comparison to the C-Li2O2 crystallites. This is not surprising since it has been well established in literature that at low current densities in TEGDME (an intermediate donor number electrolyte solvent) toroidal Li2O2 (E-Li2O2) particles are formed existing of thin platelet crystallites.7,42 In contrast, SEM measurements indicate a more isotropic shape for chemically produced, commercial Li2O2 crystallites (C-Li2O2) that present a flattened hexagon morphology consistent with the P63/mmc space group.21,32



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Figure 4. 2D peak shapes observed for various (hkl) Li2O2 reflections for (a) – (c) E-Li2O2 and (d) – (f) C-Li2O2. These reflections correspond to crystallite dimensions of: (a) 18 nm × 59 nm, (b) 37 nm × 102 nm, (c) 44 nm × 97 nm, 42 nm × 57 nm, (d) 59 nm × 86 nm, (e) 31 nm × 53 nm and (f) 30 nm × 62 nm. The pictures on the 2D detector cover a range in reciprocal space of ∆Q = 0.14 Å-1 in the horizontal and vertical direction around the reflection. The observed peak profiles vary significantly for different (hkl) reflections. The (101) reflection shown in Figure 4a, with an estimated grain dimensions of about 18 nm × 59 nm is broadened along the {101} powder ring. Several other reflections falling on the {100} and {110} powder rings that show different peak profiles and multiple overlapping peaks have been shown in Figures 4b and 4c. The aspect ratio between the short and long dimension for these E-Li2O2 crystallites is between 2.2 and 5.5. Interestingly, for C-Li2O2 the particle dimensions determined from the peak profile, examples of which are shown in Figures 4d-f for the (101), (110) and (102) reflections, also revealed anisotropic dimensions, albeit with a lower aspect ratio between 1.2 and 2.0.21


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To conclude, operando nano-beam diffraction reveals the asymmetric decomposition of individual Li2O2 platelets. The platelets become thinner more rapidly than they reduce in width as a function of charge time before they decompose completely, giving experimental evidence for the predicted decomposition mechanism via the more reactive (001) surface.40 Monitoring the volume of individual crystallites makes it possible to quantify the decomposition time, decomposition rate and local current density. The average decomposition time of 8.9 h suggests that the Li2O2 crystallites decompose highly concurrently, with a local current density of approximately 200 nA/cm2 at a constant applied charging current density of 100 µA/cm2, which is two orders of magnitude larger than the exchange current density. This reflects that the Li2O2 decomposition rate limits the charging rates of these Li-air batteries and highlights the importance of using redox mediators in solution (molecules dissolved in the electrolyte that are oxidized at a potential slightly above the equilibrium potential of Li2O2 formation which act as redox shuttles) to charge the cell.43-46 Experimental Methods Li-O2 batteries with a cathode, a glass microfiber separator soaked with the electrolyte (1M LiTFSI/TEGDME) and a Li-metal anode, were assembled in the operando X-ray diffraction (XRD) cell in the glove box. The cell was subsequently connected to O2 under a pressure of 1.5 bar where it was allowed to equilibrate before electrochemical tests were performed. Operando synchrotron X-ray diffraction experiments were performed at the ID11 beam line of the European Synchrotron Radiation Facility (ESRF Grenoble, France). Details about material preparation, battery assembly and operando synchrotron diffraction are given in the supplementary information section.



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Acknowledgements The authors would like to thank Ernst Born, Renee den Oudsten, and Kees Langelaan for their assistance in the cell design and fabrication. The assistance of Anton Lefering and Michel Steenvoorden is appreciated. We would like to acknowledge financial support from NWONANO for SG and SB. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Program (FP/20072013) / ERC Grant Agreement n. [307161] of MW. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and thank the beamline staff for assistance in using beamline ID11. Supporting Information Detailed experimental description; Methodology to determine the grain volume and dimensions; SEM image discharge product; Evolution of powder reflections during decomposition of ELi2O2 and C-Li2O2. References (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (2) Abraham, K. M.; Jiang, Z. A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery. J. Electrochem. Soc. 1996, 143, 1-5. (3) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. A Reversible and HigherRate Li-O2 Battery. Science 2012, 337, 563-566. (4) Thotiyl, M. M. O.; Freunberger, S. A.; Peng, Z.; Chen, Y.; Liu, Z.; Bruce, P. G. A Stable Cathode for the Aprotic Li–O2 Battery. Nat. Mater. 2013, 12, 1050-1056. (5) Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Bardé, F.; Novák, P.; Bruce, P. G. Reactions in the Rechargeable Lithium-O2 Battery with Alkyl Carbonate Electrolytes. J. Am. Chem. Soc. 2011, 133, 8040-8047. (6) Thotiyl, M. M. O.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The Carbon Electrode in Nonaqueous Li–O2 Cells. J. Am. Chem. Soc. 2012, 135, 494-500.


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