Mechanisms of Morphological Evolution of Li2O2 Particles during

Mar 14, 2013 - Li–O2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy...
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Mechanisms of Morphological Evolution of Li2O2 Particles during Electrochemical Growth Robert R. Mitchell,†,§ Betar M. Gallant,‡,§ Yang Shao-Horn,*,†,‡ and Carl V. Thompson*,† †

Department of Materials Science and Engineering and ‡Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Li−O2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kgLi2O2) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li−O2 batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li2O2 particles form first as stacked thin plates, ∼10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li2O2 crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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arises from spontaneous bending of the thin Li2O2 crystallites that comprise the microscale particles. Carbon nanotube and nanofiber electrodes8,11 provide a convenient platform for conducting ex situ electron microscopy observations of electrochemically formed Li2O2 owing to their high aspect ratios compared with the discharge products being imaged. Additionally, the high specific surface area of the freestanding CNT electrodes (∼ 500 m2/gC20) used in this study facilitated the investigation of a wide range of specific current densities and their resulting effects on particle morphology. We find that the morphology of Li−O2 discharge product upon first discharge is a function of both discharge rate and capacity. This rate and capacity dependence is related to the nucleation and growth process for Li2O2 on carbon. Typical SEM and TEM micrographs of electrodes discharged under low rate/low capacity and high rate/high capacity conditions are shown in Figure 1. At low rates (10 mA/gC or 2 nA/cm2C), sparse particles nucleated on CNT sidewalls grow larger into disc- and toroid-shaped particles at gravimetric capacities as low as 100 mAh/gC (Figure 1a,b). As evident from TEM imaging, the CNTs in electrodes discharged at low rates (Figure 1b) are mostly bare, and the particle sizes are similar, suggesting instantaneous nucleation at relatively few sites early in the discharge process. By contrast, at higher rates (90 mA/gC or 18 nA/cm2C), a much higher density of particles with nonuniform shapes form (Figure 1c,d) and CNT sidewalls appear to be

ithium-air (Li−O2) batteries have attracted considerable interest1−6 in the past few years because of their intrinsically high gravimetric energy densities7,8 compared with Li-ion batteries, which make them promising for electric vehicle applications. During discharge, electrons electrochemically reduce O2 on the surface of an air cathode combining with Li+ ions to form Li2O22,6,8−11 in a fundamentally different energy-storage mechanism than the intercalation reactions of Li-ion batteries.12 The nucleation, growth, and morphological evolution of Li2O2 particles have not been thoroughly investigated to date. Improved understanding of the interior structure of Li2O2 particles and these growth processes is key to engineering air cathode electrode structures that can provide high energy8,13−15 and power densities4 and lower charging voltages16 for high round-trip efficiency and obtain high Li−O2 redox reversibility for high cycle life. In previous studies, it has been demonstrated that the first discharge in Li−O2 cells containing a relatively stable electrolyte (e.g., 1,2-dimethoxyethane (DME)) and carbonbased air cathode can lead to a range of discharge product morphologies,7 including conformal films17 or disc- and toroidshaped particles.2,8,14,15,18,19 However, few studies2,19 have suggested a mechanism for growth or probed the interior structure of the Li2O2 particles. In this study, we investigated the structure of Li2O2 discharge product as a function of rate and capacity using scanning and transmission electron microscopy (SEM and TEM). We find that the disc and toroid particles previously reported8 represent different stages of particle evolution, where Li2O2 grows in a kinetically controlled regime. Furthermore, we find that the toroid shape © 2013 American Chemical Society

Received: February 17, 2013 Accepted: March 14, 2013 Published: March 14, 2013 1060

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Figure 1. SEM and TEM micrographs of Li2O2 particles on electrodes consisting of freestanding CNT carpets discharged at low and intermediate gravimetric rates. (a) SEM and (b) TEM micrographs of an electrode discharged at 10 mA/gC to 200 mAh/gC, with discshaped particles and bare CNT sidewalls. (c) SEM and (d) TEM micrographs of an electrode discharged at 90 mA/gC to 13 000 mAh/ gC, with a high density of disc particles and a thin coating of discharge product present on the sidewalls of the CNTs. Insets: Higher magnification TEM images of the CNT sidewalls, indicated by a dashed yellow line, showing the (b) absence and (d) presence of small particles (scale bar = 20 nm).

Figure 2. Evolution of the particle aspect ratio as a function of particle size for several low gravimetric rates. Disc and toroid particles represent a continuum of shapes for Li2O2 particles. As seen in both the plot and the SEM micrographs ((i) 10 mA/gC to 500 mAh/gC, (ii) 100 mA/gC to 14 000 mAh/gC, and (iii) 10 mA/gC to 1100 mAh/gC), as particles grow larger in size, their aspect ratio, defined as the ratio of particle height, h, to particle diameter, d, increases.

coated by numerous small particles (Figure 1d). Electrodes discharged at comparable gravimetric rates but to lower gravimetric capacities exhibit only coatings of small particles on the sidewalls of the CNTs (Figures S1 and S2 in the Supporting Information), implying that the coating of the carbon surface with small irregular particles precedes growth of well-defined larger disc- and toroidal-shaped particles with increasing discharge rates. To investigate the shape evolution of disc and toroid particles found at low discharge rates, ex situ SEM imaging was conducted on electrodes discharged in the range from 10 to 250 mA/gC (2−50 nA/cm2C) to a range of gravimetric capacities (200 to >10 000 mAh/gC). As seen in Figure 2, the particle aspect ratio, defined as the ratio of particle thickness, h, to particle diameter, d, increases as a function of particle size regardless of the gravimetric discharge rate. These results suggest that Li2O2 particles with disc shapes evolve to toroidal shapes as the particle size increases. This trend is further demonstrated with representative SEM micrographs of Li2O2 particles as a function of diameter in Figure 2. The origin of the observed shape evolution of Li2O2 particles was investigated using TEM imaging through the edges of the discs. The disc and toroid particles are composed of arrays of plate-like Li2O2 crystallites, as shown in Figure 3a,b; yellow overlays highlight the plate locations. (A version of this Figure without the overlays is included in Figure S3 in the Supporting Information.) These crystals grow roughly parallel to each other from the CNT at the center of the discs and appear to splay apart with increasing disc diameter. Additional plates appear to nucleate in the empty space between the splayed

Figure 3. Side-profile TEM images of disc particles discharged at (a) 50 mA/gC to 1000 mAh/gC and (b) 90 mA/gC to 13 000 mAh/gC with yellow lines highlighting the location of some of the discrete plates, which compose the microscale particle; insets show a lower magnification image of the particle. (c) Schematic illustration of the microscale shape evolution of Li2O2 particles, indicating the role of plate splaying and additional plate nucleation in the transition from disc to toroid particles.

plates, and the collective result of these two processes is the evolution of a rim around the disc circumference. This leads to 1061

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Information. Interestingly, the resulting Li2O appears to maintain an epitaxial relationship with the underlying Li2O2 matrix, as indicated in Figures S4d, S6, and S7 in the Supporting Information. In this study, beam currents and exposure times were kept short (∼1 min at low electron flux) to avoid particle damage. Combining the insights from TEM imaging, that is, microscale Li2O2 particles are composed of nearly parallel plates with nanoscale thickness, with the results from electron diffraction, it can be inferred that Li2O2 crystallites have a shape that is dominated by a large (001) crystal face represented schematically in Figure 4d. The observation of a faceted crystal with a large (001) face is consistent with theoretically calculated Wulff shapes for Li2O2,25,26 which are constructed from crystal faces minimizing the total particle surface energy. The shape of Li2O2 particles observed in this study is highly unusual because the particles exhibit single-crystalline diffraction patterns yet also have very complex microscale morphologies, attributed to the observed plate splaying and secondary nucleation of additional plates. Whereas it is not conclusively known what is driving the plate splaying mechanism, similar instances of spontaneous bending have been observed in several polar crystal systems27 (e.g., ZnO28 and some III−V semiconductors29) and Li2O2 can have polar characteristics based on its surface termination.30,31 A variety of mechanisms29,32 have been proposed to explain spontaneous bending of polar crystals, and among the proposed mechanisms, surface stress variations on opposite faces of the thin crystallites seem to be the most likely explanation for the observed bending; however, further studies are needed to examine this hypothesis in detail. (See the discussion in the Supporting Information) The particle shapes observed in this study are similar to materials formed via a proposed mesocrystallization process,33 wherein small precursor crystallites, stabilized by a polymer additive, aggregate into a larger crystal exhibiting long-range ordering. The prototypical mesocrystalline material system is CaCO3, for which there exist many reports of toroid structures.34−36 Toroidal-shaped particles have also been found in the ZnO,37 Co3O4,38 and LiFePO439 systems. Whereas a mesocrystalline aggregation mechanism has been proposed for the toroid particles reported elsewhere, these other cases do not involve electrochemical deposition, in which crystallite formation requires electrical connectivity to the electrode. A fully analogous aggregation mechanism therefore seems unlikely in the present case. The observation of either disc/toroid particles (Figure 2) or small equiaxed particles (Figure 1d inset, Figures S1 and S2 in the Supporting Information) on the CNT axes during discharge is governed by the nucleation and growth processes for Li2O2 on carbon substrates, which is analogous to electrodeposition of metals on foreign substrates,40 for which deposition occurs via a Volmer−Weber island growth mode.41 In this framework, the difference between the potential required for nucleation of Li2O2 on CNTs, U(Li2O2/CNT), and the equilibrium potential for growth, U eq(Li2 O2 /Li2 O 2), influences the resulting morphology. At low discharge rates (∼10 mA/gC or 2 nA/ cm2C), at which Uapplied exceeds U(Li2O2/CNT) by a small amount, the driving force for nucleation on the CNTs is small, resulting in nucleation and growth of Li2O2 at a small, fixed number of energetically accessible sites in the electrode, as we have observed (Figure 1a,b), with growth proceeding in a kinetically controlled regime via layer-by-layer addition of Li2O2

evolution into the characteristic toroid morphology observed for large particles, as schematically illustrated in Figure 3c. The plates observed in this study had thicknesses on the order of 10 nm, which is consistent with measured crystal sizes obtained from X-ray diffraction.11,13,21 Electron diffraction was used to investigate the crystal structure of the disc/toroid particles formed at low rates (10 mA/gC or 2 nA/cm2C) and is summarized in Figure 4. SEM and

Figure 4. Electron diffraction investigation of individual Li2O2 particles. (a) SEM and (b) bright-field TEM images of toroid particles formed electrochemically at 10 mA/gC to a gravimetric capacity of 1400 mAh/gC. (c) Simulated Li2O2 [001] zone axis (red and blue dots) superimposed over an experimental diffraction pattern for the particle pictured in panel b. (d) Side-view and top-view schematics of a stack of crystallite plates, which compose the disc and toroid particles, indicating relevant crystal planes and directions for Li2O2 and the plate rotation axis, which is related to the arcs in the experimental diffraction patterns.

TEM images of representative particles investigated using electron diffraction can be seen in Figure 4a,b, with an accompanying diffraction pattern in Figure 4c. Interestingly, the particles exhibit nearly single-crystalline diffraction patterns despite their unusual (e.g., unfaceted) microscale shape. These diffraction patterns have a series of broad arcs, which can be attributed to reflections from an array of stacked plate crystallites having a slight rotational misalignment along the c axis of Li2O2 (P63/mmc),22 as has also been previously observed in multilayer graphene sheets.23 Diffraction patterns for Li2O2 were simulated (Figure S4 in the Supporting Information, JEMS24) using the P63/mmc structure22 (additional details in the Supporting Information) and compared with experimental patterns collected from particles oriented with the [001] directions parallel to the electron beam. Experimental diffraction patterns for particles formed under a variety of electrochemical conditions were found to match the [001] zone axis of Li2O2 (Figure 4c and Figure S5 in the Supporting Information). The Li2O2 particles were found to be sensitive to the electron beam and were partially converted to Li2O after extended exposure, as shown in Figure S6 in the Supporting 1062

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sides of the thin crystallites with the plate splaying driving the evolution from disc to toroidal shapes.

on existing Li2O2 crystal faces, resulting in large faceted crystallites (Figure 4). Whereas a small driving force for Li2O2 deposition, |Uapplied − Ueq(Li2O2/Li2O2)|, leads to sparse nucleation and kinetically controlled growth of faceted crystals, at higher gravimetric rates, the driving force for nucleation is large |Uapplied − Ueq(Li2O2/CNT)| and results in progressive nucleation (Figure S2 in the Supporting Information) of particles on the sidewalls of the CNTs (Figure 1c,d and Figure S1 in the Supporting Information). In this high driving force regime, the barrier to layer nucleation is low, and growing particles are more likely to develop spherical/unfaceted shapes41 (Figure S1 in the Supporting Information). With increasing discharge capacity, the specific current density normalized to the total surface area of CNT and growing Li2O2 surfaces will decrease, which enables the growth of discs and toroids and results in the presence of both small, unfaceted particles coated on carbon and large faceted particles in the fully discharged electrodes (Figure 1d). Because the average shape and dimensions of Li2O2 particles are known from SEM measurements, the number of particles in the electrode and the magnitude of the Li2O2 specific current density on Li2O2 surfaces can be estimated to provide an order of magnitude estimate for the current density required for kinetically controlled growth of faceted crystals. The shape of the observed Li2O2 microscale particles (Figure 2) can be approximated as a disc (Figure S8 in the Supporting Information) with an evolving radius and thickness for the purpose of estimating the specific current density based on the geometric surface area of the top, bottom, and circumferential faces, corresponding roughly to the (001) and (110)/(1−10) planes, respectively. Using a simple model for disc growth (see details in the Supporting Information), we have estimated the current density on the top and bottom geometric surfaces of Li2O2 particles to be 0.16 ± 0.13 μA/cm2Li2O2. It should be noted that this disc model underestimates the true surface area of the particles because it is known from electron microscopy that microscale Li2O2 particles are composed of many discrete disc-like crystallites with nanoscale thicknesses. Therefore, the calculated value of geometric surface area for the current density represents an upper bound for the true current density on the Li2O2 particles. Comparisons of this estimated current density to reports in the literature are possible only in select studies because current is typically reported normalized versus electrode mass or electrode specific surface area. Viswanathan et al.17 conducted experiments involving galvanostatic discharge (∼1 μA/cm2) in a Li−O2 cell using a flat glassy carbon cathode on which they assumed a continuous Li2O2 film. Our findings in this study suggest that Li2O2 discs/toroids were not observed in this case because the specific current density was too high, resulting in a film having a similar structure to the small equiaxed particles we observed coating the CNTs at high rates. In summary, we have shown that at low rates electrochemically grown Li2O2 particles evolve from disc-like to toroid-like shapes as their sizes increase. Furthermore, we observe that at high rates growth of small equiaxed particles precedes growth of discs, which form only at high capacities. The disc and toroid particles are composed of thin plates of Li2O2 with the large facet of the plates having a [001] surface normal. These faceted shapes are consistent with theoretical calculations of the equilibrium Wulff shape for Li2O2 and a layer-by-layer growth mode. Plate splaying observed in the growing particles can likely be attributed to differences in surface stress on opposite

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EXPERIMENTAL SECTION Carbon Nanotube Electrode Synthesis. Freestanding carpets of carbon nanotube electrodes were prepared using an atmospheric pressure chemical vapor deposition process consisting of the catalyzed growth of multiwalled carbon nanotube (MWCNT) arrays from Fe nanoparticle catalysts supported on an Al2O3(film)/Si(substrate) with C2H4 as the carbon precursor. Electrochemical Experiments. The CNT positive electrodes were assembled in Li−O2 cells inside an Ar-filled glovebox with a lithium negative electrode and DME-based electrolyte containing 0.1 M LiClO4. The assembled cells were purged with O2 gas and discharged galvanostatically at various rates to either arbitrary intermediate depths-of-discharge or to 2.0 V versus Li, depending on the stage of particle growth under investigation. Electron Microscopy. After testing, the cells were disassembled under Ar and prepared for ex situ SEM and TEM investigation. SEM imaging was conducted using a JEOL 6320 SEM operated at 5 kV accelerating voltage and a 12 μA beam current. Zeroloss bright-field TEM imaging and selected area electron diffraction were conducted using a Zeiss Libra 120 operated at 120 kV accelerating voltage.

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ASSOCIATED CONTENT

S Supporting Information *

Further details of experimental methods (CNT synthesis and characterization), TEM images, electron diffraction, current density calculations, and growth mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.V.T.) and [email protected] (Y.S.H.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was partially supported by the MRSEC Program of the National Science Foundation under award number DMR0819762. B.M.G. acknowledges a National Science Foundation Graduate Research Fellowship and an M.I.T. Martin Family Fellowship. Notes

The authors declare no competing financial interest. § E-mail: [email protected] (B.M.G.) and [email protected] (R.R.M.).

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ACKNOWLEDGMENTS This work was partially performed at the Center for Nanoscale Systems (CNS) at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award number ECS-0335765. 1063

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