In Situ Transmission Electron Microscopy Observations of

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In Situ Transmission Electron Microscopy Observations of Electrochemical Oxidation of Li2O2 Li Zhong,†,⊥ Robert R. Mitchell,‡,⊥ Yang Liu,§ Betar M. Gallant,∥ Carl V. Thompson,‡ Jian Yu Huang,§ Scott X. Mao,*,† and Yang Shao-Horn*,‡,∥ †

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States ∥ Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: In this Letter, we report the first in situ transmission electron microscopy observation of electrochemical oxidation of Li2O2, providing insights into the rate limiting processes that govern charge in Li−O2 cells. In these studies, oxidation of electrochemically formed Li2O2 particles, supported on multiwall carbon nanotutubes (MWCNTs), was found to occur preferentially at the MWCNT/Li2O2 interface, suggesting that electron transport in Li2O2 ultimately limits the oxidation kinetics at high rates or overpotentials. KEYWORDS: Lithium peroxide, oxidation kinetics, electron-transport limited, lithium-air battery, in situ TEM study

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(diameters 2 eV in bulk26−29). The poor electronic conductivity of Li2O2 may also limit the electrochemical oxidation kinetics of Li2O2 upon charging, but the magnitude of this effect is not known. Recent studies19 have shown that the electrochemical oxidation of Li2O2 on charge exhibits a characteristic process having a plateau voltage profile with slow kinetics that are sensitive to overpotentials (∼300 mV/decade) in the voltage range from 3 to 4 V versus Li. The mechanistic details of this process are not understood but this process can be attributed to the electrochemical oxidation of the bulk of Li2O2 particles to evolve O2 via a two-phase transition, whose kinetics are limited by the nucleation of active sites.19 Our objective in this current study is to examine if the electrochemical oxidation kinetics of Li2O2 are ultimately limited by lithium ion diffusion or electronic transport in Li2O2 at very high overpotentials, where the kinetics of nucleation of active sites19 are faster than mass and electronic transport. The proliferation of in situ techniques over the past decade, which involve advances in scanning electron microscopy

he adoption of fully electric vehicles has been hindered in part by the low-energy density of conventional Li-ion batteries,1−3 resulting in a significantly smaller vehicle range than vehicles powered using conventional hydrocarbon fuels.4 Among the various high-energy density battery chemistries currently being investigated, the nonaqueous Li−O2 battery4−7 has attracted much attention because of its high theoretical energy density, which is substantially larger (2−4×)7−9 than what is achievable with present or future generation lithium-ion batteries. Unlike the intercalation reactions of Li-ion batteries,3 discharge in a Li−O2 cell involves an oxygen reduction reaction (ORR) during which molecular O2 is reduced by Li+ ions (Li+ + O2 + 2e− ⇄ Li2O2 with an equilibrium voltage of 2.96 V vs Li8,10) resulting in the formation of Li2O2 in the void volume of a porous O2 cathode.9 Despite the promising advantage of Li− O2 batteries, many issues still must be resolved before these batteries can be exploited commercially, including electrolyte instability,11−15 poor cycle life11,13 and rate capability,16 and low round-trip efficiencies,4,11,17 largely resulting from high overpotentials on charge.18,19 Little is known about the processes that govern the kinetics of Li2O2 electrochemical oxidation on charge, which hinders the development of rechargeable Li−O2 batteries with enhanced performance characteristics for practical use. Large disc and toroidal Li2O2 particles with sizes of several hundreds of nanometers9,16,20−23 can form on discharge at low rates (below ∼0.1 μA/cm2true9,16,20,21). With increasing rates, disc and toroidal particles are replaced by small Li2O2 particles © XXXX American Chemical Society

Received: February 26, 2013 Revised: April 10, 2013

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(SEM),30 transmission electron microscopy (TEM),31 X-ray diffraction (XRD), and nuclear magnetic resonance (NMR)32 among others, has yielded significant insights into the reaction kinetics and microstructural evolution of materials undergoing electrochemically induced transformations. Specifically, in situ TEM techniques have enabled high spatial and temporal resolution observations of a variety of electrochemical processes including the lithiation of SnO231,33,34 and Si nanowires (NW),35 multiwalled carbon nanotubes (MWCNT),36 and graphene nanoribbons.37 In situ techniques provide a powerful approach for exploring fundamental nanoscale processes, which impact cell-level performance. In this Letter, we report the first in situ TEM study of electrochemical oxidation of Li2O2, which has allowed us to gain considerable insights into the origin of kinetic limitations that hinder charging in Li−O2 cells. MWCNT-supported Li2O2 particles investigated in this study were formed electrochemically during discharge in a Li−O2 cell. Briefly, MWCNT carpets were synthesized via atmospheric pressure thermal chemical vapor deposition (CVD) using Fe catalysts supported on a Si (bulk)/Al2 O 3 (thin film) substrate.38,39 Further details on the synthesis can be found in the Supporting Information. After synthesis, freestanding carpets were detached from the growth substrate and used as the positive electrode in lithium cells containing 0.1 M LiClO4 in 1,2-dimethoxyethane (DME) electrolyte using a cell assembly process described previously9,13 with all cell assembly steps performed in an Ar-filled glovebox. After purging the cells with dry O2, the electrodes were discharged to 2.0 V versus Li at 90 mA/gC, resulting in a capacity of ∼12 500 mAh/gC (Figure 1a, inset). From SEM (Figure 1a) and TEM (Figure 1b and Supporting Information Figure S1a) imaging, discrete

Li2O2 particles were found to have a disc or toroidal morphology with diameters of ∼250 nm, which are in agreement with previous work.7,9,16,20−24 Li2O2 particles were distributed uniformly along the axes of MWCNTs (having no apparent orientation registry), and multiple MWCNTs were found to often intersect individual Li2O2 particles. In addition, TEM imaging of these Li2O2 particles (Figure 1b), combined with selected area electron diffraction (SAED) (Figure 1c and Supporting Information Figure S1b), revealed that the disc particles were polycrystalline and had a crystal structure in agreement with hexagonal Li2O2 (P63/mmc)40 from the indexed radial profile (Figure 1c) of the SAED pattern. Further, previous characterization using X-ray absorption near edge structure (XANES) of particles formed under similar conditions indicates that the bulk and surface of Li2O2 discs are largely free from carbonate species.13 To investigate the electrochemical oxidation process, a solidstate in situ microbattery, consisting of a MWCNT/Li2O2 positive electrode (predischarged separately as described previously) and a Si NW (delithiated) negative electrode coated with LiAlSiOx solid electrolyte (SE)41 was assembled and then placed inside a TEM. Figure 2a is a schematic representation of the assembled in situ cell. The Si NWs were grown on a Si wafer using the vapor−liquid−solid technique and were coated by an SE via atomic layer deposition (ALD).41 A small piece of the Si wafer, supporting the vertically oriented Si NWs, was attached to an aluminum rod to be mounted on the in situ setup and the MWCNT/Li2O2 electrode was attached to a gold substrate with conductive silver epoxy prior to in situ cell assembly. The prepared MWCNT/Li2O2 positive electrode and the SE-coated Si NW negative electrode were mounted onto a Nanofactory scanning tunneling microscopy (STM) TEM holder and quickly transferred into a TEM with minimal exposure to ambient atmosphere. All materials preparation and in situ cell assembly was performed inside an Ar-filled glovebox. Once inside the TEM, a single SE-coated Si NW electrode was moved toward the MWCNT/Li2O2 positive electrode, driven by a piezomanipulator, and brought into contact with individual Li2O2 particles (Figure 2a,b). The contacted Li2O2 particle was then oxidized potentiostatically through the application of a potential between the external leads of the microbattery. Typically, very large applied potentials in the range of 8−10 V are needed to induce oxidation of Li2O2 particles (visible particle size reduction and particle thinning) within the TEM experiment time frame. The applied potentials required for the electrochemical oxidation of Li2O2 in the TEM were significantly greater than those observed in macroscale Li−O2 cells,13 and these large applied potentials can be attributed to the much larger series resistance of the microbattery assembly35,42,43 compared with conventional Li−O2 cells. Despite the voltage drop due to series resistance, the overpotential, and, in turn, the charging rate in the in situ cell was still much greater than that of conventional cells, leading to much shorter times for the nearly complete oxidation of Li2O2 particles (in situ TEM, typically 30 to 60 min versus conventional Li−O2 cell testing, 100 h at 100 mA/ gC for capacities of 10 000 mAh/gc assuming particles oxidize at a uniform rate across the electrode). Although the true overpotential at the MWCNT/Li2O2 positive electrode could not be directly measured due to the nature of the experimental setup,42 it is believed that the overpotentials imposed on individual Li2O2 particles in this study are much larger than those used in conventional Li−O 2 cells reported to

Figure 1. Electrochemically formed Li2O2 particles. (a) (Inset) Voltage profile of a MWCNT electrode discharged in a lithium cell at 90 mA/gC. The particles were found to have a disc-like morphology in SEM (a) and TEM (b) imaging. (c) SAED pattern (inset) reveals a polycrystalline internal structure while the radial profile of the SAED pattern exhibits a good match to hexagonal Li2O2. Vertical lines indicate position and intensity of Li2O2 reflections in SAED expected from XRD references. B

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Figure 2. continued Particles 1 and 2 during application of a 10 V potential to the MWCNT/Li2O2 positive electrode against the Si NW negative electrode. Li2O2 close to the MWCNT bundle in Particle 1 is rapidly oxidized (c), before slowing down due to increasingly poor contact between the MWCNT bundle and the Li2O2 particle (d), indicating an electron-transport limitation. Oxidation of Particle 1 resumed when the MWCNT bundle was bent, improving physical contact (e), and oxidation of Particle 2 (f,g) occurred only when Particle 1 came into direct contact with Particle 2 where oxidation also began at the MWCNT/Li2O2 interface.

date13,15,19,21 due to much faster electrochemical oxidation of Li2O2 in a time scale of typically less than 1 h in the in situ TEM experiments, which is in contrast with full charging of toroidal particles typically occurring over several days, at overpotentials of ∼1 V and intermediate rates of ∼50−200 mA/gC,9,21−23 in conventional Li−O2 cells. Li2O2 particles were found to be very sensitive to the electron beam dose and therefore imaging conditions were optimized to minimize beam damage. Other than when recording movies, the beam was blanked during the oxidation process, except for short exposure periods, about 5 s each, occurring every ∼2 min for the purpose of recording images. Moreover, a very weak electron beam with intensity below 3 A/m2 was used in all of our experiments. A detailed study of beam irradiation was carried out and is summarized in Figure S2 and Movie M1 in the Supporting Information. An electron beam with intensity above 30 A/m2 caused quick decomposition of Li2O2 particles from the surface to the interior (Supporting Information Figure S2a−d). In addition, both SAED and electron energy loss spectroscopy (EELS) revealed that Li2O2 was converted to Li2O during beam irradiation, as evidenced by the diffraction rings (Supporting Information Figure S2e) as well as an additional peak at ∼57 eV (characteristic of Li2O44) in the EELS spectrum (Supporting Information Figure S2f). These morphological and chemical changes are distinctly different from those found during electrochemical oxidation of Li2O2, which will be discussed in detail. Taken together, these experiments support the conclusion that beam effects are not a concern in the interpretation of the results in this study. In situ TEM imaging revealed that the oxidation of individual Li2O2 particles initiated preferentially at the MWCNT/Li2O2 interface, as evidenced by the development of a light-contrast stripe beginning along the CNT axes (highlighted by red, dashed lines) of the MWCNT bundle contacting the Li2O2 particle (Particle 1 in Figure 2c). The formation of the lightcontrast stripe in TEM can be explained by thinning of Li2O2 particles at the Li2O2/MWNCT interface associated with electrochemical oxidation of Li2O2 (Li2O2 → Li+ + O2 + 2e−) and the concurrent release of O2, where lithium ions migrate through the SE into the Si NW (Figure 2a) and electrons flow through MWCNTs to the Au substrate. The preferential oxidation at the MWCNT/Li2O2 interface, but not at the interface between Li2O2/SE, suggests that the electrochemical oxidation of Li2O2 is electron-transport-limited instead of lithium-ion-transport-limited. This hypothesis is further supported by the observation that the oxidation rate decreased considerably with increasing Li2O2 removal from Particle 1, which presumably reduced the electrical contact between the MWCNT and the remaining Li2O2 in the particle (Figure 2d). However, when the MWCNT bundle was moved, coming into

Figure 2. Oxidation of Li2O2 particles. (a) Schematic illustration of the in situ TEM microbattery superimposed over a low-magnification TEM image of a SE-coated Si NW contacting a single Li2O2 particle. (b) Higher-magnification TEM image of the particles in (a), showing a MWCNT bundle contacting two physically separated Li2O2 particles labeled as Particle 1 and Particle 2, respectively. (c−g) Oxidation of C

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Information Figure S3a) revealed that the remaining material is Li2O2. Together, these observations suggest that the origin of morphological evolution observed in this study is electrochemical in nature and not an artifact of electron beam exposure. Additional experiments exhibit preferential oxidation at the MWCNT/Li2O2 interface (Supporting Information Figures S4 and S5, Movie M3), further indicating that the oxidation of Li2O2 is ultimately electron-transport limited at high overpotentials. While electron transport in Li2O2 was shown to limit the electrochemical oxidation kinetics of individual Li2O2 particles contacted by the SE-coated Si NW probe, lithium ion diffusion along the MWCNTs appears to limit the oxidation of physically separated particles. For example, in the experiment summarized in Figure 2, oxidation of Particle 2 only began after it made physical contact to Particle 1, due to movement of the MWCNTs, after which oxidation preferentially occurred at the MWCNT/Li2O2 interface (Figure 2e−g). Further, in Figure 3 particles not directly contacted by the SE-coated Si NW did not show any significant changes during biasing. Additional experiments showing similar behavior are illustrated in Figure S6 in the Supporting Information. To further understand the influence of surface diffusion of Li+ along the MWCNT on the electrochemical oxidation of Li2O2, a single SE-coated Si NW was first brought into physical contact with several MWCNTs (Figure 4a), which were in turn contacting two adjacent Li2O2 particles (i.e., no direct physical contact between the SE-coated Si NW and the Li2O2 particles). Interestingly, no visible changes were noted for these two particles during the application of a potential of 8 V for 2455 s (Figure 4b). However, once the SE-coated Si NW probe was repositioned and placed into direct contact with one of the two adjacent Li2O2 particles, preferential oxidation proceeded at the MWCNT/Li2O2 interface (Figure 4c−f), consistent with the results presented in Figures 2 and 3, suggesting Li+ ion surface diffusion along the MWCNTs limits the oxidation of adjacent Li2O2 particles that are not in direct contact with the SE-coated Si NWs. While Li+ ion surface diffusion on pristine CNTs is theorized to be rapid,45 the formation of surface species on carbon (e.g., Li2CO3),13,15 likely decreases the magnitude of surface diffusivity. However, the limitation of Li+ transport along MWCNTs is unlikely to occur in a conventional Li−O2 battery, where the Li2O2 particles are surrounded by Li+conducting liquid electrolyte, and the Li+ ions can diffuse readily through the electrolyte. The electrochemical oxidation kinetics of Li2O2 can in principle be limited by either the flux of electronic carriers, Je−/h+, or the flux of Li+ ions, JLi+, (Figure 4g) at the high rates utilized during in situ TEM experiments, where surface reaction kinetics are faster than transport processes. Carbon nanotubes are known to have extremely high electronic conductivities46 that in principle should ensure that electrons at the MWCNT/ Li2O2 particle interface flow to the external circuit with minimal resistance. A Li+-ion-diffusion-limited process can reasonably be ruled out, as such a limitation would have yielded an oxidation process starting from the Li2O2/SE interface, which was not observed in any of our experiments. Preferential oxidation occurring at the MWCNT/Li2O2 interface found in these in situ studies suggests that electronic transport in Li 2O 2 ultimately limits the electrochemical oxidation kinetics of Li2O2 particles at very high overpotentials, which is in agreement with the electronically insulating nature of bulk Li2O2 with bandgap >2 eV, reported recently.26−29 It should be

contact again with Particle 1, the oxidation rapidly resumed (Figure 2e), further confirming the important role of the MWCNTs as an electron sink for the electrochemical oxidation of Li2O2. Oxidation of Particle 2 only occurred when it came into direct contact with Particle 1, forming a path for Li diffusion to the Si NW (Figure 2e−g). Further evidence for electron-transport-limited oxidation of Li2O2 was found in another experiment, which is summarized in Figure 3. The particle in contact with the SE-coated Si NW

Figure 3. (See also Movie M2 in Supporting Information) Electrontransport limited oxidation. (a) A disc particle aligned orthogonal to the electron beam connected to two sets of MWCNTS (at the middle and bottom of the particle) and also contacted by the SE-coated Si NW. Application of a 10 V potential between the MWCNT/Li2O2 positive electrode and the Si NW negative electrode initiated the oxidation process. Two particles at the lower left corner (indicated with yellow arrows) were chosen as references. (b−d) Images captured at 230, 971, and 1963 s respectively showing preferential rapid oxidation occurring at the Li2O2/MWCNT interfaces with more gradual oxidation occurring at the top of the particle. Note that during the oxidation process, no change occurred in the two reference particles, excluding the possibility of beam irradiation effects.

probe (Figure 3a) was connected to two separate sets of MWCNTs with one bundle passing through the center of the particle and the other attached to the bottom of the particle on the exterior surface. After application of a potential (10 V) across the microbattery, the oxidation proceeded at the center of the particle and also at the bottom edge (Figure 3b, Movie M2 in Supporting Information). There was an obvious decrease in the diameter of the particle (Figure 3b,c, Movie M2) upon application of a potential due to the presence of the lower MWCNT bundle, which provided facile electron transport to the exterior surface of the Li2O2 particle, while the top side of the particle contacting the SE remained unoxidized until late in the experiment (Figure 3c,d). To ensure that the presumed electrochemical oxidation observed was not an artifact of electron beam exposure, two Li2O2 particles at the lower left corner of Figures 3a−d (marked with yellow arrows) were concurrently monitored and no visible changes were found to occur during the electrochemical oxidation of the adjacent particle. Moreover, both SAED (Supporting Information Figure S3b) and EELS spectrum (Supporting Information Figure S3c) collected from a separate partially oxidized particle (Supporting D

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Figure 4. continued more slowly through the rest of the particle. (g) Schematic illustration of the transport processes occurring during oxidation. Je−/h+ and JLi+ refer to the flux of charge carries and Li+ ions, respectively. The MWCNTs function as highly conductive electron sinks but provide poor paths for Li+ ion diffusivity, while Li2O2 particles provide facile Li+ ion diffusion with low electronic conductivity. As a result, rapid oxidation occurs only in particles directly contacted by the SE-coated Si NWs with the oxidation process always initiating at the MWCNT/ Li2O2 interface.

noted that the electronic transport limitation discussed here is not likely to be significant during charging of Li−O2 cells with liquid electrolytes at low rates reported to date, where surface electron transfer kinetics (Tafel) govern the electrochemical oxidation rate at overpotentials no greater than ∼1 V. Here we report the first in situ observation of electrochemical oxidation of Li2O2 particles, formed on MWCNTs during discharge in Li−O2 cells. Electrochemical oxidation of Li2O2 occurs preferentially at the MWCNT/Li2O2 interfaces within individual particles but not at the Li2O2/SE interface, at high overpotentials or high rates (nearly complete oxidation of ∼250 nm diameter Li2O2 nanoparticles in less than one hour). These findings indicate that electron transport, and not Li+ transport, ultimately limits the oxidation rate of Li2O2 at the high overpotentials used in this study, where the surface reaction kinetics are no longer rate-limiting. This work strongly suggests that electronic, and not Li+ transport, would govern the rate capability of rechargeable Li−O2 batteries operating at the high rates required for electrical vehicle applications.6 Future studies at lower overpotentials and longer times will be useful to study the kinetics of Li2O2 oxidation at intermediate charge rates, where both transport processes and surface electron transfer kinetics influence the oxidation rate. Our findings suggest that electrodes with large specific surfaces, which maximize the Li2O2/electrode interfacial area, and high electronic conductivity, providing facile electron transport to the reaction sites, are desirable for the design of high rate rechargeable Li− O2 batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Further details of experimental methods (CNT synthesis and characterization), additional in situ experiments and movies, particle beam sensitivity experiments, electron diffraction, and electron energy-loss spectroscopy 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: (Y.S.-H.) [email protected]; (S.X.M.) sxm2@pitt. edu. Author Contributions

⊥ The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. L.Z. and R.R.M. contributed equally.

Figure 4. Limited Li+ diffusion along MWCNT surfaces. (a,b) Images demonstrating that oxidation did not occur when the SE-coated Si NW contacted only the MWCNTs, despite a prolonged application of an 8 V potential bias. (c−f) Oxidation occurred only when the SEcoated SiNWs were moved to make direct contact with a Li2O2 particle, also under an 8 V potential. As before, in this case oxidation proceeded rapidly at the MWCNT/Li2O2 interface before proceeding

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated E

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for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The Si NWs, used as a negative electrode in the in situ TEM microbattery, were provided by Tom Picraux at the Center for Integrated Nanotechnologies, which were then coated with SE by Professor Jane P. Chang in the Department of Chemical and Biomolecular Engineering at the University of California, Los Angeles. The work carried out at MIT was supported in part by the MRSEC Program of the National Science Foundation under award number DMR-0819762. B.M.G. acknowledges a National Science Foundation Graduate Research Fellowship. S.X.M. acknowledges NSF CMMI 08 010934 through University of Pittsburgh.

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