Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with

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Unexpected LiO Film Growth on Carbon Nanotube Electrodes with CeO Nanoparticles in Li–O Batteries 2

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Chunzhen Yang, Raymond A. Wong, Misun Hong, Keisuke Yamanaka, Toshiaki Ohta, and Hye Ryung Byon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05006 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with CeO2 Nanoparticles in Li–O2 Batteries Chunzhen Yang †, Raymond A. Wong †,‡,#, Misun Hong †,#, Keisuke Yamanaka §, Toshiaki Ohta §, and Hye Ryung Byon†,# * †

Byon Initiative Research Unit (IRU), RIKEN, 2-1 Hirosawa, Wakoshi, Saitama 351-0198, Japan

‡

Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,

Yokohama 226-8502, Japan §

Synchrotron Radiation (SR) Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

#

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-

ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Keywords: lithium-oxygen battery, Li2O2 film, adsorption, solvation, ceria

ACS Paragon Plus Environment

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ABSTRACT: In lithium-oxygen (Li–O2) batteries, it is believed that lithium peroxide (Li2O2) electrochemically forms thin films with thicknesses less than 10 nm resulting in capacity restrictions due to limitations in charge transport. Here we show unexpected Li2O2 film growth with thicknesses of ~60 nm on a 3-D carbon nanotube (CNT) electrode incorporated with cerium dioxide (ceria) nanoparticles (CeO2 NPs). The CeO2 NPs favor Li2O2 surface nucleation owing to their strong binding towards reactive oxygen species (e.g., O2 and LiO2). The subsequent film growth results in thicknesses of ~40 nm (a cut-off potential of 2.2 V vs Li/Li+), which further increases up to ~60 nm with the addition of trace amounts of H2O which enhances the solution free energy. This suggests the involvement of solvated superoxide species (LiO2(sol)) that precipitates on the existing Li2O2 films to form thicker films via disproportionation. By comparing toroidal Li2O2 formed solely from LiO2(sol), the thick Li2O2 films formed from surface-mediated nucleation/thin-film growth following by LiO2(sol) deposition provides the benefits of higher reversibility and rapid surface decomposition during recharge.


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The rechargeable Li–O2 battery has been intensively studied in view of its appreciable potential for providing superior energy density.1 The outstanding theoretical gravimetric and volumetric energy density (> 3 kWh kg-1 and ~1 kWh L-1) has in particular great advantages for vehicle propulsion, which can spur the development of future electric cars.1 However, the actual performance with respect to capacity and round-trip efficiency has significantly underperformed at the current stage,1-3 which is closely associated with the nucleation, growth and decomposition processes of Li2O2. The nucleation and growth of Li2O2 occurs during discharge (DC) through oxygen reduction reaction (ORR, O2* + e– + Li+ → LiO2*, the asterisk mark (*) denotes adsorption on the surface) and a second step via either electrochemical reduction process (LiO2* + e– + Li+ → Li2O2(s)) or disproportionation (2LiO2(sol) → Li2O2(s) + O2).2, 3 The diffusion of superoxide species (LiO2*) on electrode surface leads to the predominant surface nucleation and Li2O2 film growth (indicated as surface-mediated growth). The drawback of this process however is the sluggish charge transport in the wide-bandgap insulating Li2O2 (bandgap: 5.15-6.37 eV)4, 5 either via hole tunneling6, 7 or polaronic transport (t is determined from hole polaron concentrations at spacecharge layers)8, resulting in limited film thickness (t < 10 nm, indicated as thin film). To elude this issue causing low capacity in the Li–O2 battery, the disproportionation of the solvated superoxide species (LiO2(sol)) has been preferred (indicated as solution-mediated growth). This chemical process allows for continuous growth of Li2O2 after supersaturation in the electrolyte solution, which results in large toroidal crystals (typical diameter of 0.1~1 µm at low current rates) with enlarged capacity (Figure S1a).9, 10 To increase the concentration of LiO2(sol), the solution free energy should be more dominant than the surface-binding energy from electrode, which can be achieved from the usage of high Gutmann donor number (DN) aprotic solvents and


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electrolyte anions.11, 12 The addition of soluble impurities such as trace amounts of H2O is an alternative way, which is beneficial for use in low DNs electrolyte solutions.13, 14 However, the solvation can lead to partially irreversible processes such as the precipitation of LiO2(sol) on the separator or Li electrode.10 In addition, large toroids in conjunction with being a wide-bandgap insulator require high overpotential during recharge (RC) (Li2O2(s) → O2 + 2e– + 2Li+).10 Unlike the general nucleation/growth process resulting in either thin films or large toroids, we demonstrate thick films of Li2O2 using 3-D CNT electrodes incorporated with CeO2 NPs in Li– O2 cells. The small CeO2 NPs (diameter ≤ 5 nm) contain oxygen vacancies on the surface which have strong binding energy to reactive oxygen species (e.g., O2, O2– and O22–),15, 16 primarily leading to surface nucleation and thin-film deposition. Further, subsequent growth leads to film thicknesses of 40–60 nm, which results in increased capacity. This reflects additional disproportionation from LiO2(sol) occurring on the existing thin films (Figure S1b), where LiO2(sol) is supplied from the unpassivated CNT surfaces. The CeO2 NPs were synthesized from the thermal decomposition of cerium precursor17 and subsequently incorporated onto CNTs through blending (see Experimental Section in SI). Electron microscopy images show monodispersed CeO2 NPs (Figure 1a) with an average diameter of 5.3 ± 0.6 nm (Figure 1c inset). They have fluorite structure (space group of Fm3m, Figure 1a inset and Figure S2) with predominant Ce4+ in the bulk (Figure S3) while also containing significant Ce3+ associated with oxygen vacancies on the surface, as evidenced by the Ce 3d photoelectron spectrum in Figure 1b.18-20 The CeO2/CNT composite contained a CeO2 NP mass ratio of 25% (Figure S4), which decorates the CNTs (Figure 1c). After vacuum-filtration, the CNTs construct a free-standing 3–D framework (Figure 1d) with a high BET surface area


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(~100 m2 g–1 and a pore (void) size of ~100 nm21). The resulting binder-free CeO2/CNT electrodes have a CNT mass of ~2 mg and a thickness of 50~60 µm. In dry LiClO4/tetraglyme solution (< 20 ppm of H2O measured by Karl Fischer titration) the DC profiles for CeO2/CNT electrodes in Li–O2 cells show potential plateaus at ~2.63 V (vs. Li/Li+) and a total capacity of 3.2 mAh at a current density of 0.1 mA (~0.09 mA cm–2geometry and ~50 mA g–1-CNT) at the cut-off potential of 2.2 V (Figure 2a). Transmission electron microscopy (TEM) images display the nucleation and growth of film products (i.e., Li2O2, see below) on the electrode surfaces that are closer to the O2 inlet side, at different depths of DC (Figure 2b-f and Figure S5-8). At the initial stage of DC (up to 0.4 mAh) the seeding of Li2O2 appears at the interface of CeO2/CNT (Figure 2b). The nucleation occurring in close proximity to the CeO2 NPs is attributed to the larger surface-binding energy of O2 and O2– (i.e. LiO2, see below) to CeO2 compared to the CNT surface. It has been reported that the O2 adsorption energy for CeO2 (max. –92 kcal/mol, the negative sign denotes exothermic.22) is much stronger than that for CNT (less than –1 kcal/mol for single-walled CNT23). In addition to the nuclei, the observation of thin Li2O2 deposits in close proximity to the CeO2 NPs and adjacent CNT surfaces (Figure S5) implies subsequent Li2O2 growth through the surface-mediated nucleation/growth. At 1.0 mAh, thin Li2O2 films with thicknesses between 5-20 nm passivate most of the CeO2/CNT surfaces (Figure 2c and Figure S6) while some CNT parts are still bare (Figure S6c), indicating inhomogeneity of the Li2O2 deposition process in localized areas. It is notable that the occurrence of Li2O2 film with a thickness of 5-20 nm, i.e. over the limitation of hole tunneling,6, 7 suggests the contribution of polaronic transport in Li2O2 with a high concentration of charge carriers8 and improved ionic and electrical conductivity in poorly crystalline Li2O2.24 In addition, the similar shaped Li2O2 films are also observed on the CeO2-free CNT electrodes under the


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same condition, suggesting greater surface adsorption to even CNT surface than the solvation affinity of LiO2 in dry LiClO4/tetraglyme (DNs of tetraglyme and anion of ClO4– is 16.625 and 8.44 kcal/mol,12 respectively), which is in good agreement with previous reports.13, 14 However, unlike previous results showing the rapid termination of DC after thin-film formation, the Li2O2 films (on either CeO2/CNT or (CeO2-free) CNT) become thicker as DC continues with thicknesses of ~25 nm at 2.0 mAh and ~40 nm at 3.2 mAh (Figure 2d–e, and Figures S7–8), which cannot solely be attributed to surface-mediated growth. A switch in the growth process to form the unexpectedly thick film can be inferred from the movement of CeO2 NPs. By comparing the as-prepared CeO2/CNT electrodes (Figure 1c–d), the migration and agglomeration of CeO2 NPs are observed at the very initial stages of DC (Figure 2b). In addition, a significant portion of the NPs detaches from the CNT surface and appears to float in the Li2O2 caused from the LiO2*/Li2O2 lifting the NPs upward. They are located at the uppermost part of the thin Li2O2 films (furthest from CNT) at 1.0 mAh (Figure 2c and Figure S6) while becoming embedded and immobile Li2O2 with the progression of further film growth when DC is over 1.0 mAh (Figure 2d–e, and Figures S7–8). Notably, the floating NPs are immobilized in the Li2O2 at a distance of 8–10 nm from the CNT surface. This implies the transition of the Li2O2 growth processes, from surface-mediated electrochemical deposition leading to the floating CeO2 NPs to solution-mediated precipitation enabling the immobilization of the embedded NPs (Figure 2f). The hypothesis to account for the latter process is the supply of LiO2(sol) depositing on the existing film,14 which can be verified by increasing the concentration of LiO2(sol). Figure 3a shows the DC profiles with trace amounts of H2O (200–2000 ppm) added to the electrolyte. The capacities enlarge (up to 3.9 mAh) with increasing H2O amounts while the


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potential plateaus are comparable. The FTIR spectra identify the main product as Li2O2 (Figure S9) and the electron microscopy images show the maintained film shape (Figure 3c, 3e and Figure S10). This demonstrates that the strong surface-binding energy of CeO2 NPs still overwhelms the solvation affinity causing surface nucleation to take priority. However, the total surface-binding energy is reduced as the CeO2/CNT becomes passivated, then the high solvation ability permitting the production of LiO2(sol) can aid in thicker-film growth (Figure S10). The increase in the film thicknesses to ~60 nm obtained with the presence of 2000 ppm of H2O demonstrates the contribution of increased LiO2(sol) (Figure 3e). Such thick film growth is only allowed when the surface-binding energy is predominant at the initial stage of DC. The CeO2free CNT possessing much weaker LiO2 binding energy shows the morphology of Li2O2 transforming from films to larger and thicker toroids (Figure 3d, 3f and Figure S10).13, 14 That is, the prevailing solvation over the surface binding on CNT with additional H2O leads to the solution-mediated supersaturation following by the toroidal crystal growth of Li2O2.9 The transformation in the Li2O2 morphologies changes the characteristics of DC profiles: The potentials gradually rise (2.7 V with 2000 ppm of H2O, Figure 3b) and capacities are pronouncedly improved (5.3 mAh). The toroidal crystals are further confirmed from the high Li2O2 crystallinity (100 and 101 reflections at 2θ = 32.8 and 34.9o, respectively) with X-ray diffraction (XRD, Figure 3g), which is a contrast to the poor crystallinity of the Li2O2 films of the CeO2/CNT.26 Rotating ring disk electrode (RRDE) measurements further support the suppression of LiO2(sol) at initial stages of O2 reduction in the presence of CeO2 NPs. Figure 3h and Figure S11 show the oxidation profiles of LiO2(sol) from the gold (Au) ring (jring, top panel, LiO2(sol) → Li+ + O2 + e–), originating during ORR from the CeO2/CNT or CNT containing disk


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electrode(jdisk, bottom panel), where the jring profiles relative to the corresponding jdisk indicate the solvation affinity (LiO2(sol)). In dry electrolyte, the onset of jring in CeO2/CNT occurs at a lower potential (2.5 V) relative to the ORR onset (2.7 V). The negligible release of LiO2(sol) at 2.5–2.7 V suggests strong LiO2 adsorption on CeO2/CNT. This is distinct from the CNT electrode showing concurrent onset for ORR and LiO2(sol) oxidation occurring at 2.7 V. In addition, the higher intensity of jring (and jring/jdisk) for CNT compared with that for CeO2/CNT suggests the more prominent emergence of LiO2(sol) from the CNT electrode, which is associated with the weaker LiO2 binding towards CNT. With the presence of H2O (2000 ppm), jring of LiO2(sol) appears at higher potential (~2.6 V) with CeO2/CNT. In addition, the increasing jring (and jring/jdisk) indicates the enhancement in solvation affinity. The CNT displays most pronounced jring towards lower potentials (< 2.2 V), which is attributed to the continuous supply of LiO2(sol). Along with the modulation of H2O contents, various current rates applied for Li-O2 cells can also demonstrate the role of CeO2 NP. It has been extensively studied for carbonaceous electrodes that toroid-like Li2O2 prominently forms at lower current rates.10, 27-29 Figure S12 shows discharge profiles with different current rates at 0.04 mA (20 mA g-1), 0.1 mA (50 mA g-1) and 0.2 mA (100 mA g-1). Both CeO2/CNT and CNT electrodes exhibit increased DC capacities with decreasing current rates, indicating an enhancement in LiO2(sol). The SEM image for the 1DC CeO2/CNT electrode at 0.04 mA displays thick Li2O2 film, demonstrating the prominent surface-mediated nucleation/initial growth. By contrast, the CNT electrode shows notable increase in capacity at 0.04 mA (~2000 mAh g-1 for CeO2/CNT vs ~3000 mAh g-1 for CNT) due to the bias towards the solution-mediated process. Taken all together, it is apparent that the contribution of LiO2(sol) leads to the formation of thicker films. Such thick Li2O2 films have not been observed when carbonaceous electrodes have


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low-surface area or flat 2-D structure.6, 13, 14 Therefore, we attribute the origin of LiO2(sol) to the locally unpassivated CNT surfaces, away from CeO2 NPs (Figure S6c), which mostly remain in the interior of high-surface area and 3-D structured electrodes (Figure S13-14). The surfacemediated growth process can be delayed in the interior of electrode due to possibly different conditions from the topmost electrode closer to the O2 inlet, with regard to the slow O2 diffusion, state (depth) of discharge, local potential, electrode structure (embedded within the CNT framework (inside) or directly exposed to O2 (uppermost electrode surface)), and local concentrations of O2, LiO2(sol) and impurities in the electrolyte solution and LiO2* and Li2O2 on the electrode surface. At the same time, gradually increasing levels of impurities (e.g. H2O,30, 31 LiO2(sol) and solvated side products) during the long DC process aids in enhancement in the solvation affinity of LiO2 rather than surface passivation. Accordingly, the emerging LiO2(sol) species float out and precipitate on the existing Li2O2 film via disproportionation. The microscopy images and XRD result (Figure 3c-e and 3g) reveal that the solution-mediated nucleation and subsequent growth resulting in toroidal Li2O2 is less favorable in the existence of Li2O2 film. This is likely due to the slow LiO2(sol) supersaturation (nucleation) process in electrolyte solution. Instead, the deposition of LiO2(sol) on the Li2O2 films leads to higher concentration of Li2O2 for the uppermost ~1 µm of the electrode (Figure S13). The nucleation and growth processes affect the decomposition rate during RC. Figure 4 shows DC and RC profiles at a fixed capacity of 2 mAh and the corresponding gas evolution rates using in situ quantitative on-line electrochemical mass spectroscopy (OEMS). It is apparent that the initial RC (up to 0.5 mAh) potentials are greater if the Li2O2 is toroid-like (i.e. increasing H2O amount on the CNT electrodes, Figure 4a-b). Correspondingly, the OEMS curves reveal lower rate of O2 gas evolution (Figure 4c-d). On the contrary, the Li2O2 films rapidly decompose with


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the evolution of larger amounts of O2 gas in the initial stages occurring below 4.0 V. After the initial RC, the Li2O2 film surfaces roughen with the appearance of voids (TEM images in Figure S15–16), indicating the predominant process of Li2O2 surface decomposition. For further RC (over 4.0 V), the O2 evolution rate is broadly constant where the bulk Li2O2 is decomposed together with formation/decomposition of side products (see X-ray absorption near edge structure (XANES) spectra in Figure S17). The total Li2O2 yield and amount of O2 gas evolved during RC in dry tetraglyme electrolyte are ~27 µmol (73–74% yield, by the chemical titration using TiOSO4,13 Figure S18) and 22–23 µmol (the average numbers of electrons for O2 evolution (e–/O2) are 3.3–3.4), respectively, for the CeO2/CNT and CNT electrodes. These results demonstrate the negligible effect of CeO2 NP for the formation/decomposition of Li2O2. The Li2O2 film formed with the addition of 2000 ppm of H2O on the CeO2/CNT also show similar RC behavior. However, the total amounts of Li2O2 and O2 gas are lower (Table S1), suggesting increased side reactions (Figure S9) and possible loss of LiO2(sol) precipitating away from the electrode during DC, which is also apparent for CNT containing toroidal Li2O2. Nevertheless, the sluggish initial decomposition is only observed for the toroid, which may be contributed to its large size and low ionic conductivity from the enhanced crystallinity.3, 4, 24, 26 Further, CO2 evolution starts at an earlier state of recharge (~60%) for the Li2O2 toroid electrode despite comparable amounts of total CO2 to all cases. This is possibly related to the collapsing of the toroidal shape,32 which may increase the amount of side products closely in contact with the CNT surface. On the contrary, the side products on the Li2O2 films remain until the films are almost completely decomposed (~80%). The overall results show the decreased reversibility for the formation and decomposition of toroidal Li2O2. Instead, the surface nucleation-based thick


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film causes the rapid surface decomposition, which can mostly be attributed to its morphological and structural characteristics. In summary, the nucleation, growth and decomposition of Li2O2 film were studied on CeO2/CNT electrodes. The LiO2 species adsorbed on the electrodes in dry tetraglyme solution leads to surface nucleation followed by electrochemical deposition. In addition, increasing the solvation affinity enhances the production of LiO2(sol) from unpassivated areas of the highsurface-area electrode, which results in the unexpectedly high thicknesses of the Li2O2 films. The concept of controlling the combination of surface-mediated nucleation and growth and subsequent solution-mediated growth introduces new aspects of Li2O2 formation that can guide future strategies to improve Li–O2 battery performance in conjunction with the development of stable catalysts33, electrodes34 and electrolytes35.

ASSOCIATED CONTENT Supporting Information. TGA, XRD, and Cerium L3-edge and O K-edge XANES spectra, SEM images of DC CNT electrode and more representative TEM images of DC and RC CeO2/CNT electrodes. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT


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This work is financially supported by RIKEN and JST ALCA-SPRING. The synchrotron XANES experiments at the SR center, Ritsumeikan University were performed with the approval of ‘project for creation of research platforms and sharing of advanced research infrastructure of MEXT’ (Proposal Nos. R1402, R1414 and R1417). Experimental support by Dr. Misaki Katayama and Prof. Yasuhiro Inada is highly appreciated. The authors thank the RIKEN Materials Characterization Support Unit and National Institute for Materials Science (NIMS) Battery Research Platform for assistance with TEM observation.

ABBREVIATIONS DC, discharge; RC, recharge; CNT, carbon nanotube; OEMS, online-electrochemical mass spectroscopy; XANES, X-ray absorption near edge structure.

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19. Reddy, B. M. K., A.; Yamada, Y.; Kobayashi, T.; Loridant, S.; Volta, J.-C. J. Phys. Chem. B 2003, 107, 5162-5167. 20. Beche, E.; Charvin, P.; Perarnau, D.; Abanades, S.; Flamant, G. Surface and Interface Analysis 2008, 40, (3-4), 264-267. 21. Bonnet-Mercier, N.; Wong, R. A.; Thomas, M. L.; Dutta, A.; Yamanaka, K.; Yogi, C.; Ohta, T.; Byon, H. R. Sci. Rep. 2014, 4, 7127. 22. Choi, Y. M. A., H.; Chen, H.-T.; Lin, M. C.; Liu, M. ChemPhysChem 2006, 7, 1957-1963. 23. Sorescu, D. C.; Jordan, K. D.; Avouris, P. J. Phys. Chem. B 2001, 105, (45), 11227-11232. 24. Tian, F.; Radin, M. D.; Siegel, D. J. Chem. Mater. 2014, 26, 2952-2959. 25. Laoire, C. O. M., S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. J. Electrochem. Soc. 2011, 158, A302-A308. 26. Yilmaz, E.; Yogi, C.; Yamanaka, K.; Ohta, T.; Byon, H. R. Nano Lett. 2013, 13, 4679-4684. 27. Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. J. Phys. Chem. Lett. 2013, 4, 1060-1064. 28. Griffith, L. D.; Sleightholme, A. E. S.; Mansfield, J. F.; Siegel, D. J.; Monroe, C. W. ACS Appl. Mater. Interfaces 2015, 7, 7670-7678. 29. Lau, S.; Archer, L. A. Nano Lett. 2015, 15, 5995-6002. 30. Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Barde, F.; Bruce, P. G. Angew. Chem. Int. Ed. Engl. 2011, 50, (37), 8609-13. 31. Gallant, B. M.; Mitchell, R. R.; Kwabi, D. G.; Zhou, J.; Zuin, L.; Thompson, C. V.; ShaoHorn, Y. J. Phys. Chem. C 2012, 116, 20800-20805. 32. Black, R.; Lee, J. H.; Adams, B.; Mims, C. A.; Nazar, L. F. Angew Chem. Int. Ed. Engl. 2013, 52, (1), 392-6.


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33. Bergner, B. J.; Schürmann, A.; Peppler, K.; Garsuch, A.; Janek, J. J. Am. Chem. Soc. 2015, 136, 15054-15064. 34. Xie, J.; Yao, X.; Madden, I. P.; Jiang, D.-E.; Chou, L.-Y.; Tsung, C.-K.; Wang, D. J. Am. Chem. Soc. 2014, 136, 8903-8906. 35. Adams, B. D.; Black, R.; Williams, Z.; Fernandes, R.; Cuisinier, M.; Berg, E. J.; Novak, P.; Murphy, G. K.; Nazar, L. F. Adv. Energy Mater. 2014, 5, 1400867.


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Figure 1. Characterization of CeO2 NPs and CeO2/CNT. (a) TEM image of CeO2 NPs. The lattice fringes of CeO2 in a high-resolution image (inset, the scale bar is 2 nm) with d-spacing of 0.31 nm indicates the 111 plane of fluorite structure. (b) Ce 3d photoelectron spectrum of the CeO2/CNT. The peaks, obtained by the spectral analysis, v0, v’, u0 and u’ (red) are addressed to Ce3+ and v, v’’, v’’’, u, u’’ and u’’’ (blue) are Ce4+. The v and u denote 3d5/2 and 3d3/2 spin-orbit splitting, respectively. (c) TEM image of CeO2/CNT and a size-distribution profile of CeO2 NPs from one hundred of NPs (inset). (d) SEM image of CeO2/CNT film and digital photo of the free-standing electrode (inset).


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Figure 2. Nucleation and growth of Li2O2 film on CeO2/CNT. (a) Galvanostatic discharge (DC) curves at a current density of 0.1 mA in dry 0.5 M LiClO4/tetraglyme. (b-e) TEM images of surface of DC electrodes (facing O2 side) at different limiting capacities of: (b) 0.4, (c) 1.0, (d) 2.0 and (e) 3.2 mAh. The yellow dashed boxes indicated as 1 and 2 in the top panels are magnified in the succeeding middle and bottom panels, respectively. The white dashed and dotted lines are a guide for the CNT surface and height of floating CeO2 NPs, respectively. (f) Schematic illustration of the nucleation and growth process.


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Figure 3. Morphological and structural characteristics of Li2O2 on CeO2/CNT and CNT with different H2O contents in 0.5 M LiClO4/tetraglyme. (a-b) Galvanostatic DC curves of (a) CeO2/CNT and (b) CNT at a current rate of 0.1 mA (~0.09 mA cm–2geometry) and a cutoff potential of 2.2 V. The H2O concentration is

Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with

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