Article pubs.acs.org/cm
Cite This: Chem. Mater. 2017, 29, 10542−10550
Bifunctional MnO2‑Coated Co3O4 Hetero-structured Catalysts for Reversible Li‑O2 Batteries Young Joo Lee,†,§ Do Hyung Kim,†,§ Tae-Geun Kang,† Youngmin Ko,‡ Kisuk Kang,‡ and Yun Jung Lee*,† †
Department of Energy Engineering, Hanyang University, 222 Wangshimni-ro, Seoungdong-gu, Seoul 04763, Republic of Korea Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea
‡
S Supporting Information *
ABSTRACT: The structural design and synthesis of effective cathode catalysts are important concerns for achieving rechargeable Li-O2 batteries. In this study, hexagonal Co3O4 nanoplatelets coated with MnO2 were synthesized as bifunctional catalysts for Li-O2 batteries. The oxygen reduction reaction catalyst (MnO2) was closely integrated on the surface of the oxygen evolution reaction catalyst (hexagonal Co3O4) so that this hetero-structured catalyst (HSC) hybrid would show bifunctional catalytic activity in Li-O2 batteries. A facile synthesis route was developed to form a unique HSC structure, with {111} facet-exposed Co3O4 decorated with perpendicularly arranged MnO2 flakes. The catalytic activity of the HSCs was controlled by tuning the ratio of Co to Mn (the ratio of OER to ORR catalysts) in the hybrids. With the optimized Co3O4-to-MnO2 ratio of 5:3, a Li-O2 cell containing the HSC showed remarkably enhanced electrochemical performance, including discharge capacity, energy efficiency, and especially cycle performance, compared to cells with a monofunctional catalyst and a powder mixture of Co3O4 and MnO2. The results demonstrate the feasibility of reversible Li-O2 batteries with bifunctional catalyst hybrids.
1. INTRODUCTION The increasing demand for higher energy and environmental safety has triggered much research to develop a system beyond the conventional Li-ion battery. Among these, lithium oxygen (Li-O2) batteries have been considered one of the most promising alternatives because these batteries have high theoretical specific energy (∼3500 Wh kg−1) based on the reaction (2Li + O2 = Li2O2, E0 = 2.96 V).1−3 Due to their promise for high energy, Li-O2 batteries have been considered extremely attractive for large-size battery systems, such as for electric vehicles and energy storage systems. However, despite their outstanding potential, there are many hurdles for pragmatic application of Li-O2 batteries, including poor cycle stability, low round-trip efficiency and low rate capacity from the sluggish oxygen reduction reaction (ORR), and low oxygen evolution (OER) kinetics and low electronic conductivity due to the insulating nature of the discharge products Li2O2.4−6 The desperate need for the rechargeable Li-O2 system has spurred tremendous research on highly efficient catalysts that can expedite the reaction kinetics.7−11 A large number of reports have suggested noble metals, transition metals, transitional metal oxides, and carbides with diverse types of structures as effective catalysts to reduce overpotential and enhance longterm stability.12−15 The cycle life of the state-of-the-art Li-O2 battery reported up to now is summarized in Table S1. Although the direct comparison of the performances from © 2017 American Chemical Society
different literature is not possible due to a wide variety of cell components and test conditions, the most prominent cycle number is 121 cycles from the system employing RuO2 with Mn2O3 catalysts.16 Among the candidates, transition-metal-based materials have good price competitiveness and environmental safety. MnO2 and Co3O4 are representative transition metal oxide catalysts for the Li-O2 battery. MnO2 has been regarded as an ORR catalyst in Li-O2 systems. Bruce et al. and other researchers have compared different manganese oxides, such as α-MnO2, βMnO2, γ-MnO2, commercial Mn2O3, and Mn3O4, and concluded that the ORR activity of MnO2 varies depending on the phase and crystal structure.17−20 Moreover, Cao et al. and other groups have studied the mechanism of the ORR activity in MnO2 in alkaline media, and they found that the catalytic activity is related to oxygen defects such as oxygen vacancies and hydroxyl groups.18,21,22 They suggested that the oxygen deficiencies are beneficial to surface adsorption of O2 and elongation of the O−O bond, which make the ORR activity of MnO2 stronger. On the other hand, Co3O4 has been generally known as an OER catalyst for Li-O2 batteries.23−25 Although the ORR catalytic effect of Co3O4 and its mechanism was studied by Xiao et al. and Gao et al.,26,27 Co3O4 has Received: November 18, 2017 Published: November 29, 2017 10542
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials Scheme 1. Synthesis Process of Hetero-structured Catalyst (HSC)
The autoclave was sealed and maintained at 120 °C for 10 h. After reaction, the brown product was collected by centrifugation, washed with distilled water, and then dried at 80 °C in a vacuum oven for 12 h. The dried Co(OH)2 powder was placed in a furnace to produce a Co3O4 hexagonal nanoplate. The recrystallization and oxidation was carried out at 450 °C for 2 h under an atmospheric environment. Next, to decorate the surface of the Co3O4 nanoplates with MnO2, the Co3O4 was first coated with carbon. The Co3O4 was fully immersed in 0.12, 0.28, and 0.4 M glucose solution and stirred for 24 h. The glucose solution was filtered without washing to obtain glucose-coated Co3O4, which was vacuum-dried overnight at 80 °C. The products were carbonized at 450 °C for 2 h in Ar gas, and the resulting powder was added to a 0.03 M KMnO4 solution under magnetic stirring for 1 h. The mixed solution was transferred into a Teflon-lined autoclave and heated at 160 °C for 5 h. During the hydrothermal treatment, the permanganate ions reacted with carbon and manganese oxides were produced (4MnO4− + 3C + H2O = 4MnO2 + CO32− + 2HCO3−). After cooling to room temperature, the final product was centrifuged, washed with distilled water, and dried at 80 °C in a vacuum oven for 12 h. The MnO2 powder was synthesized by a procedure described in the previous literature.32 Briefly, a 150 mL aqueous solution containing 0.8 g of MnSO4·3H2O and 2.5 g of KMnO4 was stirred for 2 h. The solution was transferred into a Teflon-lined stainless-steel autoclave, and the autoclave was sealed and maintained at 140 °C for 2 h. After cooling to room temperature, the final product was centrifuged, washed with distilled water and ethanol, and dried at 80 °C in a vacuum oven for 12 h. Li-O2 Cell Assembly and Battery Tests. The air cathodes were prepared by mixing the catalysts, Ketjen black carbon (KB), and polytetrafluoroethylene (PTFE, Sigma-Aldrich, 60 wt % dispersion in H2O) in isopropyl alcohol with a weight ratio of 7:2:1. The slurry was coated onto a stainless-steel mesh current collector and dried in a vacuum oven at 80 °C for 3 h. After punching them into circular pieces with a diameter of 1.4 cm (1−1.2 mg cm−2 loading of catalyst + KB), the electrodes were dried further in an 80 °C vacuum oven overnight. The Li-O2 batteries were assembled within an Ar-filled glovebox (O2 and H2O level < 0.1 ppm). The Li-O2 cell was assembled into a R2032 coin-type battery with a glass fiber separator (GF/D, Whatman), lithium foil (thickness, 300um, Honjo), and the air cathode. Holes (diameter of 1 mm, 21 holes) were punched in the bottom canister of the coin cell for oxygen flow. The electrolyte was 1 M LiTFSI (bis(trifluoromethane) sulfonamide lithium salt, 99.95%, SigmaAldrich) in TEGDME (tetraethylene glycol dimethyl ether, >98%, TCI). The water contents of the electrolytes were kept below 10 ppm, as determined by Mettler-Toledo Karl Fischer titration without exposure. After the cell assembly, the cells were stabilized under an oxygen atmosphere at 1.1 bar PO2 for 3 h. Electrochemical tests were conducted using a galvanostat (Maccor series 4000) in a voltage window of 2.3−4.5 V. The current density and capacities were calculated based on the mass of the KB loaded on the cathode. The linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) was measured in a three-electrode electrochemical cell with platinum (Pt) wire and 1 M NaOH Hg/HgO as the counter and reference electrodes, and the electrolyte was 0.1 M KOH solution with O2 saturated condition. The rotation speed was 1600 rpm and the scan rate was 5 mV s−1. For the electrode preparation, 12 mg of KB and 42 mg of each catalysts mixture were dispersed in isopropyl alcohol with Nafion. Next, 6 μL of prepared dilute slurry was drop-casted onto the glassy carbon working electrode of 3 mm in diameter. Cyclic voltammetry measurement was carried out in a three-
typically shown high catalytic OER activity depending on the electrical properties of the exposed surface.23,28 Su et al. reported differences in the OER catalytic effect depending on the exposed crystal planes of single crystalline Co3O4 and concluded that the {111} facet-exposed Co3O4 is the most reactive among other crystals.29 Zhu et al. attempted to unravel the catalytic mechanism on different crystal surfaces of Co3O4 through extensive density functional theory (DFT) calculations.30 Moreover, Rui et al. proposed that the higher specific capacity and good cycle performance of the Li-O2 battery catalyzed by (111) exposed Co3O4 might be attributable to the richer Co2+ and more active sites on that plane,27 and Song et al. suggested that Co3+ might play a crucial role for the high round-trip efficiency and cycle stability.23 Although the exact catalytic mechanism of Co3O4 is still under discussion, it has been proven that Co3O4 is catalytically active in the Li-O2 battery system. For the goal of synergistic integration of ORR and OER catalytic performance toward bifunctionality, Co3O4 and MnO2 have been combined and applied to metal-air battery systems.31 The best performance of the Co3O-MnO2 hybrid catalytic system in previous literature is also presented in Table S1. In this study, we synthesized {111} facet-exposed hexagonal Co3O4 nanoplatelets decorated with MnO2 as a bifunctional hetero-structured catalyst (HSC) for the Li-O2 battery. The ratio of Co3O4 to MnO2 in the HSC was controlled for optimized performance of the Li-O2 battery. Because ORR catalysts make intimate physical contact with OER catalysts, both ORR and OER catalytic effects were spatially interlaced during the discharge and charge process, resulting in superior bifunctional catalytic activity of the HSC. The superior catalytic activity of the HSC was analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM), in combination with electrochemical techniques. Li-O2 batteries employing a cathode loaded with HSC exhibited a long cycle life of 62 cycles at 200 mA g−1, higher capacity (increased to 5738.14 mAh g−1 from 3323.08 mAh g−1), and higher round-trip efficiency (76.6%) compared with cells with a noncatalyzed KB cathode (66.29%).
2. EXPERIMENTAL METHODS Synthetic Procedures. The HSC was fabricated through the steps illustrated in Scheme 1. The Co3O4 hexagonal nanoplate was made through the following procedure. First, 4.8 g of Co(NO3)2·6H2O was dissolved in a 40 mL mixture of ethanol and distilled water (1:1 by volume); then, 4 g of poly(vinyl pyrrolidine) (PVP) was added slowly as a surfactant. After 30 min of magnetic stirring, 100 mL of NaOH aqueous solution was added slowly for 2 h by a syringe pump. The color of the solution changed gradually from reddish pink to bluish green, indicating the phase change from α to β phase of Co(OH)2.29 To suppress the oxidation of Co, the precipitation reaction was conducted at low temperatures in an ice bath. Then, the β-Co(OH)2 solution was transferred into a Teflon-lined stainless-steel autoclave. 10543
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials
Figure 1. Characterization of materials: (a) FE-SEM images of Co3O4 (left), MnO2 (middle), and hetero-structured catalyst (HSC) (right) and (b) TEM image of HSC with EDS mapping. (c) and (d) are XRD patterns and FT-IR spectra each for comparing HSC with Co3O4 and MnO2. The white scale bar is 300 nm, and the yellow one is 100 nm. electrode system using a potentiostat with a voltage window of 2.3− 4.3 V at a scan rate of 0.5 mV s−1. The Li foil was used as the counter and 0.01 M Ag/Ag+ in acetonitrile was used as the reference electrode. The electrolyte, 1 M LiTFSI in TEGDME, was saturated with O2 Characterization. The crystal structure and phase were characterized by high-resolution X-ray diffraction (HR-XRD, 9k, SmartLab, Rigaku) with a Cu−Kα radiation source within 2θ = 10.0−80.0° with a step of 0.02° and scan rate of 1° min−1. The nanostructure and morphologies of the as-synthesized materials were analyzed by using a scanning electron microscope (SEM, Nova Nano SEM 450) with an energy dispersive X-ray spectroscopic (EDS) detector. The structural details of the crystal were further characterized by high-resolution transmission electron microscopy (HR-TEM, JEM 2100F, JEOL) in conjunction with the fast Fourier transform (FFT). The cathode was examined after discharge and charge using X-ray photoelectron spectroscopy (XPS, theta probe base system, Thermo Fisher Scientific Co). Fourier transform infrared spectroscopy (FT-IR) was performed with a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific Co.) to identify the chemical bonding of materials.
concentration of the carbon source and thus the amount of carbon deposit, adjusting the amount of MnO2. Characterization of Catalysts. The physical and chemical properties of the HSC as well as the Co3O4 and MnO2 were examined by SEM, TEM, XRD, and FT-IR. The SEM images of the Co3O4, MnO2, and HSC are shown in Figure 1a. The morphology of the Co3O4, after recrystallization and oxidation of Co(OH)2, is clearly shown to be hexagonal nanoplatelets (Figure 1a, left), and some mesoporous holes due to gas emission during the thermal decomposition of the precursor are present.29 Figure 1a (middle) of the MnO2 shows the layered flower-like structure with about 10 nm thick nanosheets arranged perpendicularly from the core. The {111} facetexposed Co3O4, with both Co2+ and Co3+ on the exposed surface, is known to be the most catalytically active crystal among the crystalline Co3O4 nanoparticles in Li-O2 systems.29 In Figure 1a (right), the hexagonal-shaped catalyst complex is covered with the MnO2 with the 10 nm thick nanosheets arranged perpendicularly from the core. We successfully modulated the structure so that the surfaces of the Co3O4 nanoplatelets are not completely covered and the space between MnO2 sheets is accessible. This structure effectively realizes bicatalytic activities from the physical contact between OER catalysts and the discharge products formed on ORR catalysts, which could significantly enhance the reversibility of Li-O2 cells. Moreover, the atomic ratio of Mn to Co is easily controlled by the amount of carbon deposited on the Co3O4. We named the HSC according to different Co:Mn ratios (Figure S1) as HSC-1, -2, -3 (Co:Mn = 6.89, 5.21, and 3.10, respectively). The efficient decomposition of discharge
3. RESULTS AND DISCUSSION Synthesis of HSC. As described in the Experimental Methods section, HSC composed of {111} facet-exposed hexagonal Co3O4 nanoplatelets decorated with MnO2 was synthesized through a sacrificial carbon coating method. The preformed hexagonal Co3O4 nanoplatelets were coated by carbon. The carbon coating layer reacts with permanganate, producing MnO2 on the surface of the Co3O4 nanoplatelets. The carbon coating on the Co3O4 plates (1) confines the MnO2 growth reaction specifically to the Co3O4 surface (4MnO4− + 3C + H2O = 4MnO2 + CO32− + 2HCO3−) and (2) tunes the ratio of Co to Mn in the HSC by controlling the 10544
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials
change in the Co2+-to-Co3+ ratio in the HSCs. Compared to Co3O4 only, the relative amount of Co3+ decreases in the HSCs, implying that partial reduction of cobalt ion might occur during the post process for MnO2 coating. Modulation of Catalytic Activity through Hybridization. Figure 2a−e shows the XPS O 1s spectra for the
products is expected when the distance between OER and ORR catalysts is smaller than the particle size of discharge products; however, a precise quantification of distance between two catalysts may be difficult since the size of discharge products is variable depending on the condition of discharge process, discharge capacity, and many others. The TEM images with EDS elemental mapping (Figure 1b) show more detailed observation of the HSC. In the EDS mapping, the elements cobalt, oxygen, and manganese are denoted by green, orange, and purple, respectively. In the HSC, the cobalt oxide surface is not completely covered with MnO2, as shown by the elemental distribution of Mn. The crystal structure of the Co3O4 nanoplates and the HSC was further examined by HR-TEM and the associated FFT spot patterns (Figure S2). The patterns of both the Co3O4 nanoplate and the HSC are indexed as (220), (202), and (022) crystal planes along the [111] zone axis, and the HR-TEM image shows the (011), (110), and (101) crystal planes with 0.28 nm d-spacing and 60° interfacial angle of hexagonal Co3O4. This confirms that the Co3O4 nanoplate is exposed with {111} facets. In addition, this also proves that the surface morphology and crystal structure of the hexagonal Co3O4 were maintained during the formation of the MnO2. Therefore, we speculate that the catalytic effect of Co3O4 would exist in the HSC, but it may be reduced by the reduction of effective exposed area. In the N2-sorption measurement, the shape of the isotherm curve is close to that of an IUPAC type III, which is indicative of nonporous or macroporous structures (Figure S3). The BET analysis estimated the surface area of the Co3O4, MnO2, and HSC-2 as 23.7, 26.8, and 26.6 m2 g−1, respectively. The surface areas of the MnO2 and the HSC-2 were greater than that of the Co3O4, probably due to the flower-like morphology of the MnO2. The XRD patterns of the three kinds of HSC (HSC-1, -2, and -3), showing peaks at 19° (111), 31.4° (220), 36.9° (311), 38.6° (222), 44.9° (400), 55.8° (442), 59.5° (511), and 65.4° (440) (Figure 1c), match well with the crystal structure of the hexagonal Co3O4. The XRD analysis confirms again that the crystal structure of the Co3O4 was maintained during the formation of the MnO2 on the Co3O4. The MnO2 only was identified as birnessite δ-MnO2, with broad and weak peaks at 12.5° (001), 25.2° (002), 36.2° (111), and 65.6° (020). However, the XRD patterns (Figure 1c) did not reveal the crystalline phase of the MnO2 of the HSCs of the three different Co to Mn ratios. This is indicative of an amorphous or poorly crystalline nature of the MnO2 nanoflakes deposited on the Co3O4 nanoplatelets.33,34 As shown in Figure 1d, FT-IR was used to investigate the functionalities of the materials under study. The absorption of HSC-2 is consistent with the peaks from both the Co3O4 and the MnO2 over the wavelength range of 500−4200 cm−1. The main bands of the MnO2 at 517 and 3433 cm−1 and those of the Co3O4 at 563 and 662 cm−1 are clearly found in the absorption spectrum of HSC-2. Moreover, the absorption peak at 517 cm−1 of the MnO2, which is the main characteristic band corresponding to Mn-O stretching modes of octahedral layers in the birnessite δ-MnO2 structure, is shown at the same position as in HSC-2.35 The FT-IR thus verified the coexistence of Co3O4 and birnessite-type MnO2. Figure S4 presents XPS Co 2p spectra of Co3O4 only and the HSCs. As shown by the Co3O4-only result, the XPS Co 2p peak intensity decreases as the Mn content in the HSCs increases due to the incremental surface coverage of the Co3O4 by the MnO2. In addition to the decrease in the peak intensity from the Co3O4-only case, the peak position shifts, indicating the
Figure 2. XPS O 1s spectra for (a) Co3O4, (b)−(d) HSC-1, -2, -3, and (e) MnO2. LSV curves measured in (f) ORR region and (g) OER region of KB, Co3O4, MnO2, and HSC-1, -2, -3 electrodes in O2 saturated 0.1 M KOH at a scan rate 5 mV s−1. The rotating speed was 1600 rpm. (h) Cyclic voltammetry (CV) result from 2.3 to 4.3 V at a 0.5 mV s−1 scan rate.
Co3O4, HSC1-3, and MnO2. The O 1s spectrum of each metal oxide or hybrid catalyst represents different types of oxygen species on the surface. The peak at the binding energy of 529.5−530 eV can be assigned to lattice oxygen (O2−), and the peak at 531−531.5 eV is from the surface absorbed oxygens, such as from low coordinated oxygen defects or vacancies and absorbed hydroxyls on the surface.22 Previous studies have reported that the activity of MnO2 toward oxygen reduction reaction depends on the amounts of defects, such as oxygen vacancies and OH groups, which are beneficial to surface adsorption of O2 and dissociation of O−O bonds.21 Theoretical calculations and experimental analyses indicate that oxygen adsorption on an oxygen vacancy site would cause elongation of the O−O bond, which results in the activation and partial dissociation of O−O bonds;36,37 thus, an increase of oxygen defects could indicate an enhancement of ORR activity. Moreover, there is a clear difference in the composition of absorbed oxygen between the MnO2 and the Co3O4, and the MnO2 obviously contains more absorbed oxygen peaks. For the HSCs, also, the composition of the absorbed oxygen in the XPS O 1s spectrum increases from 20.16% to 27.95% as the Co-toMn ratio decreases with the increase of the relative amount of 10545
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials
Figure 3. FE-SEM images of pristine (a, b, c, d), after discharge (e, f, g, h), and recharge (i, j, k, l) air cathodes. The white scale bar is 500 nm, and red circles indicate toroidal discharge products at each electrode after discharging.
MnO2. This implies that more oxygen defects are generated on the surface of the hybrid catalysts in the HSCs as the amount of surface MnO2 coating increases, although the exposed area of the Co3O4 is reduced due to the surface MnO2 coating. We thus suppose that these hetero-structured hybrid catalysts could show ORR activity modified from that of the MnO2. In addition, the OER catalytic performance of the Co3O4 in HSC could be lower than that of the pristine Co3O4 surface. In the XPS Co 2p spectra (Figure S4), the peak intensity of the HSCs is remarkably lower than that of the Co3O4 only due to the surface coverage by MnO2. It has been reported that the dissociation of the discharge product (Li2O2) is strongly related to the surface Co3+ and/or Co2+ sites of Co3O4 and that this process significantly reduces OER overpotentials in air cathodes with Co3O4 catalysts.27,30 Consequently, it is possible that excessive coverage of the Co3O4 surface with MnO2 for reinforcement of ORR activity could attenuate the OER catalytic effectiveness of Co3O4. However, balanced OER and ORR activities could enhance reversibility of electrochemical reactions, extending the cycle life of Li-O2 cells, thus, regulating the ratio of OER-ORR catalysts.Thus, regulating the ratio of OER-ORR catalysts is key for synergistic integration of ORR and OER catalysts. To investigate electrocatalytic activities of each catalyst, we carried out linear sweep voltammetry in O2 saturated 0.1 M KOH solution using the rotating disk electrode (RDE), at a rotation speed of 1600 rpm. In Figure 2f, there is little difference between the ORR onset potentials of KB and Co3O4, at about −0.2 V (vs 1 M NaOH Hg/HgO). On the other hand, MnO2 and MnO2 containing electrodes (HSC-1, -2, -3) exhibited positively shifted onset potentials at approximately −0.02 V. Moreover, the absolute values of kinetic currents (IK, mA cm−2) at −0.25 V increased with the following order, KB (1.33), Co3O4 (1.415), HSC-1 (2.94), HSC-2 (3.755), HSC-3 (4.675), and MnO2 (4.71). This indicates that MnO2 displays the highest ORR activity and the ORR catalytic activity among HSCs was proportional to the contents of MnO2 on Co3O4
surface. Figure 2g presents that the OER catalytic activities represented an inverse tendency with that of ORR activities of the catalysts employed. The OER IK at 1.0 V were 20.7, 12.05, 10.1, 8.15, 8.05, and 7.2 for Co3O4, HSC-1, -2, -3, and MnO2, respectively. Co3O4 showed the most active OER effect, and the OER catalytic activities among HSCs were reduced gradually as the amount of MnO2 on Co3O4 surface increases. To be noted is that Co3O4 and MnO2 certainly have ORR and OER activity, respectively, compared to noncatalyzed KB. The catalytic performance of the oxide catalysts in the Li-O2 system was further examined in cyclic voltammetry measurement in a three-electrode system (Figure 2h). Compared to the noncatalyzed KB electrode, the OER peak potentials of the Co3O4 electrode (3.6 and 4.0 V) are lower (3.8 and 4.1 V in KB) and the anodic current is much higher. In the cathodic scan, the ORR onset potential of the Co3O4 electrode is about 2.85 V, slightly higher than that of the KB (2.75 V), and the Co3O4 electrode exhibited higher cathodic current than the KB. Co3O4 thus displayed both higher ORR and OER activities compared to KB as in Figure 2f,g. The onset potential of the MnO2 during the cathodic scan is shifted higher compared with those of the KB or Co3O4. Because the MnO2 shows cathodic current in this voltage region in the CV measurement under Ar through Li+ ion intercalation and/or capacitive interaction (Figure S5),38−40 it is not clear at this moment whether the ORR onset potential of the MnO2 is shifted higher. The CV curves of the HSCs showed features of both MnO2 and Co3O4 at each cathodic and anodic curve; these varied, in particular, with the Co:Mn ratio. The lower the amount of MnO2 on the Co3O4 surface is, the more active the OER catalytic effect of the HSC is. This is consistent with LSV results in Figure 2g. We are not sure at this moment whether this is due to the higher exposed area of the Co3O4 OER catalyst or to the different ratio of Co2+ to Co3+ ions in the HSCs. On the other hand, as the MnO2 content increases, the CV curves approach that of the MnO2-only electrode similar to the result of Figure 2f. Here, we verified the successful modulation of the electrocatalytic property of the 10546
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials
catalysts-loaded electrodes are evaluated in Figure 5. Figure 5a presents the full galvanostatic discharge and recharge profiles at
HSCs between those of Co3O4 and MnO2 by formation of the hetero-structured hybrid. Characterization of Discharge Products. Ex situ SEM, XRD, and XPS investigations were carried out to identify the morphology and chemical properties of the discharge products (Figures 3 and 4). The electrodes were discharged and
Figure 4. (a) XRD patterns and (b) Li 1s XPS spectra of the discharged and charged electrodes.
recharged to a capacity of 2 mAh cm−2. Figure 3 presents SEM images of pristine, discharged, and recharged cathodes. The discharge products of all cathodes tested in this study assumed a disklike thin toroidal shape. For the MnO2-only electrodes, some larger toroids existed, probably due to the formation of discharge product on spherical, flower-like MnO2. In the XRD analysis (Figure 4a), the discharge product of the KB and Co3O4 electrodes was identified as crystalline Li2O2. On the other hand, the MnO2-containing electrodes, such as the MnO2-only and the HSC-2 cases, showed Li2O2 peaks, but the peaks suggested a poorly crystalline or almost amorphous nature. Especially, for the MnO2-only electrode, a very small and poorly crystalline LiOH peak was also identified at 2θ = 32°. The discharge product of the MnO2-only catalyzed electrodes is thus a mixture of LiOH and Li2O2, as shown by the XRD result. The existence of LiOH is not clear in the XRD data. The chemical nature of the discharge products was investigated further through XPS analysis. The XPS Li 1s spectra (Figure 4b) show the formation/decomposition of Li2O2 and other lithium compounds, such as Li2CO3, which is directly related to poor cycle performance of the Li-air battery.41 The discharge product of the KB and Co3O4 electrodes was Li2O2, consistent with the XRD result. The main discharge product of the MnO2-only and HSC-2 electrodes was also Li2O2, but with minor LiOH. The discharge product was completely decomposed after recharge of the electrodes catalyzed with Co3O4, MnO2, and HSC. However, the KB electrode showed residues corresponding mainly to Li2CO3 after recharge, as shown in Figure 4b. The instability related to carbon electrodes has also been noted in previous literature.41−44 The discharge product analysis revealed that, with surface modification of Co3O4 by MnO2 in HSC, the discharge product becomes poorly crystalline Li2O2 mixed with minor LiOH influenced by the MnO2 surface coating. Electrochemical Performance. The Li-O2 battery performances of the catalyst-free KB electrode and the oxide
Figure 5. (a) Initial discharge and charge curves of KB, Co3O4, MnO2, and HSC-2 based electrodes at a current density of 100 mA g−1 (voltage range 2.3−4.5 V). Cycling performance of (b) KB, (c) Co3O4, (d) MnO2, (e) Co3O4, + MnO2 (mechanical mix) and (f) HSC-2 cathodes with a limited capacity of 1000 mAh g−1 at a current density of 200 mA g−1 (the voltage range is from 2.3 to 4.5 V). (g) End voltage versus cycle number for each cycle behavior. (h) Rate capability at various current densities (50, 100, 200, and 500 mA g−1) at the same capacity (1000 mAh g−1).
a current density of 100 mA g−1 from 2.3 to 4.5 V vs Li+/Li. The discharge capacities of the KB, Co3O4, and MnO2 electrodes are 3323, 3506, and 5711 mAh g−1, respectively. The MnO2 electrode showed the highest capacity among the KB, Co3O4, and MnO2 electrodes, which is consistent with the literature. The HSC-2 electrode achieved the high discharge capacity of 5738 mAh g−1, similar to that of the MnO2-only electrode. Although the discharge potential is invariant, the MnO2-containing electrodes presented the highest discharge capacity, implying that MnO2 facilitates the discharge process. The charge overpotentials were compared by measuring the charge voltage at the capacity of half the full discharge capacity. The charge voltage of the Co3O4 electrode was 0.2 V lower than that of the KB electrode, suggesting the OER activity of Co3O4. More importantly, the HSC-2 electrode showed a charge voltage 0.12 V lower than that of the MnO2 electrode in the voltage profile. The Co3O4 in the HSC-2 hetero-structure might catalyze OER, lowering the overpotential. In Figure 5a, the Co3O4-only electrode showed a higher overpotential than in MnO2-only or HSC-2 electrodes, despite the higher OER 10547
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials
augmenting the probability of facilitated decomposition of the discharge product. The rate capabilities of the catalyzed and noncatalyzed electrodes are shown in Figure 5h. As the current density varied to 50, 100, and 200 mA g−1, the HSC-2 cathode maintained the lowest charge potential, yet the charge overpotential at 500 mA g−1 was elevated to the same level as the others. We assume that the low electrical conductivity of the reaction products results in high sensitivity to increase of current density.47,48 Summarizing our discussion, we materialized a heterostructured catalyst hybrid consisting of {111} facet-exposed Co3O4 with surface-coated MnO2 flakes. The catalytic activity of the hybrid catalyst was controlled by integrating two materials with different functions at varied ratios. As a result, the HSC hybrids that we designed could produce more discharge products and subsequently decompose them. With the optimized ratio between the amount of OER and ORR catalysts for balanced OER-ORR performance, the HSC hybrid demonstrated remarkably improved reversibility in Li-O2 batteries with a cycle life more than 4-fold longer than that of the noncatalyzed electrode.
catalytic activity of Co3O4. We speculate that this might be due to the large amount of discharge products in the full discharge test. With this huge amount of discharge products, a large portion of discharge products do not make direct contact with catalysts, especially with Co3O4 due to the planar nature of hexagonal Co3O4 plates. In this situation, the bulk chemical nature of the discharge products might dominate the decomposition kinetics due to the limited contact of discharge products with catalysts. The poorly crystalline discharge products in MnO2-only or HSC-2 electrodes are known to decompose better, showing lower charge overpotentials.45,46 In addition, the flower-like morphology of MnO2-only and vertical arrangement of MnO2 in HSC-2 might be beneficial for making contacts between discharge products and MnO2 catalysts that might have OER catalytic activity, though weaker than Co3O4. These all together could lead to the lower charge overpotentials in MnO2-only or HSC-2 electrodes. The first cycle reversibility is reflected in the round-trip efficiency, which is defined as the ratio of discharge to the charge voltage at the capacity of half the full discharge capacity. The KB electrode recorded an efficiency of only 66.29%, the lowest efficiency. Unlike the catalyst-free KB electrode, the catalyst-loaded electrodes reached round-trip efficiencies of 70.25%, 73.71%, and 76.60% for the Co3O4, MnO2, and HSC-2 electrodes, respectively. The cycling performance of these electrodes was evaluated at a current density of 200 mA g−1 and a cut-off capacity of 1000 mAh g−1 (Figure 5b−g and Figure S6). Figure 5g presents the end voltage vs cycles of the electrodes as an indicator of cycle life. Discharge capacity vs cycles and charge capacity vs cycles are also presented in Figure S6c,d. The HSC2 cell showed excellent cycling performance over 60 cycles, much longer than the results for KB (15 cycles), MnO2 (36 cycles), and Co3O4 (25 cycles). HSC-1 and HSC-3 were also tested under the same conditions (Figure S6a,b) and showed a cycle life of 50 and 38 cycles, respectively. All the HSC electrodes operate longer than the noncatalyzed KB and individual MnO2 or Co3O4 catalyst electrodes. The designed bifunctionality for ORR and OER is thus implemented in the HSCs. The electrodes with the bifunctional catalysts showed more stable cycle characteristics than the noncatalyzed and monofunctional catalyst electrodes, thus improving the reversibility of the electrochemical reactions. Among the HSCs of different compositions, HSC-2 with a Co-to-Mn ratio of about 5 (Co3O4:MnO2 = 5:3) showed the best cycling performance. As mentioned above, the balance between OER and ORR activity is key for achieving optimum bifunctionality for reversible Li-O2 cell operation, and here, HSC-2 has the optimum Co-to-Mn ratio. To verify the beneficial effect of hetero-structured hybrid formation between OER and ORR catalysts for bifunctionality, a powder mixture of Co3O4 and MnO2 was prepared with the same Co-to-Mn ratio as that of HSC-2 (Co-to-Mn ratio of 5). Although the powder mixture and HSC-2 had the same Co-to-Mn ratio, the powder mixture showed inferior cycling stability of 41 cycles (Figure 5e) compared to HSC-2. In a powder mixture, unlike in HSCs, physical contact between the Co3O4 and MnO2 is not guaranteed. In hetero-structured hybrid HSCs, Co3O4 and MnO2 inherently make contact; thus, this structure has higher probability that the Li2O2 formed by ORR activity of MnO2 is decomposed with the help of Co3O4. Moreover, the HSC structure developed in this study offers not only simple physical contacts between OER/ORR catalysts but also the unique perpendicular arrangement of MnO2 sheets on Co3O4,
4. CONCLUSION We report the development of a catalyst nanostructure that achieves bifunctionality in rechargeable Li-O2 batteries. The synthesized hetero-structured catalyst hybrid is a {111} facetexposed hexagonal Co3O4 coated with MnO2 flakes. The designed bifunctionality was successfully implemented in these HSCs, which showed synergistic catalytic activity for reversible Li-O2 operations. The catalytic sensitivity for OER and ORR could be modulated by controlling the ratio of Co to Mn in the HSC hybrids. The HSC-2 hybrid, with the optimum Co-to-Mn ratio of 5, showed high discharge capacity, close to that of MnO2, and outperformed in the cycle life assessment compared to the monofunctional catalysts and a powder mixture catalyst of Co3O4 and MnO2. Overall, this work confirms the feasibility of advanced designs for bifunctional catalysts in Li-O2 batteries.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04845. Elemental analysis from energy dispersive X-ray spectroscopy, HR-TEM and fast Fourier transform (FFT) data, nitrogen adsorption−desorption curve, XPS spectra for surface analysis, cyclic voltammetry, electrochemical data (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kisuk Kang: 0000-0002-8696-1886 Yun Jung Lee: 0000-0003-3091-1174 Author Contributions §
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y.J.L. and D.H.K. contributed equally to this work. 10548
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials Notes
(15) Xu, J.-J.; Wang, Z.-L.; Xu, D.; Zhang, L.-L.; Zhang, X.-B. Tailoring deposition and morphology of discharge products towards high-rate and long-life lithium-oxygen batteries. Nat. Commun. 2013, 4, 2438. (16) Yoon, K. R.; Lee, G. Y.; Jung, J.-W.; Kim, N.-H.; Kim, S. O.; Kim, I.-D. One-Dimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium−Oxygen Batteries. Nano Lett. 2016, 16 (3), 2076−2083. (17) Débart, A.; Paterson, A. J.; Bao, J.; Bruce, P. G. α-MnO2 Nanowires: A Catalyst for the O2 Electrode in Rechargeable Lithium Batteries. Angew. Chem. 2008, 120 (24), 4597−4600. (18) Cao, Y.; Yang, H.; Ai, X.; Xiao, L. The mechanism of oxygen reduction on MnO2-catalyzed air cathode in alkaline solution. J. Electroanal. Chem. 2003, 557, 127−134. (19) Cheng, F.; Su, Y.; Liang, J.; Tao, Z.; Chen, J. MnO2-based nanostructures as catalysts for electrochemical oxygen reduction in alkaline media. Chem. Mater. 2010, 22 (3), 898−905. (20) Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.; Shen, P. Facile controlled synthesis of MnO2 nanostructures of novel shapes and their application in batteries. Inorg. Chem. 2006, 45 (5), 2038− 2044. (21) Li, L.; Feng, X.; Nie, Y.; Chen, S.; Shi, F.; Xiong, K.; Ding, W.; Qi, X.; Hu, J.; Wei, Z.; Wan, L.-J.; Xia, M. Insight into the effect of oxygen vacancy concentration on the catalytic performance of MnO2. ACS Catal. 2015, 5 (8), 4825−4832. (22) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Enhancing electrocatalytic oxygen reduction on MnO2 with vacancies. Angew. Chem., Int. Ed. 2013, 52 (9), 2474−2477. (23) Song, K.; Cho, E.; Kang, Y.-M. Morphology and Active-Site Engineering for Stable Round-Trip Efficiency Li−O2 Batteries: A Search for the Most Active Catalytic Site in Co3O4. ACS Catal. 2015, 5 (9), 5116−5122. (24) Ryu, W.-H.; Yoon, T.-H.; Song, S. H.; Jeon, S.; Park, Y.-J.; Kim, I.-D. Bifunctional composite catalysts using Co3O4 nanofibers immobilized on nonoxidized graphene nanoflakes for high-capacity and long-cycle Li−O2 batteries. Nano Lett. 2013, 13 (9), 4190−4197. (25) Bergner, B. J.; Schürmann, A.; Peppler, K.; Garsuch, A.; Janek, J. r. TEMPO: a mobile catalyst for rechargeable Li-O2 batteries. J. Am. Chem. Soc. 2014, 136 (42), 15054−15064. (26) Xiao, J.; Kuang, Q.; Yang, S.; Xiao, F.; Wang, S.; Guo, L. Surface structure dependent electrocatalytic activity of Co3O4 anchored on graphene sheets toward oxygen reduction reaction. Sci. Rep. 2013, 3, 2300. (27) Gao, R.; Zhu, J.; Xiao, X.; Hu, Z.; Liu, J.; Liu, X. FacetDependent Electrocatalytic Performance of Co3O4 for Rechargeable Li−O2 Battery. J. Phys. Chem. C 2015, 119 (9), 4516−4523. (28) Wang, Z.; Wang, W.; Zhang, L.; Jiang, D. Surface oxygen vacancies on Co3O4 mediated catalytic formaldehyde oxidation at room temperature. Catal. Sci. Technol. 2016, 6 (11), 3845−3853. (29) Su, D.; Dou, S.; Wang, G. Single crystalline Co3O4 nanocrystals exposed with different crystal planes for Li-O2 batteries. Sci. Rep. 2015, 4, 5767. (30) Zhu, J.; Ren, X.; Liu, J.; Zhang, W.; Wen, Z. Unraveling the catalytic mechanism of Co3O4 for the oxygen evolution reaction in a Li-O2 battery. ACS Catal. 2015, 5, 73−81. (31) Xu, N.; Liu, Y.; Zhang, X.; Li, X.; Li, A.; Qiao, J.; Zhang, J. Selfassembly formation of Bi-functional Co3O4/MnO2-CNTs hybrid catalysts for achieving both high energy/power density and cyclic ability of rechargeable zinc-air battery. Sci. Rep. 2016, 6, 33590. (32) Zahoor, A.; Jang, H. S.; Jeong, J. S.; Christy, M.; Hwang, Y. J.; Nahm, K. S. A comparative study of nanostructured α and δ MnO2 for lithium oxygen battery application. RSC Adv. 2014, 4 (18), 8973− 8977. (33) Lee, S. W.; Kim, J.; Chen, S.; Hammond, P. T.; Shao-Horn, Y. Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors. ACS Nano 2010, 4 (7), 3889−3896. (34) Liu, R.; Lee, S. B. MnO2/poly (3, 4-ethylenedioxythiophene) coaxial nanowires by one-step coelectrodeposition for electrochemical energy storage. J. Am. Chem. Soc. 2008, 130 (10), 2942−2943.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Science & ICT (No. NRF-2014R1A2A1A11049801). This work was also supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (Grant No. 20174010201240).
■ ■
ABBREVIATIONS HSC, Hetero-structured catalyst REFERENCES
(1) Abraham, K.; Jiang, Z. A polymer electrolyte-based rechargeable lithium/oxygen battery. J. Electrochem. Soc. 1996, 143 (1), 1−5. (2) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. A reversible and higher-rate Li-O2 battery. Science 2012, 337 (6094), 563−566. (3) Jung, H.-G.; Hassoun, J.; Park, J.-B.; Sun, Y.-K.; Scrosati, B. An improved high-performance lithium−air battery. Nat. Chem. 2012, 4 (7), 579−585. (4) Li, F.; Zhang, T.; Zhou, H. Challenges of non-aqueous Li−O2 batteries: electrolytes, catalysts, and anodes. Energy Environ. Sci. 2013, 6 (4), 1125−1141. (5) Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Lithium−oxygen batteries: bridging mechanistic understanding and battery performance. Energy Environ. Sci. 2013, 6 (3), 750−768. (6) Lee, J.-H.; Black, R.; Popov, G.; Pomerantseva, E.; Nan, F.; Botton, G. A.; Nazar, L. F. The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium−oxygen batteries. Energy Environ. Sci. 2012, 5 (11), 9558−9565. (7) Lu, J.; Lee, Y. J.; Luo, X.; Lau, K. C.; Asadi, M.; Wang, H.-H.; Brombosz, S.; Wen, J.; Zhai, D.; Chen, Z.; Miller, D. J.; Jeong, Y. S.; Park, J.-B.; Fang, Z. Z.; Kumar, B.; Salehi-Khojin, A.; Sun, Y.-K.; Curtiss, L. A.; Amine, K. A lithium−oxygen battery based on lithium superoxide. Nature 2016, 529, 377−382. (8) Girishkumar, G.; McCloskey, B.; Luntz, A.; Swanson, S.; Wilcke, W. Lithium− air battery: promise and challenges. J. Phys. Chem. Lett. 2010, 1 (14), 2193−2203. (9) Kwak, W.-J.; Kang, T.-G.; Sun, Y.-K.; Lee, Y. J. Iron−cobalt bimetal decorated carbon nanotubes as cost-effective cathode catalysts for Li−O2 batteries. J. Mater. Chem. A 2016, 4 (18), 7020−7026. (10) Wang, Z.-L.; Xu, D.; Xu, J.-J.; Zhang, X.-B. Oxygen electrocatalysts in metal−air batteries: from aqueous to nonaqueous electrolytes. Chem. Soc. Rev. 2014, 43 (22), 7746−7786. (11) Jin, L.; Xu, L.; Morein, C.; Chen, C. h.; Lai, M.; Dharmarathna, S.; Dobley, A.; Suib, S. L. Titanium Containing γ-MnO2 (TM) Hollow Spheres: One-Step Synthesis and Catalytic Activities in Li/Air Batteries and Oxidative Chemical Reactions. Adv. Funct. Mater. 2010, 20 (19), 3373−3382. (12) Débart, A.; Bao, J.; Armstrong, G.; Bruce, P. G. An O2 cathode for rechargeable lithium batteries: the effect of a catalyst. J. Power Sources 2007, 174 (2), 1177−1182. (13) Jung, J.; Song, K.; Bae, D. R.; Lee, S. W.; Lee, G.; Kang, Y.-M. βFeOOH nanorod bundles with highly enhanced round-trip efficiency and extremely low overpotential for lithium-air batteries. Nanoscale 2013, 5 (23), 11845−11849. (14) Lu, Y.-C.; Xu, Z.; Gasteiger, H. A.; Chen, S.; Hamad-Schifferli, K.; Shao-Horn, Y. Platinum−gold nanoparticles: a highly active bifunctional electrocatalyst for rechargeable lithium− air batteries. J. Am. Chem. Soc. 2010, 132 (35), 12170−12171. 10549
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
Article
Chemistry of Materials (35) Huang, M.; Zhang, Y.; Li, F.; Zhang, L.; Ruoff, R. S.; Wen, Z.; Liu, Q. Self-assembly of mesoporous nanotubes assembled from interwoven ultrathin birnessite-type MnO2 nanosheets for asymmetric supercapacitors. Sci. Rep. 2015, 4, 3878. (36) Tompsett, D. A.; Parker, S. C.; Islam, M. S. Surface properties of α-MnO2: relevance to catalytic and supercapacitor behaviour. J. Mater. Chem. A 2014, 2 (37), 15509−15518. (37) Wang, F.; Wen, Z.; Shen, C.; Wu, X.; Liu, J. Synthesis of αMnO2 nanowires modified by Co3O4 nanoparticles as a highperformance catalyst for rechargeable Li−O2 batteries. Phys. Chem. Chem. Phys. 2016, 18 (2), 926−931. (38) Thackeray, M. M. Manganese oxides for lithium batteries. Prog. Solid State Chem. 1997, 25 (1), 1−71. (39) Park, H. W.; Lee, D. U.; Nazar, L. F.; Chen, Z. Oxygen reduction reaction using MnO2 nanotubes/nitrogen-doped exfoliated graphene hybrid catalyst for Li-O2 battery applications. J. Electrochem. Soc. 2013, 160 (2), A344−A350. (40) Ghodbane, O.; Pascal, J.-L.; Favier, F. Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors. ACS Appl. Mater. Interfaces 2009, 1 (5), 1130−1139. (41) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The carbon electrode in nonaqueous Li−O2 cells. J. Am. Chem. Soc. 2013, 135 (1), 494−500. (42) McCloskey, B.; Speidel, A.; Scheffler, R.; Miller, D.; Viswanathan, V.; Hummelshøj, J.; Nørskov, J.; Luntz, A. Twin problems of interfacial carbonate formation in nonaqueous Li−O2 batteries. J. Phys. Chem. Lett. 2012, 3 (8), 997−1001. (43) Xu, W.; Hu, J.; Engelhard, M. H.; Towne, S. A.; Hardy, J. S.; Xiao, J.; Feng, J.; Hu, M. Y.; Zhang, J.; Ding, F.; Gross, M. E.; Zhang, J.-G. The stability of organic solvents and carbon electrode in nonaqueous Li-O2 batteries. J. Power Sources 2012, 215, 240−247. (44) Gallant, B. M.; Mitchell, R. R.; Kwabi, D. G.; Zhou, J.; Zuin, L.; Thompson, C. V.; Shao-Horn, Y. Chemical and morphological changes of Li−O2 battery electrodes upon cycling. J. Phys. Chem. C 2012, 116 (39), 20800−20805. (45) Zhang, Y.; Cui, Q.; Zhang, X.; McKee, W. C.; Xu, Y.; Ling, S.; Li, H.; Zhong, G.; Yang, Y.; Peng, Z. Amorphous Li2O2: chemical synthesis and electrochemical properties. Angew. Chem., Int. Ed. 2016, 55 (36), 10717−10721. (46) Tian, F.; Radin, M. D.; Siegel, D. J. Enhanced charge transport in amorphous Li2O2. Chem. Mater. 2014, 26 (9), 2952−2959. (47) Guo, X.; Zhao, N. The role of charge reactions in cyclability of lithium−oxygen batteries. Adv. Energy Mater. 2013, 3 (11), 1413− 1416. (48) Gerbig, O.; Merkle, R.; Maier, J. Electron and ion transport in Li2O2. Adv. Mater. 2013, 25 (22), 3129−3133.
10550
DOI: 10.1021/acs.chemmater.7b04845 Chem. Mater. 2017, 29, 10542−10550
