Complete Decomposition of Li2CO3 in LiâO2 ... - ACS Publications

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Complete Decomposition of Li2CO3 in Li-O2 Batteries Using Ir/B4C as Non-carbon Based Oxygen Electrode Shidong Song, Wu Xu, Jianming Zheng, Langli Luo, Mark H. Engelhard, Mark E. Bowden, Bin Liu, Chongmin Wang, and Ji-Guang Zhang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04371 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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Complete Decomposition of Li2CO3 in Li-O2 Batteries Using Ir/B4C as Non-carbon Based Oxygen Electrode Shidong Song,1,2 Wu Xu,1* Jianming Zheng 1 Langli Luo,3 Mark H. Engelhard,3 Mark E. Bowden,3 Bin Liu,1 Chong-Min Wang,3 Ji-Guang Zhang1* 1

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA

99354, USA 2

School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin

300387, China 3

Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99354, USA

ABSTRACT: Instability of carbon based oxygen electrode and incomplete decomposition of Li2CO3 during charge process are critical barriers for rechargeable Li-O2 batteries. Here we report complete decomposition of Li2CO3 in Li-O2 batteries using ultrafine-iridium-decorated boron carbide (Ir/B4C) nanocomposite as a non-carbon based oxygen electrode. The systematic investigation on charging the Li2CO3 preloaded Ir/B4C electrode in an ether-based electrolyte demonstrates that Ir/B4C electrode can decompose Li2CO3 with an efficiency close to 100% at below 4.37 V. In contrast, the bare B4C without Ir electrocatalyst can only decompose 4.7% of preloaded Li2CO3. Theoretical analysis indicates that high efficiency decomposition of Li2CO3 can be attributed to the synergistic effects of Ir and B4C. Ir has a high affinity for oxygen species,

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which could lower the energy barrier for electrochemical oxidation of Li2CO3. B4C exhibits much higher chemical and electrochemical stability than carbon based electrode and high catalytic activity for Li-O2 reactions. A Li-O2 battery using Ir/B4C as oxygen electrode material shows highly enhanced cycling stability than those using bare B4C oxygen electrode. Further development of these stable oxygen-electrodes could accelerate practical application of Li-O2 batteries.

KEYWORDS: Li2CO3 decomposition, iridium catalyst, boron carbide, non-carbon oxygen electrode, lithium-oxygen battery


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Lithium-oxygen (Li-O2) batteries have attracted great attention owing to their highest theoretical energy density among various energy storage systems.1-3 However, the development of Li-O2 batteries is largely lagged by inevitable decompositions of nonaqueous electrolytes and carbonbased oxygen electrodes during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), leading to poor cycling stability and low energy efficiency of batteries. Lithium carbonate (Li2CO3), one of the main side reaction products through Li2O2 + C + 1/2O2 → Li2CO3, 2Li2O2 + C → Li2O + Li2CO3, and decomposition of electrolyte solvent,4 normally requires a high charging voltage (>4 V)5,6 to be electrochemically decomposed during charge process. Accordingly, the oxidation of Li2CO3 raises the overpotential of OER and in turn causes further oxidation of the electrolytes and carbon-based oxygen electrodes under the elevated voltage, which produces more Li2CO3 again, forming a vicious cycle of performance decay. Albeit the charge voltage of Li-O2 batteries can be remarkably reduced below 4 V with the aid of redox mediators,7-9 the parasitic reactions still exist which cause continuous accumulation of the undecomposed Li2CO3 and irreversible coverage of active sites on oxygen electrodes thus making the ORR and OER more difficult. It is noteworthy that the currently applied mediators cannot oxidize Li2CO3 due to their lower redox potentials than that for Li2CO3 decomposition (Equation 1). Li2CO3 → CO2 + 1/2O2 + 2Li+ + 2e─

3.82 V vs. Li/Li+

(1)

The use of non-carbon-based oxygen electrode materials can effectively avoid the corrosion of carbon materials during the operation of Li-O2 batteries.10-13 However, Li-O2 batteries with non-carbon-based oxygen electrodes still suffer from the poor stability of nonaqueous electrolytes.13-15 So far, no aprotic electrolyte is truly stable in Li-O2 system. Indeed, the formation and accumulation of Li2CO3 in Li-O2 batteries with cycling is still inevitable at


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present, which severely hinders the practical application of Li-O2 batteries. The complete decomposition of Li2CO3 at a relatively low charge voltage is vital for alleviating the side reactions, thereby improving the cycle life and energy efficiency of Li-O2 batteries. Very recently, in the newly emerged Li-CO2 batteries,16,17 Li2CO3 is also one of the main discharge products (Equation 2). The complete electrochemical oxidation of Li2CO3 during charging process will greatly accelerate the development of Li-CO2 batteries as well. 2Li+ + 3/2CO2 + 2e─ → Li2CO3 + 1/2C

2.80 V vs. Li/Li+

(2)

Several theoretical analyses have been conducted to reveal the energy barrier and reaction mechanism of Li2CO3 decomposition.18,19 Yang et al. indicated that Li2CO3 may not be decomposed following the Equation (1).19 Instead, it decomposed into CO2, superoxide radical anion (O2•─) and O2 that the latter two products would be consumed in oxidation of electrolyte solvent as no O2 evolution was observed during charging.19 Chen et al. reported that the rate determining step for Li2CO3 decomposition was the electrochemical extraction of Li+.18 They proposed that the incorporation of redox active species that acted as the mediator to compensate the electron loss for Li+ extraction could be an effective strategy to lower the energy barrier, that is, the polarization for electrochemical oxidation of Li2CO3. The barrier for electrochemical extraction of Li+ from Li2CO3 (0.67~0.90 eV) was much higher than that from Li2O2 (0.16~0.32 eV) probably because the oxygen species (O2, O2•─, O22─) formed during Li2O2 decomposition could provide active redox centers that compensate electron loss by varying the valence state. However, very few catalysts for Li2CO3 oxidation are reported in the literature. Wang et al. reported that Li2CO3 in NiO-Li2CO3 nanocomposite thin film prepared by pulsed laser deposition technique could be decomposed with the aid of NiO catalyst.20 However, Li2CO3 appeared to be incompletely decomposed as the charge capacity delivered was lower than the theoretical


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capacity for the preloaded Li2CO3. Additionally, the loading of Li2CO3 was low (~25 µAh cm-2) and NiO, as a semiconductor, required to work with conductive materials. Hong et al. incorporated nanoporous NiO plates on the carbon nanotube (CNT) electrode for Li-O2 batteries and demonstrated the oxidation of carbonate and carboxylate species could be remarkably enhanced by NiO catalysts.21 Subsequently, Tan et al. prepared a nano-structured RuO2/NiO cathode for Li-air batteries using LiFePO4 as anode and realized a stable cycling operation in ambient air for 400 hrs (200 cycles).22 In the latter two works mentioned above, the galvanostatic charging tests of Li2CO3-preloaded electrodes are not involved, so the decomposition efficiency of Li2CO3 cannot be evaluated. Here we report the preparation and application of iridium-decorated boron carbide (Ir/B4C) composite for electrochemical oxidation of Li2CO3. The decomposition of Li2CO3 in Li2CO3-preloaded electrodes with a relatively high loading amount (2.5~3 mgLi2CO3 cm-2, corresponding to the theoretical decomposition capacity of 1.82~2.18 mAh cm-2) has been systematically investigated. B4C has high chemical and electrochemical stability, good conductivity as well as high catalytic activity towards ORR and OER.23 It has been demonstrated as a highly stable non-carbon based oxygen electrode material for Li-O2 batteries (250 cycles, about 1600 hrs).13 Iridium (Ir) is one of the most corrosion-resistant metal and a well-known high-efficiency catalyst for OER as well as the oxidation of some organic molecules.24-27 Paticularly, Ir has a high affinity to adsorb reversible oxygen species generated during oxidation process at relatively low positive potentials, which offers the outstanding catalytic activity for electrochemical oxidation reactions.26,27 Ir can also stabilize LiO2 as the discharge product in LiO2 batteries

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due to the excellent absorbability of oxygen species on Ir surface. It is therefore


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expected that Ir/B4C can effectively lower the barrier of Li2CO3 decomposition and as a result improve the cycling performance of Li-O2 batteries. The TEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Figure 1a and b clearly reveal that ultrafine nanoparticles (NPs) are well-dispersed on the B4C particle surface and the size of these ultrafine NPs has a narrow distribution (0.7 nm ~ 2.8 nm) (also seen in Figure S1) and an average size of 1.78 nm. The HRTEM image in Figure 1c shows that these ultrafine NPs have well-matched distances of Ir (200) and Ir (111) crystal planes. Therefore, Ir/ B4C composite NPs are successfully prepared. The Ir loading was calculated to be about 1.8 mg-Ir/g-B4C from the contents of Ir and B obtained by the energy dispersive X-ray spectroscopy (EDX) elemental analysis. The conventional inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis is not feasbile for Ir/B4C because Ir is chemically resistant to most conventional acids and even aqua regia.

Figure 1. TEM (a) and HAADF-STEM (b) images of Ir/B4C NPs show ultrafine Ir NPs are welldispersed on B4C NPs. The inset in (b) shows a size distribution of Ir NPs with an average size of 1.78 nm. The HRTEM image (c) shows lattice of both B4C and Ir NPs with well-matched distances of Ir (200) and (111) crystal planes (insets) for the ultrafine NPs.


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The charge performances of Li2CO3-based cells are shown in Figure 2. The B4C-Li2CO3 cell shows a rather low charge capacity at the cut-off voltage of 4.4 V. The capacity corresponds to only about 4.7% of the theoretical capacity if the preloaded Li2CO3 is fully decomposed. In contrast, the Ir/B4C-Li2CO3 cell delivers a charge capacity corresponding to the theoretical capacity of all preloaded Li2CO3 with a charge voltage is only 4.37 V, indicating that Ir/B4C possesses an outstanding electro-catalytic activity towards Li2CO3 decomposition. The cells using blank B4C electrode and Ir/B4C electrode without Li2CO3 were charged under the same condition in Ar to reveal the capacity contributed from the electrochemical oxidation of the electrolyte. As seen in the inset of Figure 2, both cells show negligible capacities which are 0.006 mAh cm-2 for the B4C blank cell and 0.022 mAh cm-2 for the Ir/B4C blank cell, far below the theoretical capacities for complete decomposition of Li2CO3 (1.82~2.18 mAh cm-2). The blank experimental results reveal that the tetraglyme is relatively inert in the presence of B4C and Ir/B4C within the electrochemical window range from open circuit voltage (OCV) to 4.4 V, though Ir/B4C appears to promote the electrolyte oxidation to some minor extent. It is not surprising since Ir has been proved to be highly active for oxidation of some organic molecules.26,27


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Figure 2. Charge voltage profiles of B4C- and Ir/B4C-Li2CO3 electrodes in Ar. The inset shows the charge profiles of the bare B4C and the Ir/B4C electrodes without preloaded Li2CO3.

The B4C-Li2CO3 and Ir/B4C-Li2CO3 electrodes before- and after-charging are characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy EDX, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Figure 3 shows the SEM images and the EDX results of the Li2CO3 preloaded electrodes before and after charging. The surface morphologies (Figure 3a, c and g) of the before- and after-charged B4C-Li2CO3 electrode samples and the before-charged Ir/B4C-Li2CO3 electrode sample show the presence of large particles with several micrometers distributed among the submicron sized particles. The EDX oxygen dot-mapping images (Figure 3b, d, f) clearly demonstrate that these big micro sized particles contain O element and they are Li2CO3 particles. It is indicated from the surface mophologies of the before- and after-charged B4C-Li2CO3 electrodes (Figure 3a and c) that Li2CO3 cannot be effectively decomposed by B4C even after charged to 4.4 V, in agreement with the very low decomposition efficiency for the B4C-Li2CO3 cell as shown in Figure 2. On the contrary, Figure 3g shows obvious difference in surface morphology of the Ir/B4C-Li2CO3


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electrode after charging from the surface of the electrode before charging (Figure 3e). There are several pits of micron size in Figure 3g but no big Li2CO3 particles is observed. The surface morphology of the after-charged Ir/B4C-Li2CO3 electrode looks highly porous and loose due to the effective removal of Li2CO3 micro particles in the electrode. A small content of oxygen for the after-charged Ir/B4C-Li2CO3 electrode shown in Figure 3h may be from the decomposed electrolyte during charging.


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Figure 3. SEM images of B4C-Li2CO3 electrode (a, c) and Ir/B4C-Li2CO3 electrode (e, g) before (a, e) and after charging (c, g); EDX oxygen dot-mapping images captured from the regions shown in the corresponding SEM images at left side for B4C-Li2CO3 electrode before (b) and after (d) charging and Ir/B4C-Li2CO3 electrode before (f) and after (h) charging.


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The XRD patterns shown in Figure 4 reveal consistent results to those of SEM and EDX observations. The intensity and the position of Li2CO3 diffraction peaks for the B4C-Li2CO3 electrode almost do not change before and after charging, again ascribed to the low decomposition efficiency for the B4C-Li2CO3 electrode. However, for the Ir/B4C-Li2CO3 electrode, there are no evidence of Li2CO3 peaks found in the XRD pattern of charged sample, even for the strongest peak at about 21.2° (the inset figure in Figure 4). Since the Ir/B4C-Li2CO3 cell can deliver the full theoretical charge capacity associated with the complete decomposition of Li2CO3, it is concluded that the Li2CO3 preloaded in the Ir/B4C-Li2CO3 electrode can be thoroughly decomposed (at least below the detect limit of XRD).

Figure 4. XRD patterns of the Li2CO3 preloaded electrodes before and after charging. The inset shows the expanded view of the Li2CO3 peak at 21.2o. The electrocatalytic effect of B4C and Ir/B4C on charging the Li2CO3 preloaded electrodes is further evaluated by analyzing the surface compositions of B4C-Li2CO3 and Ir/B4CLi2CO3 electrodes before and after charging by XPS. The XPS results are shown in Figure 5 and S2. The deconvolution of the O1s spectra (Figure 5a) shows the existence of two possible oxygen chemical states with binding energies centered at 531.3 and 533 eV, which correspond to


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C=O and C−O bonds, respectively. Both bonds are from Li2CO3 and the content of the former (C=O) is relatively higher than that of the latter (C−O) for the B4C-Li2CO3 and Ir/B4C-Li2CO3 electrodes before charging.29 For the B4C-Li2CO3 electrode after charging, the O1s spectrum looks almost the same in peak position and intensity as compared to that for the electrode before charging, which indicates that Li2CO3 is not effectively decomposed in the B4C-Li2CO3 electrode during charging. The XPS O1s spectrum for the Ir/B4C-Li2CO3 electrode after charging shifts obviously to the high binding energy when compared with that for the electrode before charging. This change demonstrates the significant increase in relative content of C−O accompanied with considerable decrease of C=O content. Additionally, the content of the C−O becomes much higher than that of C=O. The reduction in content of C=O should be ascribed to the effective decomposition of Li2CO3. On the other hand, the enhanced content of C−O bond may indicate the formation of some C−O containing organic molecules during charging. Considering the relatively higher activity of Ir/B4C towards electrolyte oxidation as revealed in the blank experiment (inset figure in Figure 2), and the ether-based solvent (i.e. tetraglyme) applied in this work, the organic species (such as alkyl carboante or carboxylate) may be formed by the oxidation of tetraglyme, which may contribute to the small portion of C=O species in the O1s spectrum for the Ir/B4C-Li2CO3 electrode after charging.


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Figure 5. XPS spectra of the Li2CO3 preloaded B4C and Ir/B4C electrodes before and after charging. (a) O1st spectra, (b) Li1s spectra, (c) F1s spectra, (d) S2p spectra, (e) B1s spectra, and (f) C1s spectra.


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The Li1s spectra in Figure 5b show a Li2CO3 peak at 54.8 eV for the B4C-Li2CO3 electrodes before and after charging and for the Ir/B4C-Li2CO3 electrode before charging. For the B4C-Li2CO3 electrode, Li1s spectra (Figure 5b) show no obvious change in Li2CO3 peak before and after charginga similar result to those of O1s spectra (Figure 5a), again in good agreement with the poor charge performance and low Li2CO3 decomposition efficiency as shown in Figure 2. Impressively, the Li2CO3 peak in the Li1s spectra of after-charged Ir/B4C-Li2CO3 electrode cannot be found, which strongly suggests the complete extraction of Li+ from Li2CO3 or complete decomposition of the high-loading Li2CO3 when Ir/B4C was used as the electrocatalytic substrate under low charge voltage of 4.37 V (Figure 2). The oxygen species adsorbed on the surface of Ir/B4C may benefit to compensate the charge balance for Li+ extraction by redox reaction between O2 and O2•─ so that the energy barrier for Li2CO3 decomposition can be effectively lowered.18 Meanwhile, these adsorbed active surface oxygen species may also promote the electrochemical oxidation of electrolyte especially solvent to a small extent. The F1s spectra in Figure 5c show a C-F bond at 689.1 eV from PTFE binder for all the electrode samples. The Li−F bond cannot be found around 684.8 eV,30 indicating the PTFE binder is relatively stable during charging process. The S2p spectra for both Li2CO3-based B4C and Ir/B4C electrodes after charging show a small bump at 169.5 eV (Figure 5d), corresponding to S−O bond, which indicates the decomposition of LiTf salt. The peaks in B1s spetra (Figure 5e) at 188.4 and 190.5 eV correspond to B−B and B−C bonds, respectively.31 Either B4C-Li2CO3 electrode or Ir/B4C-Li2CO3 electrode after charging shows nearly the same profiles as those for the corresponding electrodes before charging. Also there is no B−O species observed at around 193 eV for both after-charged electrodes, illustrating that B4C is stable against electrochemical oxidation and/or attacks from oxygen species generated during charging of Li2CO3 (Equation


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1).32 In Figure 5f, the C1s spectra show C−F bond at 291.8eV from PTFE binder and B−C bond at 284.7 eV from B4C. Other peaks at 289.3, 286.9 and 284.8 eV correspond to C=O, C−O and C−C bonds, respectively.33 After charging, C−F bond and C=O bond has no change for the B4CLi2CO3 electrode, suggesting PTFE binder is stable and Li2CO3 cannot be effectively decomposed. However, small changes in C−O bond and C−C bond can be observed for the aftercharged B4C-Li2CO3 electrode, in which a small bulge appears at 286.9 eV, revealing the increase in the content of C−O bond. It can also be deduced that the content of C−C bond increases as well since the XPS peak between 284 eV to 285 eV shifts to higher binding energy compared with that for the B4C-Li2CO3 electrode before charging. Both increases in contents of C−O bond and C−C bond may be attributed to the oxidation of the electrolyte. For the Ir/B4CLi2CO3 electrode the changes in peaks for C−O bond and C−C bond are more evident after charging, which is consistent with the higher activity of Ir/B4C towards solvent oxidation (inset figure in Figure 2). In Figure S2, the Ir4f spectra shows no difference between before- and aftercharged Ir/B4C-Li2CO3 electrodes. The chemical state of Ir electrocatalyst shows no change, indicating Ir can be stable during electrochemical oxidation reaction. Based on the above results, the possible reacion mechanisms for Ir/B4C to catalyze Li2CO3 decomposition are suggested as below according to the theoretical analysis in 18,19. Li2CO3 → CO2 + 1/2O2 + 2Li+ + 2e─

(3)

Li2CO3 → CO2 + 2Li+ + 1/2O2•─ + e─

(4)

O2•─ → O2 + e─

(5)

O2•─ + O2 + electrolyte → organic molecules containing C−O, C−C and C=O bonds


(6)

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In these steps, step 3 represents the Li+ extraction from Li2CO3 and is the rate determining step of overal electrochemcial oxidation reaction,18 step 4 represents the decomposition of CO32− anion that produces CO2 and O2•─,19 steps 5 decribes the adsorption of O2•─ on Ir surfaces of Ir/B4C, step 6 is a fast redox reaction between O2ad and O2•─ad to release electron to compensate charge balance of Li+ extraction, and step 7 describes the potencial attack from active oxygen species (O2ad and O2•─ad) to the electrolyte, that is, the side reactions of electrolyte oxidation by Ir catalyst.

Figure 6. Discharge/charge voltage profiles (a), capacity vs. cycle number (b) and cycle curves (c: B4C cell, d: Ir/B4C cell) of Li-O2 cells using B4C and Ir/B4C oxygen electrodes under 200 mAh g-1 at 10 mA g-1 within the cut-off voltage range of 2.0 ~ 4.4 V.


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Ir/B4C is further evaluated as the oxygen electrode material in Li-O2 batteries, with B4C as a reference material. Both Ir/B4C cell and B4C cell are tested under capacity limited protocol of 200 mAh g-1 at 10 mA g-1 with the cut-off voltage range of 2.0 ~ 4.4 V, as tetraglyme is relatively stable below 4.4 V (inset figure of Figure 2).16,34 As shown in Figure 6a, the B4C cell cannot achieve the charge capacity even from the first cycle, probably due to its relatively low activity for OER and a low cut-off charge voltage of 4.4 V applied. The discharge performance of the B4C cell is slightly better than its charge performance. However, the cell degrades very fast and can only deliver a capacity of about 167 mAh g-1 in the 5th discharge process. Meanwhile, the corresponding charge capacity decreases to about 102 mAh g-1. The reason for the poor cycling performance of the B4C cell under 200 mAh g-1 may due to the low specific surface area of B4C material (12.6 m2 g-1),13 which cannot provide a large number of active sites for Li-O2 reactions to achieve a relatively high capacity. Once the electrode surface is covered by side reaction products, typically Li2CO3, B4C with the low activity to decompose Li2CO3 cannot effectively recover the electrode surface by charging under 4.4 V. The accumulation of insulating Li2CO3 will then result in a fast increase in cell impedance (Figure S3) and rapid loss of the active sites, which can be verified by SEM images of the B4C oxygen electrodes before and after cycling, shown in Figure 7a and 7b. In Figure 7a, the B4C electrode before test shows porous structure with micro-pores from tens to hundreds of nanometers and the contour of particles is clear and sharp. After 25 cycles, the surface of B4C particles are covered by a solid film composed of side reaction products and undecomposed discharge product (Li2O2) due to incomplete charge process below 4.4 V (Figure 7b). Additionally, the solid film appears to be poor conducting under SEM observation as demonstrated by the white spots (serious charging


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effect) on the sample. Though there are still some pores can be found in the electrode to transfer electrolyte and oxygen, the Li-O2 reactions may hardly occur due to the full coverage of active sites. On the contrary, the Ir/B4C cell shows much better performance than the B4C cell, either in charge or in discharge process. The discharge performance in the 5th discharge process is still overlapped with that in the first discharge process (Figure 6a). The Ir/B4C cell can achieve both the controlled discharge and charge capacity for 20 cycles (Figure 6b and 6d), in contrast, the B4C cell can only deliver the preset discharge capacity for three cycles (Figure 6b and 6c). The Ir/B4C cell can still be charged to 200 mAh g-1 for up to 23 cycles, suggesting an outstanding charge performance of the Ir/B4C oxygen electrode. Due to the long cycle time (40 hrs for each cycle) applied, the Ir/B4C cell can be operated for about 977 hrs for 25 cycles. In contrast, the B4C cell is almost dead after 200 hrs. The SEM images shown in Figure 7c and 7d demonstrate that the surface of the Ir/B4C electrode after 977 hrs cycling (Figure 7d) looks almost the same as the pristine Ir/B4C electrode (Figure 7c). The contour of Ir/B4C particles is still clear and sharp (Figure 7d). Though accumulation of side reaction products may cause the performance decay for the Ir/B4C cell as well, the solid film covering the electrode cannot be clearly observed. The SEM images under relatively low magnification shown in Figure S4(a~d) further verified the results shown in Figure 7. These results demonstrate that the use of Ir/B4C electrode can completely decompose Li2CO3 and the side reactions on the coverage of oxygen electrodes can be almost eliminated. As Ir/B4C can effectively remove the coverage of Li2CO3 on the oxygen electrode surface at relatively low charge voltages, the active sites for Li-O2 reactions on the oxygen electrode can be recovered to a large extent after each charging process. In addition, unlike the carbon material which is readily oxidized to form side reaction products, such as


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Li2CO3, both Ir and B4C are well-known stable materials and will not be corroded during cycling in Li-O2 batteries. Thus the use of non-carbon based Ir/B4C electrode material can largely eliminate the formation and accumulation of side reaction products for Li-O2 batteries. Furthermore, Ir/B4C can reduce the charge voltage polarization by suppressing the overpotential of Li2CO3 decomposition reaction so that the discharge/charge efficiency of Li-O2 batteries can be enhanced. Therefore, Ir/B4C, with excellent chemical and electrochemical stability and high electrocatalytic activity towards electrochemical decomposition of Li2CO3, can largely improve the cycling performanc of aprotic Li-O2 batteries.


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Figure 7. SEM images of B4C (a, b) and Ir/B4C (c, d) oxygen electrodes before (a, c) and after cycling (b, d) under 200 mAh g-1 for 25 cycles.


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The systematical investigation on the charging of Li2CO3 preloaded B4C electrodes with and without Ir NPs reveals that Ir/B4C electrode can completely decompose high-loading Li2CO3 at a voltage below 4.37 V, attributed to the affinity of Ir to adsorb oxygen species, which may lower the energy barrier for electrochemical oxidation of Li2CO3, as well as the inherent excellent chemical and electrochemical stability of Ir and B4C material. The Li-O2 battery using Ir/B4C as oxygen electrode material shows much improved cycling stability than that using B4C oxygen electrode with 1 M LiTf-tetraglyme electrolyte. The use of non-carbon based Ir/B4C electrode material can greatly reduce the formation and accumulation of side reaction products and lower the charge voltage by suppressing the overpotential of Li2CO3 decomposition reaction for Li-O2 batteries. The promising electrochemical properties of Ir/B4C composite validate it as a highly efficient oxygen electrode for future development of high performanc aprotic Li-O2 batteries.

ASSOCIATED CONTENT

Supporting Information. Supporting Information Available: Experimental procedures and additional characterization results including HAADF-STEM image, SEM images, XPS spectra and electrochemical impedance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]; [email protected] Author Contributions


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J.-G. Z. and W.X. perceived the experiment. S.S. prepared the electrodes, cells and Li-O2 batteries and conducted the electrochemical evaluations and measurements. J.M. Z. conducted SEM observations and EDX characterizations. L.L. carried out TEM and STEM observations. M.H.E. performed XPS experiments and analyzed the results. M.E.B. conducted the XRD measurements and analyzed the results. B.L. and C.W. contributed to the results discussion. S.S. wrote the manuscript, W.X., J.M Z. and J.-G. Z. revised the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy (DOE) under DEAC02-98CH10886 for the Advanced Battery Materials Research (BMR) program. S. S. acknowledges the Chinese Scholar Council for the financial support (201409345008). The microscopic and spectroscopic characterizations were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility located at PNNL which is sponsored by the DOE's Office of Biological and Environmental Research (BER). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.

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The table of contents entry


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Figure 1. TEM (a) and HAADF-STEM (b) images of Ir/B4C NPs show ultrafine Ir NPs are well-dispersed on B4C NPs. The inset in (b) shows a size distribution of Ir NPs with an average size of 1.78 nm. The HRTEM image (c) shows lattice of both B4C and Ir NPs with well-matched distances of Ir (200) and (111) crystal planes (insets) for the ultrafine NPs. Figure 1 57x19mm (300 x 300 DPI)


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Figure 2. Charge voltage profiles of B4C- and Ir/B4C-Li2CO3 electrodes in Ar. The inset shows the charge profiles of the bare B4C and the Ir/B4C electrodes without preloaded Li2CO3. Figure. 2 338x190mm (96 x 96 DPI)


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Figure 3. SEM images of B4C-Li2CO3 electrode (a, c) and Ir/B4C-Li2CO3 electrode (e, g) before (a, e) and after charging (c, g); EDX oxygen dot-mapping images captured from the regions shown in the corresponding SEM images at left side for B4C-Li2CO3 electrode before (b) and after (d) charging and Ir/B4C-Li2CO3 electrode before (f) and after (h) charging. Figure 3 338x190mm (96 x 96 DPI)


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Figure 4. XRD patterns of the Li2CO3 preloaded electrodes before and after charging. The inset shows the expanded view of the Li2CO3 peak at 21.2o. Figure 4 338x190mm (96 x 96 DPI)


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Figure 5. XPS spectra of the Li2CO3 preloaded B4C and Ir/B4C electrodes before and after charging. (a) O1st spectra, (b) Li1s spectra, (c) F1s spectra, (d) S2p spectra, (e) B1s spectra, and (f) C1s spectra. Figure 5 338x190mm (96 x 96 DPI)


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Figure 6. Discharge/charge voltage profiles (a), capacity vs. cycle number (b) and cycle curves (c: B4C cell, d: Ir/B4C cell) of Li-O2 cells using B4C and Ir/B4C oxygen electrodes under 200 mAh g-1 at 10 mA g-1 within the cut-off voltage range of 2.0 ~ 4.4 V. Figure 6 338x190mm (96 x 96 DPI)


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Figure 7. Discharge/charge voltage profiles (a), capacity vs. cycle number (b) and cycle curves (c: B4C cell, d: Ir/B4C cell) of Li-O2 cells using B4C and Ir/B4C oxygen electrodes under 200 mAh g-1 at 10 mA g-1 within the cut-off voltage range of 2.0 ~ 4.4 V. Figure 7 338x190mm (96 x 96 DPI)


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