Ultralong Cycle Life Achieved by a Natural Plant: Miscanthus

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Ultralong Cycle Life Achieved by a Natural Plant: Miscanthus × giganteus for Lithium Oxygen Batteries Shu Li,† Xuanxuan Bi,‡ Ran Tao,† Qingzhen Wang,† Ying Yao,*,†,§ Feng Wu,†,§ and Cunzhong Zhang*,†,§ †

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States § National High Technology Development Center of Green Materials, Beijing 100081, China ‡

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ABSTRACT: Large energy-storage systems and electric vehicles require energy devices with high power and high energy density. Lithium oxygen (Li−O2) batteries could achieve high energy density, but they are still facing problems such as low practical capacity and poor cyclability. Here, we prepare activated carbons (MGACs) based on the natural plant Miscanthus × giganteus (MG) through slow pyrolysis. It possesses a large surface area, plenty of active sites, and high porosity, which are beneficial to the utilization of oxygen electrode in Li−O2 batteries. The MGACs-based oxygen electrode delivers a high specific capacity of 9400 mAh/g at 0.02 mA/cm2, and long cycle life of 601 cycles (with a cutoff capacity of 500 mAh/g) and 295 cycles (with a cutoff capacity of 1000 mAh/g) at 0.2 mA/cm2, respectively. Additionally, the material exhibits high rate capability and high reversibility, which is a promising candidate for the application in Li−O2 batteries. KEYWORDS: Li−O2 batteries, activated carbon, oxygen electrodes, cyclability, Li2O2

1. INTRODUCTION Nowadays, energy and environmental concerns have become fast-rising issues worldwide.1−4 Owing to the instinct merits, such as decent stability and portability, secondary chemical power sources show highly efficient and reliable features among the various green energy utilization modes. The energy storage device or station requires high energy and power density to supply the high demands of the energy. Lithium oxygen (Li− O2) batteries become a good candidate due to their high theoretical energy density (11680 Wh/kg), nearly equivalent to that of gasoline.5 However, for practical usages in electric vehicles (EVs) and hybrid electric vehicles (HEVs), Li−O2 batteries still have their own problems. The practical discharge capacities are much lower than the theoretical capacities, and in particular, high capacity and high rate in Li−O2 batteries cannot be simultaneously achieved.6 For the improvement of the electrochemical performance of Li−O2 batteries, in recent years, many studies have investigated various electro-catalysts for Li−O2 batteries, including: (1) transition metal oxides;7−10 (2) metal oxide−nanocarbon hybrid materials;11−13 (3) metal−nitrogen complexes, including nonpyrolyzed and pyrolyzed types;14−16 (4) transition metal nitrides;17 (5) conductive polymers;18,19 (6) noble metals, alloys, and oxides, such as Ag, PtAu, and RuO2;20−24 and (7) various carbon materials, including graphene and doped carbons.25−27 Among them, noble metals23,24 show good © XXXX American Chemical Society

catalytic performance on the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR); however, the scarcity and high price prevent their utilization as an oxygen electrode catalyst for Li−O2 batteries. Until now, metal oxides have been paid more attention by researchers as relatively lowprice catalysts for Li−O2 batteries; however, environmental assessment based on the whole life cycle indicates that the preparation process of metal oxides catalysts is not suitable because huge energy consumption and a huge amount of waste slag discharge are involved in each part of the entire process, including mineral processing, smelting and refining. Moreover, using metal elements in oxygen electrodes is always naturalresources limited, and recovering and recycling the amount and the different metal elements from the spent batteries are also hard and energy-consuming tasks. Very recently, biochar, a kind of biomass-derived carbon material, has been used as an electrode material for the preparation of Li-ion battery anodes28 and lithium−sulfur battery cathodes.29 In the past decades, biochar has been Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: November 3, 2016 Revised: November 15, 2016 Accepted: November 16, 2016

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DOI: 10.1021/acsami.6b14071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images of the pristine MGACs (a,b) and MGNACs (c,d) at different magnifications. 2.2. Material Characterization. The materials and electrodes were characterized with various analytical techniques. The crystal structure of MGACs and MGNACs were characterized by X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku) with 2θ ranging from 10° to ∼90°. Further, the nanostructure of MGACs, the as-prepared pristine oxygen electrode and the electrodes after electrochemical tests were observed using scanning electron microscopy (SEM, Supra55, ZEISS). Raman analysis was carried out using a Raman spectroscopy (Jobin Yvon, HR800). Specific surface area was measured by BET analysis using a Tristar 3000 Surface Area and Pore Size Analyzer. The X-ray photoelectron spectroscopy (XPS, Ulvac-Phi Inc.) was performed in this study to confirm the surface component of pristine MGACs. All of the electrochemical measurements were conducted at room temperature (25 °C). 2.3. Electrode Preparation and Li−O2 Cell Assembly. The oxygen electrodes were prepared by casting a slurry containing 80 wt % of the active material, 10 wt % of polyvinylidene fluoride (PVDF) well-mixed in N-methyl-2-pyrrolidone (NMP) solvent, and 10 wt % of acetylene black onto carbon paper (Toray H060). After the pasting of slurry, the working electrode was dried in a vacuum oven at 80 °C for 12 h to remove the residual NMP solvent and then cut into disks 1.1 cm in diameter. The electrolyte was 1 M lithium trifluoromethanesulfonate (LiTFSI) in a tetraethylene glycol dimethyl ether (TEGDME). Glass fiber separators (GF/D, Whatman) were soaked with the electrolyte solution fully. Lithium metal foils (2.2 mm thickness) were used as the metal anode. For the cell testing, a Swagelok cell was used with an open oxygen electrode surface area of 0.5 cm2. All cell constructions (Li∥MGACs−O2 and Li∥MGNACs− O2) were carried out in an argon-filled glovebox (M-Braum, Labstar (1950/780)) in which the H2O level was kept below 0.5 ppm and the O2 level was kept below1 ppm. After assembly, the cell was flushed with high-purity oxygen (99.999%), and the operating pressure was 1 atm oxygen. 2.4. Electrochemical Measurements. The cells of Li∥MGACs− O2 and Li∥MGNACs−O2 were implemented on a battery tester (Land-CT2001A) with an electrochemical window of 2.0 to 4.8 V versus Li−Li+ at different current densities, accordingly. The galvanostat test was carried out with controlled capacity at various current densities (500 mAh/g at 0.02 mA/cm2 and 1000 mAh/g at 0.2 mA/cm2, respectively). Electrochemical impedance spectroscopy (EIS) was investigated by a Metrohm AUT50378 (PGSTAT204) impedance analyzer in the frequency range of 10−3 to 104 Hz. 2.5. Microcavity Powder Electrode and Three-Electrode Cell Test. The detail description regarding the preparation and the construction of a microcavity powder electrode was reported in an

extensively used as adsorbent for the treatment of organic and heavy metal pollution in wastewater due to its excellent enrichment performance30,31 and unlimited biomass raw materials.32 In this study, Miscanthus × giganteus (MG), an abundant natural plant is used as the raw material for the preparation of porous activated carbon (referred to as MGACs hereafter) via pyrolysis activation method with KOH activation agent at 900 °C in N2 atmosphere. Brunauer−Emmett−Teller (BET)-specific surface area and the large pore volume of MGACs materials are 1474 m2/g and 0.859 cm3/g, respectively. The MGACs-based oxygen electrode in an investigated Li∥MGACs−O2 cell exhibits not only excellent high specific capacity (9400 mAh/g at 0.02 mA/cm2) but also a good rate capability and a long-life performance of 601 cycles (with a limited capacity of 500 mAh/g at 0.2 mA/cm2) and 295 cycles (with a limited capacity of 1000 mAh/g at 0.2 mA/cm2), respectively. Based on the total mass of C+Li2O2, a theoretical energy density of 2600 Wh/kg, and a power density of 90 W/ kg for the Li∥MGACs−O2 cell are delivered. The results prove that MGACs, one of the routine biochar materials, can be accepted as a promising active material for oxygen positive electrodes for the development of high energy density, environmental friendly, and resource-independent Li−O2 batteries.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials Synthesis. All chemical reagents were of analytical grade or better and were used as received without any further purification. Miscanthus × giganteus (obtained from the Beijing Academy of Agriculture and Forestry) was used as the biomass precursor. First, MG was cut into pieces and thoroughly ground with KOH in a mass ratio of 1:2 in an agate mortar. Next, the mixture was heated with the activation agent KOH in a tube furnace (T60/10, SGM) to 900 °C at a rate of 10 °C/min and then kept under nitrogen flow for 2 h. After heating, the resulting biochar was cooled naturally to room temperature, collected by filtration, and washed several times with deionized water until pH ≈ 7. Finally, the obtained MGACs were dried in an oven at 80 °C for 24 h. For comparison, MG-based nonactivated carbons (named MGNACs) were also fabricated without KOH activation under the same pyrolysis conditions. The obtained MGACs and MGNACs were used as active materials for oxygen electrodes. B


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Figure 2. Nitrogen adsorption−desorption isotherms (a) and pore-size distributions (b) of MGACs and MGNACs. The inset in (a) is the parameters obtained from BET measurement. XRD patterns (c) and Raman spectra (d) of MGACs and MGNACs. C 1s (e) and O 1s spectrum (f) of MGACs. earlier publication.33 The MGACs investigated microcavity powder electrode was used as working electrode. The Ag wire and Ti wire were used as quasi-reference electrode and counter electrode, respectively. The potential of Ag quasi-reference electrode was confirmed by Fc+−Fc in investigated electrolyte. The potential value was recalculated and exhibited as the Li+−Li scale.

distributions of the MGACs and MGNACs were evaluated by the BET measurements and BJH method separately. The adsorption−desorption isotherms demonstrate that MGACs have a much higher N2 adsorptive ability than that of MGNACs, as shown in Figure 2a, and MGACs possess a high surface area of 1474 m2/g with a total pore volume of 0.859 cm3/g, while the MGNACs has a surface area of merely 17 m2/g with a pore volume of 0.047 cm3/g. Moreover, typical type I and II characteristics34 are delivered for MGACs, and the hysteresis loop in the P−P0 range from 0.4 to 1.0 is indicative of mesoporosity. From the pore-size distribution (Figure 2b), it was clearly observed that the mesopores centered at ∼3 and ∼10 nm in as-made MGACs. In addition, little amount of macro-pores centered at ∼52 nm in the same sample. In this case, the numerous channels with meso-size MGACs would be beneficial to the plentiful surface for the ORR and OER because such a unique porous honeycomb-like structure contributes to huge triphase (solid−liquid−gas) areas and much-more-active sites.35 The macropores and mesopores act

3. RESULTS AND DISCUSSION 3.1. Characterization of MGACs and MGNACs. The morphology and structure of MGACs and MGNACs were examined by scanning electron microscopy (SEM), as shown in Figure 1. Figure 1a,b shows that after activation treatment, the resulting MGACs have successfully acquired a loose and porous honeycomb-like structure consisting of numerous mesopores and macropores with different pore sizes. Figure 1c and 1d show that without KOH activation, the obtained MGNACs display a messy sheet-like and pole-like structure. The porous characteristics of MGACs and MGNACs materials were analyzed by the nitrogen adsorption−desorption technique. The surface area and the corresponding pore-size C


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volume for discharge products (Li2O2) and plentiful pathways for the mass transfer of Li+, O2, and O2-rich electrolyte. Compared to the previous and present studies of the discharge specific capacity of oxygen electrode materials, the initial discharge-specific capacity of the MGACs oxygen electrode is slightly smaller than that of the N-GNSs electrode (11660 mAh/g),42 the a-MnO2−GN hybrid positive electrode (11520 mAh/g),43 and the FHPC positive electrode (11060 mAh/g)6 and almost equal to the NPGA oxygen electrode (10081 mAh/ g)35 but apparently larger than the commercial KB carbon (5180 mAh/g).6 Besides decent performance of the specific capacity, the discharge voltage is higher than these reported results at a similar current density.6,35 This fact confirms that the MGACs could be applied as a suitable material for oxygen electrodes to achieve the high energy density of Li−O2. Quantitatively, based on the total mass of carbon and Li2O2, the theoretical gravimetric energy density of a Li∥MGACs−O2 cell can reach 2600 Wh/kg at a power density of 90 W/kg. This promising performance is attributed to the porous honeycomblike structure, which provides not only a large surface area and plenty of active sites but also the fast kinetics based on the diffusion of oxygen and lithium ions, delivering high capacity and depressing the overpotential of the ORR process. To test the cycling performance of Li−O2 batteries with MGACs as the oxygen electrode, galvanostatic cycling performance was analyzed (Figures 4 and S1). The Li∥MGACs−O2 cell exhibits excellent cycling stability of 601 cycles at a current density of 0.2 mA/cm2 with a cutoff capacity of 500 mAh/g (Figure 4c). After 600 cycles, it still achieves 93.5% Coulombic efficiency (Figure 4e), while the Li∥MGACs−O2 cell only shows 81 cycles at a current density of 0.02 mA/cm2 with a cutoff capacity of 500 mAh/g (Figure 4a). Furthermore, it clearly shows that when the cutoff capacity is 1000 mAh/g and current density is as high as 0.2 mA/cm2, the MGACs-based oxygen electrode cycled 295 times with no apparent capacity decay. This cycling performance is much better than that of previously reported for NPGA oxygen electrodes (72 cycles),35 Super P oxygen electrodes (100 cycles),44 and P-HSC deposited onto CP positive electrodes (205 cycles) in the literature.45 The fact indicates that the discharge products could be decomposed effectively and that the surface of MGACs material could also be recovered in the charge segment (that is, the process of OER). Besides cyclability, the rate performance is also a key character when evaluating batteries. The expansion of the overpotential during the first discharge is ∼75 mV for the MGACs-based oxygen electrode because current density increased 10 times, i.e., from 0.02 to 0.2 mA/cm2 (as shown in Figure S2). In comparison with results in the literature, when the current density is increased 5−10 times, larger overpotentials were found on the NPGA oxygen electrodes (160 mV),35 the Super P oxygen electrodes (150 mV),44 the FHPC oxygen electrodes (250 mV),6 the 30 wt % MMCSAs in porous catalytic electrodes (200 mV),46 the G/meso-LaSrMnO electrodes (100 mV),47 the Au microlattice positive electrodes (120 mV),48 and the PtAu/C electrodes (140 mV).49 Such a fact indicates that the faster kinetics of ORR occurs on MGACs than on each of the reported electrodes due to its unique characteristics. Some key factors were suggested as smooth 3D porous channel, larger surface area, and highly active reactive sites on the surface of MGACs, as shown in Figure 1b. Among these factors, the smooth 3D porous channel could facilitate mass transfer of O2 and oxygen-rich electrolyte to decrease the

as tunnels for oxygen transport and as assistant mass-transfer channels for lithium ions.36 The physicochemical characterizations of MGACs and MGNACs were further examined by X-ray diffraction spectroscopy (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). XRD spectra of MGACs and MGNACs present a broad reflection at 2θ ≈ 22.8°, which demonstrates that the main material is amorphous carbon (Figure 2c). Evidently, a graphite characteristic peak is newly formed at 43.8° (101). This result indicates that the graphitic structures are obtained after KOH activation process. Raman analysis results of both MGACs and MGNACs (Figure 2d) present two distinguished peaks corresponding to the D band (≈1350 cm−1) and the G band (≈1591 cm−1) of carbon, respectively, which are characteristic bands of carbonaceous materials.37 The G band corresponds to the zone center E2g mode related to phonon vibrations in sp2 carbon,38,39 while the high intensity of the D band indicates the occurrence of sp2 C with defects.40 Additionally, the ID/IG value increases from 0.85 for MGNACs to 0.93 for MGACs, revealing that the synthesized MGACs have more defects. High-resolution C 1s and O 1s XPS spectra of MGACs are shown in panels e and f of Figure 2, respectively. Through fitting, the C 1s XPS spectrum proves minor CO and O−CO components existing in C− C structures, exhibiting an oxygen element that may interact with carbon matrix.41 The O 1s spectrum is split into four main peaks as the functional groups of CO (BE ≈ 531.5 eV), C− OH/COOR (BE ≈ 532.0−532.1 eV), C−OH/C−O−C (BE ≈ 532.7−533.0 eV), and OH (BE ≈ 533.4 eV). 3.2. Electrochemical Behavior of MGACs and MGNACs. The discharge specific capacity of MGACs and MGNACs oxygen electrodes were analyzed by a galvanostatic discharge−charge test. At a current density of 0.02 mA/cm2, the MGACs-based oxygen electrode delivers 9400 mAh/g with a cutoff voltage of 2.0 V, which is much higher than the 1945 mAh/g of MGNACs-based oxygen electrodes (Figure 3). In

Figure 3. First discharge−charge voltage profiles of a Li∥MGNACs− O2 cell and a Li∥MGNACs−O2 cell at a current density of 0.02 mA/ cm2.

addition, the average discharge voltage of the Li∥MGNACs−O2 cell was obviously lower than that of the Li∥MGACs−O2 cell. The possible reason for these results should be ascribed to the formation of a porous honeycomb-like structure, a high surface area, and more defects on the surface after the process of KOH activation, as mentioned above. The high surface area and the active reactive sites (such as defects) could provide enough void D


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Figure 4. Galvanostatic cycling tests of Li∥MGACs−O2 cells using 1 M LiTFSI TEGDME at 0.02 mA/cm2 current density with limited capacity of (a) 500 and (b) 1000 mAh/g and at 0.2 mA/cm2 current density with a limited capacity of (c) 500 and (d) 1000 mAh/g, respectively. (e) Discharge capacity and Coulombic efficiency vs the cycle number for a Li−O2 cell with a MGAC oxygen electrode at 0.2 mA/cm2 with a controlling capacity of 500 mAh/g. (f) Discharge capacity and the discharge terminal voltage vs the cycle number for Li−O2 cells with MGAC oxygen electrodes at a 0.02 and 0.2 mA/cm2 current density with a controlling capacity of 500 and 1000 mAh/g, respectively.

conductivity.50−52 The larger activation energy based on the insulator property of Li2O2 contributes to the fast expansion of overpotentials cycle-by-cycle. The residue Li2O2, which could not be removed during charging, could cover active sites and change the electrochemical property of the pristine electrode, thus leading to a shorter cycle life. With a proper current density, the MGACs electrode exhibits superior cyclability and shows the potential application in Li−O2 batteries without the use of precious metal or metal oxides catalysts. It is worth noting that a relatively low charge potential of ∼3.2 V versus Li+−Li in the initial few cycles (as shown in Figure S2) regardless of the state of cycling is clearly observed. The charge overpotentials show little increase (∼30 mV) with the increase of current density from 0.02 to 0.2 mA/cm2. The charge plateau that occurred at 3.2 V (versus Li+−Li) is similar to the previous published studies.35,44,53−55 It is probably due to the onset reduction from oxygen to superoxide. This one electron-transfer reaction performs a fast kinetics and happens at a relatively low charging voltage. The first formed LiO2 might be stabilized as the core of the discharge products.56 With the increase of depth of discharge though, the superoxide starts to

concentration polarization. The high surface area and highly active reactive sites (e.g., defects) could promote the rate of charge transfer and alleviate the electrochemical polarization of ORR as the current density increased. The superior cyclability of the cells benefits from the merits of MGACs not only on ORR but also on OER at the investigated conditions. The surface and structure of the 3D porous channel of MGACs could be nearly cleaned up and recovered after each charging segment. Naturally, the excellent electro-activity of MGACs on ORR could also be retained, and the fluctuation of discharge voltage is very small in subsequent cycles. The results, shown in Figure 4f, confirm that the cycle life of MGACs-based oxygen electrodes is affected by both the current density and the depth of discharge and charge. Smaller current density and higher capacity limit lead to poor cycle performance, which indicates that the electrodes undergo sluggish kinetics during the discharge and charge processes. One or more slow steps must be involved in the total decomposition reaction of discharge products. On the basis of the fast kinetics under smaller current density, it was determined that the formation of Li2O2 is slow, and the fine structure may lead to lower electric E


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oxygen electrode is further observed after discharge and recharge (Figure 6). As shown in Figure 6a, after the first discharge to 1000 mAh/g, toroid-like products (Li2O2) are found on the MGACs-based oxygen electrode with the size around 1 μm. Figure 6b shows the MGACs-based oxygen electrode after recharging to 3.6 V. It indicates that the discharge product resulting from the MGACs positive electrode consists of mainly nanoparticles with nanosheets and toroids. The structure of the toroids becomes loose, and the number of toroids shows a little decrease after the cell charging to 3.6 V. After being recharged to 1000 mAh/g, almost all of the toroids are removed from the air electrode (Figure 6c), which further confirmed the reversibility of the MGACs-based oxygen electrode. The X-ray diffraction analysis is carried out to recognize discharge products of Li−O2 battery with the prepared MGACs positive electrode. As shown in Figure 7, the characteristic

undergo the disproportionation reaction to form peroxide, which covers the small amount of LiO2 and grows to large particles. The second OER reaction experiences sluggish kinetics, and thus, the voltage increases afterward. The fact demonstrated that the charge reaction, which occurred at ∼3.2 V versus Li+−Li, was an active-material-independent electrochemical redox reaction or named as a surface-insensitive reaction.The more possible reason for this phenomenon may be the instinct characteristics of the redox couple of O2−LiO2, according to recently published investigations.56 Meanwhile, according to normal electrochemical theory, the different between the value of the discharge voltage plateau and the charge voltage plateau should be very low due to the fact that the one-electron reversible process, O 2 −O 2 •− , is characterized by the 57 mV difference between the reduction peak and oxidation peak. The linear voltammetric behavior of the MGACs-loaded microcavity powder electrode in 1 M

Figure 7. XRD patterns of the MGACs-based oxygen electrode at different stages: (a) first discharge to 1000 mAh/g, (b) first recharge to 3.6 V, and (c) first recharge to 1000 mAh/g at a current density of 0.2 mA/cm2.

Figure 5. Steady-state linear voltammetry curve of the Li+−Li couple for ORR on a MGAC-loaded microcavity powder electrode in 1 M LiTFSI TEGDME at a scanning rate of 1 mV/s. O2-saturated (red line) and blank Ar-saturated (black line). Inset is the Tafel plots of the ORR on MGACs obtained by potential decreasing sweeps.

peaks at 32.9°, 35°, 40.6°, and 58.7° are observed after the first discharge, which could be assigned to the (100), (101), (102), and (110) lattice planes of Li2O2, confirming that Li2O2 is the dominate discharge product. The peaks at 32.9° and 40.6°, which are assigned to the (100) and (102) lattice planes for Li2O2, are missing after the subsequent charge to 3.6 V. When the charge increases to 1000 mAh/g, all peaks for Li2O2 disappear, further supporting that Li2O2 could be fully recharged on MGACs-based electrode. As shown in Figure 8, the impedance of the Li∥MGACs−O2 cell increases dramatically compared to the pristine electrode after the first discharge. This phenomenon is probably because of the poor electronic conductivity of discharge products (Li2O2) formed at the oxygen electrode side. However, the Li∥MGACs−O2 cell has the ability to recover its impedance to original state after the following recharge process, suggesting that the insulated Li2O2 could be generated and then fully

LiTFSI TEGDME (see Figure 5) was used to explain the relevant mechanism. According to the Tafel equation, η = a + b × log i

(1)

The slope of Tafel plots for the ORR on pure MGACs, ca. 121 mV/dec, was found at a low polarized potential scope. Here, EOO′2−Li2O2 was used as the potential standard to calculate the overpotential because O2−Li2O2 was the investigated total reaction of discharge and charge. Such a fact indicates that the 1e reduction species was formed in the total process of formation of peroxide. The most possible 1e reduction species and the mechanism could be described, as per the suggestion of Larire.57,58 3.3. Analysis on the Discharged and Charged Oxygen Electrodes. The morphology variation of the MGACs-based

Figure 6. SEM images of the MGACs-based oxygen electrode at different stages: (a) the first discharge to 1000 mAh/g capacity, (b) the first recharge to 3.6 V, and (c) the first recharge to 1000 mAh/g capacity at a current density of 0.2 mA/cm2. F



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REFERENCES

This research was supported by the National Natural Science Foundation of China (NSFC) through grant no. 21473011 and 51402018, and the National Key Program for Basic Research of China through grant nos. 2015CB251100 and 2014CB932300.

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Figure 8. Electrochemical impedance spectra of the pristine MGACs and MGAC electrodes after the first and 30th cycles with a cutoff capacity of 500 mAh/g at a current density of 0.2 mA/cm2.

decomposed during the discharge and charge cycles. This phenomenon becomes much more apparent after the 30th cycle. The EIS results suggest a relatively reversible reaction for Li2O2 formation and decomposition during the repeated discharge−charge process at the MGACs-based oxygen electrode.

4. CONCLUSIONS In this study, we fabricated an activated carbon material with a porous honeycomb-like structure by a KOH activation method from the earth-abundant MG. The MGACs have high surface area (1474 m2/g) and large pore volume (0.859 cm3/g), which could be used as an oxygen electrode material in Li−O2 batteries. The Li−O2 batteries with MGACs-based oxygen electrodes exhibit high specific capacity (9400 mAh/g at 0.02 mA/cm2), long-life performance (601 cycles with a cutoff capacity of 500 mAh/g at 0.2 mA/cm2), and good rate capability. All of these results proved that MGACs materials manifest a remarkable electrochemical activity toward the ORR and OER, especially with regard to the ORR. It certificates that MGACs materials will be a promising active material of oxygen positive electrode for the development of excellent performance for Li−O2 batteries. Moreover, due to the unlimited natural raw resources and convenient preparation technique, the instinctive properties of biochar-based batteries, such as being environmentally friendly and resource-independent, will become unable to be ignored and become dominant factors in the long-term future development of secondary Li−O2 batteries.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14071. Figures showing the initial 10 cycle performance of Li∥MGACs−O2 cells and the amplitude of overpotential of the first discharge of the MGACs-based oxygen electrode. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.Y. e-mail: [email protected]. *C.Z. e-mail: [email protected]. ORCID

Ying Yao: 0000-0002-0472-0852 Notes

The authors declare no competing financial interest. G


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Ultralong Cycle Life Achieved by a Natural Plant: Miscanthus

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