Electrochemically Formed Ultrafine Metal Oxide Nanocatalysts for

Letter pubs.acs.org/NanoLett

Electrochemically Formed Ultrafine Metal Oxide Nanocatalysts for High-Performance Lithium−Oxygen Batteries Bin Liu,† Pengfei Yan,‡ Wu Xu,*,† Jianming Zheng,† Yang He,§ Langli Luo,‡ Mark E. Bowden,‡ Chong-Min Wang,*,‡ and Ji-Guang Zhang*,† †

Energy and Environment Directorate and ‡Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Lithium−oxygen (Li−O2) batteries have an extremely high theoretical specific energy density when compared with conventional energy-storage systems. However, practical application of the Li−O2 battery system still faces significant challenges. In this work, we report a new approach for synthesis of ultrafine metal oxide nanocatalysts through an electrochemical prelithiation process. This process reduces the size of NiCo2O4 (NCO) particles from 20−30 nm to a uniformly distributed domain of ∼2 nm and significantly improves their catalytic activity. Structurally, the prelithiated NCO nanowires feature ultrafine NiO/CoO nanoparticles that are highly stable during prolonged cycles in terms of morphology and particle size, thus maintaining an excellent catalytic effect to oxygen reduction and evolution reactions. A Li−O2 battery using this catalyst demonstrated an initial capacity of 29 280 mAh g−1 and retained a capacity of >1000 mAh g−1 after 100 cycles based on the weight of the NCO active material. Direct in situ transmission electron microscopy observations conclusively revealed the lithiation/delithiation process of as-prepared NCO nanowires and provided in-depth understanding for both catalyst and battery chemistries of transition-metal oxides. This unique electrochemical approach could also be used to form ultrafine nanoparticles of a broad range of materials for catalyst and other applications. KEYWORDS: Lithium−oxygen battery, ultrafine catalyst, nanoparticle, NiCo2O4, prelithiation

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carbon-based air electrodes, which hinder the reversible formation/decomposition of Li2O2. Although ether-based electrolytes were found to be relatively stable against reduced oxygen species, all the electrolytes used for nonaqueous Li−O2 batteries react at different degrees with the reduced oxygen species.18,21,22 In addition, without efficient catalysts, traditional carbon-based air electrodes would decompose at a voltage >3.5 V during the charging process.3,9,13,17−20 Many strategies have been proposed to overcome the barriers to practical application of Li−O2 batteries.3,6−9,12,14,27,28 To reduce the overvoltage of Li−O2 batteries during the charging

echargeable lithium−oxygen (Li−O2) batteries have attracted worldwide interest over the past decade because of their much higher theoretical specific energy when compared to traditional Li-ion batteries.1−5 Considering the relatively low specific energy and high cost of existing Li-ion batteries, rechargeable Li−O2 batteries are being regarded as a promising candidate for next-generation energy-storage systems.6−8 However, extensive studies in recent years have revealed significant challenges that must be overcome before these batteries can be considered for use in practical applications.8−17 These challenges include limited reversible capacity, poor cyclic life, and large overpotential during charge/discharge processes. A primary failure mechanism of Li−O2 batteries results from severe irreversible side reactions between the highly reactive reduced oxygen species (especially superoxide radical anion O2−) and © XXXX American Chemical Society

Received: April 14, 2016 Revised: June 24, 2016

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Figure 1. Morphology and structure characterizations of NiCo2O4 nanowires (NCO NWs)/carbon fabric. (a,b) SEM images of NCO NWs grown on CF. Inset: High-magnification SEM image of NCO NWs. (c) TEM image of a single NCO NW. (d) High-resolution TEM image of polycrystalline NCO material. (e) SAED pattern from the single NCO NW in (c). (f) X-ray diffraction pattern of the NCO/CF composite.

process, many catalysts, including carbides,3,9,23 metal oxides,17,24−26 and noble metals,2,18,27−29 have been proposed. Recently, ternary metal oxide spinel materials, such as CoMn2O4, ZnCo2O 4, MnCo2 O4, and NiCo2 O4 (NCO) have been investigated to facilitate the oxygen reduction and oxygen evolution reaction (OER) in Li−O2 batteries.30−37 Among the ternary metal oxides, NCO exhibits richer redox chemistry from both nickel and cobalt specie. Therefore, it has been widely studied as a promising electrode material for Li-ion batteries and supercapacitors because of its higher electrical conductivity and electrochemical activity compared to binary NiO; however, the fundamental mechanism of NCO’s catalyst properties is still not well understood.32−34 For example, the electrochemical reactions of NCOs mainly have been investigated using cyclic voltammetric studies,38−40 and there still has been no direct observation of the electrochemical reaction in these ternary transition-metal oxides by probing their corresponding lithiation/delithiation processes with a combination of ex situ and in situ analyses. In this paper, we report an electrochemically formed ultrafine metal oxide nanocatalyst for high-performance Li−O2 batteries. In this new approach, NCO nanowires (NWs) with a typical domain size of ∼20 to 30 nm are first fabricated on carbon fabric (CF). The ultrafine metal oxide nanocatalyst with a domain size of ∼2 nm is formed by electrochemical prelithiation of NCO (PL-NCO) NWs in an inert atmosphere and then used in Li−O2 batteries to facilitate oxygen reduction and oxygen evolution reactions. The in situ formed PL-NCO consists of Ni and Co nanoparticles uniformly distributed among Li2O nanoparticle moiety. Under an oxygen atmosphere, these ultrafine Ni and Co nanoparticles are easily oxidized to ultrafine NiO and CoO nanoparticles and act as the catalysts for oxygen reduction and oxygen evolution reactions. This work not only improves the performance of Li−O2 batteries but also provides a new approach for forming ultrafine nanocatalysts for other broad applications.

Structural Features of the NCO-Coated Carbon Fabric. Here, ultrafine metal oxide nanocatalysts are formed by electrochemical prelithiation of the NCO NWs prefabricated on CF via a hydrothermal process and an annealing treatment. More details were disclosed in Experimental Procedures in Supporting Information. Compared to a bare CF substrate with a fiber diameter of ∼10 μm (Figure S1 in the Supporting Information), NCO NWs were densely grown on the CF surface three dimensionally to form a hierarchical and uniform structure as shown in Figure 1a,b. Figure S2 in the Supporting Information shows that these NCO NWs obtained by in situ growth stick tightly to the carbon fabric surface and achieve very good adhesion and electrical contact. More interestingly, these NWs with a diameter of 140 nm are composed of smaller nanoparticles with diameters of approximately 20−30 nm, as shown in Figure 1b (inset) and Figure 1c. The high-resolution transmission electron microscope image shown in Figure 1d reveals a lattice spacing with 0.29 and 0.24 nm, which correspond to the (220) and (311) planes, respectively, of the spinel NCO phase. The corresponding selected area electron diffraction (SAED) pattern further confirms that these NWs have polycrystalline structures (Figure 1e). The crystallographic structure of the product was further analyzed by X-ray diffraction as shown in Figure 1f. All the diffraction peaks in this pattern can be readily indexed as spinel NCO, which are consistent with the values in the standard card (JCPDS Card No. 73-1702). In addition, the typical peaks located at around 26° and 43° are found as well, which are from the CF substrate.41,42 Structure and Morphology of the Ultrafine NCO Nanocatalysts. To address concerns about the optimized oxygen diffusion and catalyst-utilization efficiency, synthesis of NCO NWs on single-side CF was first investigated in this study. A cleaned CF film was transferred into a Teflon container prior to hydrothermal processing. This free-standing CF film was immersed fully into the reaction solution containing Ni-salt, Co-salt, urea, and water and placed against the internal wall of the container as shown in Figure S3 in the Supporting Information. B

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Figure 2. Synthesis pathway of prelithiated NCO/CF composite films and corresponding lithiation curves. (a) Schematic illustration of total fabrication of prelithiated NCO/CF composite films. (b) First discharge profiles of PL-NCO/CF at a current density of 100 mA g−1 with different lithiation depths at 0.02, 0.25, 0.50, and 0.75 V versus Li+/Li.

This configuration promotes in situ growth of NCO NWs only on one side of the CF film. The electrochemical stability of this binder-free NCO/CF composite film was first evaluated in coin cells with Li metal as the anode and under argon atmosphere. As shown in Figure S4 in the Supporting Information, the NCO/CF electrode in Li-ion coin cells shows relatively stable cycling and high Coulombic efficiency (99.5%) for 100 cycles at 100 mA g−1 in a voltage window ranging from 0.5−3.0 V, indicating that the NCO NWs/CF composites can be regarded as a promising electrode for Li-ion batteries. However, we noticed that the discharge capacity of the NCO electrode is negligible when discharged to 2 V, which is the discharge cutoff voltage of the Li− O2 batteries used in this work. Therefore, the capacity of the NCO shown in Figure S4 in the Supporting Information does not contribute to the capacity of Li−O2 batteries observed in this work. The fresh binder-free NCO/CF composite films were assembled into Li metal cells to conduct the prelithiation treatment under an argon atmosphere, as shown in the schematic illustration for the previously mentioned operational procedures (Figure 2a). The corresponding lithiation curves of NCO/CF electrodes at various depths are shown in Figure 2b, which shows that the discharge profiles of Li∥NCO coin cells with the lithiation voltage stopped at 0.02, 0.25, 0.50, and 0.75 V at a 100 mA g−1 current density. The capacity of the NCO increases as the cutoff voltage decreases or the lithiation depth increases. Electrochemical Performance Characteristic of the Composite Electrode for Li−O2 Battery. After prelithiation, the cells were disassembled inside an argon-filled glovebox. The prelithiated (PL) NCO/CF electrodes were rinsed with anhydrous 1,2-dimethoxyethane solvent, dried in the glovebox, reassembled into coin cells with perforated positive cans and a Li metal anode, and then tested under an oxygen atmosphere. The cells were discharged to 2.0 V and charged to 4.5 V at a current density of 0.1 mA cm−2 for as many as 150 cycles. The voltage profiles and the cycling stability of these PL-NCO/CF electrodes with different lithiation depths are shown in Figure 3. The corresponding first-cycle discharge/charge capacities of the pristine NCO/CF and the PL-NCO/CF air electrodes with lithiation depth of 0.02, 0.25, 0.50, and 0.75 V are 1228/1414, 2445/2498, 10926/11149, 29279/30842, and 10311/10630 mAh g−1 (based on the weight of NCO), respectively (Figure 3a). Compared to pristine NCO electrode, all PL-NCO-based electrodes exhibited improved discharge capacities, and the

Figure 3. Electrochemical characterizations of PL-NCO/CF air electrodes in Li−O2 cells cycled at 0.1 mA cm−2. (a) First-cycle discharge/charge profiles of as-prepared PL-NCO/CF air electrodes with different lithiation depths (0.02, 0.25, 0.50, and 0.75 V) and the pristine NCO/CF air electrode. (b) Cyclic stability of four PL-NCO/ CF electrodes and the pristine NCO/CF electrode.

samples prelithiated at 0.50 V delivered the highest initial discharge capacity because of the significantly improved catalytic capability of PL-NCO NWs. We also noticed that the charge capacities of these samples are higher than the corresponding discharge capacities, indicating that additional materials decomposed during the charging process, which is probably C

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Figure 4. SEM morphologies of PL-NCO NWs/CF electrodes after prelithiation process at depth of 0.02 (a), 0.25 (b), 0.50 (c), and 0.75 V (d).

Figure 5. In situ TEM observation of a single NCO NW upon lithiation/delithiation. (a) Schematic illustration of the open-cell setup inside a TEM instrument, including Pt rod with Ag paste as the current collector for the NCO NW positive electrode, and W rod with Li2O/Li as the negative electrode. (b−e) Sequential lithiation processes of the NCO NW observed via in situ TEM. (f−i) Corresponding SAED patterns of the PL-NCO NW at different lithiation stages. (j) Subsequent delithiation process of the PL-NCO NW. (k) SAED pattern of the PL-NCO NW after delithiation.

related to electrolyte decomposition. At the second cycle (Figure S5 in the Supporting Information), large decreases in discharge capacity were observed for all samples, especially for the PLNCO sample prelithiated at 0.50 V.

During the subsequent cycles, different cycling stabilities are observed for various PL-NCO/CF air electrodes as shown in Figure 3b. Here, an obvious rapid capacity fading for the PLNCO/CF air electrode with a prelithiation depth of 0.02 V D

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Figure 6. Characterizations of lithiated NCO NW. (a) Schematic illustration of a NCO NW under lithiation. (b,c) TEM and high-resolution TEM images of selected lithiation areas, respectively. Inset: FFT pattern of prelithiated NCO. Moreover, STEM-HAADF images and corresponding elemental mapping images of selected areas (d and e−g, respectively) on the prelithiated NCO NW.

should be attributed to the irreversible damage to the NCO NWs during deep lithiation and the formation of a thick solid electrolyte interphase (SEI) layer during the prelithiation process as shown in Figure 4a. With a 0.25 V prelithiation depth, partial damage to PL-NCO NWs also can be observed on the carbon fibers as shown in Figure 4b. These damages naturally affect the subsequent cycling stability of the electrolyte and lead to failure of the electrode at ∼70 cycles as shown in Figure 3b. In contrast, the PL-NCO/CF electrode with prelithiation depth of 0.50 V showed an extremely high initial discharge capacity (29 279 mAh g−1) as well as a relatively enhanced stability during 100 cycles where the specific capacity over 1000 mAh g−1 was still maintained even with the full discharge/charge cycling protocol. As shown in Figure 4c, PL-NCO NWs prelithiated at 0.50 V still retain the three-dimensional architecture (compared to pristine NCO NW shown in Figure S2) although a very thin SEI layer was formed on the surface of PL-NCO/CF electrode. These structures/morphologies are critical for enhanced electrochemical characteristics shown in Figure 3. The PL-NCO/CF samples pretreated at 0.75 V have very limited lithiation and minimal morphology change as shown in Figure 4d. In addition, the OER/ORR ratios of the PL-NCO/CF and the pristine NCO/CF electrodes are summarized in Figure S6 in the Supporting Information, which reveals that the PL-NCO/CF electrodes with lithiation depths of 0.50 and 0.75 V possess relatively stable Coulombic efficiency. Besides on the above SEM characterization, the corresponding TEM images of the PLNCO/CF electrodes with different lithiation depths further demonstrate that there is no obvious morphological difference between these nanocatalysts existed in the aforementioned PLNCO electrodes once the lithiation occurs in the NCO nanowire (Figure S7 in the Supporting Information) followed by the

exposure to oxygen atmosphere. As a comparison, the pristine NCO/CF electrode exhibits much lower discharge capacities than the PL-NCO/CF electrodes with prelithiation depths of 0.75 V. The bare CF electrode with no NCO NW showed extremely low capacity due to its low specific surface area and poor OER without catalysts (Figure S8 in the Supporting Information), suggesting that the capacity contribution should be attributed to the catalytic effects of NCO or PL-NCO NWs and the increased active surface areas of the electrodes. Figure 3b also shows that only limited improvement on the capacity retention was observed for the PL-NCO/CF electrode prelithiated at 0.25 V as shown in Figure 3b. Therefore, the prelithiation depth of 0.50 V can be considered as the optimized condition to pretreat NCO/CF electrode used for rechargeable Li−O2 batteries. Lithiation and Delithiation Mechanism of NCO Based on in Situ TEM Observations. To understand the fundamental mechanism behind the enhanced electrochemical performance of prelithiated NCO NW electrode used in Li−O2 batteries, in situ TEM has been used to investigate the lithiation/delithiation processes of NCO NW in an open electrochemical cell. The schematic cell configuration is illustrated in Figure 5a. The NCO NW fixed by the Ag paste was used as the positive electrode, Li metal was used as the negative electrode as well as the Li source, and the native Li2O layer formed on the Li metal surface was used as the solid state electrolyte for Li+ transport. The NCO NW was manipulated to contact the Li2O layer on the surface of Li metal, and a bias of −2 V (or 2 V) was applied on it to drive the electrochemical lithiation (or delithiation). A sequential lithiation process of the single NCO NW was provided at 0, 40, 80, and 120 s, as shown in Figure 5b−e and Video S1 in the Supporting Information. In the case of the E

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tion.30,38−40 Therefore, based on the results of our in situ TEM and SAED analyses we believe that the entire lithiation/ delithiation processes of NCO can be clarified as following: (1) First lithiation process

pristine NCO NW (Figure 5b), the diameter of the uniform NCO NW was 140 nm, which is consistent with that in Figure 1c. When the lithiation process started, Li+ ions were driven from Li metal across the Li2O surface layer into the NCO NW. After lithiation, the PL-NCO NW only exhibited a small volume expansion (∼15%, from 140 to 150 nm), which is due to the porous polycrystalline nature of NWs enabling volume expansion to be largely accommodated by intergranular gaps (Figure 5c−e). Such small volume expansion is in sharp contrast to SnO2 and Si NWs that suffer huge volume expansions upon lithiation,43,44 making the NCO NWs to be a promising candidate for anode material. As shown in Figure 1c, the pristine NCO NW is composed of a polycrystalline structure with clear grain boundaries among small grains (20−30 nm). These grain boundaries can mitigate volume expansion during lithiation and maintain the structure integrity of PL-NCO NW. The corresponding SAED patterns selected from the pristine, lithiation/pristine interfaces, and lithiation sections of this NW were probed and are shown in Figure 5f−i, respectively. It clearly reveals that the pristine NCO has been reduced to Ni, Co, and Li2O once the Li-insertion of NCO was completed. Figure 5j,k shows TEM and SAED pattern of the PL-NCO NW after the subsequent delithiation process. To further investigate the lithiated portion of the NCO NW, a schematic drawing of a single NCO NW under lithiation is shown in Figure 6a. A clear interface between the lithiated and the pristine portion of NCO NW can be observed, as shown in Figure 5c−e. A very clear difference can be observed on the size of the primary particles within NCO NW before and after lithiation. The size of the primary particles in the pristine NCO NW is about 20−30 nm as shown in Figure 1c. However, lithiated NCO particles has been divided into the ultrasmall (around 2 nm) Ni, Co, and Li2O nanodomains as shown in Figure 6b,c. The TEM image of PL-NCO NW shown in Figure 6b exhibits no obvious profiles of polycrystalline nanoparticles, compared to the pristine NCO NW (Figure 1c). The Co-based and Ni-based species located inside the PL-NCO NW can be revealed using the ex situ scanning transmission electron microscopy (STEM) mapping characterization. The STEMHAADF (high-angle annular dark field) and corresponding mapping images of selected areas from the PL-NCO NW are shown in Figure 6d−g, and Figure S9 in the Supporting Information. Such ultrasmall Ni-based and Co-based nanophases are in the form of independent units rather than NixCoy alloy. During the subsequent delithiation process, it can be seen that the morphology of the NCO NW almost returned to its pristine state (Figure 5 panel j versus b), and NiO, CoO, and Li2O were formed as shown in Figure 5k. A large number of nanocrystalline ultrafine particles formed from the previously mentioned lithiated products, as can be seen clearly from the corresponding high-magnification TEM image (Figure S10 in the Supporting Information) when compared to the pristine NCO NW shown in Figure 1c. Figure 5k also shows that Li2O is retained after the delithiation process. This indicates that not all Li2O can react with Ni and Co during delithiation. In fact, some Li2O can still be found even after second-cycle delithiation or after prolonged delithiation (40 min) during the second cycle (Figures S11 and S12 in the Supporting Information). However, no Co3O4 was detected in the delithiated products in our studies, which is different from previously reported work on the redox reactions of NCO active materials during discharge/charge processes, including the further electrochemical reaction of CoO + 1/ 3Li2O ↔ 1/3Co3O4 + 2/3Li+ + 2/3e− during delithia-

NiCo2O4 + 8Li+ + 8e− → Ni + 2Co + 4Li 2O

(2) Follow-on reversible delithiation and lithiation processes Ni + Li 2O ↔ NiO + 2Li+ + 2e−

and

2Co + 2Li 2O ↔ 2CoO + 4Li+ + 4e−

Thus, during the delithiation process, most but not all Li2O formed at previous lithiation stage would react with Ni and Co to form NiO and CoO, respectively. The reversible electrochemical reaction mechanism of Li+ with the transition-metal oxides, such as NiO and CoO, can facilitate the high cycling capability. Although Li2O has been always considered as an electrochemically inactive material, the ultrasmall Ni- and Co-based nanophases in this study exhibit enhanced electrochemical activity toward the formation/decomposition of Li2O.38 It is worth noting that in this work we did not observe a reaction between CoO and Li2O to form Co3O4. This result is consistent with that reported by Poizot and Tarascon et al. that Li2O cannot be decomposed even by milling with Co.45 This indicates that reaction between Li2O with CoO requires a much stronger driving force. We also notice that previous reports on the formation of Co3O4 by the reaction between CoO and Li2O were based on cyclic voltammetry curves and theoretical predictions. No direct evidence has been reported on the formation of Co3O4 by this reaction route. This is a new understating regarding the Li-reactivity mechanism and the interface chemistry of many transition-metal oxides used in energy-storage systems. As discussed above, PL-NCO NWs consist of many ultrafine Ni, Co, and Li2O particles with a typical size of ∼2 nm. When PLNCO NWs/CF electrodes were used as the air electrode in Li− O2 cells and exposed to an oxygen environment, oxygen will quickly diffuse through the porous air electrodes and dissolve in the electrolyte. Then the preformed ultrasmall Ni and Co nanophases will be quickly oxidized into NiO and CoO as shown in Figure S13 in the Supporting Information. After exposure to air for 2 min, the Ni and Co nanophases were fully converted into NiO and CoO, respectively, but the size and morphology of these nanophases were not affected by this oxidation, indicating the high level of stability of the electrode structure. Therefore, numerous ultrasmall NiO/CoO nanoparticles only ∼2 nm in size could act as the highly efficient catalysts in Li−O2 cells. This is probably the reason why PL-NCO/CF electrodes exhibit greatly improved discharge/charge capacities in the first cycle. The XRD patterns of the PL-NCO/CF electrodes after first discharge and after first charge protectively loaded in an airtight specimen holder are shown in Figure 7. It is clearly seen that Li2O2 is formed as the major discharge product after the first discharge and it is decomposed after recharge. According to our previous reports,29,46,47 the formation of Li2O2 during discharge and the disappearance of Li2O2 after recharge is the electrochemistry of Li−O2 reactions, and the O2 gas is released during charging due to decomposition of Li2O2. Herein, based on results from our previous work and several other reports,8,29,15,16 after the first discharge process, the NiO/CoO nanosized catalysts will be covered by a large amount of discharge product (i.e., Li2O2) and other byproducts formed by the side reaction between the electrolyte and superoxide radical anions. After the first charge process, most of the Li2O2 and other byproducts can be oxidized F

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could be reduced to numerous NiO/CoO catalysts only ∼2 nm in size after the lithiation process and exposure to oxygen. This ultrafine catalyst is critical in improving the electrochemical performance of rechargeable Li−O2 batteries. The in situ TEM investigation revealed that the PL-NCO nanowire exhibited a volume expansion of only ∼15% because of the polycrystalline structure of NCO material, and volume changes during subsequent cycling processes remains relatively small. Moreover, this work advanced our understating regarding to the Lireactivity mechanism and interface chemistry of many transitionmetal oxides that are valuable for further development of catalysts and energy-storage technologies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01556. Detailed description of experimental methods, the schematic illustration of hydrothermal process, morphology of bare carbon fabric and NCO nanowires/carbon fabric composite, Li-ion battery performance of NCO/CF electrode, Li−O2 battery performance of PL-NCO/CF, pristine NCO/CF, and bare CF electrodes, elemental mapping characterization of the NCO NW after prelithiated, in situ characterizations of NCO nanowires, and ex situ characterizations of NCO nanowire and cycled PL-NCO/CF electrodes.(PDF) Video of in situ TEM observation of NCO nanowire during lithiation.(AVI)

Figure 7. XRD patterns of PL-NCO/CF electrodes with lithiation depth of 0.50 V after first discharge and first charge.

and released from the air electrode surface, but a small amount of residue consisting of NiO/CoO nanosized catalysts still covers the surface of the air electrode,29 as illustrated in Figure S14a,b in the Supporting Information. Therefore, although the effectiveness of the NiO/CoO nanosized catalyst is greatly suppressed during the subsequent cycles (i.e., lesser discharge products observed onto the PL-NCO/CF surface after 50th discharge, as shown in Figure S14c), enhanced reversibility of PL-NCO/CF electrode can still be maintained for long-term cycling (Figure S14). This explains why PL-NCO/CF electrodes exhibited significant capacity drop during the second cycle. After a few cycles, the air electrode surfaces are stabilized and balanced for each cycle. As seen from Figure S15 in the Supporting Information, the PL-NCO/CF electrode after prelithiation at 0.50 V shows a very stable morphological structure of the NiO/ CoO nanophases after 50 discharge/charge cycles in Li−O2 cells, which leads to a stable cycling performance of the Li−O2 cells using these NiO/CoO nanophases. Also, as shown in Figure 4c, the NW structure and high surface area of the PL-NCO NWs after prelithiation at 0.50 V are well maintained, approaching the condition of pristine NCO NWs. The minimal damage during prelithiation process is one of the reasons that PL-NCO NWs electrodes can exhibit an extremely high first discharge capacity of ∼29 280 mAh g−1 and an enhanced stability with the specific capacity >1000 mAh g−1 for more than 100 full discharge/charge cycles. This work also demonstrates a unique method to fabricate a binder-free air electrode with a uniformly distributed, ultrafine nanophase and extremely stable catalysts. Conclusions. In this work, we developed a novel method for synthesizing ultrafine NiO/CoO catalysts by prelithiation of pristine NCO nanowires at appropriate conditions. These ultrafine NiO/CoO catalysts proved to be highly efficient for stable operation of Li−O2 batteries. Among electrodes prelithiated under different conditions, PL-NCO NWs/CF electrodes prelithiated at 0.50 V delivered an extremely high initial capacity (∼29 280 mAh g−1) at room temperature, and also significantly enhanced cycling stability with a specific capacity of >1000 mAh g−1 for 100 full discharge/charge cycles. By using ex situ/in situ characterization techniques, we clearly demonstrated that each pristine NCO nanoparticle (∼30 nm)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.G.Z). *E-mail: [email protected] (W.X.). *E-mail: [email protected] (C.M.W.). Author Contributions

W.X., B.L., C.M.W., and J.G.Z. conceived and designed the experiments. B.L. performed sample fabrication, characterization, and electrochemical measurements. P.F.Y., Y.H., and L.L. conducted in situ TEM. P.F.Y. conducted ex situ TEM and SEM characterizations. J.M.Z. prepared electrolyte for lithiation. M.B. and J.M.Z. conducted X-ray diffraction characterizations. B.L. summarized the related data and drew figures. B.L., W.X., and J.G.Z. cowrote the paper with suggestions from P.F.Y. and C.M.W. All authors discussed the results and commented on the manuscript. B.L. and P.F.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. Department of Energy (DOE) under Contract no. DEAC0205CH11231 for PNNL and under DEAC02-98CH10886 under the Advanced Battery Materials Research (BMR) program. 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 DOE’s Office of Biological and Environmental Research. PNNL is operated by Battelle for DOE under Contract DE-AC05-76RLO1830. G

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DOI: 10.1021/acs.nanolett.6b01556 Nano Lett. XXXX, XXX, XXX−XXX