Understanding the Critical Role of the Ag Nanophase in Boosting the

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Understanding the Critical Role of the Ag Nanophase in Boosting the Initial Reversibility of Transition Metal Oxide Anodes for Lithium-Ion Batteries Daehee Lee,†,# Mihye Wu,‡,# Dong-Hyun Kim,§,# Changju Chae,‡ Min Kyung Cho,∥ Ji-Young Kim,∥ Sun Sook Lee,‡ Sungho Choi,‡ Youngmin Choi,‡ Tae Joo Shin,⊥ Kyung Yoon Chung,*,§ Sunho Jeong,*,‡ and Jooho Moon*,† †

Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea § Center for Energy Convergence Research and ∥Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea ⊥ Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The initial reversible capacity, a critical impediment in transition metal oxide-based anodes, is augmented in conversion-reaction-involved CoO anodes for lithium-ion batteries, by incorporating a chemically synthesized Ag nanophase. With an increase in the added amount of Ag nanophase from 5 to 15 wt %, the initial capacity loss decreases linearly up to 31.7%. The Ag nanophase maintains its pristine metallic nature without undergoing phase transformations, even during repeated vigorous electrochemical reactions of the active CoO phase. Complementary ex situ chemical/physical analyses suggest that the Ag nanophase promotes the catalytic generation of reversible gel-like/ polymeric films wherein lithium ions are stored capacitively in the low-voltage region below 0.7 V during discharging. These scientific findings would provide a heretofore unrecognized pathway to resolving a major issue associated with the critical irreversibility in conversion-type transition metal oxide anodes. KEYWORDS: Lithium-ion battery, conversion-type anode, cobalt oxide, Ag nanophase, initial Coulombic efficiency

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INTRODUCTION

methods cause significant lowering of the initial Coulombic efficiency.4,5 The irreversible capacity loss in the initial cycles generally stems from the formation of an irreversible solid electrolyte interphase (SEI) that consumes lithium ions and in turn limits the energy density of the cells. The SEI is formed by decomposition of polymeric electrolytes during the first discharging step. For conversion-type oxide anodes, suppressing the undesirable SEI formation would be a designable strategy as long as the relevant reaction mechanisms on the surfaces of transition metal oxides are revealed. However, a clear mechanism underlying these reactions is still elusive. Elaborate research on the SEI formation mechanisms has provided a plausible theory that pseudocapacitance can contribute to the overall enhancement of the reversible capacity owing to the presence of a gel-like/polymeric SEI layer in the potential range 0.02−1.9 V versus Li/Li+, so that reversible

Transition metal oxides, which store electrical charge via conversion reactions, have been regarded as promising candidates for high-energy-density anodes in lithium-ion batteries (LIBs), with superior capacities exceeding 1000 mAh g−1 (much higher than that (372 mAh g−1) of commercial graphite anodes).1−3 Despite this remarkable capacity, the commercial implementation of metal oxide anodes has been difficult because of critical impediments such as unsatisfactory reversibility (i.e., a low initial Coulombic efficiency) and longterm cycling instability.2,3 The cycle instability in conversiontype oxide anodes predominantly results from the large volumetric change during repeated electrochemical lithiation/ delithiation processes. One of the general strategies for improving the cycle stability is the construction of nanostructured composite materials with various carbon derivatives, including graphite, carbon nanotubes, and graphene. These approaches effectively contribute to an enhancement in the cycling stability, so that the composite materials meet the commercial prerequisites for LIBs; however, the aforesaid © 2017 American Chemical Society

Received: February 1, 2017 Accepted: May 31, 2017 Published: May 31, 2017 21715

DOI: 10.1021/acsami.7b01559 ACS Appl. Mater. Interfaces 2017, 9, 21715−21722

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ACS Applied Materials & Interfaces

Figure 1. Electrochemical data of the cells employing Ag−CoO−RGO composite electrodes, as a function of Ag content: (a) voltage profile, (b) initial capacity loss, and (c) normalized initial capacity loss. The Ag content represents the ratio of Ag to CoO in composite electrodes.

Figure 2. Time derivative of voltage at each cell potential for cells employing (a) Ag-free and (b) 10 wt % Ag-added electrodes. (c) Capacitance of the cell with 10 wt % Ag-added electrode at low voltages. The capacitance was calculated from the slope of the i−dV/dt plot.

storage of lithium ions is possible.6,7 This observation has also elucidated the origin of an additional capacity evolution that is commonly observed in metal oxide-based anodes, beyond the theoretical capacity. These gel-like/polymeric films are catalytically generated from the decomposition of polymeric electrolytes, with the concurrent partial formation of an irreversible SEI.8 However, till date, there are no reports on characteristic chemical methodologies based on the use of specific catalysts for facilitating the formation of these gel-like/polymeric films from the decomposition of organic electrolytes. Herein, we demonstrate that the addition of an Ag nanophase into CoO−reduced graphene oxide (RGO) anodes can promote the formation of gel-like/polymeric films and thus enhance the overall reversibility of the anodes. Electrochemical characterizations reveal that the Ag-incorporated CoO−RGO electrodes facilitate the capacitive lithium-ion storage during discharging in the voltage region, below 0.7 V. The complementary chemical/physical analyses based on transmission electron microscopy (TEM), synchrotron-sourced powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) demonstrate that the distinctive capacitive behavior in the Ag-incorporated cell results from the facile formation of gel-like/polymeric films along the surface of the electrochemically active CoO phase. Studies on the electrochemical reaction kinetics by the galvanostatic intermittent titration technique (GITT) also confirm that the Ag nanophase is responsible for the capacitive lithium-ion storage.

assembled into CR2032-type coin cells with a lithium foil as a counter/reference electrode. The voltage profiles for Agincorporated CoO−RGO composites under a constant current of 100 mA g−1 during the first charging/discharging cycle are shown in Figure 1a. All electrochemical characterizations were conducted based on the total weight of the composite materials. The Ag phase studied herein is electrochemically inactive toward lithium ions, as confirmed previously.9 A voltage plateau around 0.7 V is observed for all cells, which can be ascribed to the conversion of CoO into Co0 and Li2O. The initial capacity loss as a function of Ag content was evaluated from the Coulombic inefficiency in the first cycle, using the following expression: [100 − (charging capacity/discharging capacity)]%. The initial capacity loss of the Ag-free cell was 65.5%, whereas that of the Ag-incorporated cells diminished significantly to 31.7% (Figure 1b). The initial capacity loss is efficiently suppressed in a linear fashion as a function of the Ag nanophase content (Figure 1c). For the Ag-free and Ag-incorporated electrodes, the distinctive difference in voltage profiles during the first discharging step was observable below the plateau voltage, 0.7 V. The electrochemical decomposition of organic electrolytes is triggered at 0.8 V during the first discharging step, generating an irreversible SEI.10 The generation of SEI layers leads to a gradual drop in voltage owing to the combined contribution of the diffusion-controlled decomposition of the organic electrolyte and the insulating SEI layer-induced dielectric polarization.11 For conversion-type electrodes, reversible storage of lithium ions could occur in the low-voltage region (below 0.7 V) during discharging via the formation of space-charge layers and reversible gel-like/polymeric films, both of which allow for capacitive lithium-ion storage (other than by the Faradaic electrochemical reaction).6

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RESULTS AND DISCUSSION The electrochemically active composite materials were prepared by incorporating 20 nm Ag nanoparticles (NPs) and 100 nm CoO NPs within graphene oxide frameworks through a wet chemical synthesis and subsequent homogenization/annealing processes.9 The obtained composites were 21716


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Figure 3. Crystalline structural and microstructural evolution upon discharging/charging: (a) synchrotron-sourced PXRD patterns for pristine Agfree and 10 wt % Ag-added electrodes. (b) TEM image and (c) SAED pattern for fully discharged Ag-free electrode. (d) TEM image and (e) SAED pattern for fully discharged, 10 wt % Ag-added electrode. The yellow lines in the TEM images represent the surficial layers.

discharging/charging process was characterized by synchrotronsourced PXRD and TEM-based analyses. Figure 3a shows the phase evolution before/after the first discharging process for the Ag-free and 10 wt % Ag-added cells. As expected, after full discharging, the CoO phase in both cells disappeared and the Co metallic phase appeared. The Ag phase maintained its pristine metallic crystalline structure even after discharging. For the fully discharged cells, the unidentified peaks at around 38 and 53−63° are ascribed to the Kapton capillary used for sampling. From the similar full width at halfmaximum (FWHM) values for Co phases in both discharged cells, it is presumed that the converted Co nanophase has similar morphological dimensions. The TEM images of the composite materials after discharging clearly show the common microstructures derived by conversion reactions, in which Co NPs are embedded in the amorphous Li2O phase (Figure 3b,d). The selected-area electron diffraction (SAED) patterns indicate that the Ag nanophase retains its crystalline structure after full discharging (Figure 3e), along with a small amount of the unconverted CoO phase (Figure 3c,e). In the 10 wt % Agadded cell, the Ag metallic phase observed after full discharging was also detected in the subsequently recharged cell, as shown in Figure S1 (Supporting Information). Except for the existence of the Ag phase, there is no distinguishable structural difference between the Ag-free and Ag-added cells. Note that capacitive lithium-ion storage in the space-charge layers occurs in the interfacial vicinity between the electrochemically converted Co NPs and the Li2O phase; thus, the similar microstructures observed for the Ag-free and Ag-added cells suggest that the lithium-ion storage behavior in the space-charge layers may be similar in both cases. However, the surficial layers (yellow dotted lines in Figures 3b,d) present along the surface of the Co NPs show distinctively different morphological structures.

If the storage of lithium ions is predominantly governed in a capacitive manner in this low-voltage region, the time derivative of voltage (dV/dt) should show pseudoconstant characteristics under constant current conditions, regardless of the degree of discharging, when the current−voltage relationship in the capacitor is considered; that is, i = C(dV/dt) (i and C are the current and capacitance across the capacitor, respectively). Figure 2a,b shows the dV/dt at each cell potential in the Ag-free and 10 wt % Ag-added cells, respectively, as a function of current density during the second discharging process. When excessive Ag nanophase (>15 wt %) was incorporated, the resulting cell showed inferior cycling stability, owing to the deficiency of the stress-releasing carbon material, RGO. As the generation of irreversible SEI layers occurs dominantly onto a bare surface of the active material in the first discharging process, the change in dV/dt was analyzed during the second discharging process. The dV/dt of the Ag-free cell diminished continuously as the cell potential decreased (Figure 2a), which could be attributed to the square-root−power-law characteristic of the Faradaic reaction.6,12 On the contrary, the 10 wt % Agadded cell showed a more flattened variation in the voltage region below 0.7 V (Figure 2b). The i−dV/dt plot of the 10 wt % Ag-added cell clearly showed a linear single slope (Figure 2c). The capacitance calculated from the slope was 1173 F g−1, similar to the values reported by Tarascon’s group.6 This observation implies that the addition of the Ag nanophase induces a strong capacitive behavior, enabling capacitive storage of lithium ions. As mentioned earlier, capacitive lithium-ion storage in the low-potential region occurs in the space-charge layers between the Co/Li2O interface13−16 or gel-like/ polymeric films.6,17−19 To clarify the predominant mechanism, the morphological/crystalline structural evolution during a 21717


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ACS Applied Materials & Interfaces The thickness of the surficial layer in the Ag-added cell ranges from 5 to 15 nm, whereas the corresponding value for the Agfree cell is 25−30 nm. Interestingly, the surficial layer observed in the Ag-added cell is likely to have an organic-phase-like amorphous structure, with no lattice fringes, unlike the case of the Ag-free cell with a partially ordered, inorganic-phase-like lattice structure. In this regard, it is speculated that the origin of the lithium-ion storage behavior is associated with the generation of reversible gel-like/polymeric films in the Agincorporated composite electrodes. On the other hand, the surficial layer of the Ag-incorporated composite disappears after recharging it, whereas that of the Ag-free composite remains as shown in Figure S2. This observation implies the reversibility of the surficial layer in the Ag-incorporated composite. To gain exact information about the surficial layers, the C 1s XPS spectra were analyzed, as presented in Figure 4. Ex situ XPS studies were carried out with the electrodes prepared from slurries containing the composite material, conductive carbon, and polyvinylidene fluoride (PVDF) as a binder. The C−F chemical bond in PVDF contributes to the peak at 289.8 eV. The small amount of defects present on the surface of RGO can be detected in the form of C−O, carbonyl (CO), or carboxylate (O−CO);20 however, the contribution of these species in the XPS spectra is negligible, considering the subtle intensities of the subpeaks due to the CO and O−CO chemical bonds at 287.3 and 287.7 eV, respectively. In the fully discharged Ag-added electrode, the characteristic subpeak at 286.2 eV (red lines in the deconvoluted spectra in Figure 4) is prominent, along with the main peak due to the C−C bond at 284.7 eV. In the studies performed by Tarascon’s group on SEI layers present along the surface of the electrochemically cycled CoO phase, it was suggested that poly(ethylene oxide) (PEO, (CH2CH2O)n) structured gel-like/polymeric films are generated and decomposed reversibly during repeated cycling, thus allowing capacitive lithium-ion storage.18 The irreversible SEI layers generated by the decomposition of carbonate-based electrolytes have chemical formula Li2CO3 and/or that of an alkyl carbonate.18 On the basis of these studies, the subpeak at 286.2 eV is attributable to the C−O bond from a reversible PEO-like gel-like/polymeric film, whereas that at 288.6 eV is due to the irreversible SEI layer. The subpeak at 286.2 eV nearly disappeared upon recharging, whereas that at 288.6 eV was almost unchanged in the Ag-added electrodes. According to the results of a semiquantitative analysis (Table S1), on the basis of the areal calculation for each subpeak, the ratio of the C−O bond to the C−C bond in the subpeaks was 0.11 and 0.28 for the fully discharged Ag-free and Ag-added electrodes, respectively. A precise quantitative analysis on each chemical component is difficult because of the overlapped features of diverse carbon species, resulting from the simultaneous presence of the SEI layer, RGO, and PVDF. However, it can be stated that a unique chemical contribution of the C−O bond is evolved in the Ag-added electrode with the formation of PEO-like gel-like/polymeric films. Because the formation of irreversible SEI layers and reversible gel-like/polymeric films can occur in identical voltage ranges during discharging, the accelerated formation of gel-like/polymeric films could suppress the formation of irreversible SEI layers, enhancing the overall reversibility of the CoO-based electrode. It is believed that the incorporated Ag nanophase serves as a catalyst for the formation of reversible surficial layers along the surface of the electrochemically active CoO phase. Another interesting feature in the XPS spectra is the weakly bound carbon

Figure 4. XPS C 1s spectra for Ag-free and Ag-added electrodes after the discharging/charging process. XPS characterizations were conducted after full discharging to 0.02 V and charging to 3 V. 21718


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delithiation is kinetically retarded as long as the Ag phase is not incorporated into the electrochemically active CoO phase because the QOCV profiles are measured in the equilibration state without the involvement of the kinetic parameter, that is, diffusion-controlled confinement. To obtain further information on the reaction kinetics, the internal resistance was evaluated from the overpotentials determined in the GITT profiles (Figure 5c). The internal resistance (R) was calculated from the measured overpotential (η) and current pulse (I), as R = η/I.23 For both the Ag-free and Ag-added electrodes, the variations in internal resistance during the charging process showed a similar trend at overall voltages below 2.5 V, whereas the absolute values are higher for the Ag-free electrode. The higher overpotential observed for the Ag-free electrode indicates more sluggish delithiation kinetics as compared to that in the case of the Ag-added electrode. The slower reaction kinetics can be directly caused by inefficient current collection or a decrement in the number of active sites, which might be associated with the significant formation of irreversible SEI layers in the Ag-free electrode. The thicker, partially crystallized surficial layers observed in the TEM image for the fully discharged Ag-free electrode may cause retardation of the delithiation kinetics, whereas the addition of the Ag phase suppresses the formation of an irreversible SEI, catalyzing the generation of gel-like/polymeric films. Another evident difference between the Ag-free and Agadded electrodes was observed in the low-voltage range below 0.7 V during discharging. The internal resistance shows a gradual and continuous decrease in the Ag-free electrode but is relatively constant in the Ag-added electrode. The internal resistance in the low-voltage region reflects non-Faradaic polarization because Faradaic conversion is underway at a given constant voltage, with a plateau in the voltage profiles.8 During the Faradaic conversion at the plateau voltage, the chemical potential of lithium in the electrodes (μ*Li) increases along with the evolution of a Li2O/Co mixed phase, whereas the chemical potential of lithium in the electrolytes (μLi) remains constant, equilibrated with the cell potential. At the end of the conversion process, the huge difference between μ*Li and μLi induces a double-layer charging, lowering the cell voltage drastically.24 Subsequently, the voltage decreases gradually as the discrepancy between μ*Li and μLi diminishes, as observed in the Ag-free cell. On the other hand, the invariant behavior of the polarization of the Ag-added cell is manifested by the fact that the chemical potential gradient of the lithium ions is relaxed at a constant rate. The constant relaxation implies that the lithium ions migrate regularly under a constant current input, which can be regarded as a capacitive behavior across a capacitor. These electrochemical characterizations support our theory that the addition of the Ag nanophase leads to capacitive lithium-ion storage in the low-voltage region during discharging, which indispensably accelerates the charging kinetics. In a proof-of-concept study based on Ag− MnO2/RGO systems, no noticeable change in the initial capacity loss was observed even in the presence of the Ag nanophase (Figure S4). MnO2/RGO composite materials were prepared by an electrostatic-assembly technique, as reported in our previous study.27 The positively charged, polyethylenimine (PEI)-decorated graphene oxides were mixed with negatively charged MnO2 nanorods in an aqueous medium, resulting in electrostatically assembled MnO2/RGO composite materials after thermal annealing at 200 °C. For preparing the Ag-

component at 283.3 eV, which could be assigned to a metal− carbon bond, presumably the Li−C bond. The subpeak due to such a metal−carbon bond disappeared reversibly, similar to the subpeak due to the C−O bond. The ratios of the metal− carbon bond to the C−O bond were similar, 1.4 and 1.3 for the Ag-free and Ag-added electrodes, respectively. This implies that chemical groups with the metal−carbon bond evolve along with the generation of PEO-like gel-like/polymeric films with the C−O bond. The gel-like/polymeric film-driven capacitive behavior in the Ag-added electrodes was confirmed by GITT studies on the reaction kinetics for the Ag-free and Ag-added electrodes. The GITT measurements during the first discharging/charging processes were carried out with a series of current pulses of 100 mA g−1 for 10 min and subsequent equilibration for 2 h for each point (Figure S3a). In the GITT profiles (Figure S3b), the potential at the equilibrium state is denoted a quasiequilibrium potential or quasiopen circuit voltage (QOCV). The transient deviation of potentials between the closed circuit voltage (CCV) and the QOCV represents the potential required to break the quasiequilibrium state, that is, the overpotential (η) at each point.21,22 The QOCV profile for the Ag-added electrode during discharging showed a larger capacity (900 mAh g−1) than that of the Ag-free electrode (771 mAh g−1), similar to the CCV profiles (Figure 5a). Interestingly, in the QOCV profiles

Figure 5. (a) QOCV profiles, (b) capacity differences between the QOCV and CCV profiles, and (c) internal resistance profiles for the Ag-free and 10 wt % Ag-added electrodes during the discharging/ charging process.

for both electrodes during a subsequent recharging process, similar capacity values (733 and 749 mAh g−1 for the Ag-free and 10 wt % Ag-added electrodes, respectively) are observed (Figure 5a). However, in the CCV profiles, the Ag-added electrode exhibited a higher charging capacity (712 mAh g−1) than did the Ag-free electrode (539 mAh g−1). These capacity differences in the QOCV and CCV profiles during the discharging/charging processes are shown in Figures 5b. The capacity differences are measured to be 174 and 125 mAh g−1 during discharging for the Ag-free and Ag-added electrodes, respectively; the corresponding charging capacities are 194 and 37 mAh g−1 (Figure 5b). The huge deviation in the charging capacity was significantly mitigated by the presence of the Ag nanophase. Such a distinctive deviation in charging capacity between the Ag-free and Ag-added electrodes indicates that 21719


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(pH 11) was injected. After the reaction for 60 min at 60 °C, the synthesized Ag NPs were selectively separated by centrifugation and the obtained Ag NPs were washed with DI water by centrifugation. MnO2 nanorods were synthesized using a microwave-assisted hydrothermal method, as described in our previous work,27 using MnSO4·H2O (Sigma Aldrich, 99%) and KMnO4 (Sigma Aldrich, 99%). GOs were prepared by oxidation/exfoliation of natural graphite flakes (Sigma Aldrich), using a modified Hummers method as described in our previous work.9 All chemicals for syntheses were used without further purification. Preparation of Composite Materials. Both Ag−CoO−RGO and CoO−RGO composite materials were prepared by a sonochemical hybridizing process; detailed procedures can be found in our previous work.9 The weight ratio of CoO in the composite materials was kept at 80 wt % for all samples, whereas that of the Ag phase was varied in the range 0−15 wt %. GOs constituted the rest of the composition of the composite materials. The resulting composite materials were annealed at 300 °C for 1 h under an Ar atmosphere, for thermally reducing the GOs and triggering thermal welding of the Ag nanophase. MnO2/RGO composite materials were prepared by an electrostatic-assembly technique, as reported previously.27 Briefly, the positively charged PEI-decorated graphene oxides were mixed with negatively charged MnO2 nanorods in an aqueous medium; then, the composite materials were collected by centrifugation, followed by thermal annealing at 200 °C for 1 h under an Ar atmosphere. For preparing the Ag-nanophase-incorporated MnO2/RGO composite materials, negatively charged Ag NPs or Ag NWs (Nanopyxis) were added to a mixing batch containing PEI-decorated graphene oxides and MnO2 nanorods. The weight ratio of MnO2 in the composite materials was kept at 80 wt % for all samples, whereas that of the Ag phase was varied in the range 0−8 wt %. GOs constituted the rest of the composition of the composite materials. The products collected by centrifugation were annealed at 200 and 300 °C for 1 h under an Ar atmosphere for obtaining the Ag-NW−MnO2/RGO and Ag-NP− MnO2/RGO composite materials, respectively. Characterization. The resulting composite materials were assembled into CR2032 coin cells using a slurry composed of Nmethyl-2-pyrollidone (99.5%; Aldrich) as a solvent, PVDF (Kureha KF-1100) as a binder, and Super-P carbon black as a conducting phase. The working electrodes were prepared by casting the paste onto a copper-foil current collector. All of the working electrodes were pressed and vacuum-dried at 120 °C for 12 h. Celgard 2400 was used as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate/ diethyl carbonate (1:1 v/v) was used as the electrolyte. The cells were assembled in an Ar-filled glove box. The galvanostatic charge− discharge profiles were investigated in the voltage range 0.01−3.0 V at a current density of 0.1−2 A/g, using battery testing equipment (TOSCAT-3100; Toyo Co. Ltd). PXRD patterns were measured at the PLS-II 6D UNIST-PAL beamline of Pohang Accelerator Laboratory in Korea. The X-rays from the bending magnet were monochromated using Si(111) double crystals and focused at the detector position using a toroidal mirror. Diffraction patterns were recorded with a 2DCCD detector (MX225HS; Rayonix L.L.C.), and the X-ray irradiation time was ∼120 s depending on the saturation level of the detector. The sample was rotated during the measurements; the diffraction angles were calibrated by a lanthanum hexaboride (LaB6) and converted to the values corresponding to Cu Kα1 radiation (λ = 1.54 Å) for easy comparison. The fully discharged or charged cells were opened, and the composites were picked by a strawlike Kapton capillary (diameter: 340 μm) in an Ar-filled chamber, which was directly connected to the XRD station. TEM analyses were performed with a Cs-corrected microscope (Titan; Fei Co.). Chemical structural information was obtained by XPS (K-alpha; Thermo Fisher Scientific), and the C 1s peaks were calibrated with a binding energy of 284.7 eV

nanophase-incorporated MnO2/RGO composite materials, negatively charged Ag NPs or Ag nanowires (NWs) were added to a mixing batch containing PEI-decorated graphene oxides and MnO2 nanorods. The morphologies of the Ag NPs and NWs are shown in Figure S5. The carboxyl-terminated Ag NPs have a surficial negative charge due to partial deprotonation of the carboxyl groups in an aqueous medium with neutral pH.28 The poly(vinylpyrrolidone) (PVP)-terminated Ag NWs also have a surficial negative charge because of the chemical nature of PVP in an aqueous medium with neutral pH.29 The morphologies of the Ag-NP−MnO2/RGO and AgNW−MnO2/RGO composite materials are shown in Figure S6. According to thermodynamic phase diagrams, a couple of Ag and Co phases do not form any alloy phases and the solubility of Co in Ag is about 0.02 atom % at 600 °C. The combination of the Ag and Mn phases can form a solid solution; the solubility of Mn in Ag is about 27 atom % at 600 °C.25,26 Thus, it is hypothesized that the incorporated Ag phase should maintain its pristine metallic nature during repeated discharging/charging processes, so that the Ag nanophase in transition metal oxide-based anodes can catalyze the formation of reversible gel-like/polymeric SEI layers.

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CONCLUSIONS In summary, it is revealed that the addition of an Ag nanophase to CoO−RGO composites leads to a significant reduction in the initial capacity loss, thereby enhancing the overall reversibility of CoO-based anodes for LIBs. Electrochemical characterizations suggest that this enhanced reversibility resulted from capacitive lithium-ion storage in the low-voltage region, below 0.7 V, during discharging. Ex situ chemical analyses demonstrate that the Ag nanophase catalyzes the formation of PEO-like gel-like/polymeric films, in which the additional lithium ions can be stored capacitively in the lowvoltage region, thus suppressing the generation of irreversible SEI layers. It is strongly believed that a chemical strategy based on the use of a catalytic phase would open up a new pathway for addressing the major problem concerning conversion-type oxide electrodes with inferior reversibility.

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EXPERIMENTAL SECTION

Synthesis of Materials. CoO NPs were synthesized via chemical reduction of cobalt ions in octylamine and phenylhydrzine as described in our previous work.9 Co(III) acetylacetonate (Co(C 5 H 7O 2 )3 , 98%), octylamine (C 8 H17 NH2 , 99%), oleic acid (C18H34O2, 90%), phenylhydrazine (C6H5NHNH2, 97%), and toluene (C6H5CH3, anhydrous, 99.8%) were purchased from Aldrich and used. The details of Ag NP synthesis can be found in our previous work,9 in which Ag nitrate (AgNO3, 99.9%; Kojima Chemicals Co., Ltd.), octylamine (C8H17NH2, 99%; Aldrich), oleic acid (C18H34O2, 90%; Aldrich), phenylhydrazine (C6H5NHNH2, 97%; Aldrich), and toluene (C6H5CH3, anhydrous, 99.8%; Aldrich) were used. Carboxyl-terminated Ag NPs were synthesized via chemical reduction of Ag ions in deionized (DI) water. PVP (average MW ∼10 000) and poly(acrylic acid) (PAA; sodium salt, average MW ∼15 000, 35 wt % in H2O) were purchased from Sigma Aldrich. Sodium borohydride (98.5%) was purchased from Showa, and Ag nitrate (99.9%) was purchased from Kojima Chemicals. To prevent interparticular agglomeration, PVP and PAA were incorporated as surface-capping molecules and sodium borohydride was used as the reducing agent. Ag nitrate (4.7 g), PAA (3.8 g), and PVP (6.0 g) were added to a three-neck, round-bottomed flask containing 100 mL of DI water with pH 11. The reaction solution was heated to 60 °C and stirred with a magnetic stirrer under reflux. When the temperature reached 60 °C, 9.7 g of a mixture of sodium borohydride and DI water 21720


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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/acsami.7b01559. Quantitative analysis results of C 1s XPS spectra of Agfree and 10 wt % Ag-added electrodes, SAED pattern for fully charged 10 wt % Ag-added electrode, TEM images of the fully re-charged Ag-CoO and Ag-free composites, schematic illustration of GITT measurements and GITT profiles, voltage profiles during the first cycling of the AgNW- and Ag-NP-added MnO2/RGO cells, SEM and TEM images of Ag NWs and Ag NPs, and SEM images of Ag-NW- and Ag-NP-added MnO2/RGO composites (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.C.). *E-mail: [email protected] (S.J.). *E-mail: [email protected] (J.M.). ORCID

Daehee Lee: 0000-0001-7725-5085 Sungho Choi: 0000-0002-9834-4543 Kyung Yoon Chung: 0000-0002-1273-746X Sunho Jeong: 0000-0002-5969-1614 Jooho Moon: 0000-0002-6685-9999 Author Contributions #

D.L., M.W., and D.H.K. authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Research Laboratory Program of the National Research Foundation (NRF) funded by Ministry of Science, Information and Communication Technologies and Future Planning (NRF2015K1A1A2029679), and partially supported by the Nano Material Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015M3A7B4050306) and by the National Research Foundation of Korea funded by Korean government (MSIP) (2012R1A3A2026417).

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REFERENCES

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Understanding the Critical Role of the Ag Nanophase in Boosting the

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