100k Cycles and Beyond: Extraordinary Cycle Stability for MnO2 Nanowires Imparted by a Gel Electrolyte Mya Le Thai,† Girija Thesma Chandran,† Rajen K. Dutta,‡ Xiaowei Li,¶ and Reginald M. Penner*,† †

Department of Chemistry, ‡Department of Physics and Astronomy, and ¶Department of Chemical Engineering and Materials Science, University of California, Irvine, California 92697, United States S Supporting Information *

ABSTRACT: We demonstrate reversible cycle stability for up to 200 000 cycles with 94−96% average Coulombic efficiency for symmetrical δ-MnO2 nanowire capacitors operating across a 1.2 V voltage window in a poly(methyl methacrylate) (PMMA) gel electrolyte. The nanowires investigated here have a Au@δ-MnO2 core@shell architecture in which a central gold nanowire current collector is surrounded by an electrodeposited layer of δ-MnO2 that has a thickness of between 143 and 300 nm. Identical capacitors operating in the absence of PMMA (propylene carbonate (PC), 1.0 M LiClO4) show dramatically reduced cycle stabilities ranging from 2000 to 8000 cycles. In the liquid PC electrolyte, the δMnO2 shell fractures, delaminates, and separates from the gold nanowire current collector. These deleterious processes are not observed in the PMMA electrolyte.

F

of no reports of this extraordinary level of cycle stability for nanowires of any kind in the literature. Here, we report that the cycle stability of MnO2 all-nanowire capacitors can be extended from 2000 to 8000 cycles to more than 100 000 cycles, simply by replacing a liquid electrolyte with a poly(methyl methacrylate) (PMMA) gel electrolyte. Our investigations exploit a new Degradation and Failure Discovery Platform, consisting of a symmetrical, all-nanowire capacitor consisting of two interpenetrating arrays of 375 ultralong (5 mm) core@shell Au@δ-MnO2 nanowires1 patterned lithographically onto glass, using the LPNE process (Figure 1a).8−10 By design, the ultralong nanowires in these capacitors amplify the influence of degradation processes that culminate in breakage of the nanowire because for ultralong nanowires, breakage “disconnects” a larger fraction of the total energy storage capacity of the electrode.1 Because Lixδ-MnO2 has a low

or electrode materials that rely on ion insertion for Faradaic charge storage, a nanowire morphology can enable higher power in either batteries or capacitors than is possible using a film of the same material.1−5 However, the Achilles heal of such nanowires for energy storage is cycle stability.1,6 The diminutive lateral dimension of nanowires increases their susceptibility to dissolution and corrosion, and these processes rapidly result in a loss of electrical continuity through the nanowire and an irreversible loss of capacity. In the specific case of MnO2an insertion metal oxide that is of interest herewe have recently reported the retention of Csp to 4000 cycles for symmetrical core@shell Au@MnO2 allnanowire capacitors operating at rapid voltage scan rates of 100 mV/s across a 1.2 V window in dry LiClO4, acetonitrile electrolytes.1,2 Somewhat better cycle stability of up to 10 000 cycles has been reported7 for composites of Mn3O4 nanorods and graphene. In stark contrast, in one recent example, symmetrical capacitors based on α-Mn2O3 films produced a stable Csp for an astonishing 200 000 cycles when cycled across a 0.80 V window in aqueous Na2SO4 electrolyte.3 We are aware © XXXX American Chemical Society

Received: March 24, 2016 Accepted: April 11, 2016

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Figure 1. Degradation and Failure Discovery Platform. (a) Schematic diagram showing critical dimensions of the Au@δ-MnO2 all-nanowire capacitor investigated here. The PMMA gel layer, consisting of 20 (w/w)% PMMA in 1.0 M LiClO4 and PC, is 180 μm in thickness. (b) Lowmagnification image of a several Au@δ-MnO2 nanowires on the capacitor surface. (c) High-magnification SEM image of the gold nanowire core of the Au@δ-MnO2 nanowires with lateral dimensions of 35 nm (h) × 240 nm (w). (d) High-magnification SEM image of a Au@δ-MnO2 nanowire showing the morphology of the electrodeposited δ-MnO2 shell with a mean thickness of 124 nm.1 (e) Photograph of the capacitor containing 750 parallel nanowire loops patterned onto a glass microscope slide.

electrical conductivity for all accessible values of x (<0.005 S/ cm),11 the preparation of ultralong nanowires requires a core− shell architecture in which a gold nanowire current collector is located in the center of a δ-MnO2 shell (Figure 1b−d). As previously demonstrated,1 the central gold nanowire is able to provide electrical access to the energy storage capacity of the MnO2 shell. We evaluate the charge storage performance and cycle stability of Au@MnO2 nanowires with MnO2 shell thicknesses, tMnO2, of 143, 222, and 300 nm (Figure 2) in two electrolytes, (1) 1.0 M LiClO4 in propylene carbonate (PC) and (2) this same electrolyte with 20 (w/w)% PMMA added. Preparation of Au@δ-MnO2 Core@Shell Nanowire Capacitors. Each capacitor contained a total of 750 interdigitated nanowire loops, 375 on each electrode (Figure 1a). A small region of the capacitor showing these nanowire loops is seen in the lowmagnification SEM images of Figure 1b. These Au@δ-MnO2 nanowires were prepared using the LPNE process described previously,1 except for the replacement of a liquid PC or acetonitrile-based electrolyte with a 180 μm thick gel electrolyte layer composed of 20 (w/w)% PMMA and 1.0 M LiClO4 in PC. Process flow for the fabrication procedure is shown in Figure S1, and a description of each step is provided in the Supporting Information. The properties of as-prepared capacitors having three tMnO2 values (Figure 2i,j) show that the total energy storage capacity increases with MnO2 shell thickness across the range from 143 to 300 nm. The mass-normalized specific capacitance, Csp, (defined as Csp = Q/[(ΔE)(mMnO2)], where Q is the integrated charge, ΔE = 1.2 V, and mMnO2 is the dry mass of the MnO22) varies inversely with tMnO2 (Figure 2k). This trend, also documented previously,1 demonstrates that the energy storage capacity of the thicker MnO2 shells is not as accessible as that of thinner shells, likely a consequence of the electrical resistance of the thicker MnO2 layers. Plots of Csp versus potential scan

rate (Figure 2k) for a particular MnO2 shell thickness show decreasing Csp with increasing scan rate from 1 to 100 mV/ s.2,11,12 This behavior has been attributed to the influence of rate-limiting Li+ insertion and the solid-state diffusion of Li on charge storage, not only for MnO22,12 but also for other transition metal oxides.13−16 A key point, as demonstrated here for the 222 nm shell thickness (green traces, Figure 2k), is that the addition of PMMA to the electrolyte produces no difference in the electrochemical behavior of freshly prepared nanowire capacitors and no diminution of Csp. No penalty, in terms of energy storage performance, is imposed by the PMMA gel electrolyte. Inf luence of PMMA Gel on Cycle Stability. In liquid PC electrolyte, cycle lifetimes of 2000−8000 cycles are obtained for all three of these capacitors (Figure 3a,b). This stability is in the range of values reported recently for Au@δ-MnO2 core@shell nanowire capacitors in dry acetonitrile electrolyte.1 Dramatically greater cycle stability of Csp is seen in the PMMA gel electrolyte for all MnO2 shell thicknesses (Figure 3a,b). Stability to >100k cycles is demonstrated for capacitors containing nanowires with tMnO2 = 300 and 222 nm, whereas for tMnO2 = 143 nm, cycle stability to >200 000 cycles is demonstrated. In all cases, these cycling experiments in PMMA gel were terminated before a fade of Csp equaling 10% was observed. A slow and steady increase in Csp across 100 000 cycles was observed for tMnO2 = 222 nm and for over 200 000 cycles for the 143 nm nanowires, while tMnO2 = 300 nm showed increases in Csp for 80 000 cycles, followed by a fading of the Csp by ∼5%. To our knowledge, this level of cycle stability has not previously been reported for nanowire-based capacitor or battery electrodes that have a significant insertion capacity in addition to double-layer charge storage (as shown in Table 1).17−20 58

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Figure 2. SEM (a−d) and AFM (e−h) images of gold (a,e), and Au@δ-MnO2 core@shell (b−d, f−h) nanowires: (a,e) gold nanowire comprising the core of Au@δ-MnO2 core@shell nanowires. A height versus distance amplitude trace is shown below each AFM image. (b,f) Au@δ-MnO2 core@shell nanowire prepared by electrodepositing MnO2 onto the gold nanowire shown in (a) for 5 s. (c,g) MnO2 deposited for 10 s. (d,h) MnO2 deposited for 40 s. (i−k) Charge storage performance for all nanowire capacitors composed of Au@MnO2 nanowires. All data here were acquired using the PMMA gel electrolyte except in the case of the 222 nm shell thickness, where data for the PMMA gel electrolyte and PC-only electrolyte are both shown (k). (i) Cyclic voltammograms at 100 mV/s for capacitors prepared with three MnO2 shell thicknesses, 143, 222, and 300 nm, as indicated. (j) Galvanostatic charge/discharge curves for nanowire capacitors at 1 A/g. Total Csp values are 19, 34, and 56 F/g for tMnO2 values of 300, 222, and 143 nm, respectively. (k) Csp versus scan rate for MnO2 nanowire arrays. For the 222 nm shell thickness, data for PMMA (solid green line) and no PMMA electrolytes (dashed green line) are compared. Error bars represent ±1σ for three as-prepared capacitors at each tMnO2.

The slow increase in Csp across the first 20 000 cycles seen for capacitors in the gel electrolyte is curious as this behavior is not seen in liquid PC or acetonitrile1 electrolytes. PMMA gel was applied to dried Au@δ-MnO2 core@shell nanowires in this study. We hypothesize, that the observed “activation” of Csp involves the slow permeation of the nanoporous MnO2 shell by the viscous PMMA gel electrolyte over a period of weeks. If this permeation process occurs, as the wetting of the interior of the porous MnO2 layer proceeds, both insertion and noninsertion components of the capacity increase. For the tMnO2 = 222 nm device, the resulting increase in Csp across 100 000 cycles is readily apparent in the cyclic voltammograms (Figure 3c). Csp versus scan rate plots (Figure 3d) show the retention of high Csp values of 200 F/g at slow scan rates after 95 000 cycles. The corresponding half-cell capacity is four times this full cell value, or 800 F/g, in the range of values seen for MnO2 nanowire-based electrodes in our prior work.2,12 These data confirm that the insertion capacity of the as-prepared MnO2 is retained over the course of these ultralong experiments, lasting several months. The retention of insertion-based capacity is also supported by a deconvolution of the insertion and

noninsertion components of Csp, which is discussed in the Supporting Information. Nanowire capacitors operating in PMMA gel electrolyte exhibited a good average Coulombic efficiency (C.E.) of ∼96% over >100k cycles. As shown for tMnO2 = 143 nm nanowires in Figure 3a, a C.E. of ∼98% is measured after 30−40 kcycles. However, much lower C.E. values of 60% are seen initially, and the C.E. increases montonically over the first 20 000 cycle. Although the origin of the low values of C.E. measured initially in PMMA gel electrolyte is not apparent, low C.E. values are not seen in liquid PC electrolyte, suggesting that this phenomenon may also be related to the slow wetting of the porous MnO2 layer by PMMA, already alluded to above. Mechanism of Stabilization by PMMA Gel Electrolye. For MnO2 devices, capacity fade has been attributed to dissolution of the MnO2, caused by Mn3+ disproportionation21−23 at negative potentials. Mechanical fatigue and fracture of the MnO2, caused by the strain imposed by ion insertion/ deinsertion, is a second mechanism contributing to capacity fade.24,25 In an attempt to understand how the PMMA gel confers stability on Au@MnO2 core@shell nanowires, these nanowires 59

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electrolyte after 4000 charge/discharge cycles, short-range (100−500 nm) loss of MnO2 from the shell is readily apparent along the entire length of all of the nanowires in the capacitor (Figure 4e,f). The same capacitor cycled 100 000 cycles in the PMMA electrolyte, in contrast, shows no reduction in the shell diameter and no short-range losses of MnO2 (Figure 4g,h). The short-range loss of MnO2 is augmented by loss of MnO2 on a micron scale in other sections of the same capacitors, as shown in Figure 4i and j. Here, 1−5 μm sections of the δ-MnO2 shell have been excised from the core−shell nanowire in multiple places, revealing the underlying gold nanowire current collector (Figure 4i). This mode of shell loss is also not evidenced in SEMs of Au@δ-MnO2 nanowires cycled 100k cycles in PMMA gel electrolyte. The short-range (Figure 4e,f) and long-range (Figure 4i) losses of the MnO2 shell are collectively expected to reduce Csp qualitatively, as observed in our experiments. These images implicate a two-stage mechanism for irreversible capacity loss in Au@δ-MnO2 nanowires that involves short-range MnO2 loss, observed to occur along the entire length of the core−shell nanowire, followed by micron range separation of the entire “skeletonized” MnO2 shell from the gold nanowire current collector (Scheme 1a). Mechanical confinement of the MnO2 shell by the viscous, semisolid PMMA gel seems to be one mechanism by which fracture and long-range loss of MnO2 (Figure 4i) is averted. We speculate that the high viscosity and elasticity of the PMMA gel prevents separation of MnO2 from the current collector while remaining transparent to fluxes of Li+ involved in insertion and deinsertion. However, any mechanical stabilization conferred by the gel clearly is not the whole story: For nanowires cycled in the PMMA gel electrolyte, we also find fewer fractures of the MnO2 shell, suggesting that the PMMA may function as a plasticizer, increasing the fracture toughness of the shell. Further investigation of the mechanical properties of the MnO2 shell will be required to determine whether this “softening” of the MnO2 shell is occurring. In addition, the mechanism by which the short-range skeletonization of the MnO2 shell occurs and the corresponding mechanism of PMMA stabilization remain to be determined. A preliminary Raman microprobe study of as-prepared and cycled Au@δ-MnO2 nanowires shows no difference between the Raman spectra of Au@δ-MnO2 nanowires cycled in liquid PC and PMMA gel (Figure S3). Embrittlement of the MnO2 shell, revealed by a stiffening of Mn−O modes, would predispose the shell to fracture and mechanical loss induced by the strain imparted by ion insertion/deinsertion. This analysis is summarized in the Supporting Information. Further

Figure 3. Cycle stability of Au@δ-MnO2 core@shell nanowire capacitors. (a,b) Csp versus cycles for MnO2 shell thicknesses as indicated. Also plotted (top) is the Coulombic efficiency for the 222 nm MnO2 shell thickness. Other shell thicknesses were virtually identical. (b) Detail showing the first 20 000 cycles in (a). (c) CVs at 100 mV/s for the 222 nm MnO2 shell thickness acquired for cycle 1 and cycle 100 000, as indicated. (d) Csp versus scan rate for the 222 nm MnO2 shell thickness, for data acquired at 6000, 40,000, 75 000, and 95 000 cycles, as indicated.

were examined by SEM before and after cycling in PC and PMMA gel electrolytes (Figure 4). Two samples of as-prepared Au@MnO2 nanowires with a 222 nm shell thickness appear identical in these images (Figure 4a−d). In a liquid PC

Table 1. All Nanowire Capacitors with Field-Leading Cycle Stability anode

cathode

electrolytea

Cspb

cycles before Csp fade

literature reference

V2O5 LiMnO2 KMnO2 Mn2O4 pCNFs/G RGMA MnOx/Au LiMnO2

V2O5 PEDOT KMnO2 Mn2O4 pCNFs/G RGMA MnOx/Au LiMnO2

LiCl/PVA acetonitrile/PMMA PVA/H2SO4 PC/PMMA PVA/H2SO4 ionic liquid PVA/H2SO4 PC/PMMA

0.25 F/cm2 80 F/g 14 F/cm2 9 F/cm2 100 F/g 2.72 F/cm3 32.8 F/cm3 12−56 F/g

1000 1250 10 000 30 000 5000 6000 15 000 >100 000−>200 000

17 18 19 20 26 27 28 this work

a

Abbreviations: PEDOT - poly(ethylenedioxythiophene); PVA - poly(vinyl alchohol); PMMA - poly(methyl methacrylate); PC - propylene carbonate; RGMA - graphene oxide/MnO2/AgNW; pCNFs/G - porous carbon nanofibers/ultrathin graphite. bSpecific capacity, in specified units. 60

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Figure 4. SEM analysis of Au@δ-MnO2 nanowires before and after cycling. (a−d) SEMs at low (a,c) and higher (b,d) magnification show two identical, as-prepared Au@δ-MnO2 nanowires with shells of thickness 220 nm. (e,f) SEMs of the same nanowire shown in (a,b) after 4000 charge/discharge cycles. The short-range loss of MnO2, from 100 to 500 nm domains, is readily apparent in these images (green arrows). (g,h) SEMs of the same nanowire shown in (c,d) after 100 000 charge/discharge cycles. In contrast to (e,f), using the PMMA gel electrolyte, no shell loss is observed in this case. SEM analysis of Au@δ-MnO2 nanowires before and after cycling. In PC without PMMA, short-range loss of MnO2 (e,f) precedes long-range loss of the MnO2 shell over a length scale of microns. SEM images of a single nanowire loop of a Au@δMnO2 core@shell structure without PMMA (i) and with PMMA (j) document the loss of the MnO2 shell (green arrows) in the absence of the PMMA.

work will be required to elucidate the chemically detailed mechanism of short-range and long-range MnO2 shell loss in these systems. In summary, a cycle stability exceeding 100 000 cycles and up to 200 000 cycles has been achieved for Au@MnO2 core@shell nanowires. A mean Coulombic efficiency across 100−200 kcycles, corresponding to 6−11 weeks of cycling, was 94−96% for these systems. This performance is particularly interesting in view of the fact that the 750 Au@MnO2 core@shell nanowires contained in the Degradation and Failure Discovery Platform are each 5 mm in total length, a design feature intended to accentuate the influence of defects caused by degradation on specific capacity. The key to achieving this level of stability is simply the addition of PMMA to the normal PC electrolyte. We have further provided evidence that the MnO2 oxide shell of these nanowires continues to access insertion-based capacity during the entire experiment and does not revert to pure noninsertion charge storage. These experiments demonstrate for the first time that nanowire-based battery and capacitor electrodes are capable of providing extremely long cycle lifetimes. On the basis of SEM analysis of cycled Au@MnO2 core@ shell nanowires, one mechanism by which PMMA gel may extend cycle lifetime is simply the mechanical confinement of the MnO2 shell material on the gold nanowire current collector. The high viscosity and elasticity of the PMMA gel apparently prevents separation of MnO2 from the current collector while remaining transparent to fluxes of Li+ involved in insertion and deinsertion. From SEM images of cycled nanowires, one can also infer that the PMMA gel electrolyte reduces the propensity for fracture of the MnO2 shell, increasing its fracture toughness. Further investigation of the mechanical properties of the MnO2 shell will be required to evaluate whether this process is actually occurring.

Scheme 1. (a) Illustration of the Two-Stage Progression of Degradation for Au@δ-MnO2 Nanowires in PC Electrolyte without PMMA gela and (b) Addition of PMMA to the PC Electrolyte Forestalling Both of These Degradation Modes

a Short-range loss of MnO2 on a 100−500 nm length scale (Figure 4e,f) precedes long-range loss of the MnO2 shell over a length scale of microns (Figure 4i), both of these processes contributing to irreversible capacity loss.

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EXPERIMENTAL METHODS Chemicals and Materials. Nickel and gold pellets (5 N purity, Kurt J. Lesker Co.) were used for the preparation by thermal evaporation of ultrathin metal layers. Manganese perchlorate hydrate (Mn(ClO4)2·H2O, 99%), poly(methyl methacrylate) (PMMA, Mw = 120 000 g/mol by GPC, 99.99%), and lithium perchlorate (LiClO4, 99.99%) were used as received from Sigma-Aldrich. Acetone, nitric acid, and propylene carbonate (PC, 99.7%) were used as received from Fisher (ACS Certified). PC was stored in a glovebox until use. Au@δ-MnO2 Nanowire Fabrication. The fabrication of all nanowire capacitors using arrays of Au@δ-MnO2 core@shell nanowire arrays was accomplished using the LPNE processes previously described1 and is discussed in detail in the Supporting Information and Figure S1. Preparation of the Gel Electrolyte. The 1.0 M LiClO4, 20 (w/ w) %, PMMA, and PC gel electrolyte was prepared by adding 1.6 g (20 wt %) of PMMA to 5 mL of 1.0 M LiClO4 in dry PC. The mixture was dissolved by vigorous stirring at 115 °C. In a desiccator, the mixture slowly cooled to room temperature and transformed to the gel state. Electrochemical Characterization. Electrodeposition was accomplished using a three-electrode electrochemical cell with Princeton Applied Research 2263 and 2273 potentiostats using an SCE reference electrode. Prior to each measurement, the cell holder was presaturated with N2 gas and sealed with parafilm to eliminate moisture in air. Structural Characterization. Scanning electron micrographs were acquired using a FEI Magellan 400L XHR scanning electron microscope operating at 10 keV. Before imaging, samples were sputter-coated with ∼2 nm of iridium. AFM images and amplitude traces were acquired using an Asylum Research, MFP-3D AFM equipped with Olympus AC160TS tips in laboratory ambient air. Raman Spectroscopy. Raman spectra were collected at room temperature using a Renishaw inVia Raman microscope equipped with the EasyConfocal optical system (spatial resolution: 1 μ m) and green laser (wavelength of 532 nm and 22 mW laser power). WiRE 3 software was used to acquire the data and images. Other Raman measurement specifications are the objective lens 50× , 0.5% laser power on the sample, laser exposure time of 120 s.

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(EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. Valuable discussions with Professor Phil Collins and Tim Plett are gratefully acknowledged. SEM data were acquired at the LEXI facility (lexi.eng.uci.edu/) at UCI. Raman microprobe spectra and maps were acquired at the Laser Spectroscopy Facility (LSF) at the Department of Chemistry at UCI. The expert assistance of Dr. Dmitry Fishman, director of the LSF facility, is gratefully acknowledged.

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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/acsenergylett.6b00029. Details of Au@MnO2 core@shell nanowires fabrication using LPNE, deconvolution of Csp, and Raman spectroscopy study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Nanostructures for Electrical Energy Storage (NEES II), an Energy Frontier Research Center 62

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