Extending the Battery Life Using an Al-Doped Li[Ni0.76Co0.09Mn0.15

Jul 24, 2017 - The cycling stability of a Ni-enriched compositionally graded Li[Ni0.76Co0.09Mn0.15]O2 cathode doped with Al (1 and 2 mol %) was explic...
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Extending the Battery Life Using an Al-Doped Li[Ni0.76Co0.09Mn0.15]O2 Cathode with Concentration Gradients for Lithium Ion Batteries Un-Hyuck Kim,† Seung-Taek Myung,‡ Chong S. Yoon,*,§ and Yang-Kook Sun*,† †

Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea Department of Nano Science and Technology, Sejong University, Seoul 05006, Republic of Korea § Department of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The cycling stability of a Ni-enriched compositionally graded Li[Ni0.76Co0.09Mn0.15]O2 cathode doped with Al (1 and 2 mol %) was explicitly demonstrated by cycling the cathodes in a full cell against a graphite anode up to 1000 cycles. Without Al doping, the pristine gradient cathode retained 88% of the initial discharge capacity, whereas the 2 mol % Al-doped gradient cathode retained 95% of its original capacity. Meanwhile, Li[Ni0.82Co0.14Al0.04]O2 (NCA), representing a typical cathode for commercialized electric vehicles, retained only 80% of the initial capacity. It was shown that Al doping together with the unique morphology of the compositionally graded cathode was able to suppress the microcracking and helped to preserve the mechanical integrity of the cathode particles, whereas the benchmark NCA cathode sustained continuous capacity loss during cycling and was completely pulverized. The remarkable long-term cyclability of the Al-doped gradient cathodes was attributed to the enhanced structural and the surface stabilization, which also improved the thermal stability.

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have good rate capability and relatively low cost, remain the most practical candidates for satisfying the 300 miles threshold.4−6 To maximize the capacity of NCM and NCA cathodes, progressive Ni enrichment has been a recent trend.7−9 However, the capacity gained by Ni enrichment of NCM and NCA cathodes is accompanied by degradation of cycle life and thermal characteristics, mainly because of the formation of a large amount of reactive Ni4+ when the cathode becomes highly delithiated during charging.7,10,11 The reactive Ni4+ ions destabilize the crystal structure by reducing to NiOlike layered phase compounds on the cathode surface.12,13 To overcome the inherent instability of Ni-rich NCM cathodes, our group developed a unique synthesis strategy to minimize the formation of reactive species, introducing a concentration gradient into the cathode particles such that the core composition is highly enriched in Ni, whereas the surface is Ni-depleted and Mn-enriched.14−18 Our group demonstrated that a cell composed of the compositionally graded NCM

ue to the foreseeable depletion of easily recoverable fossil fuels and growing global CO2 emissions, electromobility is rapidly becoming a necessity for personal transportation. Moving from internal combustion engines to electrical vehicles (EVs) requires replacing the portable high-energy source, that is, petroleum, with an efficient energy storage device. Although various types of batteries (e.g., lead−acid and Ni metal hydride) are available, Li-ion batteries (LIBs), as testified by the success of the LiCoO2−graphite cell, are presently the most suitable energy storage medium for EVs and have already been used to provide electrical propulsion for recent EVs.1 Current state-of-the-art LIBs deployed in EVs are, however, still inadequate in meeting the driving range requirement of 300 miles per single charge, which is recognized as the threshold for popular public acceptance of future EVs2,3 Because the energy density of a LIB is severely limited by the specific capacity of the cathodes, to extend the drive range, recent work on EV batteries has mostly focused on identifying and developing a high-capacity cathode. Although myriad materials have been tested over the past 2 decades, Ni-rich layered lithium−transition metal oxides, Li[Ni1−x−yCoxMy]O2 (M = Al (NCA) or M = Mn (NCM); 1− x − y ≥ 0.6), which © 2017 American Chemical Society

Received: July 11, 2017 Accepted: July 24, 2017 Published: July 24, 2017 1848

DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854

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http://pubs.acs.org/journal/aelccp

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Figure 1. (a) SEM images of as-prepared FCG76 particles at different magnifications, showing the spherical morphology and nanoscale primary particles (inset). (b) XRD spectra of as-prepared FCG76, Al-1 FCG76, and Al-2 FCG76 powders. (c) EPMA composition profile of a single Al-1 FCG76 cathode particle. (d) Bright-field TEM image of an Al-1 FCG76 primary particle; (inset) corresponding electron diffraction pattern.

% was also introduced into the FCG76 cathode (yielding cathodes termed Al-1 FCG61 and Al-2 FCG76, respectively) to systematically study the effect of Al substitution on the cycling stability of the compositionally graded NCM cathode during extended cycling. ICP analysis was used to determine the respective average chemical compositions of the synthesized FCG76, Al-1 FCG76, and Al-2 FCG76 as Li[Ni 0 . 7 7 Co 0 . 0 9 Mn 0 . 1 4 ]O 2 , Li[Ni0.76Co0.09Mn0.14Al0.01]O2, and Li[Ni0.75Co0.09Mn0.14Al0.02]O2, respectively, which well matched the designed average compositions. Figure 1a shows a typical SEM image of the cathode powder, showing its morphology. Regardless of the Al content, each powder was composed of spherical particles with an average diameter of 10 μm, and each individual particle was made up of ∼250 nm sized primary particles (Figure 1a, inset). XRD spectra of the FCG76, Al-1 FCG76, and Al-2 FCG76 cathodes are shown in Figure 1b. All three spectra were indexed to the R3̅m space group, consistent with the typical layered structure of NCM cathodes without impurity phases, indicating that Al up to 2 mol % was successfully introduced into the lattice. Using the lattice parameters estimated from refinement of the XRD spectra, unit cell volumes were calculated to be 101.62, 101.56, and 101.53 Å3 for FCG76, Al-1 FCG76, and Al2 FCG76, respectively. The observed reduction in unit cell volume with increasing Al content is indicative of successful incorporation of the Al ions, whose ionic radii are smaller than those of the transition metal ions in FCG76. In addition, Rietveld refinement suggested a slight decrease in cation mixing by transition metal ion occupation (mainly by Ni ions) of Li sites. The Rietveld refinement results are summarized in Figure

cathode and a carbon nanotube−Si composite anode can carry an energy density exceeding the requirement for meeting the 300 miles threshold.17 We perceive this NCM compositional gradation technique to be the most practical solution proposed so far to allow the realization of widespread EV use. One of the key battery criteria for EVs that is often overlooked in evaluating NCM and NCA cathodes is long-term stability and prolonged cycle life. Considering the long service life of an automobile compared to electronic devices, batteries for EVs are required to withstand extended charge/discharge cycling without significant capacity fading. Despite the importance of long-term cycling behavior of Ni-rich NCM cathodes intended for EV applications, relatively little work has been published in the literature on the subject, largely due to the inherent instability of Ni-rich NCM cathodes and the difficulty in assembling a reliable full cell. Following our successful demonstration of superior long-term cycling stability in compositionally graded Ni-rich Li[Ni0.61Co0.12Mn0.27]O2 cathodes,18 in the present work, we increased the Ni fraction to an average composition of Li[Ni0.76Co0.09Mn0.15]O2, with full concentration gradients (FCGs) of Ni and Mn spanning the particle core to the surface, yielding a cathode termed FCG76. The FCG76 cathode was cycled for 1000 cycles in a pouch-type full cell with a graphite anode to verify that it had long-term structural stability, despite its Ni fraction close to that of the well-studied convention Li[Ni0.8Co0.1Mn0.1]O2, for which extended cycling data are nonexistent. In addition, because Al doping of NCM cathodes enhances structural stability and consequently cycling stability by replacing Ni ions and forming strong chemical bonds with oxygen ions,18−20 Al of 1 and 2 mol 1849

DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854

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Figure 2. (a) Initial charge and discharge curves of FCG76, Al-1 FCG76, Al-2 FCG76, and NCA 82 cathodes at 0.1 C in 2032 coin-type halfcells using Li metal anodes. (b) Cycling performance of FCG76, Al-1 FCG76, Al-2 FCG76, and NCA 82 at 0.5 C. (c) Discharge capacity of FCG76, Al-1 FCG76, Al-2 FCG76, and NCA 82 cathodes. (d) Extended cycling performance of FCG76, Al-1 FCG76, Al-2 FCG76, and NCA 82 cathodes tested in pouch-type full cells with graphite anodes at 1 C.

FCG76 (208 mAh g−1) at 0.1 C was only slightly decreased by the addition of Al (205 mAh g−1 for Al-1 FCG76; 203 mAh g−1 for Al-2 FCG76). Capacity retention, on the other hand, improved progressively with Al addition: FCG76, Al-1 FCG76, and Al-2 FCG76 maintained 93, 94, and 95% of their initial capacity, respectively, after 100 cycles at 0.5 C (Figure 2b). To estimate the rate capability of the cathodes, the coin-type halfcells were charged at 0.2 C and discharged at different C rates (0.2−5 C). Al addition progressively degraded the rate capability of the FCG76 cathode; 83.8% of the 0.2 C capacity was recovered at 5 C for FCG76, whereas only 82.4% of the 0.2 C capacity was extracted for Al-2 FCG76 at 5 C (Figure 2c). The observed effect of Al addition was in full agreement with previous data on the Al-doped graded Li[Ni0.61Co0.12Mn0.27]O2 cathode,18 and the beneficial effect of Al doping has also been validated in other types of NCM cathodes.19,20 In general, introduction of small amounts of Al ions into NCM cathodes appears to reduce their discharge capacities but progressively enhance their cycling stabilities; the substituted Al ions tend to retard Li+ ion mobility. To benchmark the cycling stability of the Al-doped FCG76 cathodes against a commercial Li[Ni0.82Co0.14Al0.04]O2 (NCA82) cathode, fundamental electrochemical data obtained for NCA82 are included in Figure 2a−c. The discharge capacity, capacity retention, and rate capability of NCA82 did not deviate greatly from those of FCG76 as both cathodes contained similar Ni fractions. Long-term cycling stability was evaluated in pouch-type full cells using graphite anodes at 1 C, with charging and discharging between 3.0 and 4.2 V; the results are shown in Figure 2d. During the first 100 cycles, as expected from Figure 2b, all four cathodes (FCG76, Al-1 FCG76, Al-2 FCG76, and the benchmark NCA82) exhibited nearly identical capacity retention. Beyond 100 cycles, the enhanced cycling stability of the Al-doped FCG76 cathodes

S1 and Table S1. The concentration gradients of Ni, Co, and Mn within the cathode particles of each type were quantitatively determined by electron probe microanalysis (EPMA) of cross-sectioned particles, with a probe diameter of 1 μm. The hydroxide precursor (Figure S2) prior to lithiation had a Ni concentration that decreased from 95.9 to 66.5% toward the surface of the particle, whereas the Co and Mn concentrations increased from 1.5 and 11.5% to 2.6 and 22.0%, respectively. The resulting surface composition was [Ni0.665Co0.115Mn0.220](OH)2, representing substantial depletion of Ni compared to the particle core ([Ni0.959Co0.015Mn0.026](OH)2), and well-established FCGs of Ni, Co, and Mn within the particles as designed. After lithiation under 790 °C heat treatment, the concentration of Ni decreased smoothly from 85.2 to 70.2%, and Co and Mn increased from 6.9 to 9.6% and from 6.8 to 19.1%, respectively (Figure 1c), while the concentration of Al remained invariant throughout the particle, giving the final surface composition of Li[Ni0.7Co0.10Mn0.19Al0.01]O2. The primary particles had elongated shapes and formed a spoke-like morphology in which they were tightly packed and oriented radially (Figure 1d). In addition to the unique compact morphology, each primary particle exhibited strong crystallographic texture with layer planes (i.e., (003) planes) running parallel to the longitudinal axis of each primary particle (Figure 1d, inset). The particle morphology and crystallographic texture of the Aldoped FCG76 cathodes were consistent with those of our previously reported compositionally graded cathodes;18 hence, Al doping of the FCG76 cathode appeared to hardly affect the unique microstructure of the graded cathodes. Electrochemical properties of the FCG76, Al-1 FCG76, and Al-2 FCG76 cathodes were characterized in 2032 coin-type half-cells (Figure 2a). The initial discharge capacity delivered by 1850

DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854

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Figure 3. SEM images of cathodes after 1000 cycles: (a) NCA82, (b), FCG76, (c) Al-1 FCG76, and (d) Al-2 FCG76.

became clearly evident. FCG76 retained 87.9% of its initial capacity after 1000 cycles, compared to 79.6% for the benchmark NCA82, thus verifying the superior stability of the compositionally graded cathode having a Ni-depleted surface. Even more remarkable was the improvement brought about by the Al addition: use of the Al-1 FCG76 and Al-2 FCG76 cathodes led to 93.7 and 95.0% retention after 1000 cycles, respectively. To guarantee battery life, commercial NCA cathodes are usually cycled well below their full depth of discharge, at the expense of reduced discharge capacity.21−23 The excellent cycling stability of the Al-doped FCG76 cathodes under cycling to their full depth of discharge, as demonstrated in Figure 2d, translates to increased energy density and extended life in their applications to EV batteries. The structural stability engendered by Al substitution was first verified at the particle level using SEM, by examining each cathode recovered from the cycled full cell (after 1000 cycles). The low-magnification image in Figure 3a shows that the NCA82 cathode particles were completely pulverized, with no visible evidence of the spherical particles present prior to cycling (Figure 3a). Rather loosely held primary particles due to microcracking21−23 can be clearly observed in the accompanying magnified image of a fractured NCA particle. Contrastingly, the FCG76 cathode maintained its spherical particle morphology, albeit with large visible cracks (Figure 3b). Meanwhile, spherical particles with no externally visible cracks were observed for Al-1 FCG76 and Al-2 FCG76, even after 1000 cycles (Figure 3c,d), evincing that the mechanical integrity of the cathode particles was well preserved. The cycled electrodes were also analyzed using TEM by preparing thin sections of a single particle from each of the cycled electrodes. A scanning TEM image of the cycled FCG76 particle (Figure 4a) contained, in addition to voids present at the core from its as-prepared state, large cracks that traversed nearly the entire particle. Moreover, a magnified image showed hairline cracks along the boundaries between primary particles (Figure 4b). It appeared that most of the cracks were initiated from the particle core and propagated outward. This observation is consistent with the explanation that the Ni-enriched core suffered relatively high lattice strain due to anisotropic volume change during charging and discharging. The random orientation of the primary particles at the core would increase the buildup of stress and likely lead to crack formation. In fact, one of the primary reasons for mechanical degradation of NCM cathodes during cycling is anisotropic lattice strain, whose magnitude worsens with increasing Ni fraction.24 In the case of the Al-doped FCG76 cathodes, no large cracks were observed

Figure 4. Mosaic scanning-TEM image of cycled electrode after 1000 cycles in a full cell (a) FCG 76, (b) magnified image marked area in (a), (c) Al-1 FCG 76 and (d) Al-2 FCG76.

(Figure 4c,d), suggesting that the presence of Al ions might increase the grain boundary strength, as previously demonstrated regarding the particle fracture strength of Al-doped NCM cathodes.18 Retardation of Li+ ion migration, which takes place mainly along the grain boundaries, provides tentative support for this conjecture. Alternatively, the suppression of crack formation might have arisen from the reduced anisotropic lattice strain of Al doping.20 This is especially applicable in Nirich NCM cathodes, regarding that T. Ohzuku25 demonstrated that Al substitution of Ni in LiNiO2 inhibits a detrimental phase transition that would otherwise cause abrupt lattice contraction during charging and thereby improves the cycling stability of LiNiO2. Because the core of the FCG76 cathode contains ∼90% Ni, the present Al doping could have a similar effect to that observed for LiNiO2. To substantiate the improved structural stability of the Al-doped FCG76 cathodes, in situ time-resolved (TR)-XRD experiments (30−600 °C) were carried out using a partially deintercalated cathode (Li0.33[Ni0.76Co0.09Mn0.15]O2), as presented in Figure 5a. The partially deintercalated FCG76 cathode maintained the R3̅m structure, which transformed to a disordered spinel structure (Fd3̅m) at 243 °C, as evidenced by merging of the (108)R and 1851

DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854

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Figure 5. Full range (10° ≤ 2θ ≤ 80°) of TR-XRD patterns of partially deintercalated FCG76 and Al-2 FCG76 cathodes during heating from 30 to 600 °C.

Figure 6. (a) High-resolution TEM image of a cycled NCA82 cathode with its corresponding electron diffraction pattern (inset) and Fourier transform images from the regions marked 1, 2, and 3. (b) TEM image of cycled NCA82 surface; (inset) corresponding electron diffraction pattern displaying an intragranular crack. (c) High-resolution TEM image of a cycled NCA82 cathode from the marked region in (b). (d) Bright-field TEM image of a cycled FCG76 cathode; arrows indicate cracks along the particle boundaries. (e) High-resolution TEM of a cycled FCG76 cathode with its corresponding electron diffraction pattern and (inset) FFT images of regions marked 1 and 2. (f) Highresolution TEM of a cycled Al-1 FCG76 cathode with its corresponding electron diffraction pattern and (inset) FFT images of regions marked 1−4.

(110)R peaks into a single peak attributed to spinel (440)S. The disordered spinel phase gradually changed to the rock salt structure (Fm3̅m) starting at 364 °C; (222)S and (400)S shifted to the lower angle and intensity of (311)S, and the (511)S peak started to decrease in intensity. At 592 °C, the cathode completed its transformation to the rock salt structure. The series of phase transitions and the onset temperatures detected from the TR-XRD data are in general agreement with previous reports on other Ni-rich NCM and NCA cathodes.26,27 In the case of Al-2 FCG76, the onset temperatures for the transitions to the disordered and rock salt structures increased to 258 and 410 °C, respectively (Figure 5b). The TR-XRD data clearly attest to the increased structural and thermal stability of the FCG76 cathode brought about by Al doping. To estimate the extent of the surface damage incurred during cycling, highresolution (HR)-TEM was used to study the surface structure

of the Al-doped FCG cathode after 1000 cycles, in comparison with that of the benchmark NCA82 cathode. Figure 6a presents a [010] zone HR-TEM image of a primary particle from the cycled NCA82 cathode, exemplifying the extensive structural damage incurred by repeated Li+ insertions and removals. Contrastingly, a Fourier-filtered image of region 1 in Figure 6a shows the perfect R3̅m structure of alternating layers of Ni and Li ions (3a and 3b sites). In comparison, (003) lattice fringes from a region in proximity to the particle edge were twisted and contained dislocations (region 2, Figure 6a inset). Streaks were also observed along the [001] direction in the diffraction pattern included in Figure 6a, testifying to the existence of thin plates of stacking faults in the layer planes. At the very edge of the particle, a NiO-like surface layer was observed (region 3, Figure 6a inset), resulting from the presence of the reactive Ni4+ species, as often observed for cycled Ni-rich NCM and 1852

DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854

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ACS Energy Letters NCA cathodes.12,13 Hence, in the cycled NCA82 cathode, the damaged area that extended to a more than 10 nm depth from the surface exhibited an assortment of structural defects: distortion of layer planes including dislocations and stacking faults and severe cation mixing. In addition, an incipient intraparticle crack developing from the particle surface was also observed (Figure 6b). A magnified image of the region marked in Figure 6b evinced crack initiation arising from material loss (Figure 6c). On the other hand, a bright-field image of the cycled FCG76 cathode particle indicated that the cracks propagated along the particle boundaries (Figure 6d), as evinced by grain boundary separation (marked with arrows). A [010] zone HR-TEM image together with Fourier-transformed images of the marked regions verifies that the R3m ̅ structure remained intact close to the particle edge; the Fouriertransformed image of region 1 well matched the selected area diffraction pattern shown as an inset in Figure 6e. However, region 2, similar to the NCA cathode, developed a NiO-like layer that was confined to ∼5 nm thickness. In the Al-1 FCG76 cathode, on the other hand, no noticeable grain boundary separation was found among the observed primary particles, and as illustrated in Figure 6f, the NiO-like damage layer observed in the Al-1 FCG76 cathode could not be detected using Fourier transformation of the selected regions 1−4 because the NiO-like layer was confined to the thin (1 nm) section along the particle edge. In summary, the NCA82 cathode was completely pulverized during 1000 cycles through a well-documented mechanism: primarily from initial microcrack formation along the grain boundaries and also from subsequent electrolyte penetration that further accelerated the deterioration of mechanical integrity. As the surfaces exposed to the electrolyte increased, the particle surface incurred increasingly severe microstructural damage (Figure 6a−c); also, cracks formed and propagated within primary particles. On the other hand, the unique morphology of the FCG76 cathode was able to somewhat suppress the microcrack formation and propagation, thereby preventing total destruction of the cathode particles; this explains why 88% of the initial discharge capacity was maintained after 1000 cycles, which is impressively high for an NCM cathode containing a high Ni fraction of 76%. The remarkable long-term cyclability of the Aldoped FCG76 cathodes arose from their structural stability, as evidenced by the TR-XRD data and from their surface stabilization, as demonstrated by the TEM analysis. The surface stabilization thus also improved the thermal stability of the Al-doped FCG cathodes (see DSC data in Figure S3), in agreement with previous reports that the thermal stability of Li[Ni0.5Mn0.5]O2 and Li[Ni0.33Co0.33Mn0.33]O2 cathodes is improved by Al substitution.28,29 It was explicitly demonstrated in a full cell that the cycling stability of Ni-enriched, compositionally graded FCG76 (average composition of Li[Ni0.76Co0.09Mn0.15]O2) can be increased through Al doping (up to 2 mol %) to a level that has not been previously attained in NCM cathodes of similar Ni fraction. A full cell including an Al 2-FCG76 cathode and a graphite anode were cycled for 1000 cycles to demonstrate the cathode’s suitability for deployment in EVs, after which the cell retained 95% of its initial discharge capacity. It was shown that the application of Al doping, in combination with the unique morphology of the compositionally graded FCG76 cathode, suppressed microcracking and helped to preserve the mechanical integrity of the cathode particles even after 1000 cycles, whereas the benchmark NCA82 cathode sustained

continuous capacity loss during cycling and was found to be completely pulverized after 1000 cycles. The remarkable longterm cyclability of the Al-doped FCG76 stemmed from its enhanced structural stability, as evidenced by TR-XRD data, and from its surface stabilization, as evidenced by TEM analysis. Al doping also improved the thermal stability of the FCG76 cathode. Thus, the Al-doped FCG76 cathode studied herein represents a base material to be improved upon for application to EV batteries requiring high energy density, long life, and safe operation.

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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.7b00613. Rietveld refinement of XRD data, EPMA composition profile of a single FCG76 precursor particle, DSC profiles of FCG76 and Al-2 FCG76 cathodes, and table of lattice parameters and cation mixing (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Seung-Taek Myung: 0000-0001-6888-5376 Chong S. Yoon: 0000-0001-6164-3331 Yang-Kook Sun: 0000-0002-0117-0170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by the Global Frontier R&D Program (2013M3A6B1078875) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, the Commercializations Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 2016K000210).

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REFERENCES

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DOI: 10.1021/acsenergylett.7b00613 ACS Energy Lett. 2017, 2, 1848−1854