A Robust Hybrid Zn-Battery with Ultralong Cycle Life - Nano Letters

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A Robust Hybrid Zn-Battery with Ultralong Cycle Life Bing Li, Junye Quan, Adeline Loh, Jianwei Chai, Ye Chen, Chaoliang Tan, Xiaoming Ge, T. S. Andy Hor, Zhaolin Liu, Hua Zhang, and Yun Zong Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03691 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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A Robust Hybrid Zn-Battery with Ultralong Cycle Life Bing Li,a Junye Quan,b Adeline Loh,#a Jianwei Chai,a Ye Chen,b Chaoliang Tan,b Xiaoming Ge,a T. S. Andy Hor,a,c Zhaolin Liu,*a Hua Zhang*b and Yun Zong*a a

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology

and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore. b

Center for Programmable Materials, School of Materials Science and Engineering, Nanyang

Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore. c

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR,

China KEYWORDS hybrid zinc-battery, zinc-air battery, zinc-nickel battery, redox reaction, NiCo2O4 nanowire

ABSTRACT Advanced batteries with long cycle life and capable of harnessing more energies from multiple electrochemical reactions are both fundamentally interesting and practically attractive. Herein, we report a robust hybrid zinc-battery which makes use of transition-metal-based redox reaction (M-O-OH → M-O, M = Ni and Co) and oxygen reduction reaction (ORR) to deliver more

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electrochemical energies of comparably higher voltage with much longer cycle life. The hybrid battery was constructed using an integrated electrode of NiCo2O4 nanowire arrays grown on precarbon-coated nickel foam, coupled with a zinc plate anode in alkaline electrolyte. Benefitted from the M-O/M-O-OH redox reactions and rich ORR active sites in NiCo2O4, the battery has concurrently exhibited high working voltage (by M-O-OH → M-O) and high energy density (by ORR). The good oxygen evolution reaction (OER) activity of the electrode and the reversible MO ↔ M-O-OH reactions also enabled smooth recharging of the batteries, leading to excellent cycling stabilities. Impressively, the hybrid batteries maintained highly stable charge-discharge voltage profile under various testing conditions, e.g. almost no change was observed over 5000 cycles at a current density of 5 mA cm-2 after some initial stabilization. With merits of higher working voltage, high energy density and ultralong cycle life, such hybrid batteries promise high potential for practical applications.

INTRODUCTION High energy and power density, long cycle life, good safety and affordable price are desirable features for batteries to meet the fast-growing need of power in personal electronics, electrical vehicles and smart grids. Among various battery technologies, zinc-based batteries with inherent merits of non-flammability, environmental benignity and low-cost warrant special attentions.1-3 Zn-air batteries (ZnABs) promise a theoretical specific energy of ~ 1086 Wh kg-1 which is about 5 times as high as that of the prevailing Li-ion batteries;1-3 whereas Zn-nickel batteries (ZnNiBs) deliver a working voltage of 1.7 V,4-7 ~ 40% higher than other types of Znbased batteries.8-14 Despite the notable advantages, commercial success of secondary ZnABs and ZnNiBs has been very limited. A major bottleneck herein is the inadequate cycle life which diminishes their commercial competitive advantages.1, 2, 5-7 The reported 200 cycles or 800 h of


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continuous operation achieved in ZnABs with 3-cathode configuration can hardly satisfy any advanced applications,9, 15 while ZnNiBs suffer faster capacity fading and rarely survive 100 cycles.6, 16, 17 It remains a grand challenge to equip Zn-based batteries with ultralong cycle life, a critical feature required for applications with heavy upfront investment, e.g. grid energy storage. High capacity and high operating voltage (e.g. >1.5 V), two desirable features for effective size reduction of power sources, do not co-present in any single type of Zn-based batteries. Recently, Chen and co-workers demonstrated the feasibility of integrating these two features in a hybrid Zn-air/nickel battery using nickel oxide/nickel hydroxide (NiO/Ni(OH)2) nanoflakes as the active electrode material.18 With fast Faradaic redox reactions of Ni2+/Ni3+ in ZnNiB and reversible oxygen reduction/evolution reactions in ZnAB, the hybrid battery outperformed any conventional Zn-based batteries in terms of power and energy densities. In addition, faster charging capability was proven without obvious capacity loss and voltage jump, and the battery cycled steadily over 70 cycles (up to 83 h). These results showed the promise of Zn-based hybrid batteries as future energy storage devices, warranting the need of further development with a focus on long-term cycling stability (e.g. >1000 cycles) to enable practical adoptions. In this contribution, we report a hybrid Zn-battery with an integrated cathode of a binary transition metal oxide of NiCo2O4 grown on pre-carbon-coated nickel foam (NiF@C) (denoted as NiCo2O4/NiF@C), which has exhibited remarkable cycling stability. In a non-stop cycling test at current density of 5 mA cm-2, the discharge/charge voltages remain almost unchanged over 5000 cycles (or a period of > 3 months) after an initial stabilization period. The extraordinary performance is benefitted from the rich active sites with well-defined redox electrochemistry of M-O/M-O-OH (M = Ni and Co) in NiCo2O419-22 and prominent catalytic activities of NiCo2O4 towards both oxygen reduction reactions (ORR)23-25 and oxygen evolution reactions (OER).24-28


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In addition, the multifunctional NiCo2O4 equips the resultant hybrid battery with an interesting 2step discharge process, i.e. a 1st plateau at 1.7 V from M-O-OH → M-O redox reaction and a 2nd one at 1.0−1.2 V from ORR, which concurrently harvest the merits of high operating voltage of ZnNiBs and high capacity of ZnABs. This work brings the hybrid Zn-air/Zn-NiCo2O4 battery one step forward towards practical applications, and may inspire new developments for advanced hybrid energy storage systems.

RESULTS AND DISCUSSION Representative scanning electron microscopy (SEM) images of the NiCo2O4/NiF@C in Figure 1a and b reveal the morphology of NiCo2O4 as uniform and dense nanowires (NWs) on NiF@C surfaces, with diameters of < 100 nm and lengths of ~3.5 µm (Figure 1b). The fine structure of NiCo2O4 NWs, as shown in a representative transmission electron microscopy (TEM) image (Figure 1c, the inset), is a porous assembly of interconnected nanoparticles. The high resolution TEM (HR-TEM) image (Figure 1c) displays 2 sets of lattice fringes with interplanar spacing of 0.24 and 0.17 nm, corresponding to (311) and (511) planes of spinel NiCo2O4, respectively. X-ray diffraction (XRD) pattern in Figure 1d further confirms the crystalline phase and structure of NiCo2O4. Ignoring the 3 strongest diffraction peaks of Ni foam, the pronounced peaks centered at 36.7°, 59.1° and 64.9° can be indexed to (311), (511) and (440) planes of spinel NiCo2O4 (JCPDS No. 20−0781),22 respectively. The X-ray photoelectron spectroscopy (XPS) data (Figure 1e and f, Figure S1 in Supporting Information) verified Ni, Co and O as the main elements, which is in good agreement with the XRD results (Figure 1d). The valence states of Ni and Co tally well with spinel NiCo2O4 in previous reports.28-31 All the characterization data have thus unambiguously confirmed the successful growth of highly crystalline spinel NiCo2O4 NWs on NiF@C substrate.


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Figure 1. Structural and compositional characterization of NiCo2O4/NiF@C. (a and b) SEM images of low and high magnifications, respectively. The inset in (b) shows the cross sectional view of NiCo2O4 NWs, scale bar: 1 µm. (c) TEM images. The lattice fringes of (311) and (511) are labeled. The inset shows individual NiCo2O4 NW consists of interconnected nanoparticles, scale bar: 50 nm. (d) XRD pattern. # corresponds to (002) phase of carbon. (e and f) XPS spectra of Ni 2P and Co 2P, respectively. The binding energy peaks centered at 853.7 and 855.2 eV for Ni 2p3/2 photoelectrons correspond to Ni2+ and Ni3+, respectively. Similarly, both Co2+ and Co3+ contribute to the Co 2p3/2 peaks.

It is worth stressing the importance of the thin layer of pre-coated-carbon on NiF substrate, which promotes the growth of dense and uniform NiCo2O4 NWs. This can be clearly seen on a control sample with NiCo2O4 NWs grown directly on bare NiF, in which the NWs detached from the substrate easily and formed noticeable defects and cracks (Figure S2). Other carbon-based conductive supports, e.g. carbon paper (CP) and carbon cloth (CC), are also found to promote the growth of dense and uniform NiCo2O4 NWs (Figure S3, the resulted electrodes are denoted as NiCo2O4/CP and NiCo2O4/CC, respectively). This thin layer of carbon coating on NiF, on one


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hand, serves as a chemically inert barrier which defines the solvothermal growth reaction of NiCo2O4 (in contrast to the Ni/NiO surface of bare Ni foam which disturbs the growth reactions unavoidably) and leads to the formation of uniform and dense NW arrays. On the other hand, it works as a conductive binder between NiCo2O4 NWs and NiF substrate to reduce the contact resistance, enabling fast and efficient transfer of electrons. In addition, the large area interfaces between the NWs and the carbon coating generate high population of active sites, promoting the ORR activities via synergetic effects.23, 32 The combination of these advantageous features with the intrinsic high catalytic activities of NiCo2O423-28 makes NiCo2O4/NiF@C a durable, robust and highly efficient electrode for Zn-based batteries. Prior to the assembly into full batteries, a polytetrafluoroethylene (PTFE) coating process was applied to furnish the NiCo2O4/NiF@C electrode a hydrophobic surface which facilitates the air diffusion and meanwhile prevents any possible leakage of electrolyte (See Experimental Section and Figure S4 for details). The PTFE coating also serves as an additional binder to firmly hold NiCo2O4 NWs on NiF@C support, and further improves the electrode stability and durability. PTFE-coated NiCo2O4/NiF@C cathodes were then taken to construct Zn-batteries in a typical 2-electrode configuration9, 11 (Figure 2a), which were evaluated by cyclic voltammetry (CV) and discharge-charge cycling tests. Despite the same battery configuration, the CV measurements revealed distinct behaviors for NiCo2O4/NiF@C cathode-based Zn battery as compared to conventional ZnABs. As shown in Figure 2b, the CV curve recorded in ambient air (red line) gives a pair of strong current peaks at 1.70/1.88 V associated with the redox reactions of M-O/M-O-OH (M = Ni and Co),19-22 in addition to the conventional cathodic-ORR and anodic-OER current at the potentials of < 1.3 V or > 1.90 V, respectively. Control experiments using bare NiF and NiF@C cathodes (Figure S5)


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Figure 2. NiCo2O4/NiF@C as integrated cathode for ZnAMBs. (a) Schematic illustration. (b) CV curves of a ZnAMB in ambient air and N2 atmosphere. (c) Voltage profile of the initial discharge-chargedischarge of the cycling test (at 5 mA cm-2, 15 min discharge followed by 15 min charge in each cycle), elaborating the transformation of the cell from a ZnAB to a ZnAMB. (d) The overall discharge-charge voltage profile of the ZnAMB cycled for >5000 cycles. (e) The detailed voltage profiles of selected cycles from (d).


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as well as other carbon-based cathodes, e.g. NiCo2O4/CP and NiCo2O4/CC (Figure S6), have confirmed the origin of these current peaks at 1.70/1.88 V as the redox reactions in NiCo2O4. Moreover, superior ORR and OER activities are observed for NiCo2O4/NiF@C as compared to its NiCo2O4-free counterpart, NiF@C. The asymmetrically shaped anodic peak and the high ratio of anodic to cathodic peak current (ipa/ipc = 1.36) indicate the presence of other source of anodic current, e.g. possible OER current from NiCo2O4 and NiF (Figure S5). In N2 atmosphere (Figure 2b, blue line), the cathodic current for ORR is hardly visible whilst the anodic current for OER and the redox couples at 1.70/1.88 V almost fully overlap with the one recorded in O2-containing environment, i.e. the ambient air (Figure 2b, red line). Using this NiCo2O4/NiF@C electrode to couple with a thin zinc plate anode, the resultant battery was able to reversibly store and deliver electrochemical energy through 2 sets of reactions, i.e. ORR and OER in ZnAB and redox reactions of M-O/M-O-OH in Zn-NiCo2O4 battery (similar to Ni2+/Ni3+ couple in rechargeable ZnNiBs).4, 5, 18 Such combination of complementary strength leads to a new powerful hybrid battery of Zn-air and Zn-NiCo2O4, denoted as ZnAMB, where ZnA and ZnM refer to the segment of Zn-air and Zn-NiCo2O4 therein, respectively. The performance of ZnAMB was investigated by pulse discharge-charge tests. The initial discharge-charge-discharge voltage profiles are presented in Figure 2c, where the smooth and flat plateau at 1.2 V in the 1st cycle of discharge resembles that of a typical ZnAB. In the 1st cycle of charge, however, one observes 2 voltage plateaus which are notably different from the typical single voltage profile for ZnABs. The plateau at 1.9 V is for M-O → M-O-OH cation oxidation, while the one at 2.15 V originates from the OER. The formation of M-O-OH in the 1st charge activates the M-O/M-O-OH pairs, leading to a 2-step discharge process from the 2nd cycle onwards. In addition to the 1.2 V plateau for ORR, the emergence of a discharge plateau at 1.7 V


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for M-O-OH → M-O cation reduction is in good agreement with the CV data shown in Figure 2b. The polarization curve of ZnAMB in the 1st scan towards lower potentials was similar to that of a typical ZnAB as a monotonously smooth curve (Figure S7a and b); while in scan towards higher potentials, a “kink” appeared in the potential window of 1.7−2.0 V (Figure S7a) due to M-O → M-O-OH cation oxidation. The reverse reaction in the subsequent scan towards lower potentials gives a peak power density of 26.2 mW cm-2 at 1.56 V (Figure S7c and d), showing distinct merit of ZnAMB as compared to conventional ZnABs (Figure S7e and f) whose voltage drops to 1.13 V for operation at same power density. To further elevate the high voltage power density of ZnAMB one may increase the loading of high quality NiCo2O4 active materials which, however, would require engineering optimization of the cathode. Hence, the ZnAMB is proven to be capable of reversibly storing and delivering electrochemical energies via reactions of M-O↔MO-OH and OER↔ORR. Impressively, ZnAMB showed remarkable cycling stability (Figure 2d and e) in terms of the discharge/charge voltage profile after a stabilization over the initial 100 cycles (or 50 h). In the followed over 3 months of continuous testing (> 5000 cycles), the voltage of 1.0/2.0 V remained essentially unchanged for the discharge/charge processes, respectively. The characteristic 2-step discharge profile was also retained throughout the cycling tests, as illustrated by that of selected cycles at different stages of the cycling test (Figure 2e). Concerning the changes during the initial stabilization over the first 100 cycles, both the charge and discharge voltage plateaus dropped for similar amplitude. It seems to suggest that an equilibrium was reached through the initial cycling, whereby the OER performance was enhanced at the cost of ORR activity to a certain extent. The SEM images taken on the air-cathode at both charged and discharged states after the initial stabilization cycling period showed that NiCo2O4 were changed from clean NWs to hierarchical


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NWs with nanosheet-like structures on the surfaces (Figure S8). Besides morphological changes, surface compositions are also found to vary to some extent between the charged and discharged states, as evidenced by the ex-situ XPS characterization data (Figure S9). Specifically, the Co 2P spectrum shifts toward higher oxidation state (Co3+) upon charging while falls back to lower oxidation state (Co2+) upon discharging, agreeing with the M-O/M-O-O-H redox reactions. The Ni 2p spectra, however, showed limited signals at either state which are difficult to be resolved meaningfully. The sharp contrast to the as-prepared NiCo2O4/NiF@C electrode (prior to battery tests) with strong signals seems to suggest a migration of cobalt element to the surface of NWs upon initial cycling, which shielded the XPS signals of element nickel. Another observation was enhanced O2 peaks in O1s spectra after the initial cycling which points to compositional change in NiCo2O4/NiF@C, and hence led to noticeable changes of charge/discharge voltage plateaus after the initial ~100 cycles (Figure 2d and e). Nevertheless, with a higher discharge voltage of 1.7 V in the 1st half of each discharge cycle followed by a 1.0 V plateau this extremely stable output over > 5000 cycles makes ZnAMB a very useful power source. Such ultrahigh cycling stability (long testing period, greater cycling numbers) has significantly outperformed the best ZnABs reported so far for both 2-electrode and 3-electrode configurations.15, 33 The facile NiCo2O4/NiF@C electrode with high durability in ZnAMB was further evidenced by a series of continuous tests on a single cell under various conditions (Figure S10-15). The characteristic voltage profile with 2 sets of well-defined charge/discharge plateaus was retained throughout these continuous serial tests at varied current densities, over prolonged or shortened charge/discharge period, or under complicated cycling tests with alternating current densities (Figure S10-15). The significant feature, especially the stable high operating voltage of 1.7 V in ZnM segment, makes ZnAMB appealing to high voltage and high power applications. It also


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allows for notable reduction in packing size of the resultant device. As an example, an operating voltage of about 3.4 V was readily achievable by 2 ZnAMBs connected in series (Figure 3a and b, see Experimental Section and Figure S16 for battery assembly details) which was sufficient to light up a white LED (Vf: 3-3.6 V) (Figure 3c). This makes a clear contrast to ZnABs (working voltage normally < 1.4 V),1, 9, 25 whereby 3 cells in series are required to power the same LED. To quantify the capacity of ZnAB segment, a full discharge test was conducted on a freshly assembled ZnAMB (Figure S17). At a current density of 5 mA cm-2 the battery was able to work continuously for ~ 580 h, giving a specific capacity of ~ 688 mAh gZn-1 which is on par with the best reported results so far.10,25 Taking into account the capacity contribution by ZnM segment, it is safe to conclude that ZnAMB has high capacity in addition to its unique high voltage.

Figure 3. Tandem device of 2 ZnAMBs connected in series. (a and b) Galvanostatic charge/discharge curves for a single cell (black) and 2 cells connected in series (red) operated at 2 mA with 0.5 h per cycle period. (c) A photo of a white LED powered up with a 2-cell tandem device. The ZnAMBs were assembled using coin cell CR2032 type with holes at the cathode side, see Experimental Section and Figure S16 for details.


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To exploit the potential of NiCo2O4/NiF@C as electrode for ZnMB, we extracted the ZnM segments (> 1.4 V) from the discharge profiles of ZnAMB in Figure 2d. The specific capacity of ZnM segment was normalized against the geometrical area of electrode and plotted in Figure 4a. Interestingly, the specific capacity is found to increase sharply by 44% (0.53 → 0.76 mAh cm-2) over the first 30 cycles (Figure 4a), followed by a much slower increase and eventually reached a peak value of 0.89 mAh cm-2 after 1000 cycles. Thereafter, a very slow and monotonous capacity fading of 0.006% (or 5.3 nAh cm-2) per cycle was observed for the subsequent cycles (Figure 4a, insert). Such highly stable cycling performance over ultra-long period has notably outperformed the high performance ZnNiBs reported previously.4, 5 In details, a NiAlCo-layered hydroxide cathode-based ZnNiB with 1M KOH electrolyte showed a capacity loss of around 10% over 600 cycles; in 6M KOH (concentration in present work) the capacity loss was as large as 40% over 500 cycles and 60% over 2000 cycles.5 Another long lifespan ZnNiB with NiO-carbon nanotubes (CNTs) cathode reported very recently also suffered a capacity decay of 35% over 500 cycles.4 Hence, in terms of cycling stability ZnAMB is superior to all the reported Zn-based batteries. In ZnAMB, the sharp increase of capacity over the initial cycles and the excellent capacity retention of the ZnM segment are likely due to the following changes and structural advantages. Firstly, the surface wetting of electrode was improved via electrochemical oxidation/reduction reactions in the initial cycles, which notably increases the electroactive sites accessible by the electrolyte.34, 35 Secondly, the presence of the 2nd charge plateau at higher voltage prolongs the charging time for the ZnMB segment, which allows more of the electroactive species of M-O in NiCo2O4 to be converted to their higher oxidation states as M-O-OH, and hence compensates the capacity loss to some extent in the cycling tests. Charging the battery at a lower cut-off voltage may lead to partial oxidation of those electroactive species and thus a lower capacity for the


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Figure 4. Battery performances of ZnM segment and ZnMB. (a) the ZnM segment in ZnAMB extracted from the cycling data in Figure 2d. (b) Representative charge-discharge curves of a ZnMB using NiCo2O4/NiF@C electrode tested at 0.25 mA cm-2 at different cut-off charge voltages from 1.9 to 1.82 V. (c) The corresponding capacity evolution when the battery in (b) was cycled between 1.5−1.9 V (black) and 1.5−1.82 V (blue), respectively. (d) Representative charge-discharge curves of a ZnMB tested between 1.5−1.9 V under varied current densities, and (e) the corresponding rate performance.

ZnMB segment. However, it helps minimize oxygen evolution reaction from charging the ZnAB segment due to the adjacency of OER onset potential and the anodic potential of M-O/M-O-OH (Figure 2b), which in turns has mitigated possible catalyst corrosion and enables high durability of the electrode.


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In Figure 4b the charge-discharge voltage profiles of cells are presented with cut-off charge voltages being 1.90, 1.85 and 1.82 V, whereby the specific charge capacity drops from 3.31 mAh cm-2 to 2.70 and 1.05 mAh cm-2, respectively. This corresponds a decrease of 18.4% and 68.3% which is a clear indication of lower conversion rate of M-O to M-O-OH. As the battery was discharged with a cut-off discharge voltage of 1.5 V, about 0.3 V above the discharge voltage plateau of the ZnAB segment (~1.2 V, cf. Figure 2c) to eliminate the contribution of ORR, the discharge specific capacity also sees a decrease from its initial 1.65 mAh cm-2 to 1.35 and 0.63 mAh cm-2, respectively. This echoes the reduced amount of M-O-OH formation and hence less is available for discharge reaction. Interestingly, the columbic efficiency (CE) was increased from 49.8% to 59.9% as the cut-off voltage was lowered from 1.90 to 1.82 V. The overall low CE values observed clearly indicates the OER participation in charging process, in good agreement with CV data (Figure 2b) which shows asymmetric shape of the redox couple for NiCo2O4 and little difference between M-O/M-O-OH cation oxidation peak and OER onset potential. Another drawback of a lower cut-off charge voltage is the faster capacity fading, about 14 and 24 µAh cm-2 per cycle for 1.90 and 1.82 V (Figure 4c), which is 2600 and 4500 times faster than that of the ZnAMB (5.3 nAh cm-2 per cycle) when sufficient charge was provided, respectively (Figure 4a). These data clearly show the importance of prolonging the charging period (e.g. by extending the charge process to the 2nd voltage plateau) to get the ZnMB segment sufficiently charged, such that both high capacity and excellent stability are harvested from the hybrid batteries. The interconnected porous structure and good mechanical stability of the NiCo2O4/NiF@C electrode also offer ZnMB or ZnAMB additional advantages, such as extremely good safety and high corrosion resistance. With interconnected pores and open cell configuration, they eliminated any possible pressure build-up by OER in case of “over-charging” or “over-voltage”,4, 6, 17 which


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is indeed a serious safety concern for the conventional sealed ZnNiBs. The use of NiF as current collector and supporting substrate of NiCo2O4/NiF@C electrode has improved the corrosion resistance to electrolyte and furnished higher conductivity for more efficient electron transfer as compared to conventional carbon paper-based electrodes. It is noteworthy that electrochemical corrosion to carbon paper based air-cathode in high concentration (e.g. 6M) KOH electrolyte was found to compromise the mechanical strength of electrode after not-long period (e.g. 350 h) of operation, resulting in electrolyte leakage and eventually termination of battery operation.36 The corrosion is often getting more prominent under cycling condition, as oxygen gas generated in OER during charging process may flush off the catalysts. This is clearly seen in a control experiment with carbon-based electrode, where the electrolyte changed from the initial colorless to dark brown after 150 cycles (Figure S18). In contrast, no color change was observed for the electrolyte of batteries using NiCo2O4/NiF@C electrode throughout the tests at even higher current density over substantially longer period (Figure S17 and 19), despite the existence of thin interface carbon layers with similar level of graphitized structure to some common carbonaceous materials (Figure S20). The least carbon corrosion issue in this case may be attributed to the dense and micrometer-long NiCo2O4 NWs on the top of carbon which restricted the access of both oxygen and KOH. The conductive thin carbon layer also facilitates the electron transfer between NiCo2O4 NWs and NiF current collector, enabling smooth battery operation with minimum perturbation to NiCo2O4/NiF@C electrode. With the merit of high corrosion resistance in high concentration alkaline electrolyte the NiCo2O4/NiF@C electrode furnishes ZnAMBs and ZnABs excellent stability in both pure discharging and charge-discharge cycling. The rate performance was evaluated on ZnMBs with NiCo2O4/NiF@C cathode in the same 2-electrode configuration as ZnAMB. The cut-off voltages were set as 1.5 and 1.9 V to minimize


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the ORR- and OER-resultant capacity contribution. Representative battery voltages at current densities of 0.25 to 2.0 mA cm-2 are plotted and shown in Figure 4e. Unsurprisingly, only one discharge plateaus at about 1.7 V was observed, and the initial specific capacity was about 1.71 mAh cm-2 at a current density of 0.25 mA cm-2. With the current density increased to 0.5, 1.0 and 2.0 mA cm-2, the capacity dropped to 1.55, 1.31 and 1.04 mAh cm-2 (capacity retention: 90.6%, 76.5% and 60.8%), respectively. As the current density was switched back to 0.25 mA cm-2 the recovery of 1.61 mAh cm-2 corresponds to capacity retention of 93.9%. It is worth noting that the capacity fading of 3 to 14 µAh cm-2 over a few cycles for battery operation at current densities of 0.25 to 2.0 mA cm-2 (Figure S21) are far much faster as compared to that of the ZnM segment in ZnAMB. Interestingly, the insufficient charging resulted capacity loss of the ZnM segment was found recoverable when the battery was provided again with sufficient charging (Figure S22). These observations unambiguously confirmed the importance of prolonged charging to ZnMB for high capacity and excellent stability.

CONCLUSIONS To summarize, using an integrated electrode of densely packed NiCo2O4 NW arrays grown on 3D conductive NiF@C, a high capacity high voltage hybrid Zn-based battery with ultralong cycle life was developed. The hybrid battery exhibits interesting 2-set charge/discharge voltage plateaus, with a high voltage of 1.7 V at the 1st discharge plateau due to M-O-OH → M-O cation reduction (similar to ZnNiB), and a higher capacity at the 2nd discharge plateau (@ ~1.0 V) from ORR in the corresponding ZnAB. The merits of rich redox sites, good ORR and OER activities, firm attachment of NiCo2O4 on the highly conductive NiF of open and interconnected porous structure, have enabled the excellent discharge-charge performance with superior stability of the hybrid battery in concentrated alkaline electrolyte. The battery was able to steadily cycle for over


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5000 cycles (> 3 months) at a current density of 5 mA cm-2, topping the cycle life of all the reported of ZnABs or ZnNiBs. Such excellent performance coupled with the known merits of environmental friendliness, cost-effectiveness and much improved safety promise a bright future for the hybrid battery. From technology adoption point of view, there are more gaps to be addressed for the present ZnAMBs. 3 months are by no means comparable to decades required for continuous operation of grid storage, whereby a number of issues become prominent, including the inherent zinc dendrite growth and surface polarization, chronic water loss and carbonation of alkaline electrolyte as the consequence of an open system, unsatisfied energy efficiency from the notable charge-discharge voltage gap, etc. It is hence hoped that this work can inspire more research on the development of efficient hybrid cathode with further improved performance from a good balance of ZnA and ZnM segments via sophisticated electrode engineering and optimization, to make ZnAMBs powerful unit cells for hybrid electrochemical energy storage system in practical applications.


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EXPERIMENTAL SECTION Chemicals and materials. Nickel nitrate (Ni(NO3)2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), zinc acetate (Zn(OAc)2·6H2O), potassium hydroxide (KOH), and urea were all Sigma-Aldrich products and used as received without further purification. All aqueous solutions were prepared from the respective chemical and de-ionized water (DI-water, resistance: >18 MΩ·cm). NiCo2O4/NiF@C electrode preparation. Nanowires (NWs) of binary metal oxide, NiCo2O4, were grown on carbon-coated NiF (NiF@C) or bare NiF via a modified solvothermal method.20 Typically, nitrate precursors of Ni and Co are dissolved with urea in a molar ratio of 1:2:12 into a mixture solvent of DI-water and ethanol (1:1, V/V) at room temperature to form a clear pink solution. 35 mL of such solution, containing 1 mmol Ni(NO3)2·6H2O, was transferred into a 50 mL Teflon-lined autoclave, followed by immersion of NiF@C or NiF substrates vertically in the reactant solution. After reaction at 100 °C for 12 h, the substrates grown with NiCo2O4 NWs were taken out and washed with DI-water under pulse-sonication, and then rinsed with ethanol to remove physisorbed and loosely attached NiCo2O4. Thereafter, the NiF@C or NiF coated with NiCo2O4 NWs were dried and annealed at 350 °C in air for 2h at a heating rate of 5 °C min-1 to obtain crystallized NiCo2O4 NWs. The loading of NiCo2O4 is estimated to be about 3.4 ± 0.3 mg cm-2 (Figure S23). Annealing at 350 °C was found to render a better ORR performance in the resultant NiCo2O4/NiF@C electrode (Figure S24). The NiF@C was obtained via a simple 2-step synthesis: 1) dip-coating a thin film of polyacrylonitrile (Mw: 150,000) on NiF from its diluted solution (5 wt.%); 2) Converting the thin polymer coating into carbon at 900 °C in a tube furnace under N2 atmosphere. The heating rate was 5 °C min-1 with a carbonization time of 1 h at 900 °C. To improve the adhesion of NiCo2O4 NWs on NiF@C and furnish a hydrophobic surface that facilitates air diffusion and prevents electrolyte from leakage, NiCo2O4/NiF@C electrodes were


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coated with polytetrafluoroethylene (PTFE) by a 5-min soaking in its aqueous solution, diluted from 60 wt.% PTFE dispersion (Sigma-Aldrich), drying at 60 °C in oven and then annealed at 300 °C in air for 1 h. The resultant electrodes possess a highly hydrophobic surface with wellretained ORR activity as compared to the uncoated control sample (Figure S25). Characterization. The morphology of NiCo2O4 NWs was characterized using a JMF 6700F scanning electron microscope (SEM) and a Philips CM300 transmission electron microscope (TEM). X–ray diffraction (XRD) patterns were recorded on a Bruker D8 Discover GADDS with a Cu Kα radiation. X–ray photoelectron spectroscopy (XPS) data was collected using a Theta Probe electron spectrometer (VG ESCALAB200i–XL, Thermo Scientific). The binding energies were calibrated using C 1s peak at 285.0 eV. Battery assembly and tests. ZnAMBs were assembled using a custom-made 2-electrode ZnAir cell (Figure S17) with NiCo2O4/NiF@C as the integrated cathode, a polished zinc plate as the anode and an alkaline aqueous electrolyte of 6M KOH containing 0.2M Zn(OAc)2·6H2O. For comparison, another ZnAB was assembled in the same configuration (Figure S18) except the air cathode was replaced by Pt/C deposited on carbon paper. The air cathode was prepared using carbon paper as the current collector and gas diffusion layer, as described in a protocol reported elsewhere.10, 11 Typically, an ink of Pt/C dispersed in ethanol/Nafion solution was carefully dropcast onto carbon paper and dried naturally to form a uniform catalyst layer, giving a mass loading of ~ 0.5 mg cm-2. The battery performances were evaluated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements on an Autolab potentiostat/galvanostat (PGSTAT302N) station, and continuous discharge-charge tests on Maccor or NEWARE battery testers in ambient air conditions (oxygen from air!). The current density for battery test was normalized to the geometric surface area of air-cathode. To facilitate assessing the performances


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of the battery in series, coin cell (CR2032) type hybrid batteries with NiCo2O4/NiF@C aircathode were also built. The assembly details are given in supplementary materials (Figure S16) with their results shown in Figure 3.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Further experimental details, characterization data and battery testing results. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected](Y. Zong) *Email: [email protected] (H. Zhang); [email protected] (H. Zhang). Website: http://www.ntu.edu.sg/home/hzhang/ *Email: [email protected] (Z.L. Liu) Present Addresses #Current address: Renewable Energy Group, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK. Notes The authors declare no competing of finical interest.


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ACKNOWLEDGMENT This research was conducted under the project IMRE/12-2P0504 with a grant from Advanced Energy Storage Research Programme (Award No.: 1229904044) supported by Science and Engineering Research Council (SERC) of A*STAR (Agency for Science, Technology and Research), Singapore. H.Z. thanks the support from MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034; ARC 19/15, No. MOE2014-T2-2-093) and AcRF Tier 1 (RGT18/13, RG5/13), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. This Research is also conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. ABBREVIATIONS ZnAB, zinc-air battery; ZnNiB, zinc-nickel battery; ZnAMB, zinc-air and zinc-NiCo2O4 battery; ORR, oxygen reduction reaction; OER, oxygen evolution reaction.

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A Robust Hybrid Zn-Battery with Ultralong Cycle Life - Nano Letters

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