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Designing Sandwiched and Crystallized NiMn2O4/C Arrays for the Enhanced Sustainable Electrochemical Energy Storage Ya Ouyang, Yangyang Feng, Huijuan Zhang, Li Liu, and Yu Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01249 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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ACS Sustainable Chemistry & Engineering

Designing Sandwiched and Crystallized NiMn2O4/C Arrays for the Enhanced Sustainable Electrochemical Energy Storage Ya Ouyang1; Yangyang Feng1; Huijuan Zhang; Li Liu; Yu Wang* The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Shapingba District, Chongqing City, P.R. China, 400044 E-mail: [email protected]; [email protected] 1

These authors equally contributed to this work.

Supporting Information: Most XRD, SEM, TEM, TGA, EDS, Raman and electrochemical data are available in the supporting information for this report. Abstract In this report, novel three-dimensional (3D) sandwiched NiMn2O4/C arrays on Ni foam are firstly synthesized through a general and simple synthetic approach. In this process, the glucose and ultrathin NiMn Layered Double Hydroxide (NiMn-LDH) arrays are used as green carbon source and sacrificial templates, respectively. This advanced nanoarchitecture obtained here can not only improve the electronic conductivity due to carbon coating and conductive substrates, but also prevent NiMn2O4 nanoparticles from agglomeration and falling off. The as-prepared sandwiched NiMn2O4/C arrays are more desirable to apply in energy storage. When evaluated as supercapacitors (SCs), it exhibits ultrahigh specific capacitance (2679 F/g at 1 A/g) and superior stability (2% decay after 6000 cycles). On the other hand, due to the unique structure design, it demonstrates excellent capacity (1346 mAh/g at 500 mA/g), remarkable rate performance and cyclability for lithium-ion batteries (LIBs). Key words: Sandwich-like, Carbon, Nickel manganese oxide, Supercapacitors, Lithium-ion batteries Introduction: In the last decades, large-scale energy-storage applications, such as sustainable energy sources, unremitting power supplies and plug-in hybrid electric vehicles (PHEV), have attracted extensive attention, especially with long cycle life, low cost and high energy density.1-3 To the knowledge, the big challenge exists among energy storage technologies is that it cannot meet the demand of continuous supply of energy, which strictly requires high power rate, high power density, low cost and safety.4-6 Therefore, it is urgent to develop high performance energy storage and conversion systems particularly with environmentally friendly and lightweight so as to meet the increasing demand.7-9 Among the promising energy-storage systems, SCs and LIBs have regarded as two main devices of electric energy storage because they possess high energy density, long lifespan, environmental friendliness and safety.10-11 Supercapacitors occupy an important position in the future electrical energy storage field owing to the longer cycle life, superior power density and outstanding mechanical properties.12-14 However, the intrinsic limitation of the electrostatic surface and easy aggregation may tend to bring in low capacitance and poor cyclability. For LIBs, although they are the effective bridge medium to recyclable store clean energies, still suffer from several serious difficulties, mainly including electrode materials’ agglomeration and aggregation during electrochemical reactions, which would definitely result in sharp decrease of capacity 1

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especially for long time running.15-16 To circumvent these shortcomings and to meet the intensive demands of practical use, many active materials have been studied to produce superior electrodes. Nanostructured metal oxides (MOs), particularly transition-metal oxides such as manganese oxide (MnO2), Cobalt oxide (Co3O4) and nickel oxide (NiO) are considered as potential electrode materials due to large reversible capacity, high power capability, safety, and long lifespan.17-25 Aside from simple binary metal oxides, in recent years, ternary transition metal oxides, such as NiCo2O4, MnCo2O4, CoFe2O4 and NiMn2O4, have raised increasing concern for electrical energy storage mainly because of their unique synergistic effect originated from the coexistence of two distinct cations in a single crystal structure, which can enhance the cyclic stabilities as well as the rate performances.26-31 Among the explored systems, NiMn2O4, as an important spinel metal oxide, is widely regarded as promising electrode for both LIBs and SCs for its higher conductivity and superior electrochemical activities when compared with binary metal oxides. 32-34 Besides, it can offer more redox active sites during electrochemical reactions due to the lattice positions of both Mn and Ni.35 Furthermore, cobalt based oxides are relative expensive and toxic in some degree.10, 26 Hence, Ni and Mn based oxides are much more desirable for applications. Nevertheless, the biggest problems of the poor electroconductivity and volume expansion during charge-discharge process still hinder the application of NiMn2O4 as energy-storage devices.36-37 Thus, it is demanding to controllably design and synthesize sophisticated nanostructural materials, particularly with designed unique structural details. In recent years, a lot of researches have been devoted to enhance its intrinsic properties, containing size reduction38-39, conductive substrate coating40 and surface cationic doping41. Unfortunately, all the strategies mentioned above are somewhat unsatisfactory when concerning the serious pulverization and aggregation during electrochemical reactions. Thus, direct growth of three-dimensional (3D) nanostructures on conducting substrates seems a feasible solution. Herein, novel hierarchical sandwich-like NiMn2O4 arrays are firstly introduced. In this method, a universal and controllable hydrothermal method is utilized to fabricate NiMn layered double hydroxide (NiMn-LDH) on Ni foam. The ultrathin NiMn-LDH is used as precursor and sacrificial templates. Importantly, the environmental-friendly glucose is used to synthesize carbon layers. Then, the glucose polymer coats onto the surface of NiMn-LDH by hydrogen-bonding. Finally, sandwiched NiMn2O4 arrays are obtained under high temperature calcinations followed by air oxidation. To the knowledge, sandwiched morphology with active nanoparticles dotted inside and the carbon layered outside possesses numerous outstanding properties. The carbon network around active materials can not only act as a barrier to suppress agglomeration and pulverization and prevent nanoparticles from peeling off thereby to maintain the structural integrity, but also increase the electrochemical conductivity of the active materials thus to reduce electrical resistance.42-43 As for the morphology, its unique 3D nanoarchitecture can shorten pathways for Li+ and electrons diffusion as well as accelerate fast ion-exchange. Additionally, the inter-spaces between each nanoparticle can efficiently isolate nanoparticles and remit the possible volume changes during the electrochemical process so as to enhance the structural stability. Furthermore, the nanoporous structure resulted from a large amount of substance loss under calcinations can promote fast diffusion of both electrolyte and ions as well as facilitate ion absorption, which results in enhanced cyclic stability and higher capacity for both SCs and LIBs. Apart from these mentioned advantages, the sandwich-like NiMn2O4 arrays on Ni foam possess higher conductivity 2


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due to introducing conductive substrates, further to enhance the electrochemical performances in energy storage. Experimental Section Materials and chemicals were directly used without purification: nickel nitrate (Ni(NO3)2, Aldrich, 99.9%), manganese nitrate (Mn(NO3)2, Aldrich, 99.9%), ammonium fluoride (NH4F, Aldrich, 99.9%), urea (Aldrich, 99.9%), nickel foam (Alfa Aesar) and glucose (Cica-Reagent, Kanto Chemical). Preparation of MnNi layered double hydroxide nanosheet arrays on Ni foam Ni foam (2 cm * 3 cm) was cleaned with ~37 wt% HCl for 20 min to get rid of the NiO layer. In this procedure, Ni(NO3)2 1 mmol and Mn(NO3)2 2 mmol were added in 30 mL deionized water to form uniform solution. 0.08 g NH4F and 0.30 g urea were than added. Then, the mixture was diverted in the autoclave of 45 mL. The Ni foam was put into the autoclave. The autoclave was left in the oven at 150 oC for 1 h. After cooling in air, the Ni foam was got out and washed seral times. Finally the Ni foam was dried in the vacuum oven overnight. Preparation of sandwiched NiMn2O4/C arrays on Ni foam Ni foam with MnNi layered double hydroxide nanosheet array was put into an autoclave. After that, 5 mL 1 M glucose aqueous solution and D.I. water (25 ml) were gradually added together to form uniform solution with ~10 min ultrasonication. Then the Teflon-lined autoclave was transferred into an oven at 180 oC for 4 h. After cooled in air, Ni foam was washed by D. I. water and ethanol. Then Ni foam was dried at 60 oC to get rid of the residual ethanol and water. Subsequently, the dried samples were calcined at 700 oC for 200 min under H2/Ar (H2 vol% =5%) and calcined at 250 oC for 200 min in air to synthesize NiMn2O4 nanoparticles. Material characterization Field-emission scanning electron microscope (SEM, 5kV, JSM-7800F) equipped with an energy dispersive spectrometer (EDS) analyzer; X-ray diffractometer (XRD, Bruker D8 Advance, Cu Kα); Transmission electron microscope (TEM, Philips, Tecnai, F30); Raman Microscope (RENISHAW Invia, UK, voltage:100-240V, Power: 150W), BET surface-area and pore-size analyzer (Quantachrome Autosorb-6B). Electrochemical testing For lithium-ion battery test, Ni foam with samples of 0.8–1.0 mg/cm2 covered was cut into 1 cm pieces. Then the pieces of Ni foam were utilized as electrode, Pure Li foil was utilized as reference and counter electrode and 1 M LiPF6 with ethylene carbonate and diethyl carbonate (EC-DEC, v/v =1:1) was utilized as electrolyte. The half-cells were assembled in a glove box with Ar-filled. The galvanostatic measurements were tested on a battery testing system (Neware). Supercapacitors test was conducted in three-electrode setup in aqueous solution (1 M KOH) (CHI660E electrochemical workstation), including Pt foil as counter electrode, NiMn2O4 arrays on Ni foam as working electrode, saturated calomel electrode (SCE) as reference electrode. Cyclic voltammetry (CV) and galvanostatic measurements were tested ranging from 0 to 0.6 V. Asymmetric capacitors (ASC) were performed by two-electrode setup with 1 M KOH as working electrolyte. ASC was assembled using active carbon (AC) and NiMn2O4 arrays as negative and positive electrons, respectively. The specific capacitance of sandwiched NiMn2O4 arrays was calculated by the following equation form the charge/discharge curves: 3



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(1) Where Cs (F·g-1) stands for the specific capacitance; I (A) is the discharge current, ∆V(V) is the potential change, ∆ t (s) refers to discharge time; m (g) represents the whole mass of the sandwiched NiMn2O4 arrays. Result and Discussion Scheme 1 illustrates the overall synthetic process. In this strategy, ultrathin NiMn-LDH arrays on Ni foam are fabricated through the simple and general hydrothermal method. The sheet-like NiMn-LDH serves as both precursor and sacrificial template. By hydrogen-bonding action, the polymer was firmly confined on the surface of NiMn-LDH nanosheets. Finally, sandwiched NiMn2O4 nanoparticles dotted in coupled carbon layers were prepared via high-temperature calcinations in Ar atmosphere followed by a second annealing in air. The sandwich-like NiMn2O4/C is regarded as electrochemical energy storage materials. General characterizations of as-synthesized NiMn-LDH arrays are described in Fig. 1. Fig. 1a displays the low-magnification scanning electron microscopy (SEM) image of a large-scale synthesis of NiMn-LDH arrays on Ni foam ligament surface. After hydrothermal reaction, a green film was coated on the surface of nickel foam (inset of Fig. 1a), suggesting the successful growth of NiMn-LDH. Fig. 1b is the magnified SEM image of the section view, from which the average size of samples is ~2 micrometers. To further analyze the as-prepared NiMn-LDH arrays, SEM images at different magnifications were also presented (Fig. 1c and Fig. 1d). It is observed that uniform NiMn-LDH nanosheets with the thickness of 20 nm are large-scale production, which is of great importance for the possible commercial applications. The corresponding X-ray diffraction (XRD) data is demonstrated in Fig. S1 (Supporting Information), which further confirms that the formed ultrathin nanosheets are pure rhombohedral NiMn-LDH.44 Through a facile in situ carbon coating followed by two-step annealing, scalability of sandwich-like NiMn2O4/C arrays is feasibly accessible. Fig. 2a reveals the large-scale synthesis of sandwiched NiMn2O4/C arrays on Ni foam, which is well inherited by the uniformly synthesis of the precursor. Fig. 2b shows the magnified SEM image, where the NiMn2O4 nanoparticles are uniformly and evenly encapsulated in coupled flexible carbon layers. By amplifying Fig. 2b (Fig. 2c), the sandwich structure is apparently observed, where many nanoparticles are tightly restricted and confined in carbon layers. Noteworthily, high-rate scanning electron beam by 5 kV acceleration voltage can exactly pierce the outside carbon sheets, which reveals the ultrathin carbon layers. To better confirm the thickness of the outside carbon, the magnified SEM image is provided (Fig. S2, Supporting Information), from which the thickness of carbon is estimated as ~10 nm, well inherited by the ultrathin precursor. To the best of the knowledge, the ultrathin carbon layer can be considered as a superior conductive substance to enhance the electric conductivity in energy storage. Besides, it is also regarded as a buffer medium for possible volume change, which leads to enhanced specific capacity and cyclability of active materials for both SCs and LIBs.45-46 The content of carbon is a vital parameter for carbon-related composites. As calculated, the carbon content of the composite is ~10 % through TGA measurement (Fig. S3, Supporting Information). A small quantity of carbon scarcely affects the specific capacity of composite. On the other hand, owing to the high conductivity of carbon and its synergic effect in 4


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electrochemical process, it can reasonably improve the electrochemical properties in energy storage.38 Under high-temperature calcinations, porous structure would be definitely obtained. To further estimate the porous structure of the sandwiched NiMn2O4/C composites, Brunauer-Emmett-Teller surface area measurement (BET) is also provided. Revealed in Fig. 2d, the specific surface area of the sandwiched composites is 271 m2/g, large enough to offer numerous active sites in electrochemical process. More importantly, it can offer large contact areas between electrode materials and electrolyte, resulting in fast ion transfer rate, which is beneficial to enhancing electrochemical properties. The pore size is mainly concentrated on 1.8 nm, as indicated in the inset of Fig. 2d, implying that porous structure has been successfully formed through a general strategy. The pores can effectively improve electron and ion exchange during charge and discharge process.47-48 XRD is effective measurement to probe the crystalline structure in the sandwiched samples as revealed in Fig. S4a. The weak peaks range from 10-90 degrees match with NiMn2O4 (JCPDS No. 84-0542). X-ray energy dispersive spectrum (EDS) (Fig. S4b) affirms that Mn&Ni exist in the final samples (obtained by cutting off from Ni foam), where the molar ratio of Mn and Ni is ~ 2:1, further confirms that the nanoparticles are NiMn2O4. Moreover, to evaluate the distribution of each element, selected area’s elemental imaging of EDX is carried out. Shown in Fig. 3, Mn and Ni uniformly exist in nanoparticles, which is in correspondence with XRD and EDS data. Importantly, the element C is uniformly dispersed in the sandwiched samples, further confirming that carbon layers are evenly wrapping around NiMn2O4 nanoparticles. To gain more insight into the detailed nanostructure, transmission electron microscopy (TEM) is introduced. As revealed in fig. 4a, sheet-like samples with thoroughly filled with tremendous quantity of tiny particles can be observed. All the nanoparticles are well restricted in outside carbon sheets, which look like sandwich. This nanoarchitecture can efficiently protect the nanoparticles form aggregation and pulverization and ensure complete touch between electrode and electrolyte, thus to improve cyclic stability during long-term cycles. In the magnified TEM image of Fig. 4b, it reveals that uniform and even NiMn2O4 nanoparticles with the size of ~5 nm are tightly confined and the average distance between interval nanoparticles is 2-10 nm, which can efficiently remit the volume expansion during electrochemical reactions. Fig. 4c shows the boundary of sandwiched NiMn2O4/C，from which several carbon layers can be seen. These carbon layers can firmly anchor nanoparticles, thus to keep from peeling off during charge/discharge process when compared to the common carbon-conjugated materials.49-50 Raman Spectroscopy (RS) is also carried out to confirm the graphitization. As revealed in Fig. S5, the ratio of G band (referring to planar vibration of graphite layer) and D band (indicating disordered carbon) is ~1, suggesting that carbon is fabricated in the final samples. The existence of carbon is beneficial for enhancing electrochemical performances. The crystal structure of NiMn2O4 (Fig. 4d) demonstrates the lattice spacing of 0.211 nm, corresponding to (400) plane of NiMn2O4. NiMn2O4 is a multi-functional material as electrode materials in energy-storage systems. To test the electrochemical performances of the sandwiched NiMn2O4/C arrays, many electrochemical tests were carried out. Fig. 5a shows the CV profiles of the sandwiched NiMn2O4/C arrays in voltage range of 0-0.6 V at a series of scan rates from 5 mV/s to 100 mV/s. Apparently, a pair of distinct redox peaks can be easily detected, where MOOH (where M represents Ni or Mn) is formed at the surface of NiMn2O4 when operated in basic solutions. The redox peaks should be derived from the Faradaic redox reactions in connection with M-O/M-O-O-H.51-52 In Fig. 5a, it also observes that the current density enhances with the scan rate increasing. Fig. 5b shows the 5



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galvanostatic charge and discharge curves within the potential of 0-0.5 V. According to discharge curves, it is observed that the specific capacitance of the composites is as high as 2679 F/g when the current is 1 A/g. Even though at 30 A/g, the specific capacitance can obtain 2258 F/g, with 84.3% capacitance retained, superior to most of reported electrodes.35, 44, 53-55 The corresponding rate performances are demonstrated in Fig. 5c, the specific capacitance ranges from 2679 to 2617, 2532, 2446, 2318, 2258 F/g with the current density raising from 1 to 2, 5, 10, 20, 30 A/g, respectively. To gain more insight into the cyclic properties of sandwiched NiMn2O4/C arrays in basic solution, cyclic life tests were also studied by using galvanostatic charge/discharge technique at 10 A/g. As observed in Fig. 5d, there is negligible attenuation (2.1 %) in the specific capacitance after operating 6000 cycles, indicating an outstanding cyclic stability for the novel sandwich-like NiMn2O4/C arrays. In order to confirm the architectural superiorities, the electrochemical behavior of NiMn2O4 nanosheets has also been measured for comparison. The sheet-like morphology is presented in Fig. S6a (Supporting Information). The CV profiles at 20 mV/s (Fig. S6b) show that the final sandwiched NiMn2O4/C arrays demonstrate enhanced current, implying the improved performances of the novel sandwiched structure. According to the galvanostatic charge/discharge plot in Fig. S6c, NiMn2O4 nanosheets show a specific capacitance of 852 F/g, which is in correspondence with the previous reports of NiMn2O4 nanosheets.35 Cyclic stability of NiMn2O4 nanosheets is also tested for comparison, as revealed in Fig. S6d, the capacitance of NiMn2O4 nanosheets show a much lower initial value and then remarkably decays, particularly in the final 1000 cycles. These observations verify that the sandwiched structure can absolutely enhance the performances of SCs, especially the rate capability and cyclic stability. To evaluate the electrochemical behavior, impedance measurements were conducted. Nyquist plots of sandwiched NiMn2O4/C arrays and NiMn2O4 nanosheets are presented in Fig. S7 (Supporting Information). EIS was tested at the 5 mV AC perturbation amplitude from 100 kHz to 0.01 Hz. In Fig. S7a, charge transfer resistance (Rct) of sandwiched NiMn2O4/C arrays is calculated as 2.13 Ω, while that of the NiMn2O4 nanosheets is 25 Ω. Fig. S7b shows the Nyquist plots of sandwiched NiMn2O4/C arrays before and after 6000 cycles. Significantly, after running for 6000 cycles, the Rct slightly increases from 2.13 to 3.97 Ω, suggesting that the sandwiched structure can markedly enhance the electro conductivity and cyclability. To further evaluate its superior properties, an asymmetric supercapacitor (ASC) was also carried out by using sandwiched NiMn2O4/C arrays and carbon as positive and negative electrode, respectively. Fig. 6a presents the CV profiles of ASC tested from 10 to 200 mV/s in the potential range of 0-1.7 V. Different from distinct redox peaks shown in the symmetric supercapacitor, these CV plots present nearly rectangular shape, suggesting the desirable rapid charge and discharge property for electric devices. To the knowledge, rate performance is a crucial factor for supercapacitors, especially in energy-storage applications. Therefore, the galvanostatic charge and discharge curves were performed at various current density from 0 to 1.7 V. Shown in Fig. 6b, the nearly symmetrical shape and highly linear correlation significantly imply the satisfied capacitive behavior of the NiMn2O4/C//AC-ASC devices. The corresponding rate performance is calculated from Fig. 6b (Fig. 6c). As revealed, the ASC device exhibits a maximum specific capacitance of 207 F/g, 196 F/g, 182 F/g, 167 F/g, 146 F/g, 132 F/g at 1 A/g, 2 A/g, 5 A/g, 10 A/g, 20 A/g, 30 A/g, separatively. The capacitance decreases 36.4% when the currents increase from 1 to 30 A/g, which further indicates that the fabricated NiMn2O4/C//AC-ASC is more desirable for SCs. To further 6


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confirm the structural advantages, the sandwiched NiMn2O4/C is also compared to the other NiMn2O4-based supercapacitors reported in the previous literatures 35, 56-58 (as shown in Table S1). As observed, the sandwiched NiMn2O4/C exhibits superior performances among the other NiMn2O4-based materials. The cyclic stability is also tested by galvanostatic charge and discharge between 0 - 1.7 V at 10 A/g. It is observed in Fig. 6d that the ASC device presents an outstanding stability of 96.1 % remained even after 3000 cycles，superior to all the NiMn2O4-based devices,56, even outperforming NiMn-based supercapacitors. This is exactly the advantages of the sandwiched structure. Due to its large surface area to offer more active sites, its ultrathin coupled carbon layers to keep active materials from peeling off and to maintain the structural integrality, as well as the porous structure to accelerate electron and ion diffusion, the sandwiched NiMn2O4/C arrays are considered as one of the most promising electrodes for SCs. Apart from SCs, NiMn2O4 is also regarded as one of the promising anode for LIBs. To test the Li-storage properties of sandwiched NiMn2O4/C arrays, a sequence of tests are conducted. CV plots of sandwiched NiMn2O4/C at the scan rate of 0.3 mV/s over the voltage range of 0.01-3.0 V are presented in Fig. S8 (Supporting Information). It is observed that two apparent oxidation peaks at ~1.26 and 1.98 V refer to the oxidation of Mn and Ni, respectively. In the reduction process, the peak at ~1.02 and 0.51 V are ascribed to the reduction of Mn3+ to Mn2+ and Mn2+ or Ni2+ to metallic Mn and Ni. Based on the CV plots, the electrochemical reactions of NiMn2O4 are presented as follows: NiMn2O4 + 8Li+ + 8eNi + 2Mn + 4Li2O (2) Ni + Li2O NiO + 2Li+ + 2e(3) Mn + Li2O MnO + 2Li+ + 2e(4) + 6MnO + Li2O Mn3O4 + 2 Li + 2e (5) Fig. S9a (Supporting Information) is the galvanostatic charge and discharge profiles performed at 500 mA/g from 0.01 to 3 V vs. Li/Li+. Noteworthily, the initial discharge capacity can obtain 1981 mAh/g, almost twice the theoretical capacity of NiMn2O4 (~ 922 mAh/g). On the one aspect, in the reactions, reversible decomposition of electrolyte and formation of solid-electrolyte-interphase (SEI) can unquestionably improve the capacity. On the other aspect, due to the simultaneously formed SEI and porous structure, the electrode can absorb extra Li ions so as to higher the capacity.59-60 Then, the discharge capacities decrease to 1346 mAh/g at the. From second cycle, the specific capacity is hardly decayed with ~98% retention after 200 cycles, much better than most of reported NiMn2O4-based anodes for LIBs. Rate performance is presented in Fig. S9b, as the current density gradually increasing from 500 mA/g to 1, 5, 10 and 20 A/g, the values of capacity change from 1346 mAh/g to 1239, 1140, 1017, 950 mAh/g, respectively. Finally the specific capacity backs to ~1336 mAh/g as the current density decreases to 500 mA/g, implying a superior rate property for the sandwiched NiMn2O4/C arrays. It is noted that the capacity of ~ 950 mAh/g can be achieved at such high current density, further affirming the structural advantages. The coulombic efficiency of sandwich-like NiMn2O4/C arrays is displayed in Fig. S9c. By calculating, the coulombic efficiency is almost 100% after 200 cycles at the current density of 500 mA/g apart from the first cycle, implying a potential LIBs anode. To investigate the rate performances and the cyclability, a series of experiments of stabilities at the various current densities are provided. Demonstrated in Fig. S9d, the final samples demonstrate an excellent stability even at high rates. The specific capacity slightly decreases from ~1283 mAh/g to 1164, 1074, 971 mAh/g as the current density increase from 1 A/g to 5 A/g, 10 A/g, 20 A/g, separatively, 7



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with a high retention of over 90 % during 100 cycles. This outstanding stability is derived from the unique sandwich-like structure. To extrude the sandwich-like structure, a comparative experiment of the durability has conducted. As Fig. S10 (Supporting Information) shows, NiMn2O4 nanosheets display a drastically decrease from 885 to 502 mAh/g, further affirms that sandwiched NiMn2O4/C array with coupled carbon layers outsides can effectively keep active materials from agglomeration and buffer possible volume change, so as to improve the cyclic stabilities. In order further affirm the superiorities, the sandwiched NiMn2O4/C arrays are also compared to other reported NiMn2O4-based materials. As shown in Table S2, the sandwiched NiMn2O4/C array exhibits better performances than that of others in previous literatures,60-62 which directly verifies the structural advantages. To confirm our structural stability, the status of the NiMn2O4/C arrays were tested again after running for 200 cycles (Fig. S11, Supporting Information). As expected, the sandwiched structure is fully remained. It is no doubt that the obtained samples possess superior stability. To evaluate the contribution of carbon in the composite, a sequence of electrochemical performances of carbon are introduced. The carbon is prepared by the same method as fabricating sandwiched NiMn2O4/C except of addition of NiMn2O4 and then removing the Ni foam. As observed in Fig. S12 (Supporting Information) the carbon exhibits the specific capacitance of 206, 198 and 185 F/g when tested at 1, 2 and 5 A/g, separatively, which is negligibly low in the whole sandwiched NiMn2O4/C arrays. Furthermore, Li storage performances of carbon are also conducted. As shown in Fig. S13 (Supporting Information), the carbon demonstrates a relative low capacity of ~ 500 and 210 mAh/g at the first and second cycle, separatively. It is concluded that the contribution of capacity/capacitance from carbon is pretty small and the whole capacity/capacitance mainly derives from NiMn2O4. On the basis of the above results, it is reasonable that the devised sandwiched NiMn2O4/C arrays are satisfied for high power energy storage, especially in SCs and LIBs. Conclusion A novel and unique sandwiched NiMn2O4/C arrays on Ni foam are successfully designed and synthesized via an easy and simple hydrothermal method followed by glucose polymer coating and two-stage calcinations. The special sandwich-like structure has numerous prominent properties, such as large specific surface area, high electroconductibility derived from coupled carbon coating and conductive substrates, and porous structure resulted from substances loss under calcinations. Based on these advantages, the sandwiched NiMn2O4/C arrays can be considered as advanced energy storage devices, particularly in SCs and LIBs. In the contributions, the as-obtained samples present outstanding electrochemical performances. It demonstrates the high specific capacities (2679 F/g for SCs and 1346 mAh/g for LIBs), excellent rate capability and stability (~98% retention). It is believed that the work would provide a novel strategy to fabricate advanced carbon-conjugated functional nanocomposites or energy storage. Acknowledgements Supported by Thousand Young Talents Program of the Chinese Central Government (Grant No.0220002102003), the National Natural Science Foundation of China (NSFC, Grant No. 21373280, 21403019), Beijing National Laboratory for Molecular Sciences (BNLMS), Fundamental Research Funds for the Central Universities (0301005202017), and Hundred Talents Program at Chongqing University (Grant No. 0903005203205). 8


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Scheme 1 Schematic illustration to introduce the whole synthesized route from flexible NiMn-LDH to NiMn2O4 nanoparticles encapsulated by coupled carbon layers on Ni foam. This novel 3D nanostructure can be regarded as effective energy storage materials for both SCs and LIBs.

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Figure 1 SEM images of NiMn-LDH grown on Ni foam. (a) Low-magnification SEM image of Ni foam ligament. Inset is the optical images of Ni foam before and after growing NiMn-LDH. (b) Cross-section of NiMn-LDH. (c) Large-synthesis of NiMn-LDH precursor on Ni foam. (d) Enlarged SEM image of the precursor to disclose the thickness of NiMn-LDH.

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Figure 2 SEM images at lower (a) and higher (b), (c) resolutions to demonstrate the sandwiched NiMn2O4 nanoparticles encapsulating in coupled carbon sheets. (d) BET profile of the novel sandwiched structure to reveal the specific surface and pore size distribution (inset) of the synthesized NiMn2O4/C.

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Figure 3 Elemental imaging via EDX reveals the element distribution of the sandwiched NiMn2O4/C. (a) TEM image and (b), (c), (d) are the distribution of C, Mn, Ni, respectively.

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Figure 4 TEM images at lower (a) and higher (b) resolutions to demonstrate the sandwich-like morphology of the final samples (c) HRTEM image to disclose the existence of several carbon layers. (d) HRTEM image to verify the crystal lattices of NiMn2O4.

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Figure 5 Electrochemical performances of sandwiched NiMn2O4/C arrays. (a) CV curves at various scan rates form 5 mV/s to 100 mV/s. (b) Galvanostatic discharge profiles at different densities. (c) Current density dependence of the specific capacitance. (d) Cyclic stability at the rate of 10 A/g (inset: charge/discharge profiles at the rate of 10 A/g).

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Figure 6 Electrochemical performances of asymmertric supercapacitor (NiMn2O4/C//AC-ASC). (a) CV curves of the device measured at various scan rates form 10 V/s to 200 V/s. (b) Galvanostatic charge/discharge profiles at different densities. (c) Rate performances of the device. (d) Cycling performance at the rate of 10 A/g (inset: charge/discharge profiles at the rate of 10 A/g).

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For Table of Contents Use Only

Designing Sandwiched and Crystallized NiMn2O4/C Arrays for the Enhanced Sustainable Electrochemical Energy Storage Ya Ouyang1; Yangyang Feng1; Huijuan Zhang; Li Liu; Yu Wang* Synopsis: Novel sandwiched NiMn2O4 nanoparticles encapsulated in coupled carbon layers are firstly fabricated for high-efficiency SCs and LIBs.

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Designing Sandwiched and Crystallized NiMn2O4 ... - ACS Publications

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