Symmetric Sodium-Ion Capacitor Based on Na0.44MnO2 Nanorods

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Symmetric Sodium-Ion Capacitor Based on Na0.44MnO2 Nanorods for Low-Cost and High-Performance Energy Storage Zhongxue Chen, Tianci Yuan, Xiangjun Pu, Hanxi Yang, Xinping Ai, Yongyao Xia, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00478 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Applied Materials & Interfaces

Symmetric Sodium-Ion Capacitor Based on Na0.44MnO2 Nanorods for Low-Cost and High-Performance Energy Storage Zhongxue Chen,*† Tianci Yuan,‡ Xiangjun Pu,† Hanxi Yang,‡ Xinping Ai,‡ Yongyao Xia,§ and Yuliang †

Cao*‡

Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power

and Mechanical Engineering, Wuhan University, Wuhan 430072, China. E-mail: [email protected] ‡

Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and

Molecular Sciences, Wuhan University, Wuhan 430072, China. Email: [email protected]. §

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and

Innovative Materials, Institute of New Energy, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai 200433, China.

ABSTRACT: Batteries and electrochemical capacitors (ECs) are playing very important roles in the portable electronic devices and electric vehicles, and have shown promising potential for large-scale energy storage applications. However, batteries or capacitors alone cannot meet the energy and power density requirements since rechargeable batteries have a poor power property, while supercapacitors offer limited capacity. Here, a novel symmetric sodium-ion capacitor (NIC) is developed based on low-cost Na0.44MnO2 nanorods. The Na0.44MnO2 with unique nanoarchitectures and iso-oriented feature offers shortened diffusion path lengths for both electronic and Na+ transport, and reduces the stress associated with Na+ insertion and extraction. Benefiting from these merits, the symmetric device achieves a high 1 ACS Paragon Plus Environment

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power density of 2432.7 W kg−1, an improved energy density of 27.9 Wh kg−1, and a capacitance retention of 85.2% over 5000 cycles. Particularly, the symmetric NIC based on Na0.44MnO2 permits repeatedly reverse-polarity characteristics, thus simplifies energy management system and greatly enhances the safety under abuse condition. This cost-effective, high safe and high-performance symmetric NIC can balance the energy and power density between batteries and capacitors, and serve as an electric power source for future low-maintenance large-scale energy storage systems. KEYWORDS: symmetric sodium-ion capacitor, Na0.44MnO2 nanorods, low cost, energy storage, high performance

1. INTRODUCTION High-efficiency energy storage system is the main approach to meet the requirements of renewable energy to replace traditional fossil fuels for solving the ever-growing global issues of energy shortage and severe environmental deterioration.1−4 Among various energy storage technologies,

rechargeable

batteries

and

supercapacitors

are

the

most

important

representatives and play critical roles in electrical vehicles (EVs), hybrid electrical vehicles (HEVs) and station grid.5−9 However, batteries or capacitors alone cannot meet the energy and power density requirements of the above systems, because rechargeable batteries offer a high energy density with a poor power property, while capacitors provide high power with low capacity. Therefore, it is best to combine these two devices in a hybrid system to make use of the benefits of both devices. Among the different types of capacitors, supercapacitor can allow fast surface (or subsurface) redox reaction, and deliver much higher energy density along with identical power density compared with traditional electrical double-layer capacitor, thus it has gained particular attentions in recent researches. In order to enhance the energy density of

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pseudocapacitors, Li or Na ions are employed as charge compensation agent to improve their pseudo-capacitance. As a result, a new class of devices is raised which is termed lithium-ion capacitors (LICs) or sodium-ion capacitors (NICs).10−19 Compared to lithium-ion based device, sodium-ion based device attracted more attentions and became the current hottest topics in energy storage field in recent years because of its low cost and resource sustainability.20−38 Hybrid NICs incorporating the V2O5/CNT nanowire composites as the anode and activated carbon as the cathode in Dunn’s work operated at a maximum voltage of 2.8 V and delivered a maximum energy of ∼40 Wh kg–1.12 Mitlin et al. obtained two different activated carbons from peanut shell and fabricated NICs with these carbons, the assembled device showed high power capability and long cycle life.13 Zhang and his co-workers reported flexible NICs based on Na2Ti3O7 nanosheet arrays/carbon textiles anodes which achieved a high energy density of 55 Wh kg–1 and high power density of 3000 W kg–1.39 A common feature of these NICs is that they all employed intercalated materials as negative electrode and surface adsorbed materials as positive electrode, called asymmetric NICs.12,15,16,40,41 In contrast to the asymmetric NICs, symmetric NICs couple identical electrodes as both the cathode and anode in the individual device. Therefore, symmetric NICs possess their own unique advantages: firstly, the production process is largely simplified so that the manufacturing cost will be minimized, because only one type of electrode needs for the device. Secondly, the volume change of battery pack is eliminated since the volume expansion or shrinkage of one electrode can be compensated by the other one during cycling to support long-term cycling performance. Thirdly, because the used materials are on the middle redox state, the device can endure overdischarge to a certain degree, even reversely charge, which immensely enhances the safety of the device.14,42−45 Motivated by the favorable features of symmetric NICs, researchers have been searching for available electrode materials regarding the potential application in NICs.46,47 However, the alternative symmetric electrode materials are scarce because few materials can maintain initial middle redox state. As far as 3 ACS Paragon Plus Environment


we know, only several titanates, vanadate-based materials and conductive polymers were reported. Among different symmetric electrodes above, vanadate-based materials show the most preferable electrochemical performance. A representative symmetric capacitor based on Na3V2(PO4)3 in Ji’s work offered a power density exceeding 5.4 kW kg−1 and a cycling life retention of 64.5% after 10000 cycles, showing its promising potential in HEV application. In spite of the fantastic performance,14 vanadium-based NICs still seem less attractive owing to their inherently toxic and expensive drawbacks. Hence, the key target for the development of symmetric NICs is to explore desirable electrode materials with low cost, high power density, high safety and mass production potential. Low-cost tunnel-type Na0.44MnO2 with open orthorhombic structure has two types of wide tunnels, which can afford fast sodium ion transportation and accommodate the structural strain from multiple phase changes during Na intercalation/deintercalation process.48 By morphology tailoring, phase purifying or surface activity controlling, the electrochemical performance of Na0.44MnO2 cathode for sodium-ion batteries is greatly improved in previous works.49−54 Additionally, Na0.44MnO2 can be considered as an intermediate product between Na0.22MnO2 and Na0.66MnO2, so as to allow both initially extraction and insertion of sodium ions from/into its host structure. Therefore, Na0.44MnO2 is very suitable to be employed as the electrodes for symmetric device. Herein, for the first time, we fabricated a novel symmetric sodium-ion capacitor (NIC) with Na0.44MnO2 as both positive and negative electrodes. The unique NIC exhibits high power density with long cycling lifespan. The charge-storage mechanism study reveals that the current response is primarily from diffusion-controlled Na+ insertion/extraction faradaic reaction at low scan rates, and is dominated by intercalation pseudocapacitance at high scan rates. Particularly, a stable cycling capacity can be maintained even during repeated reverse-polarity charging and discharging, showing potentially high safety. This low-cost and


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high-safe device can balance the energy and power density between batteries and capacitors, showing promising potential application in future large-scale energy storage fields.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Na0.44MnO2 nanorods were prepared with a phenol-formalin assisted sol-gel method using CH3COONa and Mn(CH3COO)2 as the precursor compounds. First, CH3COONa and Mn(CH3COO)2 with a molar ratio of 0.46:1 were mixed and dissolved in ethanol at 60 °C, later 0.28 g phenol and 0.25 ml formalin were added successively. Then, the solution was stirred at 80 °C vigorously for several hours until all the ethanol evaporated. After drying at 100 °C, the precursor was ground into powders, pressed into pellet, and sequentially fired in air for 15 h at 750 °C. 2.2. Morphological and Structural Characterization. X-ray diffraction (XRD) pattern was collected using a Bruker D8 advanced powder X-ray diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) and Energy dispersive x-ray spectroscopy (EDS) were conducted on a JSM-6700F (JEOL, Japan). TEM was performed with a JEM-2010FEF (JEOL, Japan). 2.3. Electrochemical Characterization. The Na0.44MnO2 electrode was fabricated by mixing 80 wt% of synthesized powders, 10 wt% of Super P, and 10 wt% polyvinylidene fluoride (PVDF) and dissolving in N -methyl pyrrolidinone (NMP). The formed slurry was coated on Al foil by a doctor blade technique and dried in a vacuum oven at 100 °C for 6 h. The electrode loading was about 2 mg cm−2. The electrochemical properties were measured with 2032-type coin cells assembled in an argon-filled glove box. In sodium half cells, sodium foil was used as the anode, a solution of 1 mol L−1 NaClO4 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:2 by wt.) was used as the electrolyte, and a microporous membrane (Celgard 2400) was used as the separator. 5 ACS Paragon Plus Environment


Galvanostatic charge-discharge measurement was performed with a multi-channel battery testing system (LAND CT2001A). Cyclic voltammetry (CV) measurement was also conducted with the coin cell on an electrochemical workstation (Autolab PGSTAT 302). EIS was performed using an impedance-measuring unit (IM 6e, Zahner) with an oscillation amplitude of 10 mV in the frequency range from 100 kHz to 0.01 Hz. The Na0.44MnO2 electrode was assembled into symmetric sodium-ion capacitor that are cathode-limited (10% excess active anode), acoording to the capacities obtained for cathode in the voltage range of 4.0-2.9 V and for anode in the voltage range of 2.9-2.0 V, the cathode/anode mass ratio in the symmetric sodium-ion capacitor was controlled at 1.07. Before charge and discharge, the NIC did not need any pre-activation.

3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) pattern of the as-obtained Na0.44MnO2 is shown in Figure 1a. All reflection peaks in the pattern are well indexed to the orthorhombic Na0.44MnO2 phase (Pbam, JCPDS no.27-0750). It has been demonstrated that the ratio of Na/Mn in the raw materials strongly affect the phase purity of Na0.44MnO2,50,51 no trace of impurities was detected in the pattern, indicating the ratio of sodium and manganese precursors were well controlled during the synthetic procedure. The morphology of the Na0.44MnO2 was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image in Figure 1b reveals that the sample is composed of many uniform, rod-like architectures ranging from 2 µm to 5 µm in length. EDS analysis indicates the Na/Mn mole ratio in some local areas is 0.437. The well-defined nanorods structure can be clearly observed from TEM images. Figure 1c shows that the nanorods possess an average diameter of ∼350 nm. The lattice fringe with d-spacing of 0.455 nm can be obviously observed in the high-resolution TEM image in Figure 1d, which 6 ACS Paragon Plus Environment

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corresponds to the (200) interplane of Na0.44MnO2. The selected-area electron diffraction (SAED) pattern (inset of Figure 1d) indicates that the particles have single-crystalline structure with a preferential growth direction of [001], this crystal growth orientation is consistent with those reported in previous works.48−52

Figure 1. (a) XRD pattern, (b) SEM image (inset is the EDS), (c) TEM image and (d) high-resolution TEM image (inset of SAED) of Na0.44MnO2 nanorods. The sodium storage properties of Na0.44MnO2 nanorods were initially tested by using Na/Na0.44MnO2 half-cell. Figure 2a displayed the cyclic voltammetry (CV) curves of the half cells at a scan rate of 0.1 mV s–1 between 4.0 and 2.0 V. As can be seen, more than six pairs of redox peaks appeared in the first cycle, implying the structure of Na0.44MnO2 undergoes multiple phase transition during charge and discharge, the small voltage gap between each redox couples suggests the high electrochemical reversibility of the cell. Except this, the



almost perfectly reproduced shape and intensity of both anodic and cathodic peaks in the first two scans indicates the good structural stability of Na0.44MnO2 upon cycling.

Figure 2. Electrochemical characterization of Na/Na0.44MnO2 half-cell in the voltage window of 2.0−4.0 V vs Na+/Na. (a) Cyclic voltammetry curves, scan rate: 0.1 mV s−1. (b) Charge/discharge profiles of the first two cycles at a current density of 0.02 A g−1. (c) Cycle performance and the corresponding columbic efficiencies at a current density of 0.12 A g−1 (inset is the EIS of the electrode before and after cycling). (d) Discharge capacity as a function of charge/discharge cycles at different charge/discharge current densities. The galvanostatic charge/discharge voltage profiles of the half-cell at a current density of 0.02 A g−1 in the voltage window 4.0–2.0 V (vs. Na/Na+) are shown in Figure 2b. The observed first charge capacity is 58.4 mAh g−1 which is related to the structural transition from Na0.44MnO2 to Na0.66MnO2, the following reversible discharge/charge capacity is about 120 mAh g−1, corresponding to the phase transformation between Na0.66MnO2 and 8 ACS Paragon Plus Environment

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Na0.22MnO2. Multiple voltage plateaus were observed in the charge/discharge curves, which are in good agreement with the above CV curves and the results reported in previous earlier works,48−50 corresponding to consecutive phase transitions during Na+ intercalation and deintercalation. The cycling performance of Na/Na0.44MnO2 half-cell was tested at a current density of 0.12 A g−1. As revealed in Figure 2c, the cell has 91.7% capacity retention after 1000 cycles, and the coulombic efficiencies keep around 100%. Moreover, the electrochemical impedance spectra (EIS) for both fresh and cycled (500th) electrodes show indiscernible difference, demonstrating excellent cycling stability. Figure 2d shows the rate performance of the half-cell at different current densities, a reversible capacity of 115.3, 98.9, and 92.2 mAh g−1 was obtained at current densities of 0.12, 1.2 and 2.4 A g−1, respectively. Notably, the capacity can be recovered when the current rate returns to 0.06 A g−1 after the rate performance tests. These results suggest that the Na0.44MnO2 electrode with nanorod morphology, high crystallinity and large S-shaped tunnels can afford fast sodium ion transport. The electrochemical performance of the Na/Na0.44MnO2 half-cell in the high voltage range of 4.0–2.9 V (vs. Na/Na+) was further evaluated. Figure 3a shows the CV profiles of the cell at a scan rate of 0.1 mV s–1. As illustrated, the curves exhibit several pairs of well-defined redox peaks, which is related to the phase transformation between Na0.44MnO2 and Na0.22MnO2, these redox peaks have the same intensity and voltage position as those observed in Figure 2a. In the subsequent scan, both the shape and intensity of the peaks remains unchanged, suggesting an excellent electrochemical reversibility of Na insertion/extraction into/from Na0.44MnO2 nanorods in the high voltage region (>2.9 V). The charge/discharge curve in Figure 3b shows that the half-cell delivered a discharge capacity of 46.8 mAh g−1 at a high current rate of 10 C (1 C = 60 mA g−1), particularly, an average discharge voltage of 3.12 V can still be maintained even under such high current density. The cycling performance at 9 ACS Paragon Plus Environment


the high current rate of 10 C was also tested and presented in Figure 3c, the cell retained a discharge capacity of 42.1 mAh g−1 after 5000 cycles, corresponding to a capacity retention of 90%, also the columbic efficiencies maintained around 100% during cycling.

Figure 3. Electrochemical characterization of Na/Na0.44MnO2 half-cell in the voltage window of 2.9−4.0 V vs Na+/Na. (a) Cyclic voltammetry curves, scan rate: 0.1 mV s−1. (b) Charge/discharge profiles at a current rate of 0.6 A g−1 (10 C). (c) Cycle performance and the corresponding columbic efficiencies at 0.6 A g−1. Also the electrochemical properties of the Na/Na0.44MnO2 half-cell in the low voltage range of 2.9–2.0 V (vs. Na/Na+) were investigated. The CV profiles in Figure 4a displayed three broad cathodic peaks during the first negative scan, implying the successive Na intercalation process in Na0.44MnO2. In the subsequent positive scan, three anodic peaks emerged accordingly, indicating the intercalated sodium ion during the initial reduction process can be 10 ACS Paragon Plus Environment

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reversibly extracted from the structure. The redox peaks in the following scan are associated with the phase transformation between Na0.44MnO2 and Na0.66MnO2, both peaks position and intensity kept stable after the initial scan, which is similar to those results obtained for the CVs in higher voltage range in Figure 3a. To further study the sodium intercalation behavior, the half-cell was firstly discharged to 2.0 V, and then charged to 2.9 V. The charge/discharge profile in Figure 4b show that the cell was able to deliver discharge and charge capacities of 55.3 and 55.1 mAh g−1 at 10 C, the average discharge and charge potentials are 2.33 and 2.65 V, respectively. Figure 4c shows that the half-cell retained 86.9 % of its initial capacity and held coulombic efficiencies around 100% in 5000 cycles at a current rate of 10 C, showing an excellent cycling stability.



Figure 4. Electrochemical characterization of Na/Na0.44MnO2 half-cell in the voltage window of 2.9−2.0 V vs Na+/Na. (a) Cyclic voltammetry curves, scan rate: 0.1 mV s−1. (b) Charge/discharge profiles at a current rate of 0.6 A g−1 (10 C). (c) Cycle performance and the corresponding columbic efficiencies at 0.6 A g−1.

Figure 5. Electrochemical characterization of Na0.44MnO2|Na0.44MnO2 symmetric NICs in the voltage window of 1.6−0 V. a) Cyclic voltammetry curves, scan rate: 0.1 mV s−1. b) Charge/discharge profiles at a current density of 0.02 A g−1. c) Cycle performance and the corresponding columbic efficiencies at 1.8 A g−1 (30 C). The above electrochemical tests of Na/Na0.44MnO2 half-cell demonstrated that the Na0.44MnO2 cathode permits both initially extraction and insertion of sodium ions from/into its host structure, and exhibit a first charge/discharge capacity of 46.8/55.3 mAh g−1 at 10 C in the high and low voltage range respectively. Therefore, in theory, a symmetric sodium-ion 12 ACS Paragon Plus Environment

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NIC can be constructed using Na0.44MnO2 as both positive and negative electrodes. Prior to charge/discharge measurements, cyclic voltammetry (CV) of the Na0.44MnO2-based NIC (NMO-NIC) was initially conducted at a scan rate of 0.1 mV s–1 in the voltage window 1.6–0 V. During the first positive scan, the cathode of the NIC transformed from Na0.44MnO2 into Na0.22MnO2, while the anode transformed from Na0.44MnO2 into Na0.66MnO2, in the following scans, the reactions for cathode and anode are Na0.22MnO2 ↔ Na0.44MnO2 and Na0.66MnO2 ↔ Na0.44MnO2, respectively. At least three pairs of redox peaks are clearly observed in Figure 5a, the voltages of the peaks are equal to the voltage gaps between the redox peaks in Figure 3a and Figure 4a. No noticeable change of the current or potential was observed for the cathodic peaks in the first two scans, implying that the sodium insertion/extraction process is highly reversible and the crystalline structure of Na0.44MnO2 remains stable. The first two charge-discharge profiles of the NMO-NICs were shown in Figure 5b. The device charged and discharged between 1.6 and 0 V delivered an initial discharge capacity of 47.8 mAh g−1 (calculated based on the mass of cathode) at a current density of 20 mA g−1, an average operating voltage of 0.6 V was obtained. The first charge curve is different from the subsequent cycle, however, the discharge curves for both cycles almost overlapped; these observations are in good agreement with the CV results in Figure 5a. The long-term cycling performance of the device was examined at a high rate of 30 C (1800 mA g−1). As shown in Figure 5c, the device presents a specific capacity of 39.7 mAh g−1, after 5000 cycles, 85.2% of its initial capacity can be maintained, demonstrating an outstanding cycling performance. Moreover, the coulombic efficiencies maintained around 100% along the cycling. The rate capability of the NMO-NICs were further studied by cycling at various charge and discharge rates ranging from 1/3 to 100 C (1C= 60 mA g−1) in a voltage range of 1.6–0 V. It can be observed in Figure 6a that the NMO-NICs exhibit remarkable rate performance, a specific capacity of 43.4 and 34.0 mAh g−1 was achieved at a high rate of 10 C and 100 C (charging or discharging in 20 seconds), respectively, which are about 92% and 72% of the 13 ACS Paragon Plus Environment


capacity (47.1 mAh g−1) at C/3. Specifically, when the current density goes back to 5 C, a capacity of 43.2 mAh g−1, close to the former value, can be recovered. Representative galvanostatic charge/discharge curves for the symmetric NMO-NICs at different current densities are shown in Figure 6b. The energy and power density of the NMO-NICs were calculated based on the charge/discharge results: a maximum energy density of 27.9 Wh kg−1 at C/3 rate and the highest power density of 2432.7 W kg−1 at 100 C rate. Figure 6c compares

Figure 6. (a) Discharge capacity of Na0.44MnO2|Na0.44MnO2 symmetric NICs as a function of charge/discharge cycles at different charge/discharge current densities (C/3, 1 C, 5 C, 10 C, 20 C, 30 C, 50 C, and 100 C). (b) Galvanostatic charge/discharge curves from 0 to 1.6 V at different current rates. (c) Ragone plots of Na0.44MnO2|Na0.44MnO2 NICs and some other


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symmetric supercapacitors recently developed. All of the data are based on the mass of electrode materials. this work with some other symmetric supercapacitor devices recently developed. Compared with active carbon (AC)-based symmetric cells, our device offers significantly higher energy and power density.12,55 Compared with metal oxides-based or Na3V2(PO4)3 (NVP)-based symmetric capacitors, the NMO-based device remains highly competitive.14 The outstanding electrochemical performances are attributed to the iso-oriented nature of nanorod architecture which shortens diffusion path lengths for both electron and Na+ transport, and provides a beneficial microstructure for Na + insertion/extraction.

Figure 7. (a) CV curves of Na0.44MnO2|Na0.44MnO2 symmetric NICs at different scan rates (0.5, 1, 2, 5, 10, and 20 mV s−1). (b-c) Relationship between anodic peak currents and the square root of scan rates (0.5, 1, and 2 mV s−1) or scan rates (5, 10, and 20 mV s−1). The point plots are experimental values, and the dash lines are fitting results. As presented above, the NMO-NICs can be charged and discharged in an extremely short time (< 20 s) and delivers such high power. Therefore, the charge-storage mechanism of this NMO-NIC was particularly investigated to reveal its fast kinetics in contrast to batteries. It has been demonstrated that the total stored charge of electrode is composed of three parts: the diffusion-controlled faradaic reaction from Na+ insertion/extraction, the faradaic contribution 15 ACS Paragon Plus Environment


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from charge-transfer reaction with both surface and subsurface (interlayer lattice planes) atoms (termed pseudocapacitance), and the non-faradaic contribution from the double-layer capacitance.56 Cyclic voltammetry measurements can be used to determine quantitatively the capacitive contribution of each part according to the relationship between peak currents and sweep speed. Figure 7a showed the CV curves conducted for the NMO-NIC at different scan rates (0.5, 1, 2, 5, 10, and 20 mV s −1) between 0 and 1.6 V. The potential gaps between the corresponding anodic and cathodic peaks did not widen very much as the scan rates increased, demonstrating superior Na-ions intercalation kinetics. The analysis results in Figure 7b showed that the peak currents had a linear relationship with the square root of the low scan rates (0.5, 1, and 2 mV s −1), implying a diffusion-controlled sodium ion intercalation process. At high sweep speed (5, 10, and 20 mV s−1), a linear relationship was found (Figure 7c) to be well fitted between the peak currents and scan rates, indicative of capacitive behavior. Based on the above calculations, the charge-storage mechanism can be concluded as follows. At low scan rates, Na ions enter the bulk of electrode and react with the electroactive material, this type of chemistry is the characteristic feature for conventional batteries, here the current response is primarily from diffusion-controlled Na+ insertion and extraction faradaic reaction. At high scan rates, the diffusion time of Na+ across the bulk electrode is far longer than that of redox reaction. In contrast, the iso-oriented nanorods enables Na+ to be facilely intercalated into interlayer gaps, accompanied by reduction of Mn ions to maintain charge neutrality. According

to

previous

works,

this

intercalation

process

can

be

considered

aspseudocapacitance in nature because the cations are faradaically stored and phase transitions do not happen. The intercalation pseudocapacitance occurs on the same timescale as redox pseudocapacitance, and is responsible for the high capacitive charge storage at such high scan rates. These observations agree well with previous researches.14,54 Two factors account for the ultrafast intercalation pseudocapacitance observed in this symmetric device. Firstly, the nanoscale dimensions of the electrode offer shortened diffusion path lengths for 16 ACS Paragon Plus Environment

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both electronic and Na+ transport. Secondly, the iso-oriented feature of the architecture reduces the stress associated with Na+ insertion and extraction. In addition, the symmetric NMO-NIC only involves Na+ intercalation/deintercalation mechanism for both positive and negative electrodes, therefore the required amount of electrolyte can be greatly reduced compared to the value for conventional supercapacitor, leading to an improvement in the energy density of the device.

Figure 8. (a) SEM and (b-c) TEM images of the Na0.44MnO2 cathode in the symmetric NICs after 5000 cycles. (d) SEM and (e-f) TEM images of the Na0.44MnO2 anode in the symmetric NICs after 5000 cycles. The structural and morphological changes of electrodes upon long-term cycling were studied to understand the superior cyclability of the Na0.44MnO2|Na0.44MnO2 symmetric NICs. The device was disassembled after cycling for 5000 cycles, both the cathode and anode were taken out and analyzed by SEM and TEM. The SEM images in Figure 8a and 8d show that the origin nanorod morphology of NMO is retained after cycling without any aggregations. The nanorods remain in good contact with conductive carbon (SP) to ensure high electronic conductivity. The nanorods shape can be distinctly observed from the TEM images in Figure 8b and 8e, both the diameter and the length of the nanorods maintain unchanged without any 17 ACS Paragon Plus Environment


expansion or attenuation. In the high magnified TEM (HRTEM) image of cycled cathode shown in Figure 8c, the lattice fringe with an interplanar spacing of 0.231 nm is clearly displayed, which is assigned to the planar distance between the (231) planes. The SAED pattern (inset in Figure 8c) confirms a crystalline orthorhombic Na0.44MnO2 structure. Also a legible lattice fringe with a basal distance of 0.455 nm, corresponding to the (200) plane of orthorhombic Na0.44MnO2, is revealed in the HRTEM image of cycled anode in Figure 8f. Moreover, the single-crystalline feature with a [001] growth direction was preserved for both cycled cathode and anode, suggesting remarkable structural stability upon long-term cycling.

Figure 9. Electrochemical characterization of Na0.44MnO2|Na0.44MnO2 symmetric NICs in the voltage window of −1.6~1.6 V. (a) Charge/discharge profiles at a current density of 20 mA g−1. (b) Cycle performance at 20 mA g−1. As the symmetry of commercially available batteries or capacitors increases, accidental reversal of battery polarity becomes a common occurrence which may cause damages of devices or even safety issues. As one of the important features of the symmetric energy storage devices, the capability to endure reversibly charge and discharge can simplify energy management system and greatly enhance the safety. The reverse polarity test was carried out by operating the NMO-NICs in a wide voltage range of −1.6~1.6 V. Figure 9a presents the first two charge/discharge profiles of the device. In the initial charge process, the cathode 18 ACS Paragon Plus Environment

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transformed from Na0.44MnO2 into Na0.22MnO2, while the anode transformed from Na0.44MnO2 into Na0.66MnO2. In the following cycles, both cathode and anode transformed between Na0.22MnO2 and Na0.66MnO2. As is seen, the NICs exhibit an initial discharge capacity of 118.2 mAh g−1, closing to the theoretical capacity of Na0.44MnO2, indicative of complete reversibility. When the device was repeatedly charge and discharge for 100 cycles (Figure 9b), a capacity retention of 86.1% was obtained, suggesting the reverse polarity operation has negligible effect on the cycling performance of NICs, which immensely improves the safety under abuse condition.

4. CONCLUSION In summary, we have demonstrated a novel symmetric sodium-ion capacitor (NICs) with Na0.44MnO2 nanorods as both positive and negative electrodes. This unique NICs exhibit high power density, improved energy density, and long cycling lifespan in a non-aqueous electrolyte, the maximum energy density and highest power density reaches 27.9 Wh kg−1 and 2432.7 W kg−1, respectively, the energy density retains 85.2% of its initial value after 5000 cycles at 1.8 A g−1 (30 C). The charge-storage mechanism study reveals that diffusion-controlled

Na+

insertion/extraction

faradaic

reaction

and

intercalation

pseudocapacitance dominate respectively at low and high scan rates. The ultrafast kinetics should be ascribed to the unique nanoarchitectures, in which nanoscale dimension of the electrode offers shortened diffusion path for both electronic and Na+ transport, and iso-oriented feature reduces the stress associated with Na+ insertion and extraction. Importantly, the NMO-NIC possesses the capability to endure repeatedly reverse polarity, thus simplify energy management system and greatly enhance the safety. With optimization, this cost-effective, high safe, and high-performance symmetric NMO-NICs can serve as an electric power source for large-scale energy storage systems. 19 ACS Paragon Plus Environment


AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank financial support by the National Key Research Program of China (No. 2016YFB0901500), the National Natural Science Foundation of China (21673165, 21373155) and the China Postdoctoral Science Foundation (2016M592383, 2017T100574).

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Symmetric Sodium-Ion Capacitor Based on Na0.44MnO2 Nanorods

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