Ultralong Layered NaCrO2 Nanowires: A Competitive Wide

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Ultralong Layered NaCrO2 Nanowires: A Competitive Wide-Temperatureoperating Cathode for Extraordinary High-Rate Sodium Ion Batteries Longwei Liang, Xuan Sun, Dienguila Kionga Denis, Jinyang Zhang, Linrui Hou, Yang Liu, and Changzhou Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20149 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Materials & Interfaces

Ultralong

Layered

NaCrO2

Nanowires:

A

Competitive

Wide-Temperature-operating Cathode for Extraordinary High-Rate Sodium Ion Batteries

Longwei Liang, Xuan Sun, Dienguila Kionga Denis, Jinyang Zhang, Linrui Hou, Yang Liu, Changzhou Yuan*

School of Materials Science & Engineering, University of Jinan, Jinan, 250022, P. R. China

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ABSTRACT The

development

of

high-rate

cathodes

particularly

with

remarkable

wide-temperature-tolerance sodium-storage capability plays a significant role in commercial applications of sodium-ion batteries (SIBs). Herein, we devise a scaled-up electrospinning avenue to fabricate nanocrystal-constructed ultralong layered NaCrO2 nanowires (NWs) towards SIBs as a wide-temperature-operating cathode. The resultant one-dimensional (1D) NaCrO2 nano-architecture is endowed with orientated and shortened electronic/ionic transport, and remarkable structural tolerance to stress change over sodiation-desodiation processes. Benefiting from these structural superiorities, the NaCrO2 NWs are featured with prominent Na+-storage behaviors in the wide operating temperature range from ‒15 to 55 °C. Promisingly, the NaCrO2 NWs exhibit extraordinary high-rate capacities of ~108.8 and ~87.2 mAh g-1 at 10 and 50 C rates at 25 °C, and even 94.6 (55 °C) and ~60.1 (‒15 °C) mAh g-1 at 10 C along with outstanding cyclic stabilities with capacity retentions of ~80.6% (‒15 °C), 88.4% (25 °C) and ~86.9 % (55 °C). The overall performance of our NaCrO2 is superior to other reported NaCrO2-based cathodes, even with conductive nano-carbon coating. Encouragingly, a competitive energy density of ~161 Wh kg-1 can be obtained by the NaCrO2 NWs-based full cell. Therefore, our NaCrO2 NWs can be highly anticipated as advanced cathode for commercial wide-temperature-tolerance SIBs. KEYWARDS:

NaCrO2;

High-rate

cathode;

Wide-temperature-tolerance; Sodium-ion batteries 2 ACS Paragon Plus Environment

Ultralong

nanowires;

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INTRODUCTION Over the past decades, sodium-ion batteries (SIBs) have gained enormous attention in response to the increasing worldwide demand for electrochemical energy storage, especially for the smart grids and electric vehicles, thanks to the widespread availability, low efficiency and similar properties with Li of the involved yet indispensable elemental sodium.1-5 In moving towards this scenario, layered transition metal oxides, NaxMO2 (M = Cr, Mn, Fe, Co, etc.) as the potential sodium intercalation cathodes for SIBs, have been extensively explored, owing to their acceptable reversible capacities, facile synthesis and environmental amity.6-16 In particular, typical O3-type layer-structured NaCrO2 stands out by virtue of its obvious superiorities including a desirable flat charge/discharge voltage plateau at ~3.0 V (vs. Na/Na+) originating from highly reversible redox reaction of Cr3+/Cr4+, large reversible discharge capacity of ~120 mAh g-1,17-20 and especially superior chemical stability of Cr4+ in CrO2 slabs, which is of huge significance to even safer SIBs for commercial applications.21 However, as far as we know, the attentions to enhancing the high-rate capacities and capacity-retention properties of NaCrO2 are still less paid. So far, only carbon coating strategy is proposed by two references to improve the rate behaviors of layered NaCrO2.22, 23 What most impressed us is that the NaCrO2 of 500 nm in size, which is coated with a carbon layer of ~10 nm in thickness, can amazingly exhibit ultrahigh rate capability of ~120 mAh g-1 at 50 C.23 Very unfortunately, a poor capacity of only 3 mA h g-1 at a high rate of 50 C can be provided by the bare NaCrO2. 3 ACS Paragon Plus Environment


Besides this, all the reported NaCrO2 cathodes, to the best of our knowledge, are commonly prepared by the solid-state reaction (SSR)19-22, 24 and emulsion-drying (ED) methods,23 as summarized in Table 1. But the two strategies always result in bulk or sub-micro/micro-sized irregular shaped NaCrO2 particles. As a consequence, the triggered long-distance solid-state diffusion paths and unfavorable transport of ions and electrolytes to electroactive sites are mainly responsible for their awful rate capabilities and high-rate cycling properties.23 Besides this, one fact still cannot be neglected at all that the commercial applications of SIBs are seriously limited by their low-temperature sodium-storage behaviors, particularly at high rates, which is always general to any secondary batteries. As established previously, the modest capacity and sluggish electrode kinetic, could be hugely enhanced by designing electrochemically active materials approaching nanoscaled dimensions to minimize the transfer distance.25-27 More encouragingly, one-dimensional (1D) nano-architecture, especially with ultralong feature, continuous and interconnected framework would ensure a large electrode/electrolyte contacting area, and provide convenient migration pathways to facilitate the rapid charge transfer along with the decreasing contacting resistance.26, 28 Meanwhile, the huge volume variations during the charge-discharge process can be effectively suppressed with the nano-strategy, hindering the pulverization and aggregation of electrode materials.25,

28, 29

Thus, the electrochemical kinetics,

reversible capacities and long-term cycling performance of the NaCrO2 cathode can be boosted as expected. 4 ACS Paragon Plus Environment

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In view of the comprehensive considerations above, in this work, we pioneeringly designed and fabricated 1D uniform and ultralong NaCrO2 nanowires (NWs) via a bottom-up yet scaled-up electrospinning (ES) methodology coupled with subsequent calcination, and further featured their viability as a competitive wide-temperature-operating cathode candidate for advanced SIBs. Physicochemical characterizations demonstrated that the as-obtained 1D NWs of ~48 nm in diameter and ~30 μm in length were constructed by ultrafine NaCrO2 nanocrystals (NCs) of ~3 ‒ ~5 nm in size. Benefiting from these prominent structural features, our pathbreaking design, i.e., 1D ultralong NaCrO2 NWs, competitively exhibited large reversible capacities, exceptional ultrahigh rate capabilities, and excellent cyclic stabilities within the wide operating temperature range from ‒15 to 55 °C for commercial SIBs applications. More impressively, a competitive energy density of ~161 Wh kg-1 can be obtained by the assembled NaCrO2 NWs//hard carbon (HC) full device at 25 °C. EXPERIMENTAL SECTION Synthesis of 1D NaCrO2 NWs. In a typical synthesis process, 2 g of polyvinyl pyrrolidone (PVP, M.W. 1300000) was dissolved into 18 mL of ethanol by vigorous stirring at 60 °C for 12 h. 5 mmol of NaNO3 and 5 mmol of Cr(NO3)3·9H2O were dissolved in 10 mL of deionized water with constant stirring for 1 h, and then mixed well with the resultant PVP solution. The obtained mixture solution was thoroughly stirred overnight and used as the precursor solution for electrospinning. After that, the precursor solution was transferred into a 10 mL of plastic syringe equipped with a 21G (gauge) stainless steel needle. The solution flow rate was set to be 18 µL min-1 5 ACS Paragon Plus Environment


controlled by a syringe pump. The metallic needle was connected to a high voltage power supply. A voltage of 15 kV was applied between the needle and a grounded aluminum foil, which was 16 cm away from the tip of the needle to collect the product. The relative humidity was controlled in the range of 25 ‒ 35%. Then, the as-collected membrane (denoted as NaCr@PVP NWs) was dried under vacuum at 110 °C for 4 h, and then decarbonized at 450 °C in air with a heating rate of 1 °C min-1 to obtain pre-oxidation NaCrOx NWs. After further crystallization at 900 °C in Ar with a heating rate of 2 °C min-1, the olive-green NaCrO2 product was finally obtained. Material Characterizations. Representative X-ray diffraction (XRD) patterns were collected by a Rigaku-TTRIII type X-ray diffractometer (Cu Kα, 40 kV, 300 mA, Japan) over the 2θ range of 10 ‒ 80 °. The XRD profile was refined via Rietveld program General Structure Analysis System (GSAS) software. Morphologies and microstructures were characterized by field-emission scanning electron microscopy (FESEM, JEOL-6300F), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning transmission electron microscopy (STEM), and high-resolution TEM (HRTEM) (JEOL JEM-2100). Energy dispersive X-ray (EDX) analysis and corresponding elemental mapping data were taken with the X-ray spectroscopy attached to the TEM instrument. X-ray photoelectron spectroscopic (XPS) measurements were implemented by using a VG Multilab 2000 spectrometer, and corresponding fitted spectra were performed by the Thermo Avantage software. Thermogravimetric (TG) analysis (STA449F5, NETZSCH, Germany) was performed 6 ACS Paragon Plus Environment

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under an air flow with a temperature ramp of 5 °C min-1. Typical crystal structure was drawn using the program VESTA. For ex situ XRD, the cells were disassembled at different states of charge and the cathodes were washed several times with dimethyl carbonate before drying, all in a glove box. Electrochemical Measurements. The working electrodes consisted of electroactive NaCrO2 NWs, acetylene black, and polyvinylidene fluoride with a mass ratio of 8 : 1 : 1, and the N-methyl-2-pyrrolidone was applied as the dispersant. Typical average mass loading of the work electrode was approximately 4.0 mg cm-2 for each cell. The electrolyte used here was 1 M NaPF6 in a mixed solvent containing ethylene carbonate and diethyl carbonate with a volume ratio of 1 : 1. The glass fiber was used as the separator and the self-made sodium metal foil as the reference electrode in CR2032 coin cell. The cells, assembled in an Ar-filled glove box, were charged and discharged under a voltage window of 2.0 − 3.6 V (vs. Na/Na+) under various current rates, which were conducted on a 8-channel Land Test System (CT2001A, Wuhan Jinnuo Electronic Co., Ltd., China). All the tests at different working temperatures of ‒15, 25 or 55 °C were achieved by keeping cells in a temperature controllable test chamber (Dongguan Kingjo equipments CO., Ltd, China). For the full cell test, the HC30 was used as an anode and paired with NaCrO2 NWs cathode. In order to enhance the stability and minimize the irreversible capacity during the first cycle, the HC anode was physically presodiated before assembly. The cell balance was controlled to a value of 1.2, which was sufficient to prevent the overcapacity of the cathode. The employed electrolyte, separator and test equipment were identical to 7 ACS Paragon Plus Environment


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those of the above coin cell. Cyclic voltammetry (CV) tests were performed on an IVIUM electrochemical workstation (The Netherlands) in a voltage range of 2.0 − 3.6 V (vs. Na/Na+) at a scan rate of 0.1 mV s-1. RESULTS AND DISCUSSION Structural and Physicochemical Characterization. The fabrication process for 1D ultralong NaCrO2 NWs is schematically illustrated in Scheme 1. Firstly, the stoichiometric precursor solution is controllably electrospun into a white membrane (Figure

S1),

which

is

constructed

randomly

by

polyvinyl

pyrrolidone

(PVP)-sustained NWs (NaCr@PVP NWs). Then, the as-obtained NaCr@PVP NWs are stabilized and decarbonized at the temperature of 450 °C in O2 atmosphere, according to the thermogravimetric (TG) data (Figure S2). Over the peroxidation process, the Na and Cr salts are self-oxidized to brown NaCrOx sample (Figure S3a) along with the high-content PVP decomposition. After further high-temperature annealing at 900 °C, the targeted NaCrO2 product with olive-green color is prepared (Figure S3b). Figure 1a displays the Rietveld refinement of the X-ray diffraction (XRD) pattern for the as-spun NaCrO2 NWs. Apparently, all of these diffraction peaks, not only their relative intensities but diffraction positions, are indexed successfully into the rhombohedral layered NaCrO2 phase (JCPDS card no. 25-0819) with a space group R m and O3-type structure (the inset in Figure 1a).19,

31

Moreover, no

detectable diffraction peaks from any Na/Cr-based impurity phases can be observed here. Besides, according to the structural refinement, corresponding lattice parameters 8 ACS Paragon Plus Environment

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are calculated to be ahex = 2.9753 Å, chex = 15.9682 Å in a hexagonal setting, which are extremely consistent with the previous reports.19, 20 More appealingly, a powerful evidence to demonstrate no existence of intermixing between the Na and Cr ions can be perceived from the small refined R factors (Rwp = 9.51% and Rp = 7.20%), indicative of a reliable structural refinement and no site occupation of Cr ions at 3b site in the interslab layer. Based on the detailed analysis above, it is easily deduced that an ideal layered O3-type structure without an antisite defect is achieved for our case of unadulterated and stoichiometric NaCrO2 NWs. In order to probe specific oxidation states of the sur-/interfacial elements in the NaCrO2 NWs, X-ray photoelectron spectroscopy (XPS) analysis is further conducted. Typical survey spectra (Figure 1b) shows the co-existence of elemental O, Cr and Na with the stoichiometric ratio in the NaCrO2 NWs. Figure 1c presents the high-resolution Cr 2p spectrum and corresponding fitted profiles. Convincingly, the six fitted peaks centered at 574.9, 575.8, 576.4, 577.4, 585.1 and 586.3 eV reveal the Cr(Ⅲ) species in the NaCrO2 NWs.32 Furthermore, the core-level O 1s spectrum is deconvoluted into three types of O species with the Gaussian-Lorentzian curve-fitting method, as plotted in Figure 1d. Specifically, the distinct O1 peak detected at ~529.6 eV is assigned to metal (Cr/Na)-oxygen lattice component. The sharply weakened O2 (~531.4 eV) can be related to other O species (‒OH, H2O or carbonates, etc.) absorbed on the NWs surface.33,

34

And the O3 peak at ~532.8 eV corresponds to

typical defect sites with low oxygen co-ordination generally discovered in nanoscaled materials.35, 36 9 ACS Paragon Plus Environment


Figure 2a demonstrates the panoramic field-emission scanning electron microscopy (FESEM) image of the resultant NaCrO2 NWs. As noted, numerous NWs align in random orientation and are interlinked each other. And these NWs apparently inherit well the ultralong wire-shape morphologies of the NaCr@PVP specimen (Figure S4a, b) and pre-oxidized NaCrOx sample (Figure S4c, d) without noticeable calcination-induced structural collapse, confirming the robust structural stability of our NaCrO2 NWs. The histogram of NWs diameter distribution (the inset in panel a), as derived from the observation in Figure 2a, confirms that the diameter of the well-developed NWs is mainly located in the range 20 ‒ 50 nm, and an average diameter of approximately 48 nm is calculated accordingly. The sharp decrease of NaCrO2 NWs in diameter, when compared to those for NaCr@PVP (~560 nm, Figure S4c) and pre-oxidized NaCrOx (~79 nm, Figure S4d) products, should result from the polymer decomposition and further crystallization over annealing in O2. More remarkably, numerous NCs can be discernable upon the rough surface of NaCrO2 NWs, as examined in Figure 2b, c, corroborating that our ultralong NaCrO2 NWs are assembled with closely linked primary NCs. It is particularly worthy to mention that the bulk NaCrO2 samples with sub-micron and/or micron size range from 500 nm to 10 μm have been reported so far,18-20, 22, 23, 31, 37 as totally summarized in Table 1. To gain more insights into the subtle microstructures of the as-prepared NaCrO2 NWs, corresponding transmission electron microscopy (TEM) are carried out. As featured in Figure 2d, lots of scattered, ordered and ultralong NWs interlink and 10 ACS Paragon Plus Environment

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construct a unique 3D network, which coincides highly with the morphologic characterizations above (Figure 2a, b). As reported, such unique 3D framework can render a superior electric conductivity for rapid electron transportation and suppresses the aggregation of active materials during charge/discharge process meanwhile, thus greatly boosting the high-rate capabilities.38,

39

The higher-magnification TEM

observation (Figure S5) further reveals the nanoscaled dimension in diameter for our NWs, which can tremendously shorten the pathway of solid-state Na+ diffusion, thus leading to the outstanding Na+-storage properties. Figure 2e, f demonstrate the high-resolution TEM (HRTEM) image of the NaCrO2 NWs. Nanocrystals of ~3 – ~5 nm in size, as shown in Figure 2f, are explicitly discerned with well-defined lattice fringes of a d-spacing of ~0.533 nm, conforming well to the (003) plane for the layered NaCrO2.31 Besides, a distinct lattice fringe with a spacing of ~0.219 nm in the margin (i.e., the blue rectangular region in panel e), corresponding to (104) plane of the NaCrO2 (Figure S6), indicates that no any coating layer and/or redundant compounds reside on the surface region. Furthermore, the selected area electron diffraction (SAED) pattern with a series of concentric diffraction rings, as displayed in Figure 2g, suggests the polycrystalline feature of the NaCrO2 NWs, and matching perfectly with the (003), (104), (012) and (116) planes (JCPDS no. 25-0819), respectively.40 Additionally, scanning TEM (STEM) image (Figure 2h) and corresponding elemental energy dispersive X-ray (EDX) mapping signals (Figure 2i-k) visually authenticate the uniform distributions of elemental Na, Cr and O in the NaCrO2 NWs. 11 ACS Paragon Plus Environment


Electrochemical Evaluation. As discussed above, the as-fabricated 1D ultralong NaCrO2 NWs with the overwhelming structural superiority undoubtedly promise its potential utilization in high-performance SIBs as a competitive cathode. Figure 3a shows typical cyclic voltammetry (CV) profiles recorded in the voltage of 2.0 ‒ 3.6 V (vs. Na/Na+) with a sweep rate of 0.1 mV s-1. Evidently, two couple of sharp cathodic/anodic peaks centered at 3.08/2.95 and 3.33/3.28 V are closely associated with the oxidation/reduction between Cr3+/Cr4+ within a potential window of 2.6 ‒ 3.4 V.22 Meanwhile, the emerged order/disorder phase transformations during the electrochemical process for the NaCrO2 cathode should be well responsible for three additional anodic peaks. Remarkably note that the CV responses, including not only current intensities but potential positions, are nearly unchanged upon cycles, undisputedly revealing the rapid redox kinetics and excellent reversibility of Na+ insertion/extraction.41,

42

Typical initial three voltage profiles of the NaCrO2 NWs

electrode at 0.1 C (1 C = 100 mA g-1) between 2.0 ‒ 3.6 V are distinctly displayed in Figure 3b. Obviously, all these charge/discharge plots, similar to the CV analysis above, are almost overlapped. Besides, two charge voltage plateaus located at 3.08 V and 3.3 V along with their discharging counterparts at 3.28 V and 2.95 V can be clearly observed, which is consistent well with two redox peaks observed in CV curves (Figure 3a). More strikingly, the as-fabricated NaCrO2 NWs cathode yields a charge capacity of ~126.2 mAh g-1 coupled with a competitive discharge capacity of ~122.6 mAh g-1 at 0.1 C rate, showing an initial Coulombic efficiency (CE) as high as 97.0%. As evidenced, the subsequent discharge-charge profiles exhibit qualitative 12 ACS Paragon Plus Environment

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resemblance to the of the 1st cycle profile, and appealingly, reversible discharge/charge capacities of ~121.9/~123.5 mAh g-1 still can be remained over the following two cycles. Figure 3c-e plot the voltage versus capacity profiles of the NaCrO2 NWs with the constant charge/discharge current rate range from 0.5 C to 50 C in the voltage window of 2.0 to 3.6 V (vs. Na/Na+), which are evaluated with a wide operating temperature range from ‒15 to 55 °C. Strikingly, the plots, no matter in charge or in discharge process, are centralized upon the augment of current rates, as reflected from the profiles at 25 °C (Figure 3c). By contrast, the voltage interval observed from the demanding conditions, i.e., at working temperature as high as 55 °C (Figure 3d) and even a low temperature of ‒15 °C (Figure 3e), is relatively aggravated, which can be ascribed to the change of electrochemical and structural stability of the electrode, as well as Na+ ions mobility.43,

44

Particularly at ‒15 °C, the ionic activity is highly

restrained in the electrolyte solution.45,

46

Furthermore, the rate capabilities of the

NWs cathode form 0.2 C to 50 C are evaluated at ‒15, 25 and 55 °C, respectively. As presented in Figure 3f, the 1D NaCrO2 electrode yields impressive Na-storage capacity retention with average discharge capacities of ~121.5, ~119.6, ~118.6, ~116.8, ~113.7, and ~108.8 mAh g-1 at 0.2, 0.5, 1, 2, 5 and 10 C, respectively, at 25 °C. Of note, with the temperature increasing or decreasing, the capacities turn out to be even smaller. However, the average discharge capacities of ~121.3 at 0.2 C showing an initial CE of 97.8% and ~94.6 mAh g-1 at 10 C still can be obtained for the case of 55 °C. More encouragingly, our NaCrO2 cathode unexpectedly renders 13 ACS Paragon Plus Environment


average capacities of ~108.3 (0.2 C) with an initial CE of 96.9% and ~60.1 (10 C) mAh g-1 at the temperature as low as ‒15 °C. It is amazingly observed that the NaCrO2 NWs electrode at 25 °C still can deliver highly competitive discharge capacities of ~104.9, ~98.7 and ~87.2 mAh g-1 at extremely high current rates of 20, 30 and 50 C, respectively, while ~82.0 (20 C), ~62.2 (30 C) and ~36.5 (50 C) mAh g-1 for the case of 55 °C. It is due to the inferior ion mobility at ‒15 °C that only ~11.6 and ~2.1 mAh g-1 can be reserved at 20 and 30 C rates, respectively. The outstanding high-rate capabilities observed for our NWs cathode should be definitely ascribed to its unique structural advantages, i.e., the localized yet ultrafine nanocrystal-assembled 1D ultralong NWs,47,

48

which ensures superior electrons diffusion coefficient and

prominent Na+-insertion/extraction dynamics, as visually depicted in Figure 3g. Furthermore, the specific 1D nanostructured structure can not only provide convenient current pathway, which is significantly beneficial to electrical transport, but also can accommodate the volume variation in consecutive sodiation/desodiation processes, thus greatly improving Na-transfer kinetics and contributing to high-rate capability, particularly at low temperature.26,

29

Consequently, we compare the

high-rate capabilities of our NaCrO2 NWs with other only two high-rate NaCrO2-based electrodes.18,

23

As distinctly profiled in Figure 3h, the high-rate

capability achieved for the NaCrO2 NWs dramatically precedes those of other bulk NaCrO2 cathodes,18,

23

particularly at ultra-high 20 and 50 C rate. Besides, the

reversible capacities of our NWs cathode are still similar to at the rate < 5 C, and even comparable with those for the NaCrO2@C hybrid cathode at the high rates of 10, 20 14 ACS Paragon Plus Environment

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and 50 C,23 which hugely highlights the enormous commercial application of our NaCrO2 cathode for advanced SIBs. More predictably, we have well-founded reasons to believe that unparalleled electrochemical sodium-storage properties, especially its rate capability when coupled with nano-carbon coating, highly promising its enormous potential as one of the high-power/energy electrode materials for wide-temperature-operating SIBs. Figure 3i demonstrates the cycling behaviors of the resultant NaCrO2 NWs cathode at 2 C rate within a potential range from 2.0 to 3.6 V (vs. Na/Na+) at ‒15, 25 and 55 °C, respectively. Encouragingly, the NaCrO2 NWs electrode delivers discharge capacities of ~115.8, ~113.5, ~107.9 and ~102.4 mAh g-1 at the 1st, 100th, 200th and 300th cycle at 25 °C, respectively, achieving a capacity decay of ~11.6% after 300 consecutive cycles. To the best of our knowledge, this is the best room-temperature cyclic performance among all the NaCrO2-based cathodes, as retrieved in Table 1. More significantly, our NaCrO2 NWs also exhibits a remarkable capacity retention even at high (55 °C) and low (‒15 °C) temperatures after the same cycles, which is also the first relevant investigation, showing capacity degradations of ~13.1 % at 55 °C and ~19.4% at ‒15 °C, respectively, indicative of its superior long-duration cyclic stability within the wide operating temperature range. Furthermore, the full sodium-ion cell, which is constituted by the presodiated HC anode and our NaCrO2 NWs cathode, as schematically shown in Figure 4a, has been assembled and tested to evaluate the Na-storage properties of NaCrO2 NWs at 25 °C. The initial charge and discharge profiles (green curves) of full cell in the voltage 15 ACS Paragon Plus Environment


range of 1.8 ‒ 3.4 V at 0.1 C rate along with the typical charge/discharge profiles of presodiated HC versus Na metal (0 ‒ 2.5 V, 0.1 C) are exhibited in Figure 4b. As distinctly shown, the full cell can yield a discharge capacity of ~112.8 mAh g-1 with an initial CE as high as ~96.2%, and an average working voltage of ~2.90 V in the discharging process can be observed. Typically, the mass loading of NaCrO2 NWs cathode and HC anode before pretreatment in the full cell are separately 2.9 mg and 1.3 mg. By employing the accepted 40% penalty factor to account for the weight of the electrolyte and of the auxiliary components (case, current collectors, and so on),49 a competitive energy density of ~161 Wh kg-1 of the full cell can be obtained.50 The voltage profiles of full cell presented in Figure 4c corroborate that the full cell can be effectively cycled via charge at 0.2 C and discharge at various current rates from 0.2 to 30 C. Moreover, with the current rate increasing, the discharge capacities decrease slowly and retain stable cycling stability upon continuous cycling (Figure 4d). What is particularly worth noting is that even at high rates of 20 and 30 C, significantly high capacities of ~88.2 mAh g-1 (78.2% of 0.1 C capacity) and~82.6 mAh g-1 (73.2% of 0.1 C capacity), respectively, can be achieved, demonstrating the outstanding electrochemical reaction kinetics of NaCrO2 NWs//HC full cell. In addition, a digital photo for the assembled full cell device is vividly exhibited in the inset in Figure 4d, which has an external dimension of about 6 × 5 cm, and a thickness of less than 1 mm. To understand the immanent origin for the outstanding electrochemical properties achieved here, post-mortem examination into the cycled electrodes is 16 ACS Paragon Plus Environment

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accordingly carried out. Figure 5a displays representative TEM image of the cycled NaCrO2 NWs after 300 cycles at 2 C at 25 °C. As distinctly shown in Figure 5a and corresponding FESEM image (Figure S7), the ultralong and continuous NWs are still well kept up without the discernable pulverization and aggregation after successive charge/discharge

process,

revealing

the

superior

structural

tolerance

and

morphological sustainability of our NaCrO2 NWs. Furthermore, close observation (Figure 5b) reveals that the layered-type NaCrO2 phase with an eye-catching (003) plane of a spacing of ~0.528 nm is finely remained. As a consequence, the upgraded sodium storage properties of NaCrO2 NWs, especially for the high-rate and cyclic performance, can be definitely endowed. As previously reported, a layered-to-rock-salt transformation caused by the Cr4+ disproportionation/comproportionation and the formation of a metastable intermediate O3 CrO2 phase always occur during the Na+ ions extraction and insertion.37, 51 The insights into the nanocrystallization on structural evolution in layered NaCrO2 are further investigated fundamentally via ex situ XRD technique. A charge current density of 0.1 C and a cut-off voltage of 3.6 V are employed for the ex situ XRD experiment. Figure 5c shows the typical voltage profile of the first desodiation process plotted as a function of Na+ content x in Na1-xCrO2. The ex situ XRD patterns associated with different charged states are plotted in Figure 5d, and corresponding enlarged diffraction patterns for the 2θ regions of 10 ‒ 19 °, 30 ‒ 39 °, and 40 ‒ 46 ° are collected in Figure 5e. As noted, continuous shift of (003) peak to the lower angles accompanying with the consecutive shift to higher angles as for (101) during 17 ACS Paragon Plus Environment


the whole desodiation process, as shown in Figure 5e, reveals that a solid solution reaction is induced with c-axis expansion.52, 53 In addition, the (006) reflection at 2θ = ~33.8 ° completely disappears, and the original (104) reflection at ~41.6° decreases in intensity and gradually becomes broader in peak shape, which also signifies a phase transition from an O3-type to a P3-type structure.24, 37 After elaborative examination into the change of (104) peak, it can be discovered that a new peak emerges at the right side of the initial (104) peak, suggesting that the material begins to form a monoclinic O3 phase as the charge voltage reaches to 3.0 V. At this stage, the original hexagonal O3 phase coexists with the emerging monoclinic O3 phase. When charged to 3.1 V, the new peak related to monoclinic O3 phase dominates the structure while the original hexagonal O3 phase vanishes, suggesting the complete phase transformation from hexagonal O3 to monoclinic O3 phase. In the meantime, another new peak at 2θ = ~44.5° can be detected with increasing intensity upon further charging, which can be ascribed to the formation of new monoclinic P3 phase. Accordingly, the coexistence of monoclinic O3 and P3 phase occurs at this stage. When further charged to 3.6 V, the broad (104) peak is wholly disappeared and the peak emerged at 44.5° is inclined to shift to the left, revealing a single monoclinic P3 phase.52,

54

Moreover, the (012) peak, standing for the rhombohedral phase, is

separated into two feeble peaks, which is also a cogent signature of the rhombohedral to monoclinic transformation.17, 51 Based on the structural analysis above, the phase transition progresses for the 1D NaCrO2 NWs evolve in the sequence from hexagonal O3 to monoclinic O3, and further to monoclinic P3 over charging, which is consistent 18 ACS Paragon Plus Environment

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with the previous investigations of the bulk NaCrO2 cathodes.24, 37, 51 CONCLUSIONS To sum up, we rationally designed and developed a 1D ultralong NaCrO2 NWs for the first time via scale-up electrospinning technique coupled with subsequent calcination. Our 1D nano-engineering design rendered several important merits required for advanced cathodes for high-performance SIBs. First, the ultrafine NaCrO2 nanocrystals subunits congenitally guaranteed high kinematics and electrochemical utilization for efficient sodium storage, especially at high rates. Secondly, the average diameter of ~48 nm and 1 D ultralong feature ensured the large electrode/electrolyte sur-/interfaces and reinforced structural tolerance, and facilitated the orientated and shortened electronic and ionic transport in electrodes. Finally, the randomly entangled NWs of ~30 μm in length formed a three-dimensional porous network, rendering easy electrolyte penetration and low diffusion resistance to ionic species. When functioned as a competitive cathode candidate for advanced SIBs, a reversible capacity as high as ~122.6 mAh g-1 at 0.1 C rate was obtained at 25 °C. More strikingly, the pathbreaking NaCrO2 NWs yielded astonishing high-rate capacity (~87.2 mAh g-1 at 50 C) and superior capacity retention of 88.4% after 300 cycles at 2 C. Furthermore, high-rate capacities and cyclic stabilities were also achieved both at 55 °C and ‒15 °C. Moreover, a remarkable energy density of ~161 Wh kg-1 and discharge capacity of ~112.8 mAh g-1 at 0.1 C can be obtained by the assembled NaCrO2 NWs//hard carbon (HC) full device at 25 °C. More significantly, the insights into the phase shift of our NaCrO2 NWs from the hexagonal O3 to 19 ACS Paragon Plus Environment


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monoclinic O3, and to monoclinic P3 over charging were put forward with ex situ XRD investigation. Expectedly, our novel electrode design concept will pave a meaningful

way

for

future

design

of

nanoscaled

NaCrO2

cathodes

for

wide-temperature-operating high-energy/power SIBs.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . FESEM images, TG data, digital images, TEM and HRTEM images of the controlled experiments (PDF)

AUTHORINFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] (Prof. Changzhou Yuan)

ORCID Changzhou Yuan: 0000-0002-6484-8970

Notes The authors declare no competing financial Interest.


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ACKNOWLEDGEMENTS The authors acknowledge the financial support from National Natural Science Foundation of China (No. 51502003, 51772127, 51772131), Taishan Scholars (No. ts201712050), Major Program of Shandong Province Natural Science Foundation (ZR2018ZB0317) and Natural Science Doctoral Foundation of Shandong Province (ZR2018BEM018).



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Pyo,

M.

Reversible

K+-Insertion/Deinsertion 27

ACS Paragon Plus Environment

and

Concomitant


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Scheme 1. Schematic illustration of the electrospinning synthetic process for 1D NaCrO2 NWs



Figure 1. (a) Typical XRD pattern and corresponding Rietveld refinement profiles, (b) XPS survey and typical high-resolution elemental (c) Cr 2p and (d) O 1s spectra of the as-obtained ultralong NaCrO2 NWs. The inset in panel (a) is for the crystallographic structure of the NaCrO2


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Figure 2. (a) Low- and (b) high-magnification FESEM images, (c-e) TEM images with various magnification, (f) HRTEM image, (g) SAED pattern, (h) STEM image and corresponding elemental (i, Na; j, Cr and k, O) EDS mapping images for the NaCrO2 NWs. The image in panel (b) is taken from the white rectangle region in panel (a). The images in panels (d) and (e) show the magnified ones of the white rectangle regions in panel (c) as indicated



Figure 3. (a) CV curves (0.1 mV s-1), (b) initial three charge/discharge profiles at 0.1 C rate, (c-e) typical charge/discharge profiles at constant charge/discharge current rates ranging from 0.2 to 50 C at various temperatures (c) 25 °C, (d) 55 °C and (e) ‒15 °C, (f) rate behaviors at various operating temperatures as indicated, (g) schematic illustration for the electron and Na+ ions transfer pathway, (h) comparisons in the rate capabilities with other high-rate NaCrO2 electrodes at 25 °C, and (i) long cycle stability performed at 2 C for 300 cycles at different temperatures for the NaCrO2 NWs cathode. Note that EES in panel (h) refers to the acronym of “Energy & 32 ACS Paragon Plus Environment

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Environmental Science”

Figure 4. Electrochemical performance of the NaCrO2 NWs//HC sodium-ion full cell. (a) Schematic illustration for the fabrication of the sodium-ion full cell. (b) The initial charge/discharge curve at 0.1 C rate over the potential range of 1.8 – 3.4 V along with the typical charge/discharge profiles of presodiated HC versus Na metal (0 ‒ 2.5 V, 0.1 C). (c) Charge/discharge voltage profiles: charged at 0.2 C (except for the 0.1 C), discharged at various current rates from 0.2 to 30 C. (d) Rate behaviors at various rates as indicated. 33 ACS Paragon Plus Environment


Figure 5. (a) TEM and (b) HRTEM images for the cycled NaCrO2 NWs at 25 °C after 300 cycles. (c) Voltage-composition profile, (d) ex situ XRD patterns collected during the first charge process (up to 3.6 V at 0.1 C rate) and (e) corresponding enlarged diffraction patterns within the various 2θ regions as indicated for a NaCrO2/Na half cell 34 ACS Paragon Plus Environment

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Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Sample 29 30 NaCrO2 31 32 33 NaCrO2 34 35 NaCrO2 36 37 38 NaCrO2 39 40 NaCrO2 41 42 43NaCrO2@C 44 45 NaCrO2 46 47 48NaCrO2@C 49 50 51 NaCrO2 52 53 54 55 56 57 58 59 60


Table 1 Comparison of the results in this study with all previously reported NaCrO2 based cathodes involving particle size, rate capability and cyclic stability at 25 °C Synthesis

Particle size

High-rate capability

Cyclic stability

Ref.

SSR

10 μm

63.0 mAh g-1 at ~20 C

~112 mAh g-1 after 100 cycles at 125 mA g-1 (2.5-3.5 V)

18

SSR

1~3 μm

–

–

19

g-1

SSR

3 μm

–

SSR

1~5 μm

–

SSR

~600 nm

100 mAh g-1 at ~20 C

SSR

200 nm~3 μm

ED

500 nm

3 mAh g-1 at ~20 C

ED

~500 nm

~113 mAh g-1 at ~20 C 106 mAh g-1 at ~50 C

~100 mAh after 50 cycles at 250 mA g-1 ~90 mAh g-1 after 50 cycles at 12.5 mA g-1 (2.5-3.6 V) ~65 mAh g-1 after 300 cycles at 100 mA g-1 ~110 mAh g-1 after 40 cycles at 5 mA g-1 ~80 mAh g-1 after 50 cycles at 20 mA g-1 ~110 mAh g-1 after 300 cycles at 20 mA g-1

ES

~48 nm in diameter

105 mAh g-1 at ~20 C 87 mAh g-1 at ~50 C

~103 mAh g-1 after 300 cycles at 200 mA g-1


20 37 31 22 23 23 This work



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