Research Article www.acsami.org
FeF3@Thin Nickel Ammine Nitrate Matrix: Smart Configurations and Applications as Superior Cathodes for Li-Ion Batteries Jian Jiang,*,†,§,∥ Linpo Li,†,§,∥ Maowen Xu,†,§ Jianhui Zhu,*,‡ and Chang Ming Li†,§ †
Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P.R. China ‡ School of Physical Science and Technology, Southwest University, Chongqing 400715, P.R. China § Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, P.R. China S Supporting Information *
ABSTRACT: Iron fluorides (FeFx) for Li-ion battery cathodes are still in the stage of intensive research due to their low delivery capacity and limited lifetime. One critical reason for cathode degradation is the severe aggregation of FeFx nanocrystals upon long-term cycling. To maximize the capacity and cyclability of these cathodes, we propose herein a novel and applicable method using a thin-layered nickel ammine nitrate (NAN) matrix as a feasible encapsulation material to disperse the FeF3 nanoparticles. Such core−shell hybrids with smart configurations are constructed via a green, scalable, in situ encapsulation approach. The outer thin-film NAN matrix with prominent electrochemical stability can keep the FeF3 nanoactives encapsulated throughout the cyclic testing, protecting them from adverse aggregation into bulk crystals and thus leading to drastic improvements of electrode behaviors (e.g., high electrode capacity up to ∼423 mA h g−1, greatly prolonged cyclic period, and promoted rate capabilities). This present work may set up a new and general platform to develop intriguing core−shell hybrid cathodes for Li-ion batteries, not only for FeFx but also for a wide spectrum of other cathode materials. KEYWORDS: FeF3@NAN matrix, hybrid cathode, smart configurations, Li-ion batteries, green synthesis
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INTRODUCTION
Despite these overwhelming advantages, worldwide research on Fe-based fluoride cathodes have been unfortunately held back. The major reasons involve the intrinsically slow ionic diffusion and poor conductivity of fluoride species, which lead to inferior rate capabilities, quite low active utilization efficiencies, very poor specific capacities, and limited cyclic lifespans.9,12−15,17 Though the use of novel electrode-design concepts, mainly by making various FeFx nanomaterials (e.g., nanowires, nanocrystals, etc.), it may be possible to circumvent the above shortcomings and enhance electrochemical kinetics through increased active reacting sites and accelerated Li+ diffusion. However, their cathode performance in LIBs is still far from meeting the criteria of practical applications.17 This is mainly caused by detrimental pulverizations and agglomerations of FeFx nanocrystals during repeated phase conversions and changes. On one hand, FeFx nanocrystals would become highly distorted and gradually lose electrical contacts to current collectors, thereby leading to rapid capacity decay upon deep cycling. On the other hand, FeFx nanocrystals may slowly dissolve into polar organic electrolytes and then deposit on electrodes during charging procedures.9 Such side reactions
Development of low-cost and earth-abundant materials into outstanding electrodes is considered to be the most rational way to overcome current issues of energy storage applications. A typical example is the case of turning manifold iron-based materials (e.g., Fe2O3, Fe3O4, FeS2, FeO, FeS, FeP, etc.) into electrode alternatives for asymmetric supercapacitor (SC) and Li-ion battery (LIB) applications because Fe is the fourth most rich element on earth.1−8 Such varieties of Fe-based species that exhibit relatively high energy densities when used in rechargeable power sources thereby possess evident price superiority over other transition-metal (e.g., Co, Cu, Ti, Mn, V, etc.)-based counterparts.9−11 As one of the classic categories, iron fluorides (FeFx) have been paid special attention over the past few decades because they are promising high-capacity and economic cathode candidates to replace the commercial lithium cobalt oxide (LiCoO2).12−16 For example, a single FeF3 molecule with superb thermal/electrochemical stability has the theoretical capability to react with three Li+, delivering a large specific capacity of 712 mA h g−1 at an average potential of ∼2.7 V and enabling an energy density of 1950 Wh kg−1, much higher than that of other mainstream cathodes (e.g., lithiated transition-metal oxides and olivine compounds are < 550 Wh kg−1).12−15 © XXXX American Chemical Society
Received: April 2, 2016 Accepted: June 7, 2016
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DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) General schematic displaying the entire evolution of the FeF3@NAN matrix. The inset SEM image shows the typical morphological feature of thin NNH layers. SEM observations of samples at distinct evolution stages: (b and c) (NH4)3FeF6 precursors, (d and e) (NH4)3FeF6@ NNH intermediates, and (f and g) FeF3@NAN hybrids.
LIBs. The products are synthesized via in situ encapsulation of (NH4)3FeF6 nanosized precursors into NNH layers in an ethanol solvent followed by a mild heating post-treatment. Note that (NH4)3FeF6 nanomaterials can be readily and abundantly produced via a simple coordination reaction between NH4F and Fe(NO3)3 at a low temperature of 70 °C; during the entire synthetic process, no hazardous chemicals (such as HF) are used. When serving as the cathode, the resulting hybrid of the FeF3@NAN matrix is capable of exhibiting a cathodic performance superior to that of bare FeF3 during battery testing with high electrochemical activity and capacity (maximum at ∼421.5 mAh g−1), excellent cyclability within a total of 400 cycles, and good rate capabilities. This work not only presents a green, novel, and scalable method for preparation of high-quality FeFx-based nanohybrids for LIBs but also offers a smart, valuable, and preferable way to promote the study of energy-storage behaviors of cathode species.
would make FeFx nanoparticles aggregate into bulk formations, which is adverse to the solid-state diffusion of Li+ and might cause dead trapping of some lithium in the FeFx lattices. The fabrication of hybrid FeFx@functionalized matrixes (FeFx@ FMs; FMs mainly include carbons, conducting polymers, or other inert/stable species 18−21 ) that can disperse FeF x nanoparticles and inhibit their negative dissolution may properly address these severe aggregation issues.22 However, this synthesis goal cannot be achieved by traditional aqueous approaches because nearly all Fe-based fluorides are highly water soluble. Moreover, the highly toxic and corrosive hydrofluoric acid (HF) is often utilized as major fluorine source for large-scale preparation of FeFx cathodes.13,14,23−25 Direct and massive use of hazardous HF for industrial production, in a long-term perspective, is undoubtedly inadvisable and should be totally eliminated. To overcome all of the above obstacles and upgrade the performance of FeFx cathodes in LIBs, a green and scalable synthesis of nanosized core−shell FeFx@FM hybrids has been urgently pursued. In previous reports, the thin nickel nitrate hydroxide (Ni3(NO3)(OH)4, denoted as NNH, average thickness per layer: ∼7 nm) has been confirmed as a feasible and effective encapsulation material for Li-S batteries due to its good semiconducting property and relatively stable electrochemical behavior over a wide potential range.26 Enlightened by this study, we developed a novel, simple, and environmentally benign strategy for the scalable preparation of a core−shell hybrid of a FeF3@thin nickel ammine nitrate (Ni3(NO3)2(NH3)6; NAN) matrix to use as the cathode for
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EXPERIMENTAL SECTION
Synthesis of FeF3@NAN Hybrids. Thin NNH layers were prepared according to prior literature procedures22,23 and directly used as the starting materials. In detail, a suspension solution containing 0.45 g of hexamethylenetetramine (HMT, C6H12N4), 0.2 g of Ni(NO3)2·6H2O, and 40 mL of distilled water was transferred into a sealed container (80 mL) and held at 95 °C for 6 h. The as-formed NNH products were washed with distilled water several times, collected by vacuum filtration, and uniformly dispersed into 45 mL of absolute ethyl alcohol (EA) by ultrasonication treatment. Afterward, 0.35 g of Fe(NO3)3·9H2O and 0.6 g of NH4F salts were added. The resulting mixture was then treated by ultrasonication for 10 min, B
DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) XRD patterns of (NH4)3FeF6 precursors and final FeF3@NAN products. (b−e) TEM images, (f) EDS detecting, and (g−l) elemental mappings for the hybrid of FeF3@NAN. transferred into a 100 mL sealed glassy container, and kept at 70 °C for 12 h. The floccus (NH4)3FeF6@NNH products were collected, washed with EA, and dried at 80 °C in a vacuum oven. The fabrication of FeF3@NAN nanohybrids was carried out in a horizontal quartz tube-furnace system. The (NH4)3FeF6@NNH intermediates created in situ were loaded into an alumina boat and then put in the center of a quartz tube. Prior to being heated, the quartz tube reactor was sealed and flushed with Ar gas (200 sccm) for 10 min. The furnace was then heated to 260 °C at a heating rate of ∼10 °C/min under a constant Ar flow of 50 sccm, held for 1 h, and allowed to cool to room temperature naturally. Synthesis of Pure FeF3 Nanoparticles. Pure FeF3 nanoparticles were synthesized by following the same synthesis procedures mentioned above, except that thin NNH films were not added into the EA solvent. Characterization Techniques and Battery Testing. The morphology and crystalline structure of the as-made products were characterized with a JEOL JSM-7800F field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) and a JEM 2010F high-resolution transmission electron microscope (HRTEM). X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) (Thermo Electron, VG ESCALAB 250 spectrometer) was also used to characterize the products. Thermogravimetric analysis (TGA) was performed on an SDT600 apparatus under a heating rate of ∼10 K/min in a N2 atmosphere. The mass of electrode materials was measured on a microbalance with an accuracy of 0.01 mg (A&D Company N92). The working electrode of the FeF3@NAN hybrids was fabricated by the conventional slurry-coating method. In detail, FeF3@NAN powders, poly(vinylidene fluoride) (PVDF) binder, and acetylene black were mixed in a mass ratio of 80/10/10 and dispersed in N-methyl-2pyrrolidone (NMP) to form slurries. The homogeneous slurry was then pasted onto an Al film (thickness: 0.5 mm) and dried at 100 °C for 10 h under vacuum. The mass loading on each current collector was controlled at 1.5−2.5 mg/cm2. Electrochemical measurements were performed using CR-2032 coin-type cells within a potential range of 1.0−4.5 V. Cells were assembled in an Ar-filled glovebox (Dellix LS800S; H2O < 0.1 ppm, O2 < 0.1 ppm) using Li foil as the counter
and reference electrode. LiPF6 (1 M) dissolved in a 1/1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. The electrochemical impedance spectroscopy and cyclic voltammetry measurements were performed on an electrochemical workstation (CorrTest CS310), and the galvanostatic charge/discharge tests were conducted using a specific battery tester (Neware). Before battery testing, all cells were aged for 8 h.
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RESULTS AND DISCUSSION Characterizations of FeF3@NAN Hybrids. Figure 1a clearly shows schematics for the overall synthesis of FeF3@ NAN hybrids. The initial step involves the ultrasonic dissolution of NH4F and Fe(NO3)3 salts into an EA solvent, wherein NNH layers were evenly dispersed beforehand. As reflected by Figure 1a, NNH films are intrinsically soft, ultrathin, flexible, and chemically stable in a neutral EA solution, possessing an intriguing graphene-like two-dimensional (2D) architecture. Such intrinsic properties are believed to be essential and fundamental prerequisites for subsequent hybrid fabrication procedures. Also note that among NNH layers, a great deal of empty space is distributed, supplying sufficient room for later in situ nucleation of (NH4)3FeF6 nanocrystals. The (NH4)3FeF6@NNH intermediates were then generated and collected after a 12 h reaction at 70 °C. This hybrid construction is greatly aided by functional hydrophilic groups (e.g., -OH, -NH2, -F) with which feasible, favorable, and tight chemical interactions and bonds would form between NNH and (NH4)3FeF6 nanocrystals. In the following step, samples were then calcined under Ar flow protection, eventually evolving into FeF3@NAN core−shell hybrids. The heating temperature of 260 °C was eventually determined by referring to TG and differential scanning calorimetry (DSC) measurements (Figure S1) on (NH4)3FeF6 in a N2 atmosphere. The TG curve combined with DSC analysis reflects that two main thermal decomposition stages are located at ∼207 and ∼252 °C (accompanied by mass remains of ∼53% and ∼24%, C
DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a) CV scan, (b) long-term cyclic performance, and (c) charge/discharge profiles of the FeF3@NAN hybrid cathode in a potential window of 1.0−4.5 V (vs Li/Li+). For comparison, cyclic behaviors for bare FeF3 are also presented. (d−g) Differential capacity plots (dQ/dV) of the FeF3@ NAN hybrid electrode at cycles 1, 100, 150, and 300. (h) EIS spectra for both electrodes (black: FeF3@NAN hybrid; gray: pure FeF3). (i) Rate behaviors of the FeF3@NAN hybrid.
Morphological observations in Figures 1b and c evidently reveal that (NH4)3FeF6 precursors have a sphere-like geometric feature. Their average diameter is statistically distributed in a range of 80−150 nm. TEM images in Figure S2 further disclose the structural characteristics of (NH4)3FeF6 nanoparticles. It is interesting to note that each individual (NH4)3FeF6 nanosphere is made up of many tiny nanocrystals (the size of each nanocrystal unit is centered at ∼8 nm). This structure formation may be tightly associated with unique crystalline growth and self-assembly properties of (NH4)3FeF6 in nonaqueous EA solvent. Figures 1d and e show the top-view SEM images of (NH4)3FeF6@NNH. Both large-area and zoomed-in SEM results confirm that such in situ derived intermediates have a well-defined core−shell hybrid configuration; nearly every (NH4)3FeF6 nanosphere has been perfectly packaged into the thin-film NNH matrix. Figures 1f and g display SEM observations toward the ultimate products of FeF3@NAN. Overall, the core−shell hybrid architecture was not altered much even if the samples were subjected to the heat treatment. However, the outer film matrix shrunk and became rough, which could be caused by its chemical composition changes and mass losses during the thermal procedure. The crystal structure and phase purity of samples at distinct evolution stages were examined by XRD. Diffraction peaks located at 2θ of 16.8°, 19.5°, 27.7°, 39.6°, 48.9°, and 52.1° in the XRD pattern (Figure 2a, blue) are in good agreement with cubic ammonium iron fluoride (JCPDS card no. 22-1040),
respectively), corresponding to stepwise deamination processes of (NH4)3FeF6, which guides us to select ∼260 °C as the proper and economic heating temperature to convert (NH4)3FeF6 precursors into FeF3. This preparation protocol is based on the following major points. First, our FeF3 synthetic strategy is more environmentally friendly in comparison to conventional approaches; no hazardous chemicals are used during the total fabrication flow. Second, NAN layers are uniformly and conformally covered on nanoscale FeF 3 products, preventing unfavorable dissolution and loss of Febased fluorides into the electrolyte phase. This encapsulation would not drastically impede the electrolytic infiltration and Li+ diffusion because such film matrixes are quite thin and penetrable. The intimate contacts between the inner FeF3 core and outer NAN shell may guarantee the electron-transfer properties of the electrode. Moreover, upon Li+ insertion/ deinsertion, each single FeF3 nanounit in the NAN matrix can better accommodate volume changes, thereby preventing the adverse aggregation of nanosized FeF3 into bulky crystals. Third, the massive production of FeF3 nanomaterials can be realized by annealing (NH4)3FeF6 precursors that are easily and richly formed in EA solvent via coordination interactions between iron salts and NH4F complexing agent: 6NH4F + Fe(NO3)3 → (NH4)3 FeF6 + 3NH4NO3
To gain insight into the synthesis procedure, the entire fabrication was purposely monitored by SEM (Figures 1b−g). D
DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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A couple of peaks appear at ∼2.32 and ∼3.34 V when the electrode is scanned in reverse from 1.0 back to 4.5 V, corresponding to stepwise phase transformations of metallic Fe0/Fe2+ and Fe2+/Fe3+, respectively. Figure 3b compares the cyclic behaviors between bare FeF3 nanopowders and FeF3@ NAN core−shell products under a current rate of 0.1 C (∼71 mA g−1; 1 C = 712 mA g−1). Overall, bare FeF3 nanopowders exhibit very poor cathodic behaviors, including inferior electrochemical stability, rapid decay in specific capacity, low Coulombic efficiency, and short cyclic lifetime. The initial discharge capacity achieves 598 mA h g−1 but immediately falls to the level of ∼276 mA h g−1. Afterward, the reversible capacity continues to decline to a low capacitive value (less than ∼80 mAhg−1 after 170 cycles). The drastic fading in capacity and Coulombic efficiency is mainly ascribed to formidable kinetic issues of metallic fluorides.9,13−15 By contrast, the FeF3@NAN cathode shows much better charge/discharge performance over 400 cycles. The discharge capacity of the FeF3@NAN electrode in the beginning drops from the original ∼426 mA h g−1 to a bottom level of ∼194 mAh g−1 (cycle 7), which is mostly due to delayed electrolyte infiltration into a well-capsulated structure, inevitable nanoactive volume expansions of FeF3 (leading to Li+ trapping in FeF3 lattices/ incomplete reconversion), and its disruption or local structural rearrangements.17 Over the next 100 cycles (cycles 10−110), the output for the FeF3@NAN hybrid was stabilized at ∼215 mA h g−1. Later, the capacity rose progressively to a maximum reversible value of ∼423 mA h g−1 (cycle 297). No similar capacity rise phenomena occurred on the bare FeF3 cathodes. We believe that the capacitive growth might be related to the partial structural damage and material fatigue of the outer NAN layers because films breaking may offer new open-up places, release some trapped FeF3, and facilitate Li+ to reach inner deep regions of the whole cathode. To study in depth the electrochemical properties of the FeF3@NAN hybrid electrode and better understand this capacity rise phenomenon, we purposely investigated the charge/discharge curves at cycles 1, 100, 150, and 300 (Figure 3c). Their corresponding differential capacity (dQ/dV) plots are also analyzed and presented in Figures 3d−g. In the first discharge, the potential reduces steeply from the open-circuit value to ∼1.42 V, wherein a plateau region sets in and continues until a specific capacity of ∼426 mA h g−1 is achieved. During the reverse charge (delithiation) step, two plateaus successively appear at ∼2.31 and ∼3.41 V, which are ascribed to complex and stepwise phase conversions of Fe/FeFx (0 < x ≤ 3).14 At cycle 100, the hybrid electrode still shows polarization/depolarization behaviors analogous to the first, capable of delivering a reversible capacity of ∼225 mA h g−1 (Figure 3e). The peak signals clearly broaden and increase, indicative of the onset of capacity growth for the hybrid electrode system. This original capacity rise may stem from the gradual activation of inner FeF3 actives along with continuous cycling. Afterward, the charge/discharge capacity begins to revive in subsequent cycles. In the dQ/dV plot for cycle 150, both anodic and cathodic peak intensities become pronounced. Note that the cathodic peak is centered at ∼1.65 V with a large peak shift (∼0.17 V) to the positive direction when compared to that of the initial case. The corresponding capacity of FeF3@ NAN hybrids continually grows to a maximum capacity value of ∼423 mA h g−1 when the electrodes run at around cycle 300. The dQ/dV plots in Figure 3g show the generation of two reduction peaks. The intense, sharp peak lying at ∼1.91 V may
corresponding to facets (111), (200), (220), (400), (422), and (511), respectively. The presence of intense diffraction signals indicates that the products of (NH4)3FeF6 exhibit good crystalline properties, though they are generated at a low temperature of 70 °C. Peaks appearing at 25.8°, 33.0°, and 42.4° are successively indexed to (002), (100), and (102) facets of hexagonal NNH film (JCPDS card no. 22-0752), which have records in line with results in previous literature.26,27 Figure 2a (black) shows the XRD pattern of derived FeF3@NAN hybrids; it appears that all diffraction peaks are highly consistent with cubic FeF3 (JCPDS card no. 38-1305) except for signals coming from NAN (JCPDS card no. 45-0027). With respect to the formation mechanism of the FeF3@NAN hybrid in the calcination process, the involved chemical reactions might be expressed by following formulas: (NH4)3 FeF6 → 3NH3 + FeF3 + 3HF
Ni3(NO3)2 (OH)4 → Ni(NO3)2 + 2NiO + 2H 2O 3Ni(NO3)2 + 6NH3 → Ni3(NO3)2 (NH3)6
TEM observations were further used to characterize the hybrid nature of FeF3@NAN. The sharp contrast in Figure 2b clearly reveals its intrinsic yolk−shell configuration and verifies our success in packaging FeF3 into thin NAN matrix. To gain insight into the interfaces between the inner FeF3 core and outer NAN shell, TEM and HRTEM examinations were performed purposely on the specific edge region (Figures 2c− e). Distinctly, both evolved FeF3 and NAN possess polycrystalline characteristics. The lattice fringes (yellow) with a spacing of 0.29 nm in Figure 2d correspond to the (222) facet of FeF3, while the ones (white) with a lattice spacing of 0.38 nm are well-indexed to the (220) plane of NAN, which highly corroborates our former XRD analysis. EDS and elemental mapping were also used to verify the hybrid composition of FeF3@NAN. Figures 2f and h−l illustrate that the synthesized products contain N, O, F, Ni, and Fe (the Al signal comes from the loading substrate); no other foreign elements are involved within the whole preparation flow. The EDS elemental mapping on a designated area of FeF3@NAN hybrids (see SEM image in Figure 2g) unquestionably discloses the largescale homogeneous element distribution and uniform NAN encapsulation (not gathering in local regions). The encapsulation of nanoscale FeF3 into a protective NAN matrix renders this system intriguing for LIB application. Figure 3a shows the CV test conducted at a slow scan rate of 50 μV s−1 in the voltage window of 1.0−4.5 V (vs Li/Li+) at ambient temperature. Within the first cathodic (lithiation) scan, an intense reduction peak appears at a lower potential of 1.23 V, which may be attributed to slower reaction kinetics between Li+ and confined FeF3 (causing delayed electrolyte penetration) and possible interfacial reactions with NAN.13−15 Such a sluggish, unpleasant reaction was clearly alleviated in subsequent scans. The reduction potential was observed at ∼1.41 (scan 2) and ∼1.60 V (scan 5) with a considerable potential shift to the positive direction. These values are comparable with previous results obtained on FeF3 nanocomposite electrodes.11 Consistent anodic peak positions were exhibited in the following cycles, demonstrating the reversibility of lithiation/delithiation conversion reactions:9 FeF3 + 3Li ↔ Fe + 3LiF E
DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces combine the Fe3+/Fe2+ and Ni3+/Ni2+ transitions, while the other is indexed to Fe2+/Fe0.14 Further, there are also a couple of anodic peaks emerging at ∼2.25 and ∼3.31 V, corresponding to three-electron transfer conversion reactions of Fe0/Fe2+ and Fe2+/Fe3+, respectively, indicative of a great improvement of the electrode kinetics. In this cyclic period, we believe that the robust and thin NAN “armors” worn by the FeF3 nanoparticles may play a significant role not only in the accommodating huge volume changes caused by Li+ insertion/extraction but also in protecting them against detrimental aggregation effects. This guarantees the good electrochemical stability and prolonged cyclic lifetime in contrast to those of bare FeF3 electrodes. Figure 3h presents the electrochemical impedance spectra (EIS) for both FeF3 and FeF3@NAN electrodes at an opencircuit voltage after 400 cycle. The hybrid electrode of FeF3@ NAN, albeit with an exterior encapsulation of NAN films, exhibits a much smaller semicircle diameter among the highfrequency region. After curve fitting, the charge-transfer (Rct) resistance value for FeF3 and FeF3@NAN electrode systems is measured to be ∼265 and ∼849 Ω, respectively. Also, the bulk electrode resistance (Rb; recorded at 0.1 M Hz) of FeF3@NAN (∼127 Ω) is much lower than that of pure FeF3 (∼239 Ω). These EIS results suggest that, after sufficient electrolyte infiltration, the integrated NAN matrix can indeed facilitate electron-transfer properties and thereby upgrade the overall electrode kinetics. In addition, the rate capability of preactivated FeF3@NAN electrodes were also evaluated. Figure 3i shows the rate performance of the FeF3@NAN electrode conducted at varied current rates from 0.1 to 2.5 C. When the current densities were increased, the hybrid electrode was able to deliver total capacities of ∼423 mA h g−1 (0.1 C), ∼287 mA h g−1 (0.3 C), ∼193 mA h g−1 (0.8 C), ∼116 mA h g−1 (1.6 C). and ∼79 mA h g−1 (2.5 C). These values are comparable or even far greater than those in literature on FeF3-based hybrid cathodes (e.g., graphene/FeF3 cathode, etc.).13−15,24,25,28 We furthermore compare the energy density between the FeF3@ NAN hybrid and commercial LiCoO2 cathode (Figure 4). After
tions, especially given the factors of manufacturing cost and cell safety. To make the relationship between the hybrid configuration and cell performance clear, SEM and TEM detections were performed based on the disassembly of cycled electrodes. Figure 5a manifests the typical SEM image of pure FeF3 electrodes after 170 charge/discharge cycles. Unlike the pristine FeF3 nanoparticles distributed in a narrow diameter range of ∼100−200 nm, nearly all of the electrode materials were changed into huge microbulks (average size: ∼5 μm). This may be attributed to unfavorable FeF3 nanocrystal aggregations and growth during repeated lithiation/delithiation processes. However, the hybrid cathode material, after going through 400 cycles, still preserved a morphology and size identical to those of the original FeF3@NAN products. Also note that each individual FeF3@NAN unit remained highly dispersive, implying the outer NAN shell is capable of protecting the inner FeF3 core against undesired bulk aggregations. The contrast in the TEM observations (Figures S3a and b) unquestionably proves the fact that the cycled FeF3@NAN electrode also remains a core−shell hybrid nanostructure. Nevertheless, it is obvious that, after a long cyclic period, this outer NAN shell becomes much more porous in comparison to that in the pristine case (compact and smooth), which should be induced by repeated Li+ insertion/desertion processes. Electrode materials have almost been present in an amorphous state except for some nanodomains (Figure S3c). Whereas, without NAN matrix protection, naked FeF3 nanoactives (size: ∼90 nm) are prone to aggregate into bulk forms when suffering from phase transitions of Fe3+/Fe0, as reflected by TEM images in Figures S3d−f, which induces a remarkable degradation of electrode performance (e.g., large initial capacity losses, rapid capacity decay, poor cyclic behaviors, etc.) because such agglomerations are quite detrimental to the electrolyte penetration, solid-state Li+ diffusion, reversibility, etc. EDS detection together with elemental mapping records were also made to ensure the composition of FeF3@NAN after 400 cycles. Figures 5c−h illustrate that the cycled products involve Fe, F, Ni, and N elements as well. From EDS elemental mapping (Figures 5d−g) on a selected region (see SEM image in Figure 5c), it is noted that the electrode of FeF3@NAN maintains an integrated nanohybrid construction even after a long cyclic period. A deep investigation to understand the working functions of NAN was further conducted using surface-sensitive XPS measurements. After XPS curve fitting (background: Shirley; fitting function: asymmetric Gaussian− Lorentzian sum function), there are two peaks present in the high-resolution Fe 2p XPS spectrum with positions at binding energies (BEs) of ∼714.3 and ∼727.3 eV (Figure 5i) corresponding to the Fe 2p3/2 and Fe 2p1/2 peaks, respectively. Such high BE values nearly approach the literature data for FeF3,29,30 reflecting that the elemental Fe in cycled electrodes is still present as the Fe3+ valence state (keeping in line with the results prior to cyclic testing; see XPS spectrum in Figure S4). Note that the peak of Fe 2p3/2 is rather broad. This might originate from Plasmon loss peaks of elemental F overlapping the Fe3+ peak, as the F 1s peak is very close to the Fe 2p3/2 envelope.30 The Ni 2p XPS spectrum (Figure 5j) shows that two prominent peaks (indexed to Ni 2p3/2 and Ni 2p1/2) are located at BEs of 857.7 and 875.3 eV with satellite peaks at high binding energies of 863.6 and 880.2 eV, respectively. These peaks agree well with those of Ni3+ species (such as NiOOH), unlike records in an original sample case where the Ni 2p3/2
Figure 4. Energy density comparison between the FeF3@NAN hybrid cathode and commercial LiCoO2 cathode.
∼150 cycles of activation, the energy density of the FeF3@ NAN hybrid cathode begins to surpass the level of LiCoO2 (∼550 Wh kg−1) with a maximum value as high as ∼825 Wh kg−1 (calculated based on the total mass of FeF3@NAN); even at the end of cycling, the energy density was still retained at the level of ∼500 Wh kg−1. As a result, FeF3-based materials with smart and synergetic configurations exhibit great promise in competing with traditional LiCoO2 cathodes in LIB applicaF
DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. SEM observations on cycled electrodes: (a) bare FeF3 and (b and c) FeF3@NAN hybrid. (d−g) EDS elemental mappings and (h) spectrum of FeF3@NAN hybrid after 400 cycles. XPS analysis of the FeF3@NAN hybrid: (i) Fe 2p spectrum and (j) Ni 2p spectrum.
large electrochemical capacity (maximum: ∼423 mA h g−1), greatly extended cyclic lifespan (nearly 77.5% capacitance retention after 400 cycles), and enhanced rate behaviors. Such records reveal the outer thin-film NAN matrix can indeed play a vital role in protecting inner FeF3 nanoactives from detrimental aggregations and greatly improving the comprehensive electrode performance for LIB applications.
peak appears at a binding energy of 856.1 eV (a fingerprint value for Ni2+; see XPS spectrum in Figure S5).31 The above features are well-indexed in the literature as fingerprints of the Ni3+ valence state (rather than the pristine Ni2+ state in NAN). The variation in chemical valence suggests that the outer functionalized armors could also participate in battery reactions and make contributions to capacity. In spite of this, a full understanding of electron-transfer behaviors in hybrid electrode systems should be further performed by more in situ synchrotron radiation (SR) and TEM studies. Further, underneath the premise of electrode cyclic capability, the stepwise reduction of NAN shell thickness to an optimal level is a very smart and effective way to shorten the activation process of FeF3@NAN hybrids, which is deserving of our systematic studies in future cases.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03949. TG and DSC measurements of (NH4)3FeF6@NNH hybrid precursors, EDS spectra and TEM images of nanosized (NH4)3FeF6 products, TEM observations of both electrodes of FeF3@NAN and bare FeF3 after 300 cycles, and XPS Fe 2p and Ni 2p spectra before cycling (PDF)
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CONCLUSIONS To upgrade the cathode behaviors of FeF3 for LIBs, a facile, effective, and smart way of packaging FeF3 into a NAN matrix has been developed. The hybrid of FeF3@NAN, with an interesting and functional yolk−shell configuration, was produced via a green, scalable, and in situ encapsulation method using ultrathin NNH layers as initiating materials. When evaluated as the cathode, the derived FeF3@NAN hybrids exhibit electrode performance far superior to that of bare FeF3 nanoparticles with good electrochemical stability,
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AUTHOR INFORMATION
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[email protected]. Author Contributions ∥
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L.L. and J.J. contributed equally to this work. DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Fundamental Research Funds for the Central Universities (SWU 115027, XDJK2016C002). J.Z. would like to thank the support of the Fundamental Research Funds for the Central Universities (SWU 115029, XDJK2016C066). M.X. would like to thank the support of the Fundamental Research Funds for the Central Universities (SWU 113079, XDJK2014C051). This work was also financially supported by the National Program on Key Basic Research Project of China (973 Program) under contract 2013CB127804 and the Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under contact cstc2011 pt-sy90001.
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DOI: 10.1021/acsami.6b03949 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
