Improved Sodium-Ion Storage Performance of Ultrasmall Iron

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Improved Sodium-Ion Storage Performance of Ultrasmall Iron Selenide Nanoparticles Feipeng Zhao, Sida Shen, Liang Cheng, Lu Ma, Junhua Zhou, Hualin Ye, Na Han, Tianpin Wu, Yanguang Li, and Jun Lu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00915 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

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Improved Sodium-Ion Storage Performance of Ultrasmall Iron Selenide Nanoparticles Feipeng Zhao,1 Sida Shen,1 Liang Cheng,1 Lu Ma,2 Junhua Zhou,1 Hualin Ye,1 Na Han,1 Tianpin Wu,2 Yanguang Li,1* and Jun Lu3*

1. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for CarbonBased Functional Materials and Devices, Soochow University, Suzhou 215123, China 2. Advanced Photon Sources, X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA 3. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA Corresponding authors’ E-mail: [email protected]; [email protected]

Abstract: Sodium-ion batteries are potential low-cost alternatives to current lithium-ion technology, yet their performances still fall short of expectation due to the lack of suitable electrode materials with large capacity, long-term cycling stability and high-rate performance. In this work, we demonstrated that ultrasmall (~5 nm) iron selenide (FeSe2) nanoparticles exhibited a remarkable activity for sodium-ion storage. They were prepared from a high-temperature solution method with a narrow size distribution and high yield, and could be readily redispersed in non-polar organic solvents. In ether-based electrolyte, FeSe2 nanoparticles exhibited a large specific capacity of ~500 mAh/g (close to the theoretical limit), high rate capability with ~250 mAh/g retained at 10 A/g, and excellent cycling stability at both low and high current rates by virtue of their advantageous nanosizing effect. Full sodium-ion batteries were also constructed from coupling FeSe2 with NASICON-type Na3V2(PO4)3 cathode, and demonstrated impressive capacity and cycle ability. Keywords: Iron selenide, ultrasmall nanoparticles, sodium-ion battery, nanosizing effect, full battery

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Since their first launch in early 1990s, lithium-ion batteries (LIBs) have deeply penetrated our everyday life and become the prevailing power source for various portable electronic devices due to their high energy/power density and impressive cycle life.1,2 They have also been proposed as the major contender for the future electrification of our road transportation system as well as for utility-scale stationary energy storage.1,2 Nevertheless, one severe drawback of lithium-ion batteries is the limited lithium resource in the Earth’s crust and its uneven geographical distribution.3 There has been a continuous call to develop alternative energy storage technologies based on earth abundant materials and with performance comparable or even superior to lithium-ion.4-6 Among several potential solutions, sodium-ion batteries (SIBs) have attracted particular attention over the past 5 years.7-12 Besides the much lower cost of sodium, the interest at SIBs is driven by the fact that Na+ ions as the energy carrier have similar electrochemical properties as Li+ ions, so that the existing design principle of many LIB electrode materials could also be readily transferred to and expedite the SIB research.7,8,12 Unfortunately, it is generally observed that SIB electrode materials have much smaller capacity and inferior cycling stability compared to their lithium-ion counterpart, presumably due to their obvious difference in the ionic size.7-12 At present, most large-capacity materials (e.g. P, Sb and Sn4P3) have insufficient cycle life,13-15 whereas long cycle life materials (e.g. hard carbon) are short in capacity.16,17 The development of high-performance SIB electrode materials simultaneously with large specific capacities (>500 mAh/g), high rate capabilities (significant capacity at 10 A/g), and long cycle life (>300 cycles) remains a grand challenge.12,18 Pyrite-type FeS2 has long been considered as the electrode material of lithium batteries since 1980s for its large theoretical capacity, low cost, earth abundance and environmental benignity.19,20 2


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Its primary batteries have impressive power performance and longer working life than equivalent-sized alkaline batteries.21 By contrast, the potential of its selenide equivalent —FeSe2 in energy storage remains inadequately explored.22-25 Among few reports available (mostly for LIBs), FeSe2-based materials were usually microsized or submicrosized synthetic powders, whereas the storage behavior of nanostructured FeSe2 toward Na+ ions is still unknown. It has been widely accepted that nanostructured materials are advantageous for battery applications: they have enlarged surface areas, and shortened diffusion length for both ions and electrons; they also better accommodate the volume change during charge and discharge, and accordingly have much improved cycle life.26-30 However, the synthesis of nanostructured transition metal selenides with well controlled composition, shape and size is not an easy task. In this work, we develop a solution method to prepare ultrasmall FeSe2 nanoparticles at a high yield and reproducibility. The resulting material demonstrates remarkable electrochemical properties toward the reversible storage of Na+ ions with great specific capacity, rate capability and cycle life. FeSe2 nanoparticles (NPs) were synthesized from the controlled reaction between FeCl2 and Se in a mixed solvent of oleylamine (OM) and 1-octadecene (ODE) at a moderate temperature (150 oC) under the protective N2 atmosphere (see Experimental Methods in Supporting Information for details). The iron precursor was first dissolved in the solvent and formed a clear pale-yellow solution. Once the solution temperature was raised to and stabilized at the designated temperature, Se powder dissolved in OM was quickly injected. The solution color immediately turned black, signaling the rapid nucleation and growth of FeSe2NPs. After 30 min continuously stirring, the reaction was quenched, and solid product was collected. As-made products could be readily re-dispersed in non-polar organic solvents and formed a homogeneous solution. Figure 1a was a photograph 3


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showing FeSe2 NPs from one single reaction batch uniformly dispersed in cyclohexane to form a ~100 ml stable solution with a concentration of ~0.5 mg/ml. The solution could stand for weeks without noticeable sedimentation.

Figure 1. Structural characterizations of FeSe2 NPs. (a) A photograph showing ~100 ml FeSe2 NP hexane solution from a single reaction batch. (b) XRD of FeSe2 NPs, (c) low-magnification TEM image and (d) corresponding size distribution histogram of FeSe2 NPs, and (e) high-magnification TEM image of FeSe2 NPs.

We first carried out spectroscopic and microscopic characterizations to probe the chemical composition and microstructure of the final product. Its X-ray diffraction (XRD) pattern was depicted in Figure 1b. All diffraction peaks were assignable to orthorhombic FeSe2 (JCPDS card No. 82-0269) free of impurities. Based on the Scherrer equation, the average particle size of FeSe2 was estimated to be ~16 nm (see Supporting Information). Fe 2p and Se 3d X-ray photoelectron spectroscopy (XPS) spectra supported the presence of divalent iron cation and selenide in the product, 4


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which were consistent with those of reported FeSe2 materials (Figure S1, Supporting Information).31,32 The weak shoulder peak between 57~60 eV in the Se 3d spectrum suggested that selenide was partially oxidized on the surface. Transmission electron microscopy (TEM) provided the important size information of FeSe2 NPs. When viewed under low-magnification TEM, FeSe2 NPs were observed to form a tightly packed film with single particle thickness over a microsized area (Figure 1c). Individual NPs had a roughly spherical shape. Statistical analysis suggested that their diameters mainly distributed in the range of 4-7 nm with a mean size of ~5 nm (Figure 1d). There was no sign of extensive particle agglomeration. We believe that it was due to the effective capping and stabilization of NPs by OM ligands. Worth noting was that the NP size observed under TEM was considerably smaller than that estimated from XRD. We attributed this discrepancy to the NP agglomeration during XRD sample preparation. High-resolution TEM image of FeSe2NPs showed that they were crystalline and exhibited clear lattice fringes from the (011) plane of the orthorhombic crystal (Figure 1e). We found that the reaction time and temperature were two important experimental parameters that had great influence on the final product (Figure 2). When the reaction time was too short (5-30 min), the yield of NPs was lower; while with prolonged reaction time (1 h), NPs started to agglomerate and formed bigger and irregularly sized particles. Similarly, lowering the reaction temperature (120oC) slowed down the reaction rate, and led to fewer products after the same period of reaction time; while raising the temperature caused uncontrolled particle agglomeration (180oC), or even the formation of submicron-sized rods (240~300oC). The successful and reproducible preparation of well-dispersed, stable FeSe2 NPs therefore required the precise control over both reaction time and temperature. It laid the ground for the following electrochemical studies. 5


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Figure 2. Effects of the reaction time and temperature. (a-c) TEM images of products prepared at 150 °C for (a) 5 min, (b) 10 min and (c) 60 min; (d-f) TEM images of products prepared at (d) 120 °C, (e) 180 °C and (f) 240 °C for 30 min.

To study their electrochemical property toward Na+ ion storage, FeSe2 NPs in the cyclohexane solution were adsorbed onto Super P carbon black power, precipitated out by adding ethanol, repetitively washed and lyophilized. The solid powder was then mildly annealed to remove most of the organic residue. Such an approach was designed so as to largely preserve the nanosized feature of FeSe2 NPs, and avoid their possible agglomeration during the electrode preparation. TEM images of the resultant hybrid product (FeSe2 NPs/CB) after the heat treatment were shown in Figure S2. Thermogravimetric analysis (TGA) indicated that the content of FeSe2 in FeSe2 NPs/CB was 76.7% (see Figure S3 in Supporting Information). We subsequently processed the powder into electrode 6


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films with an areal loading of ~1 mg/cm2 FeSe2 NPs, paired them with metallic Na disks in standard 2032-type coin cells. The electrolyte in use was 1 M NaPF6 in diethylene glycol dimethyl ether (DEGDME). The choice of DEGDME over conventional carbonate electrolytes was inspired by the previous discovery that ether electrolytes could better stabilize Na metal compared to carbonate electrolytes by forming uniform and robust solid electrolyte interface (SEI) films that were impermeable to electrolyte solvent and conducive to non-dendritic growth.33 Figure 3a illustrated the cyclic voltammetry (CV) curves of FeSe2 NPs/CB for the first three cycles in the potential range of 0.01-2.4 V (versus Na+/Na, the same hereafter) at a scan rate of 0.05 mV/s. During the initial negative sweep, the cathodic current started to take off at 1.35 V, and culminated at 1.12 V, 0.90 V and 0.47 V. Like any conversion-reaction electrode materials, a permanent structure change could be inferred from the modified CV curves at ensuing cycles. They were instead featured with two main pairs of broad redox waves with the cathodic current peaked at 1.65 V and 0.46 V, respectively, signaling reversible and stepwise sodiation/de-sodiation process. Consistent information was also garnered from the galvanostatic charge and discharge profile of FeSe2 NPs/CB at 100 mA/g as depicted in Figure 3b. The first discharge curve exhibited a plateau at ~1.1 V, followed by a gradual sloping tail down to the cutoff voltage. In subsequent cycles, the original plateau immediately vanished, and instead, three individual short plateaus gradually emerged and established, located at approximately ~1.6 V, ~1.0 V and ~0.75 V. Such a multi-stage discharge profile was in perfect agreement with that of bulk FeSe2 previously reported by Chen et al.24 It was suggested that the three plateaus corresponded to the electrochemical formation of NaxFeSe2, FeSe + Na2Se, and the fully discharged product — Fe + Na2Se, respectively.24 The specific capacity during the first discharge reached 522 mAh/g, most of which was recovered and reversibly sustained at 7


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subsequent cycles. In addition to the large initial capacity, FeSe2 NPs/CB also demonstrated impressive cycling performance. At the specific current of 100 mA/g, the electrode material delivered a specific capacity of ~410 mAh/g at the second cycle (Figure 3c). The capacity was gradually increased to ~500 mAh/g within 50 cycles (a process defined as “activation”) — probably due to the increasing wetting of the electrode film by the DEGDME electrolyte,34 and then leveled off and maintained at this value for another 150 cycles. Such a large capacity value closely approached the theoretical limit of FeSe2 (501 mAh/g), and thereby attested to the very high utilization degree of the active material. The corresponding Coulombic efficiency was initially 76% due to the irreversible SEI formation, and afterwards remained ~100% during the course of evaluation. Importantly, it was worth noting that the observed combination of large capacity and great cycling stability of FeSe2 NPs/CB rendered them highly competitive among existing SIB anode materials.12,35-39 Moreover, the use of DEGDME electrolyte was also shown necessary to ensure the excellent cycle life. If FeSe2 NPs/CB were cycled in carbonate electrolytes (exemplified by 1 M NaClO4 in ethylene carbonate and diethylene carbonate), this electrode material exhibited an overall similar CV profile and identical initial discharge capacity, but experienced a gradual decline in capacity to ~360 mAh/g at the end of 100 cycles and highly scattered Coulombic efficiency (Figure S4, Supporting Information). Next, we assessed the rate capability of FeSe2 NPs/CB (Figure 3d). The electrode material after activation was evaluated at the specific current from 50 mA/g to 10 A/g. When the current was ramped stepwise, the measured capacity lowered from 520 mAh/g at 50 mA/g to 455 mAh/g, 416 mAh/g, 335 mAh/g and 257 mAh/g at 1 A/g, 2 A/g, 5 A/g and 10 A/g, respectively. It was remarkable that a significant fraction (~50%) of capacity was retained even under the largest specific 8


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current of 10 A/g. Further, we noted that the high-rate charge/discharge did not compromise the cycling stability of FeSe2 NPs/CB: after proper activation, it maintained a specific capacity of ~450 mAh/g for over 300 cycles at a specific current of 800 mA/g (Figure 3e). To further illustrate the structural advantage of ultrasmall NPs, we prepared FeSe2 from high-energy ball milling (bm-FeSe2, see Figure S5 in Supporting Information)40 and measured its electrochemical performance under the similar condition. Despite its nanosized texture, bm-FeSe2 was found to have considerably inferior specific capacity and rate capability (Figure 3c and d). It underwent the similar activation process as FeSe2 NPs/CB, but delivered a smaller stabilized specific capacity of 430 mAh/g at 100 mA/g with limited Coulombic efficiency for the first several cycles (Figure 3c). Under 10 A/g, bm-FeSe2 exhibited a reversible capacity of ~130 mAh/g toward Na+ ion storage － only about half that of FeSe2 NPs/CB (Figure 3d). Electrochemical impedance spectroscopy (EIS) analysis revealed that FeSe2 NPs/CB had significantly smaller interfacial charge transfer resistance than bm-FeSe2 before and after cycling (Figure S6).

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Figure 3. Electrochemical performance of FeSe2 NPs/CB. (a) CV curves of FeSe2 NPs/CB for the first three cycles at a scan rate of 0.05 mV/s. (b) Galvanostatic charge/discharge curves of FeSe2 NPs/CB at 100 mA/g for the first, 30th and 100th cycle. (c) Cycling stability and corresponding Coulombic efficiency of FeSe2 NPs/CB and bm-FeSe2 at 100 mAh/g. (d) Rate capability of FeSe2 NPs/CB and bm-FeSe2 under varying specific currents as indicated. (e) Long-term cycling stability of activated FeSe2 NPs/CB at 800 mA/g.

Above results evidenced that FeSe2 NPs/CB was capable of fast Na+ ion storage. The high-rate performance observed here for FeSe2 NPs/CB was also superior to most FeSe2-based and other SIB anode materials.23-25,35-37 We ascribed the excellent electrochemical performance of FeSe2 NPs/CB to their ultrasmall particle size. It has been widely recognized that with reducing particle size, electrode materials gain increasing surface areas, and require less time for ions to diffuse through.26,41 Both structural parameters greatly contributed to the observed high capacity and rate capacity. The small particle size could also effectively buffer the volume change during repetitive cycling, and ultimately resulted in prolonged cycle life. 8,10,42 Encouraged by the promising half-cell performance, we further paired FeSe2 NPs/CB anode together with Na3V2(PO4)3 (NVP) cathode, and constructed sodium-ion full batteries (Figure 4a). NVP is a NASCON-type compound and has been extensively explored as the SIB cathode material with impressive cycling stability and high-rate performance.43-45 Here, we followed a previous work for the preparation of nanostructured NVP via the solution precipitation method followed by high-temperature annealing process.46 Resulting product was analyzed to be pure-phase NVP from XRD, and unveiled to have flower-like morphology from SEM (Figure S7a and b, Supporting Information). Half-cell evaluation in DEGDME showed that its galvanostatic discharge curve had the characteristic plateau located at ~3.4 V (Figure S7c, Supporting Information). In agreement with 10


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previous reports,43-46 our NVP delivered a reversible specific capacity of 115 mAh/g at 30 mA/g. It also demonstrated great cycling stability with negligible capacity decay for at least 100 cycles (Figure S7d, Supporting Information). Next, NVP//FeSe2 full batteries were assembled using excessive NVP cathode material (40% more than calculated based on the charge balance) in order to ensure that FeSe2 was the capacity-limiting electrode. Figure 4b showed the galvanostatic charge and discharge curve of the NVP//FeSe2 full battery between 0.15~3.0 V at the specific current of 50 mA/g (with respective to the weight of FeSe2NPs). After the very first cycle, it exhibited a stable voltage plateau between 1.7~2.0 V. A reversible specific capacity of >500 mAh/g (with respective to the weight of FeSe2 NPs) was measured, well consistent with that of FeSe2 NPs/CB half cells. Cycling experiment showed that when first charged/discharged at 50 mA/g for the first several cycles, the full battery underwent a gradual activation, also in line with our above observation on FeSe2NPs/CB half cells (Figure 4c). When the specific current was subsequently raised to 100 mA/g, the battery capacity was lowered to and stabilized at ~400 mAh/g for at least 100 cycles. Such a cycling performance of the full battery would be impossible if it was not for the excellent cycling stability of individual FeSe2 anode and NVP cathode. At last, we fabricated a NVP//FeSe2 pouch cell. As displayed in Figure 4d, when fully charged, the pouch cell could provide enough voltage to turn on an OLED to glow yellow light. In summary, we developed a high-temperature solution method to prepare FeSe2 NPs with a narrow size distribution and high yield. When assessed as the anode material of sodium-ion batteries, it exhibited a large specific capacity close to the theoretical value, high rate capability with ~250 mAh/g retained even at 10 A/g, and excellent cycling stability at both low and high current rates. The superior performance of FeSe2 NPs was attributed to the large surface area, short ionic diffusion 11


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length and better stress accommodation associated with the ultra-small size. In addition, we demonstrated that FeSe2 NPs could be coupled with NVP to form full sodium-ion batteries with an average working voltage of ~2 V and large reversible capacity of ~500 mAh/g when normalized to the mass of anode. Our study here suggested the great potential of nanosized FeSe2 toward high-performance Na+ ion storage.

Figure 4. Electrochemical performance of NVP//FeSe2 full cells. (a) Schematic showing the full cell working mechanism. (b) Charge and discharge curves of the full cell at 50 mA/g. (c) Cycling stability and corresponding Coulombic efficiency of the full cell at 50 mA/g and 100 mA/g. (d) Digital picture showing the full pouch cell lighting up a commercial OLED light.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Acknowledgements We acknowledge supports from the National Natural Science Foundation of China (51472173 and 51522208, 51572180), the Natural Science Foundation of Jiangsu Province (BK20140302 and SBK2015010320), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Collaborative Innovation Center of Suzhou Nano Science and Technology and the “111” project. J. Lu gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.

Argonne National

Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.

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Nano Letters

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Nano Letters

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TOC figure:

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Improved Sodium-Ion Storage Performance of Ultrasmall Iron

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