Mechanism of Capacity Fade in Sodium Storage and the Strategies of

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Mechanism of capacity fade in sodium storage and the strategies of improvement for FeS2 anode Kongyao Chen, Wuxing Zhang, Lihong Xue, Weilun Chen, Xinghua Xiang, Min Wan, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13421 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

Mechanism of capacity fade in sodium storage and the strategies of improvement for FeS2 anode Kongyao Chen, Wuxing Zhang*, Lihong Xue, Weilun Chen, Xinghua Xiang, Min Wan, Yunhui Huang* State Key Laboratory of Material Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China, 430074 ABSTRACT: Pyrite FeS2 has attracted extensive interest as anode material for sodiumion batteries due to its high capacity, low cost and abundant resource. However, the micron-sized FeS2 usually suffers from poor cyclability, which stems from structure collapse, exfoliation of active materials and sulfur dissolution. Here, we use a synergistic approach to enhance the sodium storage performance of the micron-sized FeS2 through voltage control (0.5-3 V), binder choice and graphene coating. The FeS2 electrode with the synergistic approach exhibits high specific capacity (524 mA h g-1), long cycle life (87.8 % capacity retention after 800 cycles) and excellent rate capability (323 mA h g-1 at 5 A g-1). The results prove that a synergistic approach can be applied in the micron-sized sulfides to achieve high electrochemical performance.

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KEYWORDS: FeS2, sodium batteries, voltage control, PAA-Na binder, graphene coating 1. INTRODUCTION Transition metal sulfides have been extensively investigated as energy storage materials.1-9 Among them, pyrite FeS2 is a very cheap one to be used in Li-ion or Na-ion batteries. Especially, the primary Li/FeS2 room temperature batteries and thermal battery have been successfully commercialized.10-17 When applied in sodium ion batteries (SIBs), FeS2 electrode usually suffers from poor cyclability, which stems from the conversion reaction and drastic structure changes.18-20 In order to solve these problems, several strategies have been proposed to improve the cycling stability of FeS2. Kovalenko’s group found that the nano-sized FeS2 can exhibit excellent electrochemical performance in sodium storage. When cycling at 1 A g-1, the capacity of nano-sized FeS2 can retain 500 mA h g-1 after 400 cycles.21 Chen et al. proposed that the cycling stability of aggregated FeS2 nanoparticles can be enhanced via voltage control. When the redox voltage window of FeS2 is controlled between 0.8~3 V, the cycle life of Na/FeS2 was improved as long as 20000 cycles with only the intercalation reaction. However, the application of FeS2 in SIBs is still inhibited because of the low tap density of nanoparticles and the low capacity (250 mA h g-1) resulted from voltage control.22 Micron-sized FeS2 single crystals are seldom investigated as electrode material in SIBs because of the poor cyclability.14, 23 However, the active materials with large particle size are desirable in the practical applications to achieve high compacted density, which has been proved in LiCoO2 in lithium ion battery. In this work, we prepare the single crystalline FeS2 micron-particles via a simple solvothermal method. The fading mechanisms of the micron-sized FeS2 single crystals were investigated, and a synergistic


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approach was proposed to achieve high capacity (523.6 mA h g-1), high rate (323 mA h g1

at 5 A g-1) and excellent cyclability (87.8 % capacity retention after 800 cycles) for the

sodium storage. 2. EXPERIMENTAL SECTION In this work, graphene was purchased from Nanjing XFNANO Materials Tech Co., Ltd. All the other reagent were purchased from Sinopharm Chemical Regent Co., Ltd, China. All the chemicals were of analytical reagent grade and used as purchased without further treatment. 2.1. Preparation of single crystalline FeS2 particles. FeS2 particles were prepared by a solvothermal method.11 Firstly, 1 mmol ferric nitrate (Fe(NO3)3) and 5 mmol thioacetamide (CH3CSNH2) were dissolved in 35 mL distilled water, and then 35 mL monoethanolamine was added to form a dark solution. The solution was stirred for 10 min, and then transferred into 80 mL Teflon-lined autoclave for solvothermal reaction at 200 °C for 48 h. Finally, the product was collected, washed by deionized water, and dried at 60 °C for 10 h in vacuum. 2.2. Preparation of different kinds of electrodes. Typically, 70 wt% FeS2, 20 wt% super-P (SP) and 10% poly (vinyl difluoride) (PVDF) were mixed in N, Ndimethylformamide (NMP) to form a slurry. The slurry was casted onto a copper foil and dried at 80 °C overnight. Then FeS2 electrode with PVDF binder was achieved (named FeS electrode), and the average mass loading of the electrodes was about 2 mg cm-2. The as-prepared FeS2 electrode was immersed into the graphene solution and taken out immediately, then the electrode was dried at 60 °C in vacuum. The ‘dip and dry’ step was repeated for 5 times, then FeS2 electrode with graphene coating was achieved (named



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FeSgraphene electrode). The amount of graphene in FeS2-graphene composite was determined by measuring the weight of electrode (6 cm * 6 cm slice) before and after graphene coating. The calculated content of graphene in FeS2-graphene composite was about 2.86 wt%. When the PVDF binder was replaced by sodium polyacrylate (PAA-Na) during the preparation process, the FeS2 electrode with PAA-Na binder is labeled as FeSPAA-Na electrode). 2.3. Characterizations. The crystalline phases of the samples were identified by a X’Pertb PRO (Panalytical B.V., Holland) diffractometer with high-intensity Cu Kα1 irradiation (λ=1.5406 Å). The morphology was observed by an FEI-Sirion 200 fieldemission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy dispersion X-ray (EDX) spectra were recorded on a JEOL 2100 microscope coupled with an EDX spectrometer (Oxford Instrument). The in-situ X-ray absorption data at the Fe K-edge was recorded at room temperature in fluorescence mode using ion chambers or in the fluorescent mode with silicon drift fluorescence detector at beam line BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF), China. The particle size distribution was measured by a laser particle size analyzer (Mastersizer 3000). 2.4. Electrochemical measurements. The as-prepared FeS2 electrode was pressed and cut into disks with a diameter of 8 mm. The loading mass of each disk is about 1 mg cm2

. Then, CR2032 coin cell was assembled in Ar-filled glove box with a counter electrode

of sodium foil and a separator of glass fibers. The electrolyte was 1 M sodium trifluomethanesulfonate (NaCF3SO3) in diethylene glycol dimethyl ether (DEGDME).


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The coin cells were galvanostatically discharged/charged at different current densities by Land CT2001 battery testers (Wuhan Land ElectricCo. Ltd., China).

3. RESULTS AND DISCUSSION The XRD pattern of solvothermally prepared FeS2 is displayed in Fig. 1a, showing pure pyrite phase (JCPDS 42-1340). SEM image shows that most of the FeS2 particles have plate-like morphology. Besides, few nanocubes also exist. According to the laser particle size analysis, the D50 size of FeS2 particles is about 6.2 µm (Fig. S1). As anode material for SIBs, the micron-sized FeS2 delivers an initial capacity as high as 718 mA h g-1 at a current density of 100 mA g-1. The capacity decays slowly during the initial 15 cycles, but fades rapidly between 20th and 30th cycles. The capacity only retains 32 mA h g-1 after 50 cycles (Fig. 1c). This demonstrates that the cycle performance is very poor for the bulk FeS2. In addition, from the discharge/charge profiles, we can see that the voltage drastically drops with cycling especially after 30 cycles, indicative of an increased polarization (Fig. 1d).



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Figure 1. (a) XRD pattern of FeS2 particles; (b) SEM images of FeS2 particles; (c) cycle performance and (d) charge/discharge profiles of FeS2 electrode. It has been suggested that the capacity fade of nano-sized FeS2 is closely related with the sodium storage procedure.23 As for the micron-sized FeS2, ex-situ XRD results show that FeS2 is finally converted into Fe and Na2S via an intermediate NaxFeS2 phase (Fig. S2). This process is consistent with the previous reports by other researchers.14, 23-26 Insitu XAFS test was also performed to detect the valence change of Fe during the charge/discharge process (Fig. S3). During the discharge process, the Fe K-edge curve moves to the left, indicating the reduction of Fe2+ to Fe. During the charge process, the Fe K-edge curve moves to the right, corresponding to the oxidation of Fe.14, 27 However, the profile cannot return to its original position because the charge product is NaxFeS2 instead of pyrite.22, 27 From the ex-situ TEM, it is observed that the FeS2 particles tend to pulverize from the outside to inside. After 30 cycles, all the particles are pulverized into nanoparticles (Fig. 2). The Ex-situ EDX shows that after 30 cycles, the S/Fe ratio decreases dramatically from 2 to 0.8 (Fig. S4). This phenomenon can be ascribed to the sulfur dissolution in the ether-based electrolyte during the conversion reaction.28


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Figure 2. Morphology evolution of the FeS2 micron particles. (a) pristine; (b) discharged to 0.8 V; (c) discharged to 0 V; (d) charged to 3 V; (e) after 20 cycles; (f) after 30 cycles. Thus, we summarize the evolution of micron-sized FeS2 during sodiation/desodiation in Fig. 3a. With the Na+ insertion, pristine pyrite FeS2 is transformed to amorphous NaxFeS2, then the NaxFeS2 intermediate phase is further converted to Na2S and Fe. When recharged to 3 V, NaxFeS2 appears again. The conversion reaction in FeS2 can generate large strain inside the particles, which leads to the structure collapse after 20 cycles. Accordingly, the capacity fade of the micron-sized FeS2 can be mainly ascribed to two reasons, i.e., particle pulverization and sulfide exfoliation into the ether-based electrolyte.



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Figure 3. Schematic illustration of (a) evolution of the FeS2 micron particles during cycling and (b) strategies to inhibit the fading procedure of micron-sized FeS2. According to the former reports, three strategies has been proposed to inhibit the fading procedure of micron-sized FeS2 electrode, as illustrated in Fig. 3b. Firstly, since the drastic volume change mainly comes from the conversion reaction (0.8-0 V), we can control the redox reaction by tuning the voltage range of intercalation reaction (3-0.8 V).22 As shown in Fig. 4a and 4b, in the voltage range of 0.8–3 V, the micron-sized FeS2 exhibits an initial capacity of 380 mA h g-1 and retains 85.6% capacity after 50 cycles. If the voltage range extends to 0.5−3 V, FeS2 delivers a higher capacity of 554 mA h g-1 and only maintains 53.9% capacity after 50 cycles. These results prove that cycling within the voltage range of 0.8–3 V can effectively alleviate the volume change and particle pulverization. However, a narrow discharge depth leads to the low capacity. Secondly, employing proper binder can effectively suppress the exfoliation of active materials. Here, we compare the effects of traditional PVDF and PAA-Na binder. The latter can provide stronger adhesion, more suitable porosity and hardness.29,

30

Our

experiment shows that the FeS2 electrode with PAA-Na binder delivers a high capacity of 708 mA h g-1 and a capacity retention of 59.8% after 50 cycles (Fig. 4c). This performance is better than that of the electrode with PVDF binder, indicating that PAANa binder can improve the cyclability of FeS2. Thirdly, the sulfur dissolution can be suppressed by the interlayer absorption structure. Similar strategy has been widely used in Li-S batteries.31, 32 Here, we coat a very thin graphene film (

Mechanism of Capacity Fade in Sodium Storage and the Strategies of

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