Adapting FeS2 micron particles as an electrode material for lithium-ion batteries via simultaneous construction of CNT internal networks and external cages

Jianhao Lu a, Fang Lian *a, Liangliang Guan a, Yuxuan Zhang a and Fei Ding b
aSchool of Materials Science and Engineering, University of Science &Technology Beijing, Beijng 100083, PR China. E-mail: lianfang@mater.ustb.edu.cn
bScience and Technology on Power Sources Laboratory, Tianjin Institute of Power Sources, Tianjin 300384, PR China

Received 16th October 2018 , Accepted 22nd November 2018

First published on 22nd November 2018


Volume changes, polysulfide shuttle effects, and low Li-ion/electronic conductivity of sulfide electrodes limit their application in Li-ion batteries with high-energy density. Here, micron FeS2 particles with bifunctional carbon nanotubes (FeS2@B–CNTs) including CNT internal conductive networks and external protective cages were prepared by a one-step solvothermal method. The internal CNTs anchor FeS2via chemical binding, which much more effectively inhibited the shuttle effect than physical confinement alone. Meanwhile, reticular CNT cages were generated on the surface of the composite FeS2 micron particles, buffering volumetric changes during cycling and further restraining the shuttle of polysulfides. Additionally, a three-dimensional CNT framework offered a primarily continuous charge transfer pathway. The FeS2@B–CNTs electrode had a high initial coulombic efficiency of 91.3% and delivered a capacity of 698 mA h g−1 over 500 cycles at 1000 mA g−1. The Li-ion diffusion coefficient is two orders of magnitude higher than those of previous reports, which improved the rate performance of the FeS2@B–CNTs electrode. These studies on micron FeS2 spheres shed light on the design strategies for the application of sulfide electrodes.


Introduction

Low-cost FeS2 with a high theoretical capacity of 894 mA h g−1 is considered to be a promising electrode for high-energy-density rechargeable Li-ion batteries;1–5 however, FeS2 suffers from a significant capacity fading because of the volume change and dissolution of the polysulfide intermediates during charge/discharge. Moreover, insulating FeS2 and discharge products have a low Li-ion conductivity at room temperature, which limits their application in Li-ion batteries, particularly at a high current density.6–9 A common strategy to address the aforementioned problems has been to develop carbonaceous nanostructural material, such as carbon coated nanostructures10–12 and carbon nanotube/nanofiber hybrid structures.13–16 Downsizing FeS2 powders to the nanoscale could mitigate the volume change during the charge/discharge process and maximize the proportion of the active material involved in the lithium-storage reaction.17 However, the nanostructures lead to a serious interphase detrimental reaction and low volumetric energy density.18–20 Therefore, the micron FeS2 particles were designed to be an active material candidate for lithium-ion batteries in our study.

Nevertheless, the serious volumetric expansion,21 small ratio of reaction-involved material, low Li+ diffusion coefficient and insufficient electronic conductivity22–24 challenge the successful development of micron-sized FeS2 particles as electrode materials. Here, we introduce a one-step solvothermal method to prepare FeS2/CNT microspheres with a special cage structure. The CNTs cage acts as a strong mechanical buffer and a polysulfide trapper to maintain electrochemical activity during ultra-long cycling. Moreover, the continuous CNTs network from the surface to the interior of the micron particles shortens the ion and electron transport paths, which enhances the lithium storage reaction kinetics. Stable cycling performance and high capacity at high mass loadings are critical for the innovation of battery technology.25 In this work, we apply Li-ion batteries employing micron FeS2 particles with bifunctional carbon nanotubes (FeS2@B–CNTs) as the electrode material, which has a higher tap density (2.2 g cm−3) and mass loading (2.8 mg cm−2) than nano-structured FeS2; thus, they are expected to show a high energy density, high coulombic efficiency, good rate performance, and long cycle life.

Experimental

FeS2@B–CNTs synthesis

The FeS2@B–CNTs composite was synthesized by a facial solvothermal method. Initially, 4 mmol FeSO4·7H2O, 20 mmol sublimed sulfur, 20 mmol urea, and 2 g multi-wall CNTs were dissolved into 70 mL of a mixture of dimethylformamide (DMF, Sigma-Aldrich) and ethylene glycol (EG, Aladdin). Subsequently, the suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. Finally, the product was centrifuged, washed with distilled water and absolute ethanol, and dried in a vacuum oven at 80 °C for 6 h to obtain the FeS2@B–CNTs powder. The multi-wall carbon nanotubes were purchased from Cnano Technology Ltd., China. By soaking in a mixture solution of HNO3 and H2SO4 (1[thin space (1/6-em)]:[thin space (1/6-em)]3 in vol), the CNTs were oxidized with carboxylic and hydroxy groups on the surface, as shown in Fig. S1. The detailed characteristics of the CNTs after oxidation treatment are shown in Table S1. FeS2 particles without CNTs that were prepared by the aforementioned method exhibit a large particle size distribution with micron-sized spherical aggregates (Fig. S2).

Material characterization

The phase composition of FeS2 and FeS2@B–CNTs microspheres were examined by powder X-ray diffraction (XRD) with Cu Kα radiation. The morphology and microstructure were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Raman spectra were recorded on a confocal Raman microscope equipped with a solid-state laser (532 nm) for excitation. The content of FeS2 in the FeS2@B–CNTs composites was determined by thermogravimetric analysis (TGA) under air atmosphere from 50 °C to 700 °C with a heating rate of 10 °C min−1. X-ray photoelectron spectroscopy (XPS) analysis with a low-energy Ar neutral beam etching method was performed with an AXIS Ultra DLD spectrometer (Al Kα X-ray source) to acquire the depth-dependent chemical states of Fe, S, and C.

Electrochemical analysis

The coin cells were assembled in an Ar-filled glovebox with a metallic-Li anode, a liquid electrolyte of LiTFSI (1 mol L−1) in diethylene glycol dimethyl ether (DGE), and a cathode of 80% FeS2@B–CNTs with 10% carbon and 10% polyvinylidene fluoride (PVDF) binder. They were tested with a Land battery testing system at 25 °C. Electrochemical impedance spectroscopy (EIS), Galvanostatic Intermittent Titration Technique (GITT), and Cyclic Voltammetry (CV) were performed on an electrochemical workstation (VersaSTAT3).

Results and discussion

The powder XRD patterns of FeS2 and FeS2@B–CNTs are shown in Fig. 1a. The main peaks correspond to the cubic FeS2 phase with Fe ions occupying the corner-sharing octahedral, and the carbon nanotubes show very small diffraction peaks (e.g., 2θ = 26°) in the FeS2@B–CNTs. The 3d electrons of the Fe ions are localized because of the weak Fe–S–Fe interaction. FeS2 is a semiconductor with the Fermi level lying between the filled t2g and empty eg band of Fe ions.26 The Raman spectra of FeS2@B–CNTs in Fig. 1b shows two strong peaks at 1350 cm−1 and 1583 cm−1, which correspond to the defect-activated disordered D-band and the ordered G-band of the graphitic layers of the CNTs, respectively. The high-intensity ratio of D/G bands indicates that the CNTs in the FeS2@B–CNTs composites were highly graphitized and highly electrically conductive. The peaks at 341 cm−1 and 375 cm−1 are attributed to the out-/in-plane S–S bond vibration, and the peak at 423 cm−1 are attributed to the transverse mode of FeS2.27
image file: c8ta09955c-f1.tif
Fig. 1 XRD patterns of the FeS2 and FeS2@B–CNTs samples (a), Raman spectroscopy (b), and XPS spectra of S 2p (c) and C 1s (d) of FeS2@B–CNTs.

The sample was etched from the surface toward the particle center measuring 30 nm in the XPS measurement. The Fe peaks at 710.9 eV and 724.5 eV are in good agreement with previously reported results (Fig. S3).28Fig. 1c shows the S 2p XPS spectra of the FeS2@B–CNTs. The signal displayed at 162.2 eV corresponded to the sulfur binding energy in FeS2, while the peak at 169.2 eV corresponded to the S in the carbon frameworks.29 It is worth mentioning that the detectable peak at 164.0 eV was due to the S–C bonds in the etched FeS2@B–CNTs sample.30 The four components at 284.6 eV, 285.2 eV, 286.8 eV, and 288.9 eV in the C spectrum (Fig. 1d) are related to the sp2, sp3 hybridized carbon, and the carbon in C[double bond, length as m-dash]O and C–O bonds, respectively.31 Another peak at 285.8 eV, which corresponded to the carbon bonding to the S ions of FeS2, was observed after Ar+-beam etching. The increased number of C–S bonds in the etched sample suggests that the FeS2 were chemically bonded with the CNTs through C–S bonds inside the micron particles.

Thermogravimetric analysis (TGA) of the FeS2@B–CNTs (Fig. S4) shows that the composite electrode had a high content of FeS2 (75 wt%). FeS2@B–CNTs exhibits a homogenous spherical morphology with an average particle size of 9 μm. Small tubular microvilli exist on the particle surface (Fig. 2a and b). The element mapping shown in Fig. 2c demonstrates a uniform distribution of C, Fe, and S elements on the surface of the FeS2@B–CNTs particles. The micron particles were crushed in an agate mortar to explore their internal structure (Fig. 2d). The continuous CNTs framework was observed from the surface to the interior of the particles. The EDS results indicate that the Fe and S contents were higher inside the particles, whereas the carbon content was higher on the surface, similar to a carbon cage.


image file: c8ta09955c-f2.tif
Fig. 2 SEM images (a and b), elemental mapping images (c) of FeS2@B–CNTs, and EDS line-scanning curves of Fe, S, and C elements for the crushed FeS2@B–CNTs samples (d). TEM images (e and f), and SAED images (g) of the FeS2@B–CNT composite samples. Morphological evolutions during preparation (h), and a schematic of the in situ formation of FeS2@B–CNTs (i).

The transmission electron microscopy (TEM) results in Fig. 2e and f show that the FeS2@B–CNTs have 30 nm-thick CNTs on the particle surface. Thus, the continuous and mechanically robust CNT network inside the particles could accommodate the volume change of the FeS2@B–CNTs during cycling and provide high electronic conductivity to FeS2. Moreover, selected area electron diffraction (SAED) verifies the highly crystallized nature of the FeS2 and CNTs. The interplanar distances of 0.272 nm and 0.367 nm correspond to the (200) plane of the cubic FeS2 and CNTs, respectively (Fig. 2g).

As was expected, a specially constructed micron FeS2 powder consisting of a CNT internal network and external cage was prepared in one step. Its formation mechanism was analyzed by detecting the evolution of the morphological structure during the preparation, as shown in Fig. 2h. Fig. 2i shows that the multi-wall CNTs with –OH and C[double bond, length as m-dash]O bonds on the surface initially interacted with Fe2+ and S to form a FeS2 crystal nucleus (step III). Then, the FeS2 crystal continued to grow on the surface of CNTs, until the carbon nanotubes were covered by FeS2 (step IV). Subsequently, the CNTs aggregated with the FeS2 material to form globular micron particles (step V). Finally, the extra carbon nanotubes in the reactor accumulated on the surface and gradually formed the cage on the outside of micron FeS2 particles (step VI).

The CNT component in the FeS2@B–CNTs provided a negligible capacity between 1 V and 3 V. The FeS2@B–CNTs demonstrated two distinct peaks at 1.4 V and 1.7 V during the cathodic scan (Fig. 3), corresponding to Li-ion intercalation (eqn (1)) and the conversion reaction (eqn (2)), respectively. The phenomena differ from the previous reports that showed an obvious overlap between these two peaks for transition metal sulfide, which indicates a good lithium storage reaction kinetics of the FeS2@B–CNTs electrode. There are three oxidation peaks during the initial anodic scan, which were assigned to the multistep reactions of LiS2 and Fe (eqn (3)–(5)). The two oxidation peaks at 2.3 V and 2.5 V in the first anodic scan shift to 2.2 V and 2.4 V, respectively, which corresponded to the reduced polarization because of the relatively low volume change in the subsequent cycles of the conversion electrodes.32


image file: c8ta09955c-f3.tif
Fig. 3 CV curves of the FeS2@B–CNTs electrode at a scanning rate of 0.1 mV s−1 in the voltage range 1.0–3.0 V.

Reduction process in the first cathodic scan:

 
FeS2 + 2Li+ + 2e → Li2FeS2(1)
 
Li2FeS2 + 2Li+ + 2e → 2LiS2 + Fe(2)

Oxidation process in the first anodic scan:

 
Fe + 2Li2S → Li2FeS2 + 2Li+ + 2e(3)
 
Li2FeS2 → Li2−xFeS2 + xLi+ + xe(4)
 
Li2−xFeS2 → FeSy + (2 − y)S + (2 − x)Li+ + (2 − x)e(5)

The Nyquist plots of the FeS2@B–CNTs and FeS2 electrodes after different cycles are shown in Fig. S5. The charge transfer resistance (Rct) and surface resistance (Rs) of the electrodes are compared in Fig. 4a and b. The Rct of FeS2 dropped slightly during the first five cycles, and then increased quickly in subsequent cycles. However, the FeS2@B–CNTs showed a much smaller and more stable Rct than that of FeS2. Moreover, the Rs of FeS2 kept increasing and doubled after the 20th cycle, indicating the continuous SEI formation and the absence of the electronic conductivity on the fresh surface of cracked FeS2 particles during cycling, whereas, the Rs of the FeS2@B–CNTs was smaller than that of FeS2 and decreased slightly after the 4th cycle, suggesting FeS2@B–CNTs could accommodate the volume change during cycling.


image file: c8ta09955c-f4.tif
Fig. 4 R ct (a) and Rs (b) variations during the cycling with a discharge cut-off voltage of 1.4 V. GITT of the FeS2@B–CNTs electrode over the first and second discharge at 25 °C (c), and Raman spectroscopy of the electrode initially discharged to 1.4 V (d).

The Galvanostatic Intermittent Titration Technique (GITT) is an effective method to determine the Li-ion diffusion coefficient (DLi+). Fig. 4c shows the voltage response of the FeS2@B–CNTs electrode and the calculated apparent value of DLi+. FeS2@B–CNTs exhibited a remarkably improved Li-ion diffusion coefficient (10−5 cm2 s−1) that is two orders of magnitude higher than those of previous reports.33–35

To understand the lithium storage reaction process, Raman spectra were measured on the electrode initially discharged to 1.4 V. The stretching vibration (Ag) peak at 375 cm−1 in the Raman spectra of FeS2@B–CNTs and FeS2 (Fig. 4d) shifts to the left (17 cm−1) and right (11 cm−1), respectively. The left shift of the Ag peak is ascribed to the reversible expansion of the S–S chemical bond and increased proportion of the reaction-involved material during discharge.36 However, the right shift of the Ag peak in the discharged FeS2 sample indicates a compressive strain on the S–S stretch mode, suggesting that the active material partially participated in the reaction and the reaction products were isolated on the electrode/electrolyte interface.29

The coin cells with FeS2@B–CNTs and FeS2 electrodes were disassembled in the fully charged state after 50 cycles at 200 mA g−1 in a glove box. SEM images in Fig. 5a show that FeS2@B–CNTs possessed structural integrity, whereas significant pulverization occurred in the FeS2 sample, which is in agreement with the Raman spectra and impedance analysis of the electrode. The TEM images in Fig. S6 further reveal that the encapsulation of the reticular CNTs cage and also the intimate contact between FeS2 and CNTs ensured a robust architecture of FeS2@B–CNTs micron particles.37 The lithium metal electrodes were washed thoroughly by 1,2-dimethoxy-ethane (DME), and the S elemental distribution on the surface was studied by element mapping analysis. Fig. 5b demonstrates more sulfur elements on the lithium metal with the FeS2 electrode than that with the FeS2@B–CNTs. In addition, the FeS2@B–CNTs and FeS2 electrodes were immersed in DME for 24 h. The FeS2@B–CNTs electrode solvent was transparent and colorless; however, the solvent with FeS2 shows a yellow color, indicating the sulfur species was dissolved from FeS2 electrode during cycling. The significant suppressed polysulfide dissolution and shuttle effect in the cells contributed to the particular structure of the FeS2@B–CNTs with the C–S bonding and the reinforced protection by the external cage.


image file: c8ta09955c-f5.tif
Fig. 5 Field emission SEM image of the FeS2@B–CNTs and FeS2 electrodes after being cycled (a), elemental mapping images of S on the lithium metal anode (b), and photographs of the solvent after soaking the sulfide electrodes for 24 h (inset image).

The cycling performance of the FeS2@B–CNTs as an electrode for LIBs is shown in Fig. 6a. FeS2@B–CNTs delivered an initial discharge capacity of 811 mA h g−1 at 1000 mA g−1 (1.12C, 1C = 890 mA g−1) with an initial coulombic efficiency of 91.3%. After 500 cycles, the electrode showed a reversible capacity of 698 mA h g−1 with a capacity retention of 86.1%. Moreover, the FeS2@B–CNTs electrode exhibited excellent rate performance and electrochemical reversibility; as shown in Fig. 6b, it delivered discharge capacities of 853 mA h g−1 and 611 mA h g−1 at current densities of 200 mA g−1 and 5000 mA g−1, respectively. FeS2@B–CNTs still have a high reversible capacity of 575 mA h g−1 and a high coulombic efficiency and good cycling stability at 5000 mA g−1 (Fig. 6 c). The FeS2@B–CNTs electrode shows a much better cycling performance than the FeS2 samples in recent reports for LIBs (Table S2).


image file: c8ta09955c-f6.tif
Fig. 6 Cycling performances at 1000 mA g−1 between 1.0 V and 3.0 V (a), rate capacity at various current densities from 200 mA g−1 to 5000 mA g−1 (b), and cycling performance at 5000 mA g−1 of the FeS2@B–CNTs electrode (c).

The high lithium-storage reaction kinetics and good electrochemical reversibility of the FeS2@B–CNTs are explained in Fig. 7. In addition to the intercalation reaction, where the lithium ions simply intercalate/deintercalate in the host structure, the lithium-storage reaction of FeS2 involved chemical transformation of the material. For FeS2 materials, only a small part of micron particles participated in the lithium-storage reaction due to the low electronic conductivity and sluggish lithium diffusion kinetics. The chemical conversion processes were confined to the surface of the particles, and finally terminated with a lack of charge transfer channel. The inhomogeneous reaction domain induced compressive strain on the surface of the FeS2, leading to particle cracking. In addition, the dissolution and shuttle of the polysulfide caused further deterioration in the electrochemical performance of the batteries.38 In contrast, FeS2@B–CNTs material possessed continuous charge transfer pathways and excellent dynamic properties, which contributed to an increase of the content of active material involved in the lithium-storage reaction. The resultant uniform stress distribution in the FeS2@B–CNTs micron particles and the three-dimensional framework constructed by the CNTs ensured the structural stability of the material. Moreover, the protection from the external CNTs cage and chemical anchoring by the internal CNTs suppressed the polysulfide dissolution and shuttle effect. Thus, significant improvements on chemical stability and structural integrity were responsible for the extended cycle life of the as-prepared FeS2@B–CNTs composites.


image file: c8ta09955c-f7.tif
Fig. 7 Schematic of the structural evolution of FeS2@B–CNTs (a) and conventional FeS2 (b) in the lithium-storage reaction.

Conclusions

To address the issues of nanostructural sulfide materials for LIBs, we constructed FeS2 micron spheres encapsulated by CNT cages with a continuous conductive network. Owing to the good Li+ diffusion coefficient/electronic conductivity, superior structural stability, and high utilization of the active substances, the as-prepared FeS2@B–CNTs electrode exhibited high coulombic efficiency, superior rate capability, and excellent cycling performance. This work paves the way for developing transition metal sulfide electrodes with long cycle life and high energy density.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51872026), the National Key Research and Development Program of China (Grant No. 2018YFB0104302) and the foundation of National Key Laboratory of Science and Technology on Power Sources (Grant No. 6142808020117C02).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta09955c

This journal is © The Royal Society of Chemistry 2019