Rudan
Hu
a,
Hongan
Zhao
a,
Jianli
Zhang
a,
Qinghua
Liang
b,
Yining
Wang
a,
Bailing
Guo
a,
Raksha
Dangol
b,
Yun
Zheng
b,
Qingyu
Yan
*b and
Junwu
Zhu
*a
aKey Laboratory for Soft Chemistry and Functional Materials (Nanjing University of Science and Technology), Ministry of Education, Nanjing 210094, China. E-mail: zhujw@njust.edu.cn
bSchool of Material Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. E-mail: alexyan@ntu.edu.sg
First published on 16th November 2018
Pyrite-type FeS2 is regarded as a promising anode material for sodium ion batteries. The synthesis of FeS2 in large quantities accompanied by an improved cycling stability, as well as retaining high theoretical capacity, is highly desirable for its commercialization. Herein, we present a scalable and simple strategy to prepare a foam-like FeS2 (F-FeS2) nanostructure by combining solution combustion synthesis and solid-state sulfurization. The obtained F-FeS2 product is highly uniform and built from interconnected FeS2 nanoparticles (∼50 nm). The interconnected feature, small particle sizes and porous structure endow the product with high electrical conductivity, good ion diffusion kinetics, and high inhibition capacity of volume expansion. As a result, high capacity (823 mA h g−1 at 0.1 A g−1, very close to the theoretical capacity of FeS2, 894 mA h g−1), good rate capability (581 mA h g−1 at 5.0 A g−1) and cyclability (754 mA h g−1 at 0.2 A g−1 with 97% retention after 80 cycles) can be achieved. The sodium storage mechanism has been proved to be a combination of intercalation and conversion reactions based on in situ XRD. Furthermore, high pseudocapacitive contribution (i.e. ∼87.5% at 5.0 mV s−1) accounts for the outstanding electrochemical performance of F-FeS2 at high rates.
Pyrite-type FeS2 has been widely regarded as one of the promising anode materials for SIBs owing to its high theoretical capacity (894 mA h g−1), abundance, cost-effectiveness and eco-friendly nature.14–16 However, FeS2-based electrodes suffer from volume expansion up to 280% during the cycling process.17 Much effort has been devoted to optimize the electrochemical performance of this material. For example, some researchers tuned the cut-off voltage to avoid large volume change,18–20 and others composited FeS2 with carbon to buffer the expansion.21–23 Although improved cyclability has been achieved through reported methods, their low yield and partial capacity sacrifice have posed a great challenge towards commercialization.24,25 Thus, exploring a facile and cost effective strategy for the large-scale production of FeS2 while retaining high theoretical capacity is highly desirable.
Herein, we report a scalable and simple strategy to prepare a FeS2 nanostructure by combining solution combustion synthesis and solid-state reaction. The first step released large amounts of gaseous products and heat, yielding a porous Fe2O3 precursor by a solution combustion procedure. Then, the precursor was sulfurized to synthesize a foam-like FeS2 (F-FeS2) nanostructure. The uniform F-FeS2 exhibits high capacity, good rate capability and cyclability. It is found that the unique foam-like structure of F-FeS2 accounts for its good electrical conductivity, ion diffusion kinetics, and tolerance ability of volume expansion. Furthermore, in situ X-ray diffraction (XRD) has been conducted to study the sodium storage mechanism. This facile strategy could open opportunities for preparing other foam-like metal sulfide materials in considerable amount and with impressive performance in electrochemical fields.
The morphology of F-FeS2 was studied by SEM and TEM, and the surface area was determined using the BET method. The as-prepared F-FeS2 has foam-like interconnected 3D porous morphology, as shown in Fig. 2a and Fig. S2 (ESI†). Such a porous structure is mainly formed due to the release of a large amount of gaseous products during liquid combustion synthesis.31,32 Interestingly, the skeleton of the foam structure is composed of FeS2 nanoparticles with diameters ranging from 25 to 120 nm (Fig. 2b), and most particles stick closely to each other, building up a continuous frame. The EDX elemental mapping results (Fig. 2c) show uniform distribution of Fe and S, confirming that the porous Fe2O3 was totally sulfurized into FeS2. The SAED pattern shown in Fig. 2d indicates the polycrystalline nature of F-FeS2. Moreover, the as-observed diffraction rings can be well indexed to the (111), (200), (210), (211), (220), (311) planes, respectively, agreeing well with the XRD result. In contrast, FeS2 shows the morphology of aggregated particles with an average size of 230 nm (Fig. 2e). Meanwhile, the BET result in Fig. 2f indicates that F-FeS2 offers a larger specific surface area (9.8 m2 g−1) than FeS2 (5.3 m2 g−1). Obviously, the larger specific surface area of F-FeS2 is attributed to the porous structure and the smaller particle sizes.
A half-cell configuration was assembled to evaluate the sodium storage behavior of F-FeS2. Fig. 3a shows the initial five CV curves of the F-FeS2 electrode recorded between 0.01 and 3 V at a scan rate of 0.1 mV s−1. As can be seen from the cathodic scan, two groups of peaks can be observed. A sharp cathodic peak located at 1.07 V is observed in the first cycle, which disappears in the subsequent cycles. This indicates an irreversible reaction process. It is substituted with a new reduction peak at 2.12 V. Another group of peaks, belonging to the discharging process, can be detected below 0.76 V in all cycles. The associated in situ XRD patterns in Fig. 3b show that the intensity of the F-FeS2 peaks gradually decreases during the discharging process and no peak shift can be observed. No other crystalline compound is detected until the Na2S was formed during the discharging process. These observations indicate that the phase of F-FeS2 has changed around 1 V and the iron sulphides converted into Na2S below 0.7 V. For the charging process, two groups of peaks, associated with Na+ extraction, are located around 1.32 and 2.54 V as seen in Fig. 3a. Each group contains at least two obvious peaks, indicating the corresponding multi-stepped reactions.33 Along the same process, no other diffraction peak change occurs except for the decomposition of Na2S from the lowest potential. This decomposition accounts for the first group of reaction peaks in the charging process, whereas another group of peaks appear to correspond to a different desodiation process. Previous studies have reported that the formation of NaxFeS2 can be observed during the first cycle discharging process, as well as a charging/discharging potential above 0.8 V in all subsequent scans.14,18 Besides, the lattice fringe of NaFeS2 can be observed by ex situ HRTEM for the samples charged/discharged to the mentioned potentials (Fig. S3, ESI†) in our study. Moreover, our CV plots present a similar profile as in the previous report, which was associated with an intercalation reaction.34 It is reasonable to believe that the intercalation reaction occurs in this electrochemical process. Hence, we propose the following reaction mechanism:
FeS2 + xNa+ + xe− → NaxFeS2 (x < 2) | (1) |
Nax−yFeS2 + yNa+ + ye− ![]() | (2) |
NaxFeS2 + (4 − x)Na+ + (4 − x)e− ![]() | (3) |
Eqn (1) is associated with a sharp peak located at 1.07 V in the first cycle, and it is substituted by eqn (2) in the following cycles. The cathodic peak at 2.12 V and the anodic peaks around 2.54 V originate from this reaction. The irreversible phase change of FeS2 matches well with the previous report.21Eqn (3) describes the reduction below 0.76 V and the oxidation around 1.32 V in all cycles. In particular, we observed the peak shift of Na2S near 23.6° as seen from the magnified in situ XRD patterns in Fig. S4 (ESI†). The Na2S diffraction peak associated with 0.01 V matches exactly with that of pure Na2S. It is interesting to find that these peaks tend to appear at lower angles before discharging to 0.01 V, and keep shifting to larger angles after it. This may be attributed to the interphase generated in the conversion process, whose interplanar spacing shrinks from 3.79 to 3.74 Å during the process. In addition, the typical diffraction peaks of Be and BeO can also be seen in Fig. 3b, which are attributed to the Be window used in the in situ XRD test.
Fig. 3c presents the corresponding initial five GCD profiles of the F-FeS2 anode at 0.1 A g−1. The first cycle delivers a discharge specific capacity of 1341 mA h g−1 and a charge specific capacity of 840 mA h g−1. The initial capacity loss is due to the irreversible phase transformation reaction and the formation of a solid–electrolyte interface (SEI) layer. The SEI generating reaction also accounts for the capacity decrease between the first scan curve and the subsequent curves. This matches well with the intensity decay in the reduction peaks below 0.76 V and the oxidation peaks around 1.32 V shown in Fig. 3a. The discharge plateaus are observed around 2.1 V and 0.5 V, while the charge plateaus locate around 1.3 V and 2.5 V. This result also matches well with the redox peaks in the CV curves. Both CV curves and GCD profiles overlap well from the second cycle onward, demonstrating the good reversibility of the reaction.
The rate capabilities and cycling stabilities of the F-FeS2 and FeS2 electrodes are presented in Fig. 4a and b. The F-FeS2 anode exhibits specific capacities of 823, 771, 721, 693, 662, 630, 608, 582, and 861 mA h g−1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 0.1 A g−1, respectively. These values are much higher than those of FeS2 (Fig. 4a). Besides, the rate performance of F-FeS2 demonstrates the highest specific capacities among previous reports (Fig. S5, ESI†). For cyclability, F-FeS2 shows a specific capacity of 755 mA h g−1 at 0.2 A g−1 after 80 cycles, with 97% capacity retention. Meanwhile, the 20th, 50th, and 80th GCD curves during cycling can be found in Fig. S6 (ESI†), and the similar profiles reveal the stability of our F-FeS2. The coulombic efficiency below 99% after tens of cycles might be attributed to the side reactions between the ester-based electrolyte and sulfur anionic groups.6,18,25,35 In comparison, FeS2 only delivers 194 mA h g−1 at 0.2 A g−1 after 80 cycles, with 33% capacity retention. The slight increase of the discharge capacity of F-FeS2 during the initial fifty cycles can be ascribed to the activation process, which has been observed in other transition metal sulfides.36–38 In contrast, the specific capacity of FeS2 decreases rapidly in the first few cycles. Considering the same phase and composition of F-FeS2 and FeS2, it is reasonable to believe that the foam-like structure enables the improved electrochemical performance of F-FeS2. In addition, we also investigated the sodium storage performance of SWCNTs, and it displays a 5th-cycle specific capacity of 91.8 mA h g−1 at 0.1 A g−1 (Fig. S7, ESI†). Taking the mass percentage (10% of the loading mass) into account, the capacity contribution from the SWCNT additive was negligible.
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Fig. 4 (a) Rate capabilities of F-FeS2 and FeS2 for SIBs from 0.1–5.0 A g−1. (b) Cycling stabilities of F-FeS2 and FeS2 at 0.2 A g−1. (c) EIS of F-FeS2 and FeS2 half cells. |
To reveal the reaction kinetics for such good Na-storage performance of F-FeS2, both samples were subjected to EIS. Both samples show similar Nyquist plots obtained at open circuit voltage. A depressed semicircle at high frequency followed by a diffusion drift at low frequency can be clearly observed (Fig. 4c). Obviously, F-FeS2 has a smaller depressed semicircle diameter than FeS2 at high frequency. This suggests that the F-FeS2 electrode has better charge transfer performance than FeS2, which might be attributed to the interconnected feature of F-FeS2. The low frequency line is associated with the ion diffusion of electrodes.15 The line slope of FeS2 is smaller than that of F-FeS2, indicating the faster transport of ions in the F-FeS2 electrode cells. The porous structure and relatively small particle sizes offer a larger surface area. This allows more contact sites with the electrolyte, thus shortening the ion transporting distance.15,20 The EIS spectra have also been fitted to an equivalent circuit, and the calculated resistances and Warburg coefficients are agreeable with the analysis above (Fig. S8, ESI†). All of these account for the high discharge capacity at each scan rate. Besides, the pores of the foam structure of FeS2 offer more space for volume expansion during the sodium storing process. This guarantees the structural stability of F-FeS2, resulting in a high capacity F-FeS2 electrode with good cyclability.
To better understand the good high-rate performance of F-FeS2, the reaction kinetics was further analyzed by distinguishing the surface-controlled pseudocapacitive capacity and diffusion-controlled capacity. Fig. 5a shows the CV curves of F-FeS2 for various scan rates ranging from 0.1–5 mV s−1. It can be found that the peaks of all the CV curves show a similar shape, although broadened peaks can be observed at increased scan rates. The preserved peaks even at high scan rates indicate small polarization during the reaction process.39 The contribution of pseudocapacitive effects is qualitatively revealed by the b value from the equation log(i) = blog(v) + log(a) where both a and b are constants, and i and v represent the current and scan rate.40–42 The series of peak currents marked in Fig. 5a were chosen to determine the b value by fitting lines based on this equation. The calculated b value of F-FeS2 is 0.93. As the value of b is very close to 1, it suggests that there is a large contribution from the pseudocapacitive storage in the F-FeS2 electrode.43–45 To quantify the pseudocapacitive contribution, the equation i = k1v + k2v0.5 was used according to previous reports, where i and v represent the current and scan rate, respectively, and k is a constant related to a fixed potential.46–48 By fitting the value of k1 at different voltage stages, we calculated a series of pseudocapacitive contributions to the current. As a result, the direct view of pseudocapacitive storage contribution (red part) for F-FeS2 at 1.0 mV s−1 is shown in Fig. 5c, and it is determined to be ∼82.5%. This indicates the surface pseudocapacitance-dominated process in the F-FeS2 electrode. The overall pseudocapacitive contributions at different scan rates were obtained using the same method, as shown in Fig. 5d. As the scan rates increase, the pseudocapacitive contribution also increases gradually. A maximum value of ∼87.5% can be obtained at 5.0 mV s−1. It suggests that the pseudocapacitive behavior dominates the whole reaction process, especially at high scan rates. It is well known that the surface-controlled pseudocapacitive process is much faster than the diffusion-controlled process.19 Such high pseudocapacitive contribution may be ascribed to the unique foam-like structure with relatively large surface area and small particle sizes, resulting in good rate capability of the F-FeS2 electrode.
Besides, the good electrochemical performance of F-FeS2, which benefits from the foam-like structure, can also be explained from the structural mechanistic aspect. As shown in Fig. S9 (ESI†), Na, S and Fe elements distribute evenly throughout the 3rd-cycle discharged material, indicating a thorough chemical conversion process. This means that our porous foam-like structure consisting of small FeS2 nanoparticles is suitable for kinetically reversible reactions.34 In addition, FeS2 tends to react with alkali metals via a radial movement of a reaction front triggered by the contact of two materials.49,50 Thus, the sufficient contact area and ion transport pathway offered by the porous structure are beneficial for sodium storage. Besides, Fe ions, based on their short diffusion length,51 tend to nucleate uniformly alongside Na2S in the resultant particle during the conversion reaction process.34,49,52 As a result, the conversion products only showed a slight increase in the diameters of the original particles, retaining the initial shape. Meanwhile, the neighboring pores in our porous foam-like structure can accommodate the associated volume expansion, maintaining the initial contact of particles. Hence, the interconnected foam-like structure can be preserved, even undergoing volume expansion after the discharge/charge process (Fig. S10, ESI†).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr06675b |
This journal is © The Royal Society of Chemistry 2019 |