Elsevier

Journal of Alloys and Compounds

Volume 688, Part A, 15 December 2016, Pages 946-952
Journal of Alloys and Compounds

In-situ TEM study of the lithiation and delithiation of FeS nanosheets

https://doi.org/10.1016/j.jallcom.2016.07.008Get rights and content

Highlights

  • Lithiation and delithiation of FeS nanosheets were studied by in-situ TEM.

  • FeS nanosheets exhibited a small volume expansion.

  • FeS nanosheets converted to Fe nanoparticles of 2∼3 nm in Li2S matrix.

  • Li1.13FeS2 instead of FeS is reversible in following cycles.

Abstract

FeS nanosheets were synthesized and used as anodes to investigate their electrochemical properties and reaction mechanism during the lithiation and delithiation processes by using in-situ transmission electron microscopy. The FeS nanosheets converted to Fe nanoparticles of 2–3 nm embedded within Li2S matrix after the first lithiation. Then the Fe and Li2S transformed into Li1.13FeS2 instead of FeS during the first delithiation. The reversible phase conversion between Fe/Li2S and Li1.13FeS2 dominated the following cycles. The nanosheets structure was well held with small volume expansion during several lithiation and delithiation cycles.

Introduction

FeS exhibits many advantages when it is used as anode materials in lithium ion batteries (LIBs) [1], [2], [3]. On one hand, FeS can react with lithium ions; on the other hand, the layer structure of FeS allows lithium ions inserted between the layers. As a result, FeS is promising to exhibit high capacity and good stability. However, the theoretic volume expansion will reach 200% when FeS is transformed into Fe and Li2S during lithiation [4], [5]. The large volume expansion will pulverize the anode materials, resulting in severe capacity reduction and unsatisfactory cycling performance. Nanostructure is believed to be beneficial for the electrode materials. The nanostructured materials can greatly increase the surface area and accommodate large change in volume. This strategy is especially suitable for nanostructured FeS, since FeS with single layer or few layers is easy to be prepared due to the weak van der Waals bonding between layers.

Although the benefits of nanostructure of FeS have been widely accepted, the electrochemical reaction mechanism of FeS during lithiation and delithiation still remains in debate. Xu et al. reported that Li2−xFeS2 was generated instead of FeS after first cycle and then the transformation between Li2−xFeS2 and Li2FeS2 was reversible [6]. However, former studies reported that the transformation between Fe + Li2S and Li2−xFeS2 was reversible [7], [8]. Whether there is a volume expansion of 200% during first lithiation? What is the product of FeS transformation after first delithiation? What is the reaction during following lithiation and delithiation process?

In-situ transmission electron microscopy (TEM) is a powerful technique to reveal the dynamic microstructure evolution of electrode materials. Plenty of fresh results about the dislocation associated lithiation, layer-by-layer lithiation and even cracking were obtained by the in-situ lithiation and delithiation study in TEM [9], [10], [11].

In this work, we adopted in-situ TEM to reveal the real time change of the FeS nanosheets during lithiation and delithiation processes. It was found that the nanosheet structure was well held with small volume expansion during several lithiation and delithiation cycles. The initial FeS transformed into Li1.13FeS2 after the first cycle and the reversible phase conversion between Fe/Li2S and Li1.13FeS2 dominated the following cycles.

Section snippets

Experimental

The synthesis of FeS nanosheets has been described in our previous work [12]. Typically, FeCl3⋅6H2O (0.5 mmol), Na2S⋅9H2O (2 mmol) and S (2 mmol) were dissolved in 30 mL deionized water. Then, 10 mL of ethylenediamine was added into the solution. Afterwards, the mixture was transferred into a 50 mL Teflon-lined autoclave (filled up to 80% of its total volume) and then the iron foil was placed standing against the wall. The size and thickness of the iron foil was 50 mm × 18 mm × 0.05 mm with a

Results and discussion

Fig. 1(a), (b) clearly show the morphology of the obtained FeS product. The thickness of the FeS nanosheets is about 50 nm and the width is about 500 nm. TEM investigation further confirms the structure of the FeS nanosheets (Fig. 1(c)). The FeS nanosheets are well crystallized and the (101) lattices are clearly observed in Fig. 1(d). Two plateaus during the first discharge are found in the charge/discharge voltage profiles (Fig. 1(e)), which agrees with previous reports [6]. FeS nanosheets

Conclusions

In this work, the morphology change and phase transformation of the FeS nanosheet during lithiation and delithiation were investigated by in-situ TEM. The volume expansion is only 129% during the first lithiation and is 112% in subsequent lithiation processes. The nanosheet structure is well held and no crack or pulverization is found. The FeS nanosheet transforms into Li2S and Fe nanoparticles during the first lithiation and this transformation is irreversible. Only Li1.13FeS2 instead of FeS

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (No. LR14B060002), the Graduate Innovation Foundation of Hangzhou Dianzi University (KYJJ2014019) and the National Natural Science Foundation of China (U1332120 and U1432244).

References (13)

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