Elsevier

Solid State Ionics

Volume 318, May 2018, Pages 60-64
Solid State Ionics

FeS nanosheets as positive electrodes for all-solid-state lithium batteries

https://doi.org/10.1016/j.ssi.2017.09.007Get rights and content

Highlights

  • FeS nanosheets are synthesized by a poly (vinyl alcohol)-assisted precipitation method.

  • The synthesis procedure shows a facile and highly productive strategy.

  • FeS nanosheets are first employed as a cathode for all-solid-state lithium batteries.

  • High capacity and excellent rate capability are demonstrated for the FeS nanosheets.

Abstract

Transition metal sulfides have attracted particular interests owing to their extremely higher specific capacity and more compatible with sulfide electrolyte than lithium transition-metal oxide for all-solid-state lithium batteries. FeS nanosheets with thickness of 10 nm are synthesized by a poly (vinyl alcohol)-assisted precipitation method and further successfully employed in Li/70%Li2S-29%P2S5–1%P2O5/Li10GeP2S12/FeS all-solid-state lithium batteries. The obtained all-solid-state lithium batteries show reversible discharge capacity of 550 mAh g 1 after 50 cycles at current density of 0.1 A g 1 and exhibit superior rate performances.

Introduction

One of the major challenges for traditional lithium ion batteries is the safety issue. All-solid-state lithium batteries employing inorganic solid electrolytes instead of organic liquid electrolytes may completely solve combustion and thermal runaway problems [1], [2]. Among various inorganic solid electrolytes, sulfide solid electrolytes, such as Li10GeP2S12 [3] or Li7P3S11 [4], show very high ionic conductivities in the order of 10 2 S cm 1, which are close to conductivities of organic liquid electrolytes, making them suitable in determining the performance of all-solid-state lithium batteries. Moreover, the softness of sulfide solid electrolytes makes it possible to get compact contacts with active materials only by cold-pressing. Another limitation for traditional lithium ion batteries is relatively low energy density. Owing to their higher specific capacity and moderate voltage plateau as well as good interfacial compatibility with sulfide solid electrolytes, transition metal sulfides based on conversion or alloying reaction mechanism have been considered as promising alternatives to conventional oxide positive materials [5]. However, structural damage caused by large volume expansion will lead to poor cycling stability and low capacity retention during repeated lithium intercalation and deintercalation process [6].

Up to now, there have been many methods reported to synthesize transitional metal sulfide materials for various applications. Taking iron sulfides as examples, different stoichiometries and crystal structures of iron sulfides have been prepared [7], [8]. Among them, nanocrystalline FeS could be synthesized by a solvothermal decomposition of a ferrous precursor complex in the presence of thiourea [9]. Pure hexagonal FeS nanotubes were prepared via sulfurization of α-Fe2O3 nanowires with H2S gas at relatively low temperatures [10]. Besides, iron sulfide/carbonaceous materials composites have been synthesized to improve their electrical conductivities and structural stabilities [11]. FeS nanoparticles wrapped in reduced graphene oxide were produced via a direct-precipitation approach [12]. Moreover, FeS nanodots accommodated in porous graphitic carbon nanowires was constructed via a combination of electrospining technique and biomolecular-assisted hydrothermal method [13]. However, most of these synthesis methods are either complicated or time-consuming, even some of them are environmentally harmful.

Recently, two-dimensional nanostructured materials have received widely attention in lithium ion batteries owing to their high specific surface area and providing short pathways and high kinetics for lithium ion insertion/extraction. The high specific area can provide abundant electrochemical reaction sites and large interfacial contact area with electrolytes. Meanwhile, due to their loosely stacked structures, they can further accommodate volume expansion and alleviate inner strain during conversion or alloying reaction [6]. Hence, it is anticipated that all-solid-state lithium batteries employing two dimension sulfide electrode could exhibit excellent cycling stability and rate performances. Moreover, ultrathin nanosheets can shorten Li-ion diffusion path length and improve charge transfer and kinetics of electrode reaction according to the law of diffusion. FeS is an attractive electrode material because of its various advantages such as affordable cost, environmentally benign, abundant resources and its high theoretical specific capacity of 609 mAh g 1. To the best of our knowledge, there is no prior publication reporting FeS nanosheets in all-solid-state lithium batteries. Moreover, there are still strong demands for simple and scalable procedures for the preparation of two dimensional FeS.

Herein, we present a simple and high-yield poly(vinyl alcohol)-assisted precipitation method to synthesize homogeneous FeS nanosheets. The all-solid-state lithium batteries employing FeS nanosheets exhibit stable reversible capacity of 550 mAh g 1 after 50 cycles at a current density of 0.1 A g 1 and superior rate performances.

Section snippets

Synthesis of FeS nanosheets

FeS nanosheets were synthesized by a poly(vinyl alcohol)-assisted precipitation method. Typically, 15 g of 33.3 wt% FeSO4·7H2O (99%, Sinopharm Chemical Reagent. Co., Ltd.) aqueous solution was mixed with 30 g of 1.0 wt% polyvinyl alcohol (99%, Sinopharm Chemical Reagent. Co., Ltd.) aqueous solution under argon atmosphere at room temperature. Then, equivalent amount of Na2S·9H2O (99%, Aladdin Chemistry. Co., Ltd.) solution was added into the above mixture, yielding black precipitates immediately.

Results and discussion

Fig. 1 shows the XRD patterns of FeS nanosheets obtained by the poly(vinyl alcohol)-assisted precipitation method. The main characteristic diffraction peaks at 2θ = 17.61, 30.09, 34.50, 36.65, 38.99, 43.56,49.59 and 50.45, corresponding to diffraction from the (001), (101), (110), (002), (111), (102), (200),and (112) crystal planes, respectively, can well be indexed to the tetragonal mackinawite (FeS) phase with a space group of P4/nmm (129) (JCPDS No. 86–0389). The sharp and narrow diffraction

Conclusions

In summary, homogeneous follower-like FeS nanosheets with thickness of 10 nm were successfully synthesized by a facile poly(vinyl alcohol)-assisted precipitation method. Two-dimensional nanosheets facilitate diffusion of Li-ion between active materials and solid electrolytes. Li/70%Li2S-29%P2S5–1%P2O5/Li10GeP2S12/FeS all-solid-state lithium metal batteries employing FeS nanosheets as active material and Li10GeP2S12/70%Li2S-29%P2S5–1%P2O5 as bilayer solid electrolyte exhibited excellent

Acknowledgements

The work was supported by funding from the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010201), the National Natural Science Foundation of China (Grant No. 51502317) and and Youth Innovation Promotion Association CAS (2017342).

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