SnSe2 nanocrystals coupled with hierarchical porous carbon microspheres for long-life sodium ion battery anode

SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ARTICLES mater.scichina.com link.springer.com Published online 30 December 2019 | https://doi.org/10.1007/s40843-019-1229-0 SnSe2 nanocrystals coupled with hierarchical porous carbon microspheres for long-life sodium ion battery anode 1,2,3 1 1 1 1 1 1 Hui Chen , Zijie Mu , Yiju Li , Zhonghong Xia , Yong Yang , Fan Lv , Jinhui Zhou , 1 2 2,3 1,4,5* Yuguang Chao , Jinshu Wang , Ning Wang and Shaojun Guo ABSTRACT Tin selenides have been attracting great attention as anode materials for the state-of-the-art rechargeable sodium-ion batteries (SIBs) due to their high theoretical capacity and low cost. However, they deliver unsatisfactory performance in practice, owing to their intrinsically low conductivity, sluggish kinetics and volume expansion during the charge-discharge process. Herein, we demonstrate the synthesis of SnSe2 nanocrystals coupled with hierarchical porous carbon (SnSe2 NCs/C) microspheres for boosting SIBs in terms of capacity, rate ability and durability. The unique structure of SnSe2 NCs/C possesses several advantages, including inhibiting the agglomeration of SnSe2 nanoparticles, relieving the volume expansion, accelerating the diffusion kinetics of electrons/ions, enhancing the contact area between the electrode and electrolyte and improving the structural stability of the composite. As a result, the as-obtained SnSe2 NCs/C microspheres show a high reversible capacity −1 −1 (565 mA h g after 100 cycles at 100 mA g ), excellent rate −1 capability, and long cycling life stability (363 mA h g at −1 1 A g after 1000 cycles), which represent the best performances among the reported SIBs based on SnSe2-based anode materials. Keywords: tin selenides, nanocrystals, hierarchical, sodium-ion batteries INTRODUCTION As a promising alternative to lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have recently attracted growing interest particularly for large-scale energy storage applications owning to the abundance and uniform distribution of sodium resource in the earth crust [1–5]. However, their electrochemical performances are severely hindered by severe volume variation and slow kinetics during insertion/extraction processes of sodium ions + (Na ) due to the intrinsic larger ionic radius and heavier molar mass of sodium ion than lithium ion [6–11]. Particularly, most of anode materials that are suitable for LIBs could not be applied directly in SIBs [12–15]. Therefore, the exploitation of excellent anode materials with high specific capacity, outstanding Na-storage reversibility and excellent rate capability is urgently desirable but remains a challenge. Recently, Sn-based materials, such as SnO2 [16–19], SnS [20,21], SnS2 [22–24], and Sn4P3 [25,26], have attracted tremendous attention as promising anode materials for both LIBs and SIBs. As earth-abundant, environmental friendly, and chemically stable materials, tin selenides (including SnSe and SnSe2) have also been regarded as attractive anode materials for SIBs, yet seldom studied till now [27,28]. SnSe2 is highlighted due to its unique layered structure and high interlayer spacing (6.14 Å for SnSe2 vs. + the diameter of 1.02 Å for Na ), which provides a fast channel for the transfer of ions and electrons [29,30]. Particularly, SnSe2 as an anode material for SIBs demonstrates a high theoretical reversible capacity of −1 756 mA h g [31]. However, similar to other Sn-based materials, SnSe2 is known for several drawbacks including 1 Department of Materials Science & Engineering, College of Engineering, Peking University, Beijing 100871, China State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China 3 State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China 4 BIC-ESAT, College of Engineering, Peking University, Beijing 100871, China 5 Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China * Corresponding author (email: ) 2 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 1 ARTICLES . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials the low electrical conductivity, huge volume variation and high mechanical stress/strain upon cycling, inevitably resulting in serious kinetic problems, making it difficult to fully take advantage of the conversion reactions, and thus leading to limited reversible capacity, cycle stability, and rate capability in practical application [29–32]. To solve these issues, herein, we report a facile strategy to prepare SnSe2 nanocrystals coupled with hierarchical porous carbon (SnSe2 NCs/C) microspheres. First, uniform stanniferous solid precursor microspheres were synthesized by a one-pot solvent-thermal method in the presence of stannous chloride and ascorbic acid in a mixed isopropanol/glycerol solution, which were subsequently transferred into SnO2 NCs/C microspheres by the thermal treatment in Ar. Then, the as-prepared SnO2 NCs/C microspheres underwent a simple selenization reaction to form SnSe2 NCs/C microspheres, which were particularly attractive for solving the problems related to SIBs. The SnSe2 NCs/C microspheres inhibits the agglomeration of SnSe2 nanocrystals by separating them from each other, and greatly improves the conductivity as well as availability of electrode materials. The 3D hierarchical porous structure not only enhances the contact area between the electrode and electrolyte, but also helps to suppress the volume expansion during charge-discharge processes. The SnSe2 nanocrystals are small and uniformly anchor on the carbon networks, and thus accelerate the diffusion kinetics of electrons/ions and improve the stability of the structure. Benefiting from these structural advantages, the SIBs based on the as-prepared SnSe2 NCs/C microspheres exhibit a high reversible spe−1 −1 cific capacity of 565 mA h g at 100 mA g , an excellent −1 −1 cycling stability (363 mA h g at 1 A g after 1000 cycles), and superior rate capability. RESULTS AND DISCUSSION Morphological and structural characterization The design and synthetic process of the SnSe2 NCs/C microspheres is schematically illustrated in Fig. 1a. Firstly, Sn-precursor (Sn-P) microspheres were facilely prepared by a one-pot solvent-thermal method in the presence of stannous chloride and ascorbic acid in a mixed isopropanol/glycerol solution. Subsequently, the uniform SnO2 NCs/C microspheres were formed by annealing those Sn-precursor microspheres in Ar. Finally, Figure 1 (a) Schematic illustration of the formation process of SnSe2 NCs/C microspheres. SEM images of the as-synthesized Sn-precursor microspheres (b, e); SnO2 NCs/C microspheres (c, f) and SnSe2 NCs/C microspheres (d, g). 2 © Science China Press and Springer-Verlag GmbH Germany, part (...truncated)