Regular Article
Scalable submicron/micron silicon particles stabilized in a robust graphite-carbon architecture for enhanced lithium storage

https://doi.org/10.1016/j.jcis.2019.07.110Get rights and content

Abstract

Silicon-carbon composite is recognized as one of the most promising next-generation anodes for high-energy lithium-ion batteries, especially silicon-graphite composites. Herein, cost-efficient and scalable submicron/micron silicon particles are stabilized in a robust graphite-carbon architecture by solid-phase ball milling and liquid-phase coating methods. The obtained silicon-graphite-carbon composite with a stable encapsulated sandwich-like architecture exhibits impressive lithium storage performance, including high initial Coulombic efficiency of 83.7%, outstanding cycle stability and remarkable rate capability. Even at high loadings of 4 mg cm−2, it still exhibits great reversible capacity with 620 mA h g−1 after 100 cycles at 0.2 C. Furthermore, 8 wt% silicon-graphite-carbon composites as additives are applied into the full cell with a designed capacity of 1000 mA h, and the full cell displays superior cycle stability with high capacity retention of 85% after 100 cycles. In addition, the scalable and low-cost preparation makes it enormous application value and huge commercial prospect.

Introduction

For lithium-ion batteries (LIBs), higher energy density and longer cycle life have been demanded to meet the development of the electric vehicles, smart grids and communication devices [1], [2]. However, the theoretical capacity of commercial graphite anode is only 372 mA h g−1, which severely limits the development of LIBs [3]. Currently, it has become urgent to explore new high capacity anode instead of graphite. Silicon, as one of the most promising anodes, has the highest theoretical capacity of 4200 mA h g−1, natural abundance and low working potential [4], [5]. However, silicon faces the huge volume change and the unstable formation of SEI during the lithium ion insertion/extraction, leading to a sharp capacity decay [6]. In addition, silicon anode suffers from the low conductivity, which further worsens electrochemical performance of Si-based anode materials [1], [3], [7], [8].

In the past two decades, nanotechnology has gradually been introduced into silicon-based anode field [9], [10], [11], [12]. Numerous experimental and computational studies have revealed that nanoscale silicon materials (<150 nm) can significantly improve the lithium storage performance due to relieved stress generated by volume changes and short lithium-ion diffusion distance [13], [14]. So far, a wide variety of silicon nanostructures have been synthesized such as nanowires [15], nanotube [16], nanosheets [17], [18], nanoparticles [14]. However, silicon nanomaterials have not been industrial production and application yet [1], [7]. One of the important reasons is the high cost caused by complex preparation methods, such as chemical vapor deposition (CVD) [19], laser deposition [20]. In addition, silicon nanomaterials have severe undesirable side reactions due to high surface area, which will cause the poor initial and later-cycles Coulombic efficiencies [15]. Generally, their initial Coulomb efficiency is less than 70% due to the numerous formation of SEI, which to some extent limits the industrial application of silicon nanomaterials [9], [21]. Furthermore, the electrode of silicon nanomaterials normally has a relatively low mass loading (<1 mg cm−2), which is a huge obstacle for the practical applications.

Relative to silicon nanomaterials, the submicron or micro silicon particles has the small specific surface area, low production cost and can be industrially prepared by conventional ball milling method [13], [22], [23]. However, the submicron or micron silicon particles have an inferior electrochemical reversibility, which is mainly caused by mechanical fracture during the charge/discharge process and the poor ion/electron transport relative to silicon nanomaterials [24]. Promoting the electron/ion transport of submicron or micron silicon particles is essential for exploiting their potential lithium storage performance. At present, there are some efforts to attempt to improve the electrochemical reversibility of submicron or micro silicon particles by incorporating conductive matrix materials such as graphene, pyrolytic carbon and so on [1], [22], [23], [25], [26]. Cui et al. encapsulated silicon microparticles (1–3 µm) using the multi-layered graphene cages, and the chemically inert graphene cage forms a stable solid electrolyte interface [1]. Wang et al. successfully prepared submicron silicon-carbon composites by spray-drying method, and the composites exhibit an initial Columbic efficiency of 82.8% and a capacity retention of 428.1 mA h g−1 after 100 cycles [23]. However, these methods generally suffer from complex multistep and expensive preparation, which is highly unfavorable for large-scale production. In addition, it is also worth pointing out that the submicron or micron silicon anode faces a more severe high loading problems. Therefore, the development of scalable low-cost submicron or micron silicon-based anode still encounters enormous challenges.

Herein, we reported a facile two-step synthesis of the high-energy silicon-graphite-carbon (Si/G@C) composite using submicron/micron silicon particles (450 nm–4 μm) by combining conventional ball milling and liquid phase coating method. The Si/G@C with stable and robust encapsulated sandwich-like architecture shows superior lithium storage performance, including high initial Columbic efficiency (83.7%), good cycling stability and impressive rate capability. Even at high loading of 4 mg cm−1, a high reversible capacity of 620 mA h g−1 still is maintained after 100 cycles at 0.2 C. Furthermore, 8 wt% Si/G@C as an additive is applied to the full cells with designed capacity of 1000 mA h by mixing with commercial graphite, and the capacity retention of full cell reaches up to 85% after 100 cycles. In addition, the scalable and cost-effective preparation makes it be a promising anode candidate for next-generation lithium ion batteries.

Section snippets

Material synthesis

The commercial silicon powders (200 mesh) were ball milled for 30 h at 500 rpm by using n-hexane as a dispersant, then the flake graphite powders were added and the ball mill continued for 2 h at 500 rpm. The weight ratio of silicon powders to graphite is 5:2. After ball milling was completed, the mixture was dried in an oven at 60 ℃ for 10 h. The obtained mixture was labelled as Si/G. A certain amount of coal tar pitch was dissolved in tetrahydrofuran, then the Si/G was added to the

Results and discussion

Fig. 1 shows the preparation process schematic diagram of Si/G@C composite. The Si/G@C was synthesized by combining ball milling and liquid coating method using submicron/micro silicon particles (450 nm–4 μm) and pitch as carbon precursor. Firstly, submicron/micro silicon powders (SM-Si) were obtained by ball milling under N-hexane as a dispersant. Then SM-Si were uniformly embedded between the graphite sheets by following ball milling, and a homogeneous mixture of graphite/silicon was obtained

In conclusion

In summary, cost-effective and scalable submicron/micron silicon particles are efficiently utilized and stabilized in a robust graphite-carbon skeleton. The silicon-graphite-carbon composite with strong architecture exhibits superior lithium storage performance, including high initial Coulombic efficiency (83.7%), remarkable cycle stability (capacity retention of 83% after 180 cycles) and outstanding rate capability (697 mA h g−1 at 1C). Even at high mass area loading of 4 mg cm−2, a high

Acknowledgements

This work was partially supported by The Natural Science Foundation of China (No. 51634003) and Heilongjiang Science & Technology Key Bidding Program (No. GA14A102).

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