Effect of electron beam irradiation on the capacity fading of hydride-terminated silicon nanocrystal based anode materials for lithium ion batteries

https://doi.org/10.1016/j.jiec.2017.04.003Get rights and content

Abstract

The selection of binder is extremely important in silicon based lithium-ion batteries (LIBs). Polyvinylidene fluoride (PVDF) has been used as a commercial binder. However, it does not accommodate a large change in volume during cell cycling. In this study, we report on the rediscovery of the use of PVDF without additional synthetic processes or further treatment. By utilizing simple and short e-beam irradiation, hydride-terminated silicon nanocrystals (H-Si NCs) and PVDF can be chemically cross-linked each other, and shows an improved cell performance. This result demonstrates a high potential of the e-beam irradiation process on Si-based anode materials in LIBs.

Introduction

The research community is currently engaging in extensive efforts to replace graphite in the anodes of lithium ion batteries (LIBs) with new materials that possess higher capacity, energy, and power density. Various approaches have been applied to investigate both carbon and non-carbon materials. Graphite is a carbon-based material that is commonly used as an active material in the anode due to its excellent characteristics, including flat and low working potential, good cycling ability and low cost [1]. However, graphite allows intercalation of only one Li-ion with six carbon atoms, and this leads to an equivalent reversible theoretical capacity of 372 mAh/g. Additionally graphite has a low diffusion rate of lithium ions (10−9–10−7 cm2/s), which results in a poor power density [2]. Hence, it is unavoidable to replace graphite with new anode materials that have a higher capacity, energy, and power density. Non-carbon candidates include nano-sized silicon, such as silicon nanocrystals (Si NCs), silicon nanoparticles (Si NPs), and silicon nanowires (Si NWs), and these have been studied aggressively as candidate materials for the anode of LIBs due their high theoretical capacity for lithium (4200 mAh/g, Li22Si5) and small size, ultimately reducing the mechanical stress originating from the large volume expansion/contraction during lithiation/delithiation [3], [4], [5], [6], [7], [8], [9], [10], [11].

However, there are some limitations to the applicability of nano-sized Si as an anode material. Primarily, the large expansion/contraction in volume (∼400%) during charge/discharge results in the consecutive pulverization of the anode active materials, thereby induce a poor cyclic life and irreversible capacity. In addition, the formation of Si compounds at the solid electrolyte interface (SEI) inhibits lithiation/delithiation during the cycle [12]. To overcome these issues, Si-based materials need to have the following properties: an optimum surface area accessible to the electrolyte, a short diffusion length for lithium ions, a large space available to accommodate the change in volume, and a high electron conductivity. Although various methods have been developed to facilitate the use of high-performance Si-based anode material, existing methods to manufacture nano-sized Si anode materials still have limitations due to the high cost of raw materials, processing costs, low scalability, and inevitable cost of additive such as conducting carbon black and polymer binders, in contrast with commercial graphite anode materials. In this point of view, Park et al. reported 3-D paper-type Si-carbon nanofiber-composite electrode (Si/CNF-P) as a binder/current collector-free anode for LIBs, which prepared using an electrospinning method [13].

Of various options, the cheapest, most scalable, and most simple route to overcome these issues is to search for a new binder system. For instance, PVDF is a commercial binder used in LIBs, but it does not provide competent cyclic stabilities in the Si-based LIB systems because the PVDF binder just attaches to the surface of the Si active material with weak van der Waals forces and does not accommodate a large change in volume during cell cycling. Consequently, finding a new and efficient binder is very important assignment to facilitate production of high-performance Si-based LIBs. In this respect, many researchers have focused on finding or synthesizing new effective binders for Si-based LIB systems. Magasinski et al. reported that polyacrylic acid (PAA) can offer superior performance as a binder for Si anodes due to the higher concentration of carboxylic functional group. Their Si active material was coated with polycarbonate (PC) and was then graphitized at 800 °C under Ar flow [14]. Song et al. reported interpenetrated gel polymer binder synthesized by using in-situ cross-linking of PAA and polyvinyl alcohol (PVA) precursors [15]. Recently, Chen et al. reported cross-linked chitosan (CS) as an efficient binder for Si anode of LIBs. The CS was dissolved in an acetic acid solution, then glutaraldehyde (GA) was injected into this solution. Si powder was mixed with Super P and CS or CS-GA at a weight ratio of 60:20:20 in 2 wt% acetic acid solutions [16]. A further study was performed by Liu and Yang by synthesizing an ideal polymer binder by tuning the electronic structure of the functional groups. The molecular structures of the developed polymers are based on polyfluorene (PF)-type polymers. They synthesized the polymer binder by using a Pd(PPh3)4 catalyst and four kinds of molecules: polyfluorene with octyl side chains, fluorine with triehyleneoxide monomethyl ether side chains, fluorenone, and methyl benzoate ester [17], [18]. However, these new kinds of binder require complex and uneconomic synthetic processes to provide efficiency and stability in coin cells. Hence, some groups investigated simple surface modification of PVDF membrane or separator. Kim et al. reported PVDF membrane modified with glycidyl methacrylate (GMA) and sodium sulfite using a radiation-induced graft polymerization technique for unique ammonia removal process [19]. Lim et al. reported the effect of electron beam irradiation on pure PVDF films at the melting temperature by varying the irradiation dose. The PVDF films irradiated at the melting temperature show highly cross-linked characteristics, as confirmed by gel fraction and FT-IR, while the films irradiated at the room temperature did not [20]. And Ahn et al. adopt surface-modified polyethylene separator via oxygen plasma treatment to meet requirements for high performance LIBs [21].

In this study, we report on the rediscovery of the use of PVDF binder. By utilizing simple and short (90 s) e-beam irradiation processes between H-Si NCs and PVDF binder at the melting temperature through partial dehydrofluorination, H-Si NCs and PVDF can be chemically interconnected and then cross-linked with each other without additional synthetic processes or further treatment. This cross-linked network system can accommodate a large volume expansion and can form an optimized SEI layer during lithiation/delithiation. The chemical bond between Si and C minimizes separating the binder from the active materials (H-Si NCs) during cycling. Thereby, the decomposition number of electrolytes on the surface of the H-Si NCs could be controlled to not go further. Consequentially, the H-Si NCs cross-linked through e-beam irradiation show an improvement in cyclic stability and a decrease in resistance relative to those that are not irradiated with the e-beam. To the best of our knowledge, this is the first work that demonstrates the improved physical and electrical properties of anode materials in LIBs based on H-Si NCs and commercial PVDF binder processed with e-beam irradiation.

Section snippets

Chemicals

Tetraethyl orthosilicate (≥99.0%), anhydrous toluene (99.8%), andhydrous methyl alcohol (99.8%), sodium chloride (≥98.0%), polyvinylidene fluoride (Typical Mw ∼534,000) were obtained from Sigma–Aldrich and were used as received. Ammonium hydroxide solution (25.0% NH3 in water) was obtained from Acros Organics, DI-water and magnesium powder (99.6%) were obtained from Alfa Aesar. Hydrochloric acid (35.0–37.0%) and ethyl alcohol (99.9%) were obtained from Duksan (Ansan City, South Korea).

Characteristics of SiO2 NPs, Si NCs@SiOx, and H-Si NCs

The crystallinity of the SiO2 NPs, Si NCs@SiOx, and H-Si NCs synthesized using the solvent-controlled method, magnesiothermic reduction, and chemical etching process were characterized via XRD and are represented in Fig. 1(a). The diffraction peaks were assigned to the (111), (220), (311), (400), (331), and (422) lattice planes. The XRD patterns reveal the formation of diamond cubic-structured Si NCs after the magnesiothermic reduction, which means that the SiO2 phases (at 2θ≈23°) are nearly

Conclusions

In conclusion, a thermally-stable cross-linked H-Si NCs-PVDF anode system was investigated via e-beam irradiation at a melting temperature of PVDF through partial dehydrofluorination. The e-beam irradiated H-Si NCs-PVDF anode shows an improvement in the electrical performance, including the mitigation of capacity fading, increased diffusion coefficient of lithium-ion (DLi), exchange current density (i0) value, and dramatic decline in the SEI layer resistance (Rsei) compared to that of the

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP: Ministry of Science, ICT and Future Planning) (NRF-2015M2B2A4029368). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B01009523).

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