Synthesis of High Performance Si Nanoflakes and Nanorod Anode Morphologies Using Water Soluble Recyclable Templates for Lithium Ion Batteries

Abstract

Silicon based anode systems have attracted considerable attention as alternative materials to replace graphite as a high energy density anode for lithium ion batteries due to silicon’s high theoretical capacity (~4200mAh/g). However, the material suffers from colossal volumetric expansion (~400%) associated with the formation of Li4.4Si phase during the lithiation reaction. Over the past few years, different approaches have been developed to address this issue which causes mechanical degradation and results in rapid loss in capacity of silicon anodes. These approaches involve the use of silicon nanoparticles [1], composite structures of amorphous silicon [2], active-inactive matrices [3], electrodeposited thin films and VACNT-Si heterostructures [4-6]. Nanostructures of silicon have been shown to withstand the mechanical pulverization by providing stress – strain relaxation mechanisms thus, showing improved cycling stability. However, most of the synthesis methods used to generate these Si nanostructures involve high processing costs, complex machinery/equipment, utilization of expensive templates and large number of processing steps. Additionally these materials show higher first cycle irreversible (FIR) loss and have poor areal loading densities. In the present work, Si nanostructures are generated using a simple two-step procedure employing a low-cost recyclable template. The template was generated using a high throughput high energy mechanical milling process (HEMM) utilizing an abundant, cheap and water soluble material. Si nanoflakes and Si nanorods (deposited using low pressure chemical vapor deposition) were obtained by varying the template synthesis conditions. The template is then recovered and reused by washing in water making the procedure amenable for commercialization. Slurry based electrodes of these nanostructures mixed with binder and conductive additives were then fabricated and tested in a half cell configuration against lithium foil between the voltage range 0.01V – 1V vs. Li/Li+ in 1M LiPF6(dissolved in EC:DEC:FEC=45:45:10) electrolyte. Fig. 1a shows the XRD patterns of the two different morphologies of silicon obtained using this simple two-step procedure. It can be seen therein that the procedure can be tailored to obtain either amorphous (Si flakes) or nanocrystalline (Si rods) silicon materials. The aim of this study is to understand the effect of the synthesis conditions on the morphology, crystal structure and subsequent electrochemical performance of low-cost template-derived silicon nanostructures. It can be seen in Fig. 1b that both morphology and crystallinity affect the silicon capacity and cycling behavior of both silicon morphologies. Though Si rods show a superior first cycle discharge capacity (~2790 mAh/g at current rate of 50mA/g; FIR loss ~12%-13%) as compared to Si flakes (~2930 mAh/g at current rate of 50mA/g; FIR loss ~17%-20%), the long term cycling behavior is in stark contrast, with Si nanoflakes and nanorods showing a fade rate of ~0.03% and ~0.4% loss per cycle, respectively, due to the differences in morphology and crystallinity. Additionally, electrodes with areal loading density of ~1.3 – 1.5mg/cm2were obtained independent of the morphology of the silicon nanostructures. Furthermore, Si nanoflakes and nanorods showed a specific discharge capacity of ~1150mAh/g and ~1025mAh/g, respectively (at a current rate of 1A/g), at the end of 150 cycles. Results of these studies will be presented and discussed. Acknowledgement: The authors gratefully acknowledge financial support of DOE contract administered through PNNL and NSF-CBET 1511390. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.