Silicon based anode has attracted considerable attention to replace graphite as a high energy density anode material for lithium ion batteries due to the high theoretical specific capacity of silicon (~4200 mAh/g). However, bulk crystalline silicon exhibit poor performance due to colossal volumetric expansion (~400%) on reaction with lithium. The past decade has witnessed different approaches to address and counter the issue of mechanical degradation of silicon anodes. These approaches involve the use of silicon nanoparticles [1], composite structures of amorphous silicon [2], active-inactive matrix [3], electrodeposited thin films and VACNT-Si heterostructures [4-6]. However, the systems developed are either based on high cost synthesis approaches or utilize pre synthesized silicon nano particles and still suffer from poor capacity retention. Silica and silicon oxides are commercially cheap sources to synthesize silicon and high energy mechanical milling (HEMM) is also a well-known commercial processing technology to generate nano particles/nano-composites.

In the present work, nanocrystalline Si, nc-Si/metal oxide/carbon composite (nc-Si/MO/C) has been synthesized and studied as a promising anode material for Li-ion batteries. Reduction of commercially available silicon monoxide has been carried out with suitable metal based reducing agents using an in-situ high energy mechanochemical reduction (HEMR) followed by low temperature heat treatment to enable completion of the reduction reaction. This directly generates silicon and metal oxide based nanocomposite which acts as an electrochemically active/inactive composite for use in lithium ion batteries. The obtained nanocomposite material was embedded in different carbon (C) based matrices to further improve the performance of the system. Slurry based electrodes of these nanocomposites mixed with binder and conductive additives were then fabricated and tested in a half cell configuration within the voltage range of 0.01V – 1V vs. Li/Li⁺ in 1M LiPF₆ (dissolved in EC:DEC:FEC=45:45:10) electrolyte.

XRD pattern (Fig 1a) shows the evolution of the nanocrystalline silicon (nc-Si) and decrease in intensity of SiO amorphous broad peak with the increase in milling time from 20h to 40h, hence, indicating the reduction of SiO by the metal based reducing agent to form nc-Si. The heat treatment of the material obtained after 40h HEMR (Fig 1a) increases the crystalline nature of silicon and improves the formation of Si by completing the reaction. The XRD pattern (Fig 1a) after heat treatment shows no unreacted SiO phase indicating the completion of reduction process. FTIR, Raman spectroscopy, SEM and TEM analysis has been conducted on the material at different stages of reduction and heat treatment process to study the evolution of the phases in the nanocomposite.

The nanocomposite embedded in carbon (nc-Si-MO/C) system shows a first and second cycle discharge capacity ~1500mAh/g and ~1340mAh/g at a current rate of ~50mA/g with a first cycle irreversible (FIR) loss of ~25%-35% (Fig 1b). The obtained capacities are in agreement with theoretical calcluated specific capacity of the composite. Following long term cycling, the system shows a stable specific capacity of ~730mAh/g after 120 cycles at a charge/discharge rate of ~500mA/g with a columbic efficiency of ~99.65-99.82% and a fade rate of ~0.15% capacity loss per cycle. Retention of the MO matrix in the HEMR generated composite coupled with the interconnected carbon acts as an efficient buffer to relieve the stresses generated during alloying/dealloying and also maintains the electrical continuity during expansion of the nc-Si. Results of these studies combined with the extensive electrochemical characterization including electrochemical impedance, rate capability and SEM analysis of the electrodes before and after electrochemical cycling will be presented and discussed.

Acknowledgement:

The authors gratefully acknowledge financial support of the DOE-BATT (DE-AC02-05CHl1231), DOE-PNNL and NSF (CBET-0933141) programs. 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.

References:

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