Scalable solvent-free mechanofusion and magnesiothermic reduction processes for obtaining carbon nanospheres-encapsulated crystalline silicon anode for Li-ion batteries

Abstract It is unquestionable that Si nanostructures i.e., nanosheets, nanowires, nanoparticles become more and more importance in high-energy lithium ion batteries (>300 Wh kg−1). However, the current commercial Si nanostructures are rather expensive, which is not yet able to complete with the graphite anode in term of USD/Wh kg−1. Herein, the conventional food grade non-porous silicon dioxide (SiO2) which costs 270-times lower than Si micron-sized and 3000-times lower than Si nano-sized (10 nm) is selected as a precursor for synthesizing pure crystalline Si nanoparticles. A novel scalable solvent-free mechanofusion method was firstly introduced to synthesize carbon nanospheres-encapsulated SiO2 (SiO2@C) followed by the reduction process producing silicon-carbon nanoparticles core-shell materials (Si@C) with interparticle void space. The carbon shell can prevent the volume expansion (>400%) of inner Si particles, which is a critical drawback of Si anode. In addition, the pre-lithiated Si@C anode obtained by a direct contact with Li counter electrode can address the poor coulombic efficiency at the first cycle. The pre-lithiated Si@C anode can deliver a reversible discharge specific capacity of 1390 mAh g−1 with a remarkable capacity retention of 90.4% and coulombic efficiency of ∼100% after 1000 cycles at a high rate of 1C. The ex situ TEM and XPS investigated confirm that the inner Si is well-confined within carbon buffer shell without being directly exposed to the electrolyte. Besides, an in operando XRD shows the reversible phase transformation during cycling for which Li15Si4 alloy is the product indicating that Si@C prepared in this work may be an ideal practical anode of high-energy Li-ion batteries.