Optimization and Constant Capacity Cycling of the Core-Shell Carbon Silicon Composite Anode for Lithium Ion Batteries

Accommodating the 3-fold volume expansion of silicon during lithiation is necessary to take advantage of its 10-fold larger capacity compared to industry-standard graphite. ¹In this report, we describe recent experiments at using core- -shell C-Si composites that showed significant improvements in cycling performance over conventional C-Si composites.

The core- -shell structure is evident in FESEM, as shown in Figure 1. The ~100 nm Si cores are encapsulated inside larger carbon shells, leaving ~50 nm of hollow space. Previous studies showed that these structures are capable of accommodating the volume expansion of Si during lithiation without distorting the carbon shell, while maintaining a stable SEI layer.²Thermal gravimetric analysis showed that Si constitutes 75 wt% of the composite, close to the nominal 77 wt% as calculated from synthesis procedure.

Cycling to a fixed capacity of 1000 mAh/g, the cycling efficiency of the composite anode was mostly above 90% after the initial SEI formation, as shown in Figure 2. After the first cycle, subsequent cycling loops appeared to have a consistent shape, indicating reversible reaction chemistry.

Optimization of the core-hollow-shell composite is achieved by fine-tuning the fabrication process and cycling conditions. Constant capacity cycling controls the state of charge by limiting the integrated charge flow into the cell. The cutoff state of charge and the current density both have a significant effect on the cell performance. XRD reveals factors contributing to the irreversible charge loss at every cycle.

Improvement of the homogeneity of chemical coating and mixing was also studied. The precursor polymer used to produce the coating of carbon on the silicon nanoparticles directly impacts the geometry of the final product. The procedure for etching away the silicon dioxide shell with hydrofluoric acid was improved as well. 

Reference

1.           Li, X. & Zhi, L. Managing voids of Si anodes in lithium ion batteries. Nanoscale(2013). doi:10.1039/c3nr03197g

2.           Liu, N. et al. A yolk-shell design for stabilized and scalable li-ion battery alloy anodes. Nano Lett. 12, 3315–21 (2012).