Cycling performance of silicon-carbon composite anodes enhanced through phosphate surface treatment

Silicon (Si)-based anodes have long been viewed as the next promising solution to improve the performance of modern lithium-ion batteries. However, the poor cycling stability of Si-based anodes impedes their application and calls for solutions for further improvements. In the present work, the incorporation of phosphate groups on the surface of an amorphous Si-carbon composite (a-Si/C) has been achieved by a hydrothermal reaction using phosphoric acid and sodium dihydrogen phosphate at pH = 2. Different levels of the surface P-doping have been realized using reaction times (2, 4, and 8 h) at two different phosphate concentrations. The presence of phosphate groups on the particle's surface has been confirmed by energy-dispersive X-ray, infrared, and Raman spectroscopy. The cycling stability of the P-treated a-Si/C composites has been significantly improved when using lithium bis(trifluoromethanesulfonyl)imide as a salt in ether-based solvents mixture compared to a conventional electrolyte for Si-based anodes (LiPF₆ in carbonate-based solvents). Coulombic efficiencies as high as 99% have been reached after five charge/discharge cycles for almost all phosphate-treated materials. The 4 h P-treated a-Si/C composite electrode exhibits the best reversible capacity of 1598 mAh g⁻¹ after 200 cycles demonstrated in half-cells using an ether-based electrolyte.