A hybrid Si@FeSiy/SiOx anode structure for high performance lithium-ion batteries via ammonia-assisted one-pot synthesis

Abstract

Synthesised via planetary ball-milling of Si and Fe powders in an ammonia (NH₃) environment, a hybrid Si@FeSi_y/SiO_x structure shows exceptional electrochemical properties for lithium-ion battery anodes, exhibiting a high initial capacity of 1150 mA h g⁻¹ and a retention capacity of 880 mA h g⁻¹ after 150 cycles at 100 mA g⁻¹; and a capacity of 560 mA h g⁻¹ at 4000 mA g⁻¹. These are considerably high for carbon-free micro-/submicro-Si-based anodes. NH₃ gradually turns into N₂ and H₂ during the synthesis, which facilitates the formation of highly conductive FeSi_y (y = 1, 2) phases, whereas such phases were not formed in an Ar atmosphere. Milling for 20–40 h leads to partial decomposition of NH₃ in the atmosphere, and a hybrid structure of a Si core of mixed nanocrystalline and amorphous Si domains, shelled by a relatively thick SiO_x layer with embedded FeSi nanocrystallites. Milling for 60–100 h results in full decomposition of NH₃ and a hybrid structure of a much-refined Si-rich core surrounded by a mantle of a relatively low level of SiO_x and a higher level of FeSi₂. The formation mechanisms of the SiO_x and FeSi_y phases are explored. The latter structure offers an optimum combination of the high capacity of a nanostructural Si core, relatively high electric conductivity of the FeSi_y phase and high structural stability of a SiO_x shell accommodating the volume change for high performance electrodes. The synthesis method is new and indispensable for the large-scale production of high-performance Si-based anode materials.