Nanofiber-in-microfiber carbon/silicon composite anode with high silicon content for lithium-ion batteries

Silicon-rich anodes are desired to leverage the energy capacity of lithium-ion batteries (LIBs) towards critical markets. We prepared new silicon-rich composite anodes with a nanofiber-in-microfiber architecture using a co-axial electrospinning setup. A polyvinyl alcohol (PVA) solution that allows high silicon content serves as the central stream, which holds silicon nanoparticles into short, branched composite nanofibers. These nanofibers were wrapped by long, ductile microfibers made of polyacrylonitrile (PAN) that is supplied in the sheath fluid. After carbonization, the received carbon/silicon composites were tested as the anode of LIBs, in which the silicon-rich nanofibers host the majority of lithium ions while their thin carbon skin originated from PVA promotes the conductivity and charge transfer. The outside PAN-derived microfibers provide needed structural support for those encapsulated silicon-rich nanofibers, making the final composites also an integrated, three-dimensional current collector. The nanofibrous morphology and the void space in between help accommodate the notorious volume expansion issues during lithiation/delithiation. The new composites were confirmed on their nanofiber-in-microfiber configuration. With a Si content of 40%, this unique fibrous anode material achieves ∼900 mAh g−1 specific capacity and ∼90% capacity retention from cycle 50 to cycle 250 by effectively balancing some major challenges associated with silicon-rich anodes.