A novel sol-gel process to encapsulate micron silicon with a uniformly Ni-doped graphite carbon layer by coupling for use in lithium ion batteries

Abstract

Silicon-based materials (e.g., nanosilicon and porous silicon) are being actively researched for use as anodes in high-energy Li-ion batteries (LIBs), which have high specific capacities and long charging/discharging cycling lifespans. However, the enormous specific surface area of nanosilicon and porous silicon results in a high preparation cost, agglomeration, and a low initial coulombic efficiency (ICE), all of which hinder industrialization. Low-cost micron bulk silicon has an extremely huge volume change and a low conductivity during alloying/dealloying and therefore cannot be directly used as an anode. Here, we develop a novel sol-gel method that couples low-cost micron silicon (Si) to a uniformly Ni-doped carbon layer. The Ni-mediated polymerization of sodium alginate (SA) by coulombic self-assembly and produces ionic self-locking, and is followed by in situ freeze-drying and low-temperature carbonization to produce the desired composite, Si/Ni@C. Si/Ni@C exhibits a crack-free graphite carbon layer, which increases the Si ICE of 75.5% to 84.6% and reduces the impedance; this carbon layer also promotes long-cycle stability, resulting in a Si/Ni@C capacity of 516 mAhg−1 after 100 cycles at 0.1 Ag−1, which is higher than that of Si (84 mAhg−1). This result is attributed to the increased cross-linking of SA by Ni-doping, which enhances the mechanical strength of the carbon layer and the conductivity and reduces side reactions and the volume change of Si during alloying/dealloying.