Negative Fading Phenomenon in Si/C Composite Anode with a Mixed Carbon Layer

The research into the causes of negative fading, an anomalous reversible cycling phenomenon where the cycling capacity increases with the number of cycles, is considered to potentially offer insights into achieving a leap in the cycling performance of anode materials. However, for the carbon-coated Si composite anode, which is regarded as the most promising alternative to a graphite-based anode, this phenomenon has received little attention. In this study, the Si nanoparticles coated with a mixed carbon layer derived from polydopamine and waterborne polyurethane were fabricated, and the negative fading behavior of the carbon-coated Si composite was systematically investigated. The discharging specific capacity of this anode gradually increased from 222.91 mAh g^–1 in the fourth cycle to 770.63 mAh g^–1 in the 210th cycle, and finally declined to 559.93 mAh g^–1 after 300 cycles. Morphological and electrochemical analyses indicate that the observed negative fading phenomenon can be attributed to the transition of flower-like carbon-coated Si aggregates from micrometer-scale to nanoscale during the cycling process, thereby enabling lithium ions to gradually intercalate into the internal Si nanoparticles. This work elucidates the correlation between morphological evolution and negative fading behavior in Si/C anodes, offering a valuable reference for the study of the abnormal cycling behavior in Si-based anodes.