Theoretical mechanisms and experimental validation of hard vs soft carbon coatings for enhanced silicon anode performance

Silicon (Si)-based anodes are finding their niche in high-energy Li-ion batteries due to their overwhelming lead on capacity compared to graphite anodes. However, the low conductivity and drastic volume expansion during the lithiation process block their large-scale applications. Although the carbon coating methods have been investigated extensively and acknowledged as the most effective strategies, so far, their mechanisms are still only roughly attributed to the high conductivity and stability of the amorphous carbon shells. Especially the unique functional characteristics of hard carbon (HC) and soft carbon (SC) remain elusive, thus restricting the full utilization of Si. In this perspective, under the guidance of theoretical calculation and the assistance of characterization, we analyzed the various attributes of ionic-electronic conductivity, electrolyte selective permeation, and mechanical stability of the HC and SC coatings during the de-/lithiation processes of electrodes. It is concluded that the SC-coated Si demonstrates superior comprehensive electrochemical performance compared to the HC-coated Si. This work offers a comprehensive insight into the correlation between the physicochemical properties of various carbon coatings and the electrochemical performance of their composites. By elucidating these relationships, it paves the way for the rational design, selection, and optimization of carbon-coated Si-based materials, facilitating their application across diverse scenarios in grid-scale energy storage.