Understanding the Electrochemical Performances of Si Anodes Incorporating Mechanically Interlocked Binders Prepared from α-Cyclodextrin-Based Polyrotaxanes

Abstract

Rechargeable Li-ion batteries with larger autonomy are needed to meet increasing market demands, hence the intensive research effort to switch from graphite to silicon (Si) anodes, despite their detrimental massive volume changes upon cycling. Many elegant polymer chemistries have addressed this issue by designing smart binders capable of buffering Si electrodes fracturing and maintaining their overall structure. In this sense, self-healing PR–PAA binders relying on the α-cyclodextrin (α-CD) supramolecular chemistry and prepared by cross-linking poly(acrylic acid) (PAA) with α-CD based polyrotaxanes (PR) have recently been proposed. We herein further explore this binder chemistry to understand the proper function of such mechanically interlocked networks. We successfully synthesized a wide range of PR–PAA binders by varying their structural parameters: the doping ratio of PR, the cross-linking density, as well as the polymer molecular weight and the PR ring coverage. Then, their electrochemical performances were tested in nano-sized Si composite electrodes, and a structure/property correlation was evidenced. By promoting the α-CD sliding motion through both an increase of the PR doping fraction and a decrease of the PR ring coverage, we succeeded in making a PR−PAA-based Si electrode having an initial capacity of >3000 mA h/g and showing 82% capacity retention after 100 cycles as opposed to only 42% for PAA-based Si electrodes. A resulting better stress dissipation was evidenced by ex situ scanning electron microscope analysis as well as operando internal stress monitoring experiments via optical sensors. Altogether, this work emphasizes the benefits that CD-based supramolecular architectures can offer to the battery community for designing self-healing binders.