A Polysaccharide Binder with Carbon Quantum Dots for Improved Flexibility of Si Anodes in Lithium–Ion Batteries

A polymer binder with high adhesive strength and flexible mechanical properties was developed by introducing carbon quantum dots (CQDs) into carrageenan (CGN), a type of polysaccharide obtained from marine red algae. The 350 nm silicon-based electrode containing CGN-CQD as a binder showed improved cell performance compared to Si@CGN, fabricated with CGN binder, due to the improved adhesive property and enhanced flexibility of CGN-CQD. These properties were due to the formation of multiple hydrogen-bonding networks between the numerous polar functional groups of the CQDs and the CGN. This 3D network imparted flexible properties to the electrode, accommodating the irreversible volume changes of Si that occur during charging and discharging and demonstrating high electrode integrity. The optimized electrode, Si@CGN-CQD5, has 5 wt % CQD relative to CGN and exhibited an initial discharge capacity of 3486 mA h g–1 at a current density of 0.2 C and a high capacity retention of 61.7% after 100 cycles. This is a significantly superior performance to the Si@CGN electrode, which showed an initial discharge capacity of 3205.5 mA h g–1 and capacity retention of only 12.2% after 100 cycles. Furthermore, the difference in capacity retention becomes more evident at a higher current density of 0.5 C. That is, the Si@CGN electrode showed a capacity retention of 4% after 100 cycles, while the Si@CGN-CQD5 electrode showed a capacity retention of 48%. The CGN-CQD binder exhibits electrode integrity even at higher charge and discharge rate of 0.5 C due to its improved flexibility and mechanical properties.