A Polysaccharide\nBinder with Carbon Quantum Dots for\nImproved Flexibility of Si Anodes in Lithium–Ion Batteries

A polymer binder with high adhesive strength and flexible\nmechanical\nproperties was developed by introducing carbon quantum dots (CQDs)\ninto carrageenan (CGN), a type of polysaccharide obtained from marine\nred algae. The 350 nm silicon-based electrode containing CGN-CQD as\na binder showed improved cell performance compared to Si@CGN, fabricated\nwith CGN binder, due to the improved adhesive property and enhanced\nflexibility of CGN-CQD. These properties were due to the formation\nof multiple hydrogen-bonding networks between the numerous polar functional\ngroups of the CQDs and the CGN. This 3D network imparted flexible\nproperties to the electrode, accommodating the irreversible volume\nchanges of Si that occur during charging and discharging and demonstrating\nhigh electrode integrity. The optimized electrode, Si@CGN-CQD5, has\n5 wt % CQD relative to CGN and exhibited an initial discharge capacity\nof 3486 mA h g<sup>–1</sup> at a current density of 0.2 C and\na high capacity retention of 61.7% after 100 cycles. This is a significantly\nsuperior performance to the Si@CGN electrode, which showed an initial\ndischarge capacity of 3205.5 mA h g<sup>–1</sup> and capacity\nretention of only 12.2% after 100 cycles. Furthermore, the difference\nin capacity retention becomes more evident at a higher current density\nof 0.5 C. That is, the Si@CGN electrode showed a capacity retention\nof 4% after 100 cycles, while the Si@CGN-CQD5 electrode showed a capacity\nretention of 48%. The CGN-CQD binder exhibits electrode integrity\neven at higher charge and discharge rate of 0.5 C due to its improved\nflexibility and mechanical properties.