The role of graphene oxide and graphene quantum dots synthesized from spent lithium-ion batteries for Highly efficient supercapacitors

• Graphene oxide (GO) and graphene quantum dots (GQDs) were synthesized from graphite recycled from spent LIBs • Adding GQDs to SCs increased the capacitance and energy-density by 131% and 275%, respectively • Adding GQDs to SCs provide extra oxygen-vacancies/defects and oxygen species that increased the charge storage • An ultimate capacitance of 2130 F g -1 was obtained in devices containing GQDs derived from LIB electrodes • The capacitance of SCs made with GO/GQDs was 130-150% higher than these made with only GO GO and GQDs synthesized from commercial graphite were named GO com and GQDs com , respectively. Furthermore, the GO and GQDs synthesized from recycled graphite (extracted from Lithium-ion-batteries) were named GO LIB and GQDs LIB , respectively. Electrochemical tests indicated that graphene supercapacitors (SCs) containing in their electrodes GO com and GO LIB produced maximum capacitances of 1131.4 F/g and 922.7 F/g, respectively. Those devices also generated energy densities of 33-35 Wh/kg. Later, SCs were made with electrodes containing GO com +GQDs com and GO LIB +GQDs LIB , consequently, the capacitance was enhanced up to outstanding values of 2130 F/g and 2519.1 F/g, respectively. This means an increase of capacitance by 122%-131% after adding the GQDs to the SCs (compared to the devices made with only GO). Surprisingly, energy densities also increased significantly by 263%-275% after adding the GQDs to the SCs. Furthermore, these devices exhibit remarkable electrochemical stability, since flexible supercapacitors using GO LIB and GQDs LIB retained 88% of capacitance after 1000 charge/discharge cycles and over 80% of capacitance after 500 bending cycles. XPS, Raman and FTIR analyses demonstrated the increment of oxygen vacancies defects and chemical species (peroxides, carboxyl groups, esters, carbonyl and alkoxy) after the introduction of GO and GQDs on the SC electrodes. All those species/defects contributed to the storage of charge by enhancing the conductivity and redox centers on the SC electrodes. Overall, this work demonstrates the promising potential of reusing graphite from spent lithium-ion batteries as a sustainable and effective material for next-generation flexible, high-performance SCs. At the same time, the battery waste is converted into value added products.