The electrochemical-grade natural graphite flake prices are increasing day by day. Reusing and recycling graphite materials from the spent lithium-ion battery (LIB) is a prospective way to overcome the issue. This report presents the synthesis of reduced graphene oxide (RGO) from spent LIB by the improved Hummers method followed by calcination at 600 °C (RGO-600). S, N-RGO-600 was prepared by doping sulfur and nitrogen with RGO-600 through hydrothermal synthesis. Assynthesized S, N-RGO-600s have sheet-like morphology having uniform heteroatom doping. S- and N-doped RGO-600 delivers 375 F g−1 at 5 A g−1 compared to RGO-600 of 233 F g−1 and retains > 98% capacitance over 20,000 cycles. The lead–carbon hybrid ultracapacitors fabricated using in-situ activated PbO2 as cathode and S, N-RGO-600 composite electrode as anode deliver a specific capacitance of 564 F g−1 at 5 A g−1 and retain 90% capacitance after 15,000 cycles. The high capacitance and stable cycle life of RGO and S, N-RGO are due to easy access of electrolyte ions through mesoporous and layered graphitic carbons with redox-active functional moieties of sulfur and nitrogen. This work illustrates an easy and scalable synthesis root for RGO and S, N-RGO. Graphical abstract: [Figure not available: see fulltext.]. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.