Dual lignin valorization enabled by carbon quantum dots and lithium-sulfur cathode

• Easily available lignin with a high abundance of β-aryl ether and hydroxyl groups. • Upgrading lignin into porous carbon and carbon quantum dots via a NaCl-template. • Fluorescence CQDs served as nanoprobe for the selective sensing of Fe 3+ ions. • Diffusion of molten sulfur within LPC voids to assemble a cathode of Li-S battery. • LPC@S cathode offers a stable cycling lifespan with acceptable specific capacity. Biomass-related technologies hold great promise to tackle the double crises of environmental expense and energy scarcity. How to exert the trash-to-treasure was a strong driving force upon lignocellulose biorefinery that emphasizes a request for upgrading lignin. As a guiding light for two-in-one valorization of low-cost biomass, we reported a NaCl-templated strategy to convert well-defined lignin into porous carbon (LPC) and carbon quantum dots (CQDs). Besides, the affluent pores and channels of LPC can be easily penetrated by molten sulfur to assemble a cathode of lithium-sulfur battery. The LPC@S-installed cathode was able to provide a high sulfur utilization up to 66 wt%, stable cycling lifespan with an initial specific capacity of 1066 mA h g −1 at 1 C (capacity decay of 0.095 % per cycle over 400 cycles), and acceptable rate capability at a charge/discharge current from 0.1–3 C. The lignin-based CQDs exhibited an average size of 4.18 nm, along with bright fluorescence, long-term stability, and water solubility. Benefitting from these alluring properties, CQDs was efficiently served as the fluorescence-switchable nanoprobes for sensitively detecting common metal ions, which presented a significant fluorescence quenching in response to the presence of ferric iron (Fe 3+ ) with a wide concentration range of 1–220 μM. It is reasonable to imagine that the present work delivers a versatile settlement to the end-up issue of technical lignin towards a circular material economy.