High-yield synthesis of graphene quantum dots from spent graphite and application in hydrogel Zn batteries

With the widespread development of electric vehicles, the number of spent lithium-ion batteries (LIBs) is steadily increasing each year. Meanwhile, the spent graphite anode, with low regeneration value and significant environmental impact, has emerged as an increasingly concerning issue. In this study, we adequately utilize the characteristic features of spent graphite, such as large interlayer spacing, abundant defects and a broken crystal structure resulting from repeated charge-discharge cycles, to fabricate uniform and single-layer graphene quantum dots (GQDs). Even more impressively, the yield of spent graphite derived GQDs is as high as 61.3%, which is much higher than that of GQDs from pristine graphite (11.8%). These GQDs with abundant hydrophilic functional groups are anchored onto a polyvinyl alcohol (PVA) matrix to construct a composite hydrogel electrolyte (0.8GQDs@PVA@Zn). When employed in zinc-ion batteries, ultra-stable cycling for 5500 h in Zn‖Zn symmetric cells and 9000 cycles in Zn‖Cu half-cells is realized. The outstanding battery performance can be attributed to the ability of GQDs to enhance the mechanical properties of the hydrogel electrolyte, regulate the composition and distribution of the solid electrolyte interphase, and modify the Zn²⁺ flux. This work offers a high-value recycling strategy for graphite anodes from spent LIBs.