Hydrophilic and nanocrystalline carbon quantum dots enable highly reversible zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have great potential in the field of stationary energy storage due to their low manufacturing cost and high safety. However, the zinc dendrite growth arising from the uneven deposition of zinc ions has been inhibiting the application of AZIBs. In this work, hydrophilic and nanocrystalline carbon quantum dots (CQDs) are synthesized and used as electrolyte additives to improve the dendrite issue. The abundant hydrophilic groups on CQDs are favorable for homogeneous Zn deposition, whereas the improved conductivity due to the high graphitization of CQDs reduces the solid-electrolyte interface impedance and lowers the polarization voltage, resulting in better rate capability of Zn-ion batteries. Owing to the reduced energy barrier of Zn2+ nucleation and the enhanced kinetics of Zn2+ plating/stripping derived from the hydrophilic and nanocrystalline CQDs, the Zn//Zn symmetrical batteries with CQDs can operate stably for more than 1000 h at 1 mA cm-2 and 1 mA h cm-2, and show superior rate performance with low voltage hysteresis of 98.3, 120.5, 156.4, and 210.2 mV at current densities of 1, 2, 3 and 4 mA cm-2. Moreover, Zn//Cu half-cells with CQDs achieve a high average coulombic efficiency of 99.7% at 1 mA cm-2 and 0.5 mA h cm-2, and Zn//MnO2 full batteries can be cycled more than 500 times with a capacity retention of 74%. Carbon quantum dots (CQDs) with rich functional groups and good nanocrystalline features are developed as an electrolyte additive for zinc-ion batteries, endowing the zinc anode with both high reversibility and fast kinetics.