Differentiating contribution to desolvation ability from molecular structure and composition for screening highly-effective additives to boost reversibility of zinc metal anode

Abstract

Optimizing aqueous electrolytes with additives is a practical strategy to postpone dendrite formation on Zn metal anode (ZMA) and therefore boost rechargeability of full zinc metal batteries (ZMBs). However, the fundamental screening principles of additives diverse in molecular structure and composition are still elusive. Herein, thiourea, urea, and allantoin are investigated as additives to regulate ZnSO4 baseline electrolyte. By combining systematical electrochemical measurements with detailed numerical simulation analysis, two fundamental principles for screening the additives with stronger desolvation ability toward hydrated zinc ions have been identified: (i) in molecular composition, carbonyl (urea) is better than thiocarbonyl (thiourea), and (ii) in molecular structure, bidentate coordination mode (allantoin) outperforms monodentate mode (urea). Consequently, an electrostripping/plating lifespan over 600 h in the Zn||Zn symmetric cell cycling under a depth of discharge up to 5.2% is realized with the optimum allantoin-ZnSO4 hybrid electrolyte, significantly boosting the rechargeability (85.7% capacity retention over 2000 cycles) of assembled ZMA||carbon-cloth@MnO2 full ZMBs than that with additive-free ZnSO4 electrolyte (40.5%). This work provides closer insights into the correlation between desirable dendrite-free behavior of ZMA and molecular characteristics of additives, and practical guidance for rational selection of more efficient additives to inhibit hydration of Zn2+ and suppress dendrite grown on ZMA.