Dr FU Jimin, Member of the Research Institute for Intelligent Wearable Systems (RI-IWEAR), and his collaborators from PolyU and Anhui University, China, have proposed a chemical mechanism for the effect of the Helmholtz plane on zinc mental anodes (ZMAs) in zinc sulphate (ZnSO4) electrolyte activated by zinc pyrrolidone carboxylate (PCA-Zn) additive. Their study provides insights on improving the life of aqueous zinc metal batteries (AZMBs) via (i) stabilisation of ZMAs and (ii) the use of additives to optimise diluted salt electrolytes for postponing dendrite formation on ZMAs and, hence, battery corrosion.
Battery performance is dependent on the cycling of ions. Battery corrosion occurs when prolonged electrolysis (i.e., water splitting) or zinc ion consumption reduces the plating/stripping of ZMAs. The Helmholtz plane (HP) can improve the stability and long-term cycling of ZMBs. The HP is a double-layer structure comprising two layers of opposite charges around a particle. The study by Dr Fu and his colleagues unravelled the chemistry that underlies the relationship between the HP and ZMAs’ stability.
Experimental and numerical simulations yielded three observations. First, as PCA-Zn dissociates in solution, the dissociated PCA− anions are zincophilic (i.e., Zn2+-attracting) and favourably adhere to the ZMA surface, and the carboxyl terminals of PCA- ions serve as anchors to replace H2O and SO42− ions. This process changes the structure of an H2O/SO42−-repellent HP to prevent water-triggered side reactions that contribute to battery corrosion. Second, the restructured HP has a spatial confinement effect on Zn2+ ions, effectively suppressing rampant Zn dendrite growth on ZMAs. Third, the HP exhibits self-repair behaviour against unexpected electrode surface damage upon cycling.
Through HP reconstruction and the PCA-Zn additive, the researchers developed ZMAs with greater stability, achieving an excellent Zn||Zn symmetric electroplating/stripping cyclic durability that exceeding 300 h under a depth of discharge up to 40% (20 mA cm−2/20 mAh cm−2). They significantly boosted the rechargeability of the AZMBs to an 89.7% capacity holding ratio after 5,000 cycles, as compared to AZMBs without PCA-Zn additive, which malfunctioned after 960 cycles.
This work helps to inform the selection of additives that optimise diluted salt electrolytes for the prevention of dendrite deposition on zinc mental anodes, thus enhancing the rechargeability of aqueous zinc metal batteries.
