pH buffer KH₂PO₄ boosts zinc ion battery performance via facilitating proton reaction of MnO₂ cathode

Zinc-ion batteries (ZIBs) are rapidly emerging as safe, cost-effective, nontoxic, and environmentally friendly energy storage systems. However, mildly acidic electrolytes with depleted protons cannot satisfy the huge demand for proton reactions in MnO₂ electrodes and also cause several issues in ZIBs, such as rapidly decaying cycling stability and low reaction kinetics. Herein, we propose a pH-buffering strategy in which KH₂PO₄ is added to the electrolyte to overcome the problems caused by low proton concentrations. This strategy significantly improves the rate and cycle stability performance of zinc-manganese batteries, delivering a high capacity of 122.5 mAh/g at a high current density of 5 A/g and enabling 9000 cycles at this current density, with a remaining capacity of 70 mAh/g. Ex-situ X-ray diffraction and scanning electron microscopy analyses confirmed the generation/dissolution of Zn₃PO₄·4H₂O and Zn₄(OH)₆(SO₄)·5H₂O, byproducts of buffer products and proton reactions. In-situ pH measurements and chemical titration revealed that the pH change during the electrochemical process can be adjusted to a low range of 2.2-2.8, and the phosphate distribution varies with the pH range. Those results reveal that H₂PO₄^- provides protons to the cathode through the chemical balance of HPO₄^2-, HPO₄^2-, and Zn₃PO₄·4H₂O. This study serves as a guide for studying the influences and mechanisms of buffering additives in Zn-MnO₂ batteries.