Controlled Solvation Structure of a Zn Ion in an Aqueous Electrolyte by Amine Additives for Long Cycle Life of a Large Capacity Zn-Air Rechargeable Battery

A rechargeable Zn-air battery is expected as a large capacity battery alternative to the Li ion battery; however, short cycle life is the current issue. In this study, it was found that the addition of 3,3-diaminodipropylamine was effective for preventing the formation of a Zn dendrite in the anode resulting in a long cycle life with more than 300 cycles and a discharge capacity larger than 700 mAh/g (>85% Zn used for discharge) when Ni_0.8Fe_0.2Co₂O₄ and 8 M KOH-H₂O were used for the air electrode and electrolyte, respectively. The increased cycle stability could be assigned to the change in the solvated structure of Zn²⁺ in the electrolyte by the strong interaction between 3,3-diaminodipropylamine and solvated water, which is suggested by Raman and Fourier transform infrared spectroscopic analyses. Ab initio molecular dynamics simulations suggest that the amine is strongly interacted with OH of solvated water molecules and changes the coordination number of Zn²⁺, and as result, the size of the solvated Zn²⁺ cluster was increased, resulting in enhanced Zn²⁺ mobility in the electrolyte for the uniform deposition of Zn on the anode during charge by increased mobility.