The Doctoral Thesis is a thematically unified collection of four original research articles and one review paper. The study focuses on improving rechargeable aqueous Zn-MnO2 batteries by investigating the interaction between electrodes and electrolytes and their impact on the voltage. The study develops a pH-gradient, electrolyte concept that enables the extension of the water-splitting voltage window beyond 2 V. Additionally, the stability of the MnO2 cathode was enhanced through Bi3+ and Mo6+ ion doping, and the Zn anode cyclability was improved by laser-induced surface structuring. The results contribute to the development of safer, more sustainable energy storage technologies.