Aqueous rechargeable Li/Na-ion batteries have shown promise for sustainable large-scale energy storage due to their safety, low cost, and environmental benignity. However, practical applications of aqueous batteries are plagued by water's intrinsically narrow electrochemical stability window, which results in low energy density. In this perspective article, we review several strategies to broaden the electrochemical window of aqueous electrolytes and realize high-energy aqueous batteries. Specifically, we highlight our recent findings on stabilizing aqueous Li storage electrochemistry using a deuterium dioxide-based aqueous electrolyte, which shows significant hydrogen isotope effects that trigger a wider electrochemical window and inhibit detrimental parasitic processes.