Emerging trends and prospects in aqueous electrolyte design: Elevating energy density and power density of multivalent metal-ion batteries

Aqueous batteries, using multivalent metallic charge carriers (Zn2+, Mg2+, Ca2+, Al3+), show promise as next-generation electrochemical energy storage due to their adequate energy density, high power density, and cost-effectiveness. The electrolyte, serving as a bridge between the cathode and anode, plays a crucial role in functionality. However, the use of aqueous electrolytes introduces issues such as limited voltage range, slow kinetics of multivalent cations, and side reactions on metal surfaces. Electrolyte engineering has emerged as a key solution to overcome major challenges in multivalent metal-ion batteries. This innovative approach involves optimizing factors such as electrolyte concentration, specific additives, pH management, and manipulating the ligand effect of cation salts. Additionally, incorporating anti-freezing agents is crucial. A systematic review of this domain is crucial for further development and refinement of electrolyte engineering strategies. This review should primarily focus on assessing how these modifications can enhance energy and power densities, broaden the operational range of the batteries (by adjusting water activity and LUMO/HOMO energy levels), improve cation transport (via enhanced ion conductivity, dielectric constant, transference number, and solvation structure), and effectively mitigate detrimental side reactions, such as metal deposition, interphase formation, and hydrogen bonding regulation. The electrolyte design strategies in multivalent metal-ion systems are outlined in terms of their effectiveness in smoothing metal dendrites, managing solvation shells and free water molecules, and minimizing side reactions between the metal surface and electrolyte. The article proposes future innovation directions and development prospects for enhancing electrolyte design engineering in these systems.