Multiscale Ion Transport Optimization in High-Mass-Loading Cathodes for Aqueous Zinc-Ion Batteries

High-mass-loading cathodes are essential for translating aqueous zinc-ion batteries (AZIBs) from laboratory demonstrations to practically relevant energy-storage devices because they increase the fraction of electrochemically active materials and thereby improve device-level energy density. However, increasing cathode loading and thickness simultaneously aggravates charge transport limitations, electrolyte infiltration insufficiency, and structural degradation, leading to severe polarization, poor active-material utilization, and rapid performance decay. In this review, we present a multiscale framework for understanding and optimizing high-mass-loading AZIB cathodes, spanning the microscale, mesoscale, and macroscale. At the microscale, we discussed how doping, vacancy, and interlayer-spacing modulation regulate electronic structure, Zn²⁺ diffusion, and structural stability of active materials. At the mesoscale, we examine pore architecture, cathode components, and fabrication strategies, with emphasis on porosity, tortuosity, electrolyte accessibility, and integrated charge transport in thick cathodes. At the macroscale, we highlight the importance of electrode balancing and electrolyte management for translating material-level advances into practical cell performance. We further outline emerging directions, including operando characterization, cross-scale modeling, data-driven materials discovery, and scalable manufacturing. By connecting material chemistry, electrode architecture, and device integration, this review provides a coherent roadmap for the rational design of high-mass-loading cathodes and the development of energy-dense, commercially relevant AZIB systems.