Built-In Electric Fields in Rechargeable Batteries: Formation Mechanisms, Functional Roles, and Design Principles.

Rechargeable batteries (lithium-ion, sodium-ion, zinc-ion) are core technologies for large-scale renewable energy storage, featuring high energy density, conversion efficiency, and application flexibility. However, their development remains hindered by structural instability, sluggish ion/electron transport, and uncontrolled interfacial reactions during cycling. The built-in electric field (BIEF) is an intrinsic physical field that forms spontaneously without external bias, which could regulate the local potential distribution and guide the migration behavior of electrons and ions, showing significant potential for improving reaction kinetics and suppressing structural instability. Through the rational design of heterointerfaces, polarized structures, and composition or defect gradients, BIEFs effectively alleviate volume expansion, homogenize reaction fronts, accelerate carrier transport, and stabilize interfacial evolution. In this review, we provide a systematic overview of BIEF formation mechanisms, characterization methods, theoretical calculations, and functional roles in anodes, cathodes, and electrolytes. Recent progress in characterization techniques and theoretical calculations for clarifying BIEF mechanisms is summarized, along with key engineering strategies and design principles for BIEF construction. We hope that this review could provide useful guidance for the predictive design and practical implementation of BIEF in next-generation batteries with high energy density and long cycle life.