The field of rechargeable batteries has witnessed significant advancements driven by the increasing demand for efficient and sustainable energy technologies. As a key component of rechargeable battery systems, electrolytes play a crucial role in determining the battery reversibility and stability. Nevertheless, the unsatisfactory ion conductivity and limited low-temperature behaviors of prevailing electrolytes greatly hinder the battery application scenarios. High-entropy electrolytes (HEEs) have attracted extensive attention due to their potential to solve the above issues. However, the ambiguous concept of HEEs, the lack of guidance for electrolyte component screening and optimization, and the unclear impact of HEEs on the electrode|electrolyte interface seriously impede the practical viability of HEEs. Herein, for the first time, we present a survey of emerging HEEs, spanning from design principles to performance optimization. We summarize the ion-transport mechanisms and fundamental properties of various classes of HEEs, including liquid, quasi-solid and all-solid HEEs, and review the recent advances in rechargeable alkali metal (e.g., Li and Na)-based battery and multivalent-ion (e.g., Mg and Zn) battery systems in which their performances can be intrinsically enhanced by HEEs. In particular, the interaction between the high-entropy solvation/crystal structure and battery performance is highlighted. Finally, we point out the main challenges encountered in developing batteries coupled with HEEs and provide a perspective for future breakthroughs.