Universal Phase Engineering of High‐Entropy Sulfides for Stable Sodium‐Ion Storage With Ultra‐High Capacity and Ultra‐Fast Kinetics

High‐entropy transition metal sulfides (HESs) exhibit great potential as anodes for sodium‐ion batteries owing to their synergistic entropy stabilization, lattice distortion and cocktail effects. However, potential phase separation caused by multi‐component incompatibility severely limits their performance. Herein, we propose a low‐mixing‐enthalpy strategy through regulation of element chemical compatibility to precisely design high‐performance HES anodes. This strategy enables the successful synthesis of a single‐phase Co‐Fe‐Ni‐Mn‐Cr HES solid solution (HES‐Cr). In contrast, inferior compatibility among components in Co‐Fe‐Ni‐Mn‐Mo HES (HES‐Mo) leads to its phase separation. The electron delocalization in HES‐Cr enhances conductivity and metal‐sulfur bond covalency, while moderate lattice distortion alleviates volume changes and stress concentration during Na + insertion/extraction and lowers the Na + migration barrier. Consequently, the HES‐Cr delivers excellent Na + storage performance, including a high reversible capacity of 845.2 mAh g −1 at 0.2 A g −1 and ultra‐high rate property of 497.5 mAh g −1 even at 40.0 A g −1 along with long stability, outperforming HES‐Mo and most HES‐based anodes. Furthermore, we propose a three‐parameter descriptor to predict single‐phase high‐entropy materials across a broader compositional range. This work provides a new approach for rational design of single‐phase HESs and deepens understanding of their composition‐phase‐performance relationships.