Cation Disorder‐Driven d‐Band Center Engineering and Dual‐Mode Phonon Coupling Enable Ultrastable, High‐Rate K+ Storage in Ge–Sn Chalcogenides

Abstract Cation disorder presents a compelling strategy to simultaneously tailor lattice dynamics, electronic structure, and ion transport in alloy‐type anode materials. Herein, a Ge–Sn co‐substituted chalcogenide, Cu 2 Sn 0.5 Ge 0.5 S 3 , is designed that leverages compositional disorder to activate synergistic phonon–electron–ion coupling for high‐performance potassium‐ion storage. Isovalent substitution of Sn with Ge induces pronounced lattice distortion and coordination asymmetry, generating dual‐frequency phonon modes that combine soft Sn‐derived vibrations with Ge‐induced rigidity. This engineered phononic landscape facilitates stress‐adaptive structural responses, effectively accommodating large volume changes during cycling. Simultaneously, disrupted cation ordering introduces band tail states and enhances electronic delocalization, increasing the DOS near E F ​(transport‐relevant) and thereby facilitating charge transfer. Density functional theory calculations reveal a shallow d‐band center (−4.38 eV), which enhances orbital overlap with K 2 S x intermediates, promotes interfacial adsorption, and accelerates redox kinetics. Additionally, a low K + adsorption energy (−0.568 kcal mol −1 ) and the emergence of low‐electron‐density regions contribute to fast K + migration and efficient charge transfer. These combined effects yield a high reversible capacity (503.1 mAh g −1 ), excellent rate capability (10 A g −1 ), a high K + diffusion coefficient (6.17 × 10 −9 cm 2 s −1 ), and stability over 2000 cycles, establishing Cu 2 Sn 0.5 Ge 0.5 S 3 as a model system for stress‐resilient and kinetically optimized potassium‐ion batteries.