ABSTRACT The transient intermediate phase Na x Cu y S z formed during the sodiation of copper sulfide anodes has recently been recognized as a key species governing fast ion/electron transport and exceptional structural resilience in sodium‐ion batteries. However, its direct synthesis remains elusive due to intrinsic kinetic instability. Herein, we unveil that the kinetic instability of this class of phases arises from the calculation of phonon dispersion, explaining the failure of conventional solid‐state routes. Furthermore, we establish a rational design framework based on lattice compatibility and anion‐binding energy deviation, followed by systematic phonon spectrum calculations to screen not only the dopant element but also the optimal doping concentration, stabilizing the thermodynamics and suppress kinetic instabilities. Guided by this principle, we achieve the first successful solid‐state synthesis of a long‐sought intermediate phase, NaCu 1.5 Co 0.5 S 2 . NaCu 1.5 Co 0.5 S 2 exhibits outstanding rate capability (337.9 mAh g − 1 at 50 C) with a high capacity of 589.9 mAh g − 1 at 0.1 C and robust cycling stability (>5000 cycles at 30 C). Our study not only realizes the targeted synthesis of a critical but previously inaccessible phase but also provides a generalizable design paradigm for stabilizing metastable functional materials, opening new avenues for next‐generation fast‐charging battery chemistries.
Sun et al. (Fri,) studied this question.
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