Unveiling Copper‐Induced Phase Transitions and Degradation Mechanisms of Transition Metal Sulfide Anodes for Sodium‐Ion Batteries

Transition metal sulfides (TMSs) are promising anodes for sodium‐ion batteries (SIBs) owing to their high theoretical capacity and natural abundance. However, their electrochemical performance is strongly affected by the formation of sodium polysulfides (NaPSs), which induce complex phase transformations and electrode degradation. In particular, NaPSs react with the copper (Cu) current collector, leading to the formation of Cu‐incorporated sulfides, yet the underlying mechanisms remain insufficiently understood. Here, we unveil the fundamental origins of phase evolution in TMS‐based anodes. We reveal that NaPSs formed during early cycling react with Cu substrates to yield NaCu 5 S 3 and Cu 2 S interfacial phases, initiating a progressive transition toward Cu‐incorporated sulfides. This process is driven by the preferential substitution of Na + in the Na 2 S matrix with Cu ions during desodiation, ultimately evolving into Cu 1.8 S. Furthermore, we demonstrate that the deactivation and ionization of transition metals, followed by their redeposition on the counter electrode, constitute critical degradation pathways. Consequently, our investigation suggests Cu 1.8 S as the ultimate phase of TMS with the most stable phase. Our findings not only establish a mechanistic framework for Cu‐induced phase transitions in TMS anodes but also provide design principles for realizing durable and high‐performance SIB systems.