Internal Space Modulation of Yolk‐Shell FeSe ₂ @Carbon Anode with Peanut‐Shaped Morphology Enabling Ultra‐Stable and Fast Potassium‐Ion Storage

The poor cycling stability and rate performance of transition metal selenides (TMSs) are caused by their intrinsic low conductivity and poor structural stability, which hinders their application in potassium-ion batteries (PIBs). To address this issue, encapsulating TMSs within carbon nanoshells is considered a viable strategy. However, due to the lack and uncontrollability of internal void space, this structure cannot effectively mitigate the volume expansion induced by large K⁺, resulting in unsatisfactory electrochemical performance. Herein, peanut-shaped FeSe₂@carbon yolk-shell capsules are prepared by modulation of the internal space. The active FeSe₂ is encapsulated within a robust carbon shell and an optimal void space is retained between them. The outer carbon shell promotes electronic conductivity and avoids FeSe₂ aggregation, while the internal void mitigates volume expansion and effectively ensures the structural integrity of the electrode. Consequently, the FeSe₂@carbon anode demonstrates exceptional rate performance (242 mAh g^-1 at 10 A g^-1) and long cycling stability (350 mAh g^-1 after 500 cycles at 1 A g^-1). Furthermore, the effect of internal space modulation on electrochemical properties is elucidated. Meanwhile, ex situ characterizations elucidate the K⁺ storage mechanism. This work provides effective guidance for the design and the internal space modulation of advanced TMSs yolk-shell structures.