Revealing the Origin of Improved Reversible Capacity of Dual-Shell Bismuth Boxes Anode for Potassium-Ion Batteries

Nanostructured alloy anodes have been successfully used in several kinds of rocking-chair batteries. However, a full picture of the origin of their improved reversible capacity remains elusive. Here, we combine operando synchrotron-based X-ray powder diffraction and ex situ X-ray absorption near-edge structure spectroscopy to study the double-shell structured bismuth boxes as anodes in potassium-ion batteries to reveal the origin of their improved capacity. The nanostructured bismuth anode offers an enhanced capability to tolerate the volume expansion under a low current density of 0.2 C, resulting in a more complete alloy reaction. Additionally, under a high current density of 2 C, nanostructured bismuth anode with larger surface area offers more sites to electrochemically alloy with potassium and results in a lower average oxidation state of bismuth. These findings offer guidance for the rational design and engineering of electrode materials according to the current density for rocking-chair batteries.