Revealing the Magnesium Storage Mechanism in Mesoporous Bismuth via Spectroscopy and Ab Initio Simulation.

Bismuth is a promising alternative to magnesium metal anode. It permits non-corrosive magnesium electrolytes with high oxidative stability, and encourages investigation of high-voltage cathode materials for magnesium ion batteries (MIBs). An in-depth understanding of the mechanism of Mg storage in bismuth is crucial for the search of reliable approaches to boost electrochemical performance of MIBs. Here, we present mesoporous bismuth nanosheets as a model to study the charge storage mechanism of Mg/Bi system. Using a systematic spectroscopy investigation of combined synchrotron-based operando X-ray diffraction, near-edge X-ray absorption fine structure and Raman, for the first time, we demonstrate a reversible two-step alloying reaction mechanism of Bi↔MgBi↔Mg 3 Bi 2 . Ab initio simulation methods disclose formation of the MgBi intermediate, and confirm its high electronic conductivity. This intermediate serves as buffer for the significant volume expansion (204%) and acts to regulate Mg storage kinetics. The mesoporous bismuth nanosheets, as an ideal material for investigation of Mg charge storage mechanism, effectively alleviate volume expansion and endow significant electrochemical performance in lithium-free electrolyte. These findings will benefit mechanistic understandings and advance material designs for MIBs.