Boosting electrochemical reactivity of tin as an anode for Mg ion batteries through introduction of second phase

Tin (Sn)-based anodes attract extensive attention for magnesium ion batteries however challenges and issues exist, such as relatively difficult alloying reaction, sluggish diffusion kinetics and rapid capacity decay. Herein, we introduce a second phase of bismuth (Bi) with varying contents into Sn via magnetron co-sputtering and get insight into how the Bi introduction boasts the electrochemical reactivity of Sn through experimental and density functional theory (DFT) calculations. The experimental results reveal that the introduction of Bi can effectively trigger the alloying reaction of Sn with Mg and the further increasing of Bi significantly improves the electrochemical performance of Sn-Bi electrodes. Moreover, the activated function of introducing the Bi phase into Sn is found in the bulk rolled Sn-Bi system. The DFT calculations demonstrate that the introduction of Bi into Sn obviously lowers the defect formation energy of Mg insertion in Sn, which further rationalizes the experimental results. Additionally, operando X-ray diffraction is performed to probe the activation process and magnesiation/demagnesiation mechanisms of the Sn-Bi electrode. The activation mechanism of Bi on Sn alloying with Mg is discussed, considering the function of second phase, the influence of Bi solid solution, as well as the size effect.