Unraveling the Evolution of Transition Metals during Li Alloying–Dealloying by In-Operando Magnetometry

In view of the long-standing controversy over the reversibility of transition metals in Sn-based alloys as an anode for Li-ion batteries, an in situ real-time magnetic monitoring method was used to investigate the evolution of Sn–Co alloy during the electrochemical cycling. Sn–Co alloy film anodes with different compositions were prepared via magnetron sputtering without using binders and conductive additives. The magnetic responses showed that the Co particles liberated by Li insertion recombine fully with Sn during the delithiation to reform Sn–Co alloy into stannum-richer phases Sn7Co3. However, as the Co content increases, it can only recombine partially with Sn into cobalt-richer phases Sn3Co7. The unconverted Co particles may form a dense barrier layer and prevent the full reaction of Li with all the Sn in the anode, leading to lower capacities. In addition, we also showed that the Fe can recombine with Sn (Sb) during the delithiation in the Sn (Sb)–Fe alloy film anodes by operando magnetometry. These critical results shed light on understanding the reaction mechanism of transition metals and provide valuable insights toward the design of high-performance Sn (Sb)-based alloy anodes.