  In view of the long-standing controversy over the reversibility of transition
metals in Sn-based alloys as anode for Li-ion batteries, an in situ real-time
magnetic monitoring method was used to investigate the evolution of Sn-Co
intermetallic 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 intermetallic 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. These critical results shed light on understanding
the reaction mechanism of transition metals, and provide valuable insights
toward the design of high-performance Sn alloy based anodes.
