A lithiation/delithiation mechanism of monodispersed MSn5 (M = Fe, Co and FeCo) nanospheres

Abstract

A designed Sn based alloy host as a higher capacity and longer cycle life next generation lithium-ion battery, consisting of monodisperse nanospheres of intermetallic MSn₅ (M = Fe, Co and FeCo) phases was synthesized by a nanocrystal conversion chemistry method using preformed Sn nanospheres as templates. The same crystal structure, identical particle surface morphology and the similar particle size distribution (30–50 nm) of these intermetallic MSn₅ (M = Fe, Co and FeCo) phases are ideal for comparison of the electrochemical performance, reaction mechanism, thermodynamics and kinetics during lithiation/delithiation. Importantly, MSn₅ (M = Fe, Co and FeCo) phases with defect structures Fe_0.74Sn₅, Co_0.83Sn₅ and Fe_0.35Co_0.35Sn₅, exhibit the highest theoretical capacity of >917 mA h g⁻¹ among the reported M–Sn (M is electro-chemically inactive) based intermetallic anodes. The ex situ XRD and XAFS illustrate the complete reversibility of MSn₅ (M = Fe, Co and FeCo) phases during lithium insertion/extraction for the first cycle. The Fe_0.35Co_0.35Sn₅ anode can take advantage of both high capacity of Fe_0.74Sn₅ and long cycle life of Co_0.83Sn₅, providing 736 mA h g⁻¹ and maintaining 92.7% of initial capacity after 100 cycles with an average capacity loss of only 0.07% per cycle. The excellent electrochemical performance of the Fe_0.5Co_0.5Sn₅ system is attributed to higher reversibility, lower reaction resistance. This work provides a novel insight toward designing and exploring an optimal Sn based alloy anode for next generation Li-ion batteries.