Tin is a promising anode material for the storage of large amounts of lithium and sodium; it currently boasts one of the highest specific charges: 990 mAh/g and 846 mAh/g in lithium-ion and sodium-ion batteries upon conversion to Li_4.4Sn and Na_3.75Sn, respectively¹. However, storing such large quantities of these ions is typically accompanied by large volume expansions of up to 400%, which leads to particle fracture and thus electrical isolation resulting in rapid capacity fade with cycle number. By alloying an active metal with an inactive metal the reaction mechanism can be modified, allowing substantial specific charge to be achieved while mitigating the detrimental effects of the volume expansion.

Here, we alloy Sn with inactive transition metals Mn²⁵, Fe²⁶ and Co²⁷ to form the tetragonal intermetallic MSn₂ (I4/mcm). Since none of the three transition metals form alloys with Na on their own, they are expected to be inactive in the storage of Na; however the three MSn₂ materials (M = Mn, Fe, Co) do not behave identically during cycling in sodium ion batteries, demonstrating that the transition metal in fact plays an “active role” in such systems. With the three materials being isostructural and the transition metals being adjacent to each other in the periodic table, the precise influence of the electronic structure of the supposedly inactive transition metal on the active system (here Sn) could be identified.

Using electrochemical measurements (CV, GITT, galvanostatic cycling and rate capability), ex situ and operando X-ray powder diffraction (XRD) data and X‑ray absorption spectroscopy (XAS) we elucidate the reaction mechanism of all three materials and uncover that the reaction between Na and MSn₂ (M = Mn, Fe, Co) is facilitated by intermediate state formation, dependent on the transition metal. The results of this study give an insight into designing the most effective combinations of active and inactive metals that may be selected as suitable alloy hosts for the next generation of novel battery materials.

References:

(1)           Komaba, S.; Matsuura, Y.; Ishikawa, T.; Yabuuchi, N.; Murata, W.; Kuze, S. Electrochemistry   Communications 2012, 21, 65.

Acknowledgment:

The Swiss National Science Foundation is thanked for financial support (Project 200021_156597). This work was performed within the Swiss Competence Center of Energy Research Heat and Storage (SCCER) framework.