Predictions of particle size and lattice diffusion pathway requirements for sodium-ion anodes using η-Cu₆Sn₅ thin films as a model system

Geometrically well-defined Cu₆Sn₅ thin films were used as a model system to estimate the diffusion depth and diffusion pathway requirements of Na ions in alloy anodes. Cu₆Sn₅ anodes have an initial reversible capacity towards Li of 545 mA h g^-1 (Li_3.96Sn or 19.8 Li/Cu₆Sn₅), close to the theoretical 586 mA h g^-1 (Li_4.26Sn), and a very low initial irreversible capacity of 1.6 Li/Cu₆Sn₅ (Li _0.32Sn). In contrast, the reaction with Na is limited with a reversible capacity of 160 mA h g^-1 compared to the expected 516 mA h g^-1 (Na_3.75Sn). X-ray diffraction and ¹¹⁹Sn-Mössbauer spectroscopy measurements show that this limited capacity likely results from the restricted diffusion of Na into the anode nanoparticles and not the formation of a low Na-content phase. Moreover, our results suggest that the η-Cu₆Sn₅ alloy should have optimized particle sizes of nearly 10 nm diameter to increase the Na capacity significantly. An alternative system consisting of a two-phase mixture of Cu₆Sn₅ and Sn of nominal composition 'Cu ₆Sn₁₀' has been studied and is able to deliver a larger initial reversible storage capacity of up to 400 mA h g^-1. Finally, we have demonstrated that the presence of Cu in Cu₆Sn₅ and 'Cu₆Sn₁₀' suppresses the anomalous electrolyte decomposition normally observed for pure Sn.