Elastic softening of alloy negative electrodes for Na-ion batteries

► We obtain the intrinsic elastic properties of alloy anodes for Na-ion batteries. ► We have investigated sodiation of Na-M (M = Sn, Pb, Si, Ge) alloys for anodes. ► Sodiation of these alloys leads up to 75% deterioration of their elastic moduli. ► Elastic softening arises due to a transition to weak ionic interatomic bonding. ► These results provide the first step in developing models for failure in Na anodes.