Abstract
Cycling Cu2Sb films with fluoroethylene carbonate additive drastically improves the capacity retention of the electrode compared to cycling in pure PC with about 250 mAh g-1 retained capacity for about two hundred cycles. TEM photographs reveal that the pristine films are formed of nanoparticles of 5-20 nm diameters. XRD results highlight that during the first discharge the reaction leads to the formation of Na3Sb via an intermediate amorphous phase. During charge, Na3Sb crystallites convert into an amorphous phase, which eventually crystallizes into Cu2Sb at full charge, indicating a high degree of structural reversibility. The subsequent discharge is marked by a new plateau around 0.5 V at low Na/Sb content which does not correspond to the formation of a crystalline phase. XAS data show that the fully discharged electrode material has interatomic distances matching those expected for the coexistence of Cu and Na3Sb nanodomains. At 1 V charge, the structure somewhat differs from that of Cu2Sb whereas at 2 V charge, when all Na is removed, the structure is significantly closer to that of the starting material. 121Sb Mössbauer spectroscopy isomer shifts of Cu2Sb powder (-9.67 mm s-1) and thin films (-9.65 mm s-1) are reported for the first time, and agree with the value predicted theoretically. At full discharge, an isomer shift (-8.10 mm s-1) rather close to that of a Na3Sb reference powder (-8.00 mm s-1) is measured, in agreement with the formation of Na3Sb domains evidenced by XRD and XAS data. The isomer shift at 1 V charge (-9.29 mm s-1) is close to that of the pristine material and the higher value is in agreement with the lack of full desodiation at 1 V.
