Probing the mechanism of sodium ion insertion into copper antimony Cu ₂Sb anodes

We report experimental studies to understand the reaction mechanism of the intermetallic anode Cu₂Sb with Na and demonstrate that it is capable of retaining about 250 mAh g^-1 over 200 cycles when using fluoroethylene carbonate additive. X-ray diffraction data indicate during the first discharge the reaction leads to the formation of crystalline Na ₃Sb via an intermediate amorphous phase. Upon desodiation the Na ₃Sb reverts to an amorphous phase, which then recrystallizes into Cu₂Sb at full charge, indicating a high degree of structural reversibility. The structure after charging to 1 V is different from that of Cu₂Sb, as indicated by X-ray absorption spectroscopy and ¹²¹Sb Mössbauer spectroscopy, and is due to the formation of an amorphous Na-Cu-Sb phase. At full discharge, an isomer shift of -8.10 mm s ^-1 is measured, which is close to that of a Na₃Sb reference powder (-7.95 mm s^-1) and in agreement with the formation of Na₃Sb domains. During charge, the isomer shift at 1 V (-9.29 mm s^-1) is closer to that of the pristine material (-9.67 mm s ^-1), but the lower value is consistent with the lack of full desodiation, as expected from the potential profile and the XAS data.