Probing the Mechanism of Sodium Ion Insertion into Copper Antimony Cu2Sb 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 Na3Sb via an intermediate amorphous phase. Upon desodiation the Na3Sb 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 Sb 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.