Probing the Mechanism of Sodium Ion Insertion into Copper Antimony Cu₂Sb Anodes

Abstract

We report experimental studies to understand the reaction mechanism of the intermetallic anode Cu 2 Sb 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 3 Sb via an intermediate amorphous phase. Upon desodiation the Na 3 Sb reverts to an amorphous phase, which then recrystallizes into Cu 2 Sb at full charge, indicating a high degree of structural reversibility. The structure after charging to 1 V is different from that of Cu 2 Sb, as indicated by X-ray absorption spectroscopy and 121 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 Na 3 Sb reference powder (−7.95 mm s –1 ) and in agreement with the formation of Na 3 Sb 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.