Enhanced Electrochemical Properties of Bi Nanowires as Anode Materials in Lithium and Sodium Batteries

Background: Bismuth (Bi) has been studied due to its high theoretical gravimetric capacity of 385 mAh g(-1), which is as important as gravimetric capacity for the practical application of battery systems in electronic mobile devices. However, there have been limited fundamental explorations on the electrochemical performances of Bi. Furthermore, the mechanism differences for the Bi anodes in lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs) should be further investigated. Methods: The Bi nanowires were fabricated by vacuum melting and pressure injection method. Briefly, a Bi bulk was placed into an injection apparatus and heated up above 275 degrees C (melt point of Bi: 271.3 degrees C), and then the melt was injected into the AAO pores by a hydraulic force. After the injection process, the chamber was kept in vacuum to cool down slowly. Subsequently, the AAO template was dissolved away slowly in the etching solution (0.4 M H3PO4+ 0.2 M CrO3) at 60 degrees C for 72 h to expose Bi nanowires. Finally, after sonication dispersion, centrifugal sedimentation and rinsing with deionized water several times to remove the excess H3PO4-CrO3 mixture, the free-standing Bi nanowires were collected. Results: The morphologies of AAO, AAO/Bi and Bi nanowires were tested and presented in detail. It found that the Bi nanowires can be obtained by pressure injection method followed with dissolve the AAO template. After boll milling with C to form Bi/C nanocomposites, the nanocomposites were assembled as an electrode of LIBs or NIBs. It exhibited high capacities in LIBs, while for NIBs, the capacity retention was relatively low. Conclusion: Bi nanowires have been prepared by mechanical pressure injection method and thoroughly dissolution of the AAO template. After successive milling of Bi nanowires with carbon black, Bi/C nanocomposites are obtained. The Bi/C nanocomposites used as electrode in LIBs exhibit high capacities and the initial discharge/charge capacities of Bi/C anode are around 1223.4/571.9, 905.9/412.3, 829.2/ 362.6 mAh g(-1) at current densities of 20, 200 and 500 mAh g(-1), respectively. The enhanced electrical performances are attributed to the smaller size of Bi nanowires and the introduced carbon black to buffer the volume changes during discharge/charge process. In NIBs, the capacity retention after 50 cycles reaches 284.7, 196.2 and 168.3 mAh g(-1) at current densities of 20, 200 and 500 mAh g(-1), respectively. Furthermore, in LIBs, Bi and Li+ ions combine together through an alloying process, while in NIBs, only an intercalation process occurs for Bi and Na+ without indication of alloying.