SnBi Alloy Composites Via Controlling Cooling Rate for High Performance Lithium-Ion Battery Anode

Abstract

Alloying-based materials, such as tin (Sn) and bismuth (Bi), have been considered as potential anode materials for lithium-ion batteries (LIBs) due to their high theoretical volumetric capacities of 1991 and 3765 mAh cm−3, respectively. However, these materials tend to have short cycle lives due to severe volume expansion when fully lithiated, such as Li4.4Sn (300%) and Li3Bi (215%), which presents a great challenge for practical implementation. To address this issue, using two metal alloys is an effective strategy to improve electrode integrity and stability. The different working voltages of Bi (0.78−0.69V) and Sn (0.65−0.1V) hinder the extreme structural changes that occur upon cycling. At the initial discharge stage, the unreacted Sn phase acts as a buffer during the lithiation of Bi. Meanwhile, the lithiated state of Bi (LixBi) will buffer the expansion when Sn is lithiated with Li-ions. However, the definition and mechanism of the buffer effect between active-active metals are still unclear. As a result, the bismuth-tin (BiSn) alloy is designed through heat treatment by controlling the cooling rate to form micro-phase alloys (BiSnM) and nano-phase alloys (BiSnN). The BiSn nano-phase alloy anode (BiSnN) exhibits a better buffer effect and achieves a superior discharge capacity of 550 mAh g−1 at 0.1 A g−1 and 320 mAh g−1 at 2 A g−1.