In-Situ Derived Bi Alloys for High Performance Li-Ion Batteries: Effects of Conversion Family, Mesomatrix, and Electrolyte

Abstract

Despite the fact that the first commercial rechargeable Li battery was based on a Li-Al alloy in the 70s, extensive investigation of Li alloys for next generation high performance Li-ion batteries has taken place only since Fuji’s initiation of the renaissance in the 90s [1-2]. Although the majority of this research has been directed towards Si, Sn, Ge and Al [3-4], Bi possesses interesting properties that make it unique amongst the lithium alloys. Although heavy, LixBi exhibits very high volumetric capacity and exceptional Li+ diffusion resulting in super ionic properties in its second lithiated phase, Li3Bi [5]. Combined, these positive attributes present the opportunity to meet the high-rate and volumetric energy capability needs in the next generation of Li-ion batteries. However, the major obstacle encountered by Bi, ubiquitous among all other metal alloys for Li, is the considerable volume changes during cycling that impede sustained capacity retention, making them impractical for modern use.In this work we explore the effectiveness of BiF3 as an electrochemical precursor to generating a Bi nanocomposite with great cyclability. To develop an understanding of its effectiveness, we observed the crystallite sizes of Bi derived from the conversion of BiF3 and other compounds. Through electrochemical and physical characterization of BiF3, Bi2O3, and Bi2S3 conversion materials, the size of the post-conversion Bi product was found to be not a result of processing conditions but rather the choice of conversion material. The resulting Bi crystallite size post-conversion was found to provide evidence for the inverse relationship between in-situ derived crystallite size and cyclability that is consistent with theory. In order to preserve the high volumetric capacity of LixBi we explored alternative matrices to C. MoS2 proved a more effective and volumetrically efficient C substitute, with the former exhibiting gravimetric capacities on par with the latter. For further cycling improvements, the effects of novel electrolyte formulations derived from use in Bi/C nanocomposites were explored. These new electrolytes showed good stabilization of the Li–Bi alloying mechanism and were highly effective when coupled with BiF3 nanocomposites. Some nanocomposites subjected to extensive high-energy mechanical milling (HEMM) experienced systematic chemomechanical dehalogenation of BiF3 into Bi metal, mimicking its conversion during electrochemical processes.The use of a milled BiF3 nanocomposite with either C or MoS2 and incorporation of an alloy-specific optimized electrolyte enabled the demonstration of LixBi alloys with exceptional cycling stability (Fig. 1). This presentation will outline the optimization of this alloy family from a holistic perspective and discuss future pathways in context with the foundations established here along with the other relatively few publications on this interesting alloy [6-9].