Despite the fact that the first commercial rechargeable Li battery was based on a Li-Al alloy in the 1970s, 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 1990s [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. The heavy pnictogen is of relatively low toxicity with a broad range of applications from Pb replacement in solders to formulations in antacids. In spite of its high molecular weight Li_xBi exhibits very high volumetric capacity and exceptional Li⁺ diffusion in its second lithiated phase, Li₃Bi [5]. The 0.8 V alloying potential of Bi vs Li/Li⁺ allows for a more advantageous selection of electrolyte salts and solvents than the aforementioned Si, Ge, and Al. Coupled with its superionic Li⁺ diffusivity, the higher alloying potential of Bi also provides a larger safety window against Li plating from overpotentials and enables fast charge. While the considerable volume expansion of Bi impedes sustained capacity retention—as with all other Li alloy metals—the volume change from complete lithiation is significantly smaller than that of the popular Si: 108% for Li₃Bi compared to 208% for Li₁₃Si₄ at a comparable 3.25 Li⁺ insertion. 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.

In this work, we explore the effectiveness of BiF₃ as an electrochemical precursor to generating a Bi nanocomposite with excellent cycling efficiency extending beyond 250 cycles. An understanding of the nanocomposite’s effectiveness is derived from observations of Bi crystallite sizes produced from the conversion of BiF₃, Bi₂O₃, and Bi₂S₃. Through electrochemical and physical characterization of these 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 shows a correlation between the in-situ derived crystallite size and the ionic conductivity of the resulting Li salt matrix. The resulting Bi crystallite size also provides evidence for the inverse relationship between the in-situ derived crystallite size and cyclability that is consistent with theory. For further development, alternative matrices to C were explored in order to preserve the high volumetric capacity of Li_xBi. Metal sulfides proved a more effective and volumetrically efficient C substitute, with the former exhibiting gravimetric capacities comparable to the latter. Electrolyte formulations derived from use in Bi/C nanocomposites were also tested and proved effective in stabilization of the Li–Bi alloying mechanism in our BiF₃ compositions for further cycling improvements. Finally, we demonstrate the ability of BiF₃-derived Li_xBi alloy thin films to delithiate with an 80% utilization at >100C rate despite the presence of a LiF nanomatrix.

Through the development of BiF₃ nanocomposites using carbon or sulfide matrices in conjunction with an alloy-specific optimized electrolyte, we are able to demonstrate the exceptional cycling of the Li_xBi alloy (Fig. 1 a,b black) relative to using pure Bi metal in a composite (Fig. 1a,b blue). This presentation will outline the optimization of this alloy family from a holistic perspective and discuss future pathways based on the foundations established here, inclusive of the minimization of the LiF matrix, and in context with the other relatively few publications on this interesting alloy [6-9].

References

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