Unveiling Extraction Bottlenecks by Analyzing Clusters Containing Lithium-Ion Endohedral Fullerene

Lithium-ion encapsulated fullerene (Li⁺@C₆₀) represents a novel class of ionic endohedral metallofullerenes possessing distinct electronic properties, including high ionic conductivity and superior electron-accepting capabilities compared to pristine C₆₀. While the "plasma shower" ion implantation method has enabled the continuous synthesis of Li⁺@C₆₀, the industrial application of this material is currently impeded by a critical disparity between the theoretical synthesis yield and the actual recovered yield. Current extraction protocols typically recover only approximately 0.8% of Li⁺@C₆₀, which is significantly lower than theoretical predictions. This study aims to rigorously quantify the Li⁺@C₆₀ content in the postsynthesis crude soot to determine the true efficiency of the plasma shower synthesis and identify the physicochemical factors limiting extraction. We employed a multifaceted analytical approach combining Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS), solid-state ⁷Li Nuclear Magnetic Resonance (NMR) spectroscopy, and Inductively Coupled Plasma (ICP) analysis. Quantitative analysis based on three independent synthesis runs (N = 3) reveals that the crude soot contains Li⁺@C₆₀ at a mass percentage of 3.4 ± 0.1%. Furthermore, spectral analysis identified a significant abundance of oxidized derivatives (Li⁺@C₆₀O, 4.3 ± 0.1%), indicating a total encapsulation efficiency of 7.7% ± 0.1%. Additionally, Transmission Electron Microscopy (TEM) revealed the formation of robust clusters with a median diameter of approximately 8 nm. Collectively, these findings confirm that the low recovery in conventional methods (∼0.8%) is not due to synthesis failure, but rather due to the formation of insoluble aggregates and oxidative derivatives. This report provides a detailed quantitative framework for evaluating Li⁺@C₆₀ synthesis and proposes that optimizing physical disintegration techniques, such as dual-frequency ultrasonication, alongside strict oxidation control, is essential for bridging the yield gap.