The pursuit of higher energy density in Li-ion batteries (LIBs) has led to exploration of conversion-type cathodes as alternatives to intercalation-type cathodes. Metal fluorides, including those of iron, copper, bismuth, and nickel, are promising candidates due to their high oxidative stability, high lithiation potentials (>2 V vs. Li+/Li), and high theoretical capacities (302–712 mAh g−1). This offers the potential for energy densities significantly exceeding those of current LIB cathodes like LiFePO4, LiNi1/3Mn1/3Co1/3O2, and LiCoO2. However, many metal fluorides suffer from poor capacity retention and limited cycle life because of large volume changes during lithiation/delithiation, impacting the electrode's mechanical integrity. Bismuth fluoride (BiF3) stands out due to its high theoretical capacity (302 mAh g−1), a discharge voltage of ~3.0 V vs. Li+/Li, and a low theoretical volume change of ~1.7% upon lithiation, comparable to conventional intercalation cathodes. Furthermore, BiF3 displays low voltage hysteresis. This research addresses the need for a cost-effective and robust BiF3 synthesis method and investigates its potential as a high-energy-density cathode material in LIBs, focusing on the effect of different electrolytes on its performance. Traditional BiF3 synthesis methods using hazardous fluorine sources like HF and NH4F, along with aqueous media prone to forming bismuth oxofluoride impurities, have hindered its widespread adoption. This study proposes a novel synthesis route and evaluates BiF3's electrochemical performance in both conventional carbonate-based electrolytes and room-temperature ionic liquids (RTILs) to mitigate side reactions, particularly the formation of Li2CO3 at the cathode-electrolyte interface (CEI) catalyzed by metallic Bi0, which is known to negatively impact the cycling stability.

Existing literature extensively covers conversion-type cathodes for Li-ion batteries, highlighting the advantages of metal fluorides over oxides and sulfides. Studies on FeF3, CuF2, and other metal fluorides show promising theoretical capacities but suffer from issues like poor cycle life and large volume changes. BiF3 has attracted attention due to its lower volume change compared to other fluorides, but its synthesis and electrochemical performance in various electrolytes require further investigation. Prior work on BiF3 synthesis often involves the use of hazardous chemicals, such as HF, or results in impure products due to hydrolysis. The impact of different electrolytes, particularly ionic liquids, on the cycle life of BiF3 remains largely unexplored. This research aims to build upon these existing findings by developing a safer, more efficient synthetic route and thoroughly evaluating the performance of the resulting BiF3 material under different electrolyte conditions.

This study employed a novel, facile synthesis route for BiF3 via thermal decomposition of bismuth(III) trifluoroacetate [Bi(TFA)3] at 300 °C under a nitrogen atmosphere. The Bi(TFA)3 precursor was synthesized following a modified procedure detailed in the supporting information, involving the reaction of Bi2O3 with trifluoroacetic acid and trifluoroacetic anhydride. The resulting BiF3 was characterized using powder X-ray diffraction (XRD) with Rietveld refinement to confirm its phase purity and crystallinity, scanning electron microscopy (SEM) to examine its morphology and particle size, and energy-dispersive X-ray spectroscopy (EDX) to determine its elemental composition. To improve electrical conductivity and reduce particle size for optimal electrochemical performance, the synthesized BiF3 was ball-milled with multi-walled carbon nanotubes (CNTs) and carbon black (CB). The resulting BiF3/C composite was then mixed with a polyvinylidene fluoride (PVdF) binder in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was cast onto aluminum foil current collectors. Coin-type cells were assembled using lithium metal as the counter and reference electrode. Electrochemical performance was evaluated using two types of electrolytes: a conventional carbonate-based electrolyte (1 M LiPF6 in ethylene carbonate/dimethyl carbonate, EC/DMC) and an ionic liquid-based electrolyte (1 M and 4.3 M LiFSI in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, Pyr1,4TFSI). Galvanostatic cycling was performed in the 2-4 V voltage range at a current density of 30 mA g−1. The effect of electrolyte concentration was also studied using a 4.3 M LiPF6 in EC/DMC electrolyte and a 4.3 M LiFSI in Pyr1,4TFSI electrolyte. Detailed XRD analysis was done on both the as-synthesized and ball-milled BiF3 to assess structural changes. SEM and TEM imaging and EDX analysis were used to characterize the particle size, morphology and the distribution of carbon in the composite material.

The thermal decomposition method yielded phase-pure, highly crystalline orthorhombic BiF3. XRD analysis, along with Rietveld refinement, confirmed the formation of the orthorhombic phase (Pnma space group) with a R(F) factor of 7%. SEM analysis showed the BiF3 particles had a flake morphology with a mean particle size of 4.9 µm. Ball-milling reduced the particle size to 1.2 µm and induced a structural transformation to the cubic phase (Fm-3m space group), as observed by XRD, although EDX measurements indicated a relatively low oxygen content suggesting the cubic phase to be primarily α-BiF3 rather than BiOxF3−2x. Electrochemical testing revealed significant differences in performance depending on the electrolyte. In the carbonate-based electrolyte, BiF3 exhibited an initial discharge capacity close to the theoretical value, but rapid capacity fading occurred due to the formation of Li2CO3 at the CEI. The voltage profiles showed two reduction peaks and three oxidation peaks, which were attributed to different Bi reduction/oxidation states and the involvement of Bi2O3 in the electrochemical process. The initial discharge capacity in carbonate electrolyte was higher than the theoretical value, which is attributed to electrolyte reduction and Li2CO3 formation. In contrast, using the ionic liquid electrolyte significantly improved the cycling stability. The 4.3 M LiFSI-Pyr1,4TFSI electrolyte yielded a high initial discharge capacity of 208 mAh g−1 at 30 mA g−1 and retained approximately 50% of this capacity after 80 cycles. The voltage profiles showed a single reduction peak and a single oxidation peak, indicating a more straightforward electrochemical process and reduced side reactions. This superior performance in ionic liquid electrolytes is attributed to the suppression of Li2CO3 formation at the CEI, ensuring better Li-ion transport and improving electrode stability.

The findings demonstrate the success of the novel BiF3 synthesis method, producing a high-quality material suitable for use as a LIB cathode. The significant improvement in cycling stability observed with the ionic liquid electrolyte directly addresses a major limitation of BiF3 in carbonate-based systems. The formation of Li2CO3 at the CEI in carbonate electrolytes is a well-known issue that hinders performance. The results clearly show that the ionic liquid electrolyte effectively mitigates this problem. The single reduction and oxidation peaks observed in the ionic liquid electrolyte indicate a simpler electrochemical process, confirming the improved stability. These findings highlight the importance of electrolyte selection in optimizing the performance of conversion-type cathode materials. The high initial capacity and improved cycle life achieved in the ionic liquid electrolyte make BiF3 a more promising candidate for high-energy-density LIBs.

This research successfully developed a facile and scalable synthesis method for high-quality BiF3 and demonstrated that using ionic liquid electrolytes dramatically improves its cycling stability as a LIB cathode material. The observed improvement is attributed to the suppression of detrimental side reactions that occur in carbonate-based electrolytes. Future research could focus on further optimizing the BiF3/C composite material, exploring different ionic liquid electrolytes, and investigating the detailed mechanism of CEI formation in both carbonate and ionic liquid-based systems. Furthermore, studies on the long-term stability at higher current densities are warranted.

While this study demonstrates significant improvements in BiF3's electrochemical performance, several limitations exist. The electrochemical testing was primarily conducted at a relatively low current density (30 mA g−1). Further investigation at higher current densities is necessary to assess the practical applicability of BiF3 in high-power LIB applications. The sample size for particle size distribution analysis, though large, could be further increased for improved statistical significance. Long-term cycling tests over several hundreds of cycles would strengthen the conclusions regarding capacity retention.