Hydrofluoric acid (HF) is a detrimental byproduct formed in lithium-ion batteries (LIBs) that use LiPF6-based electrolytes. The decomposition of LiPF6 in the presence of trace moisture leads to HF generation, which accelerates cell degradation by corroding electrode materials, degrading the solid-electrolyte interphase (SEI), and promoting transition metal dissolution. To mitigate these effects, HF scavengers such as lithium fluoride (LiF) and aluminum oxide (Al2O3) are incorporated into electrolytes. These additives chemically neutralize HF, improving cell performance and longevity. This article examines the role of HF scavengers in LiPF6-based electrolytes, focusing on their mechanisms, impact on electrolyte purity, and influence on cell behavior.

LiPF6 remains the dominant lithium salt in commercial LIBs due to its high ionic conductivity and electrochemical stability. However, its susceptibility to hydrolysis, even at low moisture levels, results in HF formation. The reaction proceeds as follows:
LiPF6 + H2O → LiF + POF3 + 2HF
HF is highly reactive and attacks cell components, including the cathode, anode, and electrolyte. To counteract this, HF scavengers are introduced to capture HF before it induces irreversible damage.

Lithium fluoride (LiF) is a common scavenger due to its chemical compatibility with LIB components. While LiF is typically considered an inert byproduct of electrolyte decomposition, its deliberate addition serves as a sacrificial buffer. LiF reacts with HF to form stable complexes, reducing free HF concentration. Studies indicate that LiF incorporation at 1-2 wt% in the electrolyte can decrease HF levels by over 50%, significantly slowing cathode degradation. However, excessive LiF may compromise ionic conductivity due to increased electrolyte viscosity.

Aluminum oxide (Al2O3) is another effective scavenger, particularly for high-voltage applications. Al2O3 nanoparticles dispersed in the electrolyte adsorb HF through surface reactions, forming aluminum fluoride (AlF3). This not only neutralizes HF but also passivates cathode surfaces, suppressing transition metal dissolution. Research shows that cells with Al2O3 additives exhibit up to 30% improvement in capacity retention after 500 cycles compared to baseline electrolytes. The scavenging efficiency depends on particle size and dispersion; nanoparticles with high surface area provide more active sites for HF adsorption.

The introduction of HF scavengers influences electrolyte purity in several ways. First, they reduce the concentration of corrosive HF, minimizing side reactions that generate additional impurities. Second, scavengers like Al2O3 can trap other acidic byproducts, such as phosphorus oxyfluorides (POF3), further stabilizing the electrolyte. However, scavengers themselves must be of high purity to avoid introducing new contaminants. For instance, Al2O3 with residual hydroxyl groups may react with LiPF6, exacerbating HF generation. Thus, pretreatment processes, such as thermal annealing, are often employed to ensure scavenger stability.

Cell longevity is directly impacted by the effectiveness of HF scavengers. By mitigating HF-induced degradation, these additives preserve electrode integrity and SEI stability. In graphite-based anodes, HF attacks the SEI, leading to increased lithium inventory loss and capacity fade. Scavengers like LiF help maintain a robust SEI by reducing HF-driven decomposition. For cathodes, particularly those containing nickel or manganese, HF scavengers limit metal dissolution, which otherwise accelerates impedance growth and active material loss. Cells with optimized scavenger concentrations demonstrate extended cycle life, with some studies reporting up to 20% higher capacity retention after 1,000 cycles.

Side reactions are also influenced by HF scavengers. While their primary role is HF neutralization, some scavengers participate in secondary processes that affect cell performance. For example, Al2O3 can catalyze the polymerization of electrolyte solvents at high voltages, leading to increased interfacial resistance. Conversely, LiF may integrate into the SEI or cathode-electrolyte interphase (CEI), altering their ionic transport properties. These effects must be carefully balanced to avoid unintended trade-offs.

The choice of scavenger depends on the specific cell chemistry and operating conditions. For instance, high-nickel cathodes benefit more from Al2O3 due to its dual role in HF neutralization and surface stabilization. In contrast, LiF may be preferred for high-power applications where minimal impact on ionic conductivity is critical. Advanced formulations combine multiple scavengers to exploit synergistic effects, such as using LiF for bulk HF capture and Al2O3 for interfacial protection.

Despite their benefits, HF scavengers are not a panacea. They address only one aspect of electrolyte degradation and must be complemented by other strategies, such as moisture control and advanced salt formulations. Future research may explore new scavenger materials with higher selectivity and lower reactivity toward non-HF species. Additionally, in-situ monitoring techniques could provide deeper insights into scavenger behavior during cycling, enabling dynamic optimization of their concentrations.

In summary, HF scavengers play a critical role in enhancing the performance and durability of LiPF6-based electrolytes. By neutralizing HF, they preserve electrolyte purity, reduce electrode degradation, and extend cell lifespan. However, their implementation requires careful consideration of material properties, concentrations, and potential side effects. As battery chemistries evolve, the development of next-generation scavengers will remain a key focus for improving LIB reliability and safety.