21324-40-3 | Lithium hexafluorophosphate

CAS:21324-40-3 | Lithium hexafluorophosphate ,Description

Lithium hexafluorophosphate (LiPF6) is a significant compound in the field of scientific research, particularly in the development of lithium-ion batteries (LIBs). 
Decomposition Mechanisms:LiPF6 decomposes under solid electrolyte interphase (SEI) formation conditions. This process is well understood through density functional theory. The decomposition of LiPF6 results in the formation of POF3, primarily through rapid chemical reactions with Li2CO3. However, the hydrolysis of LiPF6 is kinetically limited at moderate temperatures[“].
Role in Lithium-Ion Batteries:LiPF6 has been a dominant conducting salt in lithium-ion battery electrolytes for several decades. It is known for its extreme instability even at trace levels of water. Interestingly, in pure water, PF6−, a derivative of LiPF6, does not undergo hydrolysis. This property highlights the compound's unique behavior in different environments[“].
Chemical Instability:The chemical instability of LiPF6 in non-aqueous liquid electrolytes is a major challenge, particularly in the context of large-format lithium-ion batteries used in applications like electric vehicles. The undesired chemical degradation of LiPF6 significantly impedes the practical deployment of these batteries in emerging technologies[“].
SEI Engineering:In the field of battery technology, Solid Electrolyte Interphase (SEI) engineering is recognized as an efficient approach to improve the cycling performance of lithium metal batteries. LiPF6 plays a crucial role in this context, being a popular electrolyte salt employed in SEI engineering[“].

Scientific Research Applications

Lithium hexafluorophosphate plays a significant role in scientific research, particularly in the field of electrochemistry and battery technology.  
Electrolytes in Battery Technology:Lithium hexafluorophosphate is a key component in the state-of-the-art electrolytes used for electric vehicle (EV) applications. It is typically dissolved in non-aqueous organic-based carbonate solvents like ethylene carbonate and dimethyl carbonate. These electrolytes, which constitute about 8–15% of a cell's weight, are crucial but also limit cell safety due to their combustibility and restricted operating temperature range (-10 °C to 60 °C)[“].
Impact on Cell Safety and Performance:The chemistry of electrolytes, which includes lithium hexafluorophosphate, is vital in determining the safety, cycle life, power capability, and energy content of cells. Electrolytes play a crucial role in forming the solid electrolyte interphase (SEI) at the interface between the electrolyte and the active material, especially at the negative electrode. This SEI is critical for preventing degradation in the cell's capacity and power[“].
Innovation in Electrolyte Formulations:Innovative electrolyte formulations, where lithium hexafluorophosphate is a key component, are pivotal for the development of next-generation electrode materials. These include lithium metal and silicon for negative electrodes and Mn-rich and polyanionic compounds for positive electrodes. Researchers are focusing on developing tailored liquid electrolyte formulations, including fluorinated solvents and room-temperature ionic liquids (RTILs), to enhance the cycling efficiency of lithium metal[“].
Role in Solid-State Batteries:There's a strong push in the automotive industry towards solid-state electrolytes and batteries. Lithium hexafluorophosphate is instrumental in this development. Solid-state cells can be classified into two main categories: thin film cells used in devices like medical equipment and smart electronics, and bulk-type cells comparable to current generation commercial lithium-ion batteries. These developments are significant in advancing battery technology for various applications[“].
Advantages of Inorganic Solid-State Electrolytes:Inorganic solid-state electrolytes, which utilize lithium hexafluorophosphate, are available in high-temperature rechargeable batteries and are being explored for space-based applications. They offer stability, high ionic conductivities at room temperature, and improved safety due to a lower risk of thermal events. These features could lead to increased energy density at the battery pack level[“].
Potential for Improved Battery Performance:Solid-state electrolytes can potentially enable long-term cycling of advanced electrode materials, a performance difficult to achieve with conventional electrolytes. Some offer thermodynamic stability, while others provide kinetic stability by preventing interface dissolution into a liquid or restricting solvent transport to the electrode interface. However, evaluating the interaction between the electrode and solid electrolyte is crucial during the development of these batteries[“].
 

Mechanism of Action

Reactivity with Water:Lithium hexafluorophosphate can react with water contaminations present in the battery electrolyte. This reaction leads to the release of hydrogen fluoride (HF) and potentially other harmful species. These by-products can decrease battery performance and become a health hazard in case of leakage. This reactivity highlights the sensitivity of Lithium hexafluorophosphate to moisture and its potential impact on battery safety and efficiency[“].
Instability in the Presence of Water:Lithium hexafluorophosphate is notably unstable, even at trace levels of water in the electrolyte. Despite this, it does not undergo hydrolysis in pure water. This instability at minimal moisture levels is significant as it affects the reliability and longevity of the battery in practical applications[“].
Decomposition Mechanisms:The decomposition mechanisms of Lithium hexafluorophosphate have been explored using density functional theory (DFT). It was found that water is not necessary for the formation of LiF or POF3, contrary to some assumptions. Instead, PF5, a component of Lithium hexafluorophosphate, can react rapidly with readily available Li2CO3 during the early formation of the solid electrolyte interphase (SEI). This finding is crucial in understanding the chemical behavior of Lithium hexafluorophosphate in battery electrolytes and has implications for improving battery stability and performance[“].
 

Biochemical and Physiological Effects

Cytotoxicity in Yeast Cells:A study using a yeast (Saccharomyces cerevisiae) deletion mutant collection revealed that lithium hexafluorophosphate (Lithium hexafluorophosphate) is more toxic to yeast than lithium chloride (LiCl) or sodium hexafluorophosphate (NaPF6). In yeast cells, a high concentration of Lithium hexafluorophosphate caused mitochondrial damage, reactive oxygen species (ROS) accumulation, and changes in ATP content. Genes associated with oxidative phosphorylation were found to be crucial for counteracting the toxicity induced by Lithium hexafluorophosphate, suggesting that oxidative stress and ATP imbalance may be driving factors in its cytotoxicity[“].
Potential Genotoxicity:There is a lack of comprehensive in vivo studies directly assessing the genotoxicity of lithium hexafluorophosphate due to its reactivity and instability in water and air. When it comes in contact with water, lithium hexafluorophosphate rapidly releases hydrogen fluoride, leading to potential local tissue damage and secondary tissue necrosis due to localized free fluoride ion concentrations. Ethical and practical reasons, therefore, make it inappropriate to consider genotoxicity testing of lithium hexafluorophosphate in animals. Consequently, information on the genotoxicity of its hydrolysis products, such as hydrogen fluoride, lithium fluoride, and phosphoric acid, is used instead[“].
Effects on Human Health:Exposure to lithium hexafluorophosphate, particularly through overexposure, can affect various systems in the human body, including the digestive, circulatory, respiratory, and nervous systems, as well as the skin and bones. The most dangerous path of entry into the body is ingestion, but inhalation and skin contact can also be harmful. Symptoms may include nausea, vomiting, or headaches when exposed to its vapors[“].
 

Advantages and Limitations for Lab Experiments

Advantages

Enhanced Electrolyte Performance:Lithium hexafluorophosphate (Lithium hexafluorophosphate) offers several significant advantages over conventional electrolyte salts like LiAsF6, LiBF4, and LiClO4. Primarily, it forms an effective solid-electrolyte interphase (SEI) membrane on electrodes, especially cathodes. This attribute is crucial in enhancing the performance and longevity of batteries. Additionally, Lithium hexafluorophosphate contributes to the passivation of anode current collectors, preventing their dissolution, which is vital for maintaining the battery's integrity over time[“].
High Conductivity and Stability:Lithium hexafluorophosphate is noted for its superior solubility in organic carbonate solvents, high conductivity, and good electrochemical stability. These properties make it an essential component of lithium battery electrolytes. High conductivity is particularly beneficial for applications requiring efficient ion transport. Furthermore, Lithium hexafluorophosphate balances properties such as a high dissociation constant and good electrochemical stability against aluminum corrosion, making it the most commonly used salt in lithium-ion cells[“].
Safety and Solubility:When it comes to safety and compatibility with organic solvents, Lithium hexafluorophosphate has significant advantages. Its safety profile and solubility in organic solvents surpass those of conventional electrolytes, making it the most widely used electrolyte in rechargeable lithium-ion secondary batteries[“].
 

Limitations

Sensitivity to Water:A notable limitation of Lithium hexafluorophosphate is its high sensitivity to water. In the presence of moisture, Lithium hexafluorophosphate undergoes hydrolysis, producing highly corrosive hydrofluoric acid (HF). This reaction degrades battery performance and poses a significant challenge in environments with high humidity or where the electrolyte may come into contact with water[“].
Decomposition Challenges:The decomposition of Lithium hexafluorophosphate, particularly under solid-electrolyte interphase (SEI) formation conditions, is a complex process that is not fully understood. It involves rapid chemical reactions leading to the formation of products like POF3, especially in the presence of compounds like Li2CO3. This decomposition can be kinetically limited at moderate temperatures, but the full mechanism remains a subject of ongoing research[“].
Thermal Stability:Thermal decomposition studies of Lithium hexafluorophosphate indicate that while it is stable under normal temperatures when water content is low, it reacts with trace amounts of water in the air when heated. This reaction can lead to the degradation of the salt and, consequently, the electrolyte's performance, particularly under high-temperature conditions[“].
Lithium hexafluorophosphate plays a vital role in the performance and efficiency of lithium-ion batteries due to its superior conductivity, stability, and compatibility with organic solvents. However, its sensitivity to water and challenges related to decomposition and thermal stability are important factors to consider in lab experiments and practical applications.
 

Future Directions

Advancements in Solid-State Lithium Metal Batteries (SSLMBs):The evolution of lithium-ion batteries (LIBs) is reaching its limits in terms of energy content and safety. This limitation is driving a revival in research activities for solid-state lithium metal batteries (SSLMBs). SSLMBs are gaining attention for their potential in high-energy electrochemical storage, with a focus on developing innovative electrolyte materials for future rechargeable high-energy batteries. Research is particularly centered on the properties and applications of solid polymer electrolytes (SPEs) in SSLMBs, aiming to bridge the gap between early solid-state batteries and high-performance SPE-based SSLMBs[“].
Addressing Electrolyte Limitations:Significant attention is being paid to solid polymer electrolytes (SPEs) due to their physicochemical features like high flexibility and ease of thin-film processing. Efforts are focused on overcoming the shortcomings of current nonaqueous liquid electrolytes used in LIBs, which are unstable and highly flammable. The goal is to enhance the specific energy and safety of batteries through SSLMBs, which combine high-energy electrode materials with solid electrolytes[“].
Ion Transport and Conductivity Improvements:Future research is exploring ways to improve the ionic conductivity of SPEs. This includes studying the amorphous phases of SPEs which are primarily responsible for the transport of ionic species and addressing the low ionic conductivities of crystalline phases. Innovations aim to increase conductivity, especially under different temperature conditions, and to understand the molecular and ionic transport mechanisms in SPEs[“].
Incorporation of Inorganic Fillers and Polymer Modification:Another approach is the utilization of nano-sized inorganic fillers to enhance lithium-ion transport and suppress crystallization in SPEs. These fillers can provide additional transport channels and are being explored in different morphologies such as nanoparticles and nano-wires. Additionally, strategies like polymer blends and co-polymerization are being investigated to improve the mechanical properties and suppress crystallinity of SPEs[“].
Innovations in Polymer Chemistry:Recent studies have also highlighted the potential of Jeffamine compounds and other polyether oligomers in enhancing the ionic conductivities of SPEs at room temperatures. This involves optimizing the chemical structures of these compounds to improve their electrochemical stabilities and to enable the operation of SSLMBs at or near room temperatures[“].
The future research directions for lithium hexafluorophosphate and related materials are multifaceted, focusing on improving SSLMB technologies through advancements in solid polymer electrolytes, enhanced ionic conductivity, incorporation of innovative materials, and understanding the fundamental chemistry and physics of these systems.

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