Chemistry: Electrolyte & Separator

Electrolyte - The Adaptable Battery Material Unlocking Cost, Performance, and Safety Potential Liquid electrolytes in lithium-ion batteries facilitate the transport of lithium ions between the cathode and anode. Electrolytes are optimized for ionic conductivity, thermal/voltage stability, and compatibility with electrode materials. Well-designed electrolytes form a stabilizing SEI on the electrodes that protect the electrodes while allowing ions to pass freely through both the SEI and electrolyte. Class

Purpose

Considerations

Examples

Salts

Add conductivity to allow for ion transport. Decomposition products can participate in SEI formation.

Higher conductivity and solubility increase rate capability, a key aspect for high power or fast charge. Thermal and electrochemical stability can decrease Li+ inventory and affect lifetime. Lithium salts are a major cost component of most conventional electrolytes.

LiPF6: most common due to its solubility, stability, and cost. Highly moisture sensitive, releasing deleterious fluoride. Degrades 60 °C and 4.34.5V. LiFSI, LiTFSI are favored for greater stability (thermal and electrochemical) and higher conductivity but cost prohibitive.

Medium for ion conduction, wets separator and electrodes. Participates in SEI formation.

Viscosity and freezing point: Higher viscosity slows ion conduction, which worsens with lower temperature, decreasing rate capability. Solvation: Solvents are chosen to increase ionic conductivity and solubility. Solvent blends must also efficiently de-solvate ions at the electrode surface. Compatibility with electrode materials.

Carbonates: most common for cost, compatibility with common active materials, and ability to solvate salts. Solvent blends include: Cyclic: EC (high freezing point, graphite stabilizer), FEC (adds F- to SEI, enables Si anodes).) Linear: DMC (improves viscosity and conductivity), DEC (enhances low temperature performance), EMC (similar to previous with lower volatility), fluorinated (emerging, more expensive but enables high voltage).

Tune performance. Modify SEI, enable greater stability, suppress gas formation, etc.

Compatibility: Careful balancing of cell chemistry with the needs of the application. Additives often enhance one aspect of performance while negatively affecting another. Concentration: additives are often help form the SEI, but overly thick SEI layers can deplete Li inventory and decrease rate performance.

Additives span a wide range of chemistries and often contain multiple functional groups: Unsaturated additives are the prototypical VC, VEC) and undergo electroreduction to form SEI at the anode and it is generally accepted that they play a role at the cathode as well. FEC is often used as an additive to add LiF to SEI.. Fluorination of molecules containing other functional groups is also common.

Solvents

Additives 10% by weight)

Less common: LiBF4(lower conductivity). LiClO4 (good conductivity, oxidizer), LiAsF6 (high conductivity and stable, toxic and expensive), LiNO3 Li metal stabilizer, safety concerns). Good SEI formers, stable, expensive: LiBOB, LiDFOB, LiPF 2O2. Salts are used in low concentrations as additives to offset downsides.

Esters MP, EA, etc.) and Ethers DME, DOL, glymes) have good compatibility with Li metal but high flammability. Nitriles (acetonitrile, succinonitrile, etc.) have improved high temperature performance as well and a broader voltage window with some concerns related to toxicity and reactivity with other components.

Sulfur-containing additives such as PS or DTD can form an advanced SEI and improve the stability of the system. Toxicity and regulatory concerns sometimes limit use. Phosphorous compounds mitigate flammability at high concentrations but can form too thick SEIs. Nitriles and isocyanates form films often applicable to high voltage.

Source: 1. Electrolytes and Interphases in Li-Ion Batteries and Beyond, Kang Xu, Chemical Reviews. 2014. Link

Extracted from the PDF; charts and images may read imperfectly.