Solid electrolyte interphase (SEI)-stabilizing additives play a critical role in enhancing the performance and longevity of lithium-ion batteries. These additives modify the formation and properties of the SEI layer, a passivating film that forms on the anode surface during initial cycling. The SEI layer prevents further electrolyte decomposition while allowing lithium-ion transport. However, an unstable or poorly formed SEI can lead to capacity fade, increased impedance, and reduced cycle life. Additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), and lithium difluoro(oxalato)borate (LiDFOB) are widely studied for their ability to improve SEI stability.

The SEI layer forms through the reductive decomposition of electrolyte components on the anode surface, primarily during the first charge cycle. Without additives, the SEI may consist of unstable organic compounds (e.g., ROCO2Li) that continue to decompose over time, consuming active lithium and thickening the layer. SEI-stabilizing additives are designed to preferentially decompose before the base electrolyte, forming a more inorganic-rich and mechanically stable SEI composed of compounds like LiF, Li2CO3, and polycarbonates. This modified SEI exhibits higher ionic conductivity, better mechanical integrity, and reduced susceptibility to cracking during cycling.

Vinylene carbonate (VC) is one of the most effective and widely used SEI additives. It polymerizes during the initial charge cycle, forming a poly(VC) network that integrates into the SEI. This polymerized structure enhances the elasticity of the SEI, accommodating volume changes in the anode (e.g., graphite or silicon) without cracking. VC also promotes the formation of Li2CO3 and other inorganic components, which improve interfacial stability. The optimal concentration of VC typically ranges between 1-2% by weight in the electrolyte. Higher concentrations can lead to excessive SEI growth, increasing interfacial resistance. VC is particularly effective in graphite-based anodes but may show limited compatibility with high-voltage cathodes due to oxidative decomposition.

Fluoroethylene carbonate (FEC) is another prominent additive, especially for silicon or lithium-metal anodes. FEC decomposes to form LiF-rich SEI layers, which are highly stable and conductive. The presence of fluorine in FEC contributes to a more robust SEI with lower electron tunneling probability, reducing further electrolyte reduction. FEC also suppresses gas generation (e.g., CO2 and H2) during cycling, which is critical for pouch and prismatic cell designs. The recommended concentration of FEC ranges from 3-10%, with higher amounts sometimes leading to increased viscosity and reduced rate capability. FEC is particularly beneficial in high-energy-density cells but may require co-additives to mitigate its oxidative instability at voltages above 4.3V.

Lithium difluoro(oxalato)borate (LiDFOB) functions as both a salt and an additive, contributing to SEI stabilization through dual mechanisms. Its decomposition products, including LiF and borate complexes, create a stable and conductive interface. LiDFOB also passivates cathode surfaces, reducing transition metal dissolution and improving high-voltage stability. Unlike VC and FEC, LiDFOB is effective at low concentrations (0.5-1%) and is compatible with nickel-rich cathodes (e.g., NMC811). Its bifunctional nature makes it suitable for cells operating at wide voltage ranges (2.5-4.5V).

The chemical reactions involved in SEI formation with these additives follow specific pathways. VC undergoes radical polymerization, forming poly(VC) chains that integrate with lithium alkyl carbonates. FEC decomposes via defluorination, releasing LiF and generating polycarbonate species. LiDFOB decomposes into LiF, oxalate complexes, and boron-containing compounds, which enhance both anode and cathode stability. These reactions are influenced by factors such as electrode potential, temperature, and electrolyte composition.

Optimal additive concentrations are determined by balancing SEI quality with electrochemical performance. Excessive additive amounts can lead to thick SEI layers, increasing impedance and reducing energy density. For example, VC concentrations above 2% may impair low-temperature performance, while FEC beyond 10% can elevate electrolyte viscosity. LiDFOB is effective at lower concentrations but may require adjustments in salt concentration to maintain ionic conductivity.

Compatibility with electrode materials varies among additives. VC works well with graphite but may not suffice for silicon anodes, where FEC is preferred due to its LiF-forming capability. LiDFOB is versatile, supporting both high-capacity anodes and high-voltage cathodes. In nickel-rich or lithium-rich cathode systems, LiDFOB’s ability to suppress oxidative decomposition is particularly valuable.

The impact of these additives on cycle life and degradation is well-documented. Cells with VC typically exhibit 10-20% improvement in capacity retention after 500 cycles compared to additive-free electrolytes. FEC enhances silicon anode cyclability, enabling >80% capacity retention after 300 cycles. LiDFOB improves high-voltage stability, reducing impedance growth by 30-50% in NMC-based cells. These improvements stem from the additives’ ability to form stable SEI layers that minimize parasitic reactions, reduce lithium inventory loss, and maintain low interfacial resistance.

In summary, SEI-stabilizing additives like VC, FEC, and LiDFOB are indispensable for modern lithium-ion batteries. Their targeted decomposition mechanisms enable the formation of robust, conductive SEI layers that enhance cycle life and reduce degradation. By selecting appropriate additives and optimizing their concentrations, battery manufacturers can tailor electrolyte formulations to specific electrode materials and operating conditions, unlocking higher performance and reliability.