The Rise of Electrolyte Additives in Advancing Lithium-ion Battery Technology

Graphite anodes are widely used in current battery applications; however, prior to the utilization of additives, carbonaceous anodes exhibited poor cycle lifetimes and rapid capacity loss. The main technological challenge with graphite, due to its layered structure, is the co-intercalation of certain solvents, like propylene carbonate, between graphite layers alongside lithium ions. This intercalation causes expansion of the structure and weakens the interlayer interactions. Consequently, the graphite material physically disintegrates or "exfoliates" during battery cycling, resulting in capacity loss and eventual failure. To address this challenge, researchers discovered that certain electrolyte additives, such as VC, can form a robust solid-electrolyte interphase (SEI) film on the surface of graphite. This film effectively prevents solvent molecules from co-intercalating. Even the addition of just 5% VC to the electrolyte is sufficient to suppress graphite exfoliation.^[1]

One category of additives that significantly enhance the performance of graphite anodes are carbonate additives. These carbonates are known for forming durable and high-performance SEIs. During reduction at the anode, the additives preferentially react to create a thin and insoluble film composed of lithium carbonate, lithium carboxylates, and polymerization products. The most commonly used carbonate additives for carbon anodes include VC, fluoroethylene carbonate (FEC), and vinyl ethylene carbonate (VEC). The specific structure of each carbonate additive influences the properties of the resulting SEI. For example, FEC generates an SEI similar to VC, but with a higher content of LiF, which enhances the ionic conductivity of the interphase or reduces impedance.^[2] VEC forms an SEI with free vinyl groups that effectively scavenge free radicals, thus preventing solvent decomposition. Recently, methyl (2,2,2-trifluoroethyl) carbonate (FEMC) and bis(2,2,2-trifluoroethyl) carbonate (DFDEC), both linear fluorinated carbonates, have gained attention for their ability to create high-performance SEIs on graphite, silicon, and lithium metal anodes.^[3] These fluorinated carbonates can be used at higher concentrations (up to 90%) due to their enhanced ability to solvate electrolytic salts and lower viscosity.^[4]

Another class of additives that improve the performance of graphite anodes are sulfur-containing additives, such as ethylene sulfite (ES), ethylene sulfate (DTD), and 1,3-propanesultone (PS). Through electrochemical reduction, these sulfur-containing additives form an SEI rich in lithium sulfates and lithium alkylsulfates, which facilitates lithium intercalation into graphite without co-intercalation. Graphite anodes with ES, DTD, and PS additives demonstrate exceptional cycling performance and low interfacial impedance.^[5] These sulfur-containing additives contribute to the overall improvement of graphite anode performance in battery systems.

Lithium metal is another promising anode material for lithium-ion batteries, even called the “holy grail.” However, a significant challenge associated with lithium metal anodes is the growth of lithium dendrites, which are crystalline nanorods that extend from the surface of the lithium. When these dendrites grow long enough to make contact with the cathode, it can lead to battery short-circuits, potentially resulting in runaway reactions or even explosions. To tackle this issue, one key strategy involves the utilization of additives to passivate or stabilize the lithium anode surface, inhibiting dendrite formation.

Film-forming fluorinated additives such as FEC, lithium difluorophosphate (LiPO₂F₂ or LiDFP), and lithium difluorobis(oxalate)phosphate (LiDFOP) (Table 3) have demonstrated strong capabilities in forming robust layers of solid-electrolyte interphase (SEI) on lithium metal anodes. These SEI layers contribute to improving the cycle lifetime of lithium-metal batteries. A common approach involves combining FEC and LiDFP, typically at low concentrations (around 2-3 wt% and 1-2 wt%, respectively).^[7] Furthermore, small concentrations of LiDFOP (0.5-2 wt%) aid in the formation of an SEI that is rich in LiF and fluorophosphate species, which significantly enhances cycling performance.^[8] Researchers have also borrowed a concept from Li-S battery research and shown that LiNO3 additives (1-3 wt%) are known to substantially improve the cyclability of lithium metal batteries by creating an SEI on lithium metal anodes that is rich in inorganic components, thereby further enhancing cyclability.^[9]

The addition of fluorinated carbonates, such as FEMC and DFDEC, is considered one of the most effective and cost-efficient strategies to mitigate voltage fade and maintain the structural stability of NMCs and Li-rich cathodes. These additives decompose during battery operation, forming thin CEIs that are rich in LiF and contain -CF₂- moieties.^[12] These CEIs exhibit remarkable stability even at high potentials and achieve low impedances. Furthermore, incorporating small concentrations (1 wt%) of silylated borates and silylated phosphates, ^[13,14] These boron- or phosphorus-rich CEI layers significantly enhance the rate capability and cycling performance of NMC and Li-rich cathodes.

High-concentration electrolytes (HCEs) have emerged as a promising solution to overcome the limitations of traditional electrolytes, particularly with high-voltage cathode materials. These electrolytes, with electrolytic salt concentrations exceeding 3M, offer a wider electrochemical window, better passivation of aluminum (Al) current collectors, and enhanced thermal stability and flame resistance compared to conventional electrolytes. HCEs achieve these improvements by altering the coordination environment of the electrolytic salt. Rather than forming diluted solutions with free solvent molecules, HCEs concentrate the salt and solvent to the point where they form contact ion pairs, with nearly all solvent molecules coordinated with cations. Consequently, the effective stability of the electrolyte is determined by the salt molecules rather than the solvent.^[15]

The two drawbacks to HCEs are the cost of the electrolytic salt required to achieve a 3-5M concentration and the high viscosity of the electrolyte. These challenges can be addressed cleverly by diluting the HCE with a non-solvating diluent to a concentration (~1M) closer to that of a traditional electrolyte, forming what is known as localized high-concentration electrolytes (LHCE). The chosen diluent must be miscible with the solvating solvent but non-solvating with the salt. The diluent serves to dilute the solution while still preserving the local high-concentration salt-solvent contact ion pairs.

The best diluents for LHCEs are hydrofluoroethers, like TTE, TFTFE, ETFE, and HFPM (Table 5), which perform excellently for several reasons. Firstly, the low viscosity of these hydrofluoroethers helps the LHCE achieve a viscosity appropriate for traditional separators. Secondly, these hydrofluoroethers exhibit compatibility with graphite, silicon, and lithium metal anodes, facilitating the formation of protective solid-electrolyte interphase (SEI) layers rich in LiF. Thirdly, hydrofluoroethers enable the formation of inorganic-rich cathode-electrolyte interphase (CEI) layers that effectively inhibit the dissolution of transition metals, the release of oxygen, and the undesirable layered-to-rock salt phase transformation that contributes to capacity fade in NMC and Li-rich cathodes. This combination of factors results in excellent cell performance, particularly in terms of long-term cycling stability for high-voltage batteries.^[16]