The Rise of Electrolyte Additives in Advancing Lithium-ion Battery Technology
Introduction
As the importance of batteries grows in critical applications such as electric vehicles and energy storage systems, there is a pressing need for technological advancements in performance, including longer cycle life and higher energy densities. Among the technologies available today, lithium-ion batteries with liquid electrolytes remain the most viable option.
Liquid electrolytes in lithium-ion batteries typically consist of a high-purity lithium salt such as lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a polar aprotic organic solvent. Often, that solvent consists of ethylene carbonate (EC) blended with one or several linear carbonates, with the concentration of the lithium ions around 1-2 M (Table 1). Researchers often mix in additional molecules in small quantities, usually 1-5 wt% to boost the performance of the electrolyte. These additional molecules, called electrolyte additives, play a critical role in optimizing battery performance.
To achieve the desired improvements in battery performance, scientists are increasingly recognizing electrolyte additives as an economical and promising solution. From 2018 to 2022, the annual number of academic papers and patents that employ electrolyte additives nearly doubled (Fig. 1a). In addition, the proportion of papers and patents on lithium-ion batteries (LIBs) that employ electrolyte additives has steadily grown from 5.8% to 7.4% (Fig. 1b). Notably, academic articles on Li-ion batteries that incorporate electrolyte additives receive three times more citations than those without additives (Fig. 1c), suggesting that the inclusion of electrolyte additives in research enhances the significance and relevance of the findings.
In this technical article, our aim is to provide insights into the chemistries and applications of additives used in lithium-ion batteries, specifically addressing challenges associated with graphite, silicon, and lithium metal anodes, as well as NMC, Li-rich, and LFP cathodes.
Figure 1. The increasing use of electrolyte additives in academic journal articles and patents from 2018-2022. a) The annual number of articles and patents using electrolyte additives, b) The proportion of articles and patents about Li-ion batteries (LIBs) using electrolyte additives, and c) The average number of citations for academic journal articles about LIBs that did and did not use additives. Data was collected using SciFindern.
Table 1. Common components of liquid electrolytes in Lithium-ion batteries.
Film-forming additives
In the realm of electrolyte additives for batteries, film-forming additives have emerged as a vital component in enhancing battery performance. These additives play a pivotal role by forming a protective film at the positive and negative electrodes. This protective film ensures stability at the electrode/electrolyte interface and facilitates fast ion kinetics. The application of film-forming additives brings several benefits, including the mitigation of gas generation, enhanced stability through the formation of a durable solid product, extended cycle life, and improved battery safety and stability through the uniform deposition of lithium. It is important to highlight that different electrodes may require specific additives tailored to their distinct surface chemistries.
Film-forming additives for anodes
In battery systems, the anode plays a pivotal role, and the interface between the anode and electrolyte greatly impacts the overall battery performance. An essential component of this interface is the solid-electrolyte interphase (SEI) film, which acts as a protective barrier between the electrode and electrolyte, safeguarding both components from undesirable reactions. Without proper protection, reactions such as the reduction of ethylene carbonate on the bare anode surface can lead to gas generation, causing the battery to swell. By forming a dense SEI film, the decomposition of solvent molecules like ethylene carbonate at the interface can be effectively inhibited to maintain optimal battery performance.
Film-forming additives have emerged as crucial components for controlling the composition, porosity, elasticity, and thickness of the SEI film, thereby improving battery performance. These additives are incorporated into the electrolyte in small amounts, typically ranging from 1-5% by weight. One common type of film-forming additive for anodes is the reduction-type additive, which possesses a higher reductive potential than the electrolyte solvent. During the initial cycling of the battery or when new anode surfaces are exposed, the reduction-type additive is preferentially reduced and forms an insoluble SEI film. Vinylene carbonate (VC) is a classic example of a reductive film-forming additive. Another common type of film-forming additive is the reactive-type additive. This additive reacts with the major components in the SEI to stabilize the film and modify its composition, typically to enhance ionic conductivity or slow down parasitic reactions between the anode and electrolyte.
Each anode material—most commonly graphite, silicon, or lithium metal—presents a different chemical surface and benefits from different additives. Understanding the unique characteristics of each anode material enables the selection and optimization of suitable film-forming additives to enhance battery performance.
Film-forming additives for graphite anodes
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.
Table 2. Electrolyte additives for graphite anodes. |
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Electrolyte additives for silicon anodes
Electrolyte additives play a crucial role in improving the performance of silicon anodes, which hold promise for lithium-ion batteries due to their high theoretical capacity. Since most silicon anodes are composite structures combining silicon with graphite, many of the same additives used for graphite anodes can also enhance silicon anode performance. However, silicon presents an additional technical challenge, as its substantial volume expansion (up to 300%) during lithiation imposes mechanical stress on the solid-electrolyte interphase (SEI). Therefore, the mechanical properties of the SEI are of utmost importance.
Carbonate additives such as FEC and VC are widely utilized in silicon-composite anodes due to their ability to form robust SEIs. The polymeric components of these additives contribute to a mechanically flexible SEI, while the polycarbonate components offer electrochemical stability, even at elevated temperatures. FEC, in particular, demonstrates excellent performance and is often incorporated into the electrolyte at concentrations ranging from 3-20% by weight. Linear carbonate additives with higher fluorination, such as FEMC and DFDEC, assist in creating an SEI that is richer in LiF. This enhanced LiF content has the potential to improve Coulombic efficiency, while the linear carbonates also promote the formation of a thicker SEI compared to FEC or VC alone, leading to improved capacity retention.[6]
Electrolyte additives for lithium metal anodes
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]
Table 3. Salt-type electrolyte additives for NMC and Li-rich cathodes. |
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Film-forming additives for cathodes: The cathode-electrolyte interphase
The interface between the cathode and electrolyte is crucial for battery performance, with a thin film known as the cathode-electrolyte interphase (CEI) serving as a critical component. The CEI film acts as a protective barrier, preventing oxidation of the electrolyte solvent at the cathode and inhibiting metal dissolution. Film-forming additives play a vital role in controlling the properties of the CEI film. Each cathode material presents its own unique challenges, which can be addressed by selecting suitable additives.
Electrolyte additives for NMC and Lithium-rich cathodes
The layered LiNixMnyCo1-x-yO2 (NMC) cathodes are attractive as high-voltage (4+V) cathode materials because of their high reversible capacity and good thermal stability. However, the components of the electrolyte tend to oxidize on the surface at these high potentials, which lowers battery performance over time. Additionally, surface reactions between the electrolyte and NMC can cause the decomposition of the cathode material, dissolving transition metals and generating oxygen gas, which leads to permanent capacity loss. These issues are exacerbated for lithium-rich cathode materials, which are used to achieve even higher potentials (>4.5 V). For lithium-rich cathode materials, for example, metal and oxygen loss can cause the structure to break down after a few tens of cycles.
Lithium salt additives
To protect the cathode surface from decomposition, lithium borate additives such as lithium bis(oxalate)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) have proven effective. At high voltages, LiBOB and LiDFOB sacrificially undergo oxidation, forming a Li-rich CEI. This Li-rich CEI inhibits metal dissolution, reduces corrosion of the aluminum current collectors, and ultimately extends the service life of the battery. In addition, newer lithium phosphate additives, such as lithium difluorobis(oxalato)phosphate (LiDFBP) and lithium difluorophosphate (LiDFP), serve dual purposes in extending service life and slowing capacity fading.[10,11] LiDFBP and LiDFP can sacrificially oxidize to form protective CEIs that are rich in LiF and phosphates, which are known for their low impedance and ability to inhibit metal dissolution. Furthermore, LiDFBP and LiDFP also provide protection to the anode by sacrificially reducing on graphite or lithium metal anodes, resulting in the formation of thin, protective SEIs.
CEI-improving molecular additives
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.
Table 4. Silyated electrolyte additives for NMC and Li-rich cathodes. |
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Localized High-Concentration Electrolytes
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]
Table 5. Hydrofluoroether cosolvents for liquid battery electrolytes. |
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Flame Retardants: Non-Flammable Electrolytes
Safety concerns surrounding the flammability of liquid electrolytes are one of the main obstacles to the application of batteries in applications like electric vehicles. To mitigate the risk of catastrophic failure and ensure user safety, flame-retardant additives or flame-retardant solvents are being explored.
Phosphate-based solvents were among the first investigated flame-retardants for batteries; however, they are unstable against the low reductive potentials of conventional anodes and have high viscosities. A more promising approach involves the use of hydrofluoroethers as additives or co-solvents. Hydrofluoroethers like EFTE, TFTFE, TTE, and HFPM (Table 5) have gained momentum due to their flame-retardant properties. Some localized high-concentration electrolytes (LHCEs) combine phosphate-based solvents with flame-retardant hydrofluoroethers, resulting in electrolytes that are effectively flame-resistant and have suitable viscosity.[18]
Another strategy to achieve non-flammable electrolytes is to replace flammable carbonates with fluorinated carbonates such as FEC, DFDEC, and FEMC. The substitution of fluorine on the alkyl group inhibits oxygen radical propagation, which significantly improves self-extinguishing times (SET). These solvents can be blended with hydrofluoroethers to further enhance their viscosity.[19]
Yet another approach to yielding non-flammable electrolytes is to replace the flammable solvents with non-flammable ionic liquids. While not all ionic liquids are non-flammable, specific ones such as 1-ethyl-3-methylimidazolium (EMIm)FSI and EMImPF6 have both been shown to reduce flammability when used in moderate concentrations (>10 wt%).[20]
Conclusions
This review highlights various types of electrolyte additives and how they impact battery performance. While there has been great success in improving battery performance using additives, there remains ample room for exploration and optimization of additive mixtures in liquid electrolytes. We hope that commercializing many of these additives will provide opportunities for researchers to systematically study electrolyte mixtures and expedite technological advancements in liquid electrolytes, particularly in conjunction with advancements in higher-voltage cathodes, silicon anodes, and lithium metal anodes. Continued breakthroughs in these areas hold the potential to revolutionize battery technology and further improve overall performance.
References
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