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Review

Importance of High-Concentration Electrolytes for Lithium-Based Batteries

by
Susanna Krämer
1,†,
Dominik Weintz
1,†,
Martin Winter
1,2,
Isidora Cekic-Laskovic
1,* and
Mariano Grünebaum
1,*
1
Forschungszentrum Jülich GmbH, Helmholtz-Institute Münster (IMD-4), Corrensstrasse 48, 48149 Münster, Germany
2
University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstrasse 46, 48149 Münster, Germany
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Encyclopedia 2025, 5(1), 20; https://doi.org/10.3390/encyclopedia5010020
Submission received: 9 December 2024 / Revised: 9 January 2025 / Accepted: 14 January 2025 / Published: 5 February 2025
(This article belongs to the Section Chemistry)
Browse Figures
Versions Notes

Abstract

:
Each battery cell consists of three main components: the anode, the cathode, and the separator soaked with liquid electrolyte, the medium in the battery that allows charged ions to move between the two electrodes. Besides a wide electrochemical stability window and good compatibility with both electrodes, the electrolyte should also be safe, thermally stable and environmentally benign, showing a high ionic conductivity of the charge-carrying Li ions and finally a low price. This unique combination of properties is impossible to achieve with a simple salt–solvent mixture and usually requires a combination of different electrolyte components, i.e., several liquid solvents and additives and one or more conducting salt(s). For lithium-based batteries, which are the most common electrochemical energy storage devices today, a solution based on lithium hexafluorophosphate (LiPF6) in a mixture of organic carbonates as the solvent is used. Usually, the conducting salt concentrations used for lithium-based electrolytes are in the range of ≈1 to 1.2 M, but recently, electrolytes with much higher conducting salt concentrations of 5 M and even over 10 M have been investigated as they offer several benefits ranging from increased safety to a broadened electrochemical stability window, thus enabling cheap and safe solvents, even water.

1. Introduction

Lithium-based batteries, such as lithium-ion batteries (LIBs) and lithium metal batteries (LMBs), for electrochemical energy storage and conversion have reshaped the technology sector by powering devices ranging from portable electronics, to electric vehicles to stationary (“grid”) storage. LIBs are commercially available in various sizes and chemistries, usually distinguished by their performance characteristics. For a battery cell consisting of two electrodes separated by an insulating membrane [1], the electrolyte is the key factor for ion transport and the overall battery performance. It enables the lithium ions to move between the electrodes during charging and discharging without short-circuiting the battery (Figure 1). A selection of common materials for the positive and negative electrode characterized by their operating potential range against Li|Li+ and specific capacity is shown in Figure 2.
Due to the electrolyte’s unique role as ion transport medium, it has to simultaneously provide a wide range of properties: high ionic conductivity (>1 mS cm−2) in a wide temperature range (−20 to 60 °C), high chemical and thermal stability, broad (kinetic) electrochemical stability window (ESW) (0 to up to 5 V vs. Li|Li+), sufficient wettability, compatibility with further battery components, including particularly electrodes and an effective interphase formation as well as environmental friendliness, high safety, low toxicity and cost-effectiveness (Figure 3).
In commercial lithium-based batteries, the state-of-the-art electrolytes are non-aqueous liquid-based systems. Commonly, the lithium conducting salt, in most cases lithium hexafluorophosphate (LiPF6), is dissolved in concentrations ≈1 to 1.2 mol L−1 (1 to 1.2 M) in a mixture of aprotic solvents such as linear and cyclic organic carbonates, like dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or ethylene carbonate (EC) [2,3]. Customarily, (multi)-functional additives are introduced to these electrolyte formulations to form effective and robust interphases [4]. The solid electrode interphase (SEI) at the anode side and the cathode electrolyte interphase (CEI) at the cathode side are rather solid layers formed by multiple electrolyte decomposition products at the electrode–electrolyte interface [5,6]. In the ideal case, this layer is lithium-ion-conducting and protects the electrolyte from ongoing decomposition. The benefits of moderate-concentration organic carbonate-based electrolytes are their high ionic conductivity, good wettability, compatibility with the electrode materials and sufficient electrochemical stability. Nevertheless, the chemical instability against moisture, the volatility and flammability of these electrolytes are accompanied by considerable safety risks and a limited operating temperature range for the battery cell [2]. Additionally, the performance of moderate-concentration standard organic carbonate-based electrolytes in combination with LMB based electrode chemistries is typically quite poor [7].
Increasing the conducting salt concentration in the electrolyte very much beyond 1.2 M leads to a new class of concentrated liquid electrolytes, high-concentration electrolytes (HCEs), also called solvent-in-salt electrolytes [8]. Compared to the aforementioned liquid moderate-concentration electrolytes (MCEs) with conducting salt concentrations in the range of 1 to 1.2 mol L−1, HCEs have a conducting salt concentration of up to 5 M or even exceeding 10 M. Often, the conducting salt concentration for HCEs is no longer given as molar concentration, but as molal concentration. Molarity is defined as the amount of dissolved salt ions per liter of solvent (mol L−1, M). In contrast, the molality is specified as the number of dissolved salt ions per kilogram of solvent (mol kg−1, m), which makes the molal concentration independent of temperature and pressure. The increase in conducting salt concentration goes along with a change in the coordination environment and solvation structure in the electrolyte, thus causing thermally and electrochemically different properties. Moreover, the composition of SEI and CEI is changing, too. The advantages of an HCE over an MCE comprise an increased electrochemical stability at both electrodes, enhanced Li+ transport properties in the electrolyte, lower volatility and higher thermal stability, which improve the battery’s safety aspects. However, a disadvantage for HCEs is their higher viscosity, which results in a lower ionic conductivity compared to the MCEs. Besides, due to the higher costs for the conducting salt, the cost-effectiveness of such electrolytes is decreasing in relation to the state-of-the-art electrolytes.
In line with this, finding the balance between the advantages and disadvantages of HCEs is the highest aim for designing an ideal electrolyte. Herein, the history of the development of HCEs for lithium-based batteries with non-aqueous and aqueous electrolytes up to today’s newest research results will be illuminated and the electrolyte properties will be discussed. In “beyond lithium chemistries” such as sodium-, potassium-, magnesium- or zinc-ion batteries, HCEs are also reported in the literature [9], but will not be considered here.

2. Chronology of High-Concentration Electrolytes

Liquid electrolytes for lithium-based batteries can be divided into two different electrolyte types: non-aqueous, organic and inorganic electrolytes and the aqueous electrolytes (Figure 4). Each of these types can further be divided into moderate- (MCE) and high-concentration electrolytes (HCE). For the organic solvent-based electrolyte, localized high-concentration electrolytes are also known. In the following, different sub-categories and the chronology of electrolyte development will be described more closely.
The most commonly investigated lithium-ion conducting salts and solvents as electrolyte components for MCEs, HCEs and LHCEs are depicted in Figure 5.

2.1. HCEs for Graphite-Based LIBs

Dahn and co-workers used high-concentration electrolytes for lithium-based batteries already in 1985, when they successfully intercalated Li-ions into a ZrS2 electrode by utilizing a saturated propylene carbonate (PC) solution. The increased concentration of the conducting salt lithium hexafluoroarsenate (LiAsF6) could suppress a co-intercalation of the PC molecules, typically seen for layered electrodes [10,11,12,13,14,15]. This approach was further extended to graphite electrodes, where Ogumi et al. and Yamada et al. reported reversible intercalation of lithium ions into the layered graphite structure without causing electrode exfoliation and attributed it to the decreased solvent solvation number and sufficient protection of the graphite anode [16,17,18,19,20]. Thus, solvents such as dimethoxyethane (DME), tetrahydrofurane (THF), dimethyl sulfoxide (DMSO), PC and acetonitrile (ACN), previously thought not to be suitable as sole solvents for LIB electrolytes, were successfully paired with various conducting salts (lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiPF6, lithium perchlorate (LiClO4)) and used to enable reversible charge/discharge cycling of the respective batteries [16,17,18,21,22,23,24]. By replacing the unstable LiPF6 with the oxidative more stasble LiFSI to formulate a LiFSI/DMC HCE, Wang et al. were able to extend the operating temperature to 100 °C and reversibly intercalate Li-ions into a LiNi0.5Mn1.5O4 (LNMO) cathode at high cut-off voltage up to 5.2 V as the Al dissolution commonly observed with sulfonimide salts like FSI− and TFSI− at around 3.9 V is suppressed at these higher salt concentrations [25,26].

2.2. Ionic-Liquid-like HCEs in LIBs and LMBs

Ionic liquid electrolytes, also called room-temperature molten salt electrolytes, can be considered pure salt electrolytes and thus are HCEs by definition. As early as the 1930s, molten salt electrolytes were developed for thermal batteries [27], which are mainly used for special purposes, e.g., the military, where high rates (10 s) but short lifetimes (2 h) are required [28]. Molten salt electrolytes, often alkali metal-halide mixtures, feature a low volatility, wide ESW and high ionic conductivity. For application in LIBs, ionic liquid-like HCEs were mentioned more extensively in the literature in the late 1990s to 2000s with lithium salt–glyme mixtures as an amorphous analogue to the structure of polyethylene oxide-based lithium conducting solid polymer electrolytes [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. Several different derivatives of these “solvate-ionic liquids” with a strong cation-solvent interaction are known in literature [44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. In the 2000s, room-temperature molten electrolytes of LiTFSI-urea and LiTFSI-2-oxazolidinone were reported [70,71,72,73,74]. As with most ionic liquid-like HCEs, the ionic conductivity of last mentioned ones is in range of 10−3 S cm−1 due to the strong ion interactions within the electrolyte [70,71]. From 2008 to 2014, Kubota and Matsumoto et al. worked on molten electrolytes of binary and ternary mixtures of fluorosulfonyl imide anions with alkali metal cations (Li+, Na+, K+ and Cs+) [75,76,77,78,79,80,81]. In 2013, they reported a solvent-free lithium metal battery with lithium fluorosulfonyl(trifluoromethanesulfonyl)imide (LiFTFSI) as the electrolyte [82]. The ESW of these molten electrolytes is wide and depending on the kind of anion chosen, reaching from 0 V to 5.0 V for FTFSI− and up to 5.2 V vs. Li|Li+ for TFSI− anions. Nevertheless, the operating temperature with 100 to 150 °C is quite high.

2.3. HCEs for Lithium Metal Batteries

Besides the intercalation electrodes in LIBs, HCEs demonstrated excellent cycling performance with LMB anodes as different research groups reported highly reversible lithium stripping/plating with a more compact lithium deposition morphology [8,83,84,85,86]. In 2008, Jeong et al. first reported a dense lithium deposition morphology with a LiTFSI/PC HCE and in 2013 Suo and co-workers achieved a long cycling stability with Coulombic efficiencies close to 100% in a challenging lithium-sulfur battery by employing an ether-based HCE [8,83]. They attributed this advanced charge/discharge cycling performance to the HCEs’ ability to prevent the formation of dendrites and sufficiently passivate the lithium metal anode. In addition, Suo et al. found that the HCEs do not dissolve Li polysulfides, thereby effectively hindering their formation during cycling, which is a major cause of early battery failure when using high-capacity sulfur cathodes [8]. Instead of LiTFSI, the LiFSI salt is mainly used for ether-based HCEs for lithium metal batteries due to its higher solubility and high ionic conductivity in most organic solvents [87,88,89]. In recent years, the concept of HCEs has been extended by adding a diluent into the electrolyte, which does not negatively interfere with the beneficial properties of the HCE but mitigates some of its drawbacks [90,91,92]. These so-called “localized high-concentration electrolyte (LHCE)” are widely regarded as one of the most promising to enable the use of metallic lithium-based batteries and is currently one of the highly investigated electrolyte classes.

2.4. Aqueous-Based HCE in LIBs and LMBs

Due to the environmental friendliness and affordability of water, as well as higher safety due to its intrinsic non-flammability, interest in aqueous HCEs has arisen. The first LIB with an aqueous electrolyte was introduced in 1994 by Dahn et al., a 5 mol L−1 lithium nitrate (LiNO3) electrolyte with LiMn2O4 positive and VO2 negative electrode [93]. In the following years, several materials for positive electrodes and the influence of pH value changes in aqueous MCEs with inorganic lithium salts such as LiNO3 or lithium sulfate (Li2SO4) were investigated. Altogether, these aqueous MCEs were still suffering from a narrow ESW of <2 V (2.4 V up to maximum 4.3 V vs. Li|Li+) and a low energy density of below 75 Wh kg−1 [93,94]. After a decade of silence, in 2015 the concept of “water-in-salt” electrolytes (WiSE) with high amounts of conducting salts was first mentioned by Suo et al. In their study, a 21 mol kg−1 LiTFSI-based high-concentration electrolyte was developed and applied in a LiMn2O4||Mo6S8 battery cell set-up. The high amounts of conducting salt led to an expanded ESW up to 3 V, ranging from 1.9 V up to 4.9 V vs. Li|Li+, and allowed the use of aqueous electrolytes in LIBs with higher energy density than for moderate-concentration counterparts [94,95]. In 2016, “water-in-bisalt” electrolytes (WiBS) were introduced, using either two lithium salts or one lithium and one non lithium salt [96]. The further increase in the amount of lithium conducting salt leads to super-concentrated electrolytes or even hydrate melt electrolytes of eutectic salt mixtures, which partly show ESWs up to 5 V (0 V and 5.05 V vs. Li|Li+) due to suppressed hydrogen evolution reactions through alloying reactions at the electrode|electrolyte interface and enable the use of negative electrode materials with lower redox potentials like lithium titanium oxide (Li4Ti5O12, LTO) [97,98]. Besides the change in solvation structure to decrease the amount of free water molecules, a beneficial aspect of “water-in-salt” and “water-in-bisalt” electrolytes is the ability to form an interphase between electrode and electrolyte, similar to an SEI, and to stabilize the interface. Moreover, the use of artificial SEI layers to protect the electrodes against water molecules was investigated, e.g., by pre-coating with hydrophobic material 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether for “water-in-bisalt” electrolytes, but water penetration cannot be stopped completely [99]. Nevertheless, HCEs with lithium salt-based aqueous electrolytes are not yet comparable to aprotic, organic-based moderate- and high-concentration electrolytes in terms of Coulombic efficiency, constant current cycling stability as well as cycle and calendar life.

3. Impact of High Salt Concentration on Physicochemical and Electrochemical Properties

3.1. Solvation Structure

The benefits of an HCE compared to their MCE counterparts can be mainly attributed to the change in the solvation structure. In a standard organic carbonate-based electrolyte (e.g., LiPF6 in EC/EMC), the conducting salt concentration is ≈1 to 1.2 M and the lithium-ion is rather fully coordinated by solvent molecules. In addition, there are free, non-coordinating solvent molecules in the electrolyte solution. Increasing the conducting salt concentration and thereby the lithium-to-solvent ratio, fewer free uncoordinated solvent molecules are present in the electrolyte and the solvation structure shifts from solvent-separated ion pairs (SSIP), common in MCEs, to contact ion pairs (CIP) and aggregates (AGG), where the anion is coordinating to one (CIP) or more (AGG) lithium-ions (Figure 6) [29,34,40,100,101,102]. At a certain threshold, all of the solvent molecules are coordinating ions, and the lowest unoccupied molecular orbital (LUMO), which governs the reactivity against reduction, is in most cases dominated by the anion and not the solvent as it is in most MCEs [103,104]. The resulting benefits are further discussed in the next chapter. Besides the widening of the ESW, the new solvation structure further impacts the ion transport mechanism.

3.2. Ionic Conductivity

The mobility of relevant charge carriers, here the lithium ions in the electrolytes, in an electric field is described as ionic conductivity. For a battery’s performance, a high ionic conductivity, especially of lithium ions in an LIB electrolyte, is desirable as it determines the rate capability during charge and discharge of a battery and thus power and energy density. The ionic conductivity is mainly influenced by two parameters: the lithium-ion concentration and the type of anion and solvent(s) [104]. For MCEs, the ionic conductivity is increasing with higher salt concentrations as a result of the increasing number of solvent-separated ion pairs, which are the charge carriers of the electrolyte [105]. Ion movement usually obeys the Stokes–Einstein law, and ion transport follows the “vehicle-type” mechanism, in which solvent-coordinated ions move via diffusion through the solution (Figure 7a) [104,106]. At conducting salt concentrations of ≈1–1.2 mol L−1, depending on the salt class, the maximum ionic conductivity is reached. For typical MCEs with organic carbonate solvent(s), a value of 10 mS cm−1 can be reached [104]. Due to the decreasing number of solvent-separated ion pairs and increasing number of contact ion pairs and aggregates at rather high conducting salt concentrations in HCEs [100], the viscosity of the electrolyte increases, thus decreasing the ionic conductivity discernibly. According to the Walden rule, where the molar conductivity Λ is plotted as a function of viscosity η (Figure 8), the HCEs can be classified into good ionic conductive, if located close/on the KCl-line (ideal conductivity of a fully dissociated potassium chloride (KCl) solution), poor ionic conductive for values below the KCl-line, or even “superionic” for above the KCl-line [104]. Superionic performance means that the lithium-ion movement is decoupled from the viscosity of the electrolyte, e.g., this behavior is observed for the hydrate melt LiTFSI0.7LiBETI0.3 2H2O [97]. Therefore, as the lithium ions are not just solvated by the solvents but by the anions as well, the ion transport is likely a hopping-type mechanism (Figure 7b), where the lithium ions move from one anion coordinating center to the next one, while the rest of the solvation structure remains stationary [100].
Although this mechanism is not yet completely understood, Watanabe et al. and Xu et al. could provide clear evidence for this lithium-ion hopping conduction in a concentrated sulfone-based electrolyte as pulsed-field gradient nuclear magnetic resonance (PFG-NMR) and molecular dynamic (MD) simulations predicted a faster ion movement of the lithium-ions as the anion and solvent [22,107]. Here, the oxygen of the sulfone moiety and the anions bridge different lithium-ions, thereby enabling the hopping to different coordination sites [107]. A result of this is the increase in the lithium diffusion coefficient to values higher than 0.5, which is not common in lithium-based MCEs, as the anion is mostly more mobile than the lithium ions due to its smaller solvation shell [22]. Accompanied by the hopping mechanism, the transference number of HCEs is typically higher than 0.5. In HCEs, lithium ions move faster than the anions in comparison to MCEs, with a transference number between 0.2 and 0.4 [108]. Moreover, HCEs with the low-viscosity solvent acetonitrile and highly dissociated LiTFSI or LiFSI reach ionic conductivities comparable to state-of-the-art MCEs [103]. Overall, in the case of a combination of moderate ionic conductivity with a high lithium-ion transference number, HCEs exhibit high-rate performance, indicating that the balance between ionic conductivity and the lithium-ion transference number is crucial.

3.3. Electrochemical Stability Window

The electrochemical stability window (ESW) is the potential range between a cathodic and anodic limit, in which the electrolyte neither undergoes reduction nor oxidation reactions. This means the electrolyte is electrochemically (either thermodynamically or kinetically) stable and no decomposition of electrolyte components, i.e., conducting salt, solvent or additives, is taking place. The practical potential limits can be determined by cyclic voltammetry (CV) and/or linear sweep voltammetry (LSV). During this measurement, the potential is increased with a continuous scan rate while the current is recorded. An increasing current compared to the zero-current of the battery indicates a beginning decomposition of electrolyte components (Figure 9). The potentials, where reduction of the electrolyte at the cathodic side or oxidation at the anodic side occurs, are impacted by the molecular orbitals and the band gap between energy levels. Due to largely coordinated and less free solvent molecules, the electrochemical stability window of HCEs is widened. As discussed before, the different behavior of HCEs compared to MCEs mainly occurs due to the change in solvation structure from solvent-separated ion pairs to contact ion pairs and aggregates. The reductive stability is increased because the lowest unoccupied molecular orbital (LUMO) of the solvent is shifted upwards [103]. As a consequence, the LUMO of the electrolyte solution is shifted towards the conducting salt, which now decomposes before the solvent at low potentials. Moreover, this leads to the formation of a salt-derived SEI on the negative electrode, favoring better reductive stability [103]. The higher oxidative stability can be explained by the down-shifted highest occupied molecular orbital (HOMO) of the solvent to lower energy levels [49]. Both effects enable the use of HCEs for high-voltage applications and enhance the battery’s longevity (Figure 9a) [26]. For electrolyte formulation, the strong solvent coordination means that the use of solvents with narrow ESW becomes possible, e.g., DME which is usually not stable against commonly used 4 V positive electrode materials. However, especially for aqueous-based electrolytes, the widened ESW is essential, where “water-in-salt” and “water-in-bisalt” electrolytes allow the application in higher voltage ranges and overcome low energy densities [94]. In “water-in-salt” electrolytes, the water molecules coordinate the lithium ions of the conducting salt, and the high amount of conducting salt leads to a disruption of the continuous water network and thus avoids early hydrogen and oxygen evolution reactions [95]. In addition, for non-aqueous-based electrolytes in combination with 5 V high-voltage electrodes such as LiNi0.5Mn1.5O4 (LNMO) or lithium nickel cobalt manganese oxide LiNi0.8Co0.1Mn0.1O2 (NMC811), a more stable constant current cycling behavior can be observed, which is attributed to an effective interphase formation, especially on the cathode side (CEI) [109,110,111].
Additionally, HCEs suppress the dissolution of aluminum (Al) current collectors and transition metal dissolution from the cathode [112,113,114,115]. Due to the coordination of solvent by conducting salt, the shifted energy level of the LUMO and concentrations close to the saturation limit, the solubility of Al(X)n+3−n-complexes in the HCE gets low [114]. Consequently, the passivation behavior of LiPF6 in MCEs is no longer required; the choice of conducting salts is not limited, and thus non-Al-passivating salts such as fluorosulfonyl imides like LiFSI and LiTFSI can be chosen. Transition metal dissolution, the leaching of metal cations from cathode active materials into the electrolyte, is especially common in case of cobalt-free and lithium-rich cathode active materials and leads to a low Coulombic efficiency [116]. In HCEs, transition metal dissolution at the cathode and subsequent deterioration of the SEI at the anode is not only inhibited by the low solubility of the metal complexes in the electrolyte, but also by an inorganic, robust and more effective lithium fluoride (LiF)-rich CEI [116,117].

3.4. Li Stripping/Plating

In contrast to LIBs with commonly used intercalation/insertion anodes like graphite or graphite–silicon composites, LMBs do not utilize a lithium-ion host material on the anode side. The lithium ions are, therefore, plated directly on top of the residual lithium metal or the copper (Cu) current collector. This continuous deposition of Li through the SEI usually does not take place in an ideal, smooth 2D layer, but rather, inhomogeneous high surface area lithium (HSAL) deposition emerges when standard MCEs are used. The preferable decomposition of the anion in HCEs at low potentials can considerably improve the interphase morphology, as with the right choice of the conducting salt, an effective, inorganic-rich SEI is formed. Combined with a high lithium-ion concentration, which is thought to facilitate a homogeneous lithium deposition morphology and suppresses HSAL growth, an advanced cycling and rate performance is achieved [84]. This is based on the widely accepted Chazalviel model, which states that a space charge layer with an ion-concentration gradient is formed on the electrodes when a current is applied. If a threshold of a particular current density is reached and a critical amount of lithium is deposited and thereby removed from the electrolyte in proximity to the electrode, the local lithium-ion concentration can reach zero and thereby induce dendrite growth [118]. Thus, the increase in the ion-concentration in the electrolyte by using more conducting salt naturally raises the critical current density, the limit that leads to the formation of dendrites and thus promotes more homogeneous lithium deposits (Figure 10). Qian et al. demonstrated this by utilizing a 4 M LiFSI in DME electrolyte to get reversible lithium stripping/plating at high rates of up to 10 mA cm−2 [84].
The cycling experiments in Cu‖Li cells further proved the superior electrochemical performance of HCEs in comparison to various MCEs as the Coulombic efficiency surpassed 99% and dense, homogeneous lithium deposits with a passivation layer rich in inorganic fluoride, nitrogen and sulfur species were formed [84]. In recent years, several different conducting salt and solvent combinations have been investigated [26,92,116] but the combination of LiFSI and DME resulted in the best comparability with a lithium metal anode. LiFSI is highly soluble in most polar aprotic solvents, leads to a high ionic conductivity and generates a highly protective layer on the anode and cathode; three crucial properties for the conducting salt used in HCEs [84,87,119]. The solvent DME combines the advantages of a low viscosity with a high stability against lithium. The low reduction potential of −1.68 V vs. Li|Li+ reduces the probability of a reaction with lithium and further promotes the formation of a favorable inorganic anion-derived SEI as proven by advanced theoretical calculations [87,120,121]. Experimental findings back this hypothesis as the galvanostatic cycling performance of diluted DME-based HCEs outperformed other HCEs based on dimethyl carbonate, triethyl phosphate (TEP) and trimethyl phosphite (TMS), in terms of Coulombic efficiency in half- and full-cell setups [122]. The dilution of the HCE with an inert solvent to generate a so-called localized high-concentration electrolyte (LHCE) has been the research direction in recent years to further boost the performance of the HCE and circumvent some of its drawbacks, which concept will be explained in the next chapter.
Similar to the organic solvent-based HCEs, ionic liquid electrolytes also promote an effective inorganic SEI formation due to their high electrochemical stability at high and especially low potentials vs. Li|Li+. Due to their high viscosity, the ionic conductivity of organic solvent-free liquid electrolyte solutions is usually very poor; however, at high conducting salt concentrations, this issue can be avoided, and high CE at current densities as high as 20 mA cm−2 can be reached. Besides the organic-cation ILs, a low-melting pure alkali FSI-based molten salt electrolyte has been developed for LMBs by blending LiFSI with KFSI and CsFSI, which achieved a remarkable CE of 99.8% with very low overpotentials, but only at elevated temperatures of 60 and 80 °C [123]. IL-based HCE are a promising concept, especially for LMBs; however, their very high costs, high viscosity and operating temperatures make them currently unviable for broader, industrial-size use.

4. Safety Aspects

The safety of a battery cell plays a crucial role in the adaptability and usage of energy storage systems. The highly flammable organic carbonate-based electrolyte in today’s LIBs can cause fires and even explosions in the event of an accident or misuse of the battery, which is why the research is directed in a way to minimize or completely eradicate such risks by using less-flammable electrolytes. By increasing the conducting salt concentration in the electrolyte, the relative amount of solvent compared to the conducting salt is reduced and its volatility decreased, which in combination leads to a considerable decrease in the solution’s flammability, which makes concentrated electrolytes in itself safer than their moderate-concentrated counterparts [124]. In addition, novel nonflammable concentrated electrolyte formulations have been developed by substituting the flammable solvent with flame-retardant solvents like trimethyl phosphate (TMP) [125,126] and triethyl phosphate [127]. The solvent co-intercalation into graphite by a trimethyl phosphate-based electrolyte could successfully be prevented by utilizing high concentrations of LiFSI. The formation of a robust inorganic protective layer on the anode expands their scope of application further to more challenging battery chemistries like LMBs and respective batteries have shown significantly improved long-term cycling performance with a nonflammable phosphate-based electrolyte. Besides the organic flame-retardant solvent, the broad ESW opens up the possibility of using water as a solvent, which is intrinsically nonflammable and safe. As mentioned earlier such aqueous electrolytes have been used in LIBs with voltage operation windows up to 3 V (1.9 to 4.9 V vs. Li|Li+), making them a viable option for highly safe batteries with a respectable energy density [96,99].
Thus, with the enhanced safety aspects and different properties the three types of HCEs - organic HCE, aqueous HCE and ionic-liquid HCEs - can be applied for different use cases, which are summarized in Figure 11.

5. Remaining Challenges of HCEs

The benefits of high-concentration electrolytes, however, come with certain challenges hindering a broad commercial application. The primary issue regarding these electrolytes is their huge price tag. Currently used organic carbonate-based electrolytes make up about 4% of the price of LIBs. A majority of these costs can be attributed to the conducting salt (roughly 60%), even for the relatively low concentrations of only 1 mol L−1 used today, as LiPF6 is currently priced ten times higher than the organic carbonate solvent according to the latest fluctuating market data. The HCEs investigated in recent years require at least four times the conducting salt amount compared to a standard organic carbonate-based electrolyte (c.f. 1.0 M LiPF6 in EC/EMC (3:7 by wt.)). By assuming a similar price per kg for the respective Li salts and solvents, the material costs for those electrolytes dramatically increase, becoming a major cost factor of the price of a battery with a substantial contribution of over 10%. The second problem of the HCEs arises during cell manufacturing, specifically the wetting of active materials after the electrolyte was added. This process often poses a bottleneck during the production of the battery, as it usually takes several days before the electrolyte fills every hole and active particle of the electrodes evenly [26,128]. Minimizing this wetting period as much as possible is crucial to keep the production output high. Due to their higher salt concentration, HCEs possess a much higher viscosity than conventionally used electrolytes, which is in direct correlation with the wetting time of the cell. This would prolong the process during cell manufacturing, which is already considered rate-determining, even more, and although there are counter measurements to reduce the wetting time, like increasing the temperature, this would still worsen the economics of the manufacturing process [128,129].

6. Modifications of HCEs to Face Challenges

To circumvent the aforementioned challenges, the idea of diluting the electrolyte to reasonable conducting salt concentrations (1 to 2 M) has been investigated and successfully applied. In order to preserve the advantages of the HCEs, which mainly arise from the unique solvation structure, the so-called diluent should interfere with the lithium solvation shell as little as possible but still form a homogeneous solution after the addition (Figure 12).
Besides that, the diluent should possess most of the major attributes of a typical electrolyte solvent including a sufficient electrochemical stability within the operating voltage, low cost and viscosity, as well as ideally be nonflammable and not volatile to keep the safety benefits of the HCEs. In recent years, several candidates for non-aqueous HCEs have been identified as possible diluents, with the most prominent member being fluorinated ethers including 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE) and tris(2,2,2-trifluoroethyl)orthoformate (TFEO) as they combine all of the mentioned requirements for a diluent to a sufficient extent. By introducing TTE into the HCE consisting of 2.8 m LiTFSI in tetraglyme (G4), Dokko et al. could attain a one molar so-called LHCE with a considerably reduced viscosity compared to its high-concentration counterpart [53]. Ren et al. investigated a similar LHCE consisting of LiFSI in DME/TTE (1:1.2:3 molar ratio) in a lithium metal battery chemistry and achieved a considerably improved constant current cycling performance with a dense lithium deposition morphology that was even better than with the respective HCE [92]. In recent years, several other possible diluents have been investigated, further tuning specific electrolyte properties, like flammability [130]. A novel approach to especially enhance the high-rate performance of an LMB has recently been introduced by Kim et al. by employing a so-called high-entropy electrolyte. By simply using several different solvents and diluents instead of just one each, the ion conductivity could be increased leading to a better cycling performance especially at higher charging rates (>1 mA cm−2). This could be attributed to smaller ion-clusters formed due to the rise in entropy within the electrolyte [131]. However, most of the investigated diluents are highly fluorinated and will likely be included in the highly debated ban of per- and polyfluoroalkyl substances (PFAS) by the European Union initiated in early 2023 to minimize the release of these so-called “forever chemicals” due to their long half-lives and bioaccumulation risk [132,133]. However, the closed system of a battery and the current lack of suitable material alternatives, especially for electrode binders and conducting salts, might delay the PFAS ban for these use cases and even completely be deferred if the remaining challenges during the recycling can be solved [134,135]. Nonetheless, researchers have tried to find PFAS-free diluent alternatives in recent years to bypass this ban altogether, including anisole [136] and benzene [137], which enabled the use of PFAS-free electrolytes in LMBs. However, they still have some shortcomings compared to their fluorinated counterparts in terms of oxidative stability (anisole) and volatility (benzene) [136,137].

7. Conclusions and Outlook

High-concentration electrolytes (HCEs) represent a promising research approach in the field of liquid electrolytes, tackling numerous challenges and limitations associated with today’s conventional moderate-concentration liquid electrolytes (MCEs). In lithium-based batteries, HCEs find application throughout various battery chemistries, from graphite-based lithium-ion battery (LiB) setups to lithium metal-based batteries (LMBs). Their unique solvation structure, with contact ion pairs and aggregates has several benefits, including widening the electrochemical stability window, increasing the lithium-ion transference number, enhancing the rate and long-term cyclability by their tendency to form effective anion-derived solid electrode interphases and cathode electrolyte interphases at the corresponding electrode|electrolyte interfaces and improving the safety aspects due to lower volatility and flammability. Thus, HCEs with organic-based solvents extend the operating range to high-voltage batteries (>5 V) and enable aqueous electrolytes with a remarkable electrochemical stability window of 3 V (1.9 V to 4.9 V vs. Li|Li+). In contrast, facing challenges with lower ionic conductivity and prolonged wetting process due to the high viscosity as well as the high concentrations of cost-intensive conducting salts, arouse the concept of dilution. The concept of localized high-concentration electrolytes combines the benefits of HCEs and MCEs in a compromise to make high-concentration electrolytes available for battery electrolytes in the industry. Meanwhile, understanding the solvation and transport mechanisms with and without dilution is fundamental in electrolyte research. Here, a combination of experimental and theoretical techniques is required to obtain deeper insights.

Author Contributions

Conceptualization, S.K. and D.W.; methodology, S.K. and D.W.; validation, S.K., D.W., I.C.-L., M.G. and M.W.; resources, M.W.; data curation, S.K. and D.W.; writing—original draft preparation, S.K. and D.W.; writing—review and editing, S.K., D.W., I.C.-L., M.G. and M.W.; visualization, S.K. and D.W.; supervision, I.C.-L., M.G. and M.W.; project administration, I.C.-L. and M.G. All authors have read and agreed to the published version of the manuscript.

Funding

This review received no external funding.

Acknowledgments

The authors thank Andre Bar for art design and creation of Figure 3.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic battery setup and working principle of a lithium-based battery during the discharge process.
Figure 1. Schematic battery setup and working principle of a lithium-based battery during the discharge process.
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Figure 2. Common electrode materials for lithium-based batteries and their capacities and potentials.
Figure 2. Common electrode materials for lithium-based batteries and their capacities and potentials.
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Figure 3. Properties that an ideal electrolyte for lithium-based batteries should provide.
Figure 3. Properties that an ideal electrolyte for lithium-based batteries should provide.
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Figure 4. Types of liquid electrolytes differentiated by salt concentration and solvent chemistry.
Figure 4. Types of liquid electrolytes differentiated by salt concentration and solvent chemistry.
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Figure 5. Structures of common lithium-ion conducting salts and organic solvents for high-concentration electrolytes.
Figure 5. Structures of common lithium-ion conducting salts and organic solvents for high-concentration electrolytes.
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Figure 6. Solvation structures of solvent-separated ion pairs (SSIP) in MCEs, contact ion pairs (CIP) and aggregates (AGG) in organic-based HCEs and water-in-salt electrolytes as aqueous-based HCEs.
Figure 6. Solvation structures of solvent-separated ion pairs (SSIP) in MCEs, contact ion pairs (CIP) and aggregates (AGG) in organic-based HCEs and water-in-salt electrolytes as aqueous-based HCEs.
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Figure 7. Schematic illustration of two different lithium-ion transport mechanisms in electrolytes: (a) vehicle-type mechanism by mass diffusion, which is common in MCEs, and (b) hopping-type mechanism of lithium ions in HCEs.
Figure 7. Schematic illustration of two different lithium-ion transport mechanisms in electrolytes: (a) vehicle-type mechanism by mass diffusion, which is common in MCEs, and (b) hopping-type mechanism of lithium ions in HCEs.
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Figure 8. Walden plot of the molar conductivity Λ against viscosity η to classify electrolytes into good ionic conductive (green), poor ionic conductive (grey) and superionic (red).
Figure 8. Walden plot of the molar conductivity Λ against viscosity η to classify electrolytes into good ionic conductive (green), poor ionic conductive (grey) and superionic (red).
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Figure 9. (a) Schematic of the electrochemical operating window of batteries based on the highest occupied molecular orbital- lowest unoccupied molecular orbital (HOMO-LUMO) of the electrolyte and the redox energies of the electrodes. (b) Schematic cyclic voltammetry measurement to determine the reductive (grey) and oxidative (green) stability limit and thus the ESW of an electrolyte.
Figure 9. (a) Schematic of the electrochemical operating window of batteries based on the highest occupied molecular orbital- lowest unoccupied molecular orbital (HOMO-LUMO) of the electrolyte and the redox energies of the electrodes. (b) Schematic cyclic voltammetry measurement to determine the reductive (grey) and oxidative (green) stability limit and thus the ESW of an electrolyte.
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Figure 10. SEM images of the morphologies of lithium metal after plating on Cu substrates in different electrolytes. (a,b) 1 M LiPF6-PC. (c,d) 4 M LiFSI-DME. (Scale bar, 10 µm). Reproduced with permission [84]. Copyright 2015, Nature Publishing Group.
Figure 10. SEM images of the morphologies of lithium metal after plating on Cu substrates in different electrolytes. (a,b) 1 M LiPF6-PC. (c,d) 4 M LiFSI-DME. (Scale bar, 10 µm). Reproduced with permission [84]. Copyright 2015, Nature Publishing Group.
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Figure 11. Relevant properties and use-cases of high-concentration electrolytes.
Figure 11. Relevant properties and use-cases of high-concentration electrolytes.
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Figure 12. Solvation structure of high-concentration electrolytes (HCE) and localized high-concentration electrolytes (LHCE).
Figure 12. Solvation structure of high-concentration electrolytes (HCE) and localized high-concentration electrolytes (LHCE).
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Krämer, S.; Weintz, D.; Winter, M.; Cekic-Laskovic, I.; Grünebaum, M. Importance of High-Concentration Electrolytes for Lithium-Based Batteries. Encyclopedia 2025, 5, 20. https://doi.org/10.3390/encyclopedia5010020

AMA Style

Krämer S, Weintz D, Winter M, Cekic-Laskovic I, Grünebaum M. Importance of High-Concentration Electrolytes for Lithium-Based Batteries. Encyclopedia. 2025; 5(1):20. https://doi.org/10.3390/encyclopedia5010020

Chicago/Turabian Style

Krämer, Susanna, Dominik Weintz, Martin Winter, Isidora Cekic-Laskovic, and Mariano Grünebaum. 2025. "Importance of High-Concentration Electrolytes for Lithium-Based Batteries" Encyclopedia 5, no. 1: 20. https://doi.org/10.3390/encyclopedia5010020

APA Style

Krämer, S., Weintz, D., Winter, M., Cekic-Laskovic, I., & Grünebaum, M. (2025). Importance of High-Concentration Electrolytes for Lithium-Based Batteries. Encyclopedia, 5(1), 20. https://doi.org/10.3390/encyclopedia5010020

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