Recent Progress of the Crystalline Organic Electrolytes for Solid-State Battery Applications
Article information
Abstract
Crystalline organic electrolytes (COEs) have recently emerged as promising alternatives to conventional solid-state electrolytes, including oxide, sulfide, and polymeric electrolytes. This interest arises from the limitations of traditional solid-state electrolytes, which often suffer from inadequate ionic conductivity, poor electrochemical stability, and difficulty in establishing intimate contact with cathode particles. In this review, COEs are introduced with a focus on their classification, unique characteristics, and case studies highlighting their application in solid-state batteries. COEs are fundamentally composed of alkali metal salts and organic crystalline solvents. Based on the type of solvent, they are classified into three categories: organic ionic plastic crystal electrolytes (OIPCs), non-ionic plastic crystal electrolytes (NIPCs), and non-plastic crystal organic electrolytes (NOPCs). COEs offer several advantageous properties, including high ionic conductivity, low-to-negligible flammability, and excellent compatibility with electrodes achieved through meltcasting processes. These features position COEs as a transformative solution for advancing solid-state battery technologies, enabling the development of safe, high-performance, and energy-dense devices for electrified applications.
INTRODUCTION
Over the past few decades, lithium-ion batteries (LIBs) have been extensively studied to meet the soaring demand for energy devices such as mobile phones, electric vehicles, and energy storage systems [1]. However, conventional LIBs face significant safety challenges due to the highly flammable alkyl carbonate electrolytes they utilize [2,3]. These electrolytes are popular for their low cost, high ionic conductivity, and long lifecycle performance. Yet, their inherent flammability poses a severe risk, especially during thermal runaway events, where battery vent gases like H2, CH4, C2H4, and C2H6 are generated—highly hazardous under such conditions [4].
To address these safety concerns, various solid-state electrolytes have been investigated, including sulfide, polymeric, and oxide electrolytes [5–14]. While polymer electrolytes generally suffer from low ionic conductivity, sulfide electrolytes, despite their relatively high conductivity, lack adequate electrochemical stability. Oxide electrolytes, on the other hand, face challenges in forming close interfacial contact with electrodes, a critical factor for efficient charge transport [15–17].
In this context, crystalline organic electrolytes (COEs) have emerged as a potential alternative due to their advantageous properties, such as high ionic conductivity, the ability to form intimate contact with porous electrodes, and superior electrochemical stability. COEs are composed of organic crystalline solvents combined with alkali metal salts to facilitate ion conduction. Notably, oxide electrolytes or sulfide electrolytes, while crystalline, do not qualify as COEs since they are not organic materials [18–20]. Similarly, polymeric electrolytes, though composed of organic materials, are amorphous, and their ion conduction primarily depends on the segmental motion of polymer chains [21]. Therefore, oxides, sulfides, and polymeric electrolytes differ from COEs in terms of solvent type and ion conduction mechanisms.
COEs can be categorized into three types based on the nature of their solvent molecules: organic ionic plastic crystal electrolytes (OIPCs), non-ionic plastic crystal electrolytes (NIPCs), and non-plastic crystal electrolytes (NOPCs), based on solvent molecules (Fig. 1). OIPCs contains ionic solvent consisting of bulky cation and anion such as alkyl pyrrolidinium and bis(fluorosulfonyl)imide. NOPCs comprise neutral plastic crystal molecules, such as succinonitrile (SN). Both OIPCs and NOPCs are plastic crystals (PCs), which are crystalline materials with unique properties. PCs are formed by certain types of molecules or ions with reorientational freedom and offer both exceptional mechanical plasticity and long-range order, hence they are attractive for many mechano-adaptable technologies [22,23]. Ion conduction in PCs occurs in manners of “revolving door mechanism” where proton or alkali ions are conducted by rotational motions of the molecules in crystal structure (Fig. 2a) [24]. Dielectric relaxation spectroscopy (DRS) is one of the useful techniques which can determine whether some material is PC or not. Owing to their reorientational freedom, PCs exhibits relaxation behavior on DRS [22]. Unlike OIPCs, NIPCs consists of non-ionic plastic crystals [25]. There are several types of neutral plastic crystal molecules, but only SN is reported as NIPCs, because neutral plastic crystal molecules except SN possess very low melting point, which is not suitable for battery applications [26–28]. NOPCs consist of organic crystalline materials without plastic crystalline properties. Recently, several NOPCs have been reported utilizing dimethyl formamide or dimethyl sulfone by Zdilla’s group and Lee’s group [29–34].
Categorization of COEs based on their solvent types.
Both OIPCs and NIPCs are plastic crystals, which are crystalline materials with rotational freedom for their molecules or ions. This property imparts exceptional mechanical plasticity and long-range order, making them highly adaptable for various mechano-adaptable technologies [24]. Ion conduction in plastic crystals occurs via a “revolving door mechanism”, wherein alkali ions are transported through the rotational motions of the crystal’s constituent molecules (Fig. 2a). This behavior can be identified using DRS, which detects the relaxation behavior of materials with reorientational freedom [22]. Unlike OIPCs and NIPCs, the solvent molecules in NOPCs lack rotational freedom [33]. Ion conduction in NOPCs occurs through ion channels in the crystal structure, often supported by liquid-like layers on the crystal surface (Fig. 2b) [29–31,34].
This review introduces the three classifications of COEs and examines their physicochemical and electrochemical properties in detail. It also explores their potential applications in energy devices, highlighting their promise for safe, high-performance, solid-state battery technologies.
CLASSIFICATION AND PROPERTIES OF COEs
As described in the introduction, COEs can be classified into OIPCs, NIPCs, and NOPCs based on the type of solvent, each with distinct properties depending on their materials (Fig. 3). Both OIPCs and NIPCs consist of PCs, where ion conduction follows a revolving door mechanism, enabling relatively high ionic conductivity. However, due to the revolving motion of solvents, OIPCs and NIPCs have slightly low melting points, resulting in insufficient mechanical strength. In contrast, ion conduction in NOPCs is not facilitated by the fast revolving door mechanism, leading to relatively lower ionic conductivity compared to OIPCs and NIPCs. However, the absence of solvent motion in NOPCs allows for higher melting points and improved mechanical strength. Most importantly, COEs are well-suited for the melt-casting process due to their moderate melting points (50−100℃). In their molten state, COEs can readily penetrate the pore structure of composite electrodes and subsequently solidify upon cooling to ambient temperature, ensuring intimate contact with the composite electrode. This melt-castability of COEs effectively addresses the chronic contact issues commonly encountered with conventional solid-state electrolytes. Based on their classification, the physicochemical properties and applications of each type of COE will be discussed.
Comparative evaluation of OIPCs, NIPCs, and NOPCs based on key properties, including ionic conductivity, electrochemical stability, mechanical strength, cost-effectiveness, manufacturability, and interfacial contact.
Organic ionic plastic crystal electrolytes (OIPCs)
OIPCs consist of large organic cations and anions, such as pyrrolidinium and bis(fluorosulfonyl)imide [27,28]. The physical properties of OIPCs are highly influenced by the specific combination of cation and anion. In general, cations with shorter alkyl chains and smaller anions yield lower melting points, higher ionic conductivity, and improved diffusivity [35]. Certain combinations of cations and anions can produce melting points below 10℃, categorizing them as ionic liquids [36]. Consequently, the careful selection of cations and anions is critical for tailoring OIPCs as effective solid-state electrolytes. Commonly employed cations include alkyl pyrrolidinium, alkyl ammonium, and alkyl phosphonium, while widely used anions include hexafluorophosphate, bis(trifluoromethanesulfonyl) imide (TFSI), bis(fluorosulfonyl)imide (FSI), and dicyanamide, as summarized in Table 1.
Chemical structure and abbreviations of various cations and anions of OIPCs. “n” denotes the number of carbon in the alkyl chain
OIPCs are emerging as promising candidates for next-generation solid-state electrolytes due to their outstanding thermal and electrochemical stability, as well as their non-flammability and non-volatility [25–28]. Their unique liquid-like behavior within a crystalline lattice supports high Li+ diffusivity, driven by the rotational and translational motions of matrix ions. While pure OIPCs typically exhibit modest ionic conductivity at room temperature, the addition of lithium salts such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium bis(fluorosulfonyl)imide (LiFSI) can significantly enhance their ionic conductivity to exceed 10−3 S cm−1 at room temperature.
These distinctive properties position OIPCs as highly attractive electrolytes for a variety of electrochemical energy applications, LIBs, sodium-ion batteries (SIBs), fuel cells, and dye-sensitized solar cells. Examples of OIPCs, presented in Table 2, consistently demonstrate high ionic conductivity and excellent electrochemical stability, making them particularly well-suited for high-voltage battery technologies.
Electrolyte compositions, physical properties, and classification of various COEs
Danah et al. proposed OIPCs composed of LiFSI and [Py1i3][FSI] in a 1:9 molar ratio [37]. They observed that 10 mol% LiFSI in [Py1i3][FSI] exhibited significantly higher ionic conductivity compared to 90 mol% LiFSI in [Py1i3][FSI], with values of 4 × 10−4 S cm−1 and 2.5 × 10−4 S cm−1 at 30℃, respectively (Fig. 4a). Moreover, the formulation with 10 mol% LiFSI facilitated the formation of a protective Li+ conducting solid electrolyte interphase (SEI) layer on the lithium metal electrode, contributing to stable battery cycling performance. Interestingly, despite both [Py1i3][FSI] and [Py13][FSI] having the same number of carbons in their alkyl chains, the linear alkyl chain in [Py13][FSI] resulted in a lower melting point than the branched alkyl chain in [Py1i3][FSI] when mixed with lithium salts. This lower melting point makes [Py13][FSI] less suitable for use as a solid-state electrolyte (Fig. 4b). These findings underscore the importance of carefully designing the cation and anion components of OIPCs to optimize their performance as solid-state electrolytes.
(a) The ionic conductivity of the OIPC [Py1i3][FSI] and the quasisolid state electrolytes containing 10 mol% and 90 mol% LiFSI are presented with red symbols. The black symbols present the analogous ionic liquid [Py1i3][FSI] and the liquid electrolyte composed of [Py1i3][FSI] with 10 mol% LiFSI. (b) The structures and appearance of the organic ionic plastic crystal [Py1i3][FSI], the ionic liquid [Py1i3][FSI] and the quasi-solid state electrolytes formed by addition of 10 mol% and 90 mol% LiFSI to [Py1i3][FSI]. Reprinted from Ref [37] with permission. [Py1i3][FSI] refers to 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide. “i3” of [Py1i3] means isopropyl alkyl chain.
The influence of alkyl group length, size, and symmetry of cations and anions on the physicochemical properties of OIPCs has been the subject of extensive research [38]. These properties are highly dependent on the specific combinations of cations and anions. Forsyth et al. investigated three phosphonium-based OIPCs— [Ph1i444][FSI], [Ph1i444][TFSI], and [Ph111i4][TFSI]—revealing that shorter alkyl chains in phosphonium cations, paired with smaller anions, result in lower melting points [35]. These combinations also enhance ionic conductivity, improve cation and anion diffusivity, and increase current densities during sodium-ion deposition and stripping in mixtures containing sodium salts. Passerini et al. examined the effect of alkyl group variation on pyrrolidinium-based OIPCs by modifying the length and structure of the side alkyl chains (CnH2n+1, where n ranges from 1 to 10) [38]. Unlike the trends observed in phosphonium-based OIPCs, increasing the length and branching of alkyl chains in pyrrolidinium cations generally leads to reduced ionic conductivity and lower melting points. These findings underscore the critical role of tailoring alkyl group configurations in optimizing the performance of OIPCs for specific applications.
The integration of a polymer matrix into OIPCs has been investigated as a strategy to increase melting points and enhance both mechanical and electrochemical properties (Fig. 5). Xiaoen Wang et al. studied composite OIPCs comprising LiFSI, [Py12][FSI], and poly(vinylidene fluoride) (PVDF) [39]. Among the tested compositions, the formulation with 40 wt% Li0.1[Py12][FSI]/PVDF exhibited the highest ionic conductivity, achieving 6 × 10−5 S cm−1 at 30℃. The composite’s compatibility with lithium metal and the stability of SEI layer were evaluated through lithium metal symmetric cell cycling at current densities ranging from 0.1 to 0.5 mA cm−2. The symmetric cell demonstrated a reduced overpotential of less than 100 mV, even at the higher current density of 0.5 mA cm−2. This result indicates the polymer composite OIPC’s excellent compatibility with lithium metal and its ability to form a stable SEI layer on the Li metal electrode. These properties contributed to the remarkable cycling stability of Li‖LiFePO4 batteries, which maintained consistent performance over 1200 cycles at a 2C rate. This study underscores the potential of polymer composite OIPC electrolytes for next-generation all-solid-state lithium batteries, offering a promising balance of ionic conductivity, mechanical robustness, and long-term electrochemical stability.
(a) Schematic of the Li0.1[Py12]0.9[FSI]/PVDF composite electrolyte preparation procedure. (b) The effect of Li0.1[Py12]0.9[FSI] loading on the conductivity at 30℃ and (inset) the composite appearance, and (c) the temperature dependence of the conductivity of the composite electrolytes. The wt% is the weight fraction of Li0.1[Py12]0.9[FSI] in the total OIPC/PVDF composite. The room temperature pressed sample and the heat treated sample are labled as RT and HT, respectively. Reprinted from Ref [39] with permission. [Py12][FSI] refers to 1-ethyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide.
Non-ionic plastic crystal electrolytes (NIPCs)
NIPCs share the same plastic crystalline structure as OIPCs, but their solvents are composed of neutral molecules rather than ionic compounds. While a variety of non-ionic plastic crystal materials exist—such as hydrogen halides, halogenated cyclohexane, diacetylene, and SN (Fig. 6)—only SN has proven suitable for use as an electrolyte in energy devices [26]. This suitability arises from the fact that other non-ionic plastic crystalline materials are either gaseous under moderate conditions or exhibit high toxicity or acidity, rendering them inappropriate as electrolyte solvents (e.g., HF, HCl, and cyclohexyl chloride). Thus, NIPCs are predominantly represented by SN-based solid-state electrolytes. SN stands out as a promising solvent for solid-state electrolytes due to its exceptional electrochemical stability (up to 5.5 V vs. Li/Li+), high dielectric constant, and excellent solubility with alkali metal salts. These characteristics enable SN to function as a versatile and effective medium for ion transport. Various NIPC formulations have been explored, capitalizing on their advantageous properties, such as high ionic conductivity, outstanding electrochemical stability, and straightforward preparation methods (Table 2). These attributes position SN-based NIPCs as a compelling candidate for next-generation solid-state electrolyte applications.
Various types of non-ionic plastic crystalline materials.
Chen et al. explored the development of NIPCs for LIBs by doping SN with lithium salts [40]. In the LiClO4-doped system, the material remains solid, while the LiTFSI-doped system demonstrates two distinct states. This difference is attributed to a reduction in melting point and the emergence of a “crystallinity gap” caused by the addition of LiTFSI [41]. Remarkably, the 5 mol% LiTFSI/SN system exhibits a transparent quasi-solid-state appearance, indicative of nano dispersion of the lithium salt within the plastic-crystalline phase. This phenomenon results in a homogeneous, liquid-like eutectic structure. Room-temperature X-ray diffraction (XRD) analysis of LiClO4/SN and LiTFSI/SN reveals patterns closely resembling that of pure SN, suggesting that the phase behavior is predominantly determined by SN’s crystalline structure. Below the “crystallinity gap”, the material maintains its plastic-crystalline characteristics [41]. The addition of lithium salts reduces the intensity of diffraction peaks without introducing new ones, confirming that the dopants do not alter SN’s underlying crystalline phase. The ionic conductivity of these systems varies significantly; the LiTFSI/SN composite achieves a conductivity of 10−3 S cm−1, two orders of magnitude greater than LiClO4/SN (σ = 10−5 S cm−1). The ionic conductivity of LiTFSI/SN increases significantly with higher salt concentrations, approaching values near 10−2 S cm−1 in the liquid state at 7.5 and 10 mol%. In contrast, LiClO4/SN exhibits minimal conductivity enhancement with increasing salt loading. The interfacial compatibility of NIPCs with porous cathodes was assessed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results confirm that NIPCs effectively infiltrate the cathode structure, forming intimate contact with active materials. This property underscores their potential for improving ion transport and overall battery performance in solid-state systems.
SN, while an effective component for NIPCs, suffers from inadequate mechanical strength, necessitating the use of a separator in battery applications. To overcome this limitation, NIPCs can be hybridized with a polymer matrix to enhance their mechanical and electrochemical properties (Fig. 7). Park et al. demonstrated the development of NIPCs by hybridizing them with a polymer matrix, including PVDF-HFP (PVDF-co-hexafluoropropylene) or PEGDMA (poly ethyleneglycol dimethaacrylate) enabled by using UV irradiation or solution casting methods [23]. This integration enabled the formation of self-standing electrolytes, eliminating the need for separators (Fig. 7a, b). The impact of UV exposure on SN was evaluated using FT-IR spectroscopy. SN was subjected to UV radiation under the same conditions used for the preparation of polymer electrolytes (8mWcm−2, 365 nm, 10 minutes). As depicted in Fig. 7c, the band at 2252 cm−1, corresponding to the cyano functional group (C≡N), remained unchanged following UV exposure. This indicates that the cyano functional group of SN is largely unaffected by UV treatment. Additionally, the thermal stability of the acrylate functional groups in PEGDMA was investigated. PEGDMA was subjected to excessive heating (80℃ for 24 hours) and analyzed using FT-IR spectroscopy. As shown in Fig. 7d, the band at 1729 cm−1, corresponding to the C=C double bond, was preserved even after prolonged heating. This suggests that the acrylate functional groups did not undergo crosslinking reactions during heat exposure. Therefore, it can be concluded that thermal treatment has minimal impact on the properties of PEGDMA. Notably, electrolyte incorporating an PEGDMA polymer matrix exhibited improved ionic conductivity of 0.1 mS cm−1 at 30℃, making them suitable for battery applications. Similarly, Tu et al. highlighted the advantages of cross-linked networks in enhancing the electrochemical performance of NIPCs. By utilizing a cross-linked matrix of pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and ethylene glycol dimethacrylate (EGDMA) with SN, they achieved a robust electrolyte with superior mechanical integrity [42]. These cross-linked NIPCs demonstrated a high lithium-ion transference number of 0.70 and an ionic conductivity of 1.3 mS cm−1 at 30℃. When employed in lithium‖Li(Ni0.8Co0.1Mn0.1)O2 (NCM811), the electrolyte delivered exceptional cycling performance, maintaining stability over 200 cycles. These findings underscore the potential of polymer-integrated NIPCs as advanced solid-state electrolytes, offering enhanced mechanical stability, high ionic conductivity, and compatibility with high-voltage and high-performance battery systems.
(a) Digital camera images of polymer electrolytes based on (a) PVDF-HFP (PVDF-co-hexafluoropropylene) and (b) PEGDMA (poly ethyleneglycol dimethaacrylate). FT-IR spectrum of (c) SN before/after UV exposure (8 mW cm−2, 365 nm, 10 min) and (d) PEGDMA before/after heat treatment (80℃, 24 h). Reprinted from Ref [23] with permission.
SIBs face significant challenges with Na metal anodes due to the high reactivity of sodium with SN-based NIPCs. This reactivity often leads to undesirable side reactions that compromise the electrolyte’s stability and the overall battery performance. To mitigate these issues, the incorporation of fluoroethylene carbonate (FEC) as an additive to NIPCs has shown remarkable efficacy. Huang et al. investigated the influence of FEC on stabilizing the Na metal anode in the presence of NIPCs [43]. Their study revealed that immersing the Na metal anode in an FEC-containing solution formed a robust protective layer that effectively inhibited side reactions between the Na metal and NIPCs. In experiments, pristine Na metal immersed in an NIPC solution of 5 mol% NaClO4 in SN resulted in the solution turning black within 60 hours, indicating severe side reactions. In contrast, Na metal pretreated with FEC retained the solution’s clarity, confirming the protective effect. Infrared spectroscopy revealed that the darkening of the solution with pristine Na metal was caused by the polymerization of C≡N triple bonds into C=N double bonds due to side reactions with sodium. The incorporation of FEC-treated Na metal into SIBs utilizing NIPCs as electrolytes led to significant improvements in cycling stability and performance. Specifically, SIBs equipped with an FEC-treated Na metal anode and NIPCs exhibited excellent cycling stability, maintaining superior performance over 1000 cycles at a rate of 0.2 C. This approach demonstrates the potential of electrolyte additives like FEC to address the intrinsic reactivity challenges of sodium metal anodes, paving the way for more stable and efficient SIBs.
NIPCs have also demonstrated potential in enhancing the interfacial contact between ceramic electrolytes and cathode particles, addressing a significant challenge in all-solid-state battery designs (Fig. 8). Inorganic ceramic electrolytes, while offering excellent ionic conductivity and stability, often struggle to establish intimate contact with cathode particles due to their rigid nature. NIPCs can be employed at the cathode to form a plastic crystal interphase that bridges the solid electrolyte and solid cathode materials. Goodenough et al. reported that the introduction of NIPCs into the cathode significantly improved the contact area between a ceramic solid electrolyte (Na3Zr2Si2PO12) and cathode particles (Na3V2(PO4)3, NVP) [16]. This enhancement reduced the interfacial resistance, leading to improved ionic transport across the interface. Consequently, Na‖NVP batteries incorporating NIPCs exhibited markedly improved electrochemical performance. The ability of NIPCs to form adaptable interphases highlights their versatility in advancing the design of high-performance SIBs and other energy storage technologies reliant on ceramic electrolytes.
Illustrations of solid-state sodium batteries: (a) The conventional solid-state sodium battery with solid electrolyte particles in the cathode. (b) The solid-state sodium battery with plastic–crystal electrolyte in the cathode. Carbon black particles in the electrodes are omitted in the illustration for clarity. Reprinted from Ref [16] with permission.
Non-plastic crystal electrolytes (NOPCs)
OIPCs and NIPCs share the commonality of employing plastic crystalline solvents, characterized by rotational freedom within their molecular structures. In contrast, NOPCs are composed of organic crystalline materials that lack such rotational freedom, such as dimethyl sulfone (DMS) and dimethyl formamide (DMF). The absence of rotational motion in NOPCs can be confirmed through DRS [33]. For instance, mixtures of SN and glutaronitrile (GN) demonstrate dielectric relaxation behavior, indicative of molecular rotation. However, 2N8D, a specific NOPC formed from non-plastic DMS, exhibits no such relaxation behavior, underscoring the absence of molecular rotation in NOPCs (Fig. 9). Instead, ion conduction in NOPCs proceeds through two primary mechanisms: (1) ion transport along channels within the crystal structure, and (2) ion conduction facilitated by liquid-like layers on grain surfaces [29–31,34]. Table 2 provides a comprehensive overview of notable NOPCs, detailing their chemical composition, ionic conductivity, melting points, and electrochemical stability. These attributes highlight the unique properties and potential applications of NOPCs in advanced energy storage systems.
(a) Permittivity, ε′, and (b) dielectric lose, ε′′, spectra for SN0.8GN0.2 and 2N8D at 298 K. SN0.8GN0.2 displays a distinct dielectric relaxation behavior associated with molecular rotation, supporting its plastic crystalline nature. In contrast, 2N8D exhibits no dielectric relaxation, indicating the absence of molecular rotation in the given frequency region. Reprinted from Ref [33] with permission. GN refers to glutaronitrile.
The Zdilla group has reported several NOPCs tailored for lithium-ion batteries (LIBs) and SIBs, formulated with dimethyl formamide (DMF), adiponitrile (ADN), and salts such as LiCl, NaClO4, and LiPF6. One example is the NaClO4(DMF)3 co-crystal NOPC, synthesized via slow vapor diffusion of diethyl ether into a solution of NaClO4 in anhydrous DMF. Due to the insolubility of the salt mixture in ether, a white crystalline precipitate formed, which was identified through X-ray diffraction as NaClO4·(DMF)3 in the hexagonal space group (Fig. 10a) [30]. Structural analysis revealed that this co-crystal features linear ion channels for Na+ ions located at the intersections of its crystallographic three-fold and two-fold axes. SEM images of NaClO4·(DMF)3 further revealed the presence of a liquid-like layer on the grain boundaries (Fig. 10b). When this liquid-like layer was rinsed away, the ionic conductivity at room temperature decreased significantly from 2.2 × 10−4 S cm−1 to 2.2 × 10−6 S cm−1, while the interfacial resistance in a sodium metal symmetric cell dramatically increased. These observations indicate that the liquid-like layer plays a critical role in facilitating Na+ migration and reducing interfacial resistance. Additionally, NOPCs exhibit excellent compatibility with electrodes due to their ability to form intimate interfacial contact through melt-casting. With suitably moderate melting points, molten NOPCs can infiltrate porous cathodes and solidify upon cooling, achieving seamless contact between the electrolyte and the electrode. These advantageous properties enable stable battery performance, as demonstrated in a Li4Ti5O12 (LTO)‖Li(Ni0.6Co0.2Mn0.2)O2 (NCM622) full cell, which retained consistent cycling stability over 100 cycles [34].
(a) X-ray crystal structure of NaClO4·(DMF)3 Hydrogen atoms mitted for clarity. Top: Thermal-ellipsoid plot of five consecutive symmetric units. Ellipsoids set at 50% probability. Bottom: Crystal acking diagram illustrating the ion channels in the crystallographic direction. (b) SEM images of NaClO4 ·(DMF)3. Top: Crystals bound by smooth liquid interfaces. Middle: Mosaic structure of crushed crystals showing fused microcrystalline fragment domains. Bottom: A pellet of NaClO4 ·(DMF)3 resolidified from melt. Reprinted from Ref [30] with permission. DMF refers to dimethyl formamide.
Inspired by the work of the Zdilla group, Lee et al. explored sulfone-based crystalline organic electrolytes (SCOEs) for SIBs and potassium-ion batteries (KIBs). Sulfone-based electrolytes, originally pioneered by the Angell group, have garnered significant interest due to their diverse advantages, including high anodic stability, low-to-negligible flammability, and unique solvating properties. Among various sulfone compounds, DMS stands out as a promising solvent for battery electrolytes owing to its reasonable cost, non-toxicity, and favorable dielectric constant and dipole moment. However, the practical use of DMS-based electrolytes has been hindered by its high melting point of 110°C, which limits its usability under standard operating conditions. To address this issue, researchers have attempted to lower the melting point of DMS by doping it with suitable alkali metal salts. Lee et al. synthesized SCOEs using DMS and various sodium salts, such as sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), NaPF6, and NaClO4 [33]. Interestingly, SCOEs could only be successfully formed with NaFSI (Fig. 11). This observation underscores the importance of structural compatibility between the host molecule and the dopant salt in the formation of SCOEs. The inability to form SCOEs with other sodium salts highlights the critical role of molecular and structural similarity in the successful synthesis of sulfone-based crystalline organic electrolytes.
Sulfone-based crystalline organic electrolytes (SCOEs) can be synthesized by melting a mixture of sodium bis(fluorosulfonyl)imide (NaFSI) and DMS at 90°C, followed by cooling to 25℃. These SCOEs are denoted as xN(10−x)D, where x : (10−x) represents the molar ratio of NaFSI to DMS (Fig. 12a and 12b). The crystal structures of SCOEs such as 1N9D, 2N8D, and 3N7D were analyzed using X-ray diffraction (XRD). The XRD pattern of 1N9D revealed differences in peak positions and intensities compared to pure DMS, with several new peaks emerging (Fig. 12c). These structural changes became more pronounced with increasing NaFSI content, as evidenced by the unique diffraction patterns of 2N8D and 3N7D. These observations suggest that the SCOEs are not simple mixtures of NaFSI and DMS but rather co-crystals with distinct crystal structures. The formation of these new structures likely arises from complex coordination interactions among DMS, Na+, and FSI−, which vary with NaFSI content. Thermal properties of the SCOEs were investigated using differential scanning calorimetry (DSC) (Fig. 12d). The DSC curves of 1N9D, 2N8D, 3N7D, and 4N6D revealed melting points significantly lower than those of pure DMS (109°C) and NaFSI (112°C). Among these, 3N7D exhibited the lowest melting point at 38℃, potentially due to eutectic behavior in the NaFSI/DMS binary system. To assess safety, flammability tests were performed by exposing 2N8D to direct flame, compared to a conventional liquid electrolyte (LE) composed of 1 M NaPF6 in ethylene carbonate/diethyl carbonate (1:1 by volume). While LE ignited within 1 second and continued to burn after flame removal, 2N8D resisted ignition even after prolonged exposure (>5 seconds), underscoring the low flammability of sulfone compounds (Fig. 12e). 2N8D, identified as a representative SCOE for SIBs, demonstrated promising electrochemical properties, including high ionic conductivity (7.0 × 10−4 S cm−1 at 25℃), excellent oxidation stability (>5.5 V vs. Na/Na+), and a sodium ion transference number of 0.54.
(a) Schematic illustration of SCOEs and the melt-casting process. (b) Photographs of NaFSI, DMS and SCOE (NaFSI/DMS, 2:8 by mol, 2N8D) at 90℃ and 25℃. (c) Powder X-ray diffraction patterns of SCOEs. (d) DSC heating curves of SCOEs. The melting points are denoted. (e) Snapshots of flammability test of LE and 2N8D. Reprinted from Ref [33] with permission.
SCOEs can establish intimate contact with porous electrodes through a melt-casting process (Fig. 13). In this method, an NVP electrode is immersed in molten 2N8D, followed by cooling to 25℃. SEM images of the meltcasted NVP electrode reveal a significantly smoother surface compared to the rough texture of a pristine NVP electrode (Fig. 13), indicating uniform dispersion of 2N8D throughout the composite cathode [32,33]. This uniform distribution was further validated using EDS, which detected nitrogen and sulfur on the surface of the melt-casted NVP electrode but not on the pristine electrode. These results confirm the effective formation of intimate contact at the electrolyte-electrode interface, a challenge often faced with conventional solid-state electrolytes. The enhanced interfacial contact, combined with the superior electrochemical properties of SCOEs, leads to exceptional cycling performance. Na‖NVP cells utilizing SCOEs demonstrated stable operation at room temperature, maintaining excellent capacity retention over 200 cycles, and significantly outperforming cells with conventional carbonate-based electrolytes.
Due to the similar properties of potassium bis(fluorosulfonyl)imide (KFSI) and NaFSI, SCOEs for KIBs can be conveniently prepared by substituting NaFSI with KFSI [32,33]. Eutectic behavior was observed between KFSI and DMS, enabling the formation of SCOEs with favorable characteristics. Among these, 1K9D, a SCOE composed of KFSI and DMS in a molar ratio of 1:9, demonstrated excellent ionic conductivity (4.0 × 10−4 S cm−1 at 25℃) and exceptional oxidation stability (>5.8 V vs. K/K+). The compatibility of 1K9D with a potassium metal anode was significantly superior to that of a conventional liquid electrolyte (LE, 0.7 M KPF6 in ethylene carbonate/propylene carbonate). Electrochemical impedance spectroscopy (EIS) and galvanostatic polarization tests on potassium metal symmetric cells revealed that 1K9D exhibited much lower charge transfer resistance and reduced polarization compared to LE. These advantageous properties of 1K9D enabled stable operation of 5 V-class potassium batteries, pairing a KVPO4F (KVP) cathode with a potassium metal anode (Fig. 14). As a high-voltage cathode material, KVP can be charged up to 5 V (vs. K/K+); however, KIBs using KVP have been limited by the oxidative instability of LE above 4 V (vs. K/K+). In contrast, the superior oxidation stability and compatibility of 1K9D with the K metal anode allowed KVP‖K batteries to achieve excellent cycling performance, maintaining high Coulombic efficiency (99.6%) over 100 cycles. This demonstrates the potential of SCOEs like 1K9D to advance high-voltage KIB technologies, overcoming the limitations of conventional liquid electrolytes.
While COEs show great promise, several challenges remain before their widespread adoption in commercial solid-state batteries. One significant hurdle is the optimization of ionic conductivity and mechanical strength across all three categories of COEs. OIPCs and NIPCs, with their plastic crystal structures, often exhibit lower melting points, which can compromise their mechanical integrity. NOPCs, while mechanically stronger, tend to have lower ionic conductivity compared to their plastic crystal counterparts. Therefore, further research is needed to tailor the chemical compositions of COEs to achieve an optimal balance between these two critical properties. Additionally, the long-term stability of COEs under practical battery operating conditions, including high current densities and extended cycling, needs more investigation. The interfacial compatibility of COEs with diverse electrode materials is another area requiring attention, as some COEs may exhibit reactivity with certain electrode materials or form unstable SEI layer, potentially leading to capacity fading and reduced battery lifespan. Overcoming these limitations will be essential for realizing the full potential of COEs in commercial cells.
Looking ahead, COEs are poised to play a pivotal role in the development of next-generation batteries. Their inherent safety, due to low flammability, positions them as a compelling alternative to conventional carbonate electrolytes. The unique melt-casting capability of COEs facilitates intimate contact with electrodes, enabling the fabrication of high-energy-density solid-state batteries. COEs also offer tunability through the selection of different organic solvents and salt combinations to optimize the electrochemical window and ionic conductivity for specific battery chemistries, including lithium-ion, sodium-ion, and potassium-ion batteries. Future research should focus on developing novel COEs with enhanced ionic conductivity, mechanical strength, and long-term stability, as well as on exploring cost-effective synthesis and manufacturing processes to enable their mass production. The use of COEs in combination with advanced electrode materials could pave the way for safer, high-performance, and more sustainable energy storage solutions.
CONCLUSIONS
This review provides a comprehensive overview of crystalline organic electrolytes (COEs), categorizing them into organic ionic plastic crystal electrolytes (OIPCs), non-ionic plastic crystal electrolytes (NIPCs), and non-plastic crystal electrolytes (NOPCs). Each class exhibits unique structural and physicochemical properties that make them promising candidates for next-generation solid-state battery applications. OIPCs are notable for their high ionic conductivity, excellent thermal and electrochemical stability, and adaptability to diverse energy devices. Innovations in OIPC formulations, such as doping with lithium salts or integrating polymer matrices, have significantly enhanced their mechanical robustness and ionic transport properties, making them suitable for high-voltage lithium-ion batteries. NIPCs, primarily based on SN, offer exceptional electrochemical stability and straightforward preparation. Their plastic crystalline nature facilitates intimate electrode contact and efficient ion transport, which are critical for stable cycling performance in sodium-ion and lithium-ion batteries. The hybridization of NIPCs with polymer matrices further improves their mechanical properties, enabling applications in high-performance energy storage systems. NOPCs, characterized by their non-plastic crystalline structure, demonstrate unique ion transport mechanisms through ion channels and liquid-like grain boundary layers. Advances in NOPC research, including their application to SIBs and KIBs, underscore their potential for stable, high-capacity performance, particularly in high-voltage environments. Collectively, these findings highlight the transformative potential of COEs in overcoming the limitations of traditional solid-state electrolytes. Their tunable properties, combined with innovative processing techniques like melt-casting, pave the way for safer, more efficient, and higher-performance batteries. Future research focused on optimizing molecular structures and improving compatibility with various electrode materials will further advance the practical application of COEs in energy storage technologies.
Notes
ACKNOWLEDGEMENTS
This work was supported by the Technology Development Program (RS-2024-00446888) funded by the Ministry of Trade, Industry & Energy (MOTIE, Republic of Korea). This work was supported by the National Research Foundation of Korea (NRF) grants (RS-2024-00343264 & RS-2024-00428511) funded by the Ministry of Science, ICT, and Future Planning (MSIT, Republic of Korea).
