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.

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.

Danah et al. proposed OIPCs composed of LiFSI and [Py_1i3][FSI] in a 1:9 molar ratio [37]. They observed that 10 mol% LiFSI in [Py_1i3][FSI] exhibited significantly higher ionic conductivity compared to 90 mol% LiFSI in [Py_1i3][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 [Py_1i3][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 [Py_1i3][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.

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— [Ph_1i444][FSI], [Ph_1i444][TFSI], and [Ph_111i4][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 (C_nH_2n+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% Li_0.1[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.

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.

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(Ni_0.8Co_0.1Mn_0.1)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.

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.

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.

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(Ni_0.6Co_0.2Mn_0.2)O₂ (NCM622) full cell, which retained consistent cycling stability over 100 cycles [34].

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.

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.