Recent developments of nanocomposite ionogels as monolithic electrolyte membranes for lithium‐based batteries

Abstract

To utilize intermittent renewable energy to achieve carbon neutrality, rechargeable lithium‐based batteries have been deemed to be the most promising electrochemical systems for energy supply and storage. However, there still exist safety issues and challenges, especially originating from the intrinsic volatility and flammability of the electrolytes used in lithium‐based batteries. Due to the unique advantages of better safety, (quasi) solid‐state electrolytes have been exploited. Ionogel (IG), known as ionic liquid (IL) based monolithic quasi‐solid‐state electrolyte separator, consists of IL and gelling matrix and has become an active area of research in lithium‐based battery technology, owing to fascinating exotic characteristics including high safety (thermal stability) under extreme operating conditions, wide processing compatibility, and decent electrochemical performances. Among various gelling matrices, nanomaterials are very promising to simultaneously enhance ionic conductivity, mechanical strength, and thermal and electrochemical properties of IGs, which make the nanocomposite ionogels (NIGs). Herein, several significant advantages of NIGs as monolithic electrolyte membranes are briefly described. Also, recent advances in the NIGs for Li‐ion batteries, Li‐metal batteries, Li‐S batteries, and Li‐O₂ batteries are timely and systematically overviewed. Finally, the remaining challenges and perspectives on such an interesting and active field are discussed. To the best of our knowledge, there are rare review articles focusing on the NIGs for Li‐based batteries till now. This work could offer a comprehensive understanding of recent advances and challenges of NIGs for advanced lithium storage.

INTRODUCTION

To achieve the net-zero carbon future committed by more and more countries, energy storage technologies utilizing intermittent renewable energy such as wind, tidal energy, and solar power (excess solar photovoltaic generation) are extremely significant to reduce the consumption of limited fossil fuels.^1–6 Due to the relatively high energy and/or power density, rechargeable lithium-based batteries, including the Li-ion battery (LIB), Li-metal battery (LMB), Li-S battery (LSB), and Li-O₂ battery (LOB), show great promise for electrochemical energy storage (EES) systems.^7–11 In particular, LIBs are leading EES systems widely used in both portable consumer smart electronics and the automotive industry, making energy use convenient, green, and continual.^12–16 As the majority of current commercial LIBs contain liquid organic electrolytes composed of ionic salts dissolved in organic solvents, electrolyte leakage, and even combustion accidents occasionally occur, which hinders further reliable applications, especially in flexible and/or wearable devices.^17–19 Taking into account the safety concerns, much worldwide research effort has been made to the development of solid-state electrolyte (SSE) for high energy and better safety battery that not only eliminates the use of flammable organic solvents but also could remove the separator in the battery design.²⁰ However, most SSEs suffer from drawbacks such as high interfacial resistance, inferior room temperature (RT) ionic conductivity, and cumbersome processing.^21–23 Including mixed phases of both solid and liquid, quasi-SSEs have been proposed, among which the ionogel (IG) has flourished in recent years.^24–26 Also, this could be verified by the continuously increasing number of publications in recent decades about IG electrolytes in the Web of Science as displayed in Figure 1A.

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IGs are hybrid materials, with ionic liquids (ILs) entrapped/confined/immobilized by gelling solid matrices.^27,28 Typically, IL, consisting of self-dissociated cation and anion, is RT molten salt with a melting point below 100°C. Compared with organic solvents used in traditional liquid electrolytes, ILs have such unique properties as nonflammability, high ionic conductivity, negligible vapor pressure, good thermal stability, and broad electrochemical stable potential window (ESPW) of about 5–6 V (vs. Li/Li⁺).^29,30 Note that except for outflow, IGs keep the most exotic properties of the ILs used. Thereof, IGs possess high ionic conductivity, broad ESPW, and good thermal stability.³¹ Most importantly, the nonflammability of ILs endows the IGs with high safety in battery operations. On the other hand, to prevent deleterious reactions with ILs, solid matrices are expected to be chemically inert. Also, to serve as the battery separators, the matrices of the IGs should not only be electronically insulating but also provide enough high mechanical strength. There are various solid matrices explored in the IGs, including polymers, inorganic and hybrid materials. Overall, nanostructured matrices are promising for rechargeable batteries ascribing to the high specific surface area (SSA) to promote interactions with ILs, resulting in much-improved properties including mechanical strength, ionic conductivity, and thermal and electrochemical stability.

Here, we delve into recent developments focused on nanocomposite ionogels (NIGs) for advanced lithium storage (Figure 1B). In contrast to plain IGs, several significantly improved properties, that is, mechanical strength, ionic conductivity, and thermal and electrochemical stability, of NIGs are described. Furthermore, the applications of NIGs in LIBs, LMBs, LSBs, and LOBs are timely and systematically summarized. Finally, challenges and perspectives on the NIGs in widespread rechargeable lithium-based battery applications are discussed.

For convenience, the abbreviations and corresponding full names of the IL ions and the polymers frequently appeared in this article are summarized in Table 1.

Table 1 Corresponding full names for the abbreviations of the IL ions and the polymers frequently appeared in this article.

Abbreviations	Full names
FSI	bis(fluorosulfonyl)imide
TFSI	bis(trifluoromethylsulfonyl)imide
TFSA	bis(trifluoromethanesulfonyl)amide
AMIM	1-allyl-3-methylimidazolium
BMIM	1-butyl−3-methylimidazolium
C1C3IM	1-methyl-3-propylimidazolium
CPIM	1-(trimethoxysilylpropyl)-3-butylimidazole
DEME	N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium
EMIM	1-ethyl-3-methylimidazolium
mpImSi(OMe)3	1-methyl 3-propyl (trimethoxysilyl) imidazolium
P111i4	trimethyl(isobutyl)phosphonium
PDADMA	poly(diallyldimethylammonium)
PMIM	1-propyl-3-methylimidazolium
PMMI	1, 2-dimethyl-3-propylimidazolium
PP13	N-methyl-N-propyl piperidinium
PYR13	N-methyl-N-propyl-pyrrolidinium
PYR14	N-butyl-N-methyl-pyrrolidinium
PYRA12O1	N-ethyl(methylether)-N-methylpyrrolidinium
PDMA	poly(2-(dimethylamino)ethyl methacrylate)
PEGDA	poly(ethylene glycol) diacrylate
PEO	poly(ethylene oxide)
P(MMA-AN-EA)	poly(methyl methacrylate-acrylonitrile-ethyl acrylate)
P(MMA-AN-VAc)	poly(methyl methacrylate-acrylonitrile-vinyl acetate)
PVDF-HFP	poly(vinylidene fluoride-co-hexafluoropropylene)
PTFE	polytetrafluoroethylene

THE ADVANTAGES OF NIGS AS MONOLITHIC ELECTROLYTE MEMBRANES

As above-mentioned, IGs are hybrid materials with ILs immobilized by gelling solid matrices. ILs could disperse or dissolve diverse materials such as polymers, nanoparticles, nanowires, and nanosheets, offering the possibility and new opportunity to synthesize multifunctional IGs with unique properties.³² And the incorporation of nanocomponents into them creates NIGs, which could provide improved properties and performances compared with pure IGs. These nanocomponents could be of various forms in nanoscale size, including nonmetal oxide, metal oxide, boron nitride (BN), and carbon nanotube (CNT), which will be discussed in the following section. In short, NIGs have been rather attractive for various research areas and potential technological applications in actuators,^33,34 ionic thermoelectric capacitors,³⁵ solid-state batteries,³⁶ transistors,³⁷ wearable sensors,^38–43 flexible supercapacitors,^44–50 and so forth.^51–54 This is attributed to the unique combination of several significant improved properties that NIGs offer, compared with plain IGs and conventional materials. As aforementioned, ascribing to high SSAs, nanostructured matrices in the NIGs could promote greater interactions with ILs, resulting in much-improved properties such as high ionic conductivity, outstanding thermal stability, enhanced mechanical strength, and robust electrochemical performances, as shown in Figure 2. Below are four significant property benefits of the NIGs as monolithic electrolyte membranes, especially in the applications for rechargeable lithium-based batteries.

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High ionic conductivity

The ionic conductivity of the NIG is a significant factor for the application in lithium storage. The existence of the IL components enables the NIGs with high ionic conductivity since the ILs inherently possess high ion mobility for effective ion transport in the NIGs. This property of NIGs is particularly important for applications that require rapid ion transport, for example, supercapacitors and rechargeable batteries, which could enable better electrochemical performances, such as faster (dis) charging rates.

To serve as the battery separators, the NIGs should have RT ionic conductivity of no less than 0.1 mS cm⁻¹, while maintaining the electronic insulation to avert self-discharge.⁵⁹ Generally, the ionic conductivity of the IGs is mainly affected by temperature, concentration of the salt, viscosity of the IL, and matrix loading. For the NIGs comprising of ILs (Li-ILs) incorporated in nanomaterial matrices, nanocomponents of the host matrices connect/cluster to each other mainly via weak interactions such as hydrogen bonds. Also, the interactions between ILs and gelling matrices are greater due to the large SSAs of the nanoscale materials, and thus promote ionic dissociation of the IL and the lithium salt, resulting in an increase in the number of the free ions. Such unique benefits of the nanoscale matrices could improve Li⁺ transference number as aforementioned.⁶⁰ Furthermore, the ionic conductivity of NIG could also be improved. For instance, Delacroix and colleagues reported NIGs consisted of the substituted imidazolium-based ILs with grafted Stöber-type nano-SiO₂ and the LiTFSI salts, which exhibited RT ionic conductivity of 0.001 mS cm⁻¹.⁶¹ With the SiO₂ loading of 0.23 mmol g⁻¹, the ionic conductivity of NIGs could be further improved by decreasing the SiO₂ particle size from 50 to 35 nm. In addition, Garaga and colleagues synthesized NIGs composed of Al₂O₃ nanofibers (NFs, diameter: 2–6 nm, length: 200–400 nm), the IL [EMIM][TFSI], the LiTFSI salt and the PVDF-HFP host via the method of solution casting, with the RT ionic conductivity of 0.078 mS cm⁻¹, almost three times of the one without Al₂O₃ NFs (0.027 mS cm⁻¹).⁵⁵ As reported, this was ascribed to the well-distributed Al₂O₃ NFs in the matrix, as the Al₂O₃ NFs possibly located in the mobile regime greatly improved ionic species dynamics in the mobile phase between PVDF-HFP chains. Incorporating organically modified SiO₂ nanoparticles and poly(ethylene glycol) with two ILs (cations of [BMIM]⁺ and [EMIM]⁺), Blensdorf and colleagues synthesized NIGs with boosted RT ionic conductivity of as high as 1.24 mS cm⁻¹.⁶² Also, Buchtová and colleagues reported the biopolymer-based NIGs with SiO₂ NFs, which could increase the RT ionic conductivity to 4.5 mS cm⁻¹, higher than the counterparts of the bare ILs and the pristine IGs without SiO₂ NFs.⁶³ Moreover, in contrast to the neat IL electrolyte, the ionic conductivity of the NIG consisting of nano-SiO₂ with the interfacial ice layer was twice higher.⁶⁴

Such high ionic conductivity indicates that the NIGs as monolithic electrolyte membranes could be promising candidates for applications in high-power lithium-based batteries.

Outstanding thermal stability

When immobilized within the solid matrices, the ILs endow their intrinsic high thermal stability to the resulting NIGs. This property is extremely crucial for applications that require high-temperature stability, such as solid-state batteries operating in harsh circumstances. The onset deposition temperature of the NIGs is mainly determined by the ILs, and the confinement of nanoscale matrices could retard the decomposition of ILs, further raising the thermal stability of the NIGs. Inorganic nanoparticles have been widely acknowledged for their excellent thermal stability.⁶⁵ For instance, NIGs prepared by tethering the IL [C1C3IM][TFSI] to the SiO₂ nanoparticles (7 nm) showed good thermal stability (up to 400°C).⁶⁶ Also, with a modified sol-gel method, Archer and colleagues reported NIGs, covalently tethering imidazolium-based ILs to ZrO₂ nanoparticles (86 ± 2 nm), could be stable up to 400°C.⁶⁷ In addition, NIGs based on TiO₂ nanoparticles achieved thermal stability up to 400°C, with higher decomposition temperature than that of the pure IL electrolyte.⁶⁸ And NIGs consisting of the attapulgite nanorods also exhibited outstanding thermal stability, with the decomposition temperature >400°C.⁵⁶

Such good thermal stability suggests NIGs as monolithic electrolyte membranes could be promising candidates for applications in high-temperature lithium-based batteries.

Enhanced mechanical strength

The incorporation of nanocomponents within the gelling matrices in the NIGs could enhance mechanical strength and/or stability, as the nanocomponents act as reinforcing agents to provide resistance against varied deformations, swelling, and shrinkage. This improved mechanical strength makes NIGs more suitable and durable for multifarious applications such as structural materials and flexible electronics.

Mechanical properties of the NIGs play a crucial role in the battery design, especially during charging/discharging cycling. Good endurance and high mechanical strength could not only suppress the lithium dendrite growth but also withstand the compressive pressure changes between the positive and the negative electrodes in battery operations. Theoretically, dense and solid electrolyte separators that possess a shear modulus greater than 6 GPa could hinder lithium dendrite growth.⁶⁹ Increasing the loading of the matrix could enhance mechanical properties; however, it negatively affects the ionic conductivity resulting from impeding the ion motion. In contrast to microsized matrices, the nanoscale matrices could delay the onset of such a tradeoff and provide significantly enhanced mechanical properties at a fixed ionic conductivity. For instance, retaining the RT ionic conductivity >1 mS cm⁻¹, NIG with exfoliated hexagonal boron nitride (BN) nanoplatelets possessed much improve mechanical properties, two orders of magnitude higher than the counterpart composed of the bulk hexagonal BN microparticles with the equivalent matrix loading.⁵⁸ In addition, both the mechanical strength and the Li⁺ transference number of the NIG with the amine-functionalized BN nanosheets reported by another group were two-fold higher than the one without BN nanosheets.⁶⁰

Also, many oxide nanocomponents have exhibited enhanced mechanical properties of the NIGs for lithium storage as will be presented in the following section. Favorable mechanical properties suggest NIGs as monolithic electrolyte membranes could be promising candidates for applications in flexible and/or wearable lithium-based batteries.

Robust electrochemical performances

The nanocomponents incorporated could increase the effective SSAs of the electrodes and offer extra electrochemical functionalities of the NIGs. For example, metal oxide nanoparticles could enhance battery performances such as capacity, cycling stability, and rate capability. More importantly, the synergistic impacts of the nanocomponents and the ILs could result in enhanced energy storage (conversion) efficiency, making NIGs promising for electrochemical energy storage. Electrochemical stability is extremely critical to maximize the working voltage of the battery to increase its energy density. For instance, in contrast to the plain IG, NIG with well-dispersed BN nanosheet exhibited a broader ESPW of about 5.5 V (vs. Li/Li⁺).⁷⁰ Such excellent electrochemical stability suggests NIGs as monolithic electrolyte membranes could be promising candidates for applications in high-voltage lithium-based batteries.

Overall, the above-mentioned improved property benefits make NIGs attractive for a wide range of applications, including sensing technologies, flexible electronics, and energy storage. By selecting specific nanocomponents, matrices, and ILs, the properties of NIGs could be controllably tailored. The choice of the nanocomponents could customize the properties such as the conductivity and the electrocatalytic activity. And the solid matrices could be chosen according to desired and/or required mechanical properties, while various ILs could be selected to optimize the properties such as the thermal stability, ionic conductivity, and safety characteristics of the NIGs. Such versatility makes NIGs appropriate for a variety of applications as aforementioned. To further improve the performances in specific applications, ongoing research studies are focused on optimizing the compositions and structures of the NIGs. In this article, we focus on the NIGs for the application in lithium-based batteries (LIBs, LMBs, LSBs, and LOBs), which will be discussed and briefly summarized in the following section.

THE APPLICATIONS OF NIGS IN RECHARGEABLE LITHIUM-BASED BATTERIES

As illustrated in the second section, the unique property benefits make NIGs very attractive for applications in rechargeable lithium-based batteries. The critical nature of the IL immobilization within the NIG structures assists to overcome the primary potential issues of the volatilization and the leakage of liquid electrolytes, resulting in enhanced safety and stability in lithium-based battery systems. Also, NIGs provide merits of good processability and scalability, making NIGs fit for the large-scale battery manufacturing. Moreover, the nanostructures of the NIGs provide high enough mechanical strength to eliminate the need for the separator between the anode and cathode in the battery, making NIGs work as monolithic electrolyte membranes in lithium-based batteries. In addition, the ILs involved in gelling matrices impart effective contact with the electrode and the electrolyte/electrode surfaces, resulting in low interfacial resistance and rapid Li⁺ transport.⁷¹ By contrast, to reduce the interfacial resistance, the rigid inorganic SSE requires an extra thin interlayer between the electrode and the electrolyte.^72–74 Herein, corresponding outstanding merits of NIGs used in LIBs, LMBs, LSBs, and LOBs are outlined, followed by specific summaries of the closely related examples in the reported works of literature based on the nanocomponents of the NIGs.

Since its initial commercialization in 1991, LIB has become a significant technology driving various applications, and as such the LIB pioneers, Goodenough, Whittingham, and Yoshino, were awarded the 2019 Nobel Prize in Chemistry.^75–78 Unfortunately, there still exist such shortcomings as the limited rate capability and safety issues primarily originating from the combustible electrolytes that need to be solved for current LIB technology. Due to the large potential to improve both performance and safety, NIGs have achieved important attention in LIBs. Moreover, the fascinating interfacial properties of the NIGs enable the direct assembly of solid-state LIBs with various electrodes. Along with the favorable interfacial properties, high ionic conductivity has made NIGs endow outstanding rate performances in LIBs. In general, ionic conductivity could be enhanced by increasing the temperature. So, to raise the ionic conductivity, LIBs based on solid polymer electrolytes are usually operated at elevated temperatures.^79–81 In contrast, NIGs could allow LIBs at RT with favorable ionic conductivity.

Up to now, Li metal has the highest theoretical capacity of up to 3860 mAh g⁻¹ at RT environment and the lowest potential of −3.04 V (vs. standard hydrogen electrode).^82–84 Thereof, rechargeable LMB featuring Li metal anode is the most promising battery technology and has been gaining increasing attention in next-generation energy storage systems to replace mature LIBs. However, to ensure high safety and good performances of LMBs, challenges still remain. NIGs have emerged as promising materials for LMBs to solve such problems because of the outstanding property benefits including high Li-ion conductivity, improved safety, stable electrode-electrolyte interface, enhanced Columbic efficiency (CE), good processability, and scalability. LMBs face safety issues mainly due to the formation and growth of needle-like structured lithium dendrites during cycling, which would lead to short circuits or thermal runaway. NIGs could assist to address such safety concerns by offering robust mechanical stability and suppressing the formation and growth of lithium dendrite. Nanocomponents in the NIG matrices serve as the physical barrier, inhibiting the lithium dendrite growth and improving the overall battery safety. The interface between the lithium metal anode and the electrolyte is very significant for the electrochemical performances and cycling stability of the LMBs. NIGs could form stable interface layers with Li metal anodes, reducing the reactivity between the electrodes and the electrolytes. This could mitigate side reactions, minimize the formation of solid electrolyte interphase (SEI) layers that impede Li⁺ transport, and promote uniform lithium deposition and stripping, resulting in high CEs and better overall electrochemical performances. The NIGs could be facilely prepared and even processed into the desired or required shapes (such as films and coatings), enabling the integration into the LMB architectures. Such a charming feature is particularly critical for practical applications and further commercialization of LMBs. To suppress lithium dendrite growth on Li anodes, it is very effective to use the NIGs.

NIGs also show vital promise for the use of LSBs, which have caught considerable attention for next-generation energy storage because of their low cost and high energy density (with the theoretical value of 2600 Wh kg⁻¹) surpassing that of conventional LIBs.⁸⁵ However, practical implementations of the LSBs are still confronted with several key issues including high dissolution of the lithium polysulfide intermediates into the electrolytes and large volume changes of sulfur during battery cycling, causing irreversible loss of active sulfur, severe self-discharge, and inferior cycling stability.^86,87 Exploring novel electrolytes with moderate solubility of the lithium polysulfides has shown promise to address such challenges.⁸⁸ NIGs could overcome the shortcomings and enhance the overall LSB performances, mainly attributing to enhanced sulfur confinement, restraint of the shuttle effect, and catalyst integration, except for the same increased ionic conductivity and mechanical stability as in LIBs and LMBs. As known, sulfur will dissolve into the LSB electrolytes during cycling, resulting in capacity loss and short battery cycle life. NIGs could immobilize sulfur and polysulfide intermediates within the matrices to inhibit dissolutions, enhancing overall sulfur utilizations. During LSB (dis)charging, sulfur undergoes vast volume changes, causing structural degradation and mechanical stress. NIGs with robust mechanical stability could withstand such huge volume expansions/shrinkages and maintain structural stability after multiple repeated cycles. Note that the shuttle effect in LSBs refers to polysulfide species migrations between cathodes and anodes, leading to capacity fading. NIGs could limit such migration of the polysulfides, thus reducing the shuttle effect and resulting in the improvement of LSB cycling stability. Meanwhile, NIGs could integrate the catalyst nanocomponents such as metal oxides and carbon-based nanomaterials to promote sulfur redox reactions, electrochemical reaction kinetics, and hence improve the overall LSB performances during cycling.

In addition, NIGs are promising in LOBs. Compared with traditional LIBs, LOBs possess much higher theoretical energy density (~3500 Wh kg⁻¹), owning to the theoretical merit of utilizing oxygen as one of the active reactants from the atmosphere air.⁸⁹ However, LOBs face such challenges as oxygen diffusion, formations of insulating reaction products, and the O₂ cathode instability, hindering their practical implementations.^90,91 NIGs are one of the efficient strategies to address these challenges and improve the LOB performances. NIGs designed with high ionic conductivity could facilitate the diffusion of both Li⁺ and the oxygen through the electrolytes. There is no doubt that this enhanced ion and gas transport could lead to much better LOB performances and minimize the formation of the insulating solid reaction products that could clog the O₂ cathodes. During LOB cycling, the cathodes experience huge morphology changes, causing structure degradations and instability. NIGs with nanocomponents could offer mechanical reinforcement to the O₂ cathodes, thus improving the structure stability during cycling. Also, NIGs could incorporate catalyst nanocomponents to promote electrochemical oxygen reduction and evolution reactions (ORR and OER) at the O₂ cathodes, and decrease overpotential to enhance the LOB efficiency. Furthermore, NIGs assist to reduce the side reactions contributing to the electrolyte decomposition and the cathode degradations. Such controlled stable electrolyte environments in NIGs could extend the cycling life of LOBs. In addition, NIGs involving ILs and hydrophobic nanoscale matrices are competitive candidates as the nanoscale matrices could also protect Li anodes from atmosphere moisture and thereby improve the entire cycle life and the overall performances of the LOBs.

To further comprehend the above-mentioned merits of the NIGs used in LIBs, LMBs, LSBs, and LOBs, it is urgent to summarize recent developments of the NIGs applied in such promising lithium-based batteries. As known, the nanocomponents could be in various forms in nanoscale size, such as nonmetal oxide (e.g., SiO₂), metal oxides (e.g., TiO₂, Al₂O_3, and ZrO₂), BN, CNT, and halloysite nanotubes (HNT). Based on the nanocomponents of the NIGs used in lithium-based batteries, we discuss some typical examples reported in works of literature as follows.

SiO₂-based NIGs

The ILs could be stabilized on the surfaces of oxide particles. Because of unique properties such as easily tuned orientation, shape, and pore size during synthesis, SiO₂, one of the most common natural fillers, is a suitable and widely used matrix to solidify various ILs.⁹² Also, as the strongly coordinating additive with rich Lewis acidic groups, nano-SiO₂ could contribute to lithium salt dissociation and thus increase the number of free ions. At present, the NIGs based on nano-SiO₂ have been widely applied in LIBs, LMBs, LSBs, and LOBs.

SiO₂-based NIGs for LIBs

In 2010, Ferrari and colleagues reported the synthesis of NIGs containing 5 wt.% nano-SiO₂ (~20 nm, 240 m² g⁻¹) filled in the IL [PYRA_12O1][TFSI], the LiTFSI salt, and the PVDF-HFP framework by the solution casting technique and their applications in the LIBs, which showed the ionic conductivity of 0.33 mS cm⁻¹ at 20°C.⁹³ The as-fabricated Li//LiFePO₄ (LFP) Swagelok cells using such unique NIGs as both electrolytes and separators exhibited a specific capacity of 150 and 50 mAh g⁻¹ at 0.05C and 0.1C, respectively. However, the cells did not work at higher C rates. Also, Unemoto and colleagues prepared NIGs (200 μm in thickness) consisting of the fumed SiO₂ nanoparticles (7 nm), the LiTFSA salts, the TFSA-based ILs, and the PTFE.⁹⁴ The Li//LiCoO₂ (LCO) cells assembled with the NIGs based on the IL [EMIM][TFSA] exhibited the most stable cycling performances at relatively low current of 0.1C in 3.4–4.2 V. By the nonaqueous sol-gel method, Bideau and colleagues prepared NIGs by immobilizing the IL [PYR₁₃][TFSI] with the LiTFSI in the nanocomposite matrices of tetraethylorthosilicate (TEOS) hydrolyzed SiO₂ and the poly(vinylidene fluoride) (PVDF)-hydroxyethyl acrylate (HEA).⁹⁵ The Li//LFP cells assembled with such NIGs showed the same performances as those using conventional commercial Celgard separators filled with the same IL electrolytes. In addition, Zhou and colleagues reported NIGs by immersing the host of SiO₂ nanoparticles (10–20 nm) attached PVDF in the solution of the IL [PYR₁₄TFSI] and the LiTFSI.⁹⁶ By interacting with anions from lithium salts and ILs via the Lewis acid–base interactions, the surface-attached nano-SiO₂ played as the salt dissociation promoters, and thus improved both the Li⁺ transference number and the ionic conductivity of the NIGs. Furthermore, the Li//LCO coin cells using such NIGs showed improved cycling stability and rate performances. Another example was the NIGs incorporated SiO₂ nanoparticles with the [BMIM][TFSI] ILs and the LiTFSI, reported by Wu and colleagues, with improved electrochemical anodic stability (5.2 V, vs. Li/Li⁺).⁹⁷ Moreover, all full cells (Figure 3A) assembled with the NIGs, mesocarbon microbeads (MCMB) anodes, and three cathode active materials (LCO, LiNi_1/3Co_1/3Mn_1/3O_2, and LFP) exhibited excellent battery performances (Figure 3B). Similarly, in contrast with conventional liquid electrolytes, NIG based on SiO₂ nanoparticles achieved comparable rate performances in half-cells, that is, 150 and 113 mAh g⁻¹ at 0.1C and 1C, respectively.⁹⁸

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Owning to excellent thermal stability at elevated temperatures as aforementioned, NIGs have been developed for high-temperature LIBs, in combination with improved rate performances as the temperature increases, so does the ionic conductivity. Typically, Ito and colleagues obtained NIGs (500 μm in thickness) using fumed SiO₂ nanoparticles as the matrix to entrap the Li⁺ conducting ILs, [EMIM][TFSA] containing dissolved LiTFSA (1 M).⁹⁹ The ionic conductivity was 4.4 mS cm⁻¹ at 75°C. Despite being tested at 0.1C in Li//LCO coin cells at 65°C, with a voltage window of 3.4–4. 2 V, the NIGs endowed the assembled batteries with an initial capacity as high as 126 mAh g⁻¹. However, it showed a significant decay above the eighth cycle, which may be ascribed to the hygroscopic nature of the fumed SiO₂ resulting in some side reactions. Note that the dimension of nanoparticles used in the NIGs affects the ionic conductivity. For instance, Wang and colleagues prepared NIGs consisting of the polymeric IL [PDMA][TFSI] functionalized mesoporous SiO₂ nanoplates with the IL [PY₁₄][TFSI] and the LiTFSI, which showed the ionic conductivity of 1.0 mS cm⁻¹ at 130°C and the ionic conductivity increased as the SiO₂ contents increase.⁵⁷ The Li//LFP cells with the NIGs containing 8 wt.% mesoporous SiO₂ nanoplates delivered the specific capacity of 135.8 mAh g⁻¹ at 0.1C and 60°C, without obvious capacity decay after 30 cycles, much superior to the counterparts without the modified mesoporous SiO₂ nanoplates (50.0 mAh g⁻¹). In addition, Kimura and colleagues reported NIGs with the electrospun SiO₂ nanofiber incorporated into the LiTFSI/[PYR₁₄][TFSI]/PEO, exhibiting the ionic conductivity of 0.1 mS cm⁻¹ at 30°C and the Li⁺ transference number of 0.014 at 60°C.¹⁰⁰ Such a relatively low Li⁺ transference number at 60°C could be ascribed to the size of the silica nanofiber (700 nm).¹⁰¹ A capacity of 150–160 mAh g⁻¹ in the prototype Li//LFP cells using such NIGs was achieved at 0.05–0.5C at 60–80°C. As shown in Figure 4A–C, Li//LFP coin cells using the NIGs constructed with the surface amino-functionalized SiO₂ nanofibers (400 nm) demonstrated remarkable rate performances and impressive cycling stability at a temperature of as high as 125°C, much better than the counterparts operated in common conditions using the liquid electrolytes and the polyolefin separators.¹⁰² These examples indicate the potential of NIGs for high-temperature LIBs.

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Modifying the surface of nano-SiO₂ is efficient in resolving the agglomerations in high concentrations. Recently, Niu and colleagues prepared NIGs composed of IL [CPIM][TFSI] grafted dendritic mesoporous SiO₂ nanoparticles (7.5 wt.%) with the PVDF, the IL [BMIM][TFSI] and the LiTFSI, with the mechanical strength of 1.23 MPa, the Li⁺ transference number of 0.40 and the ionic conductivity of 1.69 mS cm⁻¹ (30°C).¹⁰⁴ Furthermore, the assembled Li//LFP coin-cells exhibited a capacity of 147 mAh g⁻¹ at 1C and a capacity retention of 96.57% at the 120th cycle at 0.5C.

SiO₂-based NIGs for LMBs

Using the electrospinning process, Cheng and colleagues prepared NIG with nanostructured IL [PP₁₃][TFSI] immobilized SiO₂ nanoparticles confined in fluoroethylene carbonate (FEC) plasticized PVDF-HFP backbones.¹⁰⁵ Such NIGs showed RT ionic conductivity of 0.64 mS cm⁻¹, Li⁺ transference number of 0.60 and ESPW of 5.1 V (vs. Li/Li⁺). Also, good mechanical properties were achieved, effectively suppressing lithium dendrite formation and endowing the assembled symmetric Li//Li cells with stable stripping/platting cycling for 1200 h. Compared with the counterparts of gel polymer electrolytes based on the PVDF-HFP or the commercial Celgard separators with the conventional liquid electrolytes, the Li//LiNi_0.5Mn_1.5O₄ full cells using such NIGs exhibited better rate capability and cycling stability. In addition, Guo and colleagues prepared NIGs based on SiO₂ nanoparticles and LiTFSI salts dispersed in the IL [EMIM][TFSI] and the PVDF-HFP via solution casting method, owning the RT ionic conductivity of 0.74 mS cm⁻¹.¹⁰⁶ Based on such NIGs, the assembled Li//LFP coin cells delivered good interfacial and cycling stability. In the same year 2018, Guo and colleagues in the same group reported the synthesis of another NIGs using poly(acrylic acid)-lithium (PAALi) grafting core-shell SiO₂ nanoparticles (200–250 nm), with the ionic conductivity (at ambient temperature) of 0.74 mS cm⁻¹.¹⁰⁷ The core of the nano-SiO₂ could not only enhance the compatibility between the electrolyte and the Li anode but also improve the thermal stability of the NIGs. While the shell could boost the ionic conductivity and the Li⁺ transference number, the as-fabricated Li//LFP batteries with such NIGs delivered satisfied stable cycling performances (138 mAh g⁻¹ at a rate of 0.05C and a capacity retention of 87% at 100th cycle). In addition, Huang and colleagues fabricated the NIGs consisting of the IL [EMIM][TFSI] tethered SiO₂ nanoparticles (7 nm), the PVDF-HFP and the LiTFSI, with Li⁺ transference number of 0.45, RT ionic conductivity of 1.69 mS cm⁻¹ and an ESPW of up to 5.5 V.¹⁰⁸ The assembled Li//LFP pouch-type battery using such NIGs delivered a capacity of 133.7 mAh g⁻¹ (0.1C, RT), with a capacity retention of 91.9% at 400th cycle.

SiO₂-based NIGs for LSBs

NIGs based on nanocomponent matrices and ILs have been employed in LSBs because the nanoscale matrices could physically confine the lithium polysulfides intermediates, and meanwhile, the ILs could be chosen with relatively low solubility of lithium polysulfides intermediates. For instance, Ogawa and colleagues prepared NIGs (200 µm in thickness) comprised of the fumed SiO₂ nanoparticles (7 nm, with SSA of 390 m² g⁻¹), the IL [DEME][TFSA], the LiTFSA salts, and the PTFE, which exhibited the ionic conductivity of 0.12 mS cm⁻¹ at 35°C.¹⁰⁹ The assembled LSBs displayed a higher first capacity of 1370 mAh g⁻¹ at 0.05C than that of using the commercial separators (1070 mAh g⁻¹), with a cut-off voltage of 1.8­–3.0 V at 35°C but decreased to 600 mAh g⁻¹ after 10 cycles. Later, with the same method, they further synthesized three NIGs of 30–200 µm in thickness by adopting three ILs with the same anions (TFSA⁻) but different cations ([EMIM][TFSA], [DEME][TFSA], and [PP₁₃][TFSA]).¹¹⁰ After assembling into LSB and testing under the same conditions, they concluded that LSBs using the NIGs (30 µm in thickness) with the IL [DEME][TFSA] exhibited the best performances, with 1st and 45th capacity of 1100 and 690 mAh g⁻¹ at 0.05C in the cut-off voltage of 1.8–3.0 V, respectively, comparable to those using the electrolytes comprised of the same ILs and lithium salts. In addition, Kim prepared NIGs with SiO₂ nanoparticles, IL [PMIM][TFSI], LiTFSI, and PVDF-HFP, showing the ionic conductivity of 1.1 mS cm⁻¹ at 20°C and the oxidation stability to 5.0 V (vs. Li/Li⁺).¹¹¹ The assembled LSB utilizing such NIGs showed 1st and 30th specific capacity of 1029 and 885 mAh g⁻¹, respectively, at 0.1C with the cut-off voltage of 1.5–2.8 V at 30°C.

SiO₂-based NIGs for LOBs

Zhang and colleagues synthesized the NIGs composed of nano-SiO₂ (7 nm), the IL [PMMI][TFSI], the LiTFSI, and the PVDF-HFP, with RT ionic conductivity of 1.83 mS cm⁻¹.¹¹² Without oxygen catalysts, the assembled LOBs using such NIGs as the electrolyte membranes exhibited a capacity of 2.8 Ah g⁻¹ of carbon, higher than the counterpart of the LOB adopting the electrolyte with the same IL (1.5 Ah g⁻¹). As shown in Figure 4D, Wu and colleagues prepared NIGs by infiltrating the nonwoven fabrics, coating them with commercial super-hydrophobic nano-SiO₂ and polyisobutylene (PIB) in the IL [C1C3IM][TFSI] with dissolved LiTFSI (0.5 M).¹⁰³ Such NIGs showed the RT ionic conductivity of 0.91 mS cm⁻¹, the ESPW of >5.5 V (Li/Li⁺), and good hydrophobicity (with contact angle >150°, as shown in Figure 4D). The LOBs assembled with such NIGs exhibited long-term discharge/charge performances at the relative humidity (RH) of 45%, without evident overpotential increasement for 150 cycles; while LOB with glass fiber membrane in the same conditions could work for only 22 cycles because of poor capability for preventing H₂O permeation to corrode the Li metal anodes. In addition, Peng and colleagues prepared NIGs consisted of nano-fumed SiO₂ (7 nm), the IL [PYR₁₄][TFSI], the LiTFSI salts and the PTFE, which demonstrated RT ionic conductivity of 0.46 mS cm⁻¹ and the ESPW of 5 V.¹¹³ The coin-type LOBs assembled with such NIGs could work at RT and 80°C, showing long-term cycling stability for 500 cycles at 80°C. When the LOBs were tested at RT and 80°C, the overpotential for the initial cycle was, respectively, 0.6 and 0.45 V, with round-trip efficiency of 80% and 85.8%, respectively. Also, Xing and co-workers synthesized NIG consisting of SiO₂ nanoparticles with the hydrophobic IL [PYR₁₃][TFSI] involved in the LiTFSI salt, with the ionic conductivity of 1.45 mS cm⁻¹ at 30°C.¹¹⁴ The LOBs fabricated with such NIGs showed long life of up to 3000 h and stable performance at 100 mA g⁻¹ for 300 cycles (with a limited capacity of 500 mAh g⁻¹), without obvious capacity decay at 60°C under dry oxygen conditions.

Metal oxide-based NIGs

At present, NIGs based on nanoscale metal oxides have been applied in LIBs and LMBs.

Al₂O₃-based NIGs for LIBs

Raghavan and colleagues synthesized a series of NIGs comprising of nano ceramic fillers (14 nm SiO₂, 40–47 nm Al₂O_3, or 30–50 nm BaTiO₃), the IL [BMIM][TFSI], the LiTFSI salts, and the electrospun PVDF-HFP.¹¹⁵ All NIGs exhibited good interfacial stability with lithium and oxidation stability (>5.5 V), and the one with BaTiO₃ possessed the highest value (6.0 V). The Li//LFP cell assembled with the NIG involved BaTiO₃ showed the least specific capacity fading after 20 cycles at 0.1C in the voltage range of 2.0–4.5 V at RT. In addition, Li's group prepared NIGs by using larger SiO₂ or Al₂O₃ nanoparticles (100 nm)/P(MMA-AN-VAc) incorporating with the IL [PYR₁₄][TFSI]/vinylene carbonate (VC) containing LiTFSI salts.¹¹⁶ The SiO₂-based NIG had RT ionic conductivity of 1.2 mS cm⁻¹ and the value of the Al₂O₃ ones was 1.1 mS cm⁻¹. Moreover, the Li//LFP batteries using such SiO₂ and Al₂O₃-based NIGs showed good electrochemical performances. Owing to the stronger effect on the Lewis acid–base reactions with the P(MMA-AN-VAc) segments, NIGs based on SiO₂ nanoparticles exhibited better battery performances (almost no capacity decay after 50 cycles) than that of the Al₂O₃-based NIGs (a capacity retention of 88.6% after 50 cycles). Later, using the IL [EMIM][TFSI] as plasticizers, they developed nano-SiO₂ and nano-Al₂O₃ particle (in the weight ratio of 1:1) co-decorated P(MMA-AN-EA) based NIGs, with good flame retardation property, ambient temperature ionic conductivity of 3.2 mS cm⁻¹, and a higher fracture strength (160 MPa) than that of the polyethylene (PE) separators (138 MPa).¹¹⁷ Ascribing to excellent compatibility with the Li anodes and improved oxidative stability with the LFP cathodes, such NIGs assembled Li//LFP coin-type cells achieved a first capacity of 159.3 mAh g⁻¹ and a capacity retention of 95.1% at 100th cycle (RT).

Al₂O₃-based NIGs for LMBs

Wang and colleagues used the solution casting method to synthesize the NIGs with 5 wt.% Al₂O₃ nanoparticles incorporated in phosphonium IL [P_111i4][FSI], the LiFSI, and polymerized IL [PDADMA][TFSI].¹¹⁸ The mechanical stability was effectively boosted by the Al₂O₃ nanoparticles and the ionic conductivity of such NIGs was 0.28 mS cm⁻¹ at 30°C, endowing the assembled Li//Li cells with ultra-stable battery performance. Recently, Gandolfo and colleagues prepared solvent-less NIGs composed of Al₂O₃ nanoparticles (150 nm) with the IL [PYR₁₄][TFSI], the LiTFSI, and the PEGDA matrix, showing the Li⁺ transference number of 0.17, the ionic conductivity (20°C) of 0.205 mS cm⁻¹, and the ESPW of 5.12 V (vs. Li/Li⁺).¹¹⁹ Due to the acidic sites on the surface, the Al₂O₃ nanoparticles could coordinate the anionic species and thus promote the motion of Li⁺, thereby enhancing the ionic conductivity. As a result, compared with the counterpart using the glass fiber separators (Whatman) soaked with the same IL and LiTFSI, the Li//LFP coin cells assembled with such NIGs presented an improved capacity at 1 C and a good capacity retention at 0.2C after 500 cycles.

Most recently, Ruan and colleagues prepared the gradient NIGs composed of one layer of the γ-Al₂O₃ (20 nm, 30 wt.%) with the PVDF-HFP/[EMIM][TFSI]/LiTFSI (Ionogel-dual30) to the cathodes and the other layer of γ-Al₂O₃ (20 nm, 50 wt.%) with the PVDF-HFP/[EMIM][TFSI]/LiTFSI (Ionogel-dual50, Young's modulus: 749 MPa) toward the Li metal anodes (Figure 5A).¹²⁰ After being provided electrons at the Li metal anodes, the Ionogel-dual50 layer could offer a substantial number of Al₂O₃ nanoparticles for the formation of Li₃AlF₆ and AlF₃ and also the carbon-fluorine bond cleavage in the TFSI⁻, leading to accelerated Li⁺ transport and insulated e⁻ transfer at the interface (Figure 5A). As displayed in Figure 5B, the N atoms of TFSI⁻ and the Al atoms on the Al₂O₃ surface show bonding formation, whereas the Li⁺ and the [EMIM]⁺ exhibit no bonding behavior with the Al₂O₃, suggesting preferential TFSI⁻ adsorption on Lewis acidic sites of the Al₂O₃. Both the experimental characterizations and the simulations indicated that the as-reported tidal-flow-like ion transfer pathway could provide the [Li⁺-N-methyl-2-pyrrolidone (NMP)]-(PVDF-HFP) dominant route in the layer of the Ionogel-dual30 and the [Li(TFSI)_x]⁺-Al₂O₃ interface dominant route in the layer of the Ionogel-dual50. Moreover, such gradient NIGs could endow the assembled Li//LFP and Li//NMC (811) full cells with good cycling performances, as shown in Figure 5C,D. To demonstrate the practicality and safety of such NIGs, Li//Li_1.17Ni_0.27Co_0.05Mn_0.52O₂ (LRMO) pock batteries were fabricated, which exhibited long cycling stability of 110 cycles at 4.5 V and could supply stable power to light up the LEDs under extreme conditions such as bending, cutting, and punching (Figure 5E–J).

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TiO₂-based NIGs for LIBs

As one of the strongly coordinating additives, nano-TiO₂ with rich Lewis acidic groups could also contribute to lithium salt dissociation and thus increase the free ion number.

In 2015, Kim and colleagues prepared NIGs by incorporating the solution of the IL [PYR₁₄][TFSI] and the LiTFSI salt on TiO₂ nanoparticles (70 nm), with high ionic conductivity (1.5 mS cm⁻¹) at 20°C and good thermal stability.⁶⁸ As the TiO₂ nanoparticles could trap the anions and thus increase the Li⁺ transference number, the assembled pouch-type LIBs based on the LFP cathodes exhibited high rate capability (106 mAh g⁻¹ at 1C, RT). Also, half-cells with another NIG based on the TiO₂ nanoparticles exhibited much higher rate performance at RT (138 mAh g⁻¹ at 1C).¹²¹ Chen and colleagues synthesized NIGs composed of the TiO₂, the IL [PYR₁₃][TFSI], the LiTFSI, and the PVDF-HFP, with the RT ionic conductivity of 7.4 mS cm⁻¹ (Figure 6A) and the anodic electrochemical stability up to 5.5 V (vs. Li/Li⁺).¹²² Such NIGs had good mechanical strength, endowing the assembled Li//Li coin cells with stable stripping/platting cycling for 800 h at 0.1 mA cm⁻², as shown in Figure 6A. Furthermore, the assembled Li//LFP with such NIGs could work at 0.1C for 500 days, with no evident voltage hysteresis increase (Figure 6A).

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ZrO₂-based NIGs for LMBs

Ascribing to the merits of nontoxicity, excellent biocompatibility, attractive mechanical properties, weak acid–base characteristics, thermal stability, and suitable redox potential, nanoscale ZrO₂ was suitable for NIGs to meet the requirement of high safety. As an example, Chen and co-workers synthesized NIG composed of nanoscale ZrO₂ (20–30 nm) matrices, the IL [EMIM][TFSI], and the LiTFSI (Figure 6B).¹²³ To better understand the interaction mechanisms between the IL electrolytes and the ZrO₂ nanoparticles, density functional theory (DFT) calculations (Figure 6B) were performed, which showed that the dissociation energy (DE) of the Li⁺ and the TFSI⁻ in the optimized molecule of the LiTFSI was 5.426 eV and decreased to 0.955 eV on the ZrO₂ surface, attributing to the interactions between ZrO₂ nanoparticles and the surrounding TFSI⁻. The ZrO₂-supported NIGs with high thermal stability were prepared via a process involving sol-gel reactions and were finally applied in Li//LFP coin-type full cells. Cycling with cut-off voltages of 2.7 and 4.2 V (vs. Li/Li⁺) at 0.1C, the as-assembled batteries using such NIGs exhibited good specific capacities at a series of temperatures (–10°C, 0°C, 10°C, 30°C, 60°C, and 90°C), as shown in Figure 6B.

BN-based NIGs

With highly abundant nanoporous structures, boron nitride (BN) nanosheets, as the 2D graphene analog, have been developed as the NIG hosts. At present, NIGs based on the nanoscale BN have been applied in LIBs, LMBs, and LSBs.

BN-based NIGs for LIBs

Li and colleagues reported the template-free method for the incorporation of the LiTFSI and the IL [EMIM][TFSI] in the host of BN nanosheets (2 nm).¹²⁴ The resultant NIGs exhibited good ionic conductivity of 0.23 mS cm⁻¹ at a low temperature of −20°C. Furthermore, Li//LFP button cells consisting of such NIGs showed good cycling and rate performances at RT. In addition, Hersam and colleagues prepared NIGs based on the hexagonal BN nanoplatelet matrix with the IL [EMIM][TFSI] and the LiTFSI, showing enhanced electrochemical anodic stability up to 5.3 V (vs. Li/Li⁺), much higher than the IL electrolytes (4.2 V, vs. Li/Li⁺).⁵⁸ More importantly, the as-assembled solid-state LIBs using such NIGs could operate at a high temperature of 175°C, with superlative rate characteristics and a capacity retention of 90% after 100 cycles at 10C.

Later, researchers come from Hersam's group developed layered heterostructure NIGs with two imidazolium ILs ([EMIM][TFSI] and [EMIM][FSI]), also containing LiTFSI in hexagonal BN nanoplatelet matrices, which combined both high anodic stability (>5 V, vs. Li/Li⁺) and cathodic stability (<0 V, vs. Li/Li⁺), as the hexagonal BN nanoplatelets offered large SSAs to immobilize the ILs and thus minimized the intermixing at the heterointerface.¹²⁵ The achieved layered heterostructure NIGs possessed an extended ESPW and RT ionic conductivity of >1.0 mS cm⁻¹. Using such NIGs, full-cell LIBs assembled with the LiNi_0.33Mn_0.33Co_0.33O₂ (NMC) cathodes and the graphite anodes demonstrated relatively high voltages, which could not achieve individually with either low or high-potential ILs. Moreover, as shown in Figure 7A, graphite//NCM LIB full-cells adopting such layered heterostructure NIGs exhibited significantly improved cycling performances under the same conditions, compared with that using NIGs based on the mixture of the same ILs. Recently, also based on hexagonal BN nanoplatelets, their group presented and reported a screen-printable NIG formulation, mixed with the same imidazolium IL [EMIM][TFSI] and LiTFSI, which owned the mechanical moduli >1 MPa and the RT ionic conductivity >1 mS cm⁻¹.¹²⁶ Using such NIGs, screen-printed LIBs assembled with LFP cathodes and Li₄Ti₅O₁₂ (LTO) anodes delivered desirable battery performances such as good rate performances and excellent cycling stability under harsh conditions including at elevated temperatures, bending deformations, and external compressive forces (Figure 7B).

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BN-based NIGs for LMBs

Due to the improved mechanical modulus endowed by the nanoscale matrix, NIGs composed of the BN nanoplatelets exhibited improved resistance to lithium dendrites, as compared to the IGs with bulk BN microparticles.⁵⁸ Also, it was reported that NIGs involving BN nanosheets inhibited lithium dendrites growth during cycling in the Li//LFP battery ascribing to the formed stable and robust interface, which was both mechanically strong and highly tortuous, enabled by the impermeable BN nanosheets.⁷⁰ As reported, BN nanosheets possess merits of higher Li⁺ transference number and more uniform lithium deposition and thus mitigate the lithium dendrite growth.^60,71 Moreover, BN nanosheet could be functionalized with various functional groups. For instance, Kim and colleagues fabricated NIGs based on the PVDF-HFP with the addition of the amine-functionalized BN nanosheets in the IL [PYR₁₃][TFSI] and the LiTFSI salts.⁶⁰ Due to the addition of the amine-functionalized BN nanosheets, the NIGs showed much-improved thermal stability, mechanical strength (2.22 MPa), and Li⁺ transference number (0.23). The assembled Li//LFP coin cells using such NIGs (205 ± 5 µm in thickness) retained 92.2% of the initial capacity at the 60th cycle, much higher than that of the cells without the amine-functionalized BN nanosheets (53.5%). Using such NIGs, the assembled Li//LFP coin cells delivered a capacity of 132.8 mAh g⁻¹ (at 0.2C), as well as excellent performances in Li//NMC and Li//LCO coin cells.

BN-based NIGs for LSBs

Kim and colleagues prepared NIGs by facile solution casting and subsequent hot roll pressing (Figure 8A), which were composed of the inorganic Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP), the functionalized BN nanosheet (BNNS), the IL [PYR₁₃][TFSI], the LiTFSI, and the PVDF-HFP.¹²⁷ The LATP and BNNSs could provide high Li⁺ conduction. To further enhance interfacial compatibility and reduce the interfacial resistance, two artificial interlayers, poly (1, 3-dioxolane) (pDOL) synthesized through the in-situ polymerization of DOL, were designed and inserted between such BN-based NIGs and the electrodes to construct the resulting layered NIGs in the lithium batteries (IGHE-pDOL, Figure 8B,C). Such interlayers acted as the protective barrier could not only promote better wetting and adhesion of the NIGs to the lithium metal surface to facilitate the efficient Li⁺ transport and minimize the interface resistance but also prevent direct contact between the NIGs and the lithium metal, inhibiting side reactions, and the dendritic structure growths. Such unique NIGs (IGHE-pDOL) were further applied in LSBs. The assembled LSBs exhibited satisfying interfacial connection (Figure 8D) and delivered a capacity of 686.7 mAh g⁻¹ at 0.2C after 100 cycles at RT (a capacity retention of 81.2%, Figure 8E). Also, stable CE with an average CE of 95.7% after 100 cycles was achieved, much better than that of the LSB assembled with the pDOL SSE without ILs. As reported, such excellent cycling performances of LSB using the BN-based NIGs with in-situ prepared pDOL interlayers implied no detrimental effects such as the polysulfide shuttle and the dendritic lithium deposition occurred.

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Other nanocomponent-based NIGs

Besides the nanoscale SiO₂, metal oxides (Al₂O₃, TiO_2, and ZrO₂) and BN, NIGs based on other nanocomponents, such as halloysite nanotube (HNT), Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP), vermiculite (Vr) nanosheet, CNT, and metal-organic framework (MOF), have also been applied in LIBs and LMBs.

HNT-based NIGs for LIBs

Zhao and colleagues prepared NIGs composed of the halloysite nanotubes (HNTs, length: 300–1500 nm, outer diameter: 30–60 nm, lumen diameter: 10–25 nm), the IL [BMIM][BF₄], the LiBF₄ salts and the PVDF backbones in a mass ratio of 3:5:1:1, with the RT anisotropic ionic conductivity of 3.8 mS cm⁻¹.¹²⁸ The Li//LFP cells with aluminum–plastic laminated film packages using such NIGs delivered a capacity of 144 mAh g⁻¹ at 0.1C (RT), a capacity retention value of 95.9% and the CE of 96% after 50 cycles.

LAGP-based NIGs for LIBs

Guo et al. prepared NIGs comprised of 50 wt.% active ceramic nanoparticles, Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP, 850–950 nm), the IL [EMIM][TFSI], the LiTFSI and the PVDF-HFP, with the Young's modulus of 13.96 MPa and RT ionic conductivity of 0.92 mS cm⁻¹.¹²⁹ The assembled Li//LFP cells using such NIGs showed the initial specific capacity of 157.8 mAh g⁻¹ at 0.05C in the voltage of 2.7–3.85 V and a capacity retention ratio of 89.5% at 50th cycle. Later on, based on the same ILs, LiTFSI and PVDF-HFP, they prepared another NIGs with 10 wt.% active Li_1.5Al_0.5Ge_1.5(PO₄)₃ (850–950 nm), which showed the RT ionic conductivity of 0.76 mS cm⁻¹ and the ESPW of 4.8 V (vs. Li/Li⁺).¹³⁰ In contrast to the inert SiO₂ nanoparticles, the addition of LAGP was more effective as the LAGP particles could reduce the polymer crystallinity and also act as the Li⁺ conductor to provide Li⁺. And the Li⁺ transference number was 0.54, higher than the one without the LAGP. Also, such NIGs had good compatibility with Li metal anode, without lithium dendrite. The assembled Li//LFP cells showed good cycling stability.

Vr-based NIGs for LIBs

Recently, Zhang and colleagues prepared NIGs (~20 μm in thickness, Figure 9A) by stacking the NH₂-functionalized 2D vermiculite (Vr-NH₂) nanosheets with good wetting ability (~23°, Figure 9B) and subsequently percolating the IL [EMIM][BF₄] and the LiTFSI salts (1:1 by mol) into the interlayer nanochannels with the spin-coating method.¹³¹ The achieved NIGs possessed good mechanical stability (Young's modulus: 2.03 GPa; tensile strength: 84 MPa). And the ionic conductivity of the NIGs was 0.09–1.35 mS cm⁻¹ at the temperature of −40°C to 100°C, with a high Li⁺ transference number of 0.89, comparable with the single-ion conductors. Moreover, the fabricated Li//LFP half-cells using such NIGs exhibited good battery performances over a temperature range of −40°C to 100°C (Figure 9C,E). When assembled into the Li//NMC (811) half-cells, outstanding cycling stability at 0.2C was achieved at −20°C and 60°C (Figure 9D). In particular, as shown in Figure 9F,G, the assembled Li//LFP pouch cells (0.1 Ah) exhibited good flexibility and safety, as well as high capacity at specific current of 5 mA cm⁻² at the temperature of −20°C and 60°C.

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CNT-based NIGs for LMBs

Recently, NIGs based on CNTs modified by the IL [EMIM][TFSI] entrapping into the polymer matrices of poly(vinylene carbonate) (PVCA), PEO, and PVDF-HFP, as shown in Figure 9H, were prepared by the method of physical cross-linking and the solution casting, which delivered the ionic conductivity of 0.71 mS cm⁻¹ and the Li⁺ transference number of 0.51 at 30°C, with an ESPW of 4.74 V.¹³² The Li//LFP full cells assembled with such NIGs possessed good initial capacity (111 mAh g⁻¹) and long cycling stability (retention of 85.8% at 1000th cycle) at 4C (Figure 9I).

MOF-based NIGs for LMBs

Wang and colleagues reported the NIGs comprised of the IL [EMIM][TFSI] (55.4 wt.%) impregnated MOF nanocrystals (34.5 wt.%) and the LiTFSI (10.1 wt.%) salts, with the RT ionic conductivity of 0.3 mS cm⁻¹ and the Li⁺ transference number of 0.36.⁷¹ When integrated into the Li//LFP Swagelok cells, remarkable performances were exhibited in the temperature ranges of −20°C to 150°C (Figure 10A), which was attributed to efficient 3D Li⁺ transport network and nanowetted interfaces of the NIGs.

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Recently, Ouyang and colleagues prepared NIGs based on the zwitterionic nanochannels (BZNs) incorporated MOF (MOF-BZN, 200–500 nm) with the IL [AMIM][TFSI], the LiTFSI, and the PTFE (1.0 wt.%).¹³³ The MOF-BZN was constructed by grafting multicationic oligomers (MCOs) onto the anionic MOF channels. Through both the chemical immobilization and the nanoconfinement effects, MCO dynamics were efficiently suppressed, allowing for ion transport decoupling from MCO relaxation. Furthermore, competing interactions between the negative adsorption sites in the MOF-BZN and the cationic MCOs promoted the Li⁺ dissociation from the binding state, while the anion migration was restricted due to the charge interactions. As a result, immobilized MCOs extending from pore walls played as the ion-selective gates to only facilitate the Li⁺ transport (Figure 10B). Such NIGs demonstrated the RT ionic conductivity of 0.876 mS cm⁻¹, Li⁺ transference number of 0.75, an ESPW of 4.9 V (vs. Li/Li⁺), high thermal stability, and good flame retardance. Both the assembled Li//LFP and Li//NMC (811) coin-type and pouch-type cells using such NIGs (42.9 μm in thickness) showed high capacity and good cycling stability at 0.1C. Moreover, foldable pouch cells could work after being cut (Figure 10C). More recently, Wei and colleagues reported the NIGs (180 μm in thickness) based on the Fe-containing MOF (2800 m² g⁻¹) with the IL [EMIM][TFSI], the LiTFSI, and the PEO.¹³⁴ The assembled Li//LFP coin cells exhibited a capacity of 139.1 mAh g⁻¹ (0.5C, 60°C) at 250th cycle, with a capacity retention of 93.3%.

To sum up, the investigation of the NIGs for LIBs has become a rather active research area in recent years. The nanocomponents used for the NIGs should be appropriate in such as the size, the surface, and the acid–base characteristics. To optimize the safety and the electrochemical performances of LIBs, worldwide engineers and scientists endeavor to explore diverse nanomaterials, different IL compositions, and solid matrices. By tailoring nanocomponent types, concentrations, and dispersion in the NIG matrices, researchers are aimed at addressing the cost and the scalability for practical implementation, simultaneously achieving superior electrochemical performances of LIBs with high energy/power density, enhanced long-term cycling stability, and improved safety features.

Note that the NIGs used for LMBs are still the active research area, and further optimizations (nanocomponent dispersions, compositions, and overall performances) are still needed to address such remaining challenges as high Li-ion conductivity at relatively low temperatures, long-term cycling stability, the cost, and the scalability. However, the unique combinations of outstanding properties make the NIGs promising to enable safe and high-performance LMBs, holding huge potential for applications in advanced energy storage.

It is vital to note that research on NIGs for LSBs is relatively novel and to optimize the performances, multiple designs and material compositions are being explored. Though holding big potential, practical challenges such as long-term stability, scalability, and cost should be addressed before the NIGs enable commercialized applications in next-generation LSBs and large-scale energy storage.

While the usage of NIGs in LOBs shows huge potential, it is vital to acknowledge that LOBs are still in the initial development stage. Before LOBs become commercially viable, several fundamental challenges, such as the issue of lithium dendrite formation on Li anodes and the inefficient O₂ cathodes, along with practical considerations of the cost and the safety, need to be addressed. In short, NIGs not only represent promising for improving LOB performances but also play a significant role in realizing the potential of next-generation energy storage systems with high energy density. However, further research innovations are required to overcome the remaining hurdles and advance the technology.

SUMMARIES AND PERSPECTIVES

Advantages and deficiencies of Ionogels (IGs)

IGs, the hybrid materials with ILs immobilized by gelling solid matrices, have flourished in recent years for various research areas and potential technological applications. Compared to other types of polymer electrolytes (PEs), IGs have specific advantages and deficiencies (Figure 11), which will be discussed briefly in the following sections.

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Major advantages of IGs

• (1)

  High ionic conductivity. IGs could exhibit high ionic conductivity, often surpassing that of traditional solid PEs. This high conductivity is due to the presence of ILs, which are known for their excellent ion mobility.

• (2)

  Wide temperature range. ILs used in IGs could have low freezing points and high thermal stability. This makes IGs suitable for a wide temperature range of applications, from cryogenic conditions to high-temperature environments.

• (3)

  Good electrochemical stability. ILs are generally chemically stable, which enhances the electrochemical stability of IGs. They could withstand a wide range of voltage and current conditions without significant degradation.

• (4)

  Low flammability. Many ILs used in IGs have low volatility and are nonflammable, which improves the safety of devices incorporating these materials, especially in battery applications.

• (5)

  Flexibility and moldability. IGs could be formulated to be flexible and moldable, allowing them to conform to various shapes and sizes. This flexibility is advantageous for designing custom-shaped electrolytes in energy storage devices.

Significant deficiencies of IGs

• (1)

  Cost. ILs, which are a key component of IGs, could be relatively expensive to produce compared to some other electrolyte materials. This cost factor could limit the widespread adoption of IGs in large-scale applications.

• (2)

  Environmental concerns. Some ILs used in IGs may not be environmentally friendly. Researchers are working on developing greener ILs, but this remains a concern for sustainability.

• (3)

  Limited mechanical strength. Depending on the specific formulation, IGs may have limited mechanical strength. This could be a drawback in certain applications where mechanical integrity is crucial, such as in flexible batteries or supercapacitors.

• (4)

  Limited compatibility. IGs may not be compatible with all electrode materials. Compatibility issues could lead to reduced performance or stability problems in devices.

• (5)

  Synthesis complexity. The synthesis of IGs could be more complex than that of some other types of PEs. Achieving the desired properties and performances often requires precise control over the synthesis processes.

Overall, IGs offer several advantages, including high ionic conductivity, wide temperature range, good electrochemical stability, low flammability, and flexibility. However, they also have some significant deficiencies related to cost, environmental concerns, mechanical strength, compatibility, and synthesis complexity. The choice of electrolyte materials depends on the specific requirements of the applications and the trade-offs between these advantages and deficiencies. Researchers are actively working on addressing these deficiencies and improving the overall performance of the IGs.

Advantages, summaries, and perspectives of NIGs

In contrast to the bare IGs, the comprehensive properties of the NIGs are enhanced by the nanoscale matrices, including mechanical strength, ionic conductivity, thermal stability, and electrochemical performance. Such improved significant properties and the intrinsic safety merits enable the NIGs as the leading candidates to serve as both the electrolytes and the separators for rechargeable lithium-based batteries, which have also been demonstrated by the recent developments of the monolithic NIGs using various nanoscale matrices and ILs. Typical NIGs applied in lithium-based batteries, including nanocomponents, preparation methods, thickness, mechanical strength, ionic conductivity, and battery performances, are summarized in Table 2.

Table 2 Summary of the nanocomponents, preparation methods, thickness, mechanical strength, ionic conductivity, and battery performances for the typical state‑of‑the‑art NIGs as monolithic electrolyte membranes applied in lithium-based batteries.

NIG	Method	Thickness (μm)	Mechanical strength (MPa)	IC (mS cm−1)	Battery RC (mAh g−1)	References
5 wt.% nano-SiO2 (20 nm, 240 m2 g−1)/[PYRA12O1][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	NA	0.33 (20°C)	Li//LFP Swagelok cells: 50 (x = NA, 0.1C, 2.5–4.0 V, RT)	[93]
fumed nano-SiO2 (7 nm, 390 m2 g−1)/[EMIM][TFSA]/LiTFSA/PTFE	Mixing	200	NA	NA	Li//LCO coin cells: 18 (x = 5, 0.5C, 3.4–4.2 V, 35°C)	[94]
TEOS hydrolyzed SiO2/[PYR13][TFSI]/LiTFSI/PVDF-HEA	Sol-gel, solution casting	25	NA	NA	Li//LFP batteries: 122 (x = NA, 0.1C, 2.0-4.2 V, 22°C)	[95]
15.4 wt.% SiO2 nanoparticles (10–20 nm) attached PVDF/[PYR14TFSI]/LiTFSI	Solution casting	NA	NA	NA	Li//LCO coin cells: 121 (x = 50, 0.5C, 2.8–4.2 V)	[96]
SiO2 nanoparticles/[BMIM][TFSI]/LiTFSI	Sol-gel	30	60.4 (Young modulus)	3.15 (RT)	Inject printing MCMB//LFP coin full cells: 143 (x = 100, 0.1C, 2.5–4.1 V, 30°C)	[97]
TEOS hydrolyzed SiO2/[EMIM][FSI]/LiFSI	Sol-gel	200–300	NA	6.2 (RT)	Drop-casting Li//LFP coin cells: 150 (x = NA, 0.1C, 2.8–4.2 V, 30°C)	[98]
Fumed SiO2 nanoparticles/[EMIM][TFSA]/LiTFSA	NA	500	NA	4.4 (75°C)	Li//LCO coin cells: 122 (x = 5, 0.1C, 3.4–4.2 V, 65°C)	[99]
8 wt.% polymeric IL [PDMA][TFSI] functionalized mesoporous SiO2 nanoplates (740.6 m2 g−1)/[PY14][TFSI]/LiTFSI	Solution casting	40–50	0.85 (elastic modulus)	1.0 (130°C)	Li//LFP coin cells: 135.8 (x = 30, 0.1C, 2.5–4.2 V, 60°C)	[57]
Electrospun SiO2 nanofiber (700 nm)/[PYR14][TFSI]/LiTFSI/PEO	Solution casting	NA	0.89 (Young's modulus)	0.1 (30°C)	Li//LFP coin cells: 150 (x = 33, 0.2C, 2.5–4.0 V, 60°C)	[100]
3.5 wt.% surface amino-functionalized SiO2 nanofibers (400 nm)/[EMIM][TFSA]/LiTFSA	NA	200	NA	1.0 (RT)	Li//LFP coin cells: 92 (x = 500, 1C, 2.5–4.0 V, 125°C)	[102]
[CPIM][TFSI] grafted dendritic mesoporous SiO2 nanoparticles (7.5 wt.%)/[BMIM][TFSI]/LiTFSI/PVDF	Solution casting	280	1.23 (tensile stress)	1.69 (30°C)	Li//LFP coin cells: 141 (x = 120, 0.5C, 2.5–4.2 V)	[104]
45 wt.% TiO2 nanoparticles (70 nm)/[PYR14][TFSI]/LiTFSI	Ball milling	NA	NA	1.5 (20°C)	Li//LFP pouch cells: 133.5 (x = 100, 0.1C, 2.5–4.0 V, RT)	[68]
12.4 wt.% TiO2 nanoparticles (50 nm, 300.5 m2 g−1)/[EMIM][TFSI]/LiTFSI	Sol-gel	38	NA	2.8 (RT)	Li//LFP coin cells: 150.9 (x = 200, 0.1C, 2.5–4.2 V, RT)	[121]
TiO2/[PYR13][TFSI]/LiTFSI/PVDF-HFP	Sol-gel	25	Without breaking at 90° flexion angle	7.4 (RT)	Li//LFP coin cells: 140 (x = 600, 0.1C, 2.5–4.2 V, RT)	[122]
6 wt.% nano-BaTiO3 (30–50 nm)/[BMIM][TFSI]/LiTFSI/electrospun PVDF-HFP	NA	150	NA	5.2 (RT)	Li//LFP cells: 163.5 (x = 20, 0.1C, 2.0–4.5 V, RT)	[115]
Al2O3 nanoparticles (100 nm)/[PYR14][TFSI]/LiTFSI containing vinylene carbonate (VC)/P(MMA-AN-VAc)	NA	22	NA	1.1 (RT)	Li//LFP cells: 125.9 (x = 50, 0.1C, 2.5–3.75 V, RT)	[116]
Nano-SiO2 & Al2O3 (100 nm, 1:1 in weight)/[EMIM][TFSI]/LiTFSI/P(MMA-AN-EA)	NA	NA	160 (fracture strength)	3.2 (ambient temperature)	Li//LFP coin cells: 151.5 (x = 100, 0.2C, 2.5–3.65 V, RT)	[117]
Nanoporous BN nanosheets (2 nm, 860 m2 g−1)/[EMIM][TFSI]/LiTFSI	Mixing	350	NA	3.85 (RT), 0.23 (−20°C)	Li//LFP coin cells: 141 (x = 30, 0.1C, 2.0–4.0 V, RT)	[124]
40 wt.% C-coated hexagonal BN nanoplatelet (143 ± 67 nm)/[EMIM][TFSI]/LiTFSI	Mixing	200–250	31 (compressive elastic modulus)	>1.0 (RT)	Li//LFP coin cells: 144 (x = 100, 10C, 2.5–4.0 V, 175°C)	[58]
Layered heterostructure: hexagonal BN nanoplatelet/[EMIM][TFSI]/LiTFSI & hexagonal BN nanoplatelet/[EMIM][FSI]/LiTFSI	Mixing	60–80	>1.0 MPa (storage modulus)	>1.0 (RT)	C//NMC coin cells: 97 (x = 100, 0.5C, 2.5–4.2 V, RT)	[125]
Hexagonal BN nanoplatelet/[EMIM][TFSI]/LiTFSI	Screen printing	~200	>1.0 (mechanical moduli)	>1.0 (RT)	Screen-printed LTO//LFP coin cells: >105 (x = 300, 0.3C, 1.0–2.5 V, RT)	[126]
30% halloysite nanotubes (HNTs, outer diameter: 30–60 nm, lumen diameter: 10–25 nm)/[BMIM][BF4]/LiBF4/PVDF	NA	NA	1.1 (tensile strength)	3.8 (RT)	Li//LFP cells with Al-plastic laminated film packages: 138.1 (x = 50, 0.1C, 2.5–4.0 V, RT)	[128]
50 wt.% nano-Li1+xAlxGe2−x(PO4)3 (LAGP, 850–950 nm)/[EMIM][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	13.96 (Young's modulus)	0.92 (RT)	Li//LFP coin cells: 141.3 (x = 50, 0.05C, 2.7–3.85 V, RT)	[129]
10 wt.% Li1.5Al0.5Ge1.5(PO4)3 (850–950 nm)/[EMIM][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	NA	0.76 (RT)	Li//LFP coin cells: 131 (x = 50, 0.05C, 2.7–3.85 V, RT)	[130]
NH2-vermiculite (Vr-NH2) nanosheets/[EMIM][BF4]/LiTFSI (1:1 by mol)	Spin-coating	20	2030 (Young's modulus)	0.13 (−20°C)	Li//NMC (811) half- cells: 113 (x = 100, 0.2C, 3.0–4.3 V, −20°C)	[131]
Nanostructured [PP13][TFSI] immobilized SiO2 nanoparticle/FEC plasticized PVDF-HFP	Electrospinning, mixing	40	7.88 (tensile strength)	0.64 (RT)	Li//LiNi0.5Mn1.5O4 cells: 109.6 (x = 460, 1C, 3.5–4.9 V)	[105]
20 wt.% SiO2 nanoparticles/[EMIM][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	NA	0.74 (RT)	Li//LFP cells: 117.9 (x = 50, 0.05C, 2.7–3.85 V, RT)	[106]
15 wt.% poly(acrylic acid)-lithium (PAALi) grafting core-shell SiO2 nanoparticles (200–250 nm)/[EMIM][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	NA	0.74 (ambient temperature)	Li//LFP cells: 120.1 (x = 100, 0.05C, 2.7–3.85 V, RT)	[107]
[EMIM][TFSI] tethered SiO2 nanoparticles (7 nm)/LiTFSI/PVDF-HFP	Phase inversion	200	NA	1.69 (RT)	Li//LFP coin cells: 122.8 (x = 400, 0.1C, 2.8–4.2 V, RT)	[108]
5 wt.% Al2O3 nanoparticles/[P111i4][FSI]/LiFSI/polymerized IL [PDADMA][TFSI]	Solution casting	200	6.4 (elastic modulus, 30°C)	0.28 (30°C)	Li//Li coin cells: 250 h (x = NA, 0.05 mA cm−2, −50–50 mV, 50°C)	[118]
5 wt.% Al2O3 nanoparticles (~150 nm)/[PYR14][TFSI]/LiTFSI/PEGDA	Solution casting	170	NA	0.205 (20°C)	Li//LFP coin cells: 62.8 (x = 500, 0.2C, 2.5–4.2 V, RT)	[119]
One layer of γ-Al2O3 (20 nm, 30 wt.%)/[EMIM][TFSI]/LiTFSI/PVDF-HFP (Ionogel-dual30) to the cathodes and the other layer of γ-Al2O3 (20 nm, 50 wt.%)/[EMIM][TFSI]/LiTFSI/PVDF-HFP (Ionogel-dual50) toward the Li metal anodes	Twice solution casting	NA	NA	NA	Li//LFP full cells: 126.6 (x = 500, 0.5C, 2.5–4.0 V, RT); Li//NMC (811) full cells: 143 (x = 50, 0.5C, 3.0–4.4 V, RT)	[120]
Nanoscale ZrO2 (20–30 nm)/[EMIM][TFSI]/LiTFSI	Sol-gel	NA	NA	0.743 (30°C)	Li//LFP coin-type full cells: 135.9 (x = 200, 0.1C, 2.7–4.2 V, 30°C)	[123]
2.0 wt.% BN nanosheet/[BMIM][TFSI]/LiTFSI/PIL	Solution casting, UV curing	~100	NA	NA	Li//LFP coin-type full cells: 114.4 (x = 200, 0.2C, 2.5–4.2 V, 60°C)	[70]
Amine-functionalized BN nanosheets/[PYR13][TFSI]/LiTFSI/PVDF-HFP	Solution casting	205 ± 5	2.22	0.647 (RT)	Li//LFP coin cells: 122.4 (x = 60, 0.2C, 2.5–4.2 V)	[60]
[EMIM][TFSI] impregnated MOF nanocrystals (1630 m2 g−1)/LiTFSI/PTFE	Hand milling	100	NA	0.3 (RT)	Li//LFP Swagelok cells: 132 (x = 100, 0.1C, 2.0–4.2 V, RT)	[71]
Zwitterionic nanochannels (BZNs) incorporated MOF (MOF-BZN, 200–500 nm)/[AMIM][TFSI]/LiTFSI/PTFE (1.0 wt.%)	Mixing, hot groove rolling	42.9	NA	0.876 (30°C)	Li//NMC (811) pouch cells: 175 (x = 55, 0.1C, 3.0–4.3 V, 30°C, N/P = 2.7)	[133]
Fe-containing MOF (2800 m2 g−1)/[EMIM][TFSI]/LiTFSI/PEO	Solution casting	~180	1.94 (tensile stress)	0.846 (60°C)	Li//LFP coin cells: 139.1 (x = 250, 0.5C, 2.5–3.8 V, 60°C)	[134]
Carbon nanotubes (CNTs)/[EMIM][TFSI]/LiTFSI/poly(vinylene carbonate) (PVCA), PEO, and PVDF-HFP (1:1:2, wt.%)	Solution casting	37	128	0.71 (30°C)	Li//LFP full coin cells: 95 (x = 1000, 4C, 2.5–4.2 V, 30°C)	[132]
Fumed SiO2 nanoparticles (7 nm, 390 m2 g−1)/[DEME][TFSA]/LiTFSA/PTFE	NA	200	NA	0.12 (35°C)	Li//S cells: 600 (x = 10, 0.05C, 1.8-­3.0 V, 35°C)	[109]
Fumed SiO2 nanoparticles (7 nm, 390 m2 g−1)/[DEME][TFSA]/LiTFSA/PTFE (5 wt.%)	NA	30	NA	NA	Li//S coin cells: 690 (x = 45, 0.05C, 1.8–3.0 V, 35°C)	[110]
SiO2 nanoparticles/[PMIm][TFSI]/LiTFSI/PVDF-HFP	Electrospinning, solution casting	NA	NA	1.1 (20°C)	Li//S cells: 885 (x = 30, 0.1C, 1.5–2.8 V, 30°C)	[111]
Li1.5Al0.5Ti1.5(PO4)3 (LATP, <1 µm)/functionalized BN nanosheets/[PYR13][TFSI]/LiTFSI/PVDF-HFP/poly (1, 3-dioxolane) (pDOL)	Solution casting, hot roll pressing	NA	NA	0.679 (RT)	Li//S cells: 686.7 (x = 100, 0.2C, 1.7–2.8 V, RT)	[127]
3 wt.% nano-SiO2 (7 nm)/[PMMI][TFSI]/LiTFSI/PVDF-HFP	Solution casting	NA	NA	1.83 (RT)	Li//O2 cells (MnO2 catalyzed): 600 (x = 3, 0.05 mA cm−2, 2.0–4.3 V, RT)	[112]
Nonwoven fabrics coated nano-SiO2 polyisobutylene (PIB)/[C1C3IM][TFSI]/LiTFSI	Doctor-blade technology	30	NA	0.91 (RT)	Li//O2 coin cells (RuO2/MnO2/Super P catalyzed): 1000 (x = 150, 500 mA g−1, 2.0–4.2 V, RT, with a limited capacity of 1.0 Ah g−1)	[103]
Nano-fumed SiO2 (7 nm)/[PYR14][TFSI]/LiTFSI/PTFE	Mixing	NA	NA	0.46 (RT)	Li//O2 coin cells (no catalysts): 1000 (x = 100, 100 mA g−1, 2.0–4.6 V, RT, with a limited capacity of 1 Ah g−1)	[113]
SiO2 nanoparticles/[PYR13][TFSI]/LiTFSI	Sol-gel	300	NA	1.45 (30°C)	Li//O2 coin cells (Pt3Co nanowires catalyzed): 500 (x = 300, 0.1 A g−1, 2.0–4.5 V, 60°C, in the dry oxygen, with a limited capacity of 0.5 Ah g−1)	[114]

At present, the nanocomponents of the NIGs applied in lithium-based batteries, as listed in Table 2, exist in the nanoscale form of a nonmetal oxide (SiO₂), metal oxides (TiO₂, Al₂O₃, ZrO₂), BN, CNT, HNT, LAGP, MOF, and Vr. And most NIGs are based on nano-SiO₂. Though NIGs work as both the electrolytes and the separators, most studies focus on their electrolyte-related properties and rarely investigated their separator-related properties. Below are brief summaries of the notable developments in the area of NIGs applied in lithium-based batteries.

• (1)

  Improved mechanical properties. Recent research efforts have focused on improving mechanical properties to withstand volume changes during lithium-based battery cycling. Reinforcing the matrices with suitable various nanocomponents has been exploited to improve the mechanical stability and the battery cycling stability of the NIGs.

• (2)

  Enhanced ionic conductivity. Researchers have made unremitting efforts on enhancing the ionic conductivity of the NIGs by incorporating a variety of strategies to enhance the ion transport properties, including the introduction of nanofillers (nano-SiO₂, metal oxide nanoparticles, BN, MOF, CNTs, etc.) and the optimization of the matrices or the IL compositions. Thereof, NIGs could achieve sufficient RT ionic conductivity for applications in rechargeable lithium-based batteries.

• (3)

  Thermal stability. As the thermal stability is significant for safe lithium-based battery operations, researchers have tried their best to develop NIGs with enhanced thermal stability to withstand high temperatures (up to 175°C), involved by incorporating thermally resistant nanocomponents and selecting thermally stable ILs as well as the matrices. These research advancements make contributions to safer lithium-based battery operations and also promote the utilization of high-temperature processing techniques in the manufacture of lithium-based batteries.

• (4)

  Dendrite suppression. Causing safety issues and reducing battery performances, the formation and growth of lithium dendrites is the most critical challenge in the practical application of lithium-based batteries. Based on the famous and widely cited model proposed by Monroe and Newman, if the shear modulus of the solid electrolytes is two-fold larger than that of the Li (4.8 GPa at RT), the lithium dendrite growth could be theoretically suppressed (in a large extent).^135,136 But only increasing the modulus could not faultlessly eliminate the safety risk caused by the lithium dendrites.¹³⁷ Recent research has focused on developing antidendrite NIGs with strategies such as using specific nanocomponents, polymers, or additives to construct the inhibitive stable interface with the Li anodes, suppressing lithium dendrite growth and improving the Li-based battery safety.

• (5)

  Scalability and fabrication techniques. Recent progress has been made in the fabrication techniques for NIGs that are scalable and cost-effective. Several methods including electrospinning, in situ polymerization, solution casting (as summarized in Table 2), drop-casting,¹²¹ spin-coating,¹³⁸ and inkjet printing,¹³⁹ have been developed to produce the NIGs possessed controlled morphology as well as desired properties. Such scalable fabrication techniques are extremely crucial for large-scale production and the commercialization of rechargeable lithium-based batteries.

Overall, recent advances in NIGs as both the electrolytes and the separators for rechargeable lithium-based batteries address the key challenges including mechanical stability, ionic conductivity, thermal stability, dendrite suppression, and scalability. Continued research development in the field is expected to optimize the safety and electrochemical performances of rechargeable lithium-based batteries, facilitating a variety of energy storage applications of NIGs. Perspectives on the NIGs are as follows.

Although the crystallization kinetics, the ionic mobility, and the spectroscopic influence have been investigated,¹⁴⁰ there are few literature works that reported the relationship between the fundamental parameters and the properties of the bulk electrolytes and the separators relevant to the Li-based battery applications. The effect of compositions and size on the properties and gelation mechanisms for the NIGs has yet been investigated and there still lacks comprehensive mechanistic understanding at molecular scale. To elucidate the fundamentals and guide the future rational optimizations for rechargeable lithium-based battery applications, more experimental studies combined with the corresponding theoretical investigations are needed, such as adopting DFT calculations to predict the ESPW of the NIGs.

The favorable RT ionic conductivity of the NIGs is mainly attributed to the confined ILs. However, the cost of the ILs is an obstacle to practical lithium-based battery applications. From the viewpoint of large-scale productions, this could be addressed by scalable and continuous manufacturing of IL for the expanded market with efforts from both academic and industrial researchers.^141,142 Although the majority of the reported NIGs possessed high anodic stability (>5 V, vs. Li/Li⁺), most lithium-based battery applications were still limited in the typical cathodes of LFP and LCO, which were with relatively milder electrochemical potentials. Thereof, the application of the NIGs in high energy density lithium-based batteries assembled with both high potential cathodes and low potential anodes is necessary to explore to make the NIGs more competitive under practical operation conditions. Besides high temperatures, more investigations on low-temperature (below RT) performances are required. Taking into account the NIGs as monolithic electrolyte membranes, researchers should pay more attention to the investigation of their separator-related properties, not just the electrolyte-related properties. Comprehensive investigations on whole properties related to both electrolytes and separators need to be strengthened. In addition, infiltration into the thick porous electrodes (with high areal capacities) and accurate control of the NIG thickness are also challenged.

In conclusion, if both the desirable performances and the wide processing compatibility are realized, NIGs are extremely compelling electrolyte/separator strategies for rechargeable high-energy lithium-based batteries and beyond lithium chemistry such as aqueous rechargeable Zn-ion batteries and Zn-ion hybrid supercapacitors.

ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of China (Nos. 52002352 and 52071295), the Scientific Research Project of the Department of Education of Guangdong Province (2020KQNCX045 and 2023KTSCX074), the Scientific Research Start-up Funds of Hanshan Normal University (QD 20191101), and the Innovation Team Program of Higher Education of Guangdong, China (No. 2017KCXTD023). J. Xie thanks the financial support from the program of the China Scholarship Council (CSC, No. 202208440279).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

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