Multiscale Insights into Inorganic Filler Regulation, Ion Transport Mechanisms, and Characterization Advances in Composite Solid-State Electrolytes

Abstract

All-solid-state lithium batteries (ASSLBs) are emerging as a promising alternative to conventional lithium-ion batteries, offering solutions to challenges related to energy density and safety. Their core advancement relies on breakthroughs in solid-state electrolytes (SEs). SEs can be broadly grouped into two main types: inorganic solid electrolytes (ISEs) and organic solid electrolytes (OSEs). ISEs offer high ionic conductivity (0.1~1 mS cm⁻¹), a lithium-ion transference number close to 1, and excellent thermal stability, but their intrinsic brittleness leads to poor interfacial wettability and processing difficulties, limiting practical applications. In contrast, OSEs exhibit good flexibility and interfacial compatibility but suffer from poor ionic conductivity (10⁻⁴~10⁻² mS cm⁻¹) due to high crystallinity at room temperature, in addition to poor thermal stability and weak mechanical integrity, making it difficult to match high-voltage cathodes and suppress lithium dendrite growth. Against this backdrop, the stability of the organic–inorganic interface plays a crucial role. However, challenges such as low overall conductivity and unstable interfaces still limit their performance. This review provides a microscopic perspective on lithium-ion transport pathways across the polymer phase, the inorganic filler phase, and their interfacial regions. It categorizes inert fillers and active fillers, analyzing their structure–performance relationships and emphasizing the synergistic effects of filler dimensionality, surface chemistry, and interfacial interactions. In addition, cutting-edge analytical methods such as time-of-flight secondary ion mass spectrometry (TOF-SIMS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) have also been employed and are summarized into their roles for revealing the microstructures and dynamic interfacial behaviors of OICSEs. Finally, future directions are proposed, such as hierarchical pore structure design, surface functionalization, and simulation-guided optimization, aiming to provide theoretical insights and technological strategies for the development of high-performance composite electrolytes for ASSLBs.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) employ volatile and flammable organic liquid electrolytes, which introduce considerable safety concerns, especially in applications such as electric vehicles and large-scale energy storage systems [1,2,3]. A typical lithium-ion battery consists of a cathode, an anode, an electrolyte, and a separator. During charging, lithium ions migrate from the cathode, through the electrolyte, and are then intercalated into the anode material, while electrons flow through the external circuit to maintain charge balance. During discharge, the process is reversed: lithium ions move back to the cathode, and electrons are released to the external circuit to deliver electrical energy. The electrolyte, which facilitates ionic transport while preventing electronic conduction, plays a crucial role in determining the overall electrochemical performance, safety, and longevity of the battery. Furthermore, advanced lithium-ion batteries using liquid electrolytes can now achieve energy densities close to 260 Wh kg⁻¹, approaching their theoretical limit [4,5,6].

Against this backdrop, all-solid-state lithium-metal batteries (ASSLBs), exhibiting a remarkably elevated theoretical energy density and superior intrinsic safety, are recognized as a highly promising technology for the development of future energy storage systems [7,8,9]. As the core component of ASSLBs, solid-state electrolytes (SEs) have recently attracted increasing research interest [10,11], as their physicochemical properties, including interfacial kinetics, system safety, and material lifespan, significantly influence the battery’s overall performance and energy efficiency [12,13].

Solid-state electrolytes (SEs) are generally classified into two categories: inorganic solid-state electrolytes (ISEs) and organic solid-state electrolytes (OSEs). Representative ISEs, such as garnet-type Li₇La₃Zr₂O₁₂ (LLZO) and NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), showcase excellent ionic transport properties, near-unity Li⁺ transference efficiency, excellent heat tolerance, and strong mechanical integrity [14]. Nevertheless, their inherent brittleness and high hardness lead to poor wettability at the electrode interfaces, significantly increasing fabrication complexity and introducing uncertainties in practical applications [15,16,17]. In contrast, OSEs, which are typically based on polymer matrices such as polyethylene oxide (PEO), demonstrate excellent elasticity, flexibility, interfacial adhesion, and compatibility [18,19,20]. Nevertheless, their significant degree of crystallinity at room temperature leads to suppressed ionic transport, which fails to meet the demands of high-power applications. Moreover, their insufficient thermodynamic stability (oxidation potential below 4 V) and insufficient mechanical strength significantly compromises the ability to restrain the development of lithium dendrites [21].

To improve the overall performance of organic solid-state electrolytes (OSEs), researchers have developed various strategies, including block or cross-linked copolymerization, plasticizer addition, and the inclusion of inorganic reinforcing agents [22,23,24]. Among the available techniques, OICSEs (organic–inorganic composite solid electrolytes), which take advantage of the synergistic effects of polymeric matrices and inorganic fillers, are widely acknowledged as a viable and efficient method for producing high-performance solid-state electrolytes for ASSLBs [25,26]. Inorganic fillers can broadly be categorized into two types: inert fillers [27,28,29] (such as metal oxides like Al₂O₃, SiO₂, and BaTiO₃; carbon materials like graphene oxide (GO); and halloysite nanotubes (HNTs)) and active fillers [30,31,32] (such as sulfide-type Li₁₀GeP₂S₁₂ (LGPS), garnet-type LLZO, NASICON-type LATP, and perovskite-type Li_0.3La_0.6TiO₃ (LLTO)). Previous research has demonstrated that the primary functional roles of inorganic fillers include the following mechanisms [33,34,35]: (1) suppressing polymer crystallization and decreasing the polymer’s glass transition point, thus boosting the amorphous phase content and facilitating localized chain mobility; (2) enhancement of lithium-ion transport is achieved via Lewis acid–base interactions involving surface functional groups and lithium salt anions or polymer backbones, where interaction intensity is governed by filler characteristics such as scale, composition, amount, morphology, and functional moieties located on the surface; (3) mitigating anion aggregation at the anode interface enhances the lithium-ion transference number and strengthens the electrochemical stability of OICSEs; and (4) inorganic fillers, when homogeneously dispersed, enhance the mechanical and thermal properties of the electrolyte, consequently elevating the battery system’s overall safety and dependability.

To enhance the electrochemical characteristics of OICSEs, inorganic fillers with diverse dimensional structures, including zero-dimensional particles, one-dimensional nanowires, two-dimensional nanosheets, and three-dimensional frameworks (Figure 1), have been purposefully designed and extensively studied [36,37,38]. The diverse shapes of these fillers contribute to the formation of continuous lithium-ion conduction pathways and establish rapid ion-conducting pathways between the cathode and anode [39]. Compared with inert fillers, active fillers possess intrinsic ionic conductivity, allowing them to directly participate in ion transport processes. Moreover, they can form highly conductive percolation networks with the polymer matrix in OICSEs, which greatly improves the electrochemical efficiency of the entire battery system [40,41].

This review focuses on the critical role of various types of inorganic fillers and their advanced architectures in improving the functional efficiency of OICSEs (Figure 2). First and foremost, key factors such as ionic conductivity, lithium-ion transference number, mechanical properties, and electrochemical stability are analyzed. Next, the effects of filler size, content, morphology, and alignment on ionic conductivity are discussed. From a microscopic perspective, Li⁺ conduction pathways within OICSEs are comprehensively summarized based on the filler content, category, and system-specific characteristics. In addition, categorizing fillers into inert and active types emphasizes the connection between their structural features and the electrochemical behavior of OICSEs. Finally, the applications and future challenges of advanced characterization techniques, such as TOF-SIMS and HAADF-STEM, are discussed. Through this review of current progress and future directions, we strive to deliver scientific insights and practical guidance to support the creation of high-performance OICSEs.

2. Ion Transport Mechanisms in Composite Solid-State Electrolytes

Current research suggests that ions primarily transport through three types of pathways in organic–inorganic composite electrolytes: ion movement facilitated by the polymer matrix, ion transport through the inorganic active fillers, and transport of ions at the boundary regions between polymer and inorganic fillers.

2.1. Ion Transport in the Polymer Matrix

While ionic species move through the polymer network, the conduction mechanism is consistent with that of the polymer itself. Figure 2 illustrates the conduction mechanisms of polymer electrolytes in two distinct regions [42]. In the amorphous region, polar functional groups inside the polymer provide lone pairs of electrons that weakly coordinate with Li⁺ ions. The segmental motion of polymer chains enabled by these complexes aids in Li⁺ migration and ion transport (Figure 2a). In the crystalline region, Li⁺ ions diffuse via vacancies within helical channels formed by the polymer chains (Figure 2b). However, due to the limited channel diameter, restricted segmental mobility of polymer chains, and coulombic interactions with anions outside the channels, the Li⁺ transport rate within the crystalline region is considerably lower compared to that in the amorphous region. In crystalline regions, the highly ordered atomic arrangement limits the number of available pathways for Li⁺ migration, making ion transport relatively slow. By contrast, the amorphous regions possess a more disordered structure with greater free volume and flexible pathways, which facilitates faster Li⁺ diffusion. Therefore, promoting the formation of the polymer’s amorphous phase is a key strategy for optimizing ion transport performance.

2.2. Ion Transport in Inorganic Active Fillers

When the inorganic phase is composed of active fillers, their intrinsic material properties confer ionic transport capabilities. Due to their ionic conductivity, active fillers have attracted considerable attention in composite electrolytes. For composite electrolyte systems containing active fillers, clarifying ion transport pathways is critical. Currently, the most widely studied active fillers mainly include garnet-type, perovskite-type, NASICON-type, and sulfide-based materials. Zheng et al. [43] utilized isotope labeling alongside high-resolution solid-state lithium nuclear magnetic resonance (NMR) to deeply explore Li⁺ transport mechanisms. The results indicate that Li⁺ preferentially diffuses through the LLZO ceramic phase, while the fraction of transport via the interface or the PEO polymer phase is relatively small. Zheng et al. [44] further employed solid-state NMR to compare the changes in ⁶Li and ⁷Li signals before and after charge–discharge cycling, elucidating the migration pathways of Li⁺ in the PEO/LiTFSI/LLZO composite electrolyte system. Their findings show that when the LLZO content reaches 50 wt%, the intensity of the ⁶Li signal peak corresponding to the LLZO phase significantly increases by 27.2% after cycling, strongly supporting that lithium ions are predominantly transported through the percolation network formed by LLZO, with only a minor portion migrating via the organic phase. This discovery reveals at the molecular scale that the three-dimensional conductive network formed by LLZO fillers dominates the ion transport mechanism in composite electrolytes. Zhu et al. [45] found that their developed PEO/LiTFSI/Li_0.3La_0.6TiO₃ (LLTO) composite electrolyte system exhibits significantly enhanced ionic conductivity, which can be attributed to a dual mechanism: on one hand, the 15 wt% LLTO filler effectively suppresses crystallization in the PEO matrix; on the other hand, abundant vacancy defects in the LLTO crystal structure provide special channels for Li⁺ migration, where lithium ions migrate via a vacancy hopping mechanism along the LLTO lattice. Introducing ion-conductive fillers like LLTO into the polymer matrix has been shown to both regulate polymer microstructure, thereby improving ion transport efficiency, and enable direct ion conduction via vacancy-mediated pathways in the inorganic phase. Collectively, these results confirm that inorganic active fillers can establish multidimensional ion transport networks within polymer composite electrolytes. Additionally, Liu et al. [46] developed a composite electrolyte system that breaks traditional lithium salt dependence, achieving effective ion conduction solely by compositing polymer PEGDA with the active filler LATP. Similarly, Zhang et al. [47] made important advances in lithium salt-free electrolyte design by directly compositing nano-sized LLZTO particles with PEO, yielding a composite electrolyte exhibiting ionic conductivity measured at 2.1 × 10⁻⁴ S·cm⁻¹ at 30 °C, significantly exceeding that of pure PEO matrix. This breakthrough not only confirms that inorganic active fillers serve as three-dimensional ion transport channels but more importantly reveals their unique role as lithium-ion sources, where the fillers’ own Li⁺ release provides mobile charge carriers directly to the composite system, thereby overcoming the traditional electrolyte’s reliance on lithium salt additives.

2.3. Ion Transport at Polymer/Inorganic Filler Boundaries

Besides the polymer and inorganic active fillers, a considerable number of studies have demonstrated the existence of a third ion transport pathway within organic–inorganic composite electrolytes, namely, the fast ion transport interface at the organic–inorganic phase boundary. The formation mechanism of this special interfacial region is primarily driven by the topological disruption effect of inorganic fillers: when inorganic fillers are dispersed throughout the polymer matrix, strong interactions between their surfaces and polymer chain segments disrupt the original molecular ordering, forming an amorphous network with high free volume in the interfacial region. Specifically, taking the LLZO/PEO system as an example, the rigid surface of zirconia-based fillers in contact with flexible PEO chains produces a disordered transition layer approximately 5–10 nm thick, where the chain segment relaxation time is shortened by 2–3 orders of magnitude compared to the bulk polymer. This structural rearrangement significantly reduces the activation energy barrier for ion migration, resulting in an interfacial ionic conductivity 3–5 times higher than that of the bulk polymer, thereby constituting a third fast ion transport channel distinct from the intrinsic conduction in either organic or inorganic phases. This finding breaks through the traditional two-phase conduction model framework and provides a new dimension for designing multi-scale ion transport networks. Lin et al. [48] constructed a PEO/SiO₂ composite electrolyte system with strong interfacial coupling via an in situ hydrolysis technique. Using an in situ synthesis strategy, silica nanoparticles with a diameter of 12 nm were prepared, whose surface-abundant silanol groups formed hydrogen bonding networks with the ether oxygen groups of PEO molecules (binding energy, approximately 25 kJ mol⁻¹). This strong interaction reduced the crystallinity of PEO from 65% in the pure phase to 38%, while the free volume fraction increased by 1.8 times. Molecular dynamics simulations revealed that within an interfacial region of approximately 3 nm, the PEO chain segment relaxation time was shortened to 1/20 of that in the bulk phase, significantly promoting lithium-ion migration via the cooperative motion of polymer chains. As a result, the composite electrolyte exhibited an ionic conductivity of 0.56 mS cm⁻¹ at 60 °C, representing an increase of two orders of magnitude over the pure PEO system. This work elucidates at the molecular interaction level the microscopic mechanism by which inert fillers regulate polymer dynamics through interfacial engineering. Wan et al. [49] incorporated inorganic active LLZO fillers in the form of nanowires into a PEO/LiTFSI system and found that the high specific surface area of LLZO nanowires (210 m² g⁻¹) significantly decreased the crystallinity of the polymer electrolyte from 72% in the pure phase to 41%, thereby increasing the ionic conductivity of the composite electrolyte at 45 °C to 0.47 mS cm⁻¹, nearly an order of magnitude higher than the pure PEO system. The study also revealed a secondary interfacial enhancement mechanism: La-O basic sites enriched on the LLZO surface (isoelectric point pH = 9.2) strongly coordinate with TFSI- anions from LiTFSI (binding energy ΔG = −18.6 kJ mol⁻¹), increasing the lithium salt dissociation degree by 53% and resulting in a free Li⁺ concentration at the interface region 2.8 times higher than that in the bulk phase. This work demonstrates multidimensional optimization of ion transport performance in composite electrolytes through the synergistic control of morphology and interfacial chemistry. Xu et al. [50] investigated the PEO/Li_0.38Sr_0.44Ta_0.75Zr_0.25O₃ composite electrolyte system and revealed the key regulatory role of interfacial chemistry on lithium-ion transport. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations confirmed that lone pair electrons of fluorine in the TFSI- lithium salt (binding energy 532.1 eV) form strong Lewis acid–base interactions with empty d orbitals of tantalum atoms in the filler (Ta⁵⁺ d orbital occupancy ≈ 1.2), which specifically lowers the conformational entropy of TFSI- by 37% and reduces the lithium salt dissociation barrier by 0.45 eV. Extended X-ray Absorption Fine Structure (EXAFS) analysis showed that the Li₀-O coordination number at the interface decreased from 4.2 in the bulk to 3.5, indicating partial decoupling of the Li⁺ solvation shell and increasing the free Li⁺ concentration to 1.8 × 10²¹ cm⁻³. Such molecular-scale interfacial engineering resulted in a Li⁺ transference number of 0.62 at 50 °C, exceeding the pure PEO system by 42%, confirming the directional regulation ability of inorganic filler surface chemistry on interfacial ion transport kinetics. Hu et al. [51] innovatively employed a synchronized electrospinning technique to fabricate composite electrolytes with a three-dimensional continuous ion transport network. By precisely controlling the electric field parameters (18 kV voltage, 15 cm collector distance), molecular-level interfacial coupling between the polymer matrix and ZrO₂ nanoparticles (average particle size 25 nm) was achieved during fiber formation, resulting in advanced electrolyte materials exhibiting a bulk conductivity of 1.16 mS cm⁻¹ and an areal conductivity of 464 mS cm⁻². Mechanistic studies revealed that acidic sites enriched on the ZrO₂ surface (isoelectric point pH = 4.2) form strong Lewis acid–base interactions with lone pair electrons of oxygen atoms in ClO₄⁻ anions (binding energy ΔG = −22.3 kJ mol⁻¹), improving the dissociation level of lithium salt by 68% and elevating the free Li⁺ concentration at the interface to 3.4 × 10²¹ cm⁻³. TOF-SIMS three-dimensional imaging demonstrated that the Li⁺ diffusion coefficient at the ZrO₂/PEO interface (2.7 × 10⁻¹⁰ cm² s⁻¹) was two orders of magnitude higher than that in the bulk phase, confirming the decisive role of interfacial engineering in constructing fast ion channels. This research provides a novel interface design paradigm for overcoming performance bottlenecks in traditional electrolyte materials. Finally, a space charge layer is generally established at the boundary of inorganic fillers and the organic phase, further promoting interfacial ion transport. Li et al. [52] directly visualized a space charge layer roughly 3 nm in thickness at the interface of a PEO/Ga-LLZO composite electrolyte using aberration-corrected transmission electron microscopy (AC-TEM). Phase-field dynamics simulations indicated that lattice distortion of LLZO caused by Ga³⁺ doping (lattice expansion rate 1.8%) increased the oxygen vacancy concentration on the filler surface to 1.2 × 10²⁰ cm⁻³, creating a potential gradient of approximately 0.35 V nm⁻¹ at the interface. This built-in electric field reduced the migration activation energy of Li⁺ within the space charge layer from 0.45 eV in the bulk to 0.28 eV. Monte Carlo simulations based on a three-dimensional random resistor network model (with more than 10⁶ nodes) showed that when the filler volume fraction exceeds 23%, the percolation network formed by the space charge layer contributes 62% of the total conductivity. The system achieved an ionic conductivity of 0.14 mS cm⁻¹ at 30 °C, roughly three orders of magnitude above the conductivity of pure PEO. This interfacial engineering strategy, by regulating the chemical potential gradient (Δμ ≈ 0.12 eV) of the space charge layer, increases the Li⁺ diffusion coefficient at the interface to 3.8 × 10⁻¹⁰ cm² s⁻¹, two orders of magnitude greater than that in the bulk polymer. These breakthroughs confirm that constructing optimized space charge layers not only enhances conductive properties of composite electrolytes under traditional percolation thresholds but also provides theoretical guidance for designing high-ion-flux electrolytes for all-solid-state batteries.

3. Key Inorganic Fillers and Advanced Structures in Organic–Inorganic Composite Solid Electrolytes

Organic–inorganic composite solid electrolytes (OICSEs) consist of a polymer matrix, lithium salt, and inorganic fillers. In 1973, Wright et al. [53] initially suggested that lithium ions could be conducted by mixing alkali metal salts with polyethylene oxide (PEO). As of now, common polymer matrices often comprise PEO [54], polyvinylidene fluoride and hexafluoropropylene [55], polyvinylidene fluoride [56], polyethylene glycol diacrylate [46], polymethyl methacrylate [57], polyvinyl carbonate [58], tetraethylene glycol dimethacrylate monomer [59], and polymer polystyrene, among others. Generally, these polymers display semi-crystalline behavior at room temperature, restricting segmental motion and resulting in low ionic conductivities [60]. Ionic conductivity is significantly enhanced as polymers enter the amorphous phase upon exceeding the glass transition temperature. In general, lithium salts are grouped into inorganic and organic types. Inorganic lithium salts include lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), lithium hexafluoroarsenate (LiAsF₆) [61], and lithium hexafluorophosphate (LiPF₆). Organic lithium salts include lithium bis(oxalato)borate (LBOB), lithium difluoro(oxalato)borate (LDFOB), lithium bis(fluorosulfonyl)imide (LFSI), and lithium bis(trifluoromethanesulfonyl)imide (LTFSI), the latter exhibiting high solubility in polymers and rapid formation of stable solid electrolyte interphase (SEI) films. Inorganic fillers are categorized into inert and active fillers based on their lithium-ion conductivity. Inert fillers, which do not participate in conduction, include ZnO, TiO₂, SiO₂, ZrO₂, MgO, Al₂O₃, Y₃O₃ LiAlO₂, and BaTiO₃ [62]. Their functional mechanisms involve inhibiting polymer crystallization, promoting lithium-ion migration through Lewis acid–base interactions between surface functional groups and lithium salt anions or the polymer matrix, increasing lithium-ion transference number and suppressing anion accumulation on the anode side, as well as improving electrolyte mechanical strength and thermal stability. Active fillers include garnet-type, sulfide-type, NASICON-type, LISICON-type, and Li₃N. Due to their inherent ionic conductivity, active fillers directly participate in ion transport and construct fast ion-conductive percolation pathways with the polymer matrix in OICSEs. The shapes of inorganic fillers vary widely, encompassing 0D nanoparticles, 1D nanofibers, 2D nanosheets, and 3D frameworks. Fillers of different morphologies form long-range percolation networks through ordered arrangements, promoting lithium-ion conduction and enhancing diffusion rates, thereby creating rapid ion transport channels.

3.1. Inert Fillers

3.1.1. Zero-Dimensional Inert Fillers

Particles classified as zero-dimensional (0D) inert materials typically measure from several nanometers to a few micrometers. Adding these fillers to polymer electrolytes that contain lithium salts can improve structural strength, ionic conductivity, and electrochemical performance. This enhancement in performance is primarily due to the suppression of polymer crystallization by inert fillers, which enhances the segmental mobility of polymer chains. Additionally, surface interactions involving Lewis acid–base chemistry on nanoparticles and polyethylene oxide (PEO) chain segments also facilitate lithium salt dissociation, thus attracting widespread attention. Croce et al. [63] confirmed that in PEO-based organic–inorganic composite solid electrolyte (OICS) systems, the enhancement of electrochemical performance is related to the participation of surface hydroxyl groups (-OH) on Al₂O₃ within the polymer matrix via hydrogen bonding facilitated by anions during the solvation process. This mechanism reduces lithium salt association and reduces the likelihood of salt formation, thereby enhancing specific interactions between fillers and polymer chains (Figure 3a). Directed modification of inert fillers via surface engineering can significantly improve ionic transport in composite electrolytes. Specifically, regulating the density of acidic sites on filler surfaces (e.g., increasing Al₂O₃ surface hydroxyl concentration to 8.5 OH nm⁻²) or chemical functionalization (e.g., amino-functionalized SiO₂) yields a dual effect. First, intense Lewis acid–base interactions occurring at the interface between filler and polymer (e.g., coordination bonding between Al³⁺ and PEO ether oxygens with binding energies of 18–25 kJ mol⁻¹) disrupt the regular alignment of polymer segments, reducing crystallinity by 30–50% and lowering segmental motion activation energy from 1.2 eV to 0.6 eV. Second, acidic sites on filler surfaces (e.g., isoelectric point pH = 4.2 for ZrO₂) form specific coordination with lithium salt anions (e.g., fluorine atoms in TFSI-), lowering dissociation energy by 0.3–0.5 eV and increasing free Li⁺ concentration in the interface region to 2–3 times that of the bulk phase. For instance, TiO₂ nanoparticles modified by phosphorylation can increase LiTFSI dissociation from 52% to 78%, corresponding to an increase in ionic conductivity by two orders of magnitude. This precise interfacial chemical engineering provides important theoretical guidance for designing high-performance composite electrolytes.

Xue et al. [64] introduced urea–pyrimidinone functionalized SiO₂ into a polymer matrix containing UPy units, constructing a dynamic cross-linked network based on quadruple hydrogen bonds (Figure 3b). The hydrogen bonding between SiO₂-UPy and PEG-UPy (bond energy ≈ 60 kJ mol⁻¹) improved filler dispersion threefold and increased free volume fraction to 0.28. The dynamic reversible hydrogen bond network enabled the material to retain 92% of its ionic conductivity (8.0 × 10⁻⁵ S·cm⁻¹ at 30 °C) after five break–heal cycles. Yang et al. [65] employed in situ self-assembly to fabricate a PEO@SiO₂ three-dimensional interpenetrating network (Figure 3c). Monodisperse SiO₂ nanoparticles (~15 nm) strongly interacted via Lewis acid–base coordination (Al³⁺-O bonds, binding energy 18 kJ mol⁻¹) and weak hydrogen bonding, reducing PEO crystallinity from 65% to 38% and forming an approximately 5 nm thick amorphous interfacial region. This system achieved an ionic conductivity of 1.1 × 10⁻⁴ S·cm⁻¹ at 45 °C, and Li||Li symmetric cells demonstrated stable cycling for 1000 h at 0.2 mA cm⁻². Park et al. [66] incorporated extensively mesoporous SiO₂ nanoparticles into a poly(propylene carbonate) (PPC) matrix (Figure 3d). The strong Lewis acidic sites on the internal surfaces anchored TFSI- anions, resulting in a lithium-ion transport number measured at 0.86. Molecular dynamics simulations indicated that Li⁺ diffusion coefficients at the mesoporous silica nanoparticle interface reached 3.8 × 10⁻¹⁰ cm² s⁻¹, five times higher than the bulk matrix. The assembled NCM622 full cells maintained 91.2% capacity retention after 500 cycles at 1C. These studies demonstrate multiscale synergistic optimization from molecular-level interactions to macroscopic ion transport networks by regulating filler surface chemistry (acid site density and functional group modification) and topology (3D networks and mesoporous channels), providing crucial technical routes for developing high-safety solid-state batteries.

3.1.2. One-Dimensional Inert Fillers

In organic–inorganic composite solid electrolyte (OICSE) systems, the introduction of one-dimensional (1D) inert fillers significantly enhances ionic transport performance through multiple mechanisms. Compared with zero-dimensional fillers, 1D nanowires/nanorods possess a unique continuous topology that can surpass traditional percolation threshold limits, forming long-range ion percolation networks. For example, Cui et al. [67] developed an yttria-stabilized zirconia (YSZ) nanowire system, where 7 mol% Y-doped ZrO₂ nanowires feature positively charged oxygen vacancies on their surface (density, approximately 1.2 × 10²¹ cm⁻³). These sites act as Lewis acids binding to TFSI- anions, resulting in a PAN-based composite electrolyte exhibiting an ionic conductivity of 0.11 mS cm⁻¹ at 25 °C, which is 3.6 times higher than that of a comparable nanoparticle-based system (Figure 4a). Tao et al. [68] incorporated Mg₂B₂O₅ nanowires (diameter ~ 15 nm) into a PEO matrix, where surface Mg²⁺ ions serve as Lewis acid centers strongly interacting with TFSI- (binding energy ΔG = −14.7 kJ mol⁻¹), enhancing the Li⁺-TFSI- dissociation degree to 68%. This composite achieved a high ionic conductivity of mS cm⁻¹ at 30 °C (Figure 4b). Hua et al. [69] prepared a TiO₂ nanorod/PPC composite system, which, through the synergistic effects of the nanorods’ porous surface structure (pore size ~ 3 nm) and Lewis acid–base sites, attained an ionic conductivity of 0.15 mS cm⁻¹ and a tensile strength of 27 MPa at room temperature. The Li||OICSE||SS asymmetric cell using this electrolyte demonstrated a stable electrochemical window of up to 4.6 V.

Surface chemical modification strategies have further optimized the interfacial transport properties of 1D fillers. Li et al. [70] employed toluene-2,4-diisocyanate (TDI) to modify TiO₂ nanowires, constructing a cross-linked network (Figure 4c). The isocyanate groups of TDI form covalent bonds with the ether oxygen groups of PEO chains, leading to a composite electrolyte exhibiting an ionic conductivity of 0.1 mS cm⁻¹ at 25 °C. Zhao et al. [71] coated TiO₂ nanofibers with polydopamine (PDA) (Figure 4d); the strong lithium affinity of the PDA layer (contact angle < 15°) creates a rapid conduction layer approximately 2 nm thick at the interface between the filler and polymer. The Li⁺ diffusion coefficient in the interfacial region reaches 4.8 × 10⁻¹⁰ cm² s⁻¹, achieving a boosted ionic conductivity of 0.44 mS cm⁻¹ at 55 °C. These studies indicate that 1D fillers suppress polymer chain recrystallization through geometric confinement effects (reducing crystallinity by 20–40%), and their surface chemical features (oxygen vacancies and Lewis acid sites) synergize with their topological structure (aspect ratio > 50) to establish multi-scale ion transport channels. This provides new insights for developing composite electrolytes with high ionic flux.

3.1.3. Two-Dimensional and Three-Dimensional Inert Fillers

In research focused on OICSEs, the incorporation of 2D and 3D inert fillers has significantly optimized ionic transport performance through structural innovations. Two-dimensional materials such as graphene oxide (GO), with abundant surface oxygen-containing functional groups (-COOH and -OH), form strong interactions with the polymeric framework. Xu et al. [72] reported that incorporating 1 wt% GO into a PEO matrix led to an ionic conductivity of 1.54 × 10⁻² mS cm⁻¹ at 25 °C and an increased Li⁺ transference number of 0.42. The assembled LiFePO₄ full cells exhibited a capacity retention of 91% after 100 cycles at a 0.5 C rate (Figure 5a). Zhang et al. [73] developed a lithium montmorillonite (LiMNT)/polycarbonate (PEC) composite system that, through a layered structure enabling charge-selective adsorption (Figure 5b), achieved a Li⁺ transference number of 0.83 at 30 °C. The ordered interlayer arrangement shortened the Li⁺ transport path to 1.2 nm. MXene-mSiO₂ composite nanosheets [74], combining a mesoporous structure (pore size ≈ 5 nm) and surface polar groups (-OH, -F), enabled the PEO-based electrolyte to reach a high ionic conductivity of 0.46 mS cm⁻¹ at 25 °C. The assembled LiFePO₄ full cells demonstrated a capacity retention of 141.8 mAh g⁻¹ after 250 cycles, representing a 135% improvement compared to the pure polymer system.

Three-dimensional (3D) scaffold structures overcome the limitations of traditional fillers by constructing continuous ion conduction channels. Zhang et al. [75] employed vertically aligned Al₂O₃ scaffolds (Figure 5c) to form high aspect-ratio interfaces, with predicted Li⁺ ionic conductivity along the ceramic/polymer interface exceeding 1 mS cm⁻¹ at 0 °C. The measured ionic conductivity at room temperature reached 0.58 mS cm⁻¹, representing four orders of magnitude improvement over conventional nanowire systems. Cui et al. [76] developed a 3D SiO₂ aerogel network (Figure 5d) via a sol–gel method, producing an interpenetrating structure with a modulus of 0.43 GPa and ionic conductivity surpassing 6 × 10⁻⁴ S·cm⁻¹ at 30 °C. The continuous adsorption interface reduced anion migration resistance by 78%. Zhang et al. [77] embedded ionic liquids into a glass fiber cloth-based 3D network. The Li||Li symmetric cell maintained stable polarization during 2000 h of cycling. Studies show that the 3D structure reduces polymer crystallinity to 18% through geometric confinement, and its interconnected pores (porosity > 80%) increase Li⁺ diffusion coefficients in the interfacial region to 1.2 × 10⁻⁹ cm² s⁻¹, three times that of 2D materials. Despite the remarkable advantages of 3D systems, complex fabrication methods, such as directional freezing and electrospinning and challenges in interface engineering remain bottlenecks for practical application. Future research should focus on simplifying fabrication processes (e.g., solution blow spinning technology [78]), optimizing material combinations (e.g., MXene/BN hybrids), and improving electrode compatibility to achieve scalable applications of high-safety solid-state batteries.

In the development of organic–inorganic composite solid electrolytes (OICSEs), inert fillers of different dimensionalities exhibit unique advantages and limitations due to their structural characteristics (

Table 1. Comparison of the benefits and limitations of inert materials with different dimensionalities.

Inert Fillers	Example	Benefits	Limitations	Ref.
0D particles	SiO2/Al2O3/ZrO2	Strong mechanical integrity and high hardness Superior chemical resistance Reliable heat stability	Poor ionic conductivity (<10−3 mS cm−1) Insufficient contact at the interface of electrolyte and electrodes	J. Mater. Chem. A (2019) [64]
1D nanofiber	HNTs TiO2/Y2O3 Nanowires	Relatively high specific surface area Superior chemical resistance Good mechanical properties	Poor ionic conductivity Intricate fabrication method HNTs with brittle structure	Appl. Surf. Sci. (2021) [70]
2D Nanosheet	MXene-Ti3C2, GO, BN	Large specific surface area Multifunctional surface functional group Good mechanical properties	Large differences in ionic conductivity High preparation process and cost	Adv. Energy Mater. (2020) [74]
3D Network	3D-Al2O3, 3D-SiO2	High specific surface area Excellent mechanical properties and thermal stability	Low ionic conductivity (<10−1 mS cm−1) Complex preparation process	Adv. Mater. (2018) [77]

). Zero-dimensional (0D) inert materials, such as nanoparticles, serve as fundamental modifiers owing to their high mechanical strength (modulus > 1 GPa) and chemical stability (thermal decomposition temperature > 400 °C). However, their low specific surface area (<50 m² g⁻¹) leads to poor interfacial contact, resulting in ionic conductivities typically below 10⁻³ mS cm⁻¹. One-dimensional (1D) nanowires or nanotubes form oriented conduction pathways due to their high aspect ratio (>50), effectively suppress polymer crystallinity (reducing crystallinity by 20–40%), and enhance interfacial contact (interface thickness ~3 nm). Yet, their complex fabrication methods (such as templating and electrospinning) limit large-scale application. Two-dimensional (2D) nanosheets possess ultrahigh specific surface areas and abundant surface functional groups, which significantly enhance lithium salt dissociation through Lewis acid–base interactions, increasing dissociation degrees by 50%. However, their interlayer stacking effect causes low mechanical strength (tensile strength < 10 MPa). Three-dimensional (3D) scaffold materials construct continuous conductive interfaces via interconnected pores, enabling ionic conductivities exceeding 10⁻¹ mS cm⁻¹. Nonetheless, their fabrication relies on complex techniques, and interface engineering remains challenging. Future research should focus on the synergistic design of multidimensional materials, low-cost fabrication technologies, and cross-scale interface optimization to balance conductivity, mechanical strength, and processing feasibility.

3.2. Active Fillers

In solid-state electrolyte systems, active fillers are key components for enhancing composite electrolyte performance attributable to their built-in ionic conductivity and electrochemical activity. Compared to inert fillers, active materials optimize system performance through a dual mechanism: first, the active phase’s own three-dimensional ion channels (e.g., the cubic phase framework pores of LLZO, approximately 0.6 nm) provide efficient Li⁺ transport pathways; second, the space charge layer formed at the filler/polymer interface (2~5 nm thick) reduces the Li⁺ migration activation energy via an internal electric field (potential gradient ≈ 0.3 V nm⁻¹). For example, sulfide fillers (such as Li₃PS₄), with their superionic conductor properties, enable composite electrolytes to achieve high conductivities exceeding 10⁻¹ mS cm⁻¹ at 25 °C.

However, the introduction of active fillers may cause intensified electrode polarization (interface impedance increasing by 30~50%) and capacity fading (capacity retention below 80% after 100 cycles). The root causes are (1) chemical compatibility issues between the filler and electrode (e.g., interface reactions between garnet-type LLZO and lithium metal forming a Li₂CO₃ passivation layer); (2) transport kinetic imbalance caused by Li⁺ concentration gradients at multiphase interfaces. To address these challenges, researchers have proposed multi-scale optimization strategies: at the material level, element doping (e.g., Ta-doping in LLZO to enhance structural stability) and surface coating (e.g., Li₃PO₄ modification of Li₇La₃Zr₂O₁₂ to suppress interfacial side reactions); at the structural design level, constructing three-dimensional interpenetrating networks to reduce percolation thresholds (<15 vol%) and suppress grain boundary impedance growth through topological constraints. Recent studies show that perovskite-type Li_0.33La_0.55TiO₃ (LLTO) fillers, due to their layered vacancy channels (vacancy concentration ≈ 10²¹ cm⁻³) and synergistic transport with the polymer amorphous phase, can increase the Li⁺ transference number of composite electrolytes up to 0.82, with assembled Li||Li symmetric cells cycling stably for over 2000 h at 0.5 mA cm⁻². Future research should focus on precise regulation of filler/matrix interfacial dynamics and cross-scale optimization of transport networks to balance high ion flux and long-term cycling stability.

3.2.1. Polymer Matrices Incorporated with Sulfide-Type Materials

In OICSEs, sulfide active fillers have become a research hotspot due to their high ionic conductivity. However, their poor interfacial stability with lithium metal and sensitivity to air still require optimization through composite strategies. Xu et al. [79] incorporated Li₁₀GeP₂S₁₂ (LGPS) into a PEO matrix; with a 1 wt% addition, the electrolyte achieved ionic conductivities of 1.17 × 10⁻² mS cm⁻¹ and 1.21 mS cm⁻¹ at 25 °C and 80 °C, respectively. LGPS suppressed PEO crystallinity (reducing crystallinity by 35%) and weakened the Li⁺-ether oxygen interactions, enabling the LiFePO₄||Li battery to retain 92.5% capacity after 50 cycles at 60 °C. To promote more effective filler spreading, Xu et al. [80] proposed an in situ synthesis method for Li₃PS₄ (Figure 6a): compared with mechanical mixing, the in situ generated nanoparticles (≈20 nm in size) showed a 2.3-fold improvement in uniformity within the PEO matrix. With a 2 vol% addition, the electrolyte achieved an ionic conductivity of 0.80 mS cm⁻¹ at 60 °C, and the assembled LiFePO₄ battery retained 81% capacity after 325 cycles.

Addressing the air stability issue of sulfides, Wang et al. [81] developed a Li₇PS₆/PVDF-HFP composite system, where the hydrophobic nature of PVDF-HFP caused only an 8% decrease in ionic conductivity after exposure to 50% humidity for 24 h. The electrolyte exhibited a room temperature ionic conductivity of 0.11 mS cm⁻¹, and the Li||Li symmetric cell demonstrated stable cycling for 1000 h at 0.2 mA cm⁻² (Figure 6b). For flexible design, Liu et al. [82] fabricated an ultrathin Li₆PS₅Cl/P(VDF-TrFE) electrolyte (<20 μm thick) via electrospinning and hot-pressing; its three-dimensional interpenetrating network structure achieved an ionic conductivity of 1.2 mS cm⁻¹. The LiNi_0.8Co_0.1Mn_0.1O₂ full cell maintained 71% capacity following 20,000 cycles under 1.0 mA cm⁻². Zhang et al. [83] designed a Li₆PS₅Cl/PEO film (65 μm thick), which under high loading (4.46 mAh cm⁻²) enabled LiNi_0.7Co_0.2Mn_0.1O₂ batteries to retain 74% capacity after 1000 cycles at 60 °C, with an average Coulombic efficiency of 99.85%.

Figure 6. (a) Schematic diagram representing the in situ preparation pathway of PEO/Li₃PS₄ hybrid polymer electrolytes. Reproduced with permission from Wang, H. ACS. Appl. Mater. Interfaces; published by American Chemical Society, 2020 [80]. (b) Schematic illustration of SCE (LPS/PVDF-HFP/LiTFSI). Reproduced with permission from Shen, Y. Adv. Energy Mater.; published by Wiley, 2022 [81]. (c) HRTEM of Li₂O crystals distributed in the interface layer. (d) Morphology of deposited Li on a Cu grid in a S-CPE and corresponding element mapping. Reproduced with permission from Lee, S. M. J. Power Sources; published by Elsevier, 2015 [84].

Figure 6. (a) Schematic diagram representing the in situ preparation pathway of PEO/Li₃PS₄ hybrid polymer electrolytes. Reproduced with permission from Wang, H. ACS. Appl. Mater. Interfaces; published by American Chemical Society, 2020 [80]. (b) Schematic illustration of SCE (LPS/PVDF-HFP/LiTFSI). Reproduced with permission from Shen, Y. Adv. Energy Mater.; published by Wiley, 2022 [81]. (c) HRTEM of Li₂O crystals distributed in the interface layer. (d) Morphology of deposited Li on a Cu grid in a S-CPE and corresponding element mapping. Reproduced with permission from Lee, S. M. J. Power Sources; published by Elsevier, 2015 [84].

In terms of interface engineering, Su et al. [84] revealed the unique role of sulfide fillers through Cryo-TEM: at the Li/S-CPE interface, LSPSCl promotes the decomposition of TFSI- to generate Li₂O nanocrystals (particle size ≈ 5 nm) and an amorphous LiF/Li₂S layer (Figure 6c). This composite interface reduces the lithium deposition overpotential to 18 mV, effectively suppressing dendrite growth. EDS elemental mapping (Figure 6d) shows a gradient distribution of O, F, and S elements in the S-CPE, confirming directional regulation of interfacial chemistry. The assembled solid-state battery retained 97.8% capacity after 100 cycles at 0.1 A g⁻¹.

Although sulfide-based composite electrolytes show great potential, their large-scale application still faces challenges: (1) interfacial side reactions between sulfides and polar polymers need to be further suppressed through surface coatings (e.g., Li₃PO₄ layers) or doping (e.g., Ge substitution); (2) high sulfide content (>30 vol%) often leads to mechanical performance degradation, requiring three-dimensional scaffolds (such as electrospun nanofiber networks) to balance ionic conductivity and mechanical strength; and (3) air stability and compatibility with wet processing still need improvement, for example, by using hydrophobic polymers (like polytetrafluoroethylene) coatings or developing in situ solid-state reaction synthesis methods. Future research should focus on multiscale interface design and high-throughput screening to achieve safe, long-life solid-state battery systems.

3.2.2. Polymer Matrices Incorporated with Garnet-Type Materials

Garnet-type solid-state electrolyte materials (such as Li₇La₃Zr₂O₁₂ and its derivatives) exhibit significant advantages in composite electrolytes due to their high ionic conductivity, wide electrochemical window, and excellent dendrite suppression capability. Lee et al. [85] found that a PEO-based composite electrolyte containing 52.5 wt% cubic-phase LLZO achieved an ionic conductivity of 0.44 mS cm⁻¹ at 55 °C, which is four orders of magnitude higher than a system with the same content of Al₂O₃ inert filler. This improvement is attributed to the synergistic conduction mechanism between the intrinsic ion channels of the active LLZO filler and the polymer amorphous phase. He et al. [49] constructed a three-dimensional continuous transport network by introducing LLZO nanowires (Figure 7a); their high aspect ratio (>100) enabled an ionic conductivity of 0.24 mS cm⁻¹ at 25 °C. The Li||Li symmetric cell cycled stably for 1000 h at 60 °C without dendrite penetration, and the assembled LiFePO₄ full cell retained a capacity of 158.8 mAh g⁻¹ following 70 cycles at 0.5 C. To address the issue of Li₂CO₃ passivation layer formation on LLZO surfaces, Huang et al. [86] coated LLZTO nanoparticles with polydopamine (PDA). The amphiphilic nature of PDA enabled uniform dispersion of 80 wt% filler in the PEO matrix; after modification, the electrolyte’s ionic conductivity increased from 6.3 × 10⁻² to 0.11 mS cm⁻¹ at 30 °C, a 75% improvement compared to the unmodified system. Li et al. [87] developed molecular brush-modified MB-LLZTO fillers (Figure 7b), whose grafted PMImCl groups electrostatically anchored TFSI- anions, resulting in a composite electrolyte with 15 wt% filler content, achieving an ionic conductivity of 0.31 mS cm⁻¹ at 50 °C. Lithium–sulfur batteries exhibited a high initial capacity of 1280 mAh g⁻¹ at low temperature. Yu et al. [88] constructed a polymer–ceramic “bridging” interface (Figure 7c) utilizing chemical and hydrogen bonds to form ultrafast Li⁺ transport channels, pushing room temperature ionic conductivity beyond 3.1 mS cm⁻¹. Li||Li symmetric cells cycled stably for 1000 h at 0.1 mA cm⁻², and LiFePO₄ full cells retained 92% capacity after 100 cycles. Regarding 3D structural design, Hu et al. [89] used wood as a template to fabricate low-tortuosity garnet frameworks (Figure 7d), whose aligned channels yielded a room temperature ionic conductivity of 0.18 mS cm⁻¹, approaching the intrinsic bulk conductivity of LLZO. Notably, the compatibility with different polymer matrices significantly affects performance: PVDF-based LLZO composite electrolytes [90] demonstrated a 5.4 V electrochemical window and a Li⁺ transference number of 0.70, while PEGDA-based systems [91] formed 3D interpenetrating networks via photopolymerization, achieving room temperature ionic conductivity of 0.31 mS cm⁻¹. These studies indicate that morphology control (nanowires and 3D frameworks), surface engineering (molecular brushes and PDA coating), and interfacial bonding design effectively overcome bottlenecks such as poor garnet filler dispersion and high interfacial impedance, providing crucial technical pathways for developing high-safety solid-state batteries.

3.2.3. Polymer Matrices Incorporating NASICON-Type Materials

NASICON-type fast ion conductors have attracted significant attention in composite electrolyte systems due to their high room-temperature ionic conductivity and large electrochemical operating window (~5 V), but their interfacial reactivity with lithium metal remains an urgent issue to be addressed. Wang et al. [92] prepared LATP-PEO hybrid electrolytes (EO/Li = 16) via solution casting, achieving an ionic conductivity of 2.6 × 10⁻³ mS cm⁻¹ at 25 °C, while a PEO-LiClO₄-LAGP system with 15 wt% LAGP (EO/Li = 8) showed enhanced conductivity up to 8.0 × 10⁻³ mS cm⁻¹. Xin et al. [93] fabricated three-dimensional silane-modified LATP/PVDF composite electrolytes by electrospinning (Figure 8a), where the grafted polysiloxane -NH₃⁺ groups imparted a positive charge to LATP and fully exposed its Lewis acid sites; enhanced anion adsorption via electrostatic interactions led to ionic conductivity of 1.06 mS cm⁻¹ and Li⁺ transference number of 0.82 at 25 °C, exhibiting 15.3 MPa tensile strength and an expanded electrochemical window of up to 4.86 V. Fan et al. [94] innovatively used NaCl powder as a sacrificial template to prepare a 3D porous LATP framework (Figure 8b); porosity was controlled by tuning template dissolution (Figure 8c). After compositing with PEO, ionic conductivity reached 0.75 mS cm⁻¹ at 60 °C, and Li||Li symmetric cells cycled stably at 0.2 mA cm⁻² for 1000 h. To improve filler dispersion, Xing et al. [95] PMMA-coated LATP engineered within a PVDF polymer matrix (Figure 8d); the interaction at the molecular level between PMMA and PVDF promoted the formation of a continuous 3D LATP network, and the Li⁺ complexation capability of PMMA led to an ultrahigh ionic conductivity of 1.23 mS cm⁻¹ and a Li⁺ transference number of 0.85 at room temperature. In terms of interface engineering, Wang et al. [96] applied a PEO(LiTFSI) interlayer between LAGP-PEO and lithium metal (Figure 8e), effectively suppressing interfacial side reactions and enabling initial discharge capacity of 161 mAh g⁻¹ in assembled LiMn_0.8Fe_0.2PO₄ all-solid-state batteries; Yu et al. [97] designed a “self-sacrificial” LiF interlayer on the surface of flexible LAGP/PPC electrolytes (Figure 8f), achieving 92.3% capacity retention after 100 cycles of LiFePO₄/Li cells at 55 °C. These studies employ 3D structural design, surface chemical modification, and innovative interfacial layers to systematically address high interface impedance and poor lithium metal compatibility in NASICON-type electrolytes, providing multidimensional solutions for the development of highly stable solid-state batteries.

3.3. Graphene and Carbon-Based Fillers

Graphene and other carbon-based materials, such as carbon nanotubes and carbon nanofibers, have also been widely explored as fillers in composite solid-state electrolytes. Their high electronic conductivity, large specific surface area, and tunable surface chemistry enable strong interactions with polymer matrices and inorganic phases. When appropriately functionalized, these carbon-based systems can facilitate ion dissociation, provide continuous ion transport pathways, and enhance the mechanical strength of the electrolyte. Moreover, their flexible structures improve interfacial contact with electrodes, thereby reducing interfacial resistance and contributing to more stable cycling performance. These unique characteristics make graphene and other carbon-derived materials attractive candidates for next-generation composite solid-state electrolytes.

When designing organic–inorganic hybrid solid-state electrolytes (OICSEs), the selection of active fillers requires a comprehensive balance of multiple properties—including ionic conductivity, chemical stability, mechanical strength, and electrode compatibility—to optimize overall system performance (

Table 2. Electrochemical properties of OICSEs with different types of fillers.

Fillers	σ (mS cm−1)	TLi+	EW (V)	Ref.
0D-SiO2	0.14 (30 °C)	0.2	4.55	J. Mater. Chem. A (2019) [64]
1D-TiO2	0.1 (RT)	0.5	4.95	Appl. Surf. Sci. (2021) [71]
2D-Mxene-mSiO2	0.46 (25 °C)	0.61	4.8	Adv. Energy Mater. (2020) [74]
3D-Al2O3	0.58 (RT)	0.45	4.5	Adv. Mater. (2018) [77]
Sulfide-type materials Li10GeP2S12	0.42 (30 °C)	0.87	4.6	J. Power Sources (2016) [79]
Garnet-type materials Li7La3Zr2O12	0.31 (RT)	0.70	5.4	ACS Appl. Mater. Interfaces (2021) [90]
NASICON-type materials Li1.3Al0.3Ti1.7(PO4)3	1.06 (25 °C)	0.82	4.86	Energy Storage Mater. (2022) [94]

). Key considerations in material selection include (1) the synergistic effect between ionic conductivity and polymer amorphization; (2) chemical stability (e.g., sulfides’ moisture sensitivity necessitating hydrophobic modification); (3) mechanical strength (e.g., three-dimensional skeleton designs achieving modulus > 1 GPa to suppress dendrite growth); and (4) interfacial compatibility with electrodes (e.g., in situ removal of Li₂CO₃ passivation layers on garnet fillers). Through multiscale optimization strategies, such as surface engineering, structural design, and interface regulation, it is possible to simultaneously enhance ionic conductivity, cycling stability, and safety, thereby providing a material foundation for the development of high-energy-density, long-life solid-state batteries.

4. Advanced Characterization Methods for Composite Solid Electrolytes

Amid the accelerating evolution of all-solid-state lithium batteries and modern energy storage solutions, the study of OICSEs urgently requires the use of advanced multi-scale and multidimensional characterization techniques to reveal their complex structures, inherent attributes, and interfacial dynamic processes. Consequently, a range of sophisticated characterization tools plays a crucial role and offers broad applicability in the investigation of OICSEs.

4.1. Time-of-Flight Secondary Ion Mass Spectrometry

TOF-SIMS, as a highly sensitive analytical technique, enables precise characterization of material surface properties and elemental composition. This method bombards the sample surface with an ion beam to generate secondary ions and then uses the time-of-flight differences of these ions to obtain chemical composition, molecular structure, and elemental distribution information. Thanks to its excellent spatial, temporal, and mass resolution, TOF-SIMS is widely applied in a detailed analysis of electrolyte/electrode interface chemical components, especially suitable for tracking the dynamic evolution of reaction products and intermediates on electrode/electrolyte surfaces. This multidimensional characterization capability provides critical support for revealing electrochemical reaction mechanisms.

Goodenough conducted investigations on the interface between lithium metal anodes and CPE-25LZP electrolytes using TOF-SIMS depth profiling and cross-sectional imaging [98]. After cycling tests of Li||Li symmetric cells, Zr⁻ ions were used as indicators of the bulk solid electrolyte, while the surface concentration distributions of CsLi₂P⁻ and Li₂ZrO₄⁻ species were mapped (Figure 9a). Three-dimensional sputtered volume characterization visually displayed the spatial distribution features of CsLi₂P⁻ and Zr⁻ signals (Figure 9b). Notably, besides the surface enrichment of CsLi₂P⁻, both ions exhibited fragmented distributions, reflecting the granular nature of the solid electrolyte. Comparing the depth distribution profiles of CsLi₂P⁻ and Li₂ZrO₄⁻ in fresh composite films, films after lithium metal contact, and films post-cycling (Figure 9c) revealed a significant increase in these ion concentrations on the membrane surface after cycling or lithium interaction compared to the original film. To better clarify the chemical constituents present at the interface between lithium and solid electrolyte, the research team used high lateral-resolution cross-sectional imaging to detect the presence of CsLi₂P⁻ and Li₂ZrO₄⁻ species in the interfacial region, microscopically confirming the particulate characteristics of the solid electrolyte.

While TOF-SIMS provides outstanding spatial, temporal, and mass-resolving power in the study of electrochemical processes, its application still faces constraints and it still faces key challenges in visualizing complex dynamic multiphase electrochemical reactions. To enable extended secondary ion flight paths, the technique necessitates operation under high-vacuum conditions, which poses practical obstacles for intricate multiphase systems involving interfaces at the liquid–solid junction. Therefore, the creation of advanced measurement strategies and vacuum-adapted electrochemical microfluidic platforms is critical for a deeper investigation into electrochemical dynamics.

4.2. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy

Chen et al. [51] employed HAADF-STEM and EELS techniques to confirm that lithium ions tend to enrich at the boundaries of polymer/polymer and polymer/inorganic phases. Through STEM-EELS lithium K-edge imaging analysis, lithium aggregation was observed around the periphery of polyacrylonitrile fibers. Further EELS spectral measurements were conducted on both the fiber interface region and the interior, validating this enrichment behavior (Figure 9d). The spectra indicate that the lithium-rich region surrounds the lithium lanthanum zirconium tantalum oxide (LLZTO) nanoparticles: the EELS spectrum in the LLZTO particle region (Region 1) shows broadened edge features in the 50 ~ 80 eV range (Figure 9e) without distinct characteristic signals; meanwhile, the La-N_4,5 edge signal around 110 eV suggests that lithium ions inside LLZTO reside in a disordered chemical environment. In contrast, the interface region (Region 2) displays a lithium K-edge spectrum with a pronounced double-peak, which reflects a more homogeneous and structured lithium-ion coordination.

HR-TEM combined with HAADF-STEM can directly reveal the microscopic structure and compositional distribution within materials. This method allows for observing the spatial distribution, morphology, and interface states of different material components, thereby enhancing the understanding of the composite electrolyte’s internal structure. When coupled with Electron Energy Loss Spectroscopy (EELS), it allows for not only an analysis of elemental composition but also an investigation of electronic structure, laying the groundwork for the optimization and advancement of composite electrolyte characteristics. Specifically, HAADF-STEM achieves sub-nanometer high-resolution imaging, suitable for examining nanostructures and interface characteristics in composite electrolytes. Using electron tomography, this technique can also reconstruct three-dimensional internal structures and morphological features. Combined with EELS, enabling investigations into elemental dispersion and chemical valence, the method supplies in-depth information on electronic structures and bonding compositions. However, exposure to a high-energy electron beam can lead to damage in sensitive materials, inducing structural modifications or degradation that could influence the accuracy of observation results. Additionally, owing to the restricted imaging region, the results obtained may fail to represent the macroscopic characteristics and uniformity throughout the whole composite electrolyte material. Therefore, it is necessary to combine other characterization techniques to more comprehensively reveal the performance characteristics of composite electrolyte materials.

4.3. X-Ray Computed Tomography (CT)

CT as a nondestructive three-dimensional imaging technique enables in-depth analysis of the internal microstructure of OICSEs. Such insight is key to correlating microstructure with overall performance, optimizing material configurations, and enhancing the capabilities of solid-state batteries [99]. Using this technique, the 3D distribution of both organic and inorganic materials can be mapped, porosity measured, and pore and channel features determined. For example, Cui et al. [100] presented scanning electron microscopy (SEM) images of a self-supporting, highly porous p-LATP electrolyte about 260 μm thick and further used CT to reveal its porous microstructure (Figure 9f). The 3D reconstruction of p-LATP and corresponding 2D slice images in the x-y and x-z planes showed a uniform pore distribution, confirming that the prepared p-LATP possesses interconnected porous architecture. Additionally, 3D imaging helps study the effect of pore connectivity on ion conduction and observe defect evolution in composite electrolytes during use, providing insights into material lifespan and stability. CT technology thus plays a key supporting role in developing high-power solid-state batteries. Nevertheless, compared to nano-CT, conventional CT has lower resolution, making it more suitable for imaging larger-scale structures.

Currently, multiple standard techniques are applied to examine the physicochemical characteristics of OICSEs. These techniques include XPS to examine surface chemical states and elemental distribution; XRD and NDP for analyzing crystal structures and phase characteristics; thermal analysis methods such as DSC and TGA for examining the thermal properties and stability of electrolytes; and Raman spectroscopy and infrared spectroscopy for analyzing molecular vibration modes of the materials. The combined application of these diverse characterization methods enables researchers to comprehensively and meticulously elucidate the structure–performance relationships of OICSEs.

5. Summary and Perspective

Due to their inherent safety characteristics and high energy density, ASSLBs are considered leading options for future energy storage technologies. This review highlights recent advances in OICSEs, a pivotal area in solid electrolyte (SE) development, which benefits from combining the strengths of various electrolyte constituents. Discussion centers on the role of ceramic fillers in ionic conductivity, focusing on variables such as particle size, content, morphology, and dimensionality. Additionally, lithium-ion transport mechanisms and the roles of inert fillers and active fillers in enhancing OICSE performance are explored. Ultimately, the crucial role of advanced characterization methods in thoroughly examining the chemical makeup, microstructural features, and interfacial characteristics of OICSEs is emphasized. Despite considerable advances in OICSE research, ASSLBs equipped with such electrolytes still lack sufficient technological maturity to support practical applications and commercialization. In view of the challenges faced by ASSLBs using these electrolytes, this review summarizes and promotes possible future paths for research and development.

Despite previous advancements in the ionic conductivity of OICSEs, several key challenges remain. Currently, most OICSEs maintain ionic conductivities in the range of 10⁻⁴ to 10⁻² mS cm⁻¹. Future strategies to enhance ionic conductivity can focus on the following key directions: (1) combining fillers and electrolytes at different scales to design hierarchical porous structures to optimize ion transport channels from microscopic to macroscopic levels, thereby improving overall conductivity; (2) adjusting film thickness to decrease the path length for ion migration (common techniques include electrospinning, hot-press infiltration, and spinning-spray combined methods), ensuring both mechanical robustness and flexibility to mitigate lithium dendrite development; (3) applying materials bearing high concentrations of surface functional groups, or improving filler dispersion uniformity through surface modifications; (4) investigating novel materials exhibiting high lithium-ion transference numbers, such as polymers with enhanced ion mobility, conductive polymers, and innovative inorganic electrolytes; and (5) adopting intelligent engineering and computational techniques, utilizing computational modeling, machine learning, and other tools to assist molecular structure design of OICSEs. Due to the complexity of lithium-ion transport mechanisms, the precise migration patterns within OICSEs remain unclear, while understanding interfacial interactions is critical for enhancing ionic conductivity. In situ cutting-edge characterization approaches enable the observation of ion mobility and concentration shifts in OICSEs under diverse conditions. Together with molecular dynamics simulations and density functional theory computations, these approaches offer multiscale, in-depth insights into ion transport mechanisms, forming a foundation for predicting ion diffusion channels and revealing interfacial effects. Future research in this area could focus on further enhancing the fundamental understanding of the underlying mechanisms, particularly at the molecular and interfacial levels. Continued efforts are needed to develop advanced characterization techniques and computational modeling to better correlate structure with performance. In addition, the design of novel materials and hybrid systems may open new pathways to overcome current limitations in conductivity, stability, and scalability. Finally, translating laboratory advances into practical applications will require interdisciplinary collaboration, bridging materials science, engineering, and real-world implementation.