Intelligent Sensing and Responsive Separators for Lithium Batteries Using Functional Materials and Coatings for Safety Enhancement

Abstract

With the increasing demand for high-energy-density lithium batteries, the role of separators has expanded significantly beyond conventional ion conduction and physical isolation. By integrating sensors and introducing functional coatings, separators have gained the ability to monitor internal states in real time and achieve adaptive regulation. This paper systematically reviews the latest research progress on separators modified with functional materials and coatings to achieve information sensing, intelligent response, and multifunctional integration. Notably, an electrochemical sensor based on MXene/MWCNTs-COOH/MOF-808 has been developed for rapid chemical detection; a fully printed ultra-thin flexible multifunctional sensor array has enabled multi-parameter synchronous monitoring; an ion-selective MOF-808-EDTA separator has induced uniform lithium-ion flux; and a PVDF-HFP/LLZTO/PVDF-HFP trilayer separator has maintained structural integrity at 300 °C. These innovative achievements fully demonstrate the enormous potential of intelligent separators in monitoring internal battery states, inhibiting dendrite growth, preventing thermal runaway, and significantly enhancing battery safety, cycle life, and energy density. This points to a transformative development path for the next generation of batteries with higher safety and intelligence.

1. Introduction

Commercialized in 1991, lithium-ion batteries have revolutionized energy storage and utilization [1], becoming the core technology underpinning modern electrochemical energy storage [2,3]. Their applications now span a broad spectrum, ranging from portable electronics and electric vehicles to large-scale energy storage and even the aerospace sector [4,5,6]. The demand for energy storage system with high energy density, long cycle life, and excellent safety has become increasingly urgent [7,8,9]. However, as energy density improves, lithium batteries face numerous severe challenges, such as thermal runaway [10], lithium dendrite growth [11], and interface side reactions [12]. These safety hazards severely restrict the further development of lithium battery technology. In recent years, there have been hundreds of accidents involving electric vehicles, posing a serious threat to the safety of passengers’ lives and property [13]. Among all safety issues, thermal runaway is assessed as the most dangerous situation, as it can lead to spontaneous combustion or even explosion of electric vehicles in an instant [14]. These events highlight the urgency of establishing a “first line of defense” within the batteries.

As one of the main components of the internal structure of a lithium battery [15,16], a separator serves as a physical barrier to isolate the cathode and anode electrodes to prevent short circuits, while also acting as a channel for ion conduction [17,18]. Its properties directly affect the overall performance of the battery. Currently, commercialized available polyolefin separators, such as polyethylene (PE) and polypropylene (PP), possess good chemical stability and relatively low cost, but they have inherent limitations such as poor thermal stability [19,20], insufficient electrolyte wettability [21,22,23], and a singular pore structure [24,25]. In particular, polyolefin separators are prone to melting and shrinking, when the internal temperature of the battery rises. This can lead to electrode contact, triggering short circuits and accelerating the thermal runaway process [26,27,28]. Additionally, their inert surfaces cannot effectively regulate lithium-ion flow distribution, making it difficult to suppress the growth of lithium dendrites [29,30]. To overcome these limitations, research on separator modification has undergone a paradigm shift from passive protection to active intelligent response. Early modification efforts focused mainly on surface coating [31,32] or material compounding [33] to enhance their thermal stability and affinity to the electrolyte. However, these strategies are essentially static protection and cannot dynamically respond to the changes in the internal battery. In recent years, with the development of nanofunctional materials, smart polymers, and micro-sensing technology, the concept of “smart responsive separators” have gradually emerged [34,35,36,37]. These separators can sense real-time changes in the internal state of the battery, such as temperature [38], pressure [39], ion concentration [40], etc., and make adaptive responses through changes in structure or properties, thereby achieving a functional upgrade from physical isolation to intelligent management.

The core concept of intelligent separators is to transform the separator into a multifunctional integrated platform, with capabilities far exceeding the traditional roles of ion conduction and physical separation (Figure 1). For example, thermoresponsive polymers can undergo phase transitions at specific temperatures, closing pores to block ion transmission and thus acting as thermal protection switches [3]. Conductive nanomaterial coatings, such as MXene and graphene, can both homogenize lithium-ion flow to suppress dendrite formation and serve as sensors to monitor changes in internal stress [41,42,43,44]. Functional materials like metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) can selectively adsorb lithium salt anions or capture polysulfides, increasing ion transference numbers and mitigating the shuttle effect [45,46,47]. These innovative designs endow the separator with the ability to sense, decide, and execute, offering the potential for intelligent regulation of the internal battery environment.

Figure 1. The core concept of intelligent separators for lithium batteries.

Figure 1. The core concept of intelligent separators for lithium batteries.

The trend towards intelligent separators has become inevitable, driven by three main factors. Firstly, high-energy-density battery systems, such as lithium metal and lithium-sulfur batteries, demand higher safety standards, necessitating separators with active protection functions. Secondly, the rise of artificial intelligence and the internet of things makes the construction of a “digital twin” for batteries possible, highlighting the importance of intelligent separators as critical data collection points. Finally, advancements in smart manufacturing technologies, such as machine vision positioning and automated coating, provide a technological foundation for the large-scale production of intelligent separators. The transformation towards intelligent separators will lead lithium battery technology into a new era, where separators will no longer be inert components but the core of active safety management systems. This shift not only promises to solve the longstanding safety challenges of the lithium battery industry but also paves the way for new approaches in smart battery design, laying a solid foundation for achieving the next generation of energy storage systems that are “perceptive, decisive, and executable.”

Numerous reviews have been conducted on lithium battery separators, primarily focusing on the physical and chemical properties of separator coating materials, their structural characteristics, and how they enhance the electrochemical performance of batteries [16,27,48,49,50]. However, reviews on the intelligent sensing and response of separators have not yet been reported. This article systematically analyzes the latest research progress on intelligent responsive separators for lithium batteries, with an emphasis on three key dimensions: sensing mechanisms, response behaviors, and functional integration. In the conclusion, we look forward to the future development directions of separators, including topics such as digital twin separators, multimodal sensor integration, artificial intelligence adaptive control, and environmentally friendly self-powered systems. These insights aim to provide a theoretical basis for advancing the next-generation lithium batteries towards greater safety, efficiency, and intelligence.

2. Materials and Techniques for Intelligent Separators

The core breakthrough of intelligent separators arises from the integration of functional materials and advanced manufacturing technologies. By incorporating sensing functional materials and responsive materials into the separators, real-time monitoring of various physical fields such as internal stress and chemical substances can be achieved [39,51,52]. Advanced preparation processes like electrospinning and atomic layer deposition provide key technical support for the precise construction and large-scale integration of these functions [53,54,55,56], collectively driving the leapfrog development of separators.

2.1. Materials and Coatings for Intelligent Separators

2.1.1. Intelligent Sensing

Real-time monitoring of mechanical stress changes inside batteries is crucial for early warning of lithium dendrite growth and electrode volume expansion [57,58]. Piezoelectric materials, such as polyvinylidene fluoride (PVDF) and its copolymers, can generate measurable electrical signals under external stress, enabling in situ stress sensing without external power supply [59,60]. The study by Manoharan et al. (2021) [61] confirmed that the piezoelectric PVDF composite electrolyte separator embedded with PTA exhibited a piezoelectric output voltage of up to 41 V (under an external force of 1 N) and an ionic conductivity of 1.928 × 10⁻¹⁰ S/cm (Figure 2a). This separator can function as a bipolar material with both separator and electrolyte capabilities for self-charging supercapacitors. When subjected to external pressure of ≥2 N, it triggers a self-charging mechanism, increasing the device voltage to 110 mV (Figure 2b,c), demonstrating significant potential for applications in integrated mechanical energy harvesting and storage devices [61].

Figure 2. (a) Schematic diagram of the piezoelectric potential generated in the PTA-PVDF piezoelectric polymer film under pressure [61]. Copyright 2021, Elsevier. (b,c) Current and Voltage Output of PTA-PVDF Films Under Compression [61]. Copyright 2021, Elsevier. (d) Piezoelectric stress and strain coefficients of MXene in the cases of fixed and relaxed ions [62]. Copyright 2019, Elsevier. (e) Schematic diagram of the sensing mechanism of the MXene/MWCNTs/MOF-808 electrochemical sensor [63]. Copyright 2025, Elsevier. (f) Working Principle of the PP@ZIF-67 Lithium Battery Separator in Suppressing Lithium Dendrites [64]. Copyright 2025, Elsevier.

Figure 2. (a) Schematic diagram of the piezoelectric potential generated in the PTA-PVDF piezoelectric polymer film under pressure [61]. Copyright 2021, Elsevier. (b,c) Current and Voltage Output of PTA-PVDF Films Under Compression [61]. Copyright 2021, Elsevier. (d) Piezoelectric stress and strain coefficients of MXene in the cases of fixed and relaxed ions [62]. Copyright 2019, Elsevier. (e) Schematic diagram of the sensing mechanism of the MXene/MWCNTs/MOF-808 electrochemical sensor [63]. Copyright 2025, Elsevier. (f) Working Principle of the PP@ZIF-67 Lithium Battery Separator in Suppressing Lithium Dendrites [64]. Copyright 2025, Elsevier.

Similarly, certain conductive composites achieve strain sensing through changes in their resistance. The study by Tan et al. (2019) [62] confirmed that oxygen-functionalized MXene (M₂CO₂) possesses the strongest out-of-plane piezoelectric performance among currently known two-dimensional materials, with a piezoelectric strain coefficient d₃₁ as high as 0.78 pm/V (Figure 2d). This material can generate significant out-of-plane piezoelectric voltage in cantilever and thin-film structures (for example, Sc₂CO₂ can reach 0.1 V under 2.5% strain). Additionally, the piezoelectric charge exhibits a six-region symmetrical distribution in circular films, providing an ideal material platform for ultra-thin piezoelectric energy harvesting and high-sensitivity mechanical sensing [62].

Detecting specific chemicals within a battery is crucial for assessing the battery’s health and diagnosing faults. Metal–organic frameworks (MOFs), with their designable porous structures and active sites, have emerged as efficient chemical sensing platforms [65]. Chen et al. (2025) [63] reported on an MXene/MWCNTs-COOH/MOF-808 electrochemical sensor that utilized the unsaturated coordination sites of Zr⁴⁺ in MOF-808 to capture and respond to catechin molecules in solution (Figure 2e). When catechin is adsorbed onto the surface of the composite material, it causes significant changes in the electrochemical signal at the electrode interface, thereby enabling sensitive and quantitative detection of catechin concentration [63]. Although this material has not yet been used for lithium battery sensing, the composite material demonstrates high sensitivity, low detection limits, and good stability, which can serve as a reference for battery sensors.

Gao et al. (2025) [66] developed a ZIF-8 modified polyphenylene sulfide woven separator, which utilized the selective interaction between Zn²⁺ sites and hydroxide ions (OH⁻) to effectively enhance hydroxide conduction and suppress gas permeation, significantly improving the energy efficiency and stability of alkaline electrolyzers [66]. Han et al. (2025) designed a ZIF-67 modified separator with anion affinity, where the preferential coordination of Co²⁺ with anions guides uniform lithium ion deposition, effectively inhibiting the growth of lithium dendrites and achieving highly stable lithium metal battery [64] (Figure 2f). Xia et al. (2025) constructed a sulfonic ligand hybrid ZIF-8 functional separator, which combines the physical confinement of ZIF-8 with the chemical adsorption of sulfonic groups to synergistically capture polysulfides and promote lithium ion transport, thereby significantly enhancing the cycling performance and sulfur utilization of lithium-sulfur batteries [67]. This advancement facilitates real-time monitoring of the internal chemistry of lithium batteries.

2.1.2. Materials for Intelligent Response

Thermal safety is the primary concern in lithium battery management [68,69]. Thermally responsive polymers can undergo significant phase changes at specific temperature thresholds, enabling autonomous thermal shutdown. The thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) (PNIPA) nanostructured hydrogels developed by Fernandez et al. (2016) is a typical example [70]. This material exhibits reversible swelling-collapse behavior near its volume phase transition temperature (approximately 36–45 °C). At lower temperatures, the PNIPA nanogel remains hydrophilic and promotes water absorption, keeping the hydrogel in a highly swollen state. When the temperature rises above the transition temperature, the PNIPA chains undergo hydrophobic collapse, causing the macroscopic hydrogel to lose water and shrink, significantly reducing the swelling ratio. This enables the material to respond quickly and reversibly to temperature changes, enhancing its mechanical properties, and provides a feasible approach for controlled release and sensing applications in lithium batteries.

Thermal regulation represents another critical layer of safety beyond the shutdown mechanism. Beyond the shutdown function, effective thermal management constitutes an essential safety mechanism. Phase change materials (PCMs) absorb or release heat through reversible solid–liquid phase transitions, smoothing out battery temperature fluctuations. Li et al. (2025) [71] developed a thermally responsive phase change material with an ultra-wide temperature range, achieving intelligent bidirectional thermal regulation within an extreme temperature range of −30 °C to 60 °C through the synergistic effect of multi-phase change components (Figure 3a). This material exhibits a phase change enthalpy exceeding 180 J/g, combining high thermal conductivity with adaptability. It can release latent heat in low-temperature environments to prevent lithium dendrite precipitation, and efficiently absorb heat under high-temperature or thermal abuse conditions to delay thermal runaway, significantly enhancing the all-weather thermal safety and cycling stability of lithium batteries [71]. Shen et al. (2025) [72] integrated a flame-retardant phase change material into a lithium iron phosphate battery module. This composite material exhibits an enthalpy of 178.5 kJ/kg within the phase transition temperature range of 40–55 °C and possesses high thermal conductivity of 1.58 W·m⁻¹·K⁻¹ (Figure 3b). The built-in nanoscale flame-retardant framework effectively suppresses electrolyte combustion at high temperatures, while the solid–liquid phase change absorbs a large amount of heat, achieving a dual “heat storage-flame retardant” protection mechanism, ensuring the operational safety of the battery under high-rate discharge and thermal abuse conditions [72].

Figure 3. (a) PCM-EGx@SEBS material design concept [71]. Copyright 2025, Cell Press. (b) Flame retardant mechanism of APP/EGA-PCM materials [72]. Copyright 2025, Elsevier. (c) Schematic diagram illustrating the optimization of ion separation performance by modulating the length of the ether-oxygen chains within the channels [73]. Copyright 2024, PNAS.

Figure 3. (a) PCM-EGx@SEBS material design concept [71]. Copyright 2025, Cell Press. (b) Flame retardant mechanism of APP/EGA-PCM materials [72]. Copyright 2025, Elsevier. (c) Schematic diagram illustrating the optimization of ion separation performance by modulating the length of the ether-oxygen chains within the channels [73]. Copyright 2024, PNAS.

Regulating lithium ion flow and stabilizing the electrode interface are critical for suppressing dendrite formation and prolonging battery cycle life. Covalent organic frameworks (COFs) have shown great potential in achieving ion-selective transport due to their precise pore structures. Meng et al. (2024) [73] designed a series of COF membranes with ether oxygen (EO) chain segments of varying lengths (COF-EO_x) to systematically explore the mechanism by which pore channel solvation capacity affects Li⁺ selective transportation (Figure 3c). They discovered that by precisely controlling the length of the ether oxygen chains within the pores, the transporting energy barrier for Li⁺ can be altered, thereby achieving efficient separation of Li⁺ from Mg²⁺. Among these, the COF-EO₂ membrane demonstrated outstanding performance [73].

Self-healing materials offer a dynamic solution to the problem of dendrite penetration. Elizalde et al. (2024) developed a light-mediated 3D-printed polymer electrolyte based on a reversible Diels-Alder reaction [74]. When the electrolyte is damaged due to dendrite growth or mechanical stress, the internal reversible covalent bond network can undergo bond dissociation and reformation under mild thermal stimulation, thereby autonomously repairing cracks and punctures. This dynamic self-healing capability not only restores the physical integrity of the electrolyte and effectively blocks the extension path of lithium dendrites but also reconstructs ion transport channels. As a result, lithium symmetric batteries can recover more than 95% of their initial cycle life even after experiencing deliberate cutting damage, significantly enhancing the reliability and safety of the batteries.

For lithium-sulfur batteries, the separator needs to possess intelligent management capabilities for polysulfides. Materials with both adsorption and catalytic functions can effectively suppress the shuttle effect and enhance reaction kinetics. Fang et al. (2019) [33] utilized a biomimetic carbon nanofiber network to construct a physical barrier for spatial confinement of polysulfides, while simultaneously anchoring polysulfides through the strong chemical adsorption of polar Co nanoparticles. This synergistic mechanism of structure and chemistry effectively suppressed the shuttle effect of polysulfides, resulting in a lithium-sulfur battery with a capacity decay rate as low as 0.063% per cycle after 500 cycles at a 1 C rate [33]. Zerrin et al. (2020) [75] reported on a magnetron sputtered TiO₂ thin film modification layer, which constructs a dense nano-barrier at the electrode-electrolyte interface to physically block the migration of polysulfides. Additionally, the strong polarity of the TiO₂ surface is utilized to achieve effective chemical anchoring of lithium polysulfides. This interface engineering approach synergistically suppresses the shuttle effect through both physical blocking and chemical adsorption, allowing the lithium-sulfur battery to maintain a capacity retention rate of up to 85% after 200 cycles at a 0.5 C rate [75].

2.2. Fabrication Techniques for Intelligent Separators

2.2.1. Electrospinning

Electrospinning technology, with its advantages of easy operation and wide material applicability, has become an important method for constructing intelligent separator frameworks. By controlling spinning parameters and receiving devices, precise control over fiber diameter, orientation, and pore structure can be achieved. Kong et al. (2024 [76] highlighted in their review that electrospinning technology can be used to prepare nanofiber separators with hierarchical structures, such as gradient pore sizes. These structures have been shown to enhance electrolyte wettability and ionic conductivity, thereby improving the cycling performance of batteries. The separator features a gradient pore distribution (ranging from 50 nm to 500 nm) and high porosity (>80%), significantly enhancing the electrolyte wettability and ionic conductivity (reaching up to 1.5 mS/cm) of lithium-ion batteries [76]. Ju et al. (2022) [77] developed an ultra-thin Zn-BDC MOF nanosheet-functionalized PAN composite separator by using electrospinning technology. The three-dimensional network structure formed by electrospinning enabled anion immobilization and lithium-ion redistribution, effectively suppressing the growth of lithium dendrites. This separator demonstrated low polarization voltage (<50 mV) and high coulombic efficiency (>99%) in lithium metal batteries, with a capacity decay rate of only 0.1% after 200 cycles (Figure 4a,b), showcasing the advantages of electrospinning in interface engineering [77].

Figure 4. (a) The relationship between Coulombic efficiency and cycling performance for the Li|Cu cell with a Zn-BDC/PAN(1/1)@PAN separator [77]. Copyright 2022, Elsevier. (b) The cycling performance of the symmetric Li|Li cell with a Zn-BDC/PAN(1/1)@PAN separator [77]. Copyright 2022, Elsevier. (c) Schematic diagram of improving battery performance using electrospun separators [78]. Copyright 2021, Wiley. (d) Schematic diagram of the preparation of cellulose acetate-based separators using electrospinning technology [79]. Copyright 2023, Elsevier.

Figure 4. (a) The relationship between Coulombic efficiency and cycling performance for the Li|Cu cell with a Zn-BDC/PAN(1/1)@PAN separator [77]. Copyright 2022, Elsevier. (b) The cycling performance of the symmetric Li|Li cell with a Zn-BDC/PAN(1/1)@PAN separator [77]. Copyright 2022, Elsevier. (c) Schematic diagram of improving battery performance using electrospun separators [78]. Copyright 2021, Wiley. (d) Schematic diagram of the preparation of cellulose acetate-based separators using electrospinning technology [79]. Copyright 2023, Elsevier.

Electrospinning technology also facilitates the integration of functional materials, such as polyoxometalates (POMs) and metal–organic frameworks (MOFs), to enhance the electrochemical performance and stability of separators. Ji et al. (2021) [78] prepared POM-modified polyvinylidene fluoride (PVDF) nanofiber separators using electrospinning for magnesium-sulfur batteries. The continuous fiber network formed by electrospinning provided a high specific surface area, allowing for uniform dispersion of POMs, thereby effectively capturing polysulfides and promoting magnesium ion migration (Figure 4c). The results showed that the initial capacity of the battery reached 1200 mAh/g at a 0.2 C rate, with a capacity retention rate of over 80% after 100 cycles, significantly outperforming traditional separators. This illustrates the flexibility of electrospinning in multifunctional modification, offering new insights into addressing the polysulfide shuttle effect [78]. Zhang et al. (2023) [79] designed a cross-linked network separator based on cellulose diacetate (CDA) through electrospinning (Figure 4d). This environmentally friendly separator features a tunable pore structure (pore size ~100 nm) and excellent thermal stability. The electrospinning process enabled uniform integration of CDA fibers with cross-linking agents, achieving high ionic conductivity (0.8 mS/cm) and low interfacial resistance, with a capacity retention rate of 90% after 300 cycles in lithium-ion batteries [79].

2.2.2. Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) technology, with its atomic-level precision in thickness control and excellent three-dimensional conformality, offers a revolutionary approach for surface modification of traditional separators. Shen et al. (2018) utilized ALD to uniformly deposit approximately 30 nm of Al₂O₃ coating on the surface of PVDF-HFP electrospun non-woven fabric separators, constructing a core–shell structured ceramic separator [80]. This ALD coating not only enhanced the thermal stability of the separator to withstand up to 270 °C without shrinkage, reduced the contact angle to 0°, and increased the ionic conductivity to 1.24 mS/cm, but also endowed it with excellent flame-retardant properties, allowing it to maintain a self-supporting structure in flames, significantly enhancing the safety and electrochemical performance of lithium-ion batteries (Figure 5a).

Figure 5. (a) Schematic diagram for the principle and the fabrication steps of ALD Al₂O₃ nonwoven separators [80]. Copyright 2018, Elsevier. (b) Charge–discharge curves of batteries using different solid electrolytes, and the relationship between the discharge specific capacity and cycle number of Li₁₀GeP₂S₁₂ electrolyte after ALD coating [81]. Copyright 2018, Elsevier. (c) A schematic diagram of a high thermal conductivity BNNS/aramid nanofiber composite separator constructed using LbL self-assembly technology [82]. Copyright 2021, Elsevier. (d) Lithium-ion battery separators based on DLC/PP can effectively control lithium dendrite growth [83]. Copyright 2021, Wiley.

Figure 5. (a) Schematic diagram for the principle and the fabrication steps of ALD Al₂O₃ nonwoven separators [80]. Copyright 2018, Elsevier. (b) Charge–discharge curves of batteries using different solid electrolytes, and the relationship between the discharge specific capacity and cycle number of Li₁₀GeP₂S₁₂ electrolyte after ALD coating [81]. Copyright 2018, Elsevier. (c) A schematic diagram of a high thermal conductivity BNNS/aramid nanofiber composite separator constructed using LbL self-assembly technology [82]. Copyright 2021, Elsevier. (d) Lithium-ion battery separators based on DLC/PP can effectively control lithium dendrite growth [83]. Copyright 2021, Wiley.

ALD technology also plays a crucial role in interface regulation in solid-state batteries. Rao et al. (2018) [81] employed ALD technology to construct a lithium tantalate protective layer on the surface of the sulfide solid electrolyte Li₁₀GeP₂S₁₂. This strategy not only effectively inhibited the growth of lithium dendrites but also significantly enhanced the interfacial compatibility between the electrolyte and the lithium metal anode (Figure 5b) [81]. The quasi-solid-state lithium-sulfur battery assembled with this modified electrolyte achieved stable cycling of the lithium metal anode, extending its cycle life from 20 weeks to 50 weeks, with a capacity retention rate increased to 41%. Additionally, the working voltage and energy efficiency of the battery were simultaneously improved.

2.2.3. Synergistic Application of Layer-by-Layer Self-Assembly (LbL), Spraying, and Coating Techniques

The layer-by-layer (LbL) self-assembly technique achieves molecular-level precision in thickness control of functional coatings by alternately depositing polyelectrolytes with opposite charges. The synergistic application of spraying and coating technologies provides a feasible path for the large-scale preparation of functional separators. Pei et al. (2021) [82] constructed a highly thermally conductive BNNS/aramid nanofiber composite separator using the LbL self-assembly technique (Figure 5c). They formed a three-dimensional thermal conductive network on the surface of a polypropylene separator, increasing the in-plane thermal conductivity to 1.76 W/m·K. This effectively reduced the surface temperature of the battery by 8.2 °C during 2 C discharge and inhibited lithium dendrite growth through the redistribution of ion flow, enhancing the capacity retention rate of lithium-sulfur batteries to 81.4% after 500 cycles at 2 C rate [82]. Shi et al. (2020) [84] employed the LbL self-assembly strategy to alternately deposit covalent triazine frameworks (CTFs) and PEDOT conductive polymers on the surface of Celgard separators, constructing a functional separator with a precise layered structure. The LbL assembly chemically adsorbs polysulfides through the positively charged CTF layer, while the PEDOT conductive layer accelerates electron transport to promote sulfur species conversion. As a result, the lithium-sulfur battery exhibited excellent cycling stability, with the capacity decay rate reducing to 0.08% per cycle after 200 cycles at a 0.5 C rate [84].

Cheng et al. (2019) [85] employed a simple doctor-blading technique to uniformly coat cobalt-embedded carbon nanosheets onto the surface of commercial separators. This coating offers enhanced chemical adsorption of polysulfides and efficient catalytic conversion, enabling lithium-sulfur batteries to maintain an areal capacity of 4.8 mAh cm⁻² even at a sulfur loading as high as 7.8 mg cm⁻². Furthermore, after 500 cycles at a 1 C rate, the capacity decay per cycle is only 0.052% [85]. Li et al. (2021) [83] utilized magnetron sputtering to apply a diamond-like carbon coating on the surface of copper current collectors, which was then converted into an ultra-strong lithium-ion conductor through in situ chemical lithiation. The amorphous carbon layer formed by this sputtering process creates continuous ion channels post-lithiation, increasing lithium-ion conductivity to 8.7 × 10⁻⁴ S cm⁻¹ and directing lithium to preferentially deposit along the (110) crystal plane. This allows lithium metal batteries to achieve stable cycling for over 1000 h at a high current density of 5 mA cm⁻² (Figure 5d) [83]. Inspired by the nacre-like layered structure, Song et al. (2019) [32] employed a blade-coating method to construct an ion-conductive porous multilayer lithium battery separator composed of highly oriented aragonite lamellae. This coating, with its “brick-and-mortar” structure, enhances the puncture resistance of the separator to 125 MPa while maintaining a lithium-ion conductivity of up to 0.52 mS cm⁻¹. This innovation enables the assembled dual-ion batteries to retain stable electrochemical performance under extreme conditions such as impact and bending, offering a new strategy for high-safety flexible batteries [32].

3. Intelligent Sensing of Separators

The continuous improvement in the energy density of lithium-ion batteries has heightened the safety risks associated with anomalies in local multi-physical fields [86]. The traditional “black-box” operation mode obscures critical internal state information, leading to hazards such as dendrite growth, electrolyte decomposition, and thermal runaway, which can ultimately result in catastrophic outcomes. Therefore, real-time sensing of internal temperature, pressure, and ion distribution is crucial. Internal intelligent sensing aims to address the following issues: The first issue is the propagation of thermal runaway, where microscopic temperature gradients exist minutes before macroscopic thermal runaway occurs, but external sensors cannot accurately identify them. The second issue is dendrite-induced short circuits, where a sudden local pressure increase indicates dendrite penetration through the separator, and an imbalance in lithium-ion flux accelerates dead lithium accumulation. The third issue is electrochemical degradation, Changes in lithium-ion concentration over time and space can lead to local depletion or accumulation, thereby exacerbating side reactions and accelerating capacity decay.

Integrating sensing functions directly onto the separator can achieve ultra-high spatial resolution and millisecond-level rapid response in temperature, strain, and chemical sensing. This paradigm could transform the separator into an embedded neural system within the battery, thereby shifting safety management from post-failure handling to preemptive intervention.

3.1. Temperature Sensing Separator

Thermistor materials are widely used in separator temperature sensing due to their characteristic of resistance changing with temperature. Research primarily focuses on materials with a high temperature coefficient of resistance (TCR), such as metal oxides [87,88], carbon-based materials [89,90], and semiconductor polymers [88]. These materials can be incorporated into separator structures in the form of coatings or nanocomposites to create distributed temperature sensors.

Le et al. (2025) [91] innovatively developed a reversible temperature-responsive membrane (RTRM) based on an LDPE/UHMWPE blend matrix, using it as a surface modification layer for the cathode current collector (Figure 6a). This design increases resistivity by 7.1 orders of magnitude at high temperatures of 110–120 °C, successfully cutting off the electron transport path. Upon cooling, the performance fully recovers, maintaining stability even after 30 thermal cycles, thus providing a reliable reversible thermal protection mechanism for lithium-ion batteries [91]. Fluorescent temperature sensing strategies offer an innovative approach by utilizing the temperature-dependent properties of luminescent materials to enable non-contact temperature monitoring (Figure 6b). This self-calibrating characteristic effectively eliminates the impact of excitation light source fluctuations on measurement accuracy, making it particularly suitable for precise temperature measurement inside batteries [92,93].

Figure 6. (a) Schematic diagram of the temperature response mechanism of RTRM [91]. Copyright 2025, Elsevier. (b) CIE diagrams of phosphors at different temperatures [92]. Copyright 2020, Elsevier. (c) Schematic diagram of the arrangement of fully printed smart sensors in a pouch battery [94]. Copyright 2025, Nature. (d) A schematic diagram of the structure of a thermally regulated lithium-ion battery and the thermally regulated separator [95]. Copyright 2021, Wiley. (e) A schematic diagram of the mechanism of action of smart hydrogels with a “dual insurance” mechanism [96]. Copyright 2025, Wiley.

Figure 6. (a) Schematic diagram of the temperature response mechanism of RTRM [91]. Copyright 2025, Elsevier. (b) CIE diagrams of phosphors at different temperatures [92]. Copyright 2020, Elsevier. (c) Schematic diagram of the arrangement of fully printed smart sensors in a pouch battery [94]. Copyright 2025, Nature. (d) A schematic diagram of the structure of a thermally regulated lithium-ion battery and the thermally regulated separator [95]. Copyright 2021, Wiley. (e) A schematic diagram of the mechanism of action of smart hydrogels with a “dual insurance” mechanism [96]. Copyright 2025, Wiley.

Significant progress has been made in monitoring technology based on multifunctional sensor arrays. Sun et al. (2025) [94] developed a fully printed flexible sensor array that can be integrated with battery packaging films. This array utilizes nanostructured functional inks to achieve in situ and simultaneous acquisition of multi-physical field signals such as temperature, stress, and electrolyte leakage vapor (Figure 6c). The sensor system adds only 49 mg in weight and possesses excellent resistance to multidimensional interference and long-term system-level stability. It is capable of providing early warnings for various typical failure modes, including overcharging and discharging, low-temperature/high-rate lithium plating, internal short circuits, mechanical damage, and thermal abuse. Furthermore, this platform integrates direct alarm functions for flammable gases and electrolyte leakage, enabling efficient risk identification under complex working conditions [94].

In the field of thermoregulatory separators, the introduction of phase change materials has opened new avenues. Liu et al. (2021) [95] innovatively proposed the encapsulation of paraffin as a heat-absorbing material into polyacrylonitrile nanofibers to create a thermoregulatory separator (Figure 6d). This separator takes advantage of the characteristic of phase change materials to absorb a large amount of heat during the phase transition (the latent heat of paraffin is approximately 240 J·g⁻¹). Under abusive conditions, it can effectively suppress the rapid rise in internal temperature of the battery while maintaining excellent electrolyte wettability and electrochemical performance [95].

Hydrogel electrolytes demonstrate unique advantages in thermal regulation. Wang et al. (2025) [96] developed an intelligent hydrogel electrolyte with a “dual insurance” mechanism. This electrolyte uses a thermosensitive polymer, PNIPAM, as the main chain, embedding nanogel microspheres (Figure 6e). At 65 °C, it rapidly blocks ion transmission through a quick phase transition that locks the ion channels and synergizes with water evaporation. Upon cooling to room temperature, the material’s structure and electrochemical performance almost completely recover, providing a smart thermal protection solution for flexible energy storage devices [96]. Similarly, the smart thermoregulatory hydrogel electrolyte designed by Meng et al. (2022) performs even more excellently, maintaining normal operation even at an ambient temperature of 100 °C, effectively suppressing thermal runaway in zinc-ion batteries [97].

3.2. Mechanical Pressure/Strain Sensing Separator

During the charge and discharge cycles of lithium batteries, the volume changes of electrode materials and the growth of lithium dendrites generate mechanical stress [98,99]. The accumulation of these stresses may lead to separator puncture and internal short circuits. Therefore, real-time monitoring of mechanical pressure/strain changes within the battery is of great significance for safety alerts. In recent years, mechanical pressure/strain sensing separators have made significant progress in in situ monitoring by integrating piezoelectric materials or constructing strain-sensitive conductive networks.

Piezoelectric materials, which can directly convert mechanical stress into electrical signals, are ideal for in situ stress monitoring. Among them, polyvinylidene fluoride (PVDF) and its copolymers have been extensively studied due to their excellent piezoelectric properties and electrochemical stability. For example, Zhang et al. (2022) [100] developed an integrated flexible pressure sensor based on a solid-state zinc-ion battery. By introducing a nanofiber isolation layer design, the sensor reduces interface resistance and isolation layer resistance under external mechanical pressure, achieving the conversion of mechanical signals into electrical signals (Figure 7a). The sensor’s recovery time for dynamic pressure response is as short as 88.0 milliseconds, demonstrating high sensitivity across an extremely wide operating range from 2 Pa to 368 kPa. It can accurately respond to different peak values of human pulse beats, showcasing its potential for internal battery monitoring applications [100]. Sun et al. (2025) [94] reported an ultra-thin flexible multifunctional sensor array that can be embedded in lithium-ion battery packaging films. This array can simultaneously monitor multiple mechanical parameters such as pressure and strain, and its anti-interference stability surpasses that of commercial sensors. The array is fabricated by CO₂ laser etching aluminum-polymer composite foils to create interconnection lines, allowing for a fully printable process with minimal impact on battery energy density and electrochemical performance [94].

Jiang et al. (2025) [101] developed a ternary composite flexible dielectric switching material based on polydimethylsiloxane/multi-walled carbon nanotubes/graphene, creating a self-responsive sensing system with dual functions of overheating warning and small deformation monitoring (Figure 7b). This composite material, by constructing a unique micro-crack structure, can generate reversible changes in conductive pathways under minor strain, exhibiting high sensitivity (gauge factor GF > 50) to small deformations within the range of 0.5%–5%. It effectively monitors electrode deformation behavior during the charge and discharge processes of batteries. This dual-responsive smart material provides a dual warning mechanism for temperature and deformation in lithium battery safety management and maintains stable response characteristics even after 100 thermal-mechanical cycles, demonstrating promising application prospects in intelligent battery systems [101].

Conductive filler-polymer composites provide an effective alternative for mechanical sensing through the characteristic change in resistance under strain. When such separators are subjected to compressive or tensile stress, the internal conductive network pathways change, leading to resistance alterations. By monitoring resistance changes, the magnitude of stress can be inferred. Xie et al. (2021) [102] systematically studied the tensile mechanical properties of polypropylene separators in an electrolyte-soaked state by establishing a micromechanical model for porous polymer separators. The study found that electrolyte infiltration significantly reduces the tensile strength and elastic modulus of the separator. Specifically, the ultimate tensile strength in the machine direction (MD) decreases by approximately 35%, while the modulus in the transverse direction (TD) drops by up to 42% (Figure 7c). This decline in mechanical performance is primarily attributed to the plasticizing effect of the electrolyte on polymer molecular chains and the lubricating effect between pores, which provides critical guidance to prevent internal short circuits in batteries caused by mechanical failure of the separator [102].

Graphene and carbon-based nanomaterials offer innovative approaches for embedding high-sensitivity sensors in membranes due to their unique electrical conductivity and flexibly designable microstructures. Chen et al. (2020) [103] developed a flexible capacitive sensor based on electron-induced vertical graphene porous carbon films, where the vertically aligned graphene layers provide efficient channels for electron transmission across layers. This enables the sensor to achieve a sensitivity of up to 0.13 kPa⁻¹ and a rapid response time of 66 ms [103]. Additionally, inspired by the airflow sensing capabilities of bat wing membranes in nature, Zhou et al. (2021) [104] constructed a graphene/single-walled carbon nanotube composite film through interfacial self-assembly. This structure utilizes the micro-spring effect formed by hybridization, making the sensor extremely sensitive to subtle physical stimuli, with an airflow detection limit as low as 0.0176 m/s [104]. These studies highlight the tremendous potential of integrating carbon-based nanomaterials into flexible substrates through sophisticated microstructural design to achieve high-performance sensing. Although these designs have not yet been applied to lithium battery membrane sensors, they provide valuable insights for real-time stress monitoring within membranes.

3.3. Chemical Substance Sensing Separator

Changes in the chemical environment within lithium batteries, such as the formation of by-products and the decomposition of electrolytes, directly impact battery performance and lifespan [105,106]. Chemical sensing separators provide a direct basis for monitoring the chemical state and diagnosing faults in batteries by specifically identifying and responding to these chemical substances.

In lithium-sulfur battery systems, the shuttle effect of soluble polysulfides (LiPSs) is a critical issue leading to the loss of active material and capacity decay. To address this challenge, researchers have developed various functional separators aimed at efficient management and in situ monitoring of polysulfides. For example, Wang et al. (2025) designed a spatially structured layered porous carbon-CoS₂ (LPC-CoS₂) composite separator [107]. In this separator, the layered porous carbon (LPC) provides physical confinement for the polysulfides, preventing corrosion of the lithium metal anode and ensuring uniform lithium ion deposition. Simultaneously, CoS₂ acts as a Lewis acid catalyst, exhibiting good adsorption energy for polysulfides (Figure 7d,e). It not only efficiently captures polysulfides but also promotes their conversion through the extension or cleavage of Li-S and S-S bonds after adsorption. Density functional theory (DFT) calculations and experiments have confirmed the adsorption capacity of CoS₂ for polysulfides and its role in promoting conversion. Thanks to these characteristics, lithium-sulfur batteries using this separator can maintain a specific capacity of 885 mAh·g⁻¹ after 200 cycles at a sulfur loading of 6.3 mg·cm⁻² and a current density of 1 mA·cm⁻².

Figure 7. (a) Schematic diagram of the response process of a single ZIB-P sensor to external pressure [100]. Copyright 2022, Wiley. (b) Applications of TPU/polymer alloy composite films in sensing under compressive and flexural stress, includes (I,II) finger bending sensing, (III,IV) finger compression sensing, and (V,VI) mobile terminal message alert. [101]. Copyright 2025, Springer Nature. (c) The stress distribution (i, ii) and deformation distribution (iii, iv) of the tensile model [102]. Copyright 2021, Elsevier. (d) Mechanism of interaction between LPC-CoS₂ and LiPSs [107]. Copyright 2025, ScienceDirect. (e) The adsorption energies of LiPSs on LPC and CoS₂ [107]. Copyright 2025, ScienceDirect. (f) Schematic diagram of TiN microelectrode for lithium-sulfur full cell [108]. Copyright 2025, Wiley.

Figure 7. (a) Schematic diagram of the response process of a single ZIB-P sensor to external pressure [100]. Copyright 2022, Wiley. (b) Applications of TPU/polymer alloy composite films in sensing under compressive and flexural stress, includes (I,II) finger bending sensing, (III,IV) finger compression sensing, and (V,VI) mobile terminal message alert. [101]. Copyright 2025, Springer Nature. (c) The stress distribution (i, ii) and deformation distribution (iii, iv) of the tensile model [102]. Copyright 2021, Elsevier. (d) Mechanism of interaction between LPC-CoS₂ and LiPSs [107]. Copyright 2025, ScienceDirect. (e) The adsorption energies of LiPSs on LPC and CoS₂ [107]. Copyright 2025, ScienceDirect. (f) Schematic diagram of TiN microelectrode for lithium-sulfur full cell [108]. Copyright 2025, Wiley.

On the other hand, the team led by Jin Wang (2022) [109] developed a crumpled MXene/MoS₂ (CM/MoS₂) heterostructure functional separator. This separator achieves chemical fixation of polysulfides through Lewis acid-base interactions and sulfur chain catenation, while the heterostructure reduces the diffusion barrier of lithium atoms, catalyzing the conversion of polysulfides to Li₂S₂/Li₂S. Attributed to these unique merits, Li–S batteries with the CM/MoS2-modified separator deliver a high capacity of 1336 at 0.1 C, a considerable areal capacity of 5.5 mA h cm⁻², an excellent rate capability of 810 mA h g⁻¹ at 2 C, and stable cycling performance over 500 cycles at 1 C with a low capacity decay of 0.056% for each cycle [109].

Ji et al. (2025) [108] developed a separator reactor based on titanium oxynitride carbon hollow carbon nanofibers (TiNOCF) as a functional separator for lithium-sulfur batteries. This separator reactor features abundant Ti and N active sites, whose chemically confined catalytic effects promote the kinetics of sulfur reduction/oxidation reactions (SRR/SOR), the reversibility of Li₂S deposition/dissolution, electron transfer, and Li⁺ diffusion dynamics (Figure 7f). Studies have shown that this design can simultaneously regulate the conversion of the cathode solid-phase S₈/Li₂S and the deposition growth of lithium dendrites at the anode, thereby significantly enhancing the rate performance and cycling stability of the battery [108].

In terms of electrolyte leakage detection, the research group led by Jia Huang (2020) [110] developed a chemical sensor centered around an ion-conducting metal–organic framework (IC-MOF) film. This sensor is prepared using an improved spray liquid–liquid interface self-assembly strategy, and its ion conduction characteristics allow it to perform optimally when capacitance is used as the output signal. It can rapidly respond to 50 ppb dimethyl carbonate (DMC) or 20 nL lithium-ion battery electrolyte within seconds. In simulated battery leakage experiments, this sensor can provide early warning several hours before significant changes in battery voltage occur, demonstrating excellent early warning capabilities [110]. Similarly, Zhang et al. (2022) [111] reported a chemical sensor based on receptor-modified bilayer organic field-effect transistors (OFETs). This sensor combines the sensitivity of OFETs with the selectivity of a biurea receptor, achieving a detection limit for the electrolyte component diethyl carbonate as low as 1.4 ppm and a 3% response to a micro leak of 200 nL electrolyte. This sensor offers a highly sensitive detection scheme for lithium-ion battery electrolyte leakage [111].

3.4. Preliminary Exploration of Ion Flow Distribution Sensing

Uneven distribution of lithium ions on the surface of the separator can lead to excessively high local current density, which in turn causes lithium dendrite growth and capacity decay. Monitoring and regulating ion flow distribution is crucial for effectively enhancing battery performance and safety. Although technical challenges remain, recent research has made significant progress in the mechanisms of ion flow regulation and monitoring methods.

Separator structure design is a direct approach to achieving uniform ion flow distribution. Jia et al. (2023) [112] highlight that the three-dimensional porous structure of the separator plays a crucial role in influencing the distribution of lithium-ion flow. This distribution, in turn, dictates the deposition behavior of lithium metal and significantly impacts the battery’s cycle life (Figure 8a). This work analyzed the pore structures of different polyolefin separators through particle injection simulations, finding that the uniformity of the pores is directly related to the uniformity of lithium ion flow. This provides a theoretical basis for regulating ion flow through separator structure design [112].

Figure 8. (a) The Influence of Pore Structure of Different Membranes on Ion Flux Distribution [112]. Copyright 2025, Wiley. (b) Comparison of traditional separators (i) and targeted ion sifter (ii) in suppressing active ion crosstalk [113]. Copyright 2023, Wiley. (c) Schematic diagram of negative electrode ion transport and dendrite growth behavior of ordinary diaphragm (top) and IMM (bottom) [114]. Copyright 2024, Wiley. (d) Three types of input datasets for classifying the external environment of battery cells, and a case for classifying the external states of EIS [115]. Copyright 2023, Wiley.

Figure 8. (a) The Influence of Pore Structure of Different Membranes on Ion Flux Distribution [112]. Copyright 2025, Wiley. (b) Comparison of traditional separators (i) and targeted ion sifter (ii) in suppressing active ion crosstalk [113]. Copyright 2023, Wiley. (c) Schematic diagram of negative electrode ion transport and dendrite growth behavior of ordinary diaphragm (top) and IMM (bottom) [114]. Copyright 2024, Wiley. (d) Three types of input datasets for classifying the external environment of battery cells, and a case for classifying the external states of EIS [115]. Copyright 2023, Wiley.

In the realm of functionalized separators, Feng et al. (2023) [113] reported an ion-selective separator modified with MOF-808-EDTA (Figure 8b). This separator, through its ordered pore structure and surface chemical properties, achieves targeted capture of transition metal ions while providing fast lithium-ion transport channels, inducing a uniform lithium-ion flow [113]. Similarly, Zhang et al. (2024) [114] developed an ion management membrane with vertically aligned nanochannels using ion track technology. These uniformly sized, negatively charged nanochannels act as “lithium-ion guides,” effectively reducing lithium-ion concentration fluctuations and enabling selective lithium-ion transport (Figure 8c). The membrane exhibits high ionic conductivity (0.73 mS cm⁻¹) and a lithium-ion transference number (0.80), achieving over 1200 h of cycle life in Li/Li symmetrical cells [114].

The monitoring of ion flow relies on advanced sensing schemes and data analysis methods. Electrochemical impedance spectroscopy (EIS) has become an effective tool for indirectly monitoring ion transport behavior due to its sensitivity to changes in the internal state of batteries. Han et al. (2023) [115] developed a convolutional neural network (CNN) model based on EIS image transformation. This study converted EIS data into images using the recurrence plot (RP) algorithm and then utilized CNN to extract features, achieving the classification of battery states under different external environments (Figure 8d). This provides a new approach for inferring changes in the internal ion transport environment through impedance information [115].

4. Intelligent Response of Separators

Intelligent sensing provides the informational foundation for achieving battery safety warnings, while transforming state perception into adaptive safety management relies on the separator’s intelligent response capabilities under various physical stimuli. Traditional separators have a single, passive response mode, making it difficult to cope with the complex and variable internal environment of high-energy-density batteries. In recent years, researchers have been dedicated to developing new separators with thermal, electrochemical, and mechanical response characteristics, aiming to achieve “active intervention” in safety prevention and control.

4.1. Thermally Responsive Intelligent Separators

Thermal runaway is a core cause of safety threats in lithium batteries. Thermally responsive intelligent separators enhance the thermal safety boundary of batteries significantly by employing material design and structural innovation to achieve multilevel protection of “shutdown-regulation-flame retardance” under thermal abuse conditions.

In the area of thermal shutdown mechanisms, the research focus is on designing materials that can trigger rapid and reliable ion transport blockage at specific temperatures. Li et al. (2023) [116] developed a composite separator with thermal shutdown functionality, composed of ethylene-vinyl acetate copolymer/polyether ether ketone/ethylene-vinyl acetate copolymer (EVA/PEEK/EVA) (Figure 9a). This separator utilizes the EVA layer to transform into a dense layer when the temperature exceeds 80 °C, thereby cutting off ion transport [116]. Simultaneously, the PEEK base layer maintains dimensional stability even at temperatures as high as 240 °C, effectively avoiding the risk of internal short circuits caused by thermal inertia after thermal shutdown. Constructing a thermally responsive interface layer on the cathode side is another effective strategy.

Le et al. (2025) [91] successfully endowed the cathode with thermal protection by introducing a reversible temperature-responsive membrane (RTRM) composed of low-density polyethylene (LDPE), ultra-high molecular weight polyethylene (UHMWPE), and short-cut carbon fibers between the aluminum current collector and active material. This functional layer increased resistivity by 7.1 orders of magnitude at high temperatures of 110–120 °C, achieving automatic reaction cessation, and fully restored performance upon cooling. It remained stable through 30 thermal cycles, demonstrating excellent reversibility [91]. Additionally, a multilayer polyethylene separator (ASPESA) reported by Jiang et al. (2024) also exhibited thermal shutdown functionality at 120 °C, significantly improving the separator’s thermal stability and electrolyte wettability through the outer layers of LDPE and Al₂O₃ (Figure 9b) [117].

Thermal regulation and flame retardant mechanisms focus on early intervention during thermal runaway to extend the safety response window. Zhang et al. (2023) designed an intelligent risk-responsive polymer membrane [118]. This separator is compatible with electrodes at room temperature, and during thermal accumulation, its phosphorus-containing functional groups spontaneously dissociate to release radicals, effectively quenching highly reactive radicals generated by electrolyte decomposition, thus terminating the chain exothermic reaction early. This design extended the time before thermal runaway by approximately 9 h in a 1.8 Ah pouch cell, creating a critical window for safety management (Figure 9c). Similarly, a composite separator prepared using polyarylethernitrile (PEN) engineering plastic matrix by Lin et al. (2022) demonstrated potential for application as a high-safety separator due to the excellent thermal stability of the matrix material itself [119].

Figure 9. (a) The design and thermal shutdown mechanism of the EVA/PEEK/EVA separator [116]. Copyright 2023, Elsevier. (b) Schematic diagram of the thermal shutdown process of the separator [117]. Copyright 2024, Elsevier. (c) Phosphorus-containing free radicals generated during TPF pyrolysis quench the exothermic process, thereby mitigating thermal runaway [118]. Copyright 2023, AAAS. (d) TF-COF membranes enable selective ion separation [120]. Copyright 2025, Wiley. (e) The role of -NH2 groups in NH2-Ti-MOF membrane modification [121]. Copyright 2025, Springer Nature. (f) EC separations provide uniform Li+ flux and inhibit anode thickness increase [122]. Copyright 2022, Elsevier.

Figure 9. (a) The design and thermal shutdown mechanism of the EVA/PEEK/EVA separator [116]. Copyright 2023, Elsevier. (b) Schematic diagram of the thermal shutdown process of the separator [117]. Copyright 2024, Elsevier. (c) Phosphorus-containing free radicals generated during TPF pyrolysis quench the exothermic process, thereby mitigating thermal runaway [118]. Copyright 2023, AAAS. (d) TF-COF membranes enable selective ion separation [120]. Copyright 2025, Wiley. (e) The role of -NH2 groups in NH2-Ti-MOF membrane modification [121]. Copyright 2025, Springer Nature. (f) EC separations provide uniform Li+ flux and inhibit anode thickness increase [122]. Copyright 2022, Elsevier.

4.2. Electrochemically Responsive Intelligent Separators

Electrochemically responsive intelligent separators establish a dynamic regulation mechanism within batteries through their functional materials, effectively addressing key issues such as ion transport, interface stability, and suppression of side reactions, thereby significantly enhancing the cycle life and safety of lithium batteries.

In terms of ion-selective transport, researchers have achieved efficient screening and transport of target ions by designing separator coatings with precise pore sizes and specific chemical environments. The research team led by Songyan Bai (2025) [120] developed a fluorine-based covalent organic framework (TF-COF) separator with high ion selectivity (Figure 9d). This separator utilizes its inherent regular microporous structure to achieve a high lithium-ion transference number and endows the separator with excellent flame-retardant properties, allowing it to self-extinguish up to three times under open flame, significantly improving battery safety [120]. Kang et al. (2025) [121] designed an amino-functionalized Ti-MOF (NH₂-Ti-MOF) modified separator for lithium-sulfur batteries. The -NH₂ groups in this separator interact with Li⁺ through Lewis acid-base interactions, effectively promoting directional rapid migration of Li⁺, while its reduced sub-nanometer pore size effectively blocks the shuttle of polysulfides of different chain lengths (Figure 9e) [121].

To achieve dendrite suppression and interface stability, smart separators with adaptive characteristics demonstrate unique advantages. Zheng et al. (2022) [123] developed a BaTiO₃/PVDF-TrFE composite porous flexible self-powered separator, which not only possesses excellent ionic conductivity (2.01 × 10⁻³ S/cm) and mechanical properties but also generates an electric field through its piezoelectric effect that helps guide uniform lithium-ion deposition, thus inhibiting dendrite growth [123]. Seo et al. (2022) [122] developed an electrode-customized separator (EC Separator) based on self-assembled chiral nematic liquid crystal cellulose nanocrystals (LC-CNC). Due to its ordered nanoporous channels and nanofluidic ion migration effects, this separator enables Li⁺ to flow uniformly toward the metal anode, improving the recyclability of lithium plating/stripping and successfully applying it in lithium metal full batteries under harsh conditions, significantly enhancing their cycling stability (Figure 9f) [122].

To address the polysulfide shuttle issue in lithium-sulfur batteries, functionalized separators achieve intelligent blocking through the synergy of physical restriction and chemical interaction. The NH₂-Ti-MOF separator developed by Kang et al. (2025) can block polysulfides via a size-sieving effect and also achieve redistribution and reutilization of polysulfides through electrostatic adsorption by -NH₂ groups on the pore surfaces [121]. Various inorganic materials have also shown great potential in modifying separators to inhibit polysulfide shuttling. For instance, metal oxides and metal sulfides exhibit strong adsorption and catalytic properties towards polysulfides due to their robust physicochemical interactions. Metal nitrides and metal phosphides not only possess high catalytic abilities but also exhibit high conductivity, which can accelerate the conversion kinetics of polysulfides [124].

4.3. Mechanical Response Intelligent Separators

When addressing the significant volume changes during the cycling of silicon-based anodes and lithium-metal batteries, separators with excellent mechanical properties can maintain interface contact through their ductile structures and adaptive pores, providing an effective strategy to mitigate performance degradation.

Sun et al. (2024) [125] developed a mechanically robust and safe separator based on inorganic nanofibers. This separator achieves a tight integration of inorganic and organic components through electrostatic interactions, hydrogen bonding, and chemical bonding, forming a hierarchical cross-linked structure (Figure 10a). Its tensile strength reaches up to 16.28 MPa, approximately 1.7 times that of traditional polypropylene (PP) separators. Thanks to the high aspect ratio and excellent thermal stability of hydroxyapatite nanowires, the separator exhibits negligible thermal shrinkage after being heated at 200 °C for 1 h. It also possesses a high-porosity nanostructure, good electrolyte affinity, and high ionic conductivity (0.87 mS cm⁻¹) [125].

Figure 10. (a) Schematic diagram of the multi-bond enhancement mechanism of BHLP separator [125]. Copyright 2024, ACS. (b) Schematic diagram of the UP3D lithium-ion battery separator design [126]. Copyright 2023, Wiley. (c) Mechanical strength of original PTFE, UP3D and other membranes [126]. Copyright 2023, Wiley. (d) Schematic diagram of the structure and preparation method of PILEs [127]. Copyright 2025, Wiley. (e) Mechanical properties of PILEs, including (i) stress–strain curves of different compositions, (ii) Young’s modulus and toughness, (iii) cyclic stress–strain curves at different strains, (iv,v) cyclic stress–strain curves at a fixed strain, and (vi) stress and dissipated energy in the first ten cycles [127]. Copyright 2025, Wiley.

Figure 10. (a) Schematic diagram of the multi-bond enhancement mechanism of BHLP separator [125]. Copyright 2024, ACS. (b) Schematic diagram of the UP3D lithium-ion battery separator design [126]. Copyright 2023, Wiley. (c) Mechanical strength of original PTFE, UP3D and other membranes [126]. Copyright 2023, Wiley. (d) Schematic diagram of the structure and preparation method of PILEs [127]. Copyright 2025, Wiley. (e) Mechanical properties of PILEs, including (i) stress–strain curves of different compositions, (ii) Young’s modulus and toughness, (iii) cyclic stress–strain curves at different strains, (iv,v) cyclic stress–strain curves at a fixed strain, and (vi) stress and dissipated energy in the first ten cycles [127]. Copyright 2025, Wiley.

Chen et al. (2023) [126] introduced a thin, ultra-porous separator named UP3D, which combines high mechanical strength with excellent electrolyte absorption capacity (Figure 10b,c). This separator is crafted by impregnating polyvinylidene fluoride-hexafluoropropylene onto ultra-thin polytetrafluoroethylene, resulting in a composite structure with high porosity (74%) and outstanding mechanical/thermal stability. Phase field and molecular dynamics simulations indicate that the ultra-porous structure of the UP3D separator not only significantly enhances Li⁺ ion migration but also achieves high-flux, uniform Li⁺ deposition, thereby suppressing the growth of lithium dendrites. The study demonstrates the promising application potential of the UP3D separator in high-flux, long-life lithium-ion batteries [126].

Liu et al. (2025) [127] developed a fully solid-state hydrophobic ionic conductor (PILEs) based on acrylamidoethyltrimethylammonium bis(trifluoromethanesulfonyl)imide. By introducing a compatible acrylate monomer, a stable “rigid-flexible” cross-linked network was constructed within the material (Figure 10d,e). This network features adjustable tensile strength (with a stress range of 0.41–8.05 MPa), high tensile performance (maximum elongation at break of 820.4%), and remarkable toughness (maximum toughness value of 27.53 MJ/m³). Based on a reversible electrostatic interaction mechanism, the material exhibits excellent energy dissipation and rapid strain self-recovery properties. Even under a large strain of 400%, the energy dissipation recovery rate remains at 91.31% after 3 min. After multiple cyclic loadings, the material maintains an 81% stress retention rate, demonstrating good fatigue resistance [127].

5. Multifunctional Integration of Separators

By integrating various functional materials, intelligent separators can initially achieve diverse functions such as thermal shutdown, dendrite inhibition, ion sieving, and polysulfide management. In particular, the introduction of the “sensing-response” integration and multilayer structural design concepts enables the separator to achieve multifunctional synergy and optimization under complex working conditions.

5.1. Integrated “Sensing-Response” Separators

Sun et al. (2025) [94] developed an ultra-thin, flexible, multifunctional sensor array that can be embedded into battery packaging films. This array is fabricated using fully printable technology and can simultaneously monitor multiple parameters such as temperature, pressure, strain, hydrogen, dimethyl carbonate vapor, and liquid electrolytes. By integrating with deep learning algorithms, the system can assess battery health and diagnose faults (such as overcharge/overdischarge, internal short circuits, thermal runaway, etc.), and it can directly trigger an alarm when a hazard is detected, demonstrating a closed-loop management capability of perception decision and execution [94].

In the field of integrated mechanical sensing and response for lithium batteries, significant progress has been made in the research of intelligent separators that can simultaneously monitor internal pressure and temperature changes. For example, Li et al. (2023) [128] developed a composite film sensor based on P (VDF-TrFE)/PEDOT:PSS. This material, through a designed interconnection interface structure, possesses both a high piezoelectric coefficient (d₃₃ ≈ −86 pC N⁻¹) and a high pyroelectric coefficient (p ≈ 95 μC m⁻² K⁻¹), with a sensitivity of 2.2 V kPa⁻¹ in the pressure range of 0.025–100 kPa and 6.4 V K⁻¹ in the temperature range of 0.05–10 K, providing a material foundation for achieving dual-mode force and thermal sensing inside batteries [128]. Yu et al. (2024) [129] utilized the piezoelectric-pyroelectric effect of PMN-PT single crystals to develop a self-powered pressure-temperature dual-mode sensor, with a pressure response time of 46 milliseconds and sensitivity of 28.4 nA kPa⁻¹, and a temperature response time of 1.98 s and sensitivity of 94.66 nC°C⁻¹. They leveraged the difference in response speeds to distinguish and monitor pressure and temperature signals in mixed force-thermal stimuli, providing a new strategy for decoupling multi-physical field signals under complex battery operating conditions [129].

In the area of integrated chemical sensing and regulation, Zheng et al. (2024) [130] synthesized a defect-free, nanoscale-thick crosslinked polyamide (PA) layer on a polypropylene (PP) separator using in situ polymerization technology. This polyamide modification layer, only 1.5 nanometers thick, can effectively block the diffusion of polysulfides. Lithium-sulfur batteries assembled with this functional separator exhibited a capacity decay rate of only 0.06% after 450 cycles at a 1 C rate, and the lithium-sulfur pouch batteries maintained 87.63% capacity after 50 cycles [130].

5.2. Intelligent Separators with Multilayer Structure Design

The multilayer structure design addresses the challenge of a single material struggling to balance multiple properties by spatially separating and synergistically integrating functional layers.

Feng et al. (2020) [131] designed a polyvinylidene fluoride-hexafluoropropylene/Li_6.75La₃Zr_1.75Ta_0.25O₁₂/polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP/LLZTO/PVDF-HFP) trilayer separator with flame retardant and thermal stability properties. The outer PVDF-HFP layer forms a compatible interface with the battery electrodes, while the inner LLZTO layer enhances thermal stability by undergoing only minor deformation at 300 °C. The LiFePO₄ half-cell using this trilayer separator achieves a discharge capacity of up to 153 mAh g⁻¹ at a 0.5 C rate, with a capacity retention rate of up to 95% after 200 cycles [131]. Jiang et al. (2024) [117] developed a multilayer separator named ASPESA. This separator is made by coating two layers of low-density polyethylene (LDPE) and a thin layer of Al₂O₃ on both sides of a polyethylene film, featuring a shutdown function at 120 °C and enhanced thermal stability at 185 °C (with a thermal shrinkage rate of only 1%). Additionally, its electrolyte wettability and absorption rate (407.23%) are improved. In LiFePO₄||Li batteries, the ASPESA separator can still provide a discharge capacity of 144.5 mAh g⁻¹ after 900 cycles, with a capacity retention rate of up to 98.9% [117].

Wang et al. (2024) [132] prepared a microfiber/nanofiber/halloysite (GOP-PH-ATP) multilayer separator with a pore size gradient structure (pore sizes ranging from tens of micrometers to hundreds of nanometers). This separator exhibits a porosity of up to 95% and an electrolyte absorption rate of 760%, with an ionic conductivity of 2.38 mS cm⁻¹ and a lithium-ion transference number of 0.62. Batteries using the GOP-PH-ATP separator maintain a capacity retention rate of 91.2% after 150 cycles at a 1 C rate [132]. Zhou et al. (2024) [133] reported a composite separator (SSCS) with an asymmetric pore structure, featuring small surface pores and large internal pores. This design not only provides the separator with high liquid storage capacity and low leakage rate but also enhances thermal stability and imparts high-temperature pore-closing characteristics. When used in LiFePO₄/Li batteries, this composite separator exhibits discharge capacity and cycling stability superior to the commercial separator Celgard 2400 [133].

6. Challenges and Future Prospects

Despite the significant potential of smart responsive separators in enhancing the safety and performance of lithium batteries, the transition from laboratory to industrialization still faces many challenges. Simultaneously, as new materials, processes, and concepts continue to emerge, this field also exhibits exciting developmental trends.

6.1. Challenges

The long-term chemical/electrochemical stability of functional materials is the primary obstacle to the practical application of smart separators. Most smart materials, such as thermo-responsive polymers and MOFs/COFs frameworks, struggle to maintain long-term stability in high-voltage, high-lithium-salt-concentration electrolytes. Developing new material systems that combine high stability with smart response is a key research direction for the future.

Within the battery, the vast amount of data generated by multi-physical field sensors (temperature, pressure, chemical concentration, etc.) often presents strong coupling and nonlinear characteristics. Traditional battery management systems find it challenging to accurately decouple and interpret these signals. For instance, changes in separator resistance may simultaneously result from temperature fluctuations, dendrite growth, or local side reactions. Distinguishing these signals requires the establishment of precise physicochemical models and intelligent algorithms. The development of specialized integrated circuits and edge computing platforms to achieve real-time processing and feature extraction of sensing signals is essential to unlocking the potential of intelligent separators.

Finally, the challenges of large-scale production processes and costs also constrain the industrialization of smart separators. Current smart separator preparation often relies on laboratory technologies such as electrospinning, atomic layer deposition, and layer-by-layer self-assembly, which have low production rates, difficult yield control, and large equipment investments. From this perspective, developing technologies compatible with existing separator production lines while reducing the production costs of high-end raw materials (such as MXene, MOFs) is crucial for promoting the commercial application of smart separators.

6.2. Future Prospects

By constructing digital models in virtual space that correspond precisely with physical separators, and integrating multi-physical field simulations and real-time sensing data, dynamic mapping and predictive control of internal battery states can be achieved. For example, based on separator temperature and pressure sensing data, combined with electrochemical–thermal coupling models, thermal runaway risks can be predicted several minutes in advance. By integrating ion flow distribution data and lithium deposition models, charging strategies can be optimized to suppress dendrite formation. Digital twin separators will break the “black box” state of batteries, enabling transparent lifecycle management.

Future intelligent separators will integrate multimodal sensors for temperature, pressure, chemistry, ion flow, and more to form a comprehensive sensing network. By incorporating microprocessors and actuators, a closed-loop control of “sensing-decision-execution” can be realized. For instance, when dendrite growth signals (slight pressure increase + local ion concentration anomaly) are detected, the charging current can be automatically adjusted, and self-healing coatings can be activated. When thermal risks are detected, the thermal shutdown mechanism can be triggered in advance. This full-chain intelligence will upgrade separators from passive components to the “autonomous safety steward” of batteries.

By analyzing historical operational data through deep learning, models correlating battery health with separator sensing signals can be established, enabling early fault diagnosis and remaining life prediction. For example, convolutional neural networks can identify dendrite growth pressure distribution patterns; recurrent neural networks can predict the probability of thermal runaway based on temperature and impedance time-series data. Combined with reinforcement learning algorithms, smart separators can adaptively adjust operational parameters, achieving an optimal balance between battery performance and safety.

The next generation of intelligent separators will pay more attention to environmental protection and sustainability. On one hand, by integrating piezoelectric/triboelectric nano-generators, energy can be harvested from mechanical vibrations during the battery charging and discharging process to power the sensing system, eliminating the need for external power sources. On the other hand, developing degradable bio-based smart separators, coupled with renewable functional materials, can significantly reduce environmental pollution. These environmentally friendly design concepts align with global strategies for sustainable development and are an inevitable trend in the development of electrochemical energy storage technologies.

Globally, incidents of lithium battery fires are frequent, leading to costly recalls, legal liabilities, and infrastructure damage. In response, China has implemented the mandatory “Safety Requirements for Power Batteries Used in Electric Vehicles” standard, setting “no fire, no explosion” as the baseline. Similarly, the European Union’s new battery regulations impose strict requirements on battery safety and sustainability. In the United States, several states have legislated to establish battery management frameworks, including Washington D.C., Vermont, California, Illinois, Washington State, and Colorado. These regulatory measures are vigorously promoting the development of intrinsic safety technologies, such as smart separators, which are expected to become standard configurations in high-risk applications like electric vehicles and energy storage systems.