Multi-scale interface regulation: a strategy for function-oriented modification of PVDF-Based composite electrolytes

Solution casting method

The solution casting method is a classic and widely used technique for manufacturing PVDF-based composite electrolytes^[50]. The core steps involve dissolving the PVDF polymer, lithium salt, and functional fillers in an organic solvent to form a homogeneous solution. Then, the solution is coated onto the substrate, followed by solvent evaporation and vacuum drying, resulting in a composite electrolyte membrane^[51]. The key to this method lies in controlling the rate of solvent evaporation and environmental conditions, thereby obtaining a dense and uniformly performing film. The basic principle of this technology is that during the solvent evaporation process, the solutes in the solution gradually accumulate and solidify into a layer of film. When preparing a composite electrolyte based on PVDF, the first step is to dissolve the PVDF polymer, the specific lithium salt, and possibly added functional fillers in an appropriate organic solvent to form a homogeneous mixed solution. The PVDF polymer serves as the matrix, providing mechanical support and the basic ion-conducting framework, while the lithium salt acts as the ion source for the electrolyte^[52]. Adding functional fillers can enhance the specific properties of the electrolyte, such as improving ionic conductivity, enhancing mechanical strength, or increasing thermal stability. Figure 4^[53] shows one type of PVDF electrolyte membrane prepared by the solution casting method.

Figure 4. Schematic illustration of the HFP-PC-FEC-Sn QSPE fabrication process^[53]. Reproduced from Ref.^[53] with permission from Wiley. PVDF: Polyvinylidene fluoride; DMC: dimethyl carbonate; HFP-PC-FEC: poly (vinylidenefluoride-co-hexafluoropropylene)-propylene carbonate-fluoroethylene carbonate; QSPE: quasi-solid polymer electrolyte.

The solution casting process has many important influencing factors. The selection of the solvent is crucial. The solvent not only needs to have good solubility for PVDF and other solutes, but also should have an appropriate evaporation rate. If the evaporation rate is too fast, it may cause membrane pores and defects, thereby affecting the performance of the electrolyte. On the other hand, if the evaporation rate is too slow, it will prolong the preparation cycle and reduce the production efficiency. Besides N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) can also be used as a solvent, but DMSO has a higher boiling point and a relatively slower evaporation rate, so more precise control of the drying conditions is required^[54]. The temperature and humidity of the environment are also important influencing factors. High temperature may cause thermal degradation or phase separation of the polymer, while low temperature will slow down the evaporation rate of the solvent^[55]. When the humidity is high, water may be mixed into the membrane, thereby affecting the ionic conductivity and chemical stability of the electrolyte, especially for lithium salts and fillers that are sensitive to water^[55]. The concentration of the solution and the thickness of the coated film will also have a significant impact on the performance of the final product. If the solution concentration is too high, it will make the solution too thick and difficult to apply evenly, and may cause large internal stress in the membrane, thereby leading to cracking and other problems^[56]. On the other hand, if the concentration is too low, it will prolong the evaporation time of the solvent, which may result in insufficient mechanical strength of the membrane^[56]. An overly thick coating film will increase the ionic transmission path, reducing ionic conductivity^[57]; an overly thin coating film may result in poor mechanical properties of the membrane, and it is prone to damage during subsequent battery assembly and use^[58].

The solution casting method has many advantages. This method is relatively simple to operate and does not require complex equipment or high costs. It offers great convenience in laboratory research and small-scale production. It can precisely control the proportions of each component, and by adjusting the formula, it can flexibly prepare composite electrolytes with different properties to meet the needs of different application scenarios. This method can also achieve a certain degree of regulation of the microstructure of the membrane. However, this method also has certain limitations. The solvent evaporation process takes a long time, resulting in relatively low production efficiency, which poses a significant constraint in large-scale industrial applications that require high output. Moreover, the consumption of a large amount of organic solvents not only increases operating costs but also raises concerns about potential environmental pollution. Proper recycling and treatment need to be carried out; during the preparation process, some microscopic defects in the membrane, such as pores and impurities, are difficult to avoid, and these defects may affect the stability and consistency of the electrolyte performance.

In practical applications, PVDF-based composite electrolytes prepared by the solution casting method have extensive applications in solid-state lithium batteries. Zhu et al.^[50] used the solution casting method to prepare PVDF-based composite solid electrolytes. With PVDF as the core matrix, they combined LiFSI lithium salt, dimethylformamide (DMF) ligand, and 5%-35% mass fraction of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) filler to obtain the PVLZ (PVDF-LLZTO composite solid electrolyte) series of electrolytes. The team partially replaced DMF with 49 mol% NMTFA (N-methyl-2,2,2-trifluoroacetamide) to obtain the PVLN series of electrolytes. Meanwhile, the good compatibility of PVDF ensures the stability of the system, and the ionic conductivity of the electrolyte is improved. This case fully demonstrates the feasibility and effectiveness of the solution casting method in preparing high-performance PVDF-based composite electrolytes. By reasonably selecting fillers and precisely controlling the preparation process, a composite electrolyte with excellent electrochemical performance can be obtained, providing strong support for the development of solid-state lithium batteries.

Electrospinning method

Currently, electrospinning technology is the mainstream method for preparing PVDF piezoelectric nanofibers^[59,60]. It can significantly increase the content of the β phase and the piezoelectric response of the material^[61,62]. The principle is to apply a high-voltage electric field between the polymer solution in the metal spinner head and the grounded collector. Under the combined effect of surface tension, gravity, and electric stress, the solution first forms a Taylor cone and then jets out as a liquid stream. During the stirring and stretching process of the liquid fluid, the solution gradually solidifies and eventually forms a nanofiber membrane^[60,63]. This special fiber structure can effectively enhance the piezoelectric properties of PVDF [Figure 5]^[59,64]. This preparation method has demonstrated unique technical advantages and broad application potential in the research and development of PVDF-based composite electrolytes^[51]. It is worth noting that key process parameters such as voltage, solution concentration, and injection speed play a decisive role in the final morphology of the fibers^[60].

Figure 5. (A) Schematic illustration of the electrospinning process and mechanism. The right part shows the formation of an electrospinning jet through a Taylor cone, a transition zone, whipping, and terminal fiber regions under complex forces (e.g., surface tension, gravity force, electrical stress, viscosity drag force, and electric polarization stress). After the transition zone, the jet follows a path through three sequential bending instabilities^[59]. Reproduced from Ref.^[59] with open access from Elsevier; (B) Electrospinning apparatus set-up and nanofiber mesh^[64]. Reproduced from Ref.^[64] with open access from Springer Nature.

Take the preparation of PVDF/LiTFSI (tetrabutyl trifluoromethanesulfonate lithium)/Al₂O₃ composite electrolyte as an example^[65]. The specific preparation steps are as follows: First, a certain amount of PVDF powder is dissolved in a mixed solvent of DMF and acetone. Then, the mixture is stirred at a certain temperature until complete dissolution is achieved, resulting in a uniform PVDF solution. Next, the PVDF solution is added to a certain proportion of lithium salt LiTFSI, and continued stirring is performed to ensure complete dissolution. Subsequently, the surface-modified nano Al₂O₃ particles are dispersed in the above solution and fully dispersed using an ultrasonic dispersion device to obtain a uniform and stable spinning solution. The spinning solution is loaded into a syringe with a 21G needle, and the electrospinning parameters are set, such as the applied voltage of 18 KV, the solution injection speed of 0.5 mL/h, and the fiber receiving distance of 15 cm. The electrospinning equipment is turned on, and after a period of spinning, the PVDF-based composite electrolyte nanofiber membrane can be collected on the receiving device.

During this process, numerous process parameters have a significant impact on the morphology and properties of the fibers^[60]. Voltage is a key parameter. Only when the voltage applied to the polymer solution exceeds the critical voltage and the electrostatic repulsive force is greater than the surface tension can the spinning process proceed smoothly. Generally speaking, the higher the voltage, the stronger the electric field force, and the greater the pulling force on the solution, which makes the fibers thinner. When the applied voltage is too high, the jet becomes unstable and forms a structure similar to beads. This is because an excessively high voltage causes the jet to carry too much charge, thereby increasing the Coulomb force and making the jet unstable. The concentration of the solution is also very important. The concentration of the solution is the key factor determining the degree of entanglement of the molecular chains in the solution. As the concentration increases, the viscosity of the solution also rises. The diameter of the fibers increases with increasing viscosity, which is beneficial for the formation of fibers. However, if the viscosity is too high, it may prevent fiber formation or result in discontinuous fibers^[60]. During the electrospinning process, if the concentration and viscosity of the solution are low, only polymer droplets can be obtained. This is because the solution jet is stretched under the force in electrospinning, and the molecular chains do not entangle or entangle insufficiently, resulting in breakage. The injection speed of the solution cannot be ignored either. The injection speed of the polymer solution to some extent determines the amount of spinnable solution in the electrospinning process. For a given voltage, a relatively stable Taylor cone will be formed at the nozzle. The injection speed will affect the state of the Taylor cone. A lower injection speed will cause the jet to be unstable; a higher injection speed will cause the Taylor cone to jump, thereby affecting the morphology and structure of the fibers. When the flow rate is too fast, the fibers will have a larger diameter and even a droplet phenomenon; when the flow rate is too low, the flow is slow, and bead structures will be obtained.

Electrospinning technology has numerous applications in energy and environmental sciences^[66]. The PVDF-based composite electrolytes prepared by this method possess a nano-fiber structure with high porosity and large specific surface area. This unique structure can open up more channels for ion transmission, which helps to increase the ion mobility. The three-dimensional network structure composed of nanofibers endows the electrolyte with excellent mechanical properties, ensuring the integrity of its structure throughout the battery assembly and usage process. It can effectively suppress the growth of lithium dendrites, thereby enhancing the safety and cycling stability of the battery. Additionally, electrospinning technology boasts advantages such as simple operation, controllable size and arrangement of the resulting materials, and excellent scalability for large-scale production. It can produce one-dimensional nanomaterials with various structures such as solid, hollow, porous, core-shell, and interconnected. By combining with other methods (such as gas-solid reactions, hot pressing, solution casting, sol-gel method, etc.), different compositions and structures of composite materials can be fabricated. This can further expand its application in the preparation of high-performance PVDF-based composite electrolytes.

The PVDF-based composite electrolyte prepared by electrospinning has demonstrated excellent performance in the practical application of solid-state lithium batteries. The inorganic fillers in the electrolyte interact with the polymer matrix through Lewis acid-base interactions, forming more interface layers, which significantly improves the ionic conductivity. The solid-state lithium metal battery based on this composite electrolyte demonstrated excellent charge-discharge performance and cycle stability. Hidayat et al.^[67] fabricated a Li₂CO₃-modified PVDF composite spinning solution and adopted electrospinning technology to construct a three-dimensional PVDF-Li₂CO₃ nanofiber network(as shown in Figure 6), serving as a lithium host on copper foil. Zheng et al.^[68] used PVDF as the core material and employed coaxial electrospinning technology to fabricate a three-dimensional flame-retardant quasi-solid-state composite electrolyte with a fibrous structure. Through this structural design, they were able to inhibit lithium dendrite growth. Chen et al.^[69] used electrospinning technology to blend the vinyl PVDF membrane (PVDF-a) with the modified mercaptocarbon nanotubes (CNT-s) to prepare a spun film. Subsequently, PDMS was coated and grafted onto the surface of the spun film, resulting in a PVDF-a/CNT-s@PDMS composite film that possesses both superhydrophobicity and photothermal/electrothermal properties. Wu et al.^[70] reported the application of the porous framework electrolyte prepared by electrospinning in lithium-oxygen batteries. The gel polymer electrolyte prepared by electrospinning has a three-dimensional fiber structure, which can enhance the adsorption capacity of the liquid electrolyte. This interconnected structure promotes strong interactions between Li⁺ and the polar groups in the polyacrylonitrile (PAN) matrix, further enhancing the ionic conductivity and structural stability. The lithium symmetric batteries and lithium-oxygen batteries assembled with this electrolyte exhibit excellent cycling performance under high current density, opening up new possibilities for practical applications.

Figure 6. Schematic illustration of the PVDF-Li₂CO₃ nanofiber network preparation^[67]. Reproduced from Ref.^[67] with open access from SciOpen. PVDF: Polyvinylidene fluoride; DMF: dimethylformamide.

Hot pressing method

The hot pressing method is a preparation technique that uses molds to shape materials under high temperature and high pressure^[71]. This method has unique process characteristics and application value in the preparation of PVDF-based composite electrolytes. The basic principle can be summarized as the structural evolution and interface optimization under the coupling effect of heat and force. Under high-temperature conditions, the thermal motion of PVDF molecular segments is significantly intensified, and the inter-chain interaction forces are correspondingly weakened, resulting in a good visco-plasticity of the system. At the same time, the synergistic effect of external pressure effectively promotes the interface wetting and uniform dispersion between the PVDF matrix and components such as lithium salts and inorganic fillers, thereby achieving densification sintering and geometric shaping of the composite electrolyte system. The synergy of pressure and temperature can effectively improve the microstructure and performance of the composite electrolyte^[51,72].

During the hot pressing process, the process parameters have a crucial impact on the performance of the electrolyte, including temperature, pressure, and curing time^[73]. Temperature is a key factor influencing the molten state of PVDF and the activity of its molecular chains. At lower temperatures, PVDF cannot fully melt, and the bonding between components is not tight enough, resulting in a low density of the electrolyte membrane, an unsmooth ion conduction channel, and thus a low ion conductivity. As the temperature rises, the melting degree of PVDF will increase, and the activity of the molecular chains will also enhance. This is conducive to the uniform dispersion of each component and the formation of ionic conductive channels. However, when the temperature exceeds a certain limit, PVDF may undergo thermal degradation, which will subsequently lead to the breakage of the molecular chains and changes in the chemical structure^[74]. This situation not only reduces the mechanical properties of the electrolyte, but also may damage its electrochemical stability, thereby causing a decrease in ionic conductivity. Studies have shown that pressure has a significant regulatory effect on the microstructure, crystal structure, and electrical properties of PVDF films. Moreover, the key process parameter, the hot pressing duration, also has an important impact on the comprehensive performance of the electrolyte. Short periods of hot pressing can lead to insufficient interdiffusion and integration between the components, resulting in uneven membrane performance, poor ion conductivity, and poor mechanical properties. As the heat pressing time increases, each component has enough time for sufficient interaction and diffusion, and the membrane performance will gradually improve. However, an excessively long heat pressing time not only increases production costs but also may cause PVDF to undergo thermal aging, thereby affecting the stability of the electrolyte performance.

The hot pressing method has significant advantages in industrial production^[75]. This method enables continuous production and is suitable for large-scale preparation of PVDF-based composite electrolytes, with high production efficiency. During the thermal pressing process, precise control of the shape and size of the mold can produce electrolyte films of specific specifications, adapting to the usage requirements of different battery systems, and showing good product consistency. The electrolyte films prepared by the hot pressing method have high density and excellent mechanical properties, which can effectively enhance the safety and stability of batteries and have broad application prospects. Nevertheless, the practical application of the hot-pressing technique remains confronted with several technical challenges. First, the intrinsic requirement for concurrent high-temperature and high-pressure operation imposes stringent demands on production equipment, entailing substantial capital investment. More critically, under such aggressive thermo-mechanical conditions, the PVDF-based composite electrolyte is susceptible to undesirable interfacial reactions with the mold material, which may induce progressive corrosion and abrasive wear of the tooling surfaces, thereby compromising both process stability and mold service life. During hot pressing, due to inhomogeneity of temperature and pressure, there may be certain differences in the performance of the electrolyte membrane. Therefore, it is necessary to further optimize the hot-pressing process and equipment to improve the uniformity of temperature and pressure, thereby ensuring the stability of product quality.

In practical applications, the PVDF-based composite electrolytes prepared by the hot-pressing method have achieved certain results in the field of solid-state lithium batteries. Qu et al.^[76] proposed a thermal-mechanical processing technique at 140 °C with a pressure of 600 MPa for 3 min. The preparation process of the above two studies is shown in Figure 7A. This process transformed the PVDF nanofibers from electrospinning into a dense and highly transparent piezoelectric film, enhancing transparency and mechanical rigidity and reducing haze. Shen et al.^[71] achieved the transformation of the α phase of PVDF film into the highly polar β phase through thermal mechanical induction [Figure 7B], resulting in a PVDF film with a β phase content of 97.5% and possessing excellent piezoelectric and flexible mechanical properties. This fully demonstrated the feasibility and advantages of the hot pressing method in preparing high-performance PVDF-based composite electrolytes, providing important technical support for the industrial production of solid-state lithium batteries. However, for the hot pressing method to be widely applied in industrial production, further solutions to issues such as high equipment costs, short mold life, and product quality stability need to be addressed. Through technological innovation and process optimization, the comprehensive performance and production efficiency of the PVDF-based composite electrolyte prepared by the hot pressing method can be continuously enhanced.

Figure 7. (A) Illustration of the electrospinning, thermomechanical pressing, and ice quenching processes^[75]. Reproduced from Ref.^[75] with open access from Springer Nature, licensed under CC BY-NC-ND, no modifications to original figure; (B)Schematic diagrams of the folding-pressing process in polymer films, as well as a schematic diagram illustrating the mechanism of β-crystalline phase formation throughout the entire process^[76]. Reproduced from Ref.^[76] with open access from Wiley. PVDF: Polyvinylidene fluoride; OAF: oriented amorphous fraction.

Gelation method

The preparation of composite electrolytes through gelation is a technique that combines the high ion transport efficiency of liquid electrolytes with the stability and safety of solid electrolytes^{[13,36,77,78]}. Through processes such as solution mixing and gradient temperature casting, PVDF can form a uniform porous three-dimensional network structure, which is used to load lithium salts, plasticizers/ion liquids, and LLZO functional fillers, thereby forming a gel-like electrolyte^[29]. Figure 8^[79] illustrates one method for constructing such a porous PVDF framework based on the gelation phase transition: Under heating conditions, the green methyl lactate dissolves PVDF through intermolecular hydrogen bonds, followed by cooling and water phase separation, resulting in interconnected pores for the immobilization of liquid ionic conductive components.

Figure 8. Schematic diagram of the preparation process of PVDF membrane and the polymer solution transformation during the cooling process. The suspension of PVDF and methyl lactate is heated from 25 °C to 95 °C. PVDF dissolves through hydrogen bond interactions to form a uniform solution; after cooling to 50 °C, a sol-gel transformation occurs^[79]. Reproduced from Ref.^[79] with permission from the Royal Society of Chemistry. PVDF: Polyvinylidene fluoride.

The merit of this preparation method lies in its multidimensional synergistic effect. Specifically, the dispersed liquid phase serves as a rapid ion-conducting pathway, endowing the composite electrolyte with favorable ionic conductivity even at ambient temperature. Concurrently, the PVDF matrix contributes superior mechanical robustness, which not only effectively suppresses lithium dendrite growth but also mitigates the risks of electrolyte leakage and combustion, thereby enhancing overall safety and operational reliability. This design achieves coordinated optimization of ion conduction, mechanical support, and electrochemical safety, expanding the application potential of high-voltage batteries. The basic principle of this preparation method is to utilize the interactions between polymer molecular chains and physical or chemical cross-linking with other components to form a stable gel structure. In this gel structure, the liquid plasticizer is fixed in the pores of the polymer network, while the lithium salt is dissolved in the plasticizer, providing channels for ionic conduction, and the PVDF polymer matrix provides mechanical support for the electrolyte.

The interaction between lithium salts, plasticizers, and the polymer matrix is of crucial importance in the preparation process of gels^[80]. The type and concentration of the lithium salt directly determine the number and activity of conductive ions in the electrolyte^[81]. Under the same polymer matrix and plasticizer system, due to the different anion properties of different lithium salts, their dissociation degree and ion migration rate are different, which affects the ionic conductivity of the electrolyte. Studies have shown that LiTFSI has a high dissociation degree and ion migration number in PVDF gel electrolytes, so the ionic conductivity of the electrolyte is relatively high^[82]. When the plasticizers ethylene glycol monobutyl ether and dimethyl carbonate are mixed in a 60:40 mass ratio (within a 30% to 40% weight ratio range), their conductivity is approximately 10^-3 S cm^-1[83]. The structure and performance of the polymer matrix also have a significant impact on the performance of the electrolyte^[36]. By adjusting the preparation process or introducing copolymer monomers, the crystallinity of PVDF can be reduced, and the proportion of the amorphous region increases, thereby improving the ionic conductivity. Compared with pure PVDF, the crystallinity of PVDF-difluoromethyl acrylate copolymer is lower, and its ionic conductivity is higher^[84], making it more suitable as the polymer matrix for gel electrolytes.

The gelation-prepared PVDF-based composite electrolyte has unique advantages in lithium-ion battery applications. This electrolyte has high ionic conductivity, which can meet the demand for rapid lithium-ion transmission during the charging and discharging process of batteries^[85]. The excellent mechanical properties and flexibility of this electrolyte can effectively inhibit the growth of lithium dendrites, improving the safety and cycle stability of the battery^[85]. It shows broad application prospects in fields such as electric vehicles and portable electronic devices. The energy storage material research team of Huaneng Energy Research Institute prepared a gel polymer electrolyte (PSGPE) based on poly(vinylidene fluoride-co-hexafluoropropylene)-butanedinitrile (PVDF-HFP-SN) using the impregnation precipitation method^[86]. This example fully demonstrates the outstanding advantages of PVDF-based composite electrolytes prepared by the gel method in lithium-ion batteries. The team successfully optimized the microstructure and crystallinity of the PVDF-HFP matrix by adjusting parameters such as the addition amount and solution concentration of high-polarity plastic crystals SN through the process shown in Figure 8B. On one hand, the electrolyte possesses a well-developed interconnected porous architecture, which furnishes ample space for the accommodation of the liquid electrolyte. On the other hand, the synergistic effects of two factors - namely, the reduced crystallinity of the PVDF-HFP matrix and the high dielectric polarity of SN - collectively diminish the energy barrier for lithium-ion migration. As a consequence, the as-prepared PSGPE achieves a room-temperature ionic conductivity exceeding 1 mS·cm^-1 alongside an electrochemical stability window surpassing 5 V, both of which adequately satisfy the stringent requirements for rapid lithium-ion transport. In the LiFePO₄ all-cell test, this electrolyte exhibited excellent charge-discharge performance and rate capability^[87]. After cycling at a current density of 0.5 C for 350 rounds, the battery capacity still remained at 119.2 mAh·g^-1, with a capacity retention rate of up to 93.1%. Meanwhile, by adjusting the dosage of SN, the mechanical properties of PSGPE were precisely optimized. Combined with the formed “lithium-rich fluorine” solid-state electrolyte interface film, it can effectively achieve uniform dispersion and deposition of lithium ions, significantly inhibiting the growth of lithium dendrites. This combination of high ionic conductivity and mechanical stability enables the battery to maintain structural safety during long-term cycling, providing a feasible solution to the requirements of high battery safety and endurance.

The influence of electrolyte structure on the lithium ion transport mechanism

The core advantage of PVDF as the electrolyte matrix for lithium-ion batteries stems from its unique molecular and crystal structure, which enables precise control of the lithium ion transport mechanism^[87,88]. The molecular chains of PVDF are composed of repeating units (CH₂-CF₂)_n, and these units are combined together in a highly regular zigzag arrangement. The strong electron-withdrawing groups (-C-F) not only give the material excellent electrochemical stability but also create a special ionic environment through the difference in electronegativity. This structural feature directly affects the dissociation efficiency and migration path of lithium ions in the electrolyte, serving as a key entry point for understanding its transport mechanism.

As a semi-crystalline polymer, the crystal zone/non-crystalline zone structure and the diversity of its crystal forms are the core dimensions for regulating lithium ion transport. Among the four known crystalline polymorphs of PVDF - namely, the α, β, γ, and δ phases - the α phase, characterized by a monoclinic lattice and a TGTG molecular chain conformation, exhibits a zero net dipole moment. Consequently, it is incapable of facilitating lithium salt dissociation via polar interactions, thereby exerting no appreciable influence on lithium-ion transport. In marked contrast, the β phase, featuring an all-trans (TTTT) chain conformation, possesses the highest dipole moment and thus emerges as the key structural motif for optimizing ion-transport kinetics^[89]. This enhancement can be attributed to a dual mechanism. On one hand, the pronounced polarization of the β phase attenuates the ionic binding within the lithium salt through dipole-ion interactions, thereby promoting the effective dissociation of Li⁺ from its counteranions and elevating the concentration of free lithium ions. On the other hand, the orderly aligned molecular chains give rise to a directional local electric field, which furnishes a channelized migratory pathway for lithium ions and effectively mitigates stochastic diffusion losses during transport^[89].

From the perspective of macroscopic structure and interface interaction, the film-forming property and swelling characteristics of PVDF indirectly regulate the lithium ion transport pathway by influencing the microstructure of the electrolyte^[90]. The intrinsically low swelling ratio of the material contributes to the structural stability of the electrolyte, effectively preventing deformation of ion-transport channels that would otherwise arise from volume expansion. Furthermore, the self-supporting thin films, fabricated via a phase-inversion method with a thickness ranging from 10 to 100 μm, are capable of establishing uniformly continuous ion-conducting pathways, thereby substantially reducing interfacial impedance during ion migration. In addition, the matching of PVDF’s surface energy with lithium metal can optimize the contact state of the electrode/electrolyte interface, reduce the interface charge transfer impedance, and further improve the efficiency of lithium ion transport across the interface^[91]. As shown in Figure 9A and B^[92], at the lithium metal interface, the heterogeneous ion flux of the electrolyte based on PVDF-HFP will cause uneven local current density and lead to dendrite growth. However, after introducing functional fillers to achieve uniform ion flux, the rich inorganic SEI film can effectively inhibit dendrites, which directly verifies the enhancing effect of interface regulation on transmission efficiency.

Figure 9. (A) Illustration of the Li ion transport pathway, LCD, SEI, and Li deposition morphology in PVDF HFP-based polymer electrolyte; (B) Diagram depicting the Li ion transport pathway in LPS_x and the interaction between PVDF-HFP and WS_x, and the representation of LCD, SEI, and Li deposition morphology in LPS_x CPE; (C) Effects of band engineering on the increase of p-band, s-p hybridization on the surface of WS_x, and the corresponding LiFSI dissociation and absorption processes^[92]. Reproduced from Ref.^[92] with permission from Wiley. LCD: Local current density; SEI: solid electrolyte interphase; PVDF: polyvinylidene fluoride; HFP: hexafluoropropylene; SV: sulfur vacancy; LPS_x: lithium polysulfides.

The dielectric properties of PVDF and the strength of the intermolecular interactions further determine the kinetics of lithium ion transport. The dielectric constant of PVDF is significantly higher than that of the traditional polyethylene oxide matrix. Its high dielectric constant and polar molecular chains can provide a favorable environment for the dissociation of lithium salts^[93-96]. Meanwhile, the weak coordination interaction formed between the fluorine atoms in PVDF and lithium ions has a “moderately adjustable” characteristic. This avoids the ion transmission blockage caused by excessive aggregation of the lithium salt, and also prevents the limitation of lithium ion desorption and migration due to overly tight binding^[91]. This balanced state is the key mechanism that ensures efficient lithium-ion transmission. As shown in Figure 9C^[92], the orbital interactions visually reveal the microscopic energy changes during the dissociation of the lithium salt.

It should be noted that the crystal structure of PVDF also has limitations: high crystallinity leads to a decrease in the proportion of the amorphous region, and lithium ions mainly diffuse and transfer in the amorphous region. Therefore, the ionic conductivity of pure PVDF electrolyte is relatively low^[97-99]. Researchers have discovered that the crystal structure of PVDF can be regulated through various means such as solvent or temperature control, mechanical stretching, electric field polarization, or nano-filler manipulation^[100]. This structural adjustment directly corresponds to the dual improvement of the concentration and transmission sequence of free lithium ions in the electrolyte, and is the core method for optimizing transmission performance. Guo et al.^[101] demonstrated that temperature plays a crucial regulatory role in defect formation by conducting a comprehensive characterization process (as shown in Figure 10A), and defect control can lead to the formation of the β phase, thereby increasing the dielectric constant of PVDF. Based on this, they developed an organic catalytic photo-redox polymerization method with a wide temperature range, enabling the controlled preparation of PVDF. Zhang et al.’s research^[102] subjected the PVDF film to uniaxial stretching and then carried out solid-state pressure molding. By evaluating the electrothermal effect and the distribution of β phases (as shown in Figure 10B), they found that the performance of PVDF was significantly improved. Sahu et al.^[103] adjusted the proportion of the amorphous region using nanofillers and improved the ionic conductivity of PVDF. It was verified by characterizing the structural evolution mechanism and ionic transport performance of PVDF-based composite solid-state electrolytes.

Figure 10. (A) Characterization of PVDF synthesized at different temperatures, including chain defects and degree of crystallinity versus polymerization temperature, XRD patterns of the PVDF powder and film, FTIR spectra of PVDF films, dielectric spectroscopies of PVDF films, dielectric constants and dielectric losses of PVDF, and dielectric constants of PVDF with different degrees of defects^[101]. Reproduced from Ref.^[101] with permission from the American Chemical Society; (B) Electrocaloric effect and β phase distribution in solid-phase pressure-formed PVDF^[102]. Reproduced from Ref.^[102] with open access from Springer Nature, licensed under CC BY-NC-ND, no modifications to original figure. PVDF: Polyvinylidene fluoride; FTIR: fourier-transform infrared spectroscopy; XRD: X-ray diffraction.

Characterization techniques

The characterization techniques for the Li⁺ transport mechanism mainly fall into three categories: electrochemical methods^[104,105], spectroscopic techniques^[106,107], and microscopic imaging techniques^[108-110]. The core lies in systematically explaining the migration paths and kinetic laws of lithium ions in three-dimensional space from three levels: macroscopic characteristics, molecular structure, and microscopic form.

Electrochemical characterization techniques are the most fundamental method for characterizing lithium ion transport. By testing the electrical response of batteries or electrolytes, key parameters of lithium-ion transport can be indirectly obtained. Electrochemical impedance spectroscopy (EIS) enables the effective deconvolution of resistance components by applying alternating-current (AC) signals across a range of frequencies. From the obtained impedance spectra, the ionic conductivity of the electrolyte can be reliably derived. Additionally, the origins of ionic transport resistance can be clearly identified [Figure 11A]^[105]. The lithium-ion transference number is a critical parameter that directly reflects the proportional contribution of Li⁺ transport to the total ionic current. This parameter can be determined via the direct-current (DC) polarization method in conjunction with a symmetric cell configuration under a constant applied current [Figure 11B and C]^[111]; Cyclic voltammetry (CV) records the current response as a function of continuously varying potential. The positions and profiles of the corresponding redox peaks allow for the quantitative evaluation of the kinetic rates governing lithium-ion intercalation and deintercalation within the electrode material [Figure 11D]^[112].

Figure 11. (A) Assigning meaning to cell resistances by examining their response to changes in temperature, time, electrolyte, and applied current density^[105]. Reproduced from Ref.^[105] with permission from the American Chemical Society; (B) DC polarization/depolarization profiles of pure A6 with 80 μm spacer^[111]. Reproduced from Ref.^[111] with permission from the American Chemical Society; (C) DC depolarization results of ln(U-U∞) vs. time with pure A6 electrolyte and the corresponding linear regime^[111]; (D) Comparison of the Li⁺ transport kinetics of LCO, P-LCO, and TP-LCO^[112]. Reproduced from Ref.^[112] with permission from Wiley. DC: Direct current; LCO: LiCoO₂; P-LCO: the Li₃PO₄ modified LCO; TP-LCO: the Ti/P co-modified LCO.

Spectroscopic characterization techniques can directly obtain information such as the chemical environment, coordination state, and dynamic migration process of lithium ions^[113,114]. These technologies are crucial for revealing the transmission mechanism. For instance, ⁷Li nuclear magnetic resonance (⁷Li NMR) uses chemical shift, linewidth, and relaxation time to analyze the coordination modes of Li⁺ with solvent molecules and anions^[115]. Figure 12A^[116] presents the typical characterization results of the coordination effect obtained from temperature-dependent ⁷Li NMR. One can directly observe the changing patterns of lithium ion chemical shift and spectral line width at different temperatures. Combined with such temperature-dependent nuclear magnetic resonance experiments, the diffusion coefficient of lithium ions can be further quantitatively calculated. X-ray photoelectron spectroscopy (XPS) can determine the composition of elements and their chemical valence states of the electrode/electrolyte interface (SEI membrane), and determine whether the SEI membrane has formed a stable phase that is conducive to lithium ion transport^[117-119]. Benayad et al.^[120] conducted XPS characterization of the interfaces under different electrochemical aging times, as shown in Figure 12B. The results clearly demonstrated the dynamic changes in the elemental composition and chemical valence of the interface from the initial state to 2.3 h, directly reflecting the formation and evolution process of the SEI membrane. Raman spectroscopy is used to monitor the ion association state in the electrolyte through the displacement or intensity changes of characteristic peaks. A lower degree of association indicates smoother lithium ion migration^[121,122]. Figure 12C^[104] shows the in-situ electrochemical Raman spectra of Li_0.33La_0.57TiO₃ (LLTO) and LLTO-4% LBSO when a 5 V positive voltage is applied in a 1 mol L^-1 LiCl aqueous solution. The two-dimensional pseudo-color plot clearly presents the positions and intensities of the characteristic peaks at different times.

Figure 12. (A) Characterization of electrolytes by ¹⁷O NMR, temperature-dependent ⁷Li NMR, and two-dimensional correlation infrared spectroscopy with different concentrations of APT additives^[116]. Reproduced from Ref.^[116] with open access from Wiley; (B) C 1s, N 1s, Li 1s, F 1s, S 2p, and O 1s recorded at the interface Li/C1C6ImTFSI-LiTFSI during the polarization at a current density of 200 μA·cm^-2 in the OXPS cell. The spectra were recorded after GEIS measurement^[120]. Reproduced from Ref.^[120] with permission from the American Chemical Society; (C) In situ electrochemical Raman spectra of LLTO and LLTO-4%LBSO in the 1 mol L^-1 LiCl aqueous solution with a 5 V forward voltage^[104]. Reproduced from Ref.^[104] with permission from Wiley. ¹⁷O NMR: Oxygen-17 nuclear magnetic resonance; APT: allyl trimethyl phosphonium bis(trifluoromethane)sulfonimide; OXPS: operando X-ray photoelectron spectroscopy; GEIS: galvanostatic electrochemical impedance spectroscopy; LLTO: Li_0.33La_0.57TiO₃.

Microscopic imaging techniques can directly observe the migration paths of lithium ions in the material^[123,124], the evolution of the electrode structure^[123,125], and the growth of dendrites^[126]. The combination of transmission electron microscopy (TEM) and in-situ electrochemical sample rods can capture the real-time changes in crystal structure^[127] and the process of lithium dendrite formation when lithium ions are embedded in the electrode^[128-130]. Scanning electron microscopy (SEM) enables the direct observation of the electrode microstructure, including pore-size distribution and particle dimensions. These morphological features can be correlated with the effectiveness of lithium-ion transport pathways. Furthermore, SEM imaging of the electrode surface after cycling allows for the assessment of SEI film coverage and its spatial uniformity^[131-133]. As shown in Figure 13A-J, Huang et al.^[134] combined SEM and TEM to characterize the morphology and internal structure evolution of S-NCM83 and T-NCM83 after 100 cycles at 25 °C. This provided an intuitive illustration of the changes in the microscopic morphology of the electrodes after cycling, as well as the differences in the crystal structures of the bulk phase and the surface. Atomic force microscopy (AFM) determines the roughness of the electrode surface and the uniformity of the SEI film thickness through detecting surface force signals^[135], and assesses the uniformity of interface transmission to avoid local current concentration leading to dendrite formation^[136,137]. The AFM height image in Figure 13K-N^[92] clearly shows the microscopic morphology of the electrode surface and its influence on ion transmission.

Figure 13. (A-J) Morphology and intraparticle structural evolution after 100 cycles at 25 °C^[134]. Reproduced from Ref.^[134] with permission from the American Chemical Society; (K-N) AFM images and corresponding KPFR interfacial potential images of LP and LPSx-5^[92]. Reproduced from Ref.^[92] with permission from Wiley. KPFR: Kelvin probe force microscopy; LP: LiFSI/PVDF-HFP composite; LiFSI: Li bis(trifluoromethanesulfonyl)imide; PVDF-HFP: poly(vinylidene fluoride-co-hexafluoropropylene); AFM: atomic force microscopy.

The above three characterization techniques complement each other and work together to form a complete analysis system that covers both macroscopic and microscopic levels, as well as static and dynamic tracking. This system not only enables the static observation of electrode morphology and crystal structure, but also captures the dynamic processes of lithium ion migration and interface structure evolution. This system provides systematic and comprehensive support for in-depth analysis of the migration mechanism of lithium ions and for clarifying the relationship between battery materials and ion transmission performance. This also provides an important experimental foundation for the targeted design and performance optimization of battery materials. In practical research and application, the integrated use of multiple characterization techniques can comprehensively analyze the ion transport dynamics and interface stability within the battery system. This insight provides a scientific basis and methodological guidance for further enhancing the overall performance of the battery.

Nanoscale regulation

Nanoscale regulation is a crucial step in implementing multi-scale interface control strategies. Its core lies in adjusting the nanoscale properties of PVDF molecular chains and the surface chemical structure, optimizing the interface compatibility between fillers and electrodes, thereby constructing an efficient microenvironment conducive to ion transport^[140]. At the nanoscale, precise modification of the surface chemical structure is a key method for achieving functional modification of the interface^[141-144]. Plasma treatment is a commonly used nanometer-level regulation method. Kim et al.^[145] employed plasma treatment to reconstruct the nanoscale surface chemistry of PVDF. This process introduced oxygen-containing functional groups, including hydroxyl and carboxyl moieties, which partially substituted the original C-F and C-H groups on the polymer surface. The newly incorporated polar groups are capable of forming hydrogen bonds with water molecules, thereby effectively suppressing H₂O-induced parasitic reactions. Consequently, the surface hydrophilicity of the PVDF membrane is notably enhanced, while charge accumulation at the electrode/electrolyte interface is simultaneously mitigated. Liu et al. [Figure 14]^[146] demonstrated that the control of chlorine plasma can also enhance the triboelectric properties by adjusting the surface C-F/C-Cl coordination.

Figure 14. Synergistic mechanism of C-F and C-Cl bonds in enhancing the tribo-electric performance of fluorinated polymers. (A) Schematic of the chlorine plasma process in the treatment of fluorinated polymers; (B) Different cases of Cl substitution in PVDF-CTFE (PC) when it is treated by chlorine plasma; (C) Charge decomposition analysis of the impact of different molecular fragment structures on the frontier orbitals of the chlorine plasma-treated PC (PC-Cl); (D and E) XPS spectra of PC-Cl samples treated for different durations; (F)The corresponding surface Cl:F atomic ratio, (G) charge density and (H) charge keeping rate of the PC-Cl samples treated for different durations of chlorine plasma^[146]. Reproduced from Ref.^[146] with open access from Springer Nature, licensed under CC BY-NC-ND, no modifications to original figure. XPS: X-ray photoelectron spectroscopy; PVDF-CTFE: poly(vinylidene fluoride-co-chlorotrifluoroethylene).

Furthermore, the surface grafting principle further optimizes the interface by modifying the molecular chains at the nanoscale^[147-150]. For instance, monomers such as styrene and glycidyl methacrylate are grafted onto the molecular chains of PVDF. The grafted molecular chains can effectively suppress excessive crystallization of the PVDF matrix via a spatial steric hindrance mechanism. Meanwhile, the introduced functional groups can form chemical bonds or hydrogen bonds with the filler surface. These interactions significantly enhance the dispersion uniformity of the fillers within the polymer matrix, thereby reducing the interfacial resistance^[151]. This type of regulation directly acts on the interactions between molecules, laying a good interface foundation for subsequent filler introduction or macroscopic processing. Wang et al.^[152] proposed an efficient and simple method for controlled grafting modification of PVDF, providing a promising strategy for the surface modification of PVDF membranes.

Surface modification is not limited to a single chemical modification; it also includes the combination of multi-scale interface engineering. For example, the surface modification of inorganic nanofillers (such as hydroxylation treatment) can enhance their interaction with the polymer matrix, thereby improving the dielectric properties and mechanical properties of the composite material^[153,154]. Moreover, by introducing multi-layer structures or gradient interface layers, the interface performance can be further optimized, and the mechanical properties and stability of the composite material can be improved^[155,156].

From the perspective of performance optimization, the nanoscale regulation effectively overcomes the room-temperature ionic conductivity bottleneck of PVDF-based electrolytes through two mechanisms: “carrier concentration enhancement” and “transmission path optimization”. In terms of filler selection, inert fillers can weaken the inter-chain interactions of PVDF, increase the free volume to delay the crystallization process, and their Lewis acidic surfaces can also interact with anions, promoting the dissociation of lithium salts^[157-160]. Active fillers (such as LiTaO₃, LLTO) themselves have ionic conductivity and can construct additional transmission channels^[160]. Chen et al.^[161] incorporated two distinct structural nanomaterials into PVDF nanofibers separately during the electrospinning process, yielding tri-phase composite films denoted as PVDF/polyaniline (PANi)/halloysite nanotubes (HNT) and PVDF/PANi@HNT. The resulting films were then employed to investigate the influence of homogeneous versus heterogeneous filler configurations on the overall performance of the PVDF-based composite membranes. Metal-organic framework materials (MOFs) achieve uniform lithium ion flux and inhibit dendrite growth by virtue of their large specific surface area and unsaturated metal sites. Zhao et al.^[162] modified PVDF-HFP with one-dimensional Cu-MOF-74 nanomaterials, providing a new research idea for the development of quasi-solid-state electrolytes with high electrical conductivity and high lithium ion migration number.

In terms of optimizing the lithium ion migration number, nanoscale regulation achieves this by “limiting anion migration”, which reduces the concentration polarization caused by anion accumulation, thereby improving the battery’s rate performance^[163,164]. Multiscale interface regulation enhances t⁺ by “limiting anion migration”^[165,166]. The interface layer of the core-shell structure of nanofillers can further hinder anion diffusion^[167].

In terms of interface stability and cycle life improvement, nano-scale regulation achieves this through the dual strategies of “interface inertization + mechanical barrier construction”, which reduces interface side reactions and lithium dendrite growth. For the interface of the lithium metal anode, the nano-regulation strategy introduces functional nano-components to form an in-situ layer rich in inorganic SEI with both passivation and mechanical reinforcement functions, thereby enhancing the interface compatibility and electrochemical stability of the electrolyte membrane (as shown in Figure 15)^[168]. This interface layer can homogenize the lithium ion flow and collaboratively construct a physical barrier with the polymer matrix, thereby inhibiting side reactions and dendrite growth^{[164,169,170]} and significantly enhancing the interface stability.

Figure 15. Electrochemical performances of solid-state Li||NCM811 full cells. (A) Rate performance of full cells at various current densities; (B) Charge-discharge profiles at corresponding C-rates for PVDF-NS-based cells; (C) Cycling stability at the rate of 0.5 C over 200 cycles at 25 °C; (D) Charge-discharge voltage profiles during the long-term cycling with PVDF-NS electrolyte; (E) Cycling performance at the rate of 2 C under 25 °C^[168]. Reproduced from Ref.^[168] with permission from the American Chemical Society. PVDF-NS: PVDF-based composite electrolyte embedded with NiSe nanosheets; PVDF: polyvinylidene fluoride.

In addition to electrochemical performance, nanoscale control also plays a crucial role in optimizing mechanical properties and thermal stability. In terms of enhancing mechanical properties, constructing a strengthened structure by adding nano-fillers can balance the strength and toughness of the PVDF-based composite electrolyte^[171]. For example, Liu et al.^[172] discovered that graphene and carbon nanotubes can be combined to form a three-dimensional network structure, thereby significantly enhancing the mechanical properties of the composite material. Xia et al.^[173] combined single-layer layered-double-hydroxide nanosheets (SLN) with PVDF-HFP and found that the SLN filler formed a strong interfacial interaction with the PVDF-HFP matrix, significantly improving the tensile strength and elongation at break, laying the foundation for flexible batteries. Shi et al.’s research^[174] used different chain-length fluorosilane compounds to graft onto the surface of BaTiO₃, making the distribution of BaTiO₃ in the PVDF matrix more uniform, effectively alleviating the stress concentration phenomenon, and significantly improving the impact resistance. To enhance thermal stability and safety, introducing high-thermal-stability nano-fillers can restrict the movement of molecular chains through strong interactions with PVDF polymer molecular chains and provide effective thermal shielding to enhance thermal stability^[175]. Furthermore, nano-scale interface regulation can inhibit the growth of lithium dendrites and reduce the occurrence of side reactions, thereby increasing the triggering temperature for battery thermal runaway. Meanwhile, flame-retardant nano-fillers can reduce the risk of combustion and achieve a synergistic flame-retardant effect^[176].

Microscale regulation

In the multi-scale functionalization modification strategy for PVDF-based composite electrolytes, the microscale control strategy holds significant importance. The core objective of this strategy is to further enhance the overall performance of the electrolyte by optimizing the uniformity of the material’s microstructure and strengthening the interfacial interactions. This approach establishes a connection between nanoscale control and the macroscopic scale. This scale strategy enables precise control over the size, morphology, interface structure, and interface chemical properties of the fillers, thereby achieving targeted optimization.

In terms of filler design and interface effect optimization, different types of micro-scale fillers, due to their unique structures and chemical properties, provide multiple ways to enhance the performance of PVDF-based composite electrolytes^[177]. Among them, the interface effect of inorganic nanofillers is particularly significant. For instance, BaTiO₃ nanograins have a high dielectric constant [Figure 16A and B] and can form a locally enhanced electric field within the PVDF matrix, increasing the dissociation degree of lithium salts from 60% to 90%^[178,179]. The organic nanofillers form a special interface with PVDF through π-π interactions: graphene oxide (GO) nanosheets carrying oxygen functional groups can form hydrogen-bond networks with PVDF, thereby increasing the dielectric constant^[180,181]. As shown in Figure 16C, with the addition of the filler, the dielectric constant of the BTO@rGO/PVDF composite film continuously increases. Figure 16D^[182] shows the dielectric breakdown strength and dielectric constant of the PVDF composite membrane when used in combination with carbon-based fillers. Carbon nanotubes enhance the charge separation and transmission capabilities of PVDF through their excellent electrical conductivity, improve the piezoelectric response and carrier density of PVDF, and promote the formation of the piezoelectric β phase of PVDF through interfacial interactions. Core-shell structured fillers achieve precise design of interface properties through the synergistic effect of the core layer and shell layer^[183]. At the interface control technology level, surface modification is a key means to enhance the compatibility between fillers and the matrix. The combination of interface engineering and filler design is the core logic for micro-scale regulation to enhance the performance of composite electrolytes^[184]. Through this logic, the multi-dimensional performance of PVDF-based composite electrolytes can be synergistically optimized.

Figure 16. (A and B) The dielectric permittivity and loss tangent measured at 1 kHz^[179]. Reproduced from Ref.^[179] with open access from Wiley; (C) Dielectric properties of BTO@rGO/PVDF nanocomposites^[180]. Reproduced from Ref.^[180] with permission from the Royal Society of Chemistry; (D) Dielectric breakdown strength and dielectric constant (at 10 kHz) of PVDF-2 wt% CB, PVDF-2 wt% MWCNT, PVDF-2 wt% SWCNT, PVDF-2 wt% rGO, and PVDF-2 wt% graphite films^[182]. Reproduced from Ref.^[182] with permission from the American Chemical Society. BTO: BaTiO₃; BT: BaTiO₃; PVDF: polyvinylidene fluoride; rGO: reduced graphene oxide; CB: carbon black; MWCNT: multi-walled carbon nanotubes; SWCNT: single-walled carbon nanotubes.

From the perspective of specific performance optimization directions, in terms of improving electrochemical performance, the micrometer-scale regulation mainly achieves this through the construction of ordered micro-scale transmission channels and stable interfaces, thereby further optimizing the efficiency of ion transmission and the cycling stability. The design of micro-scale gradient interface structures can meet the different requirements of positive and negative electrodes. This structure effectively optimizes the ionic transport properties and chemical stability of the electrode/electrolyte interface, thereby significantly reducing the interface impedance and effectively suppressing undesirable side reactions. In summary, these advantages ensure that the battery maintains stable electrochemical performance during long-term cycling^[185-187]. Based on the construction of ordered ionic channels and stable interfaces at the microscopic scale, the optimization effect of introducing high dielectric constant inorganic fillers (such as BaTiO₃, SrTiO₃, etc.) on electrochemical performance deserves in-depth analysis. Firstly, the high dielectric fillers form local enhanced electric fields in the PVDF matrix, inducing the α → β phase transition. The high dipole moment of the β phase promotes the dissociation of Li⁺ through dipole-ion interactions, thereby increasing the concentration of free Li⁺ and enhancing its mobility. Its spontaneous polarization generates a local electric field at the interface, repelling the enriched anions and inhibiting the excessive local concentration of lithium ions, thereby preventing the formation of dendrites. Secondly, the high-dielectric fillers modify the electronic band structure of PVDF, enhancing the uniformity of interface polarization, buffering voltage fluctuations, and inhibiting dendrite growth. The synergy of electron regulation and phase transition improves nucleation overpotential, homogenizes deposition, and constructs a multi-field-coupled dendrite-inhibition barrier at the microscopic scale. At the same time, the uniform dispersion of micrometer-scale inert/active fillers can construct a continuous ion transmission network on a larger scale, avoiding the disruption of transmission paths caused by the agglomeration of nano-fillers, and further enhancing the stability of the ion conductivity^[188].

Beyond the physical structure of the ionic conductive network, the micro-scale functional dielectric fillers provide more effective strategies for regulating the ion transport behavior. As a typical material with a high dielectric constant, barium titanate (BaTiO₃) has abundant Lewis acid sites on its surface. These sites can specifically adsorb anions (such as FSI^- and TFSI^-) through strong electrostatic coordination with the anion groups, thereby weakening the electrostatic coupling between Li⁺ and the anions and promoting the dissociation of the lithium salt. At the same time, the low-electronegativity C-F groups on the PVDF molecular chains can form weak coordination bonds with Li⁺, constructing a low-energy barrier ion-hopping channel, allowing Li⁺ to migrate between polymer chain segments. More importantly, the BaTiO₃ nanocrystals will exhibit significant dielectric polarization under the action of a local applied electric field, inducing a local interface polarization electric field. This interface electric field further promotes the dissociation of the lithium salt and makes the microscopic Li⁺ concentration distribution tend to be uniform, effectively alleviating the concentration polarization phenomenon caused by the aggregation of interface anions. Thanks to these synergistic effects, the PVDF composite electrolyte doped with BaTiO₃ significantly improves the ionic conductivity and Li⁺ mobility at room temperature. The uniform distribution of Li⁺ effectively inhibits the growth of lithium dendrites caused by the uneven interface ion concentration gradient. This filler modification strategy provides a feasible and valuable paradigm for regulating the ion transport performance by rationally designing the structure and function of micro-scale fillers.

In terms of improving mechanical properties, micrometer-scale regulation enhances the crack resistance and structural stability of PVDF-based composite electrolytes through filler dispersion and structural design. During battery cycling, the electrolyte is prone to cracking due to volume changes, and micrometer-sized fillers (such as micrometer-sized ceramic particles) can disperse stress concentration and reduce the risk of cracking through their synergistic effect with the PVDF matrix.

In terms of thermal stability and safety, micro-level control enhances thermal stability and safety by optimizing the microscopic thermal conduction pathways and flame-retardant structures of the materials. The introduction of micron-sized flame-retardant fillers can establish a thermal shielding network, further restricting the movement of PVDF molecular chains and enhancing the overall thermal stability of the material. At the same time, the dispersion of this filler can form a larger flame-retardant barrier area, working in synergy with nano-sized flame-retardant fillers to enhance the fire-retardant effect and significantly improve the safety of the battery in high-temperature environments.

Macro-scale regulation

Macro-level regulation is also a key means to achieve functional modification of the high-performance PVDF-based composite electrolyte. The core value of this method lies in converting micro-performance advantages into engineering applications. It is a crucial bridge that connects the micro-performance of the PVDF-based composite electrolyte with actual battery applications. In practical battery manufacturing, macroscopic regulation strategies - including mechanical processing, chemical modification, and structural design - can be implemented to ensure the compatibility between electrolyte performance and engineering requirements. These approaches serve as an effective bridge for transitioning high-performance laboratory-scale samples toward industrially viable products.

Processing technology is the fundamental means for achieving macroscopic-scale control, mainly including mechanical stretching and thermal pressing treatment. These two methods respectively enhance the performance of the electrolyte through crystal structure optimization and densification. Mechanical stretching can significantly disrupt the non-polar α-phase crystals in PVDF, promote the extension of molecular chains, and transform them into polar β-phase, thereby increasing the dielectric constant and polarization strength of the material^[189-191]. Figure 17A shows the phase transformation of PVDF under stretching. Even minor adjustments in the stretching ratio, stretching rate, and stretching direction will affect the amount of the β phase, thereby altering the ionic mobility of the electrolyte and the interface polarization effect.

Figure 17. (A) α-β Phase Transformation of PVDF Films under Stretching^[189]. Reproduced from Ref.^[189] with permission from the American Chemical Society; (B) Characterization and dispersion of EGaIn@GP for LIB half-cell applications^[192]. Reproduced from Ref.^[192] with open access from Wiley; (C) β phase content of PVDF films processed by different methods and PVDF polymer composite films processed by SC-HP-Q-R.100^[193]. Reproduced from Ref.^[193] with open access from the American Chemical Society. PVDF: Polyvinylidene fluoride; LIB: lithium-ion battery; DMSO: dimethyl sulfoxide; TrFE: trifluoroethylene; SPS: sulfonated poly (styrene); SC: solvent cast; PT: plasma treatment; UV: ultraviolet; BT: BaTiO₃; MWCNT: multi-walled carbon nanotubes.

Chemical modification mainly involves introducing polar functional groups, copolymerization modification, and crosslinking modification, etc. This method achieves breakthroughs in performance through macroscopic regulation at the molecular level. During the process of introducing polar functional groups, the grafted/doped polar groups can provide non-covalent sites, promoting the dissociation of lithium ions and anions, significantly improving ionic conductivity and the migration of lithium ions. Seo et al.^[192] utilized the sulfonic acid chains in the graft polymer to form an ion transport network on the surface of nanoparticles, significantly enhancing the migration efficiency of lithium ions and breaking through the bottleneck of limited ion transport in traditional liquid metal electrodes [Figure 17B].

The hot pressing treatment can achieve macroscopic densification. Yasar et al.^[193] simultaneously changed the heat energy and pressure, achieving a crystal transformation from the α phase to the β phase [Figure 17C], eliminating pores and residual solvents, and promoting the uniform dispersion of additives. Moreover, hot pressing is also suitable for large-scale production and is often combined with subsequent heat treatment or annealing processes to further optimize the crystallinity of PVDF and ensure the stability of the electrolyte performance.

In the context of copolymerization modification, the incorporation of comonomers such as HFP and Trifluoroethylene (TrFE) into the PVDF backbone disrupts the original ordered crystalline structure. This disruption increases the fraction of amorphous regions, which in turn provides a more extensive and continuous network of ion-conducting pathways. Furthermore, the resulting copolymer system typically exhibits reduced crystallinity and enhanced chain flexibility. These combined characteristics contribute to improved ionic conductivity over a broad temperature range, thereby offering compatibility with battery applications across diverse operating temperature conditions^[36].

In addition, there is cross-linking modification. Using cross-linking agents such as 2,5-dihydroxyterephthalic acid, triallyl isocyanurate, and a functionalized macromonomer derived from poly(ethylene oxide-co-propylene oxide), in situ cross-linking can be carried out under the induction of heating or light, thereby forming a dense cross-linked network within the PVDF matrix. This network can significantly enhance the mechanical strength, thermal stability, and dimensional retention ability of the material, while inhibiting the excessive movement of polymer chains and preventing interface deterioration. Moreover, the crosslinking network can effectively prevent phase separation within the electrolyte and avoid local blockage of ion channels, thereby improving the ionic conductivity and long-cycle performance at room temperature, providing a guarantee for long-life battery applications. Naren et al.^[194] aimed to solve the challenges faced by lithium metal anodes in practical applications through crosslinking strategies.

From the perspective of application-oriented performance optimization, macro-control plays a crucial role in enhancing electrochemical performance. This is mainly achieved through process adjustments, which jointly improve the ionic transport characteristics and the interface compatibility of the materials. This stretching-oriented treatment not only optimizes the crystal structure but also promotes the ordered arrangement of PVDF molecular chains^[195]. It not only improves mechanical strength but also maintains the fluidity of the chain segments, indirectly optimizing the ion transport efficiency. Moreover, the macroscopic-scale interface treatment can comprehensively alter the surface of the PVDF-based electrolyte, enhancing its compatibility with the electrode, reducing interface impedance, and improving the rate and cycle stability of the battery.

To improve mechanical properties by optimizing the process and designing the structure, a balance between the mechanical strength and flexibility of the PVDF-based composite electrolyte can be achieved, meeting actual packaging and usage requirements. The PVDF matrix itself has good film-forming properties, but the mechanical strength of pure PVDF films is relatively low. The macroscopic tensile orientation process can increase the tensile strength by ordering the molecular chains while maintaining the flexibility of the material^[196]. Furthermore, at the macroscopic level, the rational design of multi-layer composite structures enables the integration of distinct functional layers, each contributing specific performance attributes. This approach allows for the customized tailoring of mechanical properties to meet the demands of complex deformation scenarios, such as bending and folding in flexible battery applications. Moreover, such structural engineering effectively reinforces the mechanical stability of the battery during packaging and practical operation, thereby mitigating the risk of damage induced by mechanical deformation. Zhang et al.^[197] proposed a multi-layer composite structure design, which can synergistically enhance the mechanical strength and flexibility of PVDF-based materials, while endowing them with efficient electromagnetic shielding performance and thermal management capabilities. This multi-dimensional optimization of battery performance can significantly improve the structural stability and usage reliability of lithium-ion batteries.

For thermal stability and safety improvement, macro-scale regulation maximizes the thermal stability and safety advantages of PVDF-based composite electrolytes through molecular structure reinforcement and overall shape design. Based on molecular-scale cross-linking modification, macro processes (such as hot pressing molding) can further optimize the material’s density, reduce the release channels of thermal decomposition gases, and enhance thermal stability^[74]. At the same time, the macroscopic shape design of solid-state films intrinsically eliminates the risk of liquid electrolyte leakage^[15]. When combined with macroscopic interface passivation strategies - such as the application of an overall inert surface coating - this approach effectively mitigates undesirable side reactions and reduces the likelihood of thermal runaway. These combined safety features provide a critical guarantee for the practical deployment of solid-state batteries and flexible electronic devices.

In summary, the multi-scale interface control system for PVDF-based composite electrolytes systematically addresses issues at three length scales. At the nanoscale, plasma treatment, polymer grafting, and functional nano-filler design can optimize local ion dissociation and transport channels. At the microscale, uniform filler dispersion, core-shell structure, and gradient intermediate layer can ensure continuous ion conduction and alleviate stress concentration. At the macroscale, mechanical stretching, hot pressing, copolymerization, crosslinking, and multilayer structure can enhance the membrane’s density, lattice regulation, and overall mechanical integrity. The synergistic effects at various scales from molecular-level chemistry to overall engineering form the core logic for overcoming the inherent bottlenecks of PVDF-based electrolytes and provide a flexible and diverse framework for high-performance solid-state batteries.

Application in zinc batteries

The history of zinc batteries can be traced back to the 19th century [Figure 18]^[198]. Currently, due to the numerous advantages of the zinc metal negative electrode (such as high theoretical capacity, low redox potential, and environmentally friendly properties), it has become an ideal candidate system for large-scale energy storage and portable electronic devices^[199]. However, traditional liquid aqueous electrolytes still face problems such as zinc dendrite growth, electrode corrosion, and hydrogen evolution side reactions^[200-203]; non-aqueous liquid electrolytes have challenges such as low ionic conductivity and poor interfacial compatibility^{[199,204-206]}. Through structural modification and component optimization of PVDF-based composite solid electrolytes, these problems can be targetedly solved, and they have become one of the key materials in the field of zinc batteries.

Figure 18. History of research on zinc-ion batteries, including aqueous and non-aqueous systems^[198]. Reproduced from Ref.^[198] with open access from the Royal Society of Chemistry, licensed under CC BY-NC.

In zinc-ion batteries, the core function of PVDF-based composite solid electrolytes is “suppressing dendrites and blocking corrosion”. The strong polar C-F bonds in the PVDF molecular chains and the weak interaction with water molecules enable the formation of a “polymer matrix-water electrolyte” interpenetrating network, achieving the directional transport of Zn^2+[207]. On one hand, the semi-crystalline structure of PVDF can construct stable ionic channels, and its high dielectric constant helps the dissociation of zinc salts, increasing the concentration of free Zn²⁺; on the other hand, the introduction of inorganic fillers or organic modifiers can further optimize the interface characteristics. As shown in Figure 19, Yu et al.^[204] evaluated the dendrite inhibition and corrosion resistance of SPP-modified PVDF composite solid-state electrolyte in zinc-ion batteries through various electrochemical characterizations, in-situ morphology analysis, and theoretical calculations. Under the same constant current cycling conditions, the zinc anode using aqueous Zn(OTf)₂ as the electrolyte exhibited severe dendrite growth and parasitic corrosion by-products. In contrast, the battery using the SPP@PVDF solid-state electrolyte achieved an ultra-stable zinc deposition/peeling process after more than 3,000 h of long-term cycling, while the surface remained flat without dendrites and almost no corrosion by-products were generated.

Figure 19. Zn²⁺ ions stability/reversibility and transport performance/mechanism of Zn anode with the SPP@PVDF. (A) Zn²⁺ ions conductivity of SPP@PVDF and SPAF@PVDF at different temperatures; (B) tZn²⁺ of SPP@PVDF (0.82) at RT, the inset shows the EIS before and after polarization; (C) Comparison of tZn²⁺ and Zn²⁺ ions conductivity of SPP@PVDF with previously reported work based on SPE; (D) LSV curves of SPP@PVDF, SPAF@PVDF, and PVDF; (E) CV curves of Zn plating/stripping with SPP@PVDF; (F) SEM image and XRD pattern (the yellow curve in the illustration) of zinc foil surface after 100 cycles with SPP@PVDF based Zn symmetrical batteries, the green curve is a PDF card for zinc; (G) Cycling performance of Zn symmetric batteries using different electrolytes at 1 and 0.5 mAh cm^-2 (insets display the corresponding magnified voltage profiles); (H) Calculated frontier molecular orbital energies of SPP and SPAF; (I) Planar average potential from the Zn(002) crystal surface to the SPAF interface; (J) 45° side view of the 3D distribution of charge density of SPAF fragments adsorbed on the Zn(002) crystal surface.^[204] Reproduced from Ref.^[204] with permission from Wiley. SPP: Sulfonated porous aromatic framework- poly(2-ethyl-2-oxazoline); PVDF: polyvinylidene fluoride; SPAF: sulfonated porous aromatic framework; RT: room temperature; PPM: polymerized poly(ethylene glycol) methyl ether acrylate-based polymer; SPE: solid polymer electrolyte; PVHF: poly(vinylidene fluoride-co-hexafluoropropylene); PMA: poly(methyl acrylate); DEE: deep eutectic electrolyte; LSV: linear sweep voltammetry; CV: cyclic voltammetry; SEM: scanning electron microscopy; XRD: X-ray diffraction.

In solid-state zinc metal batteries, the PVDF-based composite solid electrolyte needs to satisfy the requirements of “high Zn²⁺ conductivity”^[208] and “low interface impedance” simultaneously. Since ion transport in the solid system relies on the movement of polymer chain segments and interface interactions, researchers usually adopt a “active filler composite + interface modification” dual strategy for optimization: dispersing inorganic fillers with Zn²⁺ conductivity into the PVDF matrix. These fillers serve a dual function in facilitating zinc-ion conduction. On one hand, they provide additional active sites for ion transport. Meanwhile, the surface hydroxyl groups are capable of forming hydrogen bonds with the C-F bonds of the PVDF matrix. This interaction suppresses excessive polymer crystallization and increases the proportion of amorphous regions, thereby enhancing the room-temperature ionic conductivity of zinc ions^[209]. On the other hand, plasma treatment introduces carboxyl groups onto the electrolyte surface. These functional groups strengthen the interfacial bonding with the zinc metal anode, effectively reducing the interfacial impedance and markedly improving the discharge performance of the battery^[210].

Furthermore, the PVDF-based composite solid electrolyte exhibits unique advantages in the field of flexible zinc batteries. The PVDF-based nanofiber membranes prepared by electrospinning have high porosity and good flexibility, which can adapt to the requirements of flexible devices such as bending and folding.

Application in sodium batteries

Sodium-ion batteries possess abundant resources and low costs, and have thus become an important development direction for large-scale energy storage at present^[211]. However, they face two major challenges: inefficient Na⁺ transport and the formation of sodium anode dendrites and side reactions^[211-214]. PVDF-based composite solid-state electrolytes can solve these problems through molecular chain regulation and interface design^[215,216]. Li et al.^[217] further verified the effectiveness of the copolymer modification strategy. They successfully prepared a gel polymer electrolyte based on the copolymer mixture of PVDF and polymethyl methacrylate (PMMA). The introduction of PMMA not only significantly reduced the crystallinity of the PVDF matrix but also promoted the formation of a more continuous ion-conductive network through cooperative interaction between the polymer molecular chains. As a result, the ionic conductivity of the resulting electrolyte at room temperature was significantly improved.

In sodium metal batteries, the PVDF-based electrolyte focuses on the design concept of “strengthening ion channels + stabilizing the sodium interface”. Through copolymer modification (such as the PVDF/PMMA system^[217]) or the addition of amorphous additives to reduce the crystallinity of PVDF, a wide amorphous zone is constructed to facilitate Na⁺ migration. The Li team confirmed that this strategy can significantly increase the room-temperature ionic conductivity; introducing high dielectric inorganic fillers can enhance Na⁺ solvation, promote sodium salt dissociation, and also improve the contact between the electrolyte and the sodium anode, inhibiting dendrites^[217]. Further support for this approach is provided by the work of Li et al.^[217]. Their findings demonstrate that the introduction of fillers not only effectively facilitates Na⁺ migration, but also that the enhanced mechanical properties positively contribute to the stability of the electrode-electrolyte interface [Figure 20A]. Zhao et al.^[218] used a control strategy to suppress the micro-crystallinity of the material and increase the amorphous regions, thereby enhancing the migration of Na⁺ and the ionic conductivity. Their excellent mechanical properties and dielectric characteristics help stabilize the electrode-electrolyte interface, inhibit the growth of sodium dendrites, and thus improve the electrochemical performance and safety of solid-state sodium batteries. From Figure 20B^[219], it can also be directly observed that the voltage and capacity performance corresponding to the PVDF-based electrolytes can meet the actual electrochemical requirements of sodium batteries.

Figure 20. (A) Rate performances of the Na//NVP cells using GM and Celgard 2400-LE and the cyclic performances of the cells at 1 C and 10 C^[217]; (B) The relationship between average voltage and capacity of different solid electrolytes for sodium-ion batteries (SPE: Polymer solid electrolyte, ISE: Inorganic solid electrolyte, CPE: Composite solid electrolyte)^[219]. Reproduced from Ref.^[217,219] with permission from Wiley. GM: Graphene matrix.

In sodium-ion all-solid-state batteries, PVDF-based electrolytes offer multiple functional advantages. Through surface modification, they can construct a stable interfacial layer that is compatible with high-voltage cathodes, thereby effectively suppressing oxidative decomposition. In addition, their high tensile strength enables them to accommodate the volume expansion of the cathode material during cycling. Furthermore, the PVDF/PMMA gel electrolyte developed by Li’s team exhibits inherent flexibility and a compact microstructure, which help minimize interfacial gaps and significantly enhance cycling stability^[217].

In low-cost and large-scale production scenarios, PVDF-based electrolytes offer distinct advantages. They can be readily fabricated as large-area membranes via the solution casting method, with a material cost amounting to only one-fifth that of conventional sulfide-based electrolytes. Moreover, these electrolytes exhibit favorable compatibility with aluminum foil current collectors and carbon-based anodes, enabling low-resistance contact without the need for additional surface modification. Li’s team further demonstrated that the same fabrication process allows for precise control over membrane thickness and uniformity, thereby providing a viable pathway toward industrial-scale production^[217].

Application in new-type alkali metal batteries

New types of alkali metal batteries (such as potassium batteries, magnesium batteries, and calcium batteries) are regarded as a key direction for breaking through the energy density and cost bottlenecks of lithium-ion batteries due to the abundant resources of alkali metal elements and their unique electrochemical properties. These emerging battery systems have different working principles: Figure 21A^[220] shows the electrode materials and the rocking chair structure of potassium-ion batteries; Figure 21B^[221] depicts the quasi-solid-state magnesium battery; and Figure 21C^[222] presents the schematic diagrams of calcium-ion batteries with different anodes. However, these batteries also face more severe challenges in practical applications: potassium metal anodes are prone to form loose dendrites and react violently with the electrolyte^[223-227]; magnesium ions have a high charge density, resulting in a significant increase in ion transport resistance^[228-231]; calcium ions are prone to form inert layers at the electrolyte interface^[232-235]. Aluminum batteries, another emerging alkali metal system, similarly suffer from severe anode corrosion issues, which recent studies have shown can be effectively suppressed through mesoscale hydrogen-bond network engineering that controls quantum-coherent proton transport^[236]. Based on the composite solid-state electrolyte of PVDF, through precise molecular design and multi-scale regulation, it can be customized to meet the transmission requirements of different alkali metal ions, providing core support for the performance improvement of new batteries.

Figure 21. (A) K-ion characterization and modeling^[220]. Reproduced from Ref.^[220] with open access from Springer Nature; (B) Schematic demonstration of the Mg metal battery modified by a COF layer and the role of CuPc-PD COF in regulating the uniform and directional Mg²⁺ transport^[221]. Reproduced from Ref.^[221] with permission from Wiley; (C) Schematic diagram of calcium ion batteries with different anodes^[222]. Reproduced from Ref.^[222] with open access from the Royal Society of Chemistry. COF: Covalent organic frameworks; DME: 1,2-dimethoxyethane; MOF: metal-organic frameworks; MOC: metal-organic cages.

The low redox potential of the potassium metal anode and the widespread distribution of potassium resources provide a cost advantage for this technology^[237]. The interlayer insertion behavior of potassium ions and graphite is similar to that of lithium ions, allowing the existing carbon-based positive electrodes to be directly transferred. However, the radius of potassium ions is much larger than that of lithium ions, making it difficult for traditional liquid electrolytes to achieve efficient ion migration. At the same time, potassium metal is prone to forming loose dendrites during charging and discharging, increasing the risk of short circuits^[223-227]. Yang et al.^[238] modified PVDF by rationally designing the chemical properties of the polymer framework and solvents in the gel polymer electrolyte. This significantly improved the performance of the gel polymer electrolyte for quasi-solid-state potassium-ion batteries. The key effect was to utilize the weak electrostatic force of K⁺ ions to enhance the ion migration number and improve the interface stability and safety. By regulating polarity defects at the molecular level and using multi-scale fillers, a universal approach can be provided for customizing transmission channels for different alkali metal ions.

The magnesium metal anode has an extremely high theoretical capacity, a relatively low redox potential, and a low risk of dendrite formation^[239]. Therefore, it is an ideal choice for manufacturing high-energy-density batteries. However, the high charge density of Mg²⁺ in the electrolyte easily forms a strong solvation shell, significantly increasing the resistance to ion migration. Moreover, the interface compatibility between the traditional electrolyte and the cathode material is poor, and an inert layer is prone to form at the interface. The PVDF-based composite solid-state electrolyte improves the transmission of Mg²⁺ through two methods. One approach is to add high dielectric constant plasticizers or metal oxide fillers [such as lipophilic lithium magnesium silicate(LLS), alumina]. Studies^[240] have shown that the magnesium ions in LLS can form Lewis acid-base interactions with the C-F groups on the PVDF chains, effectively disrupting the tight stacking structure of PVDF and reducing its crystallinity, while increasing the free volume. This structural optimization creates continuous channels for the migration of lithium ions and promotes the dissociation of the lithium salt, enabling the electrolyte to achieve an ion conductivity of up to 10^-4 S cm^-1 at room temperature. The other is to use the strong polar C-F bond in PVDF to form weak coordination with Mg²⁺, inducing the ion to migrate directionally along the polymer chain segments, reducing the activation energy for migration^[241].

The calcium metal anode also possesses the advantages of high theoretical capacity, low redox potential, and abundant reserves in the earth’s crust. Compared to Mg²⁺, the migration barrier of Ca²⁺ is lower, which theoretically facilitates faster ion transport^[232]. However, there are still two major bottlenecks in practice: Firstly, Ca²⁺ is prone to react with impurities in the electrolyte such as CO₂ and H₂O, generating electron-insulating inert layers like CaCO₃ and CaO, resulting in a sharp increase in interface impedance. Secondly, the existing solid-state electrolytes generally have low conductivity for Ca²⁺, which makes it difficult to meet the high power requirements^[242]. Although there is no direct report of systematic experimental data for the PVDF-Ca²⁺ system at present, existing studies have shown that the PVDF composite solid electrolyte has the potential to support the use of calcium batteries in the following aspects: Firstly, adding inert fillers (such as Al₂O₃^[243]) to the PVDF matrix to form a dense membrane, which can prevent CO₂ and H₂O from entering the system and thereby inhibit the formation of the inert layer; Secondly, using the higher dielectric constant of PVDF to enhance the dissociation degree of Ca²⁺ salts and increase the free Ca²⁺ concentration; Thirdly, introducing active fillers with intrinsic Ca²⁺ conductivity [such as Ca₃(PO₄)₂]^[244], which will construct continuous ionic fast channels within the electrolyte and improve the interfacial ionic conductivity. These ideas have made initial progress in the concept verification of calcium-ion batteries, but further material screening and interface engineering verification are still needed.

Due to the differences in radius, charge density and solvation ability of various ions, the ion conductivity of PVDF electrolyte varies significantly in different metal ion systems: the optimized Li⁺ and the regulated Na⁺ can reach 10^-4-10^-3 S·cm^-1; the Zn²⁺ and Ca²⁺ in the plasticized PVDF gel are 10^-5-10^-4 S·cm^-1; while the rigid all-solid-state PVDF and Mg²⁺ system can only achieve 10^-6-10^-5 S·cm^-1. The multi-scale interface control strategy, through customized design, can be universally applicable to all types of batteries mentioned above. For Na⁺, the crystallinity of PVDF can be reduced to widen the ion channels; for Zn²⁺, the hydrophobic interface can inhibit hydration side reactions; for Mg²⁺, high dielectric fillers can weaken the strong ionic coordination; for Ca²⁺, a dense barrier can avoid the formation of inert interfaces. This differentiated interface engineering simultaneously enhances the ion conductivity and interface stability of various metal ion batteries.