Engineering crystalline property of polymer solid electrolytes for boosted electrochemical performances: a critical review

Abstract

With the growing demand for safe and high-density energy storage systems, solid-state polymer electrolytes (SPEs) have attracted significant research attention because of their exceptional safety, flexibility and processability. However, their practical applications are hindered by the sluggish chain-mediated ion conduction at room temperature. While recent studies have greatly improved ion transport by regulating polymer crystallization behaviors, the underlying mechanism that enhances ion conduction and the absence of precise crystal structure design continue to pose major challenges for the rational development of advanced SPEs. This review first comprehensively examines the basic crystallization principles and ion transport mechanisms in SPEs. Then, three key strategies for engineering crystalline properties in SPEs are summarized, including salt engineering, additive mediation, and physical field regulation. These approaches illustrate the optimization of SPE performance by tailoring crystal morphology, orientation, and defect states. Finally, the limitations of current research are explored, and forward-looking perspectives highlight the critical role of precise crystallization control for developing high-performance SPEs.

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Genyuan Wang

Genyuan Wang obtained his B.E. from Nanjing Tech University in 2019 and M.S. from Xiamen University in 2022. Currently, he is pursuing a PhD degree at The Hong Kong Polytechnic University, focusing on advanced polymer electrolytes for sodium metal batteries.

Tao Zhang

Tao Zhang received his B.S. from Shandong University in 2013 and his M.S. from Xiamen University in 2016. He obtained his PhD from Shanghai Jiao Tong University in 2020. He is currently a postdoctoral researcher at The Hong Kong Polytechnic University. His research focuses on the solid-state electrolyte design for silicon-based lithium-ion batteries.

Liang Hu

Liang Hu received his B.E. from Central South University in 2014 and his M.S. from the University of Chinese Academy of Sciences in 2018. He obtained his PhD from The Hong Kong Polytechnic University in 2024. He is currently a postdoctoral researcher at The Hong Kong Polytechnic University. His research focuses on the electrolyte design for high-performance anode-free sodium metal batteries.

Kai Shi

Kai Shi obtained his B.E. from Nanjing University of Aeronautics and Astronautics in 2016, followed by an M.S. from Tsinghua University in 2019. He subsequently received his PhD from Karlsruhe Institute of Technology in 2024. Currently, he works as a postdoctoral researcher at The Hong Kong Polytechnic University, where his research focuses on safe and sustainable electrochemical energy storage technologies, with a particular emphasis on solid-state batteries and sodium batteries.

Xiaoliang Yu

Xiaoliang Yu obtained his BSc and PhD degrees both from Tsinghua University and is now leading the Smart Battery Lab at The Hong Kong Polytechnic University. His research focuses on smart sodium battery manufacturing, from electrolyte formulation and electrode preparation to cell fabrication. He has received various funding including Young Collaborative Research Grant and Innovation and Technology Fund in Hong Kong, with a total amount of over HK$ 12 million. He has published more than 50 peer-reviewed research papers in Adv. Mater., ACS Nano, Adv. Funct. Mater., etc. He serves as Lead Guest Editor of Frontiers in Chemistry and an Organizing Committee Member of IMLB2024. He was elected a Fellow of the International Association of Advanced Materials in 2024.

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1 Introduction

The global energy transition has made rechargeable batteries essential in modern society, with widespread applications in consumer electronics, electric vehicles (EVs), electric vertical take-off and landing (eVTOL) aircraft, and renewable energy storage.^1–5 Lithium-ion batteries (LIBs) have dominated the energy storage market over the past three decades because of their high energy density, long cycle life, and low self-discharge rate.⁶ However, conventional liquid electrolytes face multiple intrinsic challenges hindering their practical application in LIBs. First, the inherent properties of volatility and low flash points pose severe safety risks of combustion and explosion hazards.^7–9 Second, liquid electrolytes exhibit poor compatibility with high-energy cathode materials, leading to undesirable side reactions.¹⁰ It subsequently results in increased interfacial impedance and accelerated capacity degradation. Third, transition metal elements in LIBs tend to dissolve in the liquid electrolyte, causing irreversible active material loss.^11–13 These fundamental limitations have made (electro)chemically stable solid-state electrolytes a leading candidate for next-generation batteries.

Inorganic solid-state electrolytes have high ionic conductivities and wide electrochemical stability windows.^14,15 However, their high brittleness and rigidity result in unfavorable point-to-point contact with electrodes and high interfacial impedances.¹⁶ In contrast, solid-state polymer electrolytes (SPEs) exhibit improved contact with electrodes, superior interfacial compatibility, high flexibility, and excellent processability.¹⁷ Moreover, the diverse molecular structure designs and applicability of functional fillers offer great potential for developing high-performance PSEs.¹⁸ Polyethylene oxide (PEO) has been regarded as the most promising polymer matrix candidate in SPEs because of its exceptional solvation capability and relative stability against metal anodes.¹⁹ Since Armand's pioneering proposal utilizing PEO/salt complexes as solid-state electrolytes, extensive research efforts have been devoted to exploiting advanced PEO-based solid electrolytes.²⁰ Despite the high ionic conductivities of 10⁻⁴–10⁻³ S cm⁻¹ at elevated temperatures (>60 °C), they demonstrate insufficient room-temperature performance for practical applications.^21,22 Ionic conduction in PEO-based solid electrolytes is predominantly facilitated by segmental chain dynamics within the amorphous region.²³ At room temperature, the crystallinity of PEO-based solid electrolytes can reach 70–80%, leading to low ionic conductivities of <10⁻⁵ S cm⁻¹.²⁴ At high temperatures, the increased amorphous regions enhance ion transport, but their inadequate mechanical strength fails to prevent lithium dendrite penetration.¹⁶ Therefore, precise crystallization regulation is essential for enhancing ionic conductivity without compromising mechanical robustness.

Although multiple strategies, such as molecular engineering and interfacial modification, have been developed to regulate polymer crystallization, the absence of fundamental principles for precisely controlling crystalline characteristics to optimize electrolyte performance remains an obstacle to the rational design of advanced SPEs.^7,25 Existing reviews comprehensively cover the general progress and application-specific advances in SPEs,^26–30 insights on fundamental scientific challenges, particularly the polymer crystallization-ion transport relationship and crystal structure design principles, remain scattered. This review systematically summarizes recent advances in optimizing SPEs' performance through crystallization regulation (Fig. 1). First, the fundamental principles of crystallization regulation are elucidated by examining crystal nucleation/growth mechanisms in polymer-based electrolytes and their influence on ion transport. Then, three key optimization strategies, including salt engineering, additive mediation, and physical field regulation, are critically analyzed, focusing on how these approaches tailor the crystalline properties to enhance SPE performance. Finally, current limitations in SPE crystallization research and future development directions are identified, providing theoretical guidance and research perspectives for next-generation high-performance solid electrolyte design.

Fig. 1 Summary of crystalline property engineering of SPEs.

2 Polymer crystallization and ion transport

In PEO-based solid electrolytes, the coexistence of crystalline and amorphous phases creates a complex structure for ion transport. While the amorphous regions typically facilitate ion mobility, the crystalline domains exert multifaceted and often competing effects on conduction.³¹ Understanding the relationship between crystallization behavior and ion dynamics plays a crucial role in engineering the crystalline property of SPEs for boosted ion conduction.^32,33

2.1 Nucleation and growth of polymer crystals

The crystallization of polymers is governed by nucleation and growth mechanisms, which depend on the molecular chain structure and crystallization environment.^34,35 During polymer crystallization, disordered polymer chains are reorganized into aligned crystalline domains, first forming stable nuclei and then growing into large crystals. Nucleation in polymer systems predominantly occurs through heterogeneous pathways facilitated by impurities or additives, as the reduced energy barrier makes this mechanism more favorable than homogeneous nucleation within pure melts for most SPEs.^36,37 The interplay between nucleation density and crystal growth kinetics determines the crystalline morphology of SPEs, including lamellar thickness, spherulite size, and crystallinity, which ultimately dictates their ion transport properties.^34,38

According to classical nucleation theory, forming a crystal nucleus with radius R leads to a change in the system's free energy. The competition between the crystalline and amorphous phases' volumetric free energy density difference (Δg) and the interfacial free energy density (γ) determines whether the crystal grows or melts.³⁹ During crystallization, the total free energy change of the system can be expressed as

The critical size R* represents the minimum size threshold for stable crystal growth (Fig. 2a). Nuclei with a size smaller than R* dissolve, while those exceeding R* grow irreversibly. The critical nucleus size and its associated energy barrier can be determined by the equation:

Fig. 2 (a) The change of free energy in the system caused by crystal nucleation. (b) Schematic diagram of crystal growth under the Hoffman–Lauritzen theory.

At the molecular level, polymer nucleation exhibits two distinct modes, one is periodically folded configurations, and the other is intermolecular chain alignment. For short and rigid polymer chains, the nucleation is dominated by intermolecular chain alignment. In contrast, PEO-based solid electrolytes, with high molecular weight and flexible chain segments, primarily undergo nucleation via intramolecular chain folding.⁴⁰

Following crystal nucleation, polymer chains undergo periodic folding and attach to the crystal growth front in a lamellar manner by overcoming the nucleation energy barrier. According to the Hoffman–Lauritzen theory, the polymer chain segments are adsorbed to the crystal growth front as extended stems, and then undergo chain folding to enable secondary adsorption. The crystals grow through successive stem adsorption and folding cycles (Fig. 2b). The temperature dependence of the crystal growth rate can be expressed as

Here, the pre-exponential factor G₀ is associated with the segmental mobility of polymer chains, U* is the activation energy for chain segment diffusion, T_∞ denotes the kinetic freezing temperature, K_g is the nucleation barrier constant, and f stands for the correction factor of supercooling (ΔT).^41,42 It can be seen that the polymer crystallization is governed by the competition between nucleation and growth kinetics. Generally, crystalline structures featuring multiple nucleation centers with controlled growth kinetics are more conducive to ion transport.

The thickness of polymer lamellae (b) depends on the fold surface energy (σ_e), equilibrium melting temperature (T⁰_m), volumetric heat of fusion (ΔH_f), and an intrinsic thickness parameter (b₀), as described by

Eqn (5) reveals that deep supercooling (ΔT) significantly restricts the formation of long-range ordered chain folding and thick crystal lamellae.⁴³ Furthermore, introducing metal salts, plasticizers, and various additives can reduce lamellar thickness by modulating the fold surface energy (σ_e) towards facilitated ion transport.

The crystallization kinetics of polymers are also influenced by PEO molecular weight.⁴⁴ The high molecular weight PEO exhibits accelerated crystallization onset compared to its low molecular weight counterpart, stemming from enhanced nucleation kinetics driven by higher bulk free energy reduction during chain folding. Low molecular weight PEO chains exhibit stronger diffusion capability, enabling faster chain folding and crystal arrangement, resulting in higher crystallinity. Blending PEO of different molecular weights reduces the nucleation energy barrier. The long chains of high-molecular-weight PEO provide greater melt stability, while the enhanced chain mobility of low-molecular-weight PEO facilitates crystal growth. Therefore, selecting PEO with specific molecular weights enables precise control over the crystalline morphology of SPEs.

To sum up, polymer crystallization represents a complex phase transition influenced by various multi-scale factors. Molecularly, the chain-folding mechanism establishes the fundamental components of lamellae. On a mesoscopic level, the Hoffman–Lauritzen equation effectively predicts lamellar thickness, and the kinetic regimes highlight the trade-off between nucleation and growth rates, ultimately shaping spherulite morphology. Optimizing the performance of SPEs involves precise control over essential crystal nucleation and growth parameters that affect nucleation and growth kinetics and thus tailored crystalline properties.

2.2 Ion transport mechanism

In SPEs, the polymer matrix functions dually as both a structural scaffold and a promoter of salt dissociation via coordination between its polar functional groups and metal cations.³⁰ In typical semicrystalline SPEs, the amorphous regions are considered the primary pathways for ion conduction due to their high chain mobility.⁴⁵ Although crystalline phases are traditionally considered ionic insulators, emerging evidence demonstrates that ion diffusion can occur along crystal surfaces, and more remarkably, certain crystalline structures may possess bulk ion transport pathways.^46,47

The investigation into ionic conductivity in polymer-salt complexes dates back to 1973 when Fenton et al. first demonstrated that PEO/KSCN complexes exhibit measurable ionic conductivity in their heated state.⁴⁸ This groundbreaking work pioneered the development of soft solid electrolytes and opened new avenues for addressing interfacial challenges between electrode materials and inorganic solid electrolytes. Nuclear magnetic resonance (NMR) spectroscopy has elucidated the ion transport mechanism mediated by PEO's amorphous phase.⁴⁹ Strong quadrupolar interactions in crystalline domains significantly restrict cation mobility, effectively preventing their participation in charge transport. Specifically, cation transport in PEO's amorphous phase relies on dynamic coordination with ether oxygens, driven by chain segment rearrangement (Fig. 3a).^50,51 However, the sluggish evolution of local coordination environments and the energy-intensive process of continuous bond breaking/reformation during cation transport result in Li⁺ ions being typically less mobile than their anionic counterpart.⁵²

Fig. 3 The schematic diagram of (a) metal ions are transport depend on the movement of polymer chain segments in the amorphous region; (b) metal ions diffuse along the surface of the crystalline phase; (c) metal ions transport within crystal bulk through hopping mechanisms.

During crystallization, PEO forms a spherulitic structure with disordered regions at the grain boundaries, which may serve as ion transport pathways (Fig. 3b). Li et al. revealed that ion transport in single-crystal PEO-based solid electrolytes predominantly occurs through chain-folded regions guided by crystalline lamellae.⁴⁶ The oriented alignment of these crystalline lamellae results in significant anisotropy in ionic conduction. At low ion concentrations, segmental motion constraints dominate ionic conduction. As the ion concentration increases, structural features, such as tortuosity of conduction pathways, emerge as the predominant factor. High-energy ultrasonic vibration was reported to disrupt PEO crystallites.⁵³ This fragmentation process increases grain boundary density while modifying their spatial distribution. The resulting amorphous regions at grain boundaries form continuous ion transport channels that effectively reduce pathway tortuosity and facilitate ion conduction.

The crystalline complexes deteriorate PEO-based solid electrolytes' performance by interrupting the percolation network of amorphous conduction domains, leading to substantial conductivity reduction. Intriguingly, some unique crystalline complexes were reported to enable efficient ionic conduction via “hopping” mechanism (Fig. 3c). For instance, Bruce et al.'s pioneering work discovered that the well-ordered tunnel structures in PEO/LiAsF₆ crystalline complexes facilitate rapid Li⁺ migration along the chain direction.⁴⁷ The PEO chains form a double non-helical structure where two interlocked chains create cylindrical channels, where Li⁺ ions coordinate exclusively with PEO's ether oxygen atoms while remaining separated from AsF₆⁻ anions.

In summary, ion transport in PEO-based solid electrolytes is primarily governed by three mechanisms, including segmental movement in amorphous regions, surface diffusion at the crystal boundary, and ion hopping in crystalline phases. Segmental motion-assisted ion conduction depends on the local chain segment mobility within the polymer's amorphous domains. Surface diffusion along the crystal boundary enables rapid ion transportation with high efficiency, though the transport pathways are discontinuous. Bulk crystalline transport relies on ion hopping through the lattice framework, which offers high stability but requires substantial energy barriers to overcome, consequently resulting in relatively low conduction rates. These three mechanisms compete under different conditions and collectively govern the electrolyte's overall ionic conductivity. Therefore, fundamental understanding and precise control of crystallization behavior are crucial for developing high-conductivity SPEs.

3 Crystalline property engineering strategies

The crystallization behavior of PEO-based solid electrolytes can be effectively modulated by controlling the chain segment arrangement, crystal nucleation process, and crystal growth kinetics. Presently, the regulation strategies are categorized into three approaches: salt engineering, additive mediation, and physical field regulation.^54–57

3.1 Salt engineering

SPEs are typically composed of a polymer matrix and dissolved or dispersed metal salts. These salts provide metal ions that serve as charge carriers, enabling their reversible migration between the cathode and anode during electrochemical cycling.^58,59 Notably, incorporating metal salts can enhance ionic conductivity by suppressing polymer crystallization. Both the anions and cations of salts can play an essential role in modulating the crystalline phase transition by interacting with the PEO segments.^60–62 Furthermore, the properties and concentration of salt significantly influence the crystallization kinetics and final phase morphology of the PEO-based solid electrolytes, consequently determining their electrochemical performance.^63,64

3.1.1 Cation/anion optimization in salts. The coordination of cations with ether oxygen atoms in the PEO helical segment induces localized main-chain distortion, thereby disrupting the regularity of polymer segments and impeding lattice formation.⁶⁵ Furthermore, this coordination effect partially constrains segmental motion, effectively suppressing the crystal growth rate. Simultaneously, anions contribute to crystalline phase disruption through steric hindrance effects and hydrogen bonding interactions (Fig. 4a).^66,67

Fig. 4 (a) The interaction of each component in the PEO-based solid electrolytes. (b) The ionic radius and charge densities of Li⁺, Na⁺, and K⁺ ions, along with their corresponding (c) X-ray diffraction patterns and (d) EIS spectra in PEO-based solid electrolytes. Reproduced with permission.⁶⁸ Copyright 2024, American Chemical Society. (e) The relative sizes of common lithium salt anions. (f) The influence of single salt and double salt on PEO crystallization.

The coordination between cations and PEO chains is governed by their ionic radius and charge densities.^69,70 Among Li⁺ (0.76 Å, 83 C cm⁻³), Na⁺ (1.02 Å, 36 C cm⁻³), and K⁺ (1.38 Å, 15 C cm⁻³), the Li⁺ ion exhibits the strongest coordination with ethoxy groups owing to its smallest ionic radus and highest charge density, followed by Na⁺ ion and then K⁺ ion (Fig. 4b). As demonstrated by Kim et al., this trend directly correlates with ion transport rate in PEO-based solid electrolytes, showing the order: Li-PEO > Na-PEO > K-PEO.⁶⁸ Strong cation coordination of Li⁺ and Na⁺ disrupts the ordered arrangement of PEO chains, significantly reducing the PEO crystallinity (Fig. 4c). In contrast, the weak coordination of K⁺-PEO not only fails to suppress PEO crystallization, but also facilitates the formation of local crystalline domains through ion aggregation, thereby obstructing ion transport (Fig. 4d).

The anion of salts significantly influences the crystallization behavior of PEO-based solid electrolytes. Generally, larger anions exhibit stronger regulatory effects on PEO crystallization. Large anions influence PEO crystallization through steric hindrance effects that disrupt the ordered arrangement of polymer chains.^71,72 In parallel, their substantial size enhances charge delocalization, thereby promoting salt dissociation in the electrolyte system,⁷³ which increases the number of free metal cations available for coordination with ether-oxygen groups of PEO, consequently supressing polymer crystallization. The anions of typical lithium salts in battery applications are arranged in the order of anion size as follows: BF₄⁻ < ClO₄⁻ < PF₆⁻ < FSI⁻ < TFSI⁻ (Fig. 4e). However, through comparative studies of LiBF₄, LiPF₆ and LiClO₄, Koka et al. revealed that the ClO₄⁻ anion exhibits superior efficacy in hindering PEO chain rearrangement and facilitating cationic mobility compared to BF₄⁻ and PF₆⁻, which due to the coordination interaction between ClO₄⁻ and the PEO chain segments.⁷⁴ In an analogous study, Johansson et al. reported that LiTFSI achieves higher ionic conductivity than LiFSI because of its larger anions, which effectively suppress PEO crystallization and maintain high free Li⁺ concentration through superior dissociation capability.⁷⁵ In addition, the synergistic effect of mixed different anions can further inhibit the crystallization behavior of PEO-based solid electrolytes (Fig. 4f). Thomas et al. characterized the co-action mechanisms of TFSI⁻ and ClO₄⁻.⁷⁶ Specifically, TFSI⁻ disrupts the orderly arrangement of the PEO segments through steric hindrance, while the smaller ClO₄⁻ interferes with local chain arrangement via a stronger coordination effect. Their combination in dual-salt systems creates complementary effects that synergistically inhibit PEO crystallization to promote the Li⁺ ion transportation.

3.1.2 Salt concentration-dependent phase evolution. The crystallization behavior of PEO-based solid electrolytes exhibits significant concentration dependence. For lithium salt systems, there are three distinct regimes.^77–79 First, in the dilute regime, the limited Li⁺ coordination allows PEO crystals to dominate the microstructure in solid electrolytes. Second, in the intermediate regime, the enhanced Li⁺-ether oxygen coordination disrupts the helical ordering of PEO chains, leading to substantial crystallinity reduction. Then, in the concentrated regime, excessive salt may lead to the coexistence of multiple crystal phases. Fam et al. systematically investigated the crystallization and ion transport behavior of PEO/LiTFSI solid electrolytes.⁸⁰ The crystalline structure of PEO-based solid electrolytes undergoes significant evolution with varying EO : Li⁺ ratios, exhibiting three distinct structural regimes. At high (≥16 : 1) or extremely low (6 : 1) EO : Li⁺ ratios, the electrolyte exhibits crystalline-dominated structures, consisting of pure PEO crystals or P(EO)₈/LiTFSI complexes, respectively. Intermediate ratios between 12 : 1 and 8 : 1 favor amorphous phase formation (Fig. 5a). The crystalline composition evolution also demonstrates salt concentration dependency, progressing sequentially from: (i) pure PEO crystals when EO : Li⁺ ≥16; (ii) a eutectic mixture of PEO and P(EO)₈/LiTFSI complexes at intermediate concentrations; (iii) exclusively P(EO)₈/LiTFSI complexes when EO : Li⁺ ≤10 (Fig. 5b). An optimal EO : Li⁺ ratio of 10 : 1 yields the minimum crystallinity in PEO-based solid electrolytes, corresponding to peak ionic conductivity. This finding reveals that both the PEO crystalline phase and P(EO)₈/LiTFSI complex crystals exhibit considerable ionic transport resistance.

Fig. 5 (a) Polarized light microscope images of PEO_n/LiTFSI (n = 20, 16, 12, 10, 8, 6) after 2 days of storage at room temperature. Scale bar: 200 μm. (b) X-ray diffraction patterns of PEO_n-LiTFSI (n = 20, 16, 12, 10, 8, 6) after 1 week of storage at room temperature. Reproduced with permission.⁸⁰ Copyright 2024, Wiley-VCH GmbH. (c) Crystallinity and ionic conductivity of P(EO)_x/LiTFSI and P(EO)_x/LiBF₄ electrolytes (x = 100, 50, 20, 16, 12, 10, 8, 7, 6 and 5). High resolution TEM imaging of (d) P(EO)₁₆/LiBF₄ and (e) P(EO)₈/LiBF₄ electrolytes, and the insets correspond to the observed lattice spacing. Reproduced with permission.⁸¹ Copyright 2025, Wiley-VCH GmbH. (f) Nyquist curves of PEO₃₀, PEO_30(LN), PEO₂₀, PEO_20(LN). (g) Li⁺ transference number of PEO₃₀, PEO_30(LN), PEO₂₀, PEO_20(LN). (h) Comprehensive electrochemical performance comparison of PEO₂₀, PEO20_(LN) and PEO₁₂. Reproduced with permission.⁸² Copyright 2025, Wiley-VCH GmbH.

The crystallization and ion transport behaviors of the PEO/LiBF₄ solid electrolytes show a different lithium salt concentration dependence from those of the PEO/LiTFSI solid electrolytes. Chen et al. reported a biometric relationship between lithium salt concentration and ionic conductivity in PEO/LiBF₄ solid electrolytes.⁸¹ The electrolyte exhibited maximum ionic conductivity (2 × 10⁻⁵ S cm⁻¹) at an optimal EO : Li⁺ ratio of 8 : 1 (Fig. 5c). This conductivity enhancement stems from the formation of P(EO)₃/LiBF₄ nanocrystals (Fig. 5d and e), whose grain boundary defects and distinctive aggregate architecture facilitate rapid Li⁺ transport. In contrast to TFSI⁻, the smaller BF₄⁻ anions form dynamic coordination networks dominated by aggregate anionic structures, enabling Li⁺ transport through a hopping mechanism within the crystalline phase. Beyond the optimal concentration, the increase of long-range ordered micron-scale P(EO)₃/LiBF₄ crystals impedes ion mobility. Notably, the ionic conductivity of the P(EO)_x/LiBF₄ system remains substantially lower than that of P(EO)_x/LiTFSI solid electrolytes (x > 6), primarily due to the inferior salt dissociation capability of LiBF₄.

Variations in salt concentration simultaneously alter both the crystallinity and carrier concentration of the PEO-based solid electrolytes, making it difficult to distinguish their individual effects on ionic transportation. Recently, Yang et al. showed two PEO-based solid electrolytes with the same salt concentration but different crystallinity, synthesized by rapid cooling with liquid nitrogen (marked as LN) and natural cooling at room temperature, respectively. While the PEO/LiTFSI(LN) electrolytes exhibit significantly enhanced room-temperature ionic conductivity compared to their slowly cooled counterpart (Fig. 5f), this gain is offset by a sustaining decline in of Li⁺ ion transference number (Fig. 5g). This phenomenon arises from the fact that the mobility of TFSI⁻ is more substantially enhanced by increased free volume than Li⁺ ions, whose movement are restricted by strong coordination environments. In contrast, the PEO-based solid electrolytes with an EO : Li⁺ ratio of 12 demonstrate elevated crystallinity and improved Li⁺ conductivity. These findings suggest that merely reducing crystallinity does not necessarily enhance Li⁺ transport (Fig. 5h).⁸² However, it is important to note that the authors' comparative study was limited to low salt concentrations with EO : Li⁺ ratio of 30 or 20, which limits the generalizability of these conclusions to the PEO-based solid electrolytes with medium to high salt concentration. Furthermore, excessive crystallinity may physically obstruct ion transport pathways. Therefore, precise control of crystallization behavior remains a critical factor in optimizing the performance of PEO-based solid electrolytes.

3.2 Introduction of functional additives

Introducing functional additives into PEO-based solid electrolytes is an important and feasible strategy, which can effectively inhibit the crystallization of PEO to promote ion transportation. These additives can be classified into three categories: organic, inorganic, and organic–inorganic.^83–85 Organic additives, comprising small molecules, oligomers, and polymers, primarily disrupt the crystalline ordering of PEO chains through plasticization effects and physical chain entanglement. Inorganic additives, including various oxides, sulfides, and fluorides, predominantly exert their influence through interfacial interactions with polymer segments to suppress crystallization. Organic–inorganic hybrid materials offer unique advantages by combining these mechanisms and merits through synergistic effects.

3.2.1 Organic additives. Plasticizers, such as succinonitrile, carbonates, ionic liquids, etc., represent the most classical and extensively employed category of small-molecule organic additives.⁸⁶ These compounds intercalate between PEO chains through van der Waals forces or hydrogen bonding interactions, thereby weakening intermolecular forces within the polymer matrix. Furthermore, the incorporation of such small molecules disrupts the ordered arrangement of PEO segments at the crystalline surface, ultimately leading to reduced spherulite dimensions and suppressed crystallinity.⁸⁷ Hou et al. revealed that incorporating succinonitrile into PEO-based solid electrolytes effectively suppressed PEO crystallization to establish rapid Li⁺ conduction pathways.⁸⁸ This approach achieved rapid ionic transportation for practical operation at subambient temperatures (0–20 °C). Beyond the function of plasticizing, non-solvent-based small molecule additives can effectively regulate the properties of polymers through specific molecular interactions, rather than the high solubility of traditional plasticizers. Zhao et al. demonstrated that incorporating 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) into PEO-based solid electrolytes enhanced the ionic conductivity, from 5.4 × 10⁻⁶ S cm⁻¹ to 4.9 × 10⁻⁴ S cm⁻¹.⁸⁹ Molecular dynamics simulations revealed that TTE permeates the ethylene oxide chains via a swelling effect, effectively increasing interchain spacing (Fig. 6a and b). The significant reduction in PEO crystallinity was primarily attributed to chain disentanglement and structural reorganization. While such organic small molecules effectively increase the amorphous phase fraction to facilitate Li⁺ migration, this improvement is accompanied by a trade-off in mechanical property (Fig. 6c). Liu et al. adopted a novel multi-arm boron-containing oligomer (MBO) serving as a solid plasticizer for PEO-based solid electrolytes.⁹⁰ The polyethylene glycol side arms of MBO exhibit excellent compatibility with PEO chains, forming an interconnected physical network. The linear polyethylene glycol (MPEG) arms of MBO are compatible with the PEO chain, forming a physical cross-linked network to enhance mechanical properties (Fig. 6d). Medium and short-chain MBO significantly inhibit crystallization through the steric hindrance effect and the improvement of the motion of PEO segments. In contrast, long-chain MBO demonstrates reduced compatibility with the PEO matrix, leading to incomplete dispersion and phase separation, which promotes the crystallization of PEO (Fig. 6e) and inhibits ion transportation.

Fig. 6 (a) PEO₂₀LiTFSI and (b) PEO₂₀LiTFSI with TTE (grey: TTE; others represent the EO chain and LiTFSI). (c) DSC of PEO₂₀LiTFSI and PEO₂₀LiTFSI with TTE (the inserts are the morphology of the solid electrolytes). Reproduced with permission.⁸⁹ Copyright 2024, Royal Society Chemistry. (d) Schematic diagram of the effects of different MBO plasticizers with different MPEG side chain lengths on PEO-based solid electrolytes. (e) DMA of PEO solid electrolyte with MPEG plasticizers of different MPEG side chain lengths. Reproduced with permission.⁹⁰ Copyright 2025, Wiley-VCH GmbH. (f) The ionic conductivities of PEO/LiTFSI, PEO/LiTFSI with CNF, and PEO/LiTFSI with ZCNF. The LSCM images of (g) PEO/LiTFSI and (h) PEO/LiTFSI with ZCNF. Reproduced with permission.⁹¹ Copyright 2024, Wiley-VCH GmbH.

In striking contrast to organic small molecules that degrade electrochemical stability and mechanical strength, polymer additives typically suppress PEO crystallization while enhancing the mechanical robustness of solid electrolytes. Combining PEO with complementary polymers, such as PAN, PVDF, yields hybrid systems that enhance the ion transport capability of PEO and retain the mechanical strength of the added polymer.^92,93 For instance, Wang et al. showed the incorporation of zwitterionic cellulose nanofibers (ZCNF) into PEO matrix produced a solid electrolyte exhibiting both high ionic conductivity (Fig. 6f) and superior mechanical strength.⁹¹ Microscopy analysis reveals that uniformly dispersed ZCNF effectively disrupts PEO chain reorganization and crystallization. As shown in Fig. 6g and h, the bare PEO-based solid electrolytes exhibit a rough surface with crystalline spherulites, while after the introduction of ZCNF, the PEO-based solid electrolytes incorporating ZCNF feature a dense and smooth surface. The quaternary ammonium groups in ZCNF effectively immobilize TFSI⁻ anions through electrostatic interactions, thereby increasing the Li⁺ transference number. Although polymer blending effectively suppresses PEO crystallization, excessive polymer content may impede PEO segmental mobility or induce phase separation due to thermodynamic incompatibility, ultimately compromising ionic conductivity. Consequently, precise optimization of polymer loading and molecular interactions with PEO is critical to tailor crystallization behavior to promote ion transport.

3.2.2 Inorganic additives. When the inorganic additives are incorporated into PEO-based solid electrolytes, the surface polar groups, unsaturated metal sites, and oxygen/sulfur vacancies of inorganic additives act as functional sites, which interact with the ether oxygen groups of PEO through hydrogen bonds or Lewis acid–base interactions (Fig. 7a).^94,95 Inorganic additives act as crosslinking centers that simultaneously suppress crystalline phase formation and promote stress dissipation, while enhancing both ionic conduction and mechanical strength of the solid electrolytes. Notably, inorganic oxides such as SiO₂, Al₂O₃, and TiO₂, etc.,^96–98 represent the most widely investigated additives because of their effective surface chemistry. Wieczorek et al. systematically evaluated the influence of various inorganic fillers on the crystallization behavior and ionic transport properties of PEO-based solid electrolytes.^99,100 Their study revealed that filler incorporation reduces both the glass transition temperature and crystallinity of the PEO matrix. Notably, the PEO/NaI system containing 10 wt% Al₂O₃ exhibited a conductivity one order of magnitude higher than the pristine system at room temperature, even outperforming NASICON-type ionic conductor additives. These findings suggest that conductivity enhancement primarily arises from the regulation of PEO's crystalline properties rather than the inherent ionic conductivity of inorganic additives. Scrosati et al. investigated the interfacial interactions between PEO/LiCF₃SO₃ matrix and Al₂O₃ phases with varying surface chemistries (acidic, neutral, and basic).¹⁰¹ Their study revealed that exposed Al sites and hydroxyl groups on the surface of Al₂O₃ participate in Lewis acid–base interactions with both the ether oxygen groups of PEO and CF₃SO₃⁻ anions. These interactions disrupt the order of the PEO chain to expand the amorphous regions and enhance the dissociation of lithium salts, thereby increasing the ionic conductivity. Chen et al. synthesized SnO₂ with tunable oxygen vacancy concentrations as inorganic nano-additives.¹⁰² SnO₂ nanoparticles with elevated oxygen vacancy concentrations demonstrate superior performance in hybrid solid-state electrolytes, exhibiting enhanced crystallization suppression and improved ionic conductivity. The increased vacancy density strengthens Lewis acid–base interactions with the polymer matrix, leading to more effective disruption of crystalline domains, creating amorphous domains that facilitate ion transport. Similarly, Sun et al. employed a reducing atmosphere to prepare a vacancy-rich Li₇La₃Zr₂O₁₂ (LLZTO). The oxygen vacancies on LLZTO surfaces function as anchoring sites for ether-oxygen groups in PEO chains, strengthening LLZTO-PEO interfacial interactions.¹⁰³ These vacancy-mediated interactions effectively inhibit the agglomeration of LLZTO particles and the crystallization of PEO, forming a solid electrolyte with high mechanical strength and high ionic conductivity.

Fig. 7 (a)Schematic diagrams of three interactions between the surface of inorganic additives and PEO segments. (b) TEM image of graphene oxide nanosheets with a two-dimensional lamellar structure. The 2-D WAXD images of (c) P(EO)₁₂/LiClO₄ and (d) P(EO)₁₂/LiClO₄ with graphene oxide nanosheets. Reproduced with permission.¹⁰⁴ Copyright 2015, American Chemical Society.

In addition to inhibiting the crystallization, inorganic additives can also act as heterogeneous nucleation centers to promote the formation of PEO crystals,¹⁰⁵ which is mainly controlled by their surface properties and molecular-level polymer-additive interactions. Debdatta et al. found that montmorillonite uniformly dispersed in the PEO matrix could act as heterogeneous nucleation points to affect crystallization behavior.¹⁰⁶ Montmorillonite significantly reduces the nucleation barrier, promotes the preferred arrangement of PEO chains along the clay flakes, and enhances the crystal growth with increased nucleation density. Consequently, the system forms uniformly dispersed and small-sized PEO crystals. Similarly, Li et al. systematically investigated the crystallization regulation effects of two-dimensional graphene oxide (GO) nanosheets (Fig. 7b) in PEO-based systems.¹⁰⁴ Their study revealed two distinct crystallization mechanisms dependent on lithium salt presence. In salt-free PEO systems, the oxygen-containing functional groups (e.g., epoxy, hydroxyl) on GO nanosheets form strong intermolecular interactions with the ether-oxygen of PEO. These interactions significantly reduce the nucleation energy barrier, leading to accelerated crystallization kinetics. Conversely, in lithium salt-containing systems, Li⁺ ions preferentially coordinate with surface functional groups (primarily carboxyl and hydroxyl) of GO, generating localized amorphous PEO-Li⁺ complexes. These complexes disrupt the regular chain packing of PEO, thereby suppressing both crystallization rate and overall crystallinity. Moreover, the aspect ratio and layered structure of graphene oxide nanosheets enable them to act as templates during the PEO crystallization process, guiding PEO molecules to align in a specific direction (Fig. 7c and d). This templating effect establishes continuous, low-resistance pathways for ion transport through the aligned crystalline-amorphous interfaces.

3.2.3 Organic–inorganic composites. Organic–inorganic composites demonstrate synergistic effects in PEO-based solid electrolytes by integrating the organic and inorganic advantages.^107,108 Through multiscale interfacial interactions, these additives enable precise control over PEO crystallization behavior while overcoming the traditional conductivity-mechanical strength trade-off. As exemplified by Zhu et al., a supramolecular complex (SMP) was constructed via σ-hole bonding between SbF₃ and tetraethylene glycol dimethyl ether (G4).¹⁰⁹ In this hybrid system, G4 functions as a molecular plasticizer to enhance PEO segmental dynamics, while SbF₃ establishes a three-dimensional crosslinked network that reinforces mechanical stability (Fig. 8a). Therefore, compared with PEO/G4 and PEO/LiTFSI, PEO/SMP simultaneously has a high ionic conductivity of 2.4 × 10⁻⁴ S cm⁻¹ and a high mechanical strength of 8.04 MPa (Fig. 8b). Recently, Chen et al. developed an innovative aluminoxy complex (AlOC) as an organic–inorganic hybrid additive for PEO-based solid electrolytes.¹¹⁰ The AlOC molecular rings interact with PEO chains through dual non-covalent interactions: (1) C–H⋯π bonding between organic ligands and polymer backbones, and (2) C–H⋯O hydrogen bonding with ether oxygen groups (Fig. 8c). These interactions significantly suppress PEO crystallization, reducing the crystalline phase fraction from 66.7% to 35.4% (Fig. 8d). Furthermore, the host–guest interaction between AlOC and LiTFSI may adsorb TFSI⁻ anions, concentrating Li⁺ ions in amorphous regions to further enhance the Li⁺ ion transportation.

Fig. 8 (a) Schematic diagram of supramolecular interactions in PEO/SMP solid electrolytes. (b) Schematic diagrams of ionic conductivity and mechanical strength of PEO/G4, PEO/LiTFSI and PEO/SMP solid electrolytes. Reproduced with permission.¹⁰⁹ Copyright 2024, American Chemical Society. (c) Schematic diagram of the supramolecular interaction between aluminum oxide clusters (AlOC) and PEO segments. (d) Raman mapping characterization of lithium ions distribution in PEO/LiTFSI and PEO/LiTFSI with ALOC; the color maps represent the ratio of Li⁺ : EO. Reproduced with permission.¹¹⁰ Copyright 2024, Springer Nature. (e) Schematic diagram of dual-pathway Li⁺ transport in PEO polymer electrolyte doped with ultrafine pegylated SiO₂ nanoparticles. (f) Free volume fraction comparison of the PEO, SiO₂@PEO, and PEG-SiO₂@PEO-based solid electrolytes. Reproduced with permission.¹¹² Copyright 2024, Royal Chemistry Society.

Beyond small-molecule organic–inorganic complexes assembled through non-covalent interactions, an alternative approach involves the covalent grafting of polymer chains onto inorganic nanoparticle surfaces. In such hybrid systems, the robust chemical bonding between organic and inorganic components eliminates potential phase separation issues inherent in physically blended systems.¹¹¹ Moreover, the grafted polymer chains exert precise control over PEO crystallization through tailored topological constraints, enabling more predictable manipulation of the polymer's semicrystalline properties. Ye et al. successfully prepared PEG-SiO₂ by grafting short-chain polyethylene glycol onto nano-SiO₂ surface. The dimensions of PEG-SiO₂ nanoparticles are significantly smaller than the radius of gyration of PEO chains, thereby exhibiting enhanced mobility within the polymer matrix (Fig. 8e). These ultrafine nanoparticles can also act as nano lubricants, enhancing the chain mobility of the bulk PEO framework and significantly increasing the free volume of the PEO solid electrolyte (Fig. 8f). The synergistic combination of mobile PEG@SiO₂ nanoparticles and enhanced PEO segmental dynamics establishes rapid conduction pathways for Li⁺ transport. This cooperative mechanism yields a remarkable in ionic conductivity (3.92 × 10⁻⁴ S cm⁻¹, 30 °C) for the PEO-based solid electrolytes.¹¹²

In summary, the introduction of functional additives represents an effective strategy for mediating the crystallization behavior and enhancing the ionic transport properties of PEO-based solid electrolytes. While low-molecular-weight organic additives effectively suppress crystallinity, their plasticizing effects often compromise mechanical integrity. In contrast, high-molecular-weight organic additives, inorganic additives, and organic–inorganic composites demonstrate superior performance in controlling PEO crystallization while maintaining mechanical robustness. These advanced additives demonstrate significant potential for enabling the development of high-performance polymer-based solid electrolytes with commercial viability.

3.3 Regulation by physical fields

Crystalline property regulation by physical fields, including stress field, electric field, and magnetic field, presents a promising alternative approach for controlling PEO solid electrolyte crystallization, effectively circumventing the detrimental side effects of conventional additive strategies.^53,113,114 This technique enables significant enhancement of ionic conductivity while largely preserving the intrinsic physicochemical properties of the electrolyte matrix. Furthermore, when combined with salt concentration optimization or additive modification, physical field interactions can provide further crystallization and transport pathway optimization, representing a synergistic approach for PEO-based solid electrolyte engineering.

3.3.1 Mechanical stress. The mechanical stress field affects PEO-based solid electrolytes mainly through tensile and compressive deformation (Fig. 9a). Under applied stretch stress, PEO chains release stress through sliding and disentanglement, which leads to an increased proportion of amorphous regions while simultaneously inducing a more ordered arrangement in the remaining crystalline structures. Consequently, the ion transport pathways are optimized. Under compressive stress, the imposed constraints primarily reduce the degrees of freedom in PEO chain movement and elevate the energy barrier for nucleation. This shifts the thermodynamic equilibrium, effectively suppressing the formation of large-volume crystalline structures.¹¹⁵

Fig. 9 (a) Schematic diagram of the influence of stretch and compressive strain on the crystalline properties of PEO-based solid electrolytes. (b) DSC thermal images of PEO/NaTFSI solid electrolytes with different stretch deformations. The transformation near 37 °C corresponds to the glass transition temperature. The transformation near 57 °C corresponds to the melting of PEO crystals. (c and d) The small-angle X-ray scattering diagram as a function of strain illustrates the crystal orientation in the PEO-based solid electrolytes. Reproduced with permission.¹¹⁶ Copyright 2022, American Chemical Society. (e) X-ray diffraction for PEO-based solid electrolytes and after hot pressing for 5, 15, and 35 min. Reproduced with permission.¹¹⁷ Copyright 2011, Elsevier Ltd. (f) DSC curves of PEO-based solid electrolytes under different thermal pre-compression strains. (g) The enhancement coefficient of ionic conductivity of the electrolyte under different thermal pre-compression (TPC) strains. Reproduced with permission.¹¹⁸ Copyright 2024, Wiley-VCH GmbH.

Under stretch strain, PEO-based solid electrolytes experience concurrent slip-mediated crystal reorganization and strain-driven melt-recrystallization.¹¹⁹ Stress can induce the fragmentation or rearrangement of crystals, causing a decrease of grain size. As demonstrated by Chenlsea et al. in PEO/LiTFSI solid electrolytes, increasing stretch strain leads to progressive decreases in both melting and glass transition temperatures (Fig. 9b).¹¹⁶ This thermal behavior, consistent with the Gibbs–Thomson equation, directly results from the strain-induced thinning of PEO crystals. Hu et al. demonstrated that applying 200% stretch strain to PEO/LiTFSI solid electrolytes reduced crystallinity from 47% to 38%, while concurrently enhancing ionic conductivity from 2.7 × 10⁻⁶ S cm⁻¹ to 5.3 × 10⁻⁶ S cm⁻¹.¹²⁰ Moreover, this mechanical deformation may additionally facilitate the formation of new crystalline phases. Golodnitsky et al. revealed that mechanical stress promotes stronger coordination between LiI and PEO segments in PEO/LiI solid electrolytes, resulting in the formation of thermally stable composite crystals with elevated melting temperature.¹²¹

Stretch can also promote preferential alignment of PEO crystals along the axial direction, thereby effectively reducing the tortuosity of ion transport pathways. Golodnitsky et al. found that the ionic conductivity of the PEO/LiClO₄ solid electrolytes increased by about 3 times at 25% tensile deformation, and the randomly distributed PEO crystal spheres were transformed into a directional texture structure.¹²¹ Chenlsea et al. demonstrated that thermal stretching induces preferential orientation of PEO crystal layers. Upon tensile strain, the small-angle X-ray scattering intensity of the solid electrolytes becomes more concentrated along the meridian direction, confirming that the crystals align preferentially along the stretching axis, thereby optimizing ion transport pathways in the PEO-based solid electrolytes (Fig. 9c and d). Notably, the effects of stretch vary significantly between different salt systems. In the LiTFSI system, stretch reduces the overall crystallinity of the material but results in only limited crystal layer orientation. Conversely, in the LiCF₃SO₃ case, stretch strain enhances directional alignment of the crystal layers while leaving the overall crystallinity largely unaffected.¹¹⁶ Jonathan et al. reported that applied stress induces structural reorganization in PEO/LiCF₃SO₃ solid electrolytes, where chain segments are extracted from the original crystalline structure.¹²² This process transforms randomly oriented crystalline phases into highly aligned structures along the stretching direction, resulting in enhanced ionic conductivity. Brian et al. further revealed that the stress-induced effect is also affected by the salt concentration.¹²³ The orientation effect is most pronounced at lower salt concentrations with an EO : Li⁺ ratio of 20, while significantly diminished at higher concentrations with an EO : Li⁺ ratio of 10. This concentration-dependent behavior arises from restricted chain segment mobility in high salt concentration electrolytes during stretch deformation.

Compression exerts a predominant influence on the crystallization kinetics of PEO solid electrolytes, thereby ultimately governing the evolution of grain size.¹²⁴ The reduced crystallinity under compressive treatment was conclusively demonstrated through X-ray diffraction analysis, where phase transition monitoring during hot pressing revealed significant attenuation of crystallization peak intensity accompanied by pronounced peak broadening. In particular, the absence of new diffraction peaks confirms that, unlike tensile strain, compressive strain does not induce new crystalline phase formation (Fig. 9e).¹¹⁷ Furthermore, the magnitude of compressive strain significantly modulates the crystalline properties of PEO-based solid electrolytes.¹¹⁸ The PEO/LiTFSI solid electrolytes exhibit a non-monotonic response to compressive strain. At 20% strain, the crystallinity decreases significantly from 46.5% to 27.1%, demonstrating effective crystallization suppression. However, when strain increases to 30%, the crystallinity rebounds to 30.9%, indicating that excessive compressive stress paradoxically accelerates crystal growth (Fig. 9f). This strain-dependent behavior results in a distinct “volcano-type” relationship between ionic conductivity and compressive strain, where conductivity first increases and then decreases with applied pressure. Notably, the ionic conductivity shows an inverse correlation with crystallinity throughout this process (Fig. 9g).

3.3.2 Electric/magnetic field. In contrast to mechanical force fields, electric/magnetic fields enable molecular-scale regulation of crystallization for PEO-based solid electrolytes.^125,126 This non-contact regulation method not only avoids the material damage that may be caused by mechanical stress, but also can achieve precise control of PEO crystalline properties by adjusting parameters such as field strength and frequency. Among them, the electric field predominantly disrupts the ordered alignment of PEO molecular chains through coupled dielectric polarization and ion migration effects, consequently suppressing the formation of macroscopic spherulitic structures. Alina et al. employed atomic-scale molecular dynamics simulations to investigate the structural evolution of PEO/LiTFSI solid electrolytes under strong electric fields.¹²⁷ They found that PEO chains undergo uniaxial elongation along the electric field direction, resulting in an anisotropic structural reorganization characterized by transverse contraction and longitudinal extension (Fig. 10a). Wang et al. demonstrated that the incorporation of LiClO₄ enables electric-field-induced rearrangement of PEO chain segments, while pure PEO maintains its macroscopic morphology under electric field treatment.¹¹⁴ This structural reorganization leads to modified crystal architecture and enhanced ionic conductivity. Beyond these morphological and crystalline modifications, the electric field further facilitates polarization-mediated alignment of inorganic additives along the field direction. The synergistic effect of the applied electric field and its induced alignment of inorganic additives regulates the crystallization kinetics of PEO while simultaneously optimizing ion transport pathways within the solid electrolytes. Huang et al. found that montmorillonite particles formed an arrangement along the electric field in the PEO matrix, providing an ion conduction channel parallel to the electric field.¹²⁸ The combined effect of the electric field and montmorillonite effectively optimized the crystal structure of the PEO-based solid electrolytes, reduced the crystallinity and provided a directional ion channel, promoting ion transport (Fig. 10b).

Fig. 10 (a) Schematic diagram of the structural evolution of PEO/NaTFSI solid electrolyte under the action of electric field. Reproduced with permission.¹²⁷ Copyright 2021, American Chemical Society. (b) The ionic conductivity of PEO/MMT solid electrolytes with different montmorillonite contents at 25 °C prepared with or without an electric field. Reproduced with permission.¹²⁸ Copyright 2010, Elsevier Ltd. (c) DSC of the PEO/LiAsF₆ solid electrolytes: typically cast and cast under a magnetic field (MF). (d) The ionic conductivity within the PEO/LiI polymer electrolyte chain. Reproduced with permission.¹²⁵ Copyright 2005, Elsevier Ltd. (e) The ionic conductivity of PEO/LiTf solid electrolytes and PEO/LiTf with 1% Fe₂O₃-PNT (w/w) under MF. Reproduced with permission.¹³¹ Copyright 2012, Elsevier Ltd.

Magnetic fields can significantly influence the folding behavior and helical organization of PEO molecular chains. Golodnitsky et al. systematically investigated magnetic field effects on PEO-based solid electrolyte crystallization and ionic transportation behavior.^{125,129–131} Their studies revealed that magnetic field treatment narrows the PEO melting endotherm (Fig. 10c), suggesting enhanced structural ordering with polymer helices adopting preferential vertical alignment, resulting in a sevenfold enhancement in ionic conductivity (Fig. 10d).¹²⁵ Based on this work, they developed an advanced PEO composite electrolyte incorporating Fe₂O₃-coated diphenylalanine peptide nanotubes (Fe₂O₃-PNTs) under magnetic field processing.¹³¹ The magnetic field-induced alignment of Fe₂O₃-PNTs inhibits the formation of large spherulites, yielding evenly distributed small grains. Progressive grain regularization under a magnetic field facilitates the formation of continuous ion transport paths, resulting in further optimized performance (Fig. 10e).

Physical fields can significantly change the crystallization behavior of PEO by regulating its crystallization process. Mechanical stress fields primarily reduce crystallinity and induce crystal orientation via molecular chain alignment and stress-induced nucleation. In contrast, electric/magnetic fields influence molecular arrangement and crystallization dynamics through non-contact interactions, though their regulation efficiency is relatively low. Optimal effects from electric/magnetic fields typically require synergistic assistance from electromagnetic fillers. Collectively, physical field regulation serves as an effective approach for the tailored design of PEO crystallization. This methodology proves particularly valuable for performance optimization of PEO-based solid electrolytes during manufacturing processes.

4 Conclusion and perspectives

SPEs have emerged as promising candidates for next-generation high-energy-density battery electrolytes due to their superior safety, flexibility, and processability. PEO-based solid electrolytes have attracted significant attention owing to their exceptional lithium salt solubility and unique segmental motion characteristics. However, the complex relationship between the crystalline behavior and ionic conductivity of PEO-based solid electrolytes remains a critical bottleneck hindering their practical applications. This review systematically examines the influence of crystallization on ion transport mechanisms from the fundamental principles of crystallization kinetics and thermodynamics. The crystallinity of PEO originates from the well-ordered arrangement of polymer chains into crystalline domains, which depends on the PEO nucleation barrier, the segmental mobility and the thermodynamic equilibrium. While these crystalline regions impede the segmental motion in amorphous phases, certain special crystal structures provide alternative ion transport pathways either through grain boundaries or in the bulk crystalline. Furthermore, recent advances in tailoring crystallization behavior are summarized, such as salt engineering, additive mediation and external physical fields regulation. The introduction of lithium salts with bulky anions can disrupt the regular arrangement of PEO chains, thereby reducing crystallinity. However, excessive salt content may lead to the formation of polymer-salt complex crystals, which could conversely impede ion migration. Additives can suppress crystallization through plasticizing or physical blocking effects, though potentially at the expense of mechanical properties or by obstructing ion transport channels. External stress or electromagnetic fields may induce preferential alignment of PEO chains, creating anisotropic crystal structures that optimize ion conduction pathways, albeit within relatively narrow processing windows.

Crystallization remains the major limitation for room-temperature ionic conduction in PEO-based solid electrolytes. While the aforementioned regulation strategies have dramatically boosted ionic conductivity, the fundamental crystallization mechanisms governing polymer-salt complexes remain elusive. Moreover, prevailing crystallization regulation strategies predominantly focus on passively suppressing PEO polymer crystallization to increase amorphous regions, rather than achieving precise optimization of crystalline properties, such as crystal distribution and crystal structures. They significantly constrain the rational design and development of polymer electrolytes with high ionic conductivities.

The following four key research directions merit particular attention (Fig. 11):

Fig. 11 Perspectives for future crystalline property regulations to advance high-performance SPEs.

(1) In-depth exploration of crystallization mechanisms in polymer-salt complexes. Current research still shows significant gaps in understanding the non-equilibrium crystallization kinetics of multicomponent polymer-salt systems. This unresolved key scientific issue severely restricts precise control of crystalline phases in SPEs and optimization of their ion-conductive functions. Combining multiscale characterization with computational modeling holds promise for revealing crystallization mechanisms. At the molecular level, in situ spectroscopy (e.g., FTIR, Raman) combined with molecular dynamics simulations elucidates how coordination bonding (e.g., dynamic interplay between cations, ether oxygens and anions) regulates nucleation. At the mesoscopic level, the integration of synchrotron X-ray techniques (e.g., time-resolved SAXS, WAXS) with advanced microscopy characterization (e.g., polarized light microscopy, AFM) enables the construction of dynamic evolution models from local ordering to macroscopic crystal formation. At the macroscopic level, conventional optical equipment, such as polarized optical microscopy, can serve as a powerful tool for visualizing spherulitic growth and crystallization fronts.

(2) Uniform and controllable crystallization. Current crystallization regulation predominantly relies on stochastic nucleation processes, resulting in non-uniform crystalline domain distribution and sizes that impede precise regulation of ion transport pathways. Controlled spatial distribution of nucleation sites coupled with tailored crystal growth kinetics enables precise regulation of crystalline domain size, orientation, and interfacial characteristics, which govern ion transport efficiency in SPEs. For example, seed-induced crystallization techniques represent an effective approach for achieving uniform and controllable crystallization. The introduction of specific seeds, including PEO microcrystals, lithium salt nuclei, or functionalized nanoparticles, facilitates the ordered arrangement of PEO molecules along seed surfaces, enabling controlled crystal growth and yielding more uniform crystalline domain size distributions. Moreover, the effects of solvent polarity on PEO crystallization during solution processing merit further investigation. In addition, an alternative approach involves molecular design strategies that incorporate specific functional groups into PEO segments to modulate intermolecular interactions, thereby directing crystal growth with molecular-level precision.

(3) Synergistic integration of physical fields with salt/additive engineering. Physical field regulation (including mechanical, electric, and magnetic fields) offers distinct advantages through its non-contact nature, programmable control, and high precision, thereby creating complementary effects when combined with conventional salt engineering and additive approaches. The synergistic combination of physical fields with other regulation approaches overcomes the limitations of individual strategies, enabling precise control of crystallization behavior and directional optimization of ion transport. For instance, when electric fields are coupled with salt engineering, the applied field facilitates salt dissociation while suppressing the formation of polymer-salt complex crystalline phases at high salt concentrations. Similarly, the integration of magnetic fields with additive strategies promotes the aligned orientation of ferroelectric materials and prevents nanoparticle aggregation. These inorganic additives can act as heterogeneous nucleation sites to promote uniform polymer crystallization, thereby further optimizing ion transport pathways.

(4) Decoupling ion transport from segmental dynamics. In conventional PEO-based solid electrolytes, ionic conduction strongly depends on polymer segmental motion, which exhibits an inverse relationship with crystallinity. Resolving this fundamental compromise requires innovative polymer crystal structure designs that enable ion migration mechanisms independent of segmental dynamics. The structural design of polymer crystals with rapid ion-conduction capabilities critically depends on establishing stable ion transport channels while simultaneously weakening the interactions between salt cations and PEO chains, as well as between salt cations and anions. The incorporation of rigid conjugated structures presents a promising strategy, wherein the rigid framework forms stable ion transport channels through crystalline ordering, while the interactions between the conjugated moieties and both PEO ether oxygen groups and salt anions are anticipated to substantially lower the migration energy barriers for metal salt cations. Additionally, introducing controlled point defects (e.g., vacancies) into the crystal lattice can further reduce the energy barrier for ion migration. Similarly, in halide-based electrolytes, the substitution of Lu³⁺ with Zr⁴⁺ increases vacancy concentration within the chloride sublattice, leading to an enhancement in ionic conductivity from 0.40 mS cm⁻¹ to 1.50 mS cm⁻¹.¹³²

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from The Hong Kong Polytechnic University (U-CDCA), Natural Science Foundation of Guangdong (No. 2025A1515011149), and Innovation and Technology Fund (ITS-322-23FP).

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