In recent years, the escalating environmental crises stemming from the overuse of fossil fuels have intensified the global pursuit of clean and renewable energy sources. While wind and solar power hold immense potential, their widespread adoption is hampered by intermittency and geographical constraints. Consequently, the development of efficient, safe, and reliable energy storage and conversion technologies has become imperative. Lithium-ion batteries (LIBs) have dominated the market due to their superior electrochemical performance, powering everything from portable electronics to electric vehicles and grid storage. However, traditional LIBs employ liquid electrolytes that pose significant safety risks, including flammability, corrosiveness, and volatility, which severely limit their advancement and scalability. In response, solid-state lithium batteries (SSLBs) have emerged as a promising next-generation alternative, offering enhanced safety and higher energy density. The core component of SSLBs, the solid-state electrolyte (SSE), is pivotal in determining the overall performance of these batteries. Generally, SSEs are categorized into inorganic solid-state electrolytes (ISEs), polymer solid-state electrolytes (PSEs), and composite solid-state electrolytes (CSEs). ISEs exhibit high ionic conductivity and lithium-ion transference numbers but suffer from brittleness, complex processing, and poor electrode-electrolyte interface compatibility. PSEs, on the other hand, demonstrate excellent interfacial compatibility but are plagued by low ionic conductivity and transference numbers. The integration of inorganic fillers into polymer matrices to form CSEs is widely regarded as the most viable solution to enhance the comprehensive performance of SSEs.
Nanowires (NWs), characterized by their nanoscale diameter, high aspect ratio, and large specific surface area, have garnered significant attention in the realm of solid-state batteries. Their unique one-dimensional structure facilitates continuous carrier transport, making them ideal for improving ion conduction and interfacial properties in SSEs. Conventional synthesis methods for NWs include hydrothermal/solvothermal processes, sol-gel techniques, co-precipitation, ultrasonic spray pyrolysis, electrospinning, high-temperature solid-state reactions, chemical vapor deposition (CVD), and physical vapor deposition (PVD). Advances in nanotechnology have enabled the production of NWs with complex morphologies, such as porous, hollow, ultrafine, hierarchical, heterostructured, and arrayed NWs, which further enhance their surface area, compatibility, and performance. The advantages of NWs in solid-state batteries are multifaceted: their radial ion transport paths are short, while axial paths are continuous, enabling superior rate capabilities; their large surface area increases electrode-electrolyte contact, shortening charge/discharge times; they can accommodate volume changes during cycling, mitigating mechanical degradation and extending cycle life; they serve as scaffolds for composite materials, allowing the construction of multilayer ordered electrolytes; and their geometric properties facilitate in situ characterization in single-NW electrochemical devices. Owing to these attributes, various NW materials have been extensively applied in solid-state batteries, particularly in SSEs, to address challenges related to ion transport and electrode-electrolyte interfaces.
In this article, we delve into the mechanisms by which NWs modulate ion transport and interfaces in solid-state batteries. We explore how NWs enhance ionic conductivity through effects such as reducing the glass transition temperature and crystallinity of polymer matrices, promoting lithium salt dissociation, restricting anion motion, weakening interactions between lithium ions and polymer segments, and forming new ion transport pathways. Additionally, we examine the role of NWs in improving electrode-electrolyte interface contact and stability. Finally, we summarize the current challenges and future prospects of NW-based solid-state batteries, aiming to provide a comprehensive understanding that fosters further development in this field. Throughout our discussion, we emphasize the critical importance of solid-state battery technologies and the transformative potential of NWs in advancing their performance.
Fundamentals of Nanowires in Solid-State Electrolytes
Nanowires are one-dimensional nanostructures with diameters typically in the nanometer range and lengths extending to micrometers, resulting in high aspect ratios. This morphology endows them with unique properties that are highly beneficial for solid-state batteries. The large surface area of NWs facilitates extensive interactions with polymer matrices in CSEs, leading to the formation of numerous organic-inorganic interfaces that serve as rapid ion transport channels. Moreover, the mechanical robustness of NWs helps reinforce the electrolyte structure, preventing dendrite growth and enhancing cycling stability. The synthesis of NWs has evolved to include various techniques that allow precise control over their dimensions and composition. For instance, electrospinning can produce polymer or ceramic NWs with tunable diameters, while hydrothermal methods are effective for growing inorganic NWs like LLTO or LLZO. The ability to functionalize NW surfaces with specific groups further enhances their compatibility with electrolytes and electrodes, optimizing ion transport and interfacial properties.
In the context of solid-state batteries, the incorporation of NWs into SSEs addresses several inherent limitations. For PSEs, which typically exhibit low ionic conductivity at room temperature due to high crystallinity, NWs disrupt the polymer chain ordering, increasing amorphous regions and facilitating ion mobility. In CSEs, NWs act as active or passive fillers: active fillers, such as fast ion conductors, provide additional pathways for lithium ions, while passive fillers improve mechanical properties and interface stability. The following sections detail the mechanisms through which NWs enhance ion transport and interface performance, supported by experimental data and theoretical models. We also present tables and equations to summarize key findings and relationships, underscoring the pivotal role of NWs in advancing solid-state battery technology.
Mechanisms of Ion Transport Modulation by Nanowires
The ionic conductivity of SSEs is a critical parameter that dictates the efficiency and performance of solid-state batteries. Understanding the mechanisms of lithium-ion transport in CSEs incorporating NWs is essential for designing high-performance electrolytes. NWs enhance ion transport through multiple interconnected pathways, which we categorize into four primary mechanisms: reduction of glass transition temperature and crystallinity, promotion of lithium salt dissociation and restriction of anion motion, weakening of lithium ion-polymer interactions, and formation of new ion transport channels. Each mechanism contributes to lowering the energy barriers for ion migration and increasing the number of free lithium ions, thereby boosting overall conductivity.
Reduction of Glass Transition Temperature and Crystallinity
The addition of NWs to polymer matrices can significantly alter the thermal and structural properties of the electrolyte. Specifically, NWs interact with polymer chains through hydrogen bonding or other forces, disrupting chain packing and reducing the glass transition temperature (Tg). A lower Tg enhances segmental motion of the polymer, which is crucial for ion hopping in PSEs. For example, in polyether-based polyurethane (PEC) matrices, the incorporation of LLTO NWs breaks transient cross-links formed by hydroxyl groups, leading to a decrease in Tg and faster chain dynamics. Similarly, in PEO-based electrolytes, which are highly crystalline at room temperature, NWs like CsPbI3 or LLZTO can reduce crystallinity from over 70% to below 20%, as confirmed by X-ray diffraction (XRD) studies. This amorphization expands the ion-conducting regions, facilitating easier lithium ion movement. The relationship between crystallinity and ionic conductivity can be expressed using the following equation for semi-crystalline polymers:
$$\sigma = \sigma_{\text{amorphous}} \cdot (1 – X_c) + \sigma_{\text{crystalline}} \cdot X_c$$
where $\sigma$ is the overall ionic conductivity, $\sigma_{\text{amorphous}}$ and $\sigma_{\text{crystalline}}$ are the conductivities of the amorphous and crystalline phases, respectively, and $X_c$ is the degree of crystallinity. Since $\sigma_{\text{crystalline}}$ is typically negligible, reducing $X_c$ directly enhances $\sigma$. The table below summarizes the impact of various NWs on the crystallinity and Tg of polymer electrolytes:
| Nanowire Type | Polymer Matrix | Reduction in Crystallinity (%) | Reduction in Tg (°C) | Ionic Conductivity (S/cm) |
|---|---|---|---|---|
| LLTO NWs | PEC | ~20 | ~15 | ~10−4 |
| CsPbI3 NWs | PEO | ~60 | ~10 | ~10−3 |
| LLZTO NWs | PEO | ~50 | ~12 | ~10−3 |
| TiO2 Nanotubes | PVDF | ~30 | ~8 | ~10−4 |
These findings illustrate that NWs with smaller diameters and higher aspect ratios are more effective in disrupting polymer crystallinity, thereby improving ion transport. Furthermore, the mechanical reinforcement provided by NWs helps maintain electrolyte integrity under operational stresses, contributing to the longevity of solid-state batteries.
Promotion of Lithium Salt Dissociation and Restriction of Anion Motion
Nanowires with abundant Lewis acid sites on their surfaces can interact with anions from lithium salts, promoting salt dissociation and increasing the concentration of free lithium ions. This phenomenon is particularly evident in NWs containing oxygen vacancies or functional groups that act as anion traps. For instance, Y2O3-doped ZrO2 (YSZ) NWs with rich oxygen vacancies strongly coordinate with anions, facilitating the release of Li+ ions. Similarly, halloysite nanotubes exhibit charged surfaces that adsorb anions while providing channels for cation transport. In recent studies, cationic framework NWs have been developed to offer anion exchange platforms, where Cl− sites attract TFSI− anions, enhancing salt dissociation and yielding high ionic conductivity (0.267 mS/cm) and transference numbers (0.63). The transference number (tLi+) is a key parameter defined as:
$$t_{\text{Li}^+} = \frac{\sigma_{\text{Li}^+}}{\sigma_{\text{total}}}$$
where $\sigma_{\text{Li}^+}$ is the conductivity contribution from lithium ions and $\sigma_{\text{total}}$ is the total ionic conductivity. By restricting anion motion, NWs increase tLi+, reducing concentration polarization and improving battery efficiency. The following equation describes the effect of anion trapping on salt dissociation:
$$\text{LiX} \rightleftharpoons \text{Li}^+ + \text{X}^- \quad \text{(enhanced by NWs)}$$
where X− represents the anion. The table below compares the ion transport properties of CSEs with different NW fillers:
| Nanowire Filler | Polymer Matrix | Ionic Conductivity (S/cm) | Transference Number (tLi+) | Mechanism |
|---|---|---|---|---|
| YSZ NWs | PAN | ~10−4 | ~0.5 | Anion trapping via oxygen vacancies |
| Halloysite Nanotubes | PVDF | ~10−4 | ~0.6 | Surface charge-mediated anion adsorption |
| Cationic Framework NWs | PEO | 2.67 × 10−4 | 0.63 | Anion exchange and local positive charge fields |
| HAP NWs | PEO | ~10−3 | ~0.5 | Coordination with Ca2+ promoting dissociation |
These results underscore the importance of surface chemistry in NWs for optimizing ion transport in solid-state batteries. Density functional theory (DFT) calculations further support these mechanisms, showing strong binding energies between NW surfaces and anions.
Weakening of Lithium Ion-Polymer Interactions
In polymer electrolytes, strong interactions between lithium ions and polymer segments (e.g., Li+–O in PEO) can impede ion mobility by increasing the activation energy for transport. Nanowires can mitigate this effect by forming hydrogen bonds or other interactions with the polymer, thereby shielding the lithium ions from strong coordination. For example, ultrafine boehmite NWs (BNWs) with diameters of 3.7 nm create a confined environment through hydrogen bonding with PEO chains, reducing the free volume and lowering the energy barrier for Li+ migration. Similarly, aramid nanofibers (ANFs) in PEO-based CSEs form hydrogen bonds between amide groups and TFSI− anions, weakening Li+–O interactions and enhancing ion mobility. The activation energy (Ea) for ion conduction, derived from the Arrhenius equation, decreases in the presence of NWs:
$$\sigma = A \exp\left(-\frac{E_a}{kT}\right)$$
where A is the pre-exponential factor, k is Boltzmann’s constant, and T is temperature. Studies show that Ea can be reduced from over 0.5 eV to below 0.2 eV with NW incorporation, indicating more facile ion transport. The table below lists the activation energies and conductivity enhancements for various NW-polymer systems:
| Nanowire Type | Polymer Matrix | Activation Energy (eV) | Ionic Conductivity (S/cm) | Key Interaction |
|---|---|---|---|---|
| BNWs | PEO | 0.25 | ~10−3 | Hydrogen bonding with polymer chains |
| ANFs | PEO | 0.20 | ~10−3 | H-bonding with TFSI− and polymer |
| Li-HA-F NWs | PEO | 0.18 | 4 × 10−4 | Dual transport channels and functional groups |
These interactions not only lower Ea but also improve the mechanical properties of the electrolyte, contributing to the overall durability of solid-state batteries.
Formation of New Ion Transport Channels
Nanowires can create additional pathways for lithium ion transport, either through their intrinsic ionic conductivity or by forming percolation networks in the polymer matrix. Active NW fillers, such as LLTO or LLZO, possess high ionic conductivity and serve as highways for Li+ ions. For example, LLTO NWs in PAN-LiClO4 electrolytes increase room-temperature conductivity by three orders of magnitude to 2.4 × 10−4 S/cm. Aligned NWs further enhance this effect by providing continuous, low-resistance paths. In contrast, passive fillers like TiO2 NWs improve conductivity by forming interfacial layers that facilitate ion hopping. A novel approach involves sub-1 nm inorganic cluster chains that maximize organic-inorganic interface area, enabling homogeneous ion transport and achieving conductivities of 5.2 × 10−4 S/cm. The effective conductivity in CSEs can be modeled using percolation theory:
$$\sigma_{\text{eff}} = \sigma_m \phi_m + \sigma_f \phi_f + \sigma_i \phi_i$$
where $\sigma_m$, $\sigma_f$, and $\sigma_i$ are the conductivities of the matrix, filler, and interface, respectively, and $\phi$ represents their volume fractions. When NWs form interconnected networks, $\sigma_{\text{eff}}$ is dominated by $\sigma_i$, which is significantly enhanced. The table below highlights the performance of CSEs with different NW types:
| Nanowire Filler | Polymer Matrix | Ionic Conductivity (S/cm) | Transport Mechanism | Application in SSLBs |
|---|---|---|---|---|
| LLTO NWs | PAN | 2.4 × 10−4 | Percolation network | High-rate cycling |
| Aligned LLTO NWs | PEO | ~10−3 | Continuous axial paths | Improved rate capability |
| Cluster Chains (<1 nm) | PEO | 5.2 × 10−4 | Maximized interfaces | Stable cycling with NCM811 |
| BTO-LLTO Heterostructure | PVDF | 8.2 × 10−4 | Cross-phase migration | High-voltage stability |
These advancements demonstrate the versatility of NWs in creating efficient ion transport channels, which is crucial for high-performance solid-state batteries. Moreover, the coupling of active and passive fillers, as in BTO-LLTO heterostructures, synergistically enhances conductivity and interface stability.
Mechanisms of Interface Modulation by Nanowires
The electrode-electrolyte interface is a critical factor in the performance and longevity of solid-state batteries. Poor interfacial contact leads to high impedance, uneven current distribution, and accelerated degradation. Nanowires address these issues by enhancing physical contact and chemical stability at both anode and cathode interfaces. We discuss two primary mechanisms: enhancement of electrode-electrolyte contact and improvement of interfacial stability.
Enhancement of Electrode-Electrolyte Contact
In solid-state batteries, the rigid nature of ISEs often results in poor contact with electrodes, especially lithium metal anodes, which undergo significant volume changes during cycling. Nanowires can mitigate this by forming compliant layers or three-dimensional structures that adapt to volume changes. For instance, Cu3N NW arrays on LLZTO surfaces create lithiophilic nanogrooves that guide uniform lithium deposition, reducing interface resistance to 4 Ω·cm2 and enabling high critical current densities (1.8 mA/cm2). Similarly, in composite cathodes, the integration of NWs like LLZO into PEO-based binders forms seamless interfaces, lowering impedance and improving ion access to active materials. In situ polymerization techniques, where NW-containing electrolytes are formed directly within the battery, further enhance contact by conformally coating electrodes. For example, acid-treated carbon nanotube papers functionalized with —COOH and —OH groups induce ring-opening polymerization of dioxolane, producing flexible electrolytes that improve contact in lithium-sulfur batteries. The interfacial resistance (Rint) can be expressed as:
$$R_{\text{int}} = \frac{\delta}{\sigma_{\text{int}}}$$
where $\delta$ is the interface thickness and $\sigma_{\text{int}}$ is the interfacial conductivity. By reducing $\delta$ and increasing $\sigma_{\text{int}$} through NW incorporation, Rint is minimized. The table below summarizes interface improvements achieved with NWs:
| Nanowire Application | Electrode Type | Interface Resistance (Ω·cm2) | Improvement Mechanism | Battery Performance |
|---|---|---|---|---|
| Cu3N NW arrays | Li metal | 4 | Lithiophilic nanogrooves | High CCD (1.8 mA/cm2) |
| LLZO NWs in PEO | LiFePO4 cathode | ~10 | Integrated structure | Stable cycling at 0.1 mA/cm2 |
| In situ polymerized NWs | Sulfur cathode | ~15 | Conformal coating | Enhanced Coulombic efficiency |
These strategies highlight the role of NWs in achieving intimate electrode-electrolyte contact, which is essential for minimizing polarization and enabling high-power solid-state batteries.
Improvement of Interfacial Stability
Continuous side reactions at electrode-electrolyte interfaces, such as electrolyte decomposition or lithium dendrite growth, undermine the stability of solid-state batteries. Nanowires can enhance interfacial stability through physical barriers, catalytic effects, or surface functionalization. For example, asymmetric CSEs with polyamide NW layers on the cathode side provide oxidation resistance up to 5.3 V, while Al2O3 nanosheets on the anode side suppress dendrite formation. Catalytic NWs, like carbonate hydroxyapatite (Ca-PO4-CO3) with cation vacancies, promote the decomposition of TFSI− to form stable SEI layers, improving cycle life in Li||Li and Li||NCM811 cells. In PVDF-based electrolytes, LATP NWs adsorb residual DMF solvents, preventing their decomposition and ensuring uniform lithium deposition. Additionally, Kevlar aramid nanofiber (KANF) coatings on LATP surfaces act as barriers against lithium metal reduction, enabling stable cycling for over 2000 hours. The stability of the interface can be quantified by the cycle life and critical current density (CCD), which are greatly improved with NW incorporation. The following equation relates interface stability to the growth rate of dendrites (vdendrite):
$$v_{\text{dendrite}} = \frac{J \cdot t}{\rho \cdot F}$$
where J is the current density, t is time, ρ is density, and F is Faraday’s constant. By reducing J through homogeneous current distribution, NWs lower vdendrite. The table below compares interface stability parameters for different NW-modified systems:
| Nanowire Modifier | Electrolyte System | Electrochemical Window (V) | Cycle Life (cycles) | CCD (mA/cm2) |
|---|---|---|---|---|
| Polyamide NWs | PEO-based CSE | 5.3 | >800 | ~1.0 |
| CHANW with vacancies | PEO/LiTFSI | ~4.5 | >1000 | ~1.5 |
| LATP NWs | PVDF-based CSE | ~4.8 | 1500 (at 2C) | ~1.2 |
| KANF coating | LATP with PDO | ~4.5 | 2000 h (sym.) | 1.4 |
These examples underscore the multifaceted approaches to interface stabilization using NWs, which are crucial for developing durable solid-state batteries capable of operating under high voltages and currents.
Challenges and Future Perspectives
Despite the significant progress in using NWs to enhance ion transport and interface properties in solid-state batteries, several challenges remain. The high surface energy of NWs often leads to agglomeration within polymer matrices, reducing their effectiveness. Improving dispersion through surface functionalization or optimized processing is essential. Furthermore, the structure-function relationships between NW morphology and battery performance are not fully understood; for instance, how different NW shapes influence ion transport pathways or interface formation requires deeper investigation. In situ characterization techniques, such as X-ray microscopy or neutron depth profiling, could provide real-time insights into these mechanisms. Additionally, the scalability and cost of NW synthesis, especially for complex morphologies, pose barriers to commercialization. Future research should focus on developing low-cost, high-yield methods for NW production, such as scalable electrospinning or template-assisted growth.
From a fundamental perspective, exploring new ion transport mechanisms, such as quantum effects in ultrafine NWs, could unlock unprecedented conductivity. The integration of NWs into all-solid-state batteries must also address interface engineering at both electrodes simultaneously, possibly through multilayer or gradient structures. Machine learning and DFT simulations can aid in designing optimal NW-polymer composites by predicting interactions and transport properties. Lastly, the environmental impact of NW-based batteries, including recyclability and toxicity, should be evaluated to ensure sustainability. We believe that overcoming these challenges will accelerate the adoption of NW-enhanced solid-state batteries, paving the way for safer, higher-energy-density energy storage solutions.
Conclusion
In summary, nanowires play a transformative role in advancing solid-state batteries by modulating ion transport and electrode-electrolyte interfaces. Through mechanisms such as reducing polymer crystallinity, promoting salt dissociation, weakening ion-polymer interactions, and forming new transport channels, NWs significantly enhance ionic conductivity and transference numbers. Simultaneously, they improve interface contact and stability by forming compliant layers, catalytic surfaces, and protective barriers. These improvements lead to solid-state batteries with superior cycle life, rate capability, and safety. However, challenges related to NW dispersion, cost, and fundamental understanding persist. By addressing these issues through interdisciplinary research and advanced characterization, we can harness the full potential of NWs to realize the commercial viability of solid-state batteries. As the demand for efficient energy storage grows, the continued innovation in NW-based electrolytes will be crucial in powering a sustainable future.