In recent years, the rapid depletion of fossil fuels and escalating environmental pollution have driven intense research into clean and sustainable energy sources, such as solar, wind, and hydropower. As a key technology for energy storage and conversion, electrochemical devices like batteries and supercapacitors have gained prominence due to their reliability and efficiency. Among these, lithium-ion batteries (LIBs) with liquid electrolytes have dominated portable electronics and electric vehicles, owing to their high energy density. However, safety concerns—including dendrite growth, electrode corrosion, and electrolyte decomposition—pose significant risks. To address these issues, solid-state lithium metal batteries (SSLMBs) have emerged as a promising alternative, offering enhanced safety and potentially higher energy density. The core component of a solid-state battery is the solid-state electrolyte (SSE), which must exhibit high ionic conductivity, good electrochemical stability, and compatibility with electrodes. In this context, my research focuses on developing advanced SSEs using metal-organic frameworks (MOFs) combined with ionic liquids, aiming to overcome the limitations of traditional SSEs and advance the performance of solid-state battery systems.
Solid-state batteries represent a paradigm shift in energy storage, as they replace flammable liquid electrolytes with solid materials, thereby mitigating risks like leakage and thermal runaway. The ideal solid-state battery should feature a seamless integration of electrodes and electrolytes to minimize interfacial resistance and promote efficient ion transport. However, conventional SSEs, such as inorganic ceramics (e.g., oxides and sulfides) and solid polymer electrolytes (SPEs), often suffer from drawbacks like brittleness, low ionic conductivity at room temperature, and poor interfacial contact. For instance, inorganic SSEs typically have ionic conductivities above 10-3 S·cm-1 but require high-temperature sintering and may react with lithium metal, while SPEs offer flexibility but struggle with conductivities below 10-5 S·cm-1 and low lithium-ion transference numbers (tLi+). To bridge this gap, composite approaches have been explored, where fillers like MOFs are incorporated into polymer matrices to enhance ion conduction. MOFs, with their tunable porous structures and functional groups, provide an excellent platform for confining ionic species and facilitating selective ion transport, making them ideal candidates for next-generation solid-state battery electrolytes.
The design of MOF-based SSEs often involves impregnating MOF pores with liquid electrolytes or ionic liquids to create quasi-solid systems. This strategy leverages the MOF’s high surface area and ordered channels to host ion-conducting species while maintaining structural integrity. In my work, I aimed to enhance this approach by in-pore polymerization of ionic liquids within a functionalized MOF, specifically UiO-66-NH2. This MOF was chosen for its amine groups, which can interact with anions to restrict their movement, thereby promoting lithium-ion selectivity—a critical factor for high-performance solid-state battery operation. The resulting material, denoted as UiO-66-NH2@PIL, integrates polyionic liquid (PIL) within the pores, providing abundant conduction sites and improving electrochemical properties. This study delves into the synthesis, characterization, and performance evaluation of UiO-66-NH2@PIL as an SSE, demonstrating its potential for application in lithium metal solid-state batteries.
To understand the ion transport mechanisms in solid-state electrolytes, it is essential to consider fundamental equations. The ionic conductivity (σ) is calculated from electrochemical impedance spectroscopy (EIS) data using the formula:
$$ \sigma = \frac{L}{S \times R} $$
where L is the thickness of the electrolyte membrane (cm), S is the electrode area (cm2), and R is the bulk resistance (Ω). For temperature-dependent behavior, the Arrhenius equation describes the relationship:
$$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$
Here, σ0 is the pre-exponential factor, Ea is the activation energy (eV), k is Boltzmann’s constant, and T is the temperature (K). The lithium-ion transference number (tLi+), which indicates the fraction of current carried by Li+ ions, is determined from combined EIS and DC polarization measurements:
$$ t_{\text{Li}^+} = \frac{I_s (\Delta V – I_0 R_0)}{I_0 (\Delta V – I_s R_s)} $$
where I0 and Is are the initial and steady-state currents, respectively, ΔV is the applied voltage (typically 10 mV), and R0 and Rs are the resistances before and after polarization. These parameters are crucial for assessing SSE performance in a solid-state battery, as higher tLi+ values reduce concentration polarization and improve rate capability.
The synthesis of UiO-66-NH2@PIL involved a multi-step process. First, UiO-66-NH2 was prepared via solvothermal reaction using zirconium tetrachloride and 2-aminoterephthalic acid in a mixture of N,N-dimethylformamide (DMF) and acetic acid. The product was washed and dried to obtain a crystalline powder. Then, a lithium-containing ionic liquid precursor (Li-IL) was prepared by dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1-allyl-3-methylimidazolium TFSI ([AMIM][TFSI]). This precursor was infused into the MOF pores along with a radical initiator (AIBN), followed by thermal polymerization at 85°C under nitrogen to form PIL within the confines. The resulting composite was vacuum-dried to remove residual solvents. For electrolyte membrane fabrication, the UiO-66-NH2@PIL powder was mixed with polytetrafluoroethylene (PTFE) binder and hot-rolled into thin films (~40 μm thick). Comparative samples, such as Glass Fiber/PIL and PVDF-HFP/IL membranes, were also prepared using similar methods to benchmark performance.
Characterization techniques included X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption-desorption analysis, and electrochemical tests. XRD confirmed that the UiO-66-NH2 framework remained intact after PIL incorporation, with diffraction peaks matching the simulated pattern of UiO-66. SEM images revealed uniform particles around 200 nm in size, and the presence of PIL on surfaces suggested enhanced interparticle contact. BET surface area measurements showed a dramatic decrease from 878.1 m2·g-1 for UiO-66-NH2 to 3.0 m2·g-1 for UiO-66-NH2@PIL, indicating pore filling by PIL, which provides conductive pathways for ions. These structural insights underpin the electrochemical performance essential for a reliable solid-state battery.
Electrochemical impedance spectroscopy was conducted across temperatures from -20°C to 60°C to evaluate ionic conductivity. The Nyquist plots yielded bulk resistance values, from which σ was calculated. UiO-66-NH2@PIL exhibited a room-temperature ionic conductivity of 3.9 × 10-4 S·cm-1, which is comparable to or higher than many reported composite SSEs. The temperature dependence followed Arrhenius behavior, with an activation energy Ea = 0.32 eV, suggesting favorable ion transport kinetics. This high conductivity is attributed to the confined PIL offering continuous conduction channels and the amine groups facilitating Li+ mobility. In contrast, conventional SPEs often show lower conductivities, highlighting the advantage of MOF-based designs for solid-state battery applications.
To quantify ion selectivity, lithium-ion transference numbers were measured. UiO-66-NH2@PIL achieved tLi+ = 0.50, significantly higher than Glass Fiber/PIL (tLi+ = 0.18). This enhancement stems from interactions between the amine functionalities and TFSI– anions, which restrict anion movement and promote cation-dominated conduction. High tLi+ is critical for solid-state batteries to minimize polarization during cycling, especially at high rates. Additionally, linear sweep voltammetry (LSV) revealed an electrochemical stability window of 5.24 V for UiO-66-NH2@PIL, surpassing that of Glass Fiber/PIL (4.42 V). This wide window enables compatibility with high-voltage cathodes, potentially increasing the energy density of solid-state battery systems.
The performance of UiO-66-NH2@PIL was further assessed in symmetric Li||Li cells and full cells with LiFePO4 (LFP) cathodes. In Li plating/stripping tests at 0.2 mA·cm-2, the UiO-66-NH2@PIL-based cell maintained stable operation for over 450 hours without short-circuiting, whereas a PVDF-HFP/IL cell failed after ~60 hours due to dendrite-induced short circuits. This demonstrates the ability of UiO-66-NH2@PIL to homogenize Li+ flux and suppress dendrite growth, a key requirement for safe lithium metal solid-state batteries. For full-cell evaluations, LFP||UiO-66-NH2@PIL||Li configurations were assembled and tested under various rates. The table below summarizes the electrochemical properties:
| Parameter | UiO-66-NH2@PIL | Glass Fiber/PIL | PVDF-HFP/IL |
|---|---|---|---|
| Ionic Conductivity at 30°C (S·cm-1) | 3.9 × 10-4 | N/A | ~10-5 |
| Li+ Transference Number (tLi+) | 0.50 | 0.18 | 0.25 |
| Electrochemical Window (V) | 5.24 | 4.42 | 4.8 |
| Activation Energy Ea (eV) | 0.32 | N/A | 0.45 |
Rate performance tests showed discharge capacities of 166.7, 158.8, 146.2, and 120.3 mAh·g-1 at 0.1C, 0.2C, 0.5C, and 1.0C, respectively, indicating good rate capability. Long-term cycling at 1.0C retained 97.3% of the initial capacity after 250 cycles, underscoring excellent stability. These results surpass many reported MOF-based SSEs and align with the goals for high-performance solid-state batteries. The enhanced performance can be modeled using empirical equations for capacity retention. For instance, the capacity fade over cycles (N) can be expressed as:
$$ C_N = C_0 \times \exp(-kN) $$
where C0 is the initial capacity and k is a degradation constant. For UiO-66-NH2@PIL, k is low, indicating minimal degradation—a testament to robust electrode-electrolyte interfaces in the solid-state battery.
The success of UiO-66-NH2@PIL hinges on the synergy between MOF confinement and PIL properties. The confined polymerization ensures that PIL chains are anchored within pores, preventing leakage and enhancing mechanical stability. Meanwhile, the amine groups on UiO-66-NH2 act as Lewis basic sites, coordinating with TFSI– anions via hydrogen bonding or electrostatic interactions. This interaction reduces anion mobility, as described by the Nernst-Einstein relation for ion transport:
$$ D_i = \frac{kT \sigma_i}{n_i q^2} $$
where Di is the diffusion coefficient of species i, σi is its partial conductivity, ni is the number density, and q is the charge. By lowering Danion, the effective tLi+ increases, leading to more efficient Li+ transport. Furthermore, the porous MOF matrix accommodates volume changes during cycling, reducing stress on the electrolyte membrane and prolonging cycle life in solid-state batteries.
Comparing UiO-66-NH2@PIL with other SSEs highlights its advantages. For example, inorganic SSEs like Li7La3Zr2O12 (LLZO) offer high conductivity but require rigorous processing and may form resistive interfaces with lithium. Polymer-based SSEs, such as PEO-LiTFSI, are flexible but suffer from low conductivity at room temperature. Composite SSEs blending MOFs with polymers have shown promise, but often trade-offs exist between conductivity and mechanical strength. UiO-66-NH2@PIL addresses these by combining the benefits of both: the MOF provides structural order and ion-selective channels, while PIL ensures high ionic conductivity. This makes it a compelling candidate for next-generation solid-state battery technologies.
Future directions for this research include optimizing the PIL composition, exploring other MOF topologies, and scaling up membrane fabrication for practical solid-state battery devices. For instance, varying the ionic liquid monomer or incorporating cross-linkers could further enhance mechanical properties and ionic conductivity. Additionally, in situ characterization techniques, such as operando XRD or NMR, could elucidate ion dynamics within the confined pores. From a manufacturing perspective, roll-to-roll processing of UiO-66-NH2@PIL membranes could facilitate commercialization, potentially reducing costs and improving consistency for large-scale solid-state battery production.
In conclusion, the development of UiO-66-NH2@PIL as a solid-state electrolyte demonstrates significant progress toward high-performance lithium metal solid-state batteries. By confining polymerized ionic liquid within amine-functionalized MOF pores, this material achieves high ionic conductivity, superior lithium-ion transference, and wide electrochemical stability—key metrics for SSEs. The resulting solid-state battery cells exhibit excellent rate capability and long-term cycling stability, outperforming conventional counterparts. This work underscores the potential of MOF-based composites in advancing solid-state battery technology, offering a pathway to safer and more efficient energy storage solutions. As research continues, such innovative approaches will likely play a pivotal role in realizing the full potential of solid-state batteries for diverse applications, from electric vehicles to grid storage.