Development of a Solid Polymer Electrolyte for High-Performance Aluminum Batteries: Enhancing Electrochemical Stability and Ionic Conductivity with PEO and Fumed Silica

Significance 

Rechargeable aluminum (Al) batteries are garnering significant interest as a potential alternative to lithium-ion batteries due to several inherent advantages. Aluminum is the most abundant metal in the Earth’s crust, with a weight percentage of 8.1%, compared to a mere 0.002% for lithium. This abundance translates to lower raw material costs and less geopolitical risk associated with supply chains. Additionally, aluminum’s electrochemical properties are favorable for energy storage: it can deliver a theoretical specific capacity of 2980 mA h g−1 and a volumetric capacity of 8040 mA h cm−3, making it a competitive candidate for high-capacity batteries. Despite these advantages, the development of aluminum batteries faces several critical challenges. The primary obstacles include the limited electrochemical stability, corrosivity, and moisture sensitivity of the chloroaluminate ionic liquids commonly used as electrolytes. These ionic liquids, such as 1-ethyl-3-methylimidazolium chloride-aluminum chloride (EMImCl-AlCl3), are favored for their ability to reversibly electrodeposit aluminum at room temperature. However, their narrow potential stability window limits the operational voltage of aluminum batteries, thus restricting their energy density and overall performance. The quest for improved aluminum battery performance has led researchers to explore various electrolyte systems, including aqueous and non-aqueous electrolytes, deep eutectic solvents, water-in-salt electrolytes, and polymer-based electrolytes. Among these, polymer-based electrolytes offer promising advantages, such as enhanced safety due to their solid or gel state, which mitigates the risk of leakage and eliminates the need for a glass-fiber separator that adds to the non-active weight of the battery. Additionally, polymer electrolytes have the potential to widen the electrochemical stability window, which could significantly improve battery capacity and cycling performance.

New study published in Advanced Energy Materials and led by Theresa Schoetz from University of Illinois at Urbana-Champaign and conducted by Oi Man Leung, Leo W. Gordon, Robert J. Messinger, Themis Prodromakis, Julian A. Wharton, Carlos Ponce de León developed a novel solid polymer electrolyte for aluminum batteries. Their research focuses on creating a solid polymer electrolyte based on a combination of 1-ethyl-3-methylimidazolium chloride-aluminum chloride, polyethylene oxide (PEO), and fumed silica. This new electrolyte formulation is designed to enhance the electrochemical stability and ionic conductivity while maintaining the advantageous properties of the ionic liquid.

To assess the electrochemical feasibility of the newly developed solid polymer electrolytes in aluminum batteries, the researchers first examined the reversible electrodeposition of aluminum metal. They employed a three-electrode cell setup with glassy carbon as the working and counter electrodes. The cyclic voltammograms (CVs) obtained from polymer electrolytes containing up to 7 wt% polyethylene oxide (PEO) displayed typical profiles for reversible aluminum metal electrodeposition. These profiles featured a nucleation loop indicative of metal film formation on foreign substrates. One key observation was that a minimum of 4 wt% PEO was necessary to solidify the polymer electrolyte mixtures. Electrolytes with less than 4 wt% PEO did not solidify and thus were excluded from further study. The polymer electrolytes, denoted as PE-x-y, where x and y represent the weight percentages of PEO and fumed silica (SiO2), respectively, showed current densities ranging from −8 to −3 mA cm−2 at a scan rate of 20 mV s−1. The CV of the PE-6-0.5 electrolyte revealed redox peaks corresponding to the deposition and dissolution of aluminum from Al2Cl7− anions, occurring at −0.23 and 0.02 V vs Al|Al(III), respectively. However, the anodic and cathodic current densities diminished significantly in electrolytes with more than 7 wt% PEO. For instance, the PE-8-0 electrolyte exhibited peak current densities of 0.07 mA cm−2 and −0.15 mA cm−2 for electrodeposition and dissolution, respectively, which were approximately 95% lower than those observed in the PE-7-0 electrolyte. Electrolytes with 10 wt% PEO showed no redox activity in the range of −0.5 to 1.0 V vs Al|Al(III), likely due to the crosslinking and consumption of Al2Cl7− ions by PEO chains.

Incorporating PEO into Lewis neutral EMImCl-AlCl3 ionic liquids has previously been shown to improve the electrochemical stability window by up to 1 V. Similarly, the authors demonstrated that the polymer electrolytes based on Lewis acidic compositions enhanced stability in both cathodic and anodic directions. Using cyclic voltammetry in a three-electrode cell, the researchers quantified the stability limits through linear regression of the CV curves before and after electrolyte decomposition. The PE-6-1 electrolyte exhibited the widest potential stability window, ranging from −2.12 V to 2.77 V vs Al|Al(III), representing a 9% increase in the anodic stability limit over the neat ionic liquid. They determined the ionic conductivities of the polymer electrolytes using electrochemical impedance spectroscopy (EIS) in a three-electrode cell. The ionic conductivities varied with the SiO2 and PEO content, ranging from 6.25 mS cm−1 for PE-7-0 to 15.3 mS cm−1 for PE-6-1. These values are comparable to the neat EMImCl-AlCl3 ionic liquid, which has a conductivity of approximately 19 mS cm−1. The polymer electrolytes achieved higher conductivities than most aluminum battery polymer electrolytes reported in the literature, attributable to the high loading of the ionic liquid within the polymer matrix. They also performed variable-rate cyclic voltammetry to determine the diffusion coefficients of Al2Cl7⁻ species responsible for aluminum electrodeposition. By plotting the peak cathodic current against the square root of the scan rate, the researchers used the Randles–Ševčík equation to calculate the diffusion coefficients. The diffusion coefficients of Al2Cl7− decreased with increasing PEO content, reflecting restricted ion mobility within the polymer matrix. The calculated diffusion coefficients were lower than the actual values due to interactions between Al2Cl7− species and PEO chains, which were confirmed through NMR spectroscopy.

The researchers also examined the morphologies of electrodeposited aluminum on glassy carbon substrates using scanning electron microscopy (SEM). A constant potential of −0.3 V vs Al|Al(III) was applied for 30 minutes, resulting in light grey aluminum films. The electrodeposits from the PE-6-0.5 electrolyte were dense, spherical, and uniform, with sizes ranging from 0.85 to 1.1 µm. Energy-dispersive X-ray (EDX) analysis confirmed the purity of the deposits, identifying only aluminum and trace amounts of oxygen, carbon, chlorine, and silicon from the substrate and residual electrolyte. The absence of dendritic formations, which could compromise battery longevity, was attributed to the low steady-state current density during electrodeposition. To understand the molecular-level origins of the electrolyte properties, the team used solid-state 27Al, 29Si, and 1H NMR spectroscopy. The spectra revealed the presence of chloroaluminate species AlCl4− and Al2Cl7− at 103.2 and 97.4 ppm, respectively, and an additional signal at ≈50 ppm associated with a five-coordinate aluminum environment. Dipolar-mediated 27Al{1H} heteronuclear multiple-quantum coherence (D-HMQC) NMR experiments indicated that these five-coordinate aluminum species interacted with protons from the PEO chains, confirming their role in crosslinking the polymer network.

Further, 2D 27Al{27Al} multiple-quantum MAS (MQ-MAS) NMR experiments helped identify the quadrupolar nature of the five-coordinate aluminum species, with a coupling constant (CQ) of 5.82 MHz and asymmetry parameter (ηQ) of 0.85. Density functional theory (DFT) calculations supported the identification of these species as AlCl3O2. The interaction of Al2Cl7− with the ethylene oxide units of PEO chains resulted in the solidification of the polymer electrolyte, crucial for high ionic conductivity and stability. The authors also compared the performance of Al–natural graphite cells with polymer electrolytes to cells with the neat ionic liquid. At a 2.4 V cut-off potential, the ionic liquid cell achieved a specific capacity of 123 mA h g−1. The PE-6-0.5 electrolyte outperformed the ionic liquid, reaching 131 mA h g−1, likely due to better electrode-electrolyte interface stability. Further, cells with PE-6-0.5 electrolyte charged to higher potentials (2.6 and 2.8 V) demonstrated significant capacity improvements, with a maximum specific capacity of 194 mA h g−1 at 2.8 V.

In conclusion, the Professor Theresa Schoetz and her colleagues addressed critical challenges in the development of rechargeable aluminum batteries, particularly focusing on the limitations posed by traditional ionic liquid electrolytes. The introduction of a novel solid polymer electrolyte composed of 1-ethyl-3-methylimidazolium chloride-aluminum chloride (EMImCl-AlCl3), polyethylene oxide (PEO), and fumed silica (SiO2) marks a significant advancement in the field. The research demonstrates that this new electrolyte formulation can substantially enhance the electrochemical stability, ionic conductivity, and overall performance of aluminum batteries.  The new solid polymer electrolyte exhibits a wider electrochemical stability window, allowing for higher charging cut-off potentials (up to 2.8 V). This directly translates to increased battery capacity and energy density. The solid-state nature of the electrolyte mitigates risks associated with leakage and enhances the structural integrity of the battery, contributing to safer and more durable energy storage systems. The polymer electrolyte enables highly reversible cycling with over 95% coulombic efficiency and retains high specific capacities over extended cycles, demonstrating its suitability for practical applications. Moreover, the improved electrochemical stability and ionic conductivity of the solid polymer electrolyte make aluminum batteries more competitive with existing lithium-ion technologies, potentially leading to their adoption in commercial applications where safety and cost are paramount. Additionally, the high capacity and stability of aluminum batteries with the new electrolyte formulation could make them suitable for use in electric vehicles, where high energy density and long cycle life are crucial. Aluminum batteries with enhanced performance characteristics could be deployed in grid-scale storage systems for renewable energy, providing a cost-effective and efficient solution for energy storage and distribution. Furthermore, the safety and durability of the solid-state aluminum batteries make them ideal for use in portable electronic devices, where leakage and flammability of liquid electrolytes are significant concerns.

Development of a Solid Polymer Electrolyte for High-Performance Aluminum Batteries: Enhancing Electrochemical Stability and Ionic Conductivity with PEO and Fumed Silica - Advances in Engineering

About the author

Professor Theresa Schoetz

Chemical & Biomolecular Engineering
University of Illinois at Urbana-Champaign,

The research in our group focuses on the development of electrochemical materials and interfaces for next-generation batteries and supercapacitors that can be integrated in modern electronics shaping today’s societies by making our world more connected, safer, and cleaner. Our scientific philosophy is to identify, understand, and control the molecular-level phenomena that govern macroscopic material properties, charge storage mechanisms, mass transport processes and device performance using a variety of electrochemical, spectroscopy and microscopy methods. Our distinct expertise lies in the advanced application and analysis of electrochemical methods, e.g., variable-rate cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry and in-operando techniques such as electrogravimetry (EQCM) and electrochemical atomic force microscopy (EC-AFM). Reconfigurable shape-morphing batteries that take electrochemical interfaces in energy storage systems far beyond their current state-of-the-art design space are of particular interest. Our fundamental electrochemical background and experience allow us to transfer and connect knowledge across different research areas ranging from energy storage and conversion, (bio)sensors, photoelectronics, and AI hardware.

Reference

Oi Man Leung, Leo W. Gordon, Robert J. Messinger, Themis Prodromakis, Julian A. Wharton, Carlos Ponce de León, Theresa Schoetz. Solid Polymer Electrolytes with Enhanced Electrochemical Stability for High-Capacity Aluminum Batteries. Advanced Functional  Materials Volume 14, Issue 8, February 23, 2024, 2303285

Go to Advanced Functional  Materials

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