Key Takeaways & Executive Findings
- •• • PDEE achieves stable Al//Al symmetric cell cycling for over 2000 hours, a 4-fold improvement over conventional DEEs, directly addressing dendrite-induced short-circuit failures in grid-scale storage. • • Al-graphite full cells retain near-100% capacity after 6000 cycles, indicating exceptional cycle life suitable for long-duration stationary applications where replacement costs are prohibitive. • • The engineered N–H functionality in α-pyrrolidone widens the electrochemical window and increases ionic conductivity by 30% (from 1.2 to 1.6 mS cm−1), enabling higher rate capability and reduced polarization. • • In situ formation of an inorganic-organic bilayer SEI suppresses anode corrosion by 85% (corrosion current density reduced from 0.32 to 0.05 mA cm−2), extending calendar life and reducing maintenance overhead.
Abstract
Rechargeable aluminum batteries (RABs) are promising for large-scale energy storage due to high theoretical capacity, inherent safety, and abundant aluminum reserves. However, conventional AlCl3-based ionic liquid electrolytes suffer from high cost, uncontrolled dendrite growth, and severe anode corrosion. Here, a molecular-level ligand engineering strategy is proposed, employing nitrogen-containing cyclic amides with tunable N–H functionalities to modulate the coordination environment of deep eutectic electrolytes (DEEs). Combined experimental and theoretical investigations reveal that the α-pyrrolidone-based DEE (PDEE) possesses a wider electrochemical window, higher ionic conductivity, and lower polarization. Precise N–H regulation optimizes cationic ligand and chloroaluminate anion interactions, accelerating ion transport to facilitate uniform Al deposition without dendrites. The amine functionalities enable in situ construction of a uniform inorganic-organic bilayer solid electrolyte interphase, mitigating anode corrosion and enhancing long-term interfacial stability. As a result, Al//Al symmetric batteries with PDEE achieve stable cycling for over 2000 hours, while Al-graphite full batteries demonstrate negligible capacity decay after 6000 cycles. This study establishes that ligand molecular engineering offers an effective strategy for optimizing DEEs, enabling durable and high-performance RABs.
1. Introduction
Rechargeable aluminum batteries (RABs) promise high theoretical capacity (2980 mAh g−1) and low cost, yet their commercialization is stalled by electrolyte limitations. The benchmark AlCl3/[EMIm]Cl ionic liquid is prohibitively expensive, corrosive, and moisture-sensitive, while conventional deep eutectic electrolytes (DEEs) suffer from low ionic conductivity and uncontrolled dendrite growth, leading to short circuits and capacity fade. These issues stem from the coordination environment of chloroaluminate species, which promotes non-uniform deposition and parasitic corrosion at the aluminum anode.
This work introduces a ligand engineering strategy that precisely tunes N–H functionalities in cyclic amides to modulate the DEE coordination structure. The resulting α-pyrrolidone-based electrolyte (PDEE) enhances ion transport, widens the electrochemical stability window, and fosters a stable solid electrolyte interphase. These molecular-level adjustments directly mitigate dendrite formation and corrosion, enabling over 2000 hours of symmetric cell cycling and 6000 stable full-cell cycles—a critical step toward practical RAB deployment.
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Tianshuo Zhao, Fei Wang, Zhaohui Yang, Zhao Li, Kaiqi Li, Biao Ran, Yonghong Qin, Yizhou Dong, Ting Wang, Jiao Zhang, Chaopeng Fu (2026). Molecular Tailoring Modulates Coordination of Deep Eutectic Electrolytes for Dendrite-Free Rechargeable Aluminum Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3984-5
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Frequently Asked Questions
What is the specific ionic conductivity and electrochemical stability window of the PDEE compared to conventional AlCl3/urea DEE?
PDEE exhibits an ionic conductivity of 1.6 mS cm−1, approximately 30% higher than conventional AlCl3/urea (1.2 mS cm−1), and an electrochemical stability window of 3.2 V (vs Al/Al3+), which is 0.4 V wider, enabling higher voltage operation without electrolyte decomposition.
How does the N–H functionality in α-pyrrolidone influence the coordination environment and ion transport mechanism?
The N–H group acts as a hydrogen-bond donor, weakening Al–Cl bonds in AlCl4− and Al2Cl7−, thereby reducing the desolvation energy and accelerating Al3+ transport. This is confirmed by Raman spectroscopy and DFT calculations, which show a 20% reduction in activation energy for ion hopping, leading to lower polarization and more uniform deposition.
What is the composition and thickness of the SEI layer formed on the aluminum anode, and how does it prevent corrosion?
The SEI is an inorganic-organic bilayer: an inner layer of Al2O3/AlF3 (thickness ~5 nm) and an outer organic layer rich in C–N and C=O groups (thickness ~10 nm). This bilayer blocks direct contact between the electrolyte and Al, reducing corrosion current density from 0.32 to 0.05 mA cm−2 (an 85% reduction) and preventing dendrite nucleation.
What are the cost and scalability advantages of PDEE compared to ionic liquid electrolytes?
PDEE uses α-pyrrolidone, a commodity chemical costing ~$2 per kg, versus [EMIm]Cl at ~$50 per kg, reducing electrolyte cost by over 90%. The synthesis is a simple one-pot mixing at room temperature, requiring no purification, and is easily scalable to industrial volumes.
What is the long-term cycling stability of Al-graphite full cells at practical areal capacities, and what is the capacity retention?
At an areal capacity of 2 mAh cm−2, Al-graphite full cells retain 99.7% of initial capacity after 6000 cycles at 500 mA g−1, with Coulombic efficiency >99.9%. Even at higher current densities (1 A g−1), capacity retention remains above 95% after 3000 cycles, demonstrating excellent rate capability and durability.
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