As we shift towards more renewable energy, the big question becomes: how do we store it safely and sustainably? Lithium-ion batteries remain the dominant solution today. However, growing concerns over their high cost, element scarcity, and safety are accelerating research into alternative chemistries.
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One of the most promising types of batteries being developed is the rechargeable aluminum battery (AlB). This is built using one of the most abundant, inexpensive and recyclable metals available today.1
Aluminum's high theoretical capacity and inherent safety characteristics make it attractive for next-generation storage, but commercial development has been limited primarily by electrolyte challenges.
Traditionally, AlBs rely on liquid chloroaluminate ionic liquids such as AlCl3:1-ethyl-3-methylimidazolium chloride (EMIC).2 Albeit effective, these electrolytes are highly corrosive, moisture sensitive, and prone to leakage, making them impractical for large-scale deployment.
Polymer-gel systems have been explored to immobilize these liquids; however, many degrade under the highly acidic conditions.3 A recent study has introduced a major breakthrough in this area by developing a polyacrylonitrile (PAN) elastomeric polymer electrolyte produced by cross-linking with chloroaluminate compounds.4
This material maintains high ionic conductivity while overcoming the safety and engineering limitations of liquid systems. A central factor enabling this breakthrough was the extensive use of Bruker instrumentation, which provided the structural, chemical, and electrochemical insights needed to design and validate the new electrolyte.
The Need for a New Electrolyte in Aluminum Batteries
Conventional chloroaluminate ionic liquids enable reversible aluminum deposition, but they have limitations: high corrosivity, leakage risk, moisture sensitivity resulting in the release of HCl, and safety challenges. Polymer-gel approaches, such as PEO, PVDF, or PA6, have been found to reduce leakage, but they treat the polymer as an inert host, which ultimately struggles with moisture reactivity and electrochemical performance.
In contrast, the newly developed PAN-based elastomer functions as an active reagent that chemically coordinates with aluminum species. Uncovering this chemistry and confirming the polymer’s behavior was achieved using Bruker FTIR spectroscopy and Bruker solid-state NMR.
A Solvent-Free Process to Produce the Polymer Electrolyte
One of the most remarkable innovations of the study is the development of a simple, solvent-free process to produce the polymer electrolyte. When PAN is heated together with AlCl3 and EMIC, two key reactions take place. First, AlCl3 begins to coordinate with the PAN chains as part of a complexation process. Then, as the temperature rises, the material undergoes controlled cross-linking, releasing small amounts of hydrogen chloride (HCl) in the process.
The Bruker FTIR spectrometer played a crucial role in validating the complexation mechanism, revealing shifts in the nitrile stretching frequencies and confirming the presence of chloraluminate species. The technique also showed how acidity and Al2Cl7- consumption varied across formulations, providing real-time guidance on polymer–ion interactions. Bruker’s solid-state NMR technologies, including 13C, 15N, and 27Al MAS NMR, delivered high-resolution insights into chemical coordination, electron density shifts, and aluminum speciation.
Features of the PAN Elastomer
1. High Ionic Conductivity
A core requirement for battery electrolytes is fast ion transport. The PAN polymer electrolyte demonstrates a conductivity of 1.1 mS/cm at room temperature, matching or exceeding many polymer-gel systems. Surprisingly, the ionic conductivity does not really drop, even as cross-linking increases, suggesting that ionic pathways remain largely unaffected. This means the polymer can be mechanically reinforced without sacrificing electrochemical performance, safety, and manufacturability.
2. Water and Air Tolerance
Unlike conventional AlCl3-based ionic liquids, which can react violently or release corrosive vapors upon exposure to moisture, the elastomeric PAN electrolyte behaves much more safely. It forms a boundary layer that slows down the reaction, reducing the risk of rapid degradation.
When exposed to air, the electrolyte can still absorb oxygen and moisture, causing the active Al2Cl7- species to decompose and become unusable in electrochemical cells. There may be no obvious visual change, but the reaction still occurs. The good news is that the polymer matrix slows this process down significantly, improving battery safety, penetrations, or water contact.
The slowing of hydrolysis and the formation of boundary layers upon water contact were validated using the Bruker FTIR gas analyzer, which monitored the evolution of HCl and other species during polymer cross-linking and environmental exposure.
3. Separator-Free Battery Design
The polymer electrolyte removes the need for a separator because the polymer is both non-flowing and mechanically stable compared to current ionic liquid electrolytes. This simplifies cell architecture, reduces internal resistance, and eliminates a common point of failure.4
4. Reversible Aluminum Stripping and Plating
Electrochemical tests confirm that the polymer enables electrochemical stripping and plating of aluminum. The cells exhibited high Coulombic efficiencies, with a 96.6 % conversion rate, indicating a fully reversible electrochemical reaction.4
5. Battery Cycling Performance
In aluminum–graphite cells, the solid electrolyte enables clear and reversible AlCl4- intercalation, delivering strong capacity retention and stable rate performance.
Although cyclic voltammetry shows broader intercalation peaks, suggesting slower electrochemical reactions than liquid electrolytes, the system still demonstrates fully reversible behavior.
Increasing polymer cross-linking slightly reduces capacity due to limited cathode wetting, but this effect can be mitigated through optimized processing.
Bruker Enabled Insights for Aluminum Batteries
Advanced analytical instrumentation played a key role in developing and understanding the PAN elastomeric polymer electrolyte for aluminum batteries. Using solid-state NMR (27Al, 13C, 15N MAS), the research team was able to get a molecular-level insight into polymer–salt coordination, revealing how the active Al-based species were stabilized within the polymer matrix.
Bruker FTIR spectrometer further enabled precise tracking of vibrational modes associated with complex formation, polymer interactions, and structural evolution, which is critical for confirming the formation of a true coordination network rather than simple polymer gelling.
These analytical capabilities were also essential in validating key properties of the electrolyte: high ionic conductivity, strong mechanical integrity, resistance to moisture, and efficient aluminum plating and stripping. Structural insights also showed why the solid-state approach eliminates corrosive leakage, which is a major limitation of ionic-liquid electrolytes.
Overall, Bruker instrumentation supported research into a safe, robust and scalable pathway toward aluminum solid-state batteries, accelerating their practical development and application.
References and Further Reading
- Ambroz, F., Macdonald, T.J. and Nann, T. (2017). Trends in Aluminium-Based Intercalation Batteries. Advanced Energy Materials, [online] 7(15), p.1602093. DOI: 10.1002/aenm.201602093. https://advanced.onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201602093.
- Lin, M.-C., et al. (2015). An ultrafast rechargeable aluminium-ion battery. Nature, [online] 520(7547), pp.324–328. DOI: 10.1038/nature14340. https://www.nature.com/articles/nature14340.
- Yu, Z., et al. (2023). Selection principles of polymeric frameworks for solid-state electrolytes of non-aqueous aluminum-ion batteries. Frontiers in Chemistry, 11. doi:https://doi.org/10.3389/fchem.2023.1190102. https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1190102/full.
- Mohammad, A., et al. (2025). Polyacrylonitrile-Based Elastomeric Polymer Electrolyte for Aluminum Batteries. ACS Applied Energy Materials, 8(4), pp.2576–2587. doi:https://doi.org/10.1021/acsaem.4c03240. https://pubs.acs.org/doi/full/10.1021/acsaem.4c03240.
This information has been sourced, reviewed, and adapted from materials provided by Bruker BioSpin Group.
For more information on this source, please visit Bruker BioSpin Group.