Design and characterisation of polymer electrolytes for aluminium–graphite batteries

Abstract

Rechargeable aluminium–graphite batteries offer a safe and sustainable form of electrochemical energy storage and are promising candidates for next-generation energy storage devices. Chloroaluminate ionic liquids such as 1-ethyl-3-methylimidazolium chloride–aluminium chloride (EMImCl–AlCl₃) are commonly used electrolytes owing to their ability to electrodeposit aluminium with high reversibility. However, their narrow potential stability window limits the charging cut-off potential of the cell to ca. 2.45 V, preventing the full utilisation of the high theoretical specific capacities of the graphite and aluminium electrodes. This work advances the field of aluminium batteries by developing solid polymer electrolytes based on EMImCl-AlCl₃, polyethylene oxide (PEO) and fumed SiO₂ mixtures that provide extended electrochemical stability to enable cycling to a 0.3 V higher cut-off potential compared to conventional ionic liquids in aluminium–natural graphite batteries, resulting in a 30% higher specific capacity. The electrochemical properties of the solid polymer electrolytes were systematically studied to probe the effects of PEO and fumed silica on properties including ionic conductivity, potential stability, and ion diffusion. Molecular interactions within the polymer electrolytes were elucidated via solid-state ²⁷Al nuclear magnetic resonance (NMR) spectroscopy to understand the ionic speciation and structure of the electrolyte. NMR results revealed the crosslinking of PEO chains with a five-coordinate aluminium species, while fumed silica dissolves and reacts with chloroaluminate ions to increase the Lewis acidity of the electrolyte, increasing the specific capacity of the cell. The solid polymer electrolyte also demonstrates stable and reversible cycling behaviour at charge-discharge rates up to 2 A g^–1, due to the retention of high ionic conductivities (ca. 13 mS cm^–1) imparted by the high ionic liquid loading (>90%) within the polymer matrix. Variable-rate cyclic voltammetry of aluminium–graphite cells was performed to separate the faradaic diffusion-limited and (pseudo)capacitive current of chloroaluminate anions into graphite, revealing a partial recovery of pseudocapacitive characteristics upon the addition of <1 wt.% fumed SiO₂ to the PEO–EMImCl-AlCl₃ electrolyte that diminish upon the addition of PEO to the ionic liquid. The electrolyte developed in this work demonstrates stable cycling for over 1000 cycles at 2.7 V in aluminium–graphite batteries, as well as the capability of cycling at temperatures up to 60 °C and as low as –20 °C. The results of this work provide a new approach towards developing solid-state electrolytes for high-performance aluminium batteries.

More information

Identifiers

Catalogue record

Export record

ASCII CitationAtomBibTeXData Cite XMLDublin CoreDublin CoreEP3 XMLEndNoteHTML CitationHTML CitationHTML ListJSONMETSMODSMPEG-21 DIDLOpenURL ContextObjectOpenURL ContextObject in SpanRDF+N-TriplesRDF+N3RDF+XMLRIOXX2 XMLReferReference ManagerSimple Metadata