Elettra | XAFS, SuperESCA Full-solid-state electrolytes for aluminum batteries based on aluminated ammonium oxofluorotitanates In recent years, the global shift toward electrification has been largely driven by the widespread adoption of Lithium-ion batteries (LIBs), which are extensively used in transportation, energy storage, and portable electronics. However, concerns about the long-term sustainability of LIBs have emerged due to the limited natural abundance and uneven geographic distribution of critical elements such as Lithium (Li), Nickel (Ni), and Cobalt (Co). These concerns, along with safety issues, have prompted the search for alternative battery chemistries that are safer, more sustainable, and cost-effective. Among the alternatives, multivalent metal-based batteries, particularly those using Sodium (Na), Magnesium (Mg), Calcium (Ca), Zinc (Zn), and Aluminum (Al), have gained attention. In particular, Aluminum-ion batteries (AIBs) are especially promising due to Aluminum’s high natural abundance (8.1% by weight in the Earth’s crust), low cost, and high volumetric capacity (8046 mAh cm-3), which surpasses that of both Lithium and Magnesium. Despite these advantages, the development of AIBs faces significant challenges, primarily relating to the trivalent nature and high charge density of Al3+ ions. This results in poor intercalation performance, low mobility in the solid state, and a general lack of suitable cathode and electrolyte materials. With regard to the electrolyte, the majority of literature on AIBs focuses on the use of haloaluminate-based Ionic Liquids (ILs). While these electrolytes are non-flammable, they are also expensive, viscous, highly hygroscopic and extremely corrosive. Furthermore, their liquid nature poses risks such as leakage, evolution of gases (e.g., Cl2) upon degradation, and dendrite growth at high current densities. Clearly, the development of a fully solid electrolyte would enable extremely safe batteries to be obtained without the limitations of liquid electrolytes. Nevertheless, as of today, the literature reports only a few examples of quasi-solid systems that are almost entirely based on the immobilization of ILs in a polymeric host, which only partially mitigates the disadvantages of liquid electrolytes. In this context, the present work proposes, for the first time, a family of fully inorganic, full-solid-state Al-ion conductors to be used as electrolytes for full-solid-state Aluminum batteries. The development of this class of materials is particularly relevant, as it could enable the production of extremely safe batteries based on cheap and high energy-dense Aluminum. The investigated electrolytes exhibit an Aluminum content as high as 1.4 mol kg-1 and an exceptionally high conductivity of 2.7×10-5 S cm-1 at 25°C in the full solid state. The interplay between the structure of the proposed materials and the conductivity mechanism is investigated using a broad set of techniques, including x-ray diffraction (XRD), Raman spectroscopy, X-ray Absorption Near Edge Structure (XANES) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), impedance measurements, and solid-state NMR. The proposed family of electrolytes can be synthesized via a single-step, neat reaction between an ammonium oxofluorotitanate precursor and AlCl3 under inert atmosphere at 300°C. XRD measurements demonstrate that the bulk structure of the resulting solid-state Al3+ ion conductors corresponds to that of anatase, showing no evidence of Al3+ insertion into the TiO2 lattice and no formation of covalent or ionic Al-based compounds. The presence of Ti in the +4 oxidation state is confirmed by both XANES and XPS analyses. The surface structure of the electrolytes includes Fluorine in both bridging and terminal configurations, as highlighted by XPS and solid-state NMR measurements. Specifically, it is demonstrated that the reaction mechanism between the ammonium oxofluorotitanate precursor and AlCl3 involves the elimination of ammonium groups from the precursor surface and the neutralization of surface fluorides by Al3+. As a result, the final electrolytes can be considered as aluminated particles with an anatase core. XANES measurements, focusing on the Ti pre-edge region, reveal that the surface of the electrolytes includes defective Ti atoms with five-fold coordination, whose presence is strongly correlated with the observed high conductivity. Indeed, the electrolyte with the highest conductivity also exhibits the highest concentration of defective penta-coordinated Ti atoms. Impedance measurements demonstrate impressive Al3+ mobility in the solid state, resulting in a conductivity of 2.7×10-5 S cm-1 at 25°C and 5.3×10-5 S cm-1 at 80°C for the electrolyte with an Aluminum content of 1.4 mol kg-1. This level of performance is further confirmed by static 27Al NMR measurements, which detect a clear motional narrowing effect between 20°C and 80°C, providing additional evidence of Al3+ mobility. Taken together, the results demonstrate that the longrange charge migration of Al3+ occurs at the interfaces between anatasebased particles through a hopping mechanism between coordination sites characterized by the presence of penta-coordinated Ti4+ and terminal Fluorine atoms. This type of interfacial conductivity appears particularly suitable for Al3+ due to its high charge density and is the key factor governing the performance of the proposed electrolytes. The high conductivity values of the proposed materials, along with their ability to form a stable Solid Electrolyte Interphase (SEI) layer at the Aluminum-electrolyte interface, represent a significant breakthrough in the field of solid-state Aluminum batteries, paving the way for the realization of a full-solid-state Aluminum cell in the near future. Elettra Highlights 2024–2025 96
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