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Intercalation of multivalent ions, e.g., Al³⁺, has received increasing attention as an energy-boosting strategy for rechargeable batteries. The charge storage process of the wide-spread Li-ion batteries relies on Li⁺ intercalating electrodes with layered structures. Multivalent-ion batteries could accommodate more charges than Li-ion batteries because their ions carry more charge than Li⁺. This concept, however, has now been challenged by a research team led by BenoÎt Limoges and Véronique Balland of Université de Paris, France. Their mechanistic investigation, published in Chemical Science, revealed that Al³⁺ ions were unable to intercalate electrodes in aqueous electrolytes.

The authors selected TiO₂ arrays as the object of their study. These ~1 µm-high arrays were grown using the glancing angle deposition technique. By applying negative potentials to the TiO₂ arrays, cations such as Al³⁺ could diffuse through the inter-array slits and interact with TiO₂ (Figure 1).

Figure 1. Structure of the TiO₂ arrays. (left) Scanning electron micrographs and (right) a cartoon illustrating ion diffusion pathways.

Electrochemistry tests elucidated that the charge-storage process of TiO₂ in an Al³⁺-containing aqueous electrolyte correlated to proton intercalation. This conclusion was mainly based on the nearly identical cyclic voltammograms (Figure 2, top) and capacity vs. potential curves (Figure 2, bottom) of the TiO₂ arrays in both AlCl₃ and acetic acid aqueous electrolytes. Since acetic acid solution had no Al³⁺, the observed charge-storage activity could not be attributed to Al³⁺ intercalation. Instead, the authors argued that protons dissociated either from hydrated Al³⁺ cations, [Al(H₂O)₆]³⁺ or acetic acid must intercalate TiO₂ and result in the observed charge-storage capacities.

Figure 2. (top) Cyclic voltammograms and (bottom) capacity vs. potential curves of TiO₂ in (left) Al³⁺-containing and (right) acetic acid aqueous electrolytes. The electrolytes are 0.3 M KCl with different concentrations of AlCl₃ or acetic acid: 0 M (black), 25 mM (blue), 50 mM (purple), 100 mM (magenta), and 250 mM (red).

The authors believe that the misconception of Al³⁺ intercalation is due to the overlooking of Al³⁺ hydration, which is inevitable when Al³⁺ is present in aqueous electrolytes. Removing the water shell (a prerequisite for ion intercalation) is energy costly for Al³⁺ because of the strong binding force between water molecules and Al³⁺. Additionally, even if Al³⁺ ions intercalate TiO₂, their movement is strongly hindered by Coulombic interactions within the TiO₂ lattice. The immobilized intercalated ions would then block other ions from entering the TiO₂ lattice. Together, both factors prevent Al³⁺ from intercalating into TiO₂.

In summary, this work demonstrates that the charge-storage capacity of TiO₂ in Al³⁺-containing aqueous electrolytes is most probably due to proton intercalation. This conclusion also applies to other multivalent cations, including Zn²⁺ and Mn²⁺, as shown in this work.

To find out more, please read:

On the Unsuspected Role of Multivalent Metal Ions on the Charge Storage of A Metal Oxide Electrode in Mild Aqueous Electrolytes

Yee-Seul Kim, Kenneth D. Harris, BenoÎt Limoges, and Véronique Balland

Chem. Sci., 2019, doi: 10.1039/c9sc02397f

Tianyu Liu acknowledges Zachary L. Croft of Virginia Tech, the U.S., for his constructive comments on this post.

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about the communication of scientific endeavors and cutting-edge research to both the general public and other scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.