<i>(Invited)</i> Advanced Alloy Anodes for High Energy Density Sodium-Ion Cells

Worldwide efforts to develop sodium-ion batteries have accelerated in recent years. There are several companies that are developing sodium-ion batteries, including CATL, HiNa, LiFUN, Natron, Tiamat, Faradion, and UNIGRID. Commercial sodium-ion cells are available at this time, but their energy density may not be sufficient for practical application. Now it is necessary to create sodium-ion cells with a volumetric energy density greater than that of LFP/graphite cells. The volumetric capacity of typical sodium-ion battery negative electrodes like hard carbon is limited to less than 450 mAh/cm 3 . Alloy-based negative electrodes such as phosphorus (P), tin (Sn), and lead (Pb) more than double the volumetric capacity of hard carbon, all having a theoretical volumetric capacity above 1,000 mAh/cm 3 in the fully sodiated state. [1] These alloy materials have massive volume expansion, with P expanding by almost 300% and both Sn and Pb expanding to about 400% of their initial volumes when fully sodiated. We will show that Sn and Pb have excellent half-cell cycling performance despite this large volume change, including high Coulometric efficiency. [2, 3] Pb experiences 387% volume expansion upon full sodiation, which leads to significant changes in the electrode morphology. We will track the morphology of Pb and Pb-hard carbon blended electrodes using SEM imaging. As well, each Na-Pb phase will be examined to analyze their physical properties. These analyses will show that the Pb particles restructure into ~1 µm particles (see Figure 1), even after just a single cycle, and surprisingly do not pulverize the hard carbon in a blended electrode. Single-walled carbon nanotubes appear to be necessary to maintain active material electrical connection. [4] Furthermore, we will show how to improve the formulation of the Pb negative electrode and evaluate capacity retention in half- and full-cell configurations, while investigating the potential degradation mechanisms. The impact of oxide impurities on the cycling stability of Pb negative electrodes will be studied using a controlled heat treatment procedure. The irreversible formation of Na 2 O, a side product of the sodiation reaction of Pb oxide, results in sodium inventory loss, decreased first cycle efficiency, and a detrimental impact on the cell’s long-term cyclability. [5] Additionally, the thermal stability of fully sodiated Pb in the presence of ether-based electrolyte will be investigated. The results suggest that Pb not only provides an energy density advantage, but is also marginally more stable at elevated temperatures compared to hard carbon under the same testing conditions. Overall, we aim to provide valuable insights into the cyclability, degradation mechanisms and thermal stability of Pb, offering useful guidance for its future commercialization and deployment in sodium-ion batteries. [6] References [1] V. Chevrier and G. Ceder J. Electrochem. Soc. 158 (2011). [2] T. R. Jow and L. W. Shacklette J. Electrochem. Soc. 136 1 (1989). [3] M. Garayt, = L. Zhang, = Y. Zhang, = M. Obialor = et al. J. Electrochem. Soc. 171 070523 (2024). [4] M. Garayt et al. J. Electrochem. Soc. 171 120521 (2024). [5] M. Obialor et al. submitted (2025). [6] M. Obialor et al. manuscript in preparation (2025). Figure 1. SEM images of an ion-mill cross-section of a pristine Pb electrode (a) and desodiated Pb electrode after 106 cycles (b). Figure 1

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