Restructuring of Sodium-Lead Alloys during Charge-Discharge Cycling in Sodium-Ion Batteries

Sodium-ion batteries (NIBs) are of growing interest due to their expected lower cost than many lithium-ion batteries (LIBs). However, most NIBs suffer from lower volumetric energy density than LIBs. Lead (Pb) can replace hard carbon in the NIB negative electrode to substantially increase its volumetric energy density and has been shown to have no capacity fade over hundreds of cycles in half cells. Pb also experiences 387% volume expansion upon full sodiation, which presumably leads to significant changes in the electrode morphology. In this work, the morphology of Pb and Pb-hard carbon blended electrodes is tracked using scanning electron microscopy. As well, each Na-Pb phase is examined to analyze their physical properties. These analyses show that the Pb particles restructure into ∼1 μm particles, even after just a single cycle, and surprisingly do not pulverize the hard carbon in a blended electrode. Importantly, single-walled carbon nanotubes appear to be necessary to maintain active material electrical connection during the restructuring.