This seems to be a smoking gun:

Researchers say the most convincing evidence came from measuring how the cell expanded during charging.

When a battery charges, ions move into the anode, causing it to expand. Graphite anodes have a unique expansion pattern because of changes in graphite’s layered structure. The Donut Lab cell showed this exact pattern.

This finding matters because sodium ions are too big to fit into graphite the way lithium ions do. According to investigators, the graphite expansion pattern clearly shows that lithium is the active ion in the battery.