A conventional graphite anode in a sodium battery is like a cramped garage that barely fits a large truck. Sodium is larger than lithium and doesn't fit well between graphite layers, so batteries quickly lose capacity. Researchers found a workaround: they took a sheet of graphene and attached amino groups to one side — molecular "hooks." The result is a two-sided material: one side smooth, the other studded with hooks.
When the battery charges, sodium enters this material in three stages. First, ions attach to amino groups, like hooks on a coat rack. Then they form clusters of several sodium atoms around each group. Finally, they fill the space between graphene layers, like water seeping between the pages of a book. This mechanism differs from that in hard carbon, where sodium fills pores and defects randomly.
The material barely swells during charging, and sodium ions move through it a hundred times faster than in hard carbon. This means the battery will charge faster and last more cycles. The voltage remains stably low—0.15 volts — which is ideal for an anode.
Such an anode can be folded like a stack of paper without fear of cracking. Sodium is cheap and abundant, so these batteries could replace lithium ones in electric vehicles and grid storage.
Sodium is the sixth most abundant element in Earth's crust and can be extracted from seawater. Lithium, by contrast, is concentrated in a few countries, making it more expensive and politically sensitive.