A piggy bank where coins slip in easily but can only be retrieved by smashing it. Something similar happens with a new material for sodium batteries. It readily accepts sodium ions during charging, but during discharge, it gives back almost nothing.

The material with the formula Na4NiCr(PO4)3 belongs to the NASICON family — framework structures where sodium ions can travel through a three-dimensional network of channels. Calculations showed that the barrier for ion movement is low — only 0.468 electronvolts, roughly the energy of thermal motion at room temperature. That means ions should be able to move freely.

After a charge-discharge cycle, the crystal lattice remained intact, and calculations showed that sodium ions could still migrate. So the problem isn't ion movement. Most likely, poor electronic conductivity is to blame: electrons can't reach the sites where ions are located to complete the reaction.

It's like having functional water pipes in a house but a broken pump. The water (ions) is there, the pipes (channels) are fine, but without the pump (electrons), the water doesn't flow.

To make such a material work, its electronic conductivity needs to be improved — for example, by adding dopants or altering the structure. Then the piggy bank will not only accept but also give back.