A traditional lithium-ion battery consists of a cathode and anode, both of which store lithium ions; a separator to keep the electrodes separated on either side; and an electrolyte — the liquid through which the ions move. When lithium flows from the anode to the cathode, free electrons leave through the current collector to the device being powered while the lithium passes the separator to the cathode. To charge, the process is reversed and the lithium passes from the cathode, through the separator, to the anode.
The concept of replacing lithium with sodium and doing away with the anode isn't new. The problem has been that the anode-free battery does not have a reasonable lifetime. They always fail very quickly, have a very low capacity, or require special processing of the current collector.
Anode-free batteries tend to be unstable, growing dendrites — finger-like growths that can cause a battery to short or simply to degrade quickly. This conventionally has been attributed to the reactivity of the alkali metals involved; in this case, sodium. In the new battery, only a thin layer of copper foil was used on the anode side as the current collector, so the battery has no active anode material. Instead of flowing to an anode where they sit until time to move back to the cathode, in the anode-free battery, the ions are transformed into a metal. First, they plate themselves onto copper foil, then they dissolve away when it's time to return to the cathode.
Traditionally, when a battery fails, in order to determine what went wrong, a researcher can open it up and take a look. But that after-the-fact observation has limited usefulness.
All of the battery's instabilities accumulate during the working process. What matters is instability during the dynamic process and there's no method to characterize that. So, the team developed a unique, transparent capillary cell that offers a new way to look at batteries. Watching the anode-free capillary cell, one could clearly see that if there is not good quality control of the electrolyte, various instabilities will be seen including the formation of dendrites.
Alkali metals react with water, so the research team brought the water content down. Watching the battery in action, they saw shiny, smooth deposits of sodium. It's the smoothness of the material that eliminates morphological irregularities that can lead to the growth of dendrites. Water content must be lower than 10 parts-per-million. With that realization, the team was able to build not just a capillary cell but a working battery that is similar in performance to a standard lithium-ion battery but takes up much less space because of the lack of an anode.