“Despite their very high theoretical energy densities, only a small number of lithium-air batteries with high energy densities have actually been fabricated and evaluated,” according to NIMS. “This limited success is attributed to the fact that a large proportion by weight of lithium-air battery contains heavy components – separators and electrolytes – that do not directly participate in actual battery reactions.”
Key to the development is a positive electrode material, where oxygen from the air combines with lithium from inside the battery.
Carbon is the material of choice and high energy electrodes using graphene and carbon nanotubes have been made, but these are expensive materials compared to the carbon black used in traditional batteries.
Now, the NIMS-Softbank team has managed to build a suitable highly porous electrode from carbon black, which is self-standing and achieves discharge capacity of ~7,000mAh/g even at a current density of over 0.4mA/cm2.
According to the researchers, other attempts at creating a similar carbon black electrodes have only reached 0.1mA/cm2, and have needed far more electrically-passive supporting material – binder for example – to make them self-standing.
The NIMS electrode is 70% carbon black, with carbon fibres and polymer binder making up the rest to add strength – with the associated fabrication process highly controlled to create the correct interconnected pore structure.
It was a 6,000mAh/g version of this electrode that allowed the 500Wh/kg battery to be built.
Much work remains to be done. For example, so far, the batteries only last a handful of charge-discharge cycles.
The carbon electrode and 500Whr/kg cell is described in the paper ‘Carbon-black-based self-standing porous electrode for 500 Wh/kg rechargeable lithium-oxygen batteries‘, published in Cell Reports Physical Science.