Practical Alloy-Based Negative Electrodes for Na-ion Batteries

The volumetric capacity of typical Na-ion battery (NIB) negative electrodes like hard carbon is limited to less than 450 mAh cm −3 . Alloy-based negative electrodes such as phosphorus (P), tin (Sn), and lead (Pb) more than double the volumetric capacity of hard carbon, all having a theoretical volumetric capacity above 1,000 mAh cm −3 in the fully sodiated state. These alloy materials have massive volume expansion, with P expanding by almost 300% and both Sn and Pb expanding to about 400% of their initial volumes when fully sodiated. This work shows that despite this large volume change, Sn and Pb have excellent half-cell cycling performance, including high Coulombic efficiency. A blended P-hard carbon cell shows promising initial performance despite P having far lower electronic conductivity than either Sn or Pb. Overall, it is expected that these Na-alloying elements will be needed to increase the volumetric energy density of NIBs to compete with low-cost lithium iron phosphate-based cells. In fact, one company, UNIGRID, has recently demonstrated cylindrical Na-ion cells that use a pure tin negative electrode, dramatically increasing the volumetric energy density of Na-ion technology.