It is now established from battery pack engineering modeling calculations, that a Na-ion battery operating at 3.3 V, employing an anode (negative electrode) with a capacity of 500 mAhg^-1, and a 200 mAhg^-1 cathode (positive electrode) will provide upwards of 15-20% higher energy density (gravimetric basis) over current Li-ion batteries (i.e. 200-225 Whkg^-1).¹ Of course, reaching such capacity goals of the anodes and cathodes for Na-ion batteries remains very challenging. In this presentation, a number of anode materials’ chemistry and electrochemistry will be evaluated and discussed. These systems include sodium alloys with Sn, Sb, Bi, Ge in carbon-composite particles or in elemental thin-film form, SnSb intermetallics, nano-tube TiO₂ and Na_2-xH_xTi₂O₄(OH)₂, conversion anodes like Sb₂O₄, Co₃O₄, Fe₂O₃, Fe₃O₄, PbO, Pb₃O₄, Sn-Na₂O conversion-alloying composite, and lastly, spherically-shaped hard carbons. Notably, the Sn-Na₂O conversion-alloying composite yields promising target 500 mAhg-1 capacity at C/10 rate for 500 cycles. An interesting mechanism is at work here, whereby the Na-Sn forms an amorphous Zintl phase such that the negative charge at deep sodiation collects on the Sn atom in the compound. This process is believed to minimize overall volume changes in the (de)sodiation process which aids in the good reversibility of the material.

[1] http://www.cse.anl.gov/batpac/index.html