Optimization of Na-Ion Battery Systems Based on Polyanionic or Layered Positive Electrodes and Carbon Anodes

The revival of the Na-ion battery concept has prompted intense research activities toward new Na-based insertion compounds and their implementation in full Na-ion cells. Herein, we report the optimization of full Na-ion cells consisting of either a layered oxide Nax(Fe1/2Mn1/2)O2 or a polyanionic Na3V2(PO4)2F3 cathode associated with a hard carbon anode. From charge/discharge curves collected via 2 or 3-electrode measurements, the charge/discharge profiles of full cells are simulated to evaluate the maximum energy density these two systems can deliver. Similar energies of 235 W h kg−1 are found for both systems provided that a fully sodiated Na1(Fe1/2Mn1/2)O2 layered phase is used. Experimental cells confirm these values, and cells based on polyanionic compounds surpass the layered cathodes in terms of energy retention, average voltage and rate capabilities. By using Na sources to compensate for carbon's irreversible capacity, energy densities as high as 265 W h kg−1 can be reached with the Na3+xV2(PO4)2F3 / hard C system. Overall, such studies reveal that the gravimetric energy density advantage of layered over polyanionic compounds for Li-ion batteries vanishes by moving to Na-ion. We hope this information will be of great interest for battery manufacturers willing to enroll in the future commercialization of Na-ion batteries.