The major advantage of sodium-ion batteries (SIBs) over lithium-ion is the lower cost of materials (Na vs Li and Al current collector replacing Cu); while the biggest drawback is the lower energy density (due to larger ionic radius and lower redox potential of Na⁺). Considering the most common grid-scale energy storage Li-ion system: LiFePO₄ – Graphite (120 Wh/kg), simple replacement of Li and Cu with Na and Al, respectively, would results in only about 15% cost reduction. This means that Na-ion batteries need to have energy density higher than 102 Wh/kg in order to be competitive with LFP systems, which is hardly achievable in full cells comprising oxide-based cathodes and carbon anodes. Therefore, to commercialise SIBs for stationary energy storage, novel concept batteries with higher energy densities and considerably lower production costs need to be designed and developed.

The concept of fabricating batteries in the discharged state with only an appropriate current collector (anode free) is an elegant method to achieve these aims. On the initial charge, reactive metal (Li or Na) is electroplated at the current collector, and so, during electrochemical cycling, the cell operates as a battery which contains only the amount of metal that is supplied by the positive electrode. Since Na metal has very high specific capacity (1166 mAh/g), the lowest possible working potential for SIBs and there is no anode material, the achievable energy density is extremely high. Nevertheless, in order to realise such systems we have to overcome several obstacles such as dendritic growth, nucleation potential, homogeneous plating as well as large volume changes.

In this work we investigated anode free SIBs with particular focus on the battery’s components. We compared performance of several different current collectors and electrolytes. We also considered various cathode materials including conventional layered oxides or Prussian blue analogues. These materials differ in sodium storage mechanisms, working potentials and specific capacities as well as manufacturing costs.

For instance, our tests of Na_0.90Fe[Fe(CN)₆] cathode and carbon coated Al current collector showed outstanding capacity of ~120 mAh/g over 250 cycles (Fig. 1). The average discharge potential is 3.2 V, which results in remarkable energy density of 384 Wh/kg. This is only slightly lower than the energy density of LiMn₂O₄ (410–492 Wh/kg) or LiFePO₄ (518–587 Wh/kg). However, Li-ion values are based only on active mass of oxide cathode and in full cell configurations the graphite anode would have to be taken into account. This is not the case for anode free SIBs and therefore the energy density is already much higher than state-of-the-art LIBs. The system showed also remarkable rate capabilities up to 20C discharge rate, resulting in power density reaching 11 kW/kg. Moreover, the utilisation of inexpensive Prussian Blue analogues instead of transition metal oxides would contribute to lowering the cost even further. In conclusion, electroplating anode free sodium ion batteries can overtake LIBs in both performance and economic factors, emerging as one of the most promising and viable technologies for medium to large scale energy storage applications.