Is Sodium-Ion the next generation sustainable Battery?

Materials (Anodes, Cathodes & Electrolyte)

Due to the physical and electrochemical properties of sodium, SIBs require other materials than those used for LIBs.

Anodes

SIBs use hard carbon, a disordered carbon material consisting of a non-graphitizable, non-crystalline and amorphous carbon. Hard carbon’s ability to absorb sodium was discovered in 2000. This anode was shown to deliver 300 mAh/g with a sloping potential profile above ⁓0.15 V vs Na/Na⁺. It accounts for roughly half of the capacity and a flat potential profile (a potential plateau) below ⁓0.15 V vs Na/Na⁺. Graphite anodes for LIBs offer typical capacities of 300–360 mAh/g. The first sodium-ion cell using hard carbon was demonstrated in 2003 and showed a 3.7 V average voltage during discharge. Hard carbon is preferred due to its excellent combination of capacity, (lower) working potentials and cycling stability.

Cathodes

Sodium-ion cathodes store sodium via intercalation. Owing to their high tap density, high operating potentials and high capacities, cathodes based on sodium transition metal oxides have received the greatest attention. To keep costs low, research attempts to minimize costly elements such as Co, Cr, Ni or V. A P2-type Na_2/3Fe_1/2Mn_1/2O₂ oxide from earth-abundant Fe and Mn resources can reversibly store 190 mAh/g at average discharge voltage of 2.75 V vs Na/Na⁺ utilising the Fe^3+/4+ redox couple – on par or better than commercial lithium-ion cathodes such as LiFePO₄ or LiMn₂O₄. However, its sodium deficient nature lowered energy density. Significant efforts were expended in developing Na-richer oxides. Copper-substituted Na_0.67Ni_0.3−xCu_xMn_0.7O₂ cathode materials showed a high reversible capacity with better capacity retention. In contrast to the copper-free Na_0.67Ni_0.3−xCu_xMn_0.7O₂ electrode, the as-prepared Cu-substituted cathodes deliver better sodium storage.

Research has also considered cathodes based on polyanions. Such cathodes offer lower tap density, lowering energy density on account of the bulky anion. This may be offset by the stronger covalent bonding of the polyanion that positively impacts cycle life and safety. Among polyanion-based cathodes, sodium vanadium phosphate and fluorophosphate have demonstrated excellent cycling stability and in the latter, an acceptably high capacity (⁓120 mAh/g) at high average discharge voltages (⁓3.6 V vs Na/Na⁺).

Electrolytes

Sodium-ion batteries can use liquid and non-liquid electrolytes. The limited electrochemical stability window of water results in lower voltages and limited energy densities. Non-liquid carbonate ester polar aprotic solvents extend the voltage range. These include ethylene carbonate, dimethyl carbonate, diethyl carbonate, and propylene carbonate. The most widely used non-liquid electrolyte uses sodium hexafluorophosphate as the salt dissolved in a mixture of these solvents. Additionally, electrolyte additives can improve performance metrics.