Sodium-ion battery

Battery-grade salts of sodium are cheap and abundant, much more so than those of lithium. This makes them a cost-effective alternative especially for applications where weight and energy density are of minor importance. These cells can be completely drained (to zero charge) without damaging the active materials. They can be stored and shipped safely. Moreover, sodium-ion batteries have excellent electrochemical features in terms of charge-discharge, reversibility, coulombic efficiency and high specific discharge capacity.

In November of 2017 French Network on Electrochemical Energy Storage (RS2E) announced the intention to produce a 18650 format battery by 2020. The battery will be 3.5V, 90Wh/Kg, perform more than 2,000 charge and discharge cycles without significant loss of performance, and life expectancy of more than 10 years in continuous use.

SIBs store energy in chemical bonds of the anode. Charging the battery forces Na⁺ ions to de-intercalate from the cathode and migrate towards the anode. Charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode. During discharge the process reverses. Once a circuit is completed electrons pass back from the anode to the cathode and the Na⁺ ions travel back to the cathode.

Anode

Aquion originally used a mix of activated carbon and titanium phosphate NaTi₂(PO₄)₃ that relied mostly on pseudocapacitance to store charge, resulting in a low energy density and a tilted voltage-charge slope. In many ways, titanium phosphate is similar to iron phosphate used in some other batteries, but with a low (anodic) electrode potential.The initial electrolyte was an aqueous sodium sulphate solution. Later a more soluble <5M NaClO₄ was used

Cellulose

In one study, tin-coated wood anodes replaced stiff anode bases. The wood fibers proved withstood more than 400 charging cycles. After hundreds of cycles, the wood ended up wrinkled but intact. Computer models indicated that the wrinkles effectively reduce stress during charging and recharging. Na ions move via the fibrous cell walls and diffuse at the tin film surface.
Another study used MoS₂/graphene composite paper as an electrode, yielding 230 Ah/kg with Coulombic efficiency reaching approximately 99%

Cathode

Tests of Na₂FePO₄F and Li₂FePO₄F cathode materials indicated that a sodium iron phosphate cathode can replace a lithium iron phosphate cathode in a Li cell. The lithium-ion and sodium-ion combination would lower manufacturing costs.
P2-Na_2/3[Fe_1/2Mn_1/2]O₂ delivered 190  Ah/kg of reversible capacity in sodium cells using electrochemically active Fe³⁺/Fe⁴⁺ redox at room temperature. Triclinic Na₂FeP₂O₇ was examined as rechargeable sodium ion batteries by a glass-ceramics method. The precursor glass, also made of Na₂FeP₂O₇, was prepared by melt-quenching. Na₂FeP₂O₇ and exhibited 2.9 V, 88 Ah/kg.
Separately, chromium cathodes employed the reaction:

NaF + (1−x)VPO₄ + xCrPO₄ → NaV_1−xCr_xPO₄F

The effects of Cr doping on cathode performance materials was analyzed in terms of crystal structure, charge/discharge curves and cycle performance and indicated that the Cr-doped materials expressed better cycle stability. The initial reversible capacity was 83.3 Ah/kg and the first charge/discharge efficiency was about 90.3%. The reversible capacity retention of the material was 91.4% after the 20th cycle.