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-Na2/3[Fe1/2Mn1/2]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 → NaV1−xCrxPO4F
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.