Sodium-ion batteries are a type of reusable battery that uses sodium-ions as its charge carriers. This type of battery is in a developmental phase, but may prove to be a cheaper way to store energy than commonly used lithium-ion batteries. (As of 2014, one company, Aquion Energy, has a commercially available sodium-ion battery with cost/kWh capacity similar to a nickel-iron battery.) Unlike sodium-sulfur batteries, sodium ion batteries can be made portable and can function at room temperature (approx. 25˚C).
Like all batteries, the sodium ion battery stores energy in chemical bonds in its anode. When the battery is charging Na+ ions de-intercalate and migrate towards the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. 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. Sodium is much more abundant than the lithium that makes up 2013's highest performing rechargeable batteries, creating the potential for lower cost solutions.
Sodium ion cells have been reported with a voltage of 3.6 volts, able to maintain 115 Ah/kg after 50 cycles, giving the battery a storage capacity of approximately 400 Wh/kg Yet, sodium-ion batteries are still unable to maintain a strong charge after repeated charge and discharge. After 50 cycles most sodium-ion batteries tend to store about 50% of original capacity.
Using NaxC6 as the anode, the average voltage on the low potential plateau was higher on Na cells compared to Li cells. Unlike traditional Li cells which make use of a intercalated graphitic anode, with a fully lithiated stoichiometry of LiC6, Na cells do not have that capability as they cannot reversibly graphite. This is in large part due to the larger ionic size of the Na+ ion compared to the Li+ ion, which renders the expansion of the graphite galleries unfavorable. For this reason, carbon based anodes rely on amorphous carbons consisted of spatially disoriented graphene sheets, defects and interstitial pores. These amorphous carbons, can be categorized into two categories: hard and soft carbons. Hard carbons cannot be transformed into graphite through annealing at high temperatures, while soft carbons can. The hard carbon materials can be derived a variety of sources such as sugars, cellulose, biomass and polymers.
In addition to carbon anodes, alloying type anodes such as Antimony (Sb), Tin (Sn), Phosphorous (P), Germanium (Ge) and Lead (Pb) can also be used. As opposed to carbon anodes, which merely provide empty spaces for the storage of Na+ ions, alloying anodes go on to form complexes with the Na+ ions such as Na3Sb, Na3Sn and Na3P. This capability gives alloying type anodes a much greater theoretical capacity that traditional carbon based ones. Whereas amorphous based carbon atoms have shown capacity between 300-400 mAh g-1, a Na3P anode has a theoretical capacity of 2596 mAh g-1. Though this capacity represents a large increase, alloying type anodes do have some other significant problem. The alloying process causes an extremely large volume change, sometimes of close to 400%, in the anode. This large volume change results in the fracturing and displacement of the alloying material which causes it to passivate and effectively be turned into 'dead weight', as it cannot alloy itself with sodium ions once it has been passivated. If left unchecked these large volume changes are very detrimental to the cycle life of the battery. For this reason, much of the research conducted in the area of alloying anodes focuses on mitigating the volumes changes that happen upon sodiation, as well as reducing their negative effects.
Tin-coated wood anodes can replace stiff anode bases that are too brittle to withstand the swelling and shrinking that happens as ions move to and from the anode. Wood fibers are supple enough withstand more than 400 charging cycles. After hundreds of times, the wood ended up wrinkled but intact. Computer models showed that that the wrinkles effectively relax the stress in the battery during charging and recharging, so that the battery can survive many cycles. Na ions move via the fiber cell walls and by diffusion at the tin (Sn) film surface.
Tests of Na2FePO4F and Li2FePO4F cathode materials indicated that the sodium iron phosphate cathode easily replaces a lithium iron phosphate in a Li cell. The combined lithium-ion and sodium-ion make up would lower the overall price of the battery.
The electrode material P2-Na2/3[Fe1/2Mn1/2]O2 delivers 190 Ah/kg of reversible capacity in sodium cells using the electrochemically active Fe3+/Fe4+ redox at room temperature. Triclinic Na2FeP2O7 has been examined as rechargeable sodium ion batteries by glass-ceramics method. The precursor glass, also made of Na2FeP2O7, was prepared by melt-quenching method. Na2FeP2O7 exhibits 2.9 V, 88 Ah/kg.
Separately, chromium cathodes employed the reaction:
- NaF + (1−x)VPO4 + xCrPO4 → NaV1−xCrxPO4F
These cathodes retained more capacity through cycles of charge and discharge. 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 is about 90.3%. The reversible capacity retention of the material was 91.4% after the 20th cycles.
|Cathode materials||The first charge capacity (Ah/kg)||The first discharge capacity (Ah/kg)||Capacity loss in the first cycle (Ah/kg)||Reversible efficiency in the first cycle (%)||The discharge capacity at the 20th (Ah/kg)||The capacity retention ratio at the 20th (%)|
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