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170 B.L. Ellis, L.F. Nazar / Current Opinion in Solid State and Materials Science 16 (2012) 168–177 2.2. Sodium–air (Na–O2) cells Air electrodes operate by the reaction of oxygen with alkali me- tal ions to produce alkali oxides. The positive electrode employs porous carbon and/or porous metal as the current delivery system for O2 reduction and as host for the product. The discharge reaction fills the voids with the oxide product and terminates when these voids are filled. Oxygen oxidation and reduction benefit from the use of a catalyst, usually dispersed on the porous matrix. Thus simultaneous contact is necessary for the reaction of alkali ions and oxygen molecules (present in the electrolyte) and electrons (delivered by the conductive matrix) for oxygen reduction at the catalyst sites on discharge. A representation of the Na–O2 cell is shown in Fig. 2a. The potentials for reactions of sodium with oxy- gen are given below: Naþ þO2 þe !NaO2 E1⁄42:263V ð1Þ Unlike Li–O2 cells which normally operate at ambient tempera- tures, Peled replaced the metallic lithium anode with sodium and operated the cell above the melting point of sodium (98 °C) which prevented the formation of metallic dendrite formation on the negative electrode during charge. Additionally, at temperatures above 100 °C, the adsorption of water vapor by the cell compo- nents was negligible so minimal interference of atmospheric water was expected. The voltage profile of the high-temperature Na–O2 cell using a polymer electrolyte is shown in Fig. 2b. The Na–O2 cell exhibited a charging potential at 2.9 V and 1.8 V on discharge. With the discharge potential expected to lie around 2.3–2.4 V, the low discharge voltage suggests kinetic overpotential is a con- cern for this Na–O2 cell, possibly due to the polymer electrolyte. In Li–O2 cells, the overpotential is usually 0.3–0.4 V [23,24]. In addition, lithium metal reacts with the electrolyte to form a surface solid electrolyte interphase (SEI) layer, which is re-formed on continuous cycling [25]. Lithium dendrite formation on charge is thus a concern for Li–O2 cells. The successful cyclability demon- strated in this initial Na–O2 report will encourage the development of a new generation of high specific energy density rechargeable cells. 2.3. ZEBRA cells ZEBRA cells were developed in the 1980s and contain a liquid sodium negative electrode and a metal chloride positive electrode, usually NiCl2 [26]. More recent work has shown that addition of iron to the cell increases the power response [27–29]. The dis- charge reactions and potentials versus sodium at 300 °C are shown below: NiCl2 þ2Na!Niþ2NaCl E1⁄42:58V ð4Þ FeCl2 þ2Na!Feþ2NaCl E1⁄42:35V ð5Þ A schematic of the sodium ion transport through the cell during discharge is shown in Fig. 3. Sodium ions which result from the oxidation of sodium at the negative electrode are transported through the solid sodium b00-alumina electrolyte to the NiCl2 by a secondary electrolyte (a eutectic mixture of NaCl and AlCl3). For most of the discharge, the system functions as a Na-NiCl2 cell. If a high current pulse is applied to the cell and the working voltage falls below 2.35 V, the iron reaction augments the main nickel reaction: both discharge in parallel. This occurs at the front of the electrode and the cell therefore has its minimum resistance. When the working voltage recovers above 2.35 V, the iron pro- duced is then reoxidized to FeCl2 by the remaining nickel chloride and the FeCl2 is then available for the next high current discharge. An advantage of the ZEBRA cells is that they may be assembled in the discharged state with NaCl, Al, nickel and iron powder. Fur- 2Naþ þO2 þ2e !Na2O2 4Naþ þO2 þ4e !Na2O E1⁄42:330V ð2Þ E1⁄41:946V ð3Þ Although research on Li–O2 batteries is fervent [14–19], Na–O2 cells are in their infancy, with one on ambient temperature cells from Sun et al. [20] which followed the initial report on molten Na–O2 cells from Peled et al. [21]. Sun’s report proved to be inter- esting, however, the electrolyte used in the study was a mixture of organic carbonates and in Li–O2 cells, carbonate-based electrolytes are known to decompose [22]. The oxide product formed on dis- charge in the Na–O2 cell at ambient temperature was identified as Na2O2, possibly by decomposition of NaO2. Under certain condi- tions, it may be possible to stabilize NaO2. Fig. 2. (a) Schematic representation of Na-O2 battery on discharge and (b) voltage profile for Na-O2 battery on first discharge cycle. Reproduced with permission from [21]. Fig. 3. Schematic representation of Na-NiCl2 (ZEBRA) cell on discharge.PDF Image | Sodium and sodium-ion energy storage batteries
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