PDF Publication Title:
Text from PDF Page: 006
172 B.L. Ellis, L.F. Nazar / Current Opinion in Solid State and Materials Science 16 (2012) 168–177 unfortunately Ti, Fe, or Mn do not crystallize in this phase as single metal compositions. NaxMO2 materials, in contrast, form the ‘‘ideal’’ ordered O3 – type layered structure more readily due to the larger ionic size difference between alkaline and transition metals that drives the segregation of the A and M into alternating layers. The deintercalation and intercalation of sodium in various layered Nax- MO2 has been reported for a very wide variety of transition metals as briefly outlined in a recent report [52]. Of these, the manganese and cobalt oxides are the most viable for positive electrodes. The known phases of NaxMnO2 (x = 0.2, 0.40, 0.44, 0.70, 1) in- clude two for x = 1 [53]. The phase formed at lower temperatures, a-NaMnO2, displays an O3 layered structure with a monoclinic structural distortion due to the Jahn–Teller distortion of the Mn3+ ion, whereas high temperature orthorhombic b-NaMnO2 adopts a double stacked sheet structure. Computational studies indicate that the former is energetically more stable [54]. The electrochem- ical behavior of both a- and b-NaMnO2 were reported by Mendibo- ure et al. in 1985, indicating that only 0.22 and 0.15 Na could be reversibly extracted and re-intercalated, respectively [55]. Re- cently, the electrochemical properties of the monoclinic a-NaMnO2 have been revisited. In a significant improvement, the authors showed that about 0.8 Na can be reversibly de/intercalated with good capacity retention, corresponding to a 200 mAh/g capacity [49]. As the potential for Na de/insertion was the same, the reasons for the difference with the earlier work are not clear, although they may be related to the electrolyte. The previous report utilized Na- ClO4 in propylene carbonate, whereas the latter used NaPF6 in eth- ylene carbonate/dimethylcarbonate. The voltage profiles of monoclinic NaMnO2 reveal very pronounced structural transitions on desodiation [49], which have also been observed in NaCoO2; see below. These are much less common in Li intercalation oxides. They are likely due to Na-vacancy ordering or transitions that in- volve the gliding of oxygen planes, especially as Na likes to adopt both octahedral and trigonal prismatic environments and the latter can only be achieved in an O3 stacking by sliding some of the oxy- gen layers. The difference between the Li and Na oxides might be explained by the possibility that Na+-vacancy ordering interactions are stronger due to sodium’s larger cation radius. Alternatively, the oxygen layer gliding may be responsible, as it would allow optimi- zation of Na coordination at each stoichiometry – which would not be as strong a driving force in the lithiated oxides. Overall, the ef- fect is to create more cell hysteresis and reduce the energy effi- ciency somewhat. However, it is also interesting that 50% desodiation of NaMnO2 does not result in conversion to a spinel as it does in the case of Li0.5MnO2 where migration of lithium to a tetrahedral site serves to trigger the transformation to spinel LiMn2O4. This ultimately results in capacity fading. This difference in behavior owes to the much higher relative stability of the Na+ site in the intermediate layered Na0.5MnO2, that inhibits Na+ migration. Good cyclability of the material results. Regarding cobalt oxides, NaxCoO2 exists as either the O3, P2 or P3 types, depending on the amount of Na intercalated. Of these, the P2 phase (Na0.66CoO2) is the most easily synthesized. The voltage profile (Fig. 5) shows that sodium extraction and reintercalation into Na0.66CoO2 occurs via several steps. Cells run at higher tem- perature (40 °C) retain the most stable electrochemical features (# 3, 8 and 9 from Fig. 5) and also maintain a similar capacity which indicates greater sodium mobility (disorder) and phase sta- bility. Reduction of particle size of the synthesized compounds was found to improve electrochemical performance [56]. In contrast to the complicated mechanism of sodium extraction from the pure Co P2 phase, the P2 phase of Na0.66Co0.66Mn0.33O2 displayed solid- solution behavior on sodium extraction, except at the composition Na0.50Co0.66Mn0.33O2 where an ordered line-phase was found [57]. Another layered oxide, based on nickel(II)/manganese(IV) – Na0.85Li0.17Ni0.21Mn0.64O2 – also displays promising electrochemistry [58] as shown in Fig. 6. Assuming the Ni2+ ? Ni4+ redox reaction, the theoretical capacity is based on extraction of 0.42 Na (112 mAh/g). The voltage profile on discharge/charge was a smooth curve which Fig. 6. Electrochemical potential of Na0.85Li0.17Ni0.21Mn0.64O2 at various discharge rates. Reproduced with permission from [58]. Fig. 5. Galvanic cycling of Na0.7CoO2, showing phase transitions on electrochemical cycling. Closeup of the region between Na0.7CoO2 and Na0.85CoO2 shown in right inset. Structure of Na0.7CoO2 shown in left inset. Reproduced with permission from [51].PDF Image | Sodium and sodium-ion energy storage batteries
PDF Search Title:
Sodium and sodium-ion energy storage batteriesOriginal File Name Searched:
2012_Na-battery_review.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Salt water flow battery technology with low cost and great energy density that can be used for power storage and thermal storage. Let us de-risk your production using our license. Our aqueous flow battery is less cost than Tesla Megapack and available faster. Redox flow battery. No membrane needed like with Vanadium, or Bromine. Salgenx flow battery
CONTACT TEL: 608-238-6001 Email: greg@salgenx.com (Standard Web Page)