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B.L. Ellis, L.F. Nazar / Current Opinion in Solid State and Materials Science 16 (2012) 168–177 173 indicated intercalation within a single phase. Impressive rate capabil- ity was demonstrated with this material: the observed discharge capacity of 65 mAh/g at high rate of 25 C (discharge in about 3 min) corresponds to a power density of about 670 W/kg. The layered metal oxide structures are certainly amenable to sodium intercalation and extraction. A recent comprehensive arti- cle nicely contrasts the difference between Na and Li ion chemis- tries in a variety of compounds – including layered oxides, olivines and NASICONs – with respect to voltage, phase stability and activation energy for ion mobility [59]. The authors demon- strate that the generally lower calculated voltages for Na com- pounds are due to the smaller energy gain obtained from inserting Na into a host structure, versus that of Li. The differences, typically between 0.18 and 0.57 V, may be especially advantageous for the design of sodium battery negative electrode materials. Their calculations also show that Na+ migration barriers may be lower than the corresponding Li+ migration barriers in the layered struc- tures. Although diffusional barriers are very structure dependent, this holds promise for the development of materials with improved kinetics. 3.1.3. NASICON-type materials Compounds based on the 3-D structure of NASICON (NAtrium Super Ion CONductor) have been extensively studied for their structural stability and fast ion conduction, initially as solid elec- trolytes and more recently as insertion materials [60–62]. NaNb- Fe(PO4)3 and Na2TiFe(PO4)3 were initially examined in 1992 by Tillement et al. [63] and the latter was also reported later by Masquelier and co-workers, along with Na2TiCr(PO4)3 [64]. Iron- containing phosphate compounds are sought after as electrodes owing to their stability and environmentally benign character. The Fe3+ ? Fe2+ redox potential in these compounds is quite low, around 2.4 V versus Na/Na+ owing to the corner-shared framework which reduces the influence of the inductive effect [65]. Full reduc- tion of Fe (to Fe2+) and Ti (to Ti3+) was observed on electrochemical cycling of Na2TiFe(PO4)3 with a desirably small polarization. The intermediate voltage, however, makes them less attractive as posi- tive electrodes, but not quite low enough to function as negative electrode insertion materials. Variations in composition to further lower the insertion potential are discussed later in the context of negative electrodes. Sodium intercalation in Na3V2(PO4)3 was studied in 2002 by Ya- maki et al. [66]. When placed initially in discharge, the capacity of this compound was found to be 50 mAh/g at 1.6 V versus Na. Fur- thermore, after the material was charged to the original stoichiom- etry, the material could be further charged at 3.4 V versus Na and the discharge capacity at this step was 90 mAh/g. As with other V3+ phosphates such as LiVPO4F [67], Na3V2(PO4)3 is an ideal candidate for fabrication of symmetric cells, owing to the range of oxidation states of vanadium. With that intent, it was revisited by the same group in 2010 [68], although cycling stability over several cycles was poor. 3.1.4. Olivines The ability of the layered oxide structure and the 3D network of NASICON to flexibly accommodate ions in either their layered or large interstitial spaces allows the existence of stable versions of these compounds for both Na and Li. This is not the case with LiFe- PO4 which crystallizes in the olivine structure [69]: the corre- sponding sodium compound, NaFePO4, crystallizes in the maricite structure and is electrochemically inactive [70,71]. To evaluate the electrochemistry of the FePO4 olivines versus sodium, the lithium analogue LiFePO4 was prepared and the lithium was extracted to produce orthorhombic FePO4 which was then cycled at a very slow rate (1 Li in 50 h) in a sodium cell. First reported in 2010 [72], the structure of the electrochemically intercalated NaFePO4 confirmed that this compound retained the olivine frame- work with a unit cell volume of 320.14 Å3. It exhibits a volume con- traction on Na extraction of almost 15%, more than twice that of Li–olivine (6.7%). Such large volume changes on de/insertion are common for Na-polyanion materials owing to the larger size of the Na+ cation versus Li+, and can result in slow kinetics due to the higher energy needed to move the phase boundary in two- phase reactions. The electrochemical curve is shown in Fig. 7a. On initial discharge, a voltage plateau at 2.8 V versus Na/Na+ was observed and maintained until full insertion. On charge, a plateau indicative of a two-phase transition was observed at 3.0 V. Forma- tion of a new single phase ordered line-phase at the composition Na0.7FePO4 is signalled by a shift of the plateau up to 3.2 V. The structure has not yet been identified, and subsequent cycling showed quick capacity fade [71]. The potentials observed for elec- trochemical cycling are in good agreement with those predicted by calculations [73]. Amorphous FePO4 prepared at 100 °C was also found to intercalate sodium with a reversible capacity of 100 mAh/g [74]. Unlike the pure iron olivine, sodium iron/manganese olivine phosphate, Na(Fe0.5Mn0.5)PO4, could be synthesized directly via a molten salt reaction [75]. The electrochemical profile is shown in Fig. 7b. Interestingly, the electrochemical profile of Na(Fe0.5Mn0.5)- PO4 exhibited a sloping profile over the entire voltage range when cycled versus sodium, with a somewhat low average potential of 2.7 V suggestive of a kinetic limitation. The origin of this solid solu- tion behavior, confirmed by XRD measurements, is still under investigation, although it could be the result of transition metal Fig. 7. (a) Electrochemical profile of olivine-FePO4 intercalated with sodium. Reproduced with permission from [72] and (b) electrochemical profile of Na(Fe0.5Mn0.5)FePO4 cycled in a sodium cell. Reproduced with permission from [75].PDF Image | Sodium and sodium-ion energy storage batteries
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