PDF Publication Title:
Text from PDF Page: 009
B.L. Ellis, L.F. Nazar / Current Opinion in Solid State and Materials Science 16 (2012) 168–177 175 Fig. 9. Representation of the manner of which sodium intercalates into hard carbon, the ‘‘house of cards’’ model. Reproduced with permission from [92]. 3.2. Negative electrode materials for Na-ion batteries 3.2.1. Carbon materials Graphite, the common negative electrode in Li-ion batteries, cannot be used as an insertion electrode in Na-ion batteries as Na atoms do not intercalate between the carbon sheets [88–90]. Other carbonaceous materials have been investigated, however, and show promise. Sodium intercalation was observed in petro- leum cokes,[91] along with hydrogen-containing carbon, soft car- bons (small regions of ordered graphene) and hard (disordered) carbons [90,92]. The potential of Na insertion into hard carbon is close to that of the metal itself, which indicates there was very lit- tle carbon to sodium charge transfer. This would occur if the so- dium inserted into the carbon via pores at the surface. Sodium was found to intercalate between the disordered layers of the hard carbon, as shown in the schematic diagram in Fig. 9. Hard carbon prepared from glucose demonstrated a capacity of 300 mAh/g at a slow intercalaction rate (C/80). More recently, a non-graphitic carbon based on a porous silica template was prepared by Wenzel et al. in 2011 [93]. The high porosity enhanced the electrochemical performance at higher rates: a capacity of 180 mAh/g was achieved on the first cycle at a rate of C/5, but faded to 80 mAh/g after 125 cycles. This carbon outperformed several other industrial carbons, though, likely as a result of the carbon microstructure and porosity. 3.2.2. Low potential metal phosphates Sodium insertion into the NASICON-type NaTi2(PO4)3 for both aqueous and non-aqueous cells was reported by Park et al. in 2011 [94]. Na intercalates into this compound by a two-phase mechanism at 2.1 V versus Na/Na+. The observed capacities for Na- Ti2(PO4)3 in non-aqueous and aqueous electrolytes on cycling were 120 and 123 mAh/g respectively, which corresponds to over 90% of the theoretical capacity of 133 mAh/g. The polarization observed in the aqueous electrolyte on cycling was substantially smaller than in non-aqueous electrolyte, especially at a large current density, as a result of the lower impedance and the viscosity of the aqueous electrolyte. The Na-ion insertion/extraction potential is located at the lower limit of the electrochemical stability window of the aqueous Na2SO4 electrolyte, which makes NaTi2(PO4)3 an attractive negative electrode for aqueous sodium-ion batteries. 3.2.3. Low potential sodium metal oxides An interesting report on sodium intercalation into amorphous TiO2 nanotubes was presented in 2011 [95]. No significant interca- lation of sodium ions was observed when narrow TiO2 nanotubes (<45 nm inner diameter, 10 nm wall thickness) were cycled versus sodium. However, as the size of the nanotubes were increased to >80 nm inner diameter, wall thickness >15 nm, Na intercalation was observed: cells cycled at a rate of C/3 exhibited a capacity of 100 mAh/g between 2.5 and 0.9 V versus Na/Na+ on the first cycle, which increased gradually on cycling to reach 140 mAh/g on the 15th cycle. The improved intercalation properties for the larger tubes was attributed to the fact that they should have a greater number of Na+ charge carriers (because of the greater volume of electrolyte contained within the tube) and hence could more easily establish a critical ion concentration. A recent report on layered Na2Ti3O7 suggests it has the lowest potential of any Na intercalation compound thus far [96]. Reduc- tion takes place with the observation of a reversible plateau around 0.3 V versus Na/Na+ (Fig. 8), which is quite remarkable. At this po- tential, two additional sodium ions were intercalated into the structure at a C/25 rate which corresponds to a capacity of 200 mAh/g and reduction of 2/3 of the Ti4+ to Ti3+. Vanadium oxides are also known to exhibit low potential alkali intercalation chemistry. Sodium deinsertion from the O3 phase of NaVO2 has been examined by Delmas et al. in a detailed study that elucidated many of the complex structural changes that take place [97]. On deintercalation, the phase adopts a monoclinic distortion to form an O03 structure where the octahedral site occupation of Na+ is preserved (it is interesting to note that in contrast, chemical oxidation of NaVO2 induces a switch from octahedral to prismatic coordination of the Na+ cation [98]). Electrochemical deintercala- tion is reversible up to at least 0.5 mol Na per mole, corresponding to a capacity of 126 mAh/g of active material. Single line-phases were identified for both Na1/2VO2 and Na2/3VO2 compositions. Sim- ilar studies were later reported by Hamani et al. who confirmed the existence of these phases [99]. The voltage profile of each com- pound goes through several steps on both oxidation and reduction between 1.2 V and 2.4 V, although above 2.4 V an electrochemi- cally inactive phase formed. While both compounds exhibit good reversibility on initial cycling, Na0.7VO2 displays very low polariza- tion, a possible indicator of good performance at high rates. None- theless, the large voltage variation during cycling and the extreme oxygen sensitivity of NaVO2 present challenges for practical applications. Another vanadium oxide with a tunnel structure, NaV6O13, was prepared in nanorod morphology via hydrothermal synthesis [100]. The electrochemical profile contained two distinct plateaus at 2.7 V and 2.3 V before the main plateau at 1.7 V versus Na/Na+, making it a slightly high voltage negative electrode material. It is also prone to substantial capacity loss at low cycling rates. 3.2.4. Alloys As recently noted by Ceder [73], little research has been done thus far on sodium alloy materials as negative electrodes for so- dium-ion batteries, although silicon alloys are well-researched for Li-ion batteries. The electrochemical sodiation of lead has been reported and up to 3.75 Na per Pb were found to react [39]. The same fraction of sodium might be expected to react with Sn, as Na15Sn4 is a known compound, although Si and Ge might be ex- pected to uptake less sodium: the only known alloys of sodium/sil- icon and sodium/germanium are NaSi and NaGe. Nevertheless, since full lithiation of silicon alloys is not typically sustained in any case in order to avoid large volume changes, this may not pres- ent a problem. The potentials for these reactions were recently pre- dicted [73], and show promise for future explorations. 4. Conclusions Owing to concerns over lithium cost and sustainability of re- sources, sodium and sodium-ion batteries have re-emerged as promising candidates for both portable and stationary energy storage. Molten Na cells based on Na–S and Na–NiCl2 developed in the last decade are commercially available and are especially of use for large-scale grid-applications; but new directions include ‘‘lower temperature’’ (i.e., 100 °C) Na–S and Na–O2 cells, as well as ambient temperature Na–O2 cells. These initial reports on Na–O2PDF 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 | RSS | AMP |