In the search for new positive-electrode materials for lithium-ion batteries, recent research has focused on nanostructured lithium transition-metal phosphates that exhibit desirable properties such as high energy storage capacity combined with electrochemical stability. Only one member of this class--the olivine LiFePO(4) (ref. 3)--has risen to prominence so far, owing to its other characteristics, which include low cost, low environmental impact and safety. These are critical for large-capacity systems such as plug-in hybrid electric vehicles. Nonetheless, olivine has some inherent shortcomings, including one-dimensional lithium-ion transport and a two-phase redox reaction that together limit the mobility of the phase boundary. Thus, nanocrystallites are key to enable fast rate behaviour. It has also been suggested that the long-term economic viability of large-scale Li-ion energy storage systems could be ultimately limited by global lithium reserves, although this remains speculative at present. (Current proven world reserves should be sufficient for the hybrid electric vehicle market, although plug-in hybrid electric vehicle and electric vehicle expansion would put considerable strain on resources and hence cost effectiveness.) Here, we report on a sodium/lithium iron phosphate, A(2)FePO(4)F (A=Na, Li), that could serve as a cathode in either Li-ion or Na-ion cells. Furthermore, it possesses facile two-dimensional pathways for Li+ transport, and the structural changes on reduction-oxidation are minimal. This results in a volume change of only 3.7% that--unlike the olivine--contributes to the absence of distinct two-phase behaviour during redox, and a reversible capacity that is 85% of theoretical.

We report new solid state and hydrothermal synthetic routes to (Li,Na)2FePO4F that incorporate carbon-containing additives and result in good electrochemical properties of this Li (or Na) ion electrode material. Single crystal X-ray diffraction analysis of Na2FePO4F prepared by flux growth confirms the unusual structural features of this compound that include pairs of face-sharing metal octahedra and [6 + 1] coordination of the sodium ions. Facile Na−Li ion-exchange occurs upon reflux with lithium salts, upon electrochemical cycling in a cell (vs. Li), and also in a cell simply equilibrated at OCV. The material does not exhibit typical two-phase behavior on electrochemical cycling. A combination of a redox process which occurs with little structural strain, and ion scrambling give rise to a solid solution-like sloping voltage profile on charge−discharge, although localization of the Fe2+/3+ in the mixed valence single phase intermediate, Na1.5FePO4F drives a very small structural distortion. Temperature-dependent Mössbauer spectroscopy measurements confirm this localization, at least on short time scales (10−8 s), which persists to 370 °C. Finally, polycrystalline powders of other members of this family of compounds (Na2CoPO4F, Na2NiPO4F) were synthesized for the first time. Na2CoPO4F has a Co2+/Co3+ potential near 4.8 V. Mixed-metal phosphates of the form Na2(Fe1−x M x )PO4F, where M = Co, Mg, were also synthesized and found to be promising positive electrode materials for Li-ion or Na-ion energy storage devices.

Lithium transition metal phosphate olivines such as LiFePO 4 have been recognized as very promising electrodes for lithium-ion batteries because of their energy storage capacity combined with electrochemical and thermal stability. A key issue in these materials is to determine the synthetic conditions for optimum control of particle size and morphology, and ideally to find those that result in nanocrystalline products. Here, we report a full study that examines the synthesis of the material via hydrothermal methods to give single phase nanocrystalline materials for LiFePO 4 and LiMnPO 4 , and their solid solutions with Mg 2+ . A reaction mechanism is proposed. Variation of the synthesis parameters showed that increasing reactant concentration strongly favours the formation of nanocrystalline products, but as less defect-free materials are formed at temperatures above 180 uC, and ideally above 200 uC, control of nucleation and growth can (and should) also be effected using polymeric or surfactant additives. The nature of the precursor and carbon-containing additives in the autoclave also have profound effects on morphology and the electrochemical properties.

In the recent hunt for novel Na-ion cathode hosts, a variety of sodium analogues of classic Li-ion structures have been thoroughly explored. However, Na-ion analogues generally possess modified structures and dissimilar Na-ion energetics compared to their Li-ion analogues due to the large size of Na + (102 pm) vs Li + (76 pm), often resulting in sluggish Na + kinetics. Materials development targeted toward new and different specific host structures possessing optimum properties for Na-ion migration is crucial. Here, we report the first sodium metal fluoropyrophosphate Na-ion host with a three-dimensional frameworkNa 4 NiP 2 O 7 F 2 which is predicted to have a high voltage (∼5 V) based on its Ni 2+/3+/4+ redox couple and composition. Structure solution from single crystal diffraction data combined with atomistic simulation computation suggests the presence of low activation energy Na-ion migration pathways (<0.6 eV) in all three dimensions. The particularly low barrier of 0.36 eV calculated for migration along the [010] direction is in full accord with temperature dependent ionic conductivity measurements that yield an experimental value of 0.32 eV. Spacious Na-ion pathways endow the material with good ionic conductivity as determined by ac impedance spectroscopy, and facile exchange of three Na + ions for Li + is observed at slightly elevated temperatures. Furthermore, the polycrystalline material exhibits excellent thermal stability under ambient atmosphere up to 600 °C, crucial for the safe operation of a Na-ion battery.

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