Na-ion batteries represent an effective energy storage technology with slightly lower energy and power densities but potentially lower material costs than Li-ion batteries. Here, we report a new polyanionic intercalation cathode material of an unusual chemical class: sidorenkite (Na3MnPO4CO3). This carbonophosphate compound shows a high discharge capacity (∼125 mAh/g) and specific energy (374 Wh/kg). In situ X-ray diffraction measurement suggests that sidorenkite undergoes a solid solution type reversible topotactic structural evolution upon electrochemical cycling. Ex situ solid state NMR investigation reveals that more than one Na per formula unit can be deintercalated from the structure, indicating a rarely observed two-electron intercalation reaction in which both Mn2+/Mn3+ and Mn3+/Mn4+ redox couples are electrochemically active.

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Tetragonal alkali metal tungsten bronze and hexagonal alkali metal tungstate nanorods have been synthesized by sub-ambient reduction by alkalide solutions and subsequent oxidation. Tetragonal potassium tungsten bronze, K 0.4 WO 3 , nanorods result after post-reduction annealing at 250 C for 3 h followed by 600 C for 4 h in vacuum if the alkalide reduction is performed using K + (15-crown-5) 2 Na À and a mixed sodium/potassium tungsten bronze, Na 0.3 K 0.2 WO 3 , if K + (18-crown-6) 2 Na À is used. The rods are generally $40 nm wide and 400-500 nm long single crystals, their principal axis being in the [001] crystallographic direction. Annealing in air at 500 C changes the material to a bronze-like hexagonal alkali metal tungstate, without substantial effect on its nanorod morphology. The hexagonal alkali metal tungstate, K 0.3 W 0.95 O 3 , nanorods can be made directly by heating to 400 C rather than 600 C in the 2-step annealing process, consistent with the tungstate being the first phase formed, transforming to the bronze at higher temperature.

Tb 0.05 ) 2 O 2 CO 3 nanoparticles have been synthesized by subambient homogeneous reduction using alkalide solutions and subsequent oxidation. As synthesized, the material consists of free flowing agglomerates of ill-defined, amorphous, or subnanocrystalline nanoparticles. Thermal analysis shows that 2.83 CO 2 molecules are adsorbed per Y 2 O 3 formula unit in the washed product. The samples annealed at 200 °C or greater are crystalline, consisting of agglomerated nanocrystals. The nanocrystals grow from an average of 15.3-17.6 nm, the agglomerates decrease in size, and the surface area decreases from 88 to 78 m 2 /g as the annealing temperature is raised from 400 to 700 °C. Annealing at 800 °C results in a phase change from the carbonate to (Y 0.95 Tb 0.05 ) 2 O 2 . The crystallite size of the oxide phase increases from 21.4 to 22.6 nm, and the surface area increases from 82 to 86 m 2 /g as the annealing temperature is increased from 800 to 1000 °C. Transmission electron microscopy observations show that the increase in the surface area, even as the average crystallite size increases, is consistent with the breaking up of agglomerates and the creation of a highly textured material, probably due to the release of CO 2 during the phase transition. The washed unannealed product displays luminescence typical of the Tb 3+ ion. The photoluminescence intensity of the green 5 D 4 -7 F 4 transition increases with increasing annealing temperature, from 5% for the unannealed product to 16% for the nanophosphor annealed at 1000 °C.

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