We report the direct synthesis of powder Na3Ti2(PO4)3 together with its low-potential electrochemical performance and crystal structure elucidation for the reduced and oxidized phases. First-principles calculations at the density functional theory level have been performed to gain further insight into the electrochemistry of Ti(IV)/Ti(III) and Ti(III)/Ti(II) redox couples in these sodium superionic conductor (NASICON) compounds. Finally, we have validated the concept of full-titanium-based sodium ion cells through the assembly of symmetric cells involving Na3Ti2(PO4)3 as both positive and negative electrode materials operating at an average potential of 1.7 V.

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It has previously been reported that under high-pressure V 2 O 5 (R-V 2 O 5 ) transforms into a layered polymorph, β-V 2 O 5 , consisting of V 5+ O 6 octahedra instead of V 5+ O 5 -square pyramids. Both polymorphs have a good performance as positive electrode for lithium batteries. In this work, we investigate the pressure-induced R f β transformation combining first principles and experimental methods. Density functional theory (DFT) predicts that R-V 2 O 5 transforms to β-V 2 O 5 at 3.3 GPa with a 11% volume contraction; experiments corroborate that at a pressure of 4 GPa, V 2 O 5 (d ) 3.36 g/cm 3 ) transformed into a well-crystallized β-V 2 O 5 , with a much denser structure (d ) 3.76 g/cm 3 ). β-V 2 O 5 can be also prepared at 3 GPa, although with a substantial degree of amorphization. The calculated bulk modulus of R-V 2 O 5 (18 GPa) indicates that this is a very compressible structure; this being linked to the contraction along its b-axis (interlayer space) and to a significant decrease of a long V-O distance (V-O ≈ 2.9 Å). As a result, the vanadium coordination increases from five (square pyrmamid) in R-V 2 O 5 to six (distorted octahedron), leading to the stabilization of the high-pressure (β) polymorph. This change of the coordination environment of vanadium ions also affects the electrical conductivity. The calculated density of states shows a narrowing of 0.5 V in the band gap for the β polymorph, in comparison to the ambient-pressure material; the measured resistivities at room temperature (10 000 Ω cm in R-polymorph and 400 Ω cm in β-polymorph) reveal that β-V 2 O 5 is indeed a better electronic conductor than R-V 2 O 5 . In view of these results, similar transformations at moderate pressures are expected to occur in other V 5+ frameworks, suggesting an interesting way to synthesize novel V 5+ compounds with potential for electrochemical devices.

First principles calculations have been used to investigate the effect of N for O substitution on the electrochemical properties of Li 2 FeSiO 4 . Within the Li 2 FeSiO 4 structure, hypothetical models of the N-substituted Li 2 FeSiO 3 N and Li 2 FeSiO 3.5 N 0.5 have been analyzed. The computational results indicate that the lithium deinsertion voltage associated to the Fe 3+/ Fe 4+ redox couple can be decreased by N substitution (4.86 V in Li 2 FeSiO 4 , 4.7 V in Li 2 FeSiO 3.5 N 0.5 and 4.1 V in Li 2 FeSiO 3 N). The high theoretical specific capacity of Li 2 FeSiO 4 (330 mA h g À1 ) could be retained in N-substituted silicates thanks to the oxidation of N 3À anions. The redox activity of N ions is observed in a voltage range of ca. 3.5-4.2 V. In the light of the potential benefits of N substitution for O experimental work is encouraged, in particular to investigate the reversibility and overpotential of the N redox reaction.