Increasing Energy Density with Capacity Preservation by Aluminum Substitution in Sodium Vanadium Phosphate.

Highly Al-substituted C-coated Na3V2-xAlx(PO4)3 compounds with a sodium superionic conductor structure are synthesized by a single and easily scalable sol-gel route. The effect of the experimental conditions is examined. Their structural, chemical, and morphological features are described. The first-principles method is used to determine the theoretical voltage vs Na content profile during Na extraction. The electrochemical Na extraction is characterized by the presence of two plateaus. The first one at ca. 3.4 V is assigned to the V4+/V3+ redox pair and shows good cyclability. The second plateau at ca. 3.9-4.0 V can be assigned to the V5+/V4+ pair, as evidenced by X-ray photoelectron spectroscopy. This second plateau is less reversible during further discharge.