Low-Potential Sodium Insertion in a NASICON-Type Structure through the Ti(III)/Ti(II) Redox Couple

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.