Rhombohedral NASICON-structured Li2NaV2(PO4)3 with single voltage plateau for superior lithium storage

Abstract

A single rhombohedral NASICON-structured cathode material Li₂NaV₂(PO₄)₃ has been synthesized through a facile sol–gel route for the first time. The results suggest that the Na⁺ sources, i.e. anionic surfactants including sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfonate (SDS) and sodium oleate (NaOL) can induce the Na⁺ ions to occupy all of the A1 sites in formula unit [V₂(PO₄)₃]³⁻. Almost all the monoclinic phase of Li₃V₂(PO₄)₃ can be transformed into the rhombohedral phase of Li₂NaV₂(PO4)₃ with Li⁺ partially substituted by Na⁺ ions offered by SDBS, SDS or NaOL. In the absence of monoclinic Li₃V₂(PO₄)₃, the rhombohedral Li₂NaV₂(PO₄)₃ exhibits only one voltage plateau around 3.76 V versus lithium metal during the charge–discharge process, which is consistent with the single couple V⁴⁺/V³⁺ redox peaks in the cyclic voltammetry (CV) curves. Furthermore, the Li₂NaV₂(PO₄)₃ obtained using SDBS as a Na⁺ source displays remarkably high-rate capability (80 mA h g⁻¹ at 5 C and 68 mA h g⁻¹ at 10 C) and excellent cyclability, 500 cycles at 2 C (about 93% of the initial capacity is retained) due to no fundamental change occurring in the host three-dimensional framework and the porous nanosheets structure with carbon coating.