Structural transformations with Li+-deintercalation in cathode materials Li3VSc(PO4)3 with anti-NASICON and NASICON structures

Abstract

Although vanadium-containing phosphates with the NASICON-related structure are promising cathode materials for lithium-ion batteries, the issues of structural degradation associated with the complete extraction of Li⁺ and/or complete oxidation of vanadium remain unclear. Here, we present novel monoclinic (m-LVScP) and rhombohedral (r-LVScP) polymorphs of Li₃VSc(PO₄)₃ as model objects to study structural transformations occurring upon oxidation and reduction of vanadium cations. The structure of m-LVScP obtained by direct high-temperature synthesis belongs to the anti-NASICON type. Ion Li⁺ → Na⁺ exchange from NASICON type Na₃VSc(PO₄)₃ leads to the formation of the rhombohedral polymorph of Li₃VSc(PO₄)₃ and is accompanied by a strong distortion of the NASICON type polyanion framework. The BVSE study of Li₃(V,Sc)₂(PO₄)₃ indicates faster diffusion of Li⁺ ions within the anti-NASICON framework; moreover, the activation energy of cation migration in the substituted phase m-Li₃VSc(PO₄)₃ is lower than that of the corresponding non-substituted vanadium and scandium phosphates. Carbon-coated m-LVScP and r-LVScP demonstrate a specific capacity up to ∼175 mA h g⁻¹ which corresponds to a complete three-electron V²⁺/V³⁺/V⁴⁺/V⁵⁺ process within the voltage range of 1–4.65 V vs. Li/Li⁺, following a predominantly solid-solution mechanism, as shown by operando and ex situ powder X-ray diffraction. Electrochemical and chemical deintercalation of 2Li⁺ per formula unit from m-LVScP and r-LVScP results in the formation of a short V⁵⁺O bond, revealed by ⁵¹V NMR and Fourier-transformed infrared spectroscopy. This is accompanied by strong distortion of the frameworks, which impedes the complete reversibility of the phase transformations and leads to capacity degradation during long time cycling.