Li₃V₂(PO₄)₃-Based Cathode Materials for Li-Ion Batteries: Time Evolution of the Vanadium Valence State

Li₃V₂(PO₄)₃-based composites as a lithium-ion battery cathode material were synthesized by the hydrothermal method with subsequent annealing in an Ar atmosphere. The as-prepared samples were characterized by X-ray diffraction analysis and electron spin resonance (ESR) methods. Based on ESR, data the quantitative estimation of V⁴⁺ content was performed. The tetravalent vanadium ions arised instead of trivalent vanadium ions due to lithium nonstoichiometry in Li₃V₂(PO₄)₃ structure. This method of identifying nonstoichiometry via ESR detection of V⁴⁺ ions is very simple and demonstrative and can be used to characterize not only as-prepared samples but also samples after multiple electrochemical cycling. This technique of the lithium nonstoichiometry detection in Li₃V₂(PO₄)₃ can be used if the valence state of vanadium ions is stable and investigated samples are not degraded during a long time. To clarify the question, the additional ESR measurements were performed for Li₃V₂(PO₄)₃ samples which were air-stored over a long period (up to 2 years). The obtained data proved the sample degradation in the form of increasing in V⁴⁺ content. It was found that in the presence of LiPO₃ salt in the composite the number of magnetic centers increased significantly, so it contributed to sample degradation, while the pure Li₃V₂(PO₄)₃ sample Li₃V₂(PO₄)₃/C were most stable.