The origins of kinetics hysteresis and irreversibility of monoclinic Li3V2(PO4)3

Abstract

Monoclinic Li3V2(PO4)3 is a promising cathode material with complex charge–discharge behavior. Previous structural investigation of this compound mainly focuses on local environments; while the reaction kinetics and the driving force of irreversibility of this material remain unclear. To fully understand the above issues, both the equilibrium and the non-equilibrium reaction routes have been systematically investigated in this study. Multiple characterization techniques including X-ray diffraction, variable temperature (spinning rate) and ex/in situ7Li, 31P solid state NMR have been employed to provide comprehensive insights into kinetics, dynamics, framework structure evolution and charge ordering, which is essential to better design and application of lithium transition metal phosphate cathodes. Our results suggest that the kinetics process between the non-equilibrium and the quasi-equilibrium delithiation pathways from Li2V2(PO4)3 to V2(PO4)3 is related with a slow relaxation from two-site to one-site delithiation. More importantly, it has been demonstrated that the irreversibility in this system is not solely affected by cation and/or charge ordering/disordering, but mainly driven by framework structure distortion.