Multiple phases in the ε-VPO4O–LiVPO4O–Li2VPO4O system: a combined solid state electrochemistry and diffraction structural study

Polyanionic materials attract great interest in the field of Li-ion battery research thanks to the wide range of possible available compositions, resulting in a great amount of different properties. Tavorite type compositions offer a very rich crystal chemistry, among which LiVPO4F delivers the highest theoretical energy density. In this work we focus our interest on the homeotypic composition LiVPO4O. This oxy-phosphate shows the ability to exploit two redox couples, V5+/V4+ at 3.95 V vs. Li+/Li0 and V4+/V3+ at an average potential of 2.3 V vs. Li+/Li0 upon Li+ extraction and insertion, respectively. The two domains show marked differences both in the electrochemical signature and in the phase diagram. Here we address for the first time both topics with a combination of electrochemical techniques and ex situ/in situ X-ray and neutron diffraction and support of density functional theory (DFT) calculations, to get a deep understanding of the behavior of Li1±xVPO4O. In the low voltage region, in particular, the formation of intermediate phases and the crystal structure of the end member Li2VPO4O are reported for the first time.