The peculiar structural behaviour of β-Na0.33V2O5 upon electrochemical lithium insertion

The structural behaviour of the beta-Na0.33V2O5 active material in a composite electrode is determined during discharge in the 3.8/2.2 V potential range (0 <= x < 1.66 in LixNa0.33V2O5) using X-ray diffraction combined with Raman microspectrometry. In spite of the existence of three well defined voltage plateaus, we provide evidence here for very weak structural changes throughout the lithium intercalation process, both at the long range order and local scale, involving mainly minor unit cell parameter variations. This result contrasts with the sharp phase transitions with one-phase and large two-phase regions usually observed for Li intercalation compounds exhibiting successive voltage plateaux separated by potential drops. The present findings demonstrate the reliability of a lithium filling scheme in the beta-Na0.33V2O5 3D host lattice involving no phase transition in a remarkably wide Li composition range. Furthermore, new kinetic data for the electrochemical Li insertion reaction into beta-Na0.33V2O5 are reported using AC impedance spectroscopy, which are discussed in relation with the specific structural response. This peculiar behaviour highlights the interest of such rigid tunnel framework to minimize the structural stress induced by lithium insertion.