Fig. 136: Exterior and design of the electrochemical cell for operando X-ray
STRUCTURE OF MATERIALS
transformations in Na3+xMnxV2-x(PO4)3 samples, operando high-resolution synchrotron X-ray powder diffraction (SXRPD) experiments were carried out at beamline BM01 (l = 0.68987 Å, 2D Detector Pilatus 2M, transmission mode) using an electrochemical cell with sapphire windows (Figure 136) .
Single-crystal sapphire X-ray windows provide excellent signal-to-noise ratio, proper electrochemical contact because of the constant pressure between the electrodes, and perfect electrochemical stability at high potentials due to the inert and non-conductive nature of sapphire. The design of the cell allows the simultaneous or sequential application of diffraction and spectroscopic techniques. It is particularly important to mention that this cell design allows for fluorescence detection at the outlet as well as at the inlet side of the cell. Different materials can be studied simultaneously using periodically repeating measurements with multiple cell- positioning holders.
Two Na3+xMnxV2-x(PO4)3 materials with x = 0.8 and 1.0 were used in this study.
The electrochemical behaviour of these samples is different (Figure 137a). Sodium extraction from Na4MnV(PO4)3 proceeds via two steps in the 2.5-3.8 V voltage window, with one sloping and one flat plateau at ~3.4 and 3.6 V, which implies single-phase and biphasic deintercalation mechanisms, respectively . Contrarily, the charging curve of Na3.8Mn0.8V1.2(PO4)3 is wholly sloped without evident signatures of biphasic transition. Operando SXRPD patterns (Figure 137b), recorded during the charge of both electrochemical cells, show that at the beginning, Na+ extraction from the initial Na3+xMnxV2-x(PO4)3 leads to "Na3MnxV2-x(PO4)3" via reflection-shifting by a single-phase mechanism, followed by a biphasic reaction until the "Na1+xMnxV2-x(PO4)3" composition is reached at 3.8 V.
Utilisation of highly resolving synchrotron radiation allows to clearly observe that for the case of x = 0.8 at approximately 50% state of charge, the "Na3Mn0.8V1.2(PO4)3" reflection broadens. Concurrently, reflection of a new "Na2Mn0.8V1.2(PO4)3" phase appears and shifts to higher 2θ angles until the "Na1.8Mn0.8V1.2(PO4)3"