Electrochemical Properties and Evolution of the Phase Transformation Behavior in the NASICON-type Na3+xMnxV2-x(PO4)3 (0≤x≤1) Cathodes for Na-ion batteries, M.V. Zakharkin (a,b), O.A. Drozhzhin (a,b), S.V. Ryazantsev (a,b), D. Chernyshov (c,d), M.A. Kirsanova (a), I.V. Mikheev (b),
E.M. Pazhetnov (a), E.V. Antipov (a,b) and K.J. Stevenson (a), J. Power Sources 470, 1-8 (2020); https://doi.org/10.1016/j. jpowsour.2020.228231. (a) Skolkovo Institute of Science and Technology, Moscow (Russia) (b) Lomonosov Moscow State University,
Moscow (Russia) (c) ESRF (d) Peter the Great St. Petersburg Polytechnic University, Saint-Petersburg (Russia)
 O.A Drozhzhin et al., J. Synchrotron Radiat. 25 , 468-472 (2018).  M.V. Zakharkin et al., ACS Appl. Energy Mater. 1, 5842-5846 (2018).
PRINCIPAL PUBLICATION AND AUTHORS
composition is reached. Le Bail fitting indicates an ongoing alteration of the unit cell parameters of both phases present during the biphasic domain on the charge of Na3.8Mn0.8V1.2(PO4)3 to 3.8 V, which explains the sloped shape of the E-x curve (Figures 137b-c). In the case of Na4MnV(PO4)3, both "Na3MnxV2-x(PO4)3" and "Na2MnxV2-x(PO4)3" phases don t change their 2θ position, which is reflected in the flat plateau at 3.6 V in accordance to the Gibbs phase rule.
Through operando SXRPD, rather unusual behaviour was observed for Na3.8Mn0.8V1.2(PO4)3 and Na4MnV(PO4)3 compositions, which demonstrate almost identical phase transformation behaviour but, at the same time, show absolutely different types of voltage- composition curves. These observations and the results detailed herein illustrate the necessity of operando synchrotron diffraction experiments, since similar structural transitions may result in the different macroscopic properties.
Fig. 137: Similar structural transitions may result in different electrochemical signatures. a) Electrochemical charge curves, (b) transformations of the selected region of SXRPD patterns, and (c) unit cell parameters of Na3.8−xMn0.8V1.2(PO4)3 (left) and Na4−xMnV(PO4)3 (right) within 2.5−3.8 V voltage ranges.