phases at low C-rate (0.11 C). However, when a higher current density was applied (i.e., towards a non-equilibrium state), the Na2V2(PO4)3 phase became more isolated. Similarly, in the operando study of LiFePO4, a higher C-rate (1 C to 10 C) allowed a more intense appearance of the intermediate LixFePO4 phase. Even though the C-rates used in this study were relatively low, the battery cells already reached a non-equilibrium condition. This is probably because (i) the Na3V2(PO4)3 powder contains relatively large particles (see Figure ); and (ii) the electrodes without binder can induce a higher overpotential. This is further confirmed by the observation of the large polarization of ~150 mV using the in situ cell compared to a low polarization of 30 mV observed using a normal coin cell, cycled at the same C-rate of 0.11 C (Figure ). Figure shows operando SXRPD data collected during six cycles with the C-rates of 0.37-0.77-0.77-0.77-0.37-0.77 C (which are about three to seven times higher current densities than 0.11 C). To increase the chance of isolating a 'pure' Na2V2(PO4)3 phase, multiple cycles were measured. The SXRPD pattern which appears mostly composed by the Na2V2(PO4)3 phase was obtained during the last cycle (see Figure for more details). The powder pattern of Na2V2(PO4)3 presented in Figure was then used to solve its crystal structure. The indexing of the powder pattern, the space group determination, and the cell transformation with group-subgroup relationships were carried out using Dicvol, Chekcell, and POWDERCELL 48 software, to fully resolve the crystal structure of Na2V2(PO4)3. During the steps of indexing and space group determination, it was found that the reflection at 2θ = 3.85° excludes the rhombohedral lattices (Figure ) as well as Ccentered monoclinic-type cells (Figure ). Possible space groups are then primitive, and thus several structure models with different space groups were examined to solve the structure. The initial atomic positions of each model were deduced from the groupsubgroup relationships starting from the R3c rhombohedral cell. The highest to lowest symmetry of the examined space groups are P21/c, P2/c, P21/m, and P/m (corresponding to 22, 23, 42, and 46 independent atoms per cell, respectively). We attempted unsuccessfully to solve the structure using the two space groups (P21/m, and P/m). The structure model, namely "Model 1", using the P21/c space group showed slightly better results than that using a P2/c space group (i.e., Model 2). The detailed comparisons between Models 1 and 2 are discussed later in this paper.