Abstract
NaSICON (Na Super-Ionic CONducting) structured materials are among the most promising solid electrolytes for Li-ion batteries and `beyond Li-ion' batteries (e.g. Na and K) due to their superior ionic conductivities. Although this material has been well known for decades, its exact phase behaviour is still poorly understood. Herein, a starting material of Na3Sc2(PO4)3 single crystals is used, grown by flux methodology, where Na is subsequently chemically replaced by Ag, in order to take advantage of the higher scattering contrast of Ag. It is found that the NaSICON-type compound shows two phase transitions from a low-temperature monoclinic α-phase to a monoclinic β-phase at about 180 K and to a rhombohedral γ-phase at about 290 K. The framework of [Sc2(PO4)3]3− is rigid and does not change significantly with temperature and change of symmetry. The main driving force for the phase transitions is related to order–disorder phenomena of the conducting cations. The sensitivity of the phase behaviour on the ordering of these ions suggests that small compositional changes can have a great impact on the phase behaviour and, hence, on the ionic conductivity of NaSICON-structured materials.
Highlights
NaSICON (Na Super-Ionic CONducting)-type compounds, with the general formula AxMM0(TO4)3 (Masquelier & Croguennec, 2013), are promising materials for Li-ion and ‘beyond Li-ion’ batteries
Indexing of the Powder X-ray diffraction (PXRD) data yields a monoclinic cell [a = 15.7168 (4), b = 8.9328 (1) and c = 9.0408 (2) A, and = 126.035 (3)] that can be most accurately structurally refined with C2/c symmetry, as proposed previously (Guin et al, 2017; Redhammer et al, 2020; Ladenstein et al, 2020)
NaSICON-structured materials belong to the most promising group of solid electrolytes for Li-ion batteries and ‘beyond Liion battery’ concepts (e.g. Na and K) due to their superior ionic conductivities. Despite it being well known for decades, the exact phase behaviour is poorly understood
Summary
NaSICON (Na Super-Ionic CONducting)-type compounds, with the general formula AxMM0(TO4) (Masquelier & Croguennec, 2013), are promising materials for Li-ion and ‘beyond Li-ion’ batteries. They are built from a framework of corner-sharing tetrahedra (T-sites) and octahedra (M-sites), with well-connected large interstitial voids that can host alkalimetal ions (Goodenough et al, 1976). This three-dimensional (3D) alkali-metal ion sublattice enables a fast-ionic transport when used as a solid electrolyte and enables the insertion/ extraction of ions during charge and discharge when used as an electrode material.
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