Crystal Chemistry of NaSICONs: Ideal Framework, Distortion, and Connection to Properties

Abstract

The crystal structure of R3̅c NaSICON type A x M2(TO4)3 (M = Al, Ce, Co, Cr, Fe, Ge, Hf, In, Li, Mg, Mn, Mo, Na, Nb, Ni, Sb, Sc, Se, Sn, Ta, Ti, U, V, Y, Yb, Zn, Zr; T = As, Ge, Mo, P, S, Si, V) with its multiple internal degrees of freedom offers considerable flexibility and can accommodate dozens of different ions leading to hundreds of reported compositions. In this work, the ideal R3̅c structure with undistorted [MO6] octahedra and [TO4] tetrahedra is parametrized and used to quantify distortion of about 340 real NaSICON-type oxides consistently across the entire family. It is shown that distortion is primarily driven by the size mismatch between the A cations and M2(TO4)3 framework, that can be used to optimize the geometry of the structure to control properties, such as A-ionic conductivity or thermal expansion.

Irreversible Electrochemical Reaction at High Voltage Induced by Distortion of Mn and V Structural Environments in Na4MnV(PO4)3

“…Zakharkin et al . described the structure of Na 3 MnV­(PO 4 ) 3 using the C 2/ c space group, a typical space group for NASICON-type materials. − However, as a result of our detailed examination, two diffraction peaks were observed at 6.58° and 6.63° and only the former reflection could be explained using the C 2/ c space group (inset in Figure S7). Our new structural model for Na 3 MnV­(PO 4 ) 3 therefore uses the R 32 space group, which can successfully explain the presence of two reflections at low angle (Figure a).…”

“…The NASICON (Na-superionic-conductor)-type materials described by a 3D framework structure and the chemical formula of A x MM′(XO 4 ) 3 (A = Li and Na; X = P, S, Si, Mo, and W) are extensively studied for Na-ion batteries because of their thermal stability, rate performance, and cycle life. − Na 3 V 2 (PO 4 ) 3 , for instance, when used as a positive electrode material, is characterized by a reversible electrochemical reaction involving the exchange of two Na + through the V 4+/3+ redox couple at 3.4 V (vs Na + /Na) via a two-phase reaction for a theoretical capacity of 118 mAh/g. − To increase this moderate-capacity, new NASICON-type materials capable of multiredox reactions, i.e. , possibly allowing more than one Na + exchange per transition metal, such as Na 3+ x MM′(PO 4 ) 3 (MM′ = FeV, − MnV, − and MnCr − ), have been extensively studied in recent years.…”

NASICONs‐type solid‐state electrolytes: The history, physicochemical properties, and challenges

“…When the temperature was below 423 K, the phase transition driven by shear deformation to the monoclinic structure would happen. [48,49,73] However, only the rhombohedral phase showed superionic conductivity because of its higher symmetry. There were three different Na sites for rhombohedral Na 3 Zr 2 Si 2 PO 12 as shown in Figure 2C, the Na1 R (6b) site was located between two ZrO 6 units with sixfold oxygen coordination; the Na2 R (18e) site was located with eightfold oxygen coordination, and the Na3 R (36f) site was at midway between Na1 R and Na2 R site with fivefold oxygen coordination.…”

Rational Design of Mixed Polyanion Electrodes NaxV2P3–i(Si/S)iO12 for Sodium Batteries

“…The rhombohedral ( R 3̅ c ) and monoclinic ( C 2/ c ) NaSICON phases of Na x VP 3 can be understood as networks of “lantern” units stacked into columns perpendicular to the basal plane to form the NaSICON framework (Figure S2). ,,, Each lantern unit (see inset of Figure ) consists of two VO 6 octahedra corner-sharing with O atoms of three PO 4 tetrahedra. Each O atom in the structure is shared between VO 6 and PO 4 in this manner, forming a three-dimensional (3-D) framework with V–O–P bonding.…”