Sodium superionic conductor (NASICON) cathodes are attractive for Na‐ion battery applications as they exhibit both high structural stability and high sodium ion mobility. Herein, a comprehensive study is presented on the structural and electrochemical properties of the NASICON‐Na3+yV2−yMny(PO4)3 (0 ≤ y ≤ 1) series. A phase miscibility gap is observed at y = 0.5, defining two solid solution domains with low and high Mn contents. Although, members of each of these domains Na3.25V1.75Mn0.25(PO4)3 and Na3.75V1.25Mn0.75(PO4)3 reversibly exchange sodium ions with high structural integrity, the activity of the Mn3+/Mn2+ redox couple is found to be absent and present in the former and latter candidate, respectively. Galvanostatic cycling and rate studies reveal higher capacity and rate capability for the Na3.75V1.25Mn0.75(PO4)3 cathode (100 and 89 mA h g−1 at 1C and 5C rate, respectively) in the Na3+yV2−yMny(PO4)3 series. Such a remarkable performance is attributed to optimum bottleneck size (≈5 Å2) and modulated V‐ and Mn‐redox centers as deduced from Rietveld analysis and DFT calculations, respectively. This study shows how important it is to manipulate electronic and crystal structures to achieve high‐performance NASICON cathodes.

structure is made of so-called "lantern units" which consist of two VO 6 octahedra and three PO 4 tetrahedra. These lantern units are stacked along c-axis to make 3D framework, wherein sodium ions occupy two independent crystal sites (Na(1) and Na(2)). [8] It delivers reversible capacities closer to 110 mA h g −1 at an average intercalation voltage of 3.4 V versus Na + /Na 0 , resulting an energy density of ≈400 Wh kg −1. The corresponding electrochemical sodium (de)intercalation reaction proceeds through a two-phase transformation (i.e., Na 3 V 2 (PO 4) 3 to NaV 2 (PO 4) 3 via oxidation of V 3+ to V 4+). It is also worth mentioning that NaV 2 (PO4) 3 phase contains the remaining sodium ions in the Na(1) sites, while the Na(2) sites are completely empty. [9] Various strategies were reported to fabricate carbon coated nano-NVP cathodes which exhibited enhanced rate performances with better cycling stability. [10-14] Thanks to richness of NASCION crystal chemistry, the composition of the NVP cathode could be tuned by partially replacing vanadium ions with different alio-and isovalent cations. [15-22] In particular, Na 4 VMn(PO 4) 3 (NVMP) composition is very attractive because of its reduced cost and toxicity as well as its improved energy density (425 Wh kg −1) with respect to the NVP cathode. [22] Its sodium (de)intercalation reaction proceeds though two voltage steps (V 4+ /V 3+ at 3.4 V and Mn 3+ /Mn 2+ at 3.6 V) with reversible capacities of ≈110 mA h g −1. Further, when the charging window is extended to 4.3 V versus Na + /Na 0 , higher first charge capacity has been obtained (≈150 mA h g −1 , which is equivalent to removal of three moles of sodium per formula unit), thanks to the activity of V 5+ /V 4+ /V 3+ and Mn 3+ /Mn 2+ redox couples. [23-26] Nevertheless, the NVMP cathode still suffers from two major issues which limit its potential application in NIBs. First, the NVMP cathode prepared by solid state reaction exhibited limited reversible capacity, significant polarization and poor capacity retention. Such inferior performance could be attributed to its limited electronic conductivity and formation Jahn-Teller active Mn 3+ during cycling. To address this issue, recently a carbon coated (≈8.6 wt%) nano-NVMP cathode was developed, which had shown enhanced rate performances and cycle life. [27] However, such approach is time consuming, laborious and the resulting cathode has lower energy density for practical application (because of its higher carbon content and lower Sodium superionic conductor (NASICON)-Na 4 VMn(PO 4) 3 (NVMP) cathode is attractive for sodium-ion battery application due to its reduced cost and toxicity, and high energy density (≈425 Wh kg −1). However, it exhibits significant polarization, limited rate and cycling performances due to its lower electronic conductivity and formation of Jahn-Teller active Mn 3+ during cycling. In this report, a chemical approach is presented to partially replace Mn 2+ of the NVMP framework by Mg 2+ and Al 3+ substitutions. The Mg-and Al-substituted NVMP cat...

ter.8b04010. Experimental details, Mossbauer, FTIR, FESEM, TGA, additional electrochemical, and XRD data, Na-ion channels in the weberite phase, cavity size in the 1D precursor, tables for the structural details, and Mossbauer analysis (PDF)

Partial replacement of Ti4+ by Te4+ ions in calcium copper titanate lattice improved its dielectric behaviour mostly due to cubic-to-tetragonal structural transformation and associated distortion in TiO6 octahedra. The relative permittivity values (23–30 x 103) of Te4+ doped ceramics is more than thrice that of un-doped ceramics (8 x 103) at 1 kHz. A decreasing trend in relative permittivity with increasing temperature (50–300 K) is observed for all the samples. Barrett’s formula, as a signature of incipient ferroelectricity, is invoked to rationalize the relative permittivity variation as a function of temperature. A systematic investigation supported by temperature dependent Raman studies reveal a possible ferroelectric transition in Te4+ doped ceramic samples below 120 K. The possible ferroelectric transition is attributed to the interactions between quasi-local vibrations associated with the micro-clusters comprising TiO6 and TeO6 structural units and indirect dipole-dipole interactions of off-center B–cations (Ti4+ and Te4+) in double perovskite lattice.

Vanadium multi-redox based NASICON-Na z V 2-y M y (PO 4 ) 3 (3 ≤ z ≤ 4; M = Al 3+ , Cr 3+ and Mn 2+ ) cathodes are particularly attractive for Na-ion battery applications due to their high Na insertion voltage (>3.5 V vs. Na + /Na 0 ), reversible storage capacity (~150 mA h g -1 ) and rate performance. However, their practical application is hindered by rapid capacity fade due to bulk structural rearrangements at high potentials involving complex redox and local structural changes. To decouple these factors, we have studied a series of Mg 2+ substituted Na 3+y V 2y Mg y (PO 4 ) 3 (0 ≤ y ≤ 1) cathodes for which the only redox-active species is vanadium. Whilst X-ray diffraction (XRD) confirms the formation of solid solutions between the y = 0 and 1 end members, X-ray absorption spectroscopy and solid-state nuclear magnetic resonance reveal a complex evolution of the local structure upon progressive Mg 2+ substitution for V 3+ . Concurrently, the intercalation voltage rises from 3.35 to 3.45 V, due to increasingly more ionic V-O bonds, and the sodium (de)intercalation mechanism transitions from a two-phase for y ≤ 0.5 to a solid solution process for y ≥ 0.5, as confirmed by inoperando XRD, whilst Na-ion diffusion kinetics follow a non-linear trend across the compositional series.