High Capacity and High‐Rate NASICON‐Na_3.75V_1.25Mn_0.75(PO₄)₃ Cathode for Na‐Ion Batteries via Modulating Electronic and Crystal Structures

Abstract

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.

Impact of Mg2+ and Al3+ Substitutions on the Structural and Electrochemical Properties of NASICON‐NaxVMn0.75M0.25(PO4)3 (M = Mg and Al) Cathodes for Sodium‐Ion Batteries

“…More importantly, the Mn K-edge shows shift toward higher energies, thus confirming the participation of Mn 4+ /Mn 3+ redox couple during the high voltage charging (Figure 5c; and Figure S6b, Supporting Information). [26] Further, in-operando XRD studies were also performed on the Mg-NMVP cathode to follow its structural changes during charge and discharge processes in both voltage windows (3.8-2.75 and 4.2-2.75 V vs Na + /Na 0 ) ( Figures S8 and S9, Supporting Information). The cell parameters obtained from the refinements of the selected XRD patterns are shown in Table S2 (Supporting Information).…”

“…The anisotropic changes in the lattice parameters could be ascribed to the smaller ionic radius of Al 3+ (0.54 Å) as well as to the decreased sodium ion population in Na(1) sites, which in turn enhances the electrostatic repulsion between (V/Mn/Al)O 6 octahedra along c ‐direction. [ 26 ] Further, the average (V/Mn/Mg)O and (V/Mn/Al)O bond lengths in the Mg‐ and Al‐NVMP cathodes are also shortened as expected (Table 1).…”

“…These observations are in agreement with the earlier reports. [ 24,25,26 ] Interestingly, the Mg‐ and Al‐NVMP cathodes also present similar trends in their voltage‐capacity profiles and first cycle capacity losses. Their first cycle reversible capacities are estimated as 78 and 89 mA h g −1 , respectively.…”

“…Moreover, the substitution of aliovalent Al 3+ cation in the NVMP framework creates sodium ion vacancies, which is expected to reduce the activation energy and enhance the sodium ion mobility. [26,28] As a result, the Mg-and Al-substituted NVMP cathodes have shown higher rate performances and cycling stabilities in comparison with unsubstituted sample.…”

“…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 ]…”