Impact of Mg²⁺ and Al³⁺ Substitutions on the Structural and Electrochemical Properties of NASICON‐Na_xVMn_0.75M_0.25(PO₄)₃ (M = Mg and Al) Cathodes for Sodium‐Ion Batteries

Abstract

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...

A Novel NASICON‐Typed Na4VMn0.5Fe0.5(PO4)3 Cathode for High‐Performance Na‐Ion Batteries

“…The increase of Na1 population would in turn weaken the electrostatic repulsion between two TMO 6 octahedra, finally resulting in the decrease in c parameter. [16,36] Thus, VMF-0.5 shows slightly larger c value due to its low Na1 occupancy, which is expected to provide a facile Na-ion diffusion by the expansion of the c-axis. Fouriertransform infrared spectroscopy (FTIR) data were recorded to reveal more structural information of as-prepared samples (Figure 1b).…”

Reversible Activation of V4+/V5+ Redox Couples in NASICON Phosphate Cathodes

“…However, as the Na + ions incorporated, the increased Na1 occupations would instead reduce the electrostatic repulsion between MO 6 (M = Fe, V) octahedra in c axis direction, resulting in the lowered c values. [29] In addition, the average bond lengths of Na2O for Na 3+x V 2-x Fe x (PO 4 ) 3 were also calculated (Figure S3), which is associated with their electrochemical performances. A long Na2O bond means the facilitated Na extraction/insertion from/into the cathode, usually representing better electrochemical properties.…”

Mn-Rich Phosphate Cathodes for Na-Ion Batteries with Superior Rate Performance

“…Ghosh et al developed a Mg-doped Na 4 VMn 0.75 Mg 0.25 (PO 4 ) 3 composite cathode, which shows an improved cycling stability. 14 Kim's group also incorporated Cu 2+ for substitution of Mn 2+ to obtain a Na 4 VMn 0.9 Cu 0. electrochemical performance. 15 More recently, our group demonstrated that lowering the Jahn−Teller effect of Mn 3+ on the electrochemical performance of cathodes by reducing the Mn concentration under the employment of Fe ions as dopants is effective.…”