Understanding the Na-Ion Storage Mechanism in Na_3+xV_2–xM_x(PO₄)₃ (M = Ni²⁺, Co²⁺, Mg²⁺; x = 0.1–0.5) Cathodes

Abstract

Bivalent cations (M = Ni 2+ , Co 2+ , Mg 2+ ) with different doping contents (x = 0.1, 0.2, 0.3, 0.4, 0.5) were incorporated for vanadium in the Na 3+x V 2−x M x (PO 4 ) 3 (NVP), yielding enhanced rate performance and capacity retention. Successful doping of these cations in the NVP structure was confirmed by powder X-ray diffraction (PXRD), vibrational FT-IR spectroscopy, and scanning electron microscopy (SEM) techniques. The improved electrochemical performance of substituted NVP cathode has been correlated to effective Na ion migration, which improved kinetics of charging and discharging properties. Mg 2+ was possible to dope up to x = 0.5 in the NVP structure, which exhibited a superior electrochemical performance compared to that of Ni 2+ -and Co 2+ -doped NVP samples. The Mg 2+ -doped NVP electrode exhibited fast Na ion kinetics with a specific capacity of 70 mAh g −1 at a 20 C rate. The oxidation state of the vanadium in the Mg 2+ -substituted NVP was investigated by using X-ray photoelectron spectroscopy (XPS).

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

“…Upon subsequent sodiation/desodiation, ≈2.18 Na + reversibly inserted into and extracted from the VMF-0.5 framework. Such a reversible structure evolution involved in both solid-solution and biphase electrochemical reactions has also been demonstrated in other similar NASICON cathodes, such as typical Na 2 VTi(PO 4 ) 3 , [10] Na 3 TiMn(PO 4 ) 3 , [22] Na 4 VMn(PO 4 ) 3 , [23] Na 3 VFe(PO 4 ) 3 , [23] Na 3 V 2 (PO 4 ) 3 , [34] Na 3 VCr(PO 4 ) 3 , [59,60] etc. The total unit volume variation (Figure S21, Supporting Information) during Na + insertion and extraction processes was calculated as 8.41% (ΔV/V pristine ), which is close to that of NVP cathode (8.26%) [61] and smaller than that of VM reported (9.28%).…”

Electrochemical Performance of High‐Valence Mo6+ and Low‐Valence Mn2+ Doped‐ Na3V2(PO4)3@C Cathode for Sodium‐Ion Batteries

“…In this technique, during charging and discharging a constant current pulse is applied for fixed time period, which is shown in the Figure 10(a). The Na + diffusion coefficient (D Na+ ) of Na 3.94 V 0.98 Mo 0.02 Mn(PO 4 ) 3 @C cathode is calculated by using Weppner and Huggins derived expression as shown below: [32] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr D{_{{\rm Na}+}}={{{{4\hskip0.17em\hskip0.17em{\rm L}}^{2}{\rm \ }}\over{{\rm \pi }{\rm \tau }}}\left[{{{\Delta {\rm E}}_{{\rm s}}{\rm \ }}\over{{\Delta {\rm E}}_{{\rm t}}}}\right]}^{2}\hfill\cr}}$$$ …”

“…In this technique, during charging and discharging a constant current pulse is applied for fixed time period, which is shown in the Figure 10(a). The Na + diffusion coefficient (D Na + ) of Na 3.94 V 0.98 Mo 0.02 Mn(PO 4 ) 3 @C cathode is calculated by using Weppner and Huggins derived expression as shown below: [32]…”

“…Also, Pal et al substituted the P by silicon (Si) and found the superior sodium-ion diffusivity, enhanced electrochemical performance with a high specific capacity, excellent rate capability and stable cyclic performance. [26] Many metals such as Al, [27] Mn, [24,28,29] Cr, [30] Fe, [31] Mg, [27,32] Mo, [23] Ti [33] have been introduced into NVP at V site for improving the performance of NVP cathode. Li et al [34] have introduced high valence molybdenum (+ 6) ion (Mo 6 + ) in the V sites of NVP and investigated the electrochemical performance of Na-half cell.…”

Synergistically Boosting Sodium-Storage Performance of Na3V2(PO4)3 by Regulating Na Sites and Constructing 3D Interconnected Carbon Nanosheet Frameworks

“…For example, various cation-doped Na 3 V 2– x M x (PO 4 ) 3 (M: Al 3+ , Ni 2+ , Mg 2+ , Mn 2+ , Cr 3+ , Nb 5+ , Mo 6+ , etc.) have been developed by partially replacing V 3+ ions, − realizing remarkably enhanced electrochemical performance. Masquelier et al reported the substitution of V sites by Al, which can not only facilitate the ion conductivity but also partially activate 3rd Na + extraction/insertion, realizing higher specific capacity and energy density .…”