Stabilization of a 4.5 V Cr⁴⁺/Cr³⁺ redox reaction in NASICON-type Na₃Cr₂(PO₄)₃ by Ti substitution

Abstract

NASICON-type Na2CrTi(PO4)3 offers a stable redox reaction of Cr4+/Cr3+ at 4.5 V vs. Na/Na+.

Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions

“…Various kinds of material consisting of layered metal oxides, metal cyanides with Prussian blue structure, have been evaluated as cathodes for sodium-ion batteries with the expectation to realize high energy density. − Another important category of the cathode is polyanionic oxides with a three-dimensional structure, and the strong X–O (X = B, S, P, and Si) covalent bond offers remarkable stability and small volume variation upon sodium-ion insertion/extraction. − Furthermore, the inductive effect originating from polyanionic groups can further elevate the electrochemical potential of electroactive transition metal ions compared to the same redox couples in layered cathodes. , Among the polyanionic compounds, the sodium superionic conductor (NASICON) is of special interest because of the favorable kinetics of sodium-ion mobility and the robust three-dimensional framework. , Na 3 V 2 (PO 4 ) 3 as a typical NASICON-structured cathode renders a specific capacity of 117 mA h g –1 with an electrochemical potential plateau around 3.5 V (V 3+ / 4+ vs Na/Na + ) . Partial substitution of V 3+ by Mn 2+ , Al 3+ , and Cr 3+ can stimulate the redox couple of V 4+ / 5+ (∼4.0 V) in a Na 3 V 2 (PO 4 ) 3 cathode. , Recently, the fully reversible redox couples of Mn 2+ /Mn 3+ (3.6 V) and Mn 3+ /Mn 4+ (4.1 V vs Na/Na + ) have been realized in Na 3 MnZr­(PO 4 ) 3 and Na 3 MnTi­(PO 4 ) 3 , inspiring the further exploration of electroactive transition metals with a high operating potential. − In addition to Ni 2+ /Ni 3+ and Co 2+ /Co 3+ , the Cr 3+ /Cr 4+ redox couple warrants intensive exploration of a novel cathode with high operation voltage. − However, the introduction of these elements into the NASICON-type cathode always causes apparent capacity fading during cycling, and the reasons are still vague but necessary to understand for future improvement.…”

Rationally Designed Sodium Chromium Vanadium Phosphate Cathodes with Multi‐Electron Reaction for Fast‐Charging Sodium‐Ion Batteries

“…[ 15–17 ] Among these, sodium super‐ionic conductor (NASICON)‐structured Na x MeMe′(PO 4 ) 3 (Me/Me′ refers to transition metals) are capable of satisfying the above requirements in terms of high ionic conductivity (3D open frameworks), limited volume change (strong PO bonds), excellent air tolerance and higher redox potential (inductive effect between transition metal and phosphates). [ 18 ] Due to the ease of redox tunability, diverse NASICONs have been identified, including Na 3 V 2 (PO 4 ) 3 , [ 19 ] Na 2 CrTi(PO 4 ) 3 , [ 20 ] Na 2 TiV(PO 4 ) 3 , [ 21,22 ] Na 4 NiV(PO 4 ) 3 , [ 23 ] Na 3 MnTi(PO 4 ) 3 , [ 24,25 ] Na 3 FeV(PO 4 ) 3 , [ 23 ] Na 3 MnZr(PO 4 ) 3 , [ 26 ] Na 4 MnV(PO 4 ) 3 , [ 23,27,28 ] Na 4 MnAl(PO 4 ) 3 , [ 29 ] Na 4 MnCr(PO 4 ) 3 , [ 30–32 ] and Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 , [ 33 ] etc. Nevertheless, most NASICONs suffer from low capacity (<120 mAh g −1 ) as a result of limited (i.e., one to two) electron transfer per chemical formula unit, which reduces the overall energy density and impedes further practical application.…”

Crystal Structures and Local Environments of NASICON-Type Na3FeV(PO4)3 and Na4FeV(PO4)3 Positive Electrode Materials for Na-Ion Batteries

“…Alternatively, various substitutions on the transition-metal site were studied. Thanks to the excellent tunability of the NASICON structure, many different compositions could be explored, exemplified by Na x (M,M′) 2 (PO 4 ) 3 , with M,M′ = Al, Ti, − V, − ,− , Cr, ,,, Mn, ,− Fe, ,,,− Nb, and Zr . However, most of the materials have shown no meaningful capacity increase when the electrochemical reaction was limited to the exchange of two Na + ions.…”