Study on sodium storage properties of manganese‐doped sodium vanadium phosphate cathode materials

Abstract

Na+ superionic conductor (NASICON)‐structured Na4VMn(PO4)3 (NVMP) possesses stable cycling performance at 2.5–3.8 V by replacing V with lower cost Mn but suffers rapid capacity decay when further widening the voltage to 2.5–4.2 V, owing to a less stable V4+/V5+ redox couple. Herein, to stabilize the V4+/V5+ couple and improve the reversibility, a series of carbon‐coated NVMP (NVMP@C) with different V/Mn ratios are compared, among which, Na3.25V1.75Mn0.25(PO4)3@C delivers an additional reversible V4+/V5+ capacity of 7 mAh g−1 at the voltage of 3.9–4.2 V. Based on this, to further activate the capacity of V4+/V5+, Al‐doped Na3.25V1.75−yMn0.25Aly(PO4)3 structures are synthesized. When 0.5 Al is doped, 15 mAh g−1 capacity corresponding to V4+/V5+ can be released. In addition, it is found that the activation of V4+/V5+ is not conducive to electrochemical reversibility and C‐rate performance, owing to the shrinkage of the whole framework structure with the oxidization of V4+ to V5+.

High‐Energy‐Density Cathode Achieved via the Activation of a Three‐Electron Reaction in Sodium Manganese Vanadium Phosphate for Sodium‐Ion Batteries

“…Figure 4b,c shows the charge/discharge curves of the NVP-Mn(0) and NVP-Mn(0.5) in the first ten cycles at 1 C. Compared with those of the NVP-Mn(0), the curves of the NVP-Mn(0.5) show much better overlap. In addition, the NVP-Mn(0.5) presents three distinct pairs of voltage plateaus, matching well with the reversible redox couples of V 2+/3+ (1.6 V), V 3+/4+ /Mn 2+/3+ (3.4 V), and V 4+/5+ (3.9 V), [6,11,16,27] , respectively. Due to the close conversion voltage of V 3+ /V 4+ and Mn 2+ /Mn 3+ , only a single voltage plateau is displayed in the NVP-Mn(0.5).…”

“…[1][2][3] However, the sustainable application of LIBs is greatly hindered by the low abundance and geographically uneven distribution of lithium resources in the earth's crust. [4][5][6] Recently, as one of the promising alternative candidates for LIBs, sodium-ion batteries (SIBs) have been widely favored due to their low cost and similar physicochemical characteristics to LIBs. [7][8][9][10] However, the limited energy density of SIBs hinders the commercialization.…”

The Distance Between Phosphate‐Based Polyanionic Compounds and Their Practical Application For Sodium‐Ion Batteries

“…Besides, for different transition metal species in phosphates, the operation voltage of Na 3 V 2 (PO 4 ) 3 is much higher than that of Na 3 Fe 2 (PO 4 ) 3 (3.4 V vs 2.6 V), since V 3+ has a larger effective nuclear charge with deeper valence levels tend than that of Fe 2+ (Figure 1g). [46,47] Considering the respective superiorities of metal species and anions, Yamada's group synthesized the cathodes of Na 3 Cr 2 (PO 4 ) 3 and Na 3 Cr 2 (PO 4 ) 2 F 3 , which possess ultrahigh and stable operation voltages of 4.5 and 4.7 V, respectively, based on the redox cou-ple of Cr 4+ /Cr 3+ . [48,49] However, their practical capacities and cycling properties are not good enough probably due to poor electrochemical activity and the decomposition of electrolytes at high voltage.…”

Sodium Layered/Tunnel Intergrowth Oxide Cathodes: Formation Process, Interlocking Chemistry, and Electrochemical Performance

“…One of the main concerns regarding the practical application of SIBs is to find outstanding electrode materials, especially cathode materials, which determine the critical battery characteristics such as specific capacity, rate performance, and cyclic stability. – Recently, great efforts have been devoted to developing excellent cathode materials including layered transitional metal oxides (Na x TMO 2 ), Prussian-blue analogues, and polyanion compounds. – Among them, Na x TMO 2 ( x ≤ 1, TM = transition metal), particularly Mn-based oxides have been mostly exploited because of their low cost, easy synthesis, high specific capacity, high energy density, and environmental friendliness. – Typically, Na x TMO 2 can be divided into layered and tunnel structures. There are four main kinds of layered structures including O2, O3, P2, and P3 types according to classification depending on their Na occupation sites and the stacking sequence of O layers. – Compared with O3-type oxides, the P2-type materials exhibit better rate performance owing to large prismatic residing sites and facile diffusion paths for Na + . – So far, many P2-type sodium layered oxides have been explored as cathode materials for SIBs, such as Na 0.67 MnO 2 , Na 0.67 Fe 0.5 Mn 0.5 O 2 , Na 0.55 [Ni 0.1 Fe 0.1 Mn 0.8 ]­O 2 , Na 0.67 [Mg 0.28 Mn 0.72 ]­O 2 , Na 0.67 Mn 0.6 Ni 0.3 Cu 0.1 O 2 , Na 0.72 [Li 0.24 Mn 0.76 ]­O 2 , etc.…”