High Entropy Enabling the Reversible Redox Reaction of V⁴⁺/V⁵⁺ Couple in NASICON‐Type Sodium Ion Cathode

Abstract

NASICON‐type Na3VM(PO4)3 (M: transition metals) cathodes usually suffer from poor cycling stability in the voltage region of above 4 V versus Na+/Na owing to irreversible phase transition and severe structural distortion. Herein, the high entropy concept is extended to NASICONs and Na3VAl0.2Cr0.2Fe0.2In0.2Ga0.2(PO4)3 (NVMP) with high purity is obtained. The NVMP achieves a highly reversible specific capacity of 102 mAh g−1 (2.5–4.4 V vs Na+/Na) via the successive redox reaction of V3+/V4+/V5+ together with a long‐term lifespan of 5000 cycles at 20 C (a capacity retention of 86.8%). Even at an extreme temperature of −20 °C, the NVMP cathode can still provide excellent cycling performance (a capacity retention of 94.2% at 5 C after 1000 cycles). Moreover, the increased configurational entropy in the electrode renders a quite small cell volume change of 1.1%. The sodium ion storage mechanism containing solid solution‐type in the voltage range of 2.5–3.8 V and bi‐phasic in 3.8–4.4 V is revealed by ex situ XRD analysis. Pairing with a hard carbon anode, NVMP//HC cell offers a specific capacity of 81 mAh g−1 at 0.2 C based on the cathode mass. This high‐entropy engineering is expected to be widely applicable for the development of polyanionic electrode materials.

A zero-strain Na-deficient NASICON-type Na2.8Mn0.4V1.0Ti0.6(PO4)3 cathode for wide-temperature rechargeable Na-ion batteries

“…This process is related to the V 4+ /V 5+ conversion, which was also reported in other similar studies. 55,65,66 The discharging reaction almost returned to the pristine state with the reduction of Mn and V ions, except for the extra reduction of Ti 4+ to Ti 3+ during the D2–D3 process. These findings illustrate that the electrochemical reaction is closely connected with the redox of the four pairs, including V 3+ /V 4+ , Mn 2+ /Mn 3+ , V 4+ /V 5+ and Ti 3+ /Ti 4+ involved in the MVT-D cathode.…”

Unlocking the multi-electron transfer reaction in NASICON-type cathode materials

“…Based on the above finding, we can speculate that the transition metal migration of V 5+ is a common issue of the irreversible V 4+/5+ redox couple. It should be noted that the replacing elements of Al 3+ , Cr 3+ , and Ga 3+ make the V 4+/5+ redox couple reaction reversible, and a platform located at 4.0 V (vs. Na + /Na) occurred in the discharge curve (figure 2(d)) [63][64][65][66]. This finding can be attributed to the small Al 3+ and the eliminated Jahn-Teller effect of Mn 3+ , so the crystal structure can be stable.…”

A Multielectron-Reaction and Low-Strain Na3.5Fe0.5VCr0.5(PO4)3 Cathode for Na-Ion Batteries

“…Sodium-ion batteries (SIBs) have aroused tremendous attention in the field of large-scale energy storage owing to the cost-effective and broadly available (worldwide) sodium resources. − Nonetheless, the higher redox potential of Na + compared with that of Li + (−2.71 V vs −3.04 V relative to the standard hydrogen electrode) imposes penalties on the energy density of SIBs; meanwhile, the larger size (1.02 Å) of Na + compared with that of Li + (0.76 Å) tends to induce undesirable electrode structural failure with large volume variation upon sodiation/desodiation, resulting in unsatisfactory cycling performance. , Since the cathode plays a key role in determining the cost and electrochemical properties of battery systems, , great efforts are needed to explore advanced cathodes with high working voltage, high specific capacity, and high cycling stability. Up to now, various types of Na-storage cathode materials have been evaluated, including layered/tunnel transition-metal oxides, − polyanions, − Prussian blue analogues, , etc.…”

“…Such defective sites may be vacancies or holes, as porous structures are attested by the N 2 adsorption–desorption measurement (Figure S3), which could favor the fast ion transportation. Besides, the characteristic peak located at around 1020 cm –1 is attributed to the stretching vibration of PO 4 3– . , Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the NFVCP particles with sizes of 100–300 nm are homogeneously embedded in the continuous carbon matrix (Figure c and Figure S4). This nanostructure possesses a relatively large specific surface area (Figure S3), benefiting sufficient electrolyte infiltration.…”

“…. 7,33 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the NFVCP particles with sizes of 100−300 nm are homogeneously embedded in the continuous carbon matrix (Figure 1c and Figure S4). This nanostructure possesses a relatively large specific surface area (Figure S3), benefiting sufficient electrolyte infiltration.…”