Utilization of the V5+/V4 + Redox Reaction in Nasicon-Structured Cathode Materials for Sodium-Ion Batteries

Abstract

The energy problem has become a critical issue worldwide, and renewable energy will dominate the energy system in the near future. Electrochemical storage devices are widely used as the energy storage devices of renewable energy, and the most intuitive feeling is that electric vehicles driven by lithium-ion batteries (LIBs) have become a new generation of transportation. Nevertheless, the worrisome side though was the high cost of LIBs and the scarcity of lithium. Considering that high crustal abundances of sodium and large-scale energy storage devices are almost not restricted by the size of the device, sodium-ion batteries (SIBs) were considered as a promising alternative to the LIBs.1 Cathode materials, as the highest cost proportion of SIBs (32.4 %), have always been the bottleneck of development and high-performance cathode materials are actively being explored by researchers. Sodium super ionic conductor (NASICON)-structured materials, as a member among the most investigated cathode materials for SIBs, are a promising candidate due to the excellent stability of long-term cycling.Na3V2(PO4)3 is the most classical cathode material in NASICON-structures with the high working potential (3.4 V, corresponding to the V4+/V3+ redox reaction) and high specific capacity (117.6 mAh g-1). Recently, many studies have shown that the V5+/V4+ redox reaction can be accessed through partial substitution of V3+.2 However, two phenomena needed deep consideration: (1) the potential of the V5+/V4+ redox reaction in Na3V2(PO4)3 is different from its derivatives, and (2) the utilization of the V5+/V4+ redox reaction is very limited. In one of our work, Na3VAl(PO4)3 and Na3VGa(PO4)3 cathode materials were selected as contrasting systems to explain these two phenomena because Al and Ga are in the same group. Combining experimental tests and density functional theory (DFT) calculations, sodium ions are extracted/inserted from/into different sodium sites, causing a different potential of the V5+/V4+ redox reaction; and the full use of the V5+/V4+ redox reaction was restricted by the excessive diffusion energy barrier.3 Therefore, we proposed two possible approaches to promote the V5+/V4+ redox reaction: (1) substitution of V3+ with smaller ions and (2) utilizing the x = 2.0-3.0 region.As described previously, the access of V5+/V4+ redox reaction is difficult. Reducing the amount of substitution is an effective method to fully use of the V5+/V4+ redox reaction. We have studied successively the electrochemical properties of Na3V1+x Cr1-x (PO4)3 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5),4 Na3V1+x Al1-x (PO4)3 (x = 0.3, 0.5 and 0.7) and Na3V1+x Ga1-x (PO4)3 (x = 0.5 and 0.8). To sum up, Na3V1.7Al0.3(PO4)3 has a good trade-off in utilization of the V5+/V4+ redox reaction and the V4+/V3+ redox reaction. The reversible specific capacity of Na3V1.7Al0.3(PO4)3 is 105.8 mAh g-1 in the first cycle and the specific capacity retention is 95.6 % after 50 cycles at 20 mA g-1.References Goikolea, E.; Palomares, V.; Wang, S.; Larramendi, I. R.; Guo, X.; Wang, G.; Rojo, T., Na‐Ion Batteries—Approaching Old and New Challenges. Advanced Energy Materials 2020, 10 (44), 2002055. Gao, H.; Seymour, I. D.; Xin, S.; Xue, L.; Henkelman, G.; Goodenough, J. B., Na3MnZr(PO4)3: A High-Voltage Cathode for Sodium Batteries. J. Am. Chem. Soc. 2018, 140 (51), 18192-18199. Wang, Q.; Gao, H.; Li, J.; Liu, G. B.; Jin, H., Importance of Crystallographic Sites on Sodium-Ion Extraction from NASICON-Structured Cathodes for Sodium-Ion Batteries. ACS Appl Mater Interfaces 2021, 13 (12), 14312-14320. Wang, Q.; Zhao, Y.; Gao, J.; Geng, H.; Li, J.; Jin, H., Triggering the Reversible Reaction of V3+/V4+/V5+ in Na3V2(PO4)3 by Cr3+ Substitution. ACS Appl Mater Interfaces 2020, 12 (45), 50315-50323. Figure 1