Surface Pseudocapacitive Mechanism of Molybdenum Phosphide for High‐Energy and High‐Power Sodium‐Ion Capacitors

Abstract

Sodium‐based energy storage technologies are potential candidates for large‐scale grid applications owing to the earth abundance and low cost of sodium resources. Transition metal phosphides, e.g. MoP, are promising anode materials for sodium‐ion storage, while their detailed reaction mechanisms remain largely unexplored. Herein, the sodium‐ion storage mechanism of hexagonal MoP is systematically investigated through experimental characterizations, density functional theory calculations, and kinetics analysis. Briefly, it is found that the naturally covered surface amorphous molybdenum oxides layers on the MoP grains undergo a faradaic redox reaction during sodiation and desodiation, while the inner crystalline MoP remains unchanged. Remarkably, the MoP anode exhibits a pseudocapacitive‐dominated behavior, enabling the high‐rate sodium storage performance. By coupling the pseudocapacitive anode with a high‐rate‐battery‐type Na3V2O2(PO4)2F@rGO cathode, a novel sodium‐ion full cell delivers a high energy density of 157 Wh kg−1 at 97 W kg−1 and even 52 Wh kg−1 at 9316 W kg−1. These findings present the deep understanding of the sodium‐ion storage mechanism in hexagonal MoP and offer a potential route for the design of high‐rate sodium‐ion storage materials and devices.

Optimized Kinetics Match and Charge Balance Toward Potassium Ion Hybrid Capacitors with Ultrahigh Energy and Power Densities

“…Furthermore, these values are among the top rank compared with previous reported PIHCs, [3,6b,7,11,12,38] even with Na-ion and Li-ion hybrid capacitors (Figure 6d; and Table S3, Supporting Information). [20,39] Good cycling stability is also demonstrated for AC// NOCS PIHCs (Figure 6e). Specifically, AC//NOCS-1.5 PIHCs and AC//NOCS-1.8 PIHCs display high reversible capacities of 52.7 and 48.6 mAh g −1 after 2000 cycles with high Coulombic efficiency of above 98%.…”

Self‐Supporting Electrode Composed of SnSe Nanosheets, Thermally Treated Protein, and Reduced Graphene Oxide with Enhanced Pseudocapacitance for Advanced Sodium‐Ion Batteries

“…Figure b shows the log( )‐log( ) plots for the self‐supporting SnSe‐TP@rGO film electrode and the fitted b values of the reduction peaks and oxidation peaks based on the CV curves are calculated to be 0.89/0.77 and 0.83/0.79, respectively, implying the capacity comes from diffusion and surface‐controlled behavior. Moreover, by separating the current response (i) at a fixed voltage through the equation 5, the exact ratio of the capacitive‐controlled and diffusion‐controlled capacity at a certain scanning rate can be determined: …”

Molybdenum‐based materials for sodium‐ion batteries

“…This conversion can be described as MoP +3Na + + 3e − ↔ Na 3 P + Mo, leading to a high theoretical capacity of 633 mAh g −1 . However, MoP is very sensitive to oxygen and moisture, and therefore surface engineering should be implemented to enhance the stability of phosphides 100‐102 . As for nitrides, Mai et al have explored the sodium performance of mesoporous Mo 3 N 2 and MoN nanowires 103 .…”