Electrochemical Sodiation and Solid Electrolyte Interphase for Phosphorus Electrodes

Phosphorus (P), an element of the fifth group in the periodic table, has three main allotropes: white, red, and black. P as electrode materials for Na cells has a considerable advantage in terms of materials abundance and relatively small volume change by sodiation as Na binary compounds and alloys.^[1] However, the volumetric change of P upon sodiation/de-sodiation cycles at low voltage (0.3 - 0.7 V vs. Na/Na⁺) caused electrolyte degradation which remains the main factor limiting the cycling life of electrodes.^[1]

The surface reactions of electrolytes with P in sodium ion cells have been examined. Coin cells (red or black P // Na) were cycled in electrolytes containing NaPF₆ salt and ethylene carbonate/dimethyl carbonate solvent without and with the electrolyte additive (FEC or VC). Capacity retention is significantly better for the cells cycled with the electrolyte additive than for the cells cycled without the electrolyte additive, and the cells with the additive show high discharge capacity above 1450 mAh g^-1 with the high coulombic efficiency than 98% after 25^th cycles (Figure 1-A). To further examine the understanding of the formation of a solid electrolyte interphase (SEI) and of its evolution during cycling, powerful surface characterization techniques are combined for studying the electrode/electrolyte interphase of black P/AB electrodes, such as SEM, TEM, time-of-flight secondary ion mass spectrometry (TOF-SIMS), hard/soft X-ray photoelectron spectroscopy (HAXPES/SXPES). In particular, the effect of the electrolyte additives on the interphase chemistry was carefully explored by HAXPES and TOF-SIMS. The HAXPES analysis shows that organic species is enriched from the reduction of electrolyte solvents (EC, DEC) and electrolyte additives (FEC, VC) confirming a formation of SEI passive film. The thicker SEI layer was formed after cycling in additive-free electrolyte as compared to electrolyte with FEC or VC additive (Figure 1-B). TOF-SIMS studies reveal the SEI of the FEC-based electrolyte consists of only inorganic species, however the SEI of the VC-based electrolyte consists of organics and inorganics species. In addition, considerable modification of the surface morphology for the both electrodes was observed by SEM. Electrochemical impedance spectroscopy spectra and charge/discharge profiles of the P electrodes are analyzed to examine the kinetics evolution of electrochemical processes at different stages for sodiation. Further investigations on black or red P electrodes to improve its overall electrochemical performance (capacity, cyclability, coulombic efficiency, and voltage profile) are being carried out, including the electrode structures, effect of binders, and electrolyte/additive dependency. To summarize analytical and electrochemical results, we will discuss the potential of P as the advanced negative electrode in Na-ion batteries, concerning charge-transfer mechanisms and SEI properties.