Black Phosphorus as a High-Capacity, High-Capability Negative Electrode for Sodium-Ion Batteries: Investigation of the Electrode/Interface

For a nonaqueous sodium-ion battery (NIB), phosphorus materials have been studied as the highest-capacity negative electrodes. However, the large volume change of phosphorus upon cycling at low voltage causes the formation of new active surfaces and potentially results in electrolyte decomposition at the active surface, which remains one of the major limiting factors for the long cycling life of batteries. In this present study, powerful surface characterization techniques are combined for investigation on the electrode/electrolyte interface of the black phosphorus electrodes with polyacrylate binder to understand the formation of a solid electrolyte interphase (SEI) in alkyl carbonate ester and its evolution during cycling. The hard X-ray photoelectron spectroscopy (HAXPES) analysis suggests that SEI (passive film) consists of mainly inorganic species, which originate from decomposition of electrolyte solvents and additives. The thicker surface layer is formed during cycling in the additive-free electrolyte, compared to that in the electrolyte with fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additive. The HAXPES and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) studies further reveal accumulation of organic carbonate species near the surface and inorganic salt decomposition species. These findings open paths for further improvement for the cyclability of phosphorus electrodes for high-energy NIBs.