The electrode performance of amorphous phosphorus in aprotic Na cells is examined. Amorphous phosphorus is electrochemically reduced in the Na cells with a three‐electron redox process, crystallizing into Na3P. NaP bonds in Na3P have high covalent characteristics. Therefore, the molar volume of Na in Na3P is anomalously small in comparison to other Na–metal alloys that have been used as negative electrode materials. The theoretical volumetric capacity, calculated at full volume expansion of amorphous phosphorus electrodes through sodiation, is expected to be 27 % larger than that of metallic sodium. However, experimentally, it is found that severe electrolyte decomposition results in insufficient reversibility of the electrode materials, owing to the highly reactive Na3P surface. Electrode reversibility is successfully improved by utilizing an electrolyte additive, fluoroethylene carbonate (FEC). The surface chemistry of phosphorus in the aprotic solvent with sodium salts is examined by hard/soft X‐ray photoelectron spectroscopy (PES). PES studies reveal that FEC effectively stabilizes the solid–electrolyte interphase (SEI), containing monovalent phosphorus species and sodium fluoride, and thus electrolyte decomposition is partly suppressed by the relatively stable SEI formed on the surface of phosphorus particles.

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.

Electrochemical sodium insertion for hard carbon is examined in a cyclic alkylene carbonate based solution containing a NaClO4 or NaPF6 salt with a fluoroethylene carbonate (FEC) additive to study electrolyte dependency for sodium‐ion batteries. NaPF6‐based electrolytes provide superior reversibility and cyclability of sodium insertion into hard carbon compared with NaClO4‐based ones. The FEC‐derived passivation film improves capacity retention because of better passivation with a thinner surface layer, as revealed by hard and soft X‐ray photoelectron spectroscopy (PES). The use of both the NaPF6 salt and FEC additive results in a synergetic effect on passivation for the hard‐carbon electrode, leading to enhanced cycle performance. Hard‐carbon electrodes with polyvinylidene difluoride binder in propylene carbonate based electrolytes containing NaPF6 and FEC demonstrate excellent capacity retention with a reversible capacity of about 250 mAh g−1. The difference in capacity retention for the electrolytes is expected to originate as a consequence of the difference in the surface interphase layer formed on the hard‐carbon electrodes. Surface analyses with PES and time‐of‐flight secondary ion mass spectrometry reveal differences in surface and passivation chemistry which depend on the salts, solvents, and FEC additives used for the hard‐carbon negative electrodes.

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Graphite/silicon composite electrodes are prepared with PANa polymer as a binder. Morphological characters and electrode performance are compared with those of PVdF. The PANa layer behaves like SEI at the interface with ionic liquid, resulting in the highly reversible electrode performance.