Impact of the Cut-Off Voltage on Cyclability and Passive Interphase of Sn-Polyacrylate Composite Electrodes for Sodium-Ion Batteries

Reversibility of electrochemical sodiation for Sn-based electrodes consisting of Sn powder, graphite, and sodium polyacrylate was examined at different upper cutoff voltages of 0.65 and 0.70 V in nonaqueous Na cells. The upper cutoff voltage is one of the key factors to improve the electrochemical reversibility. In case of a cutoff voltage of 0.70 V, the sodiation/desodiation cycle performance was not stable and accompanied by capacity decay, indicating that the anodic decomposition of passivation layer is led to the dissolution and reformation at 0.68 and 0.40 V, respectively, on Sn particles that were catalyzed by pure Sn metal. The repeated dissolution and reformation brought a thicker and resistive surface layer, resulting from the accumulation of electrolyte decomposition products, which was clarified by X-ray photoelectron spectroscopy. In contrast, the capacity retention and stability were improved by simply changing the upper cutoff voltage to 0.65 V due to exclusion of the SEI decomposition at 0.68 V. The results of time-of-flight secondary ion mass spectroscopy measurements suggests that the surface passivation layer containing polymer/oligomer on the Sn electrode was successfully formed and enhanced the SEI functionality for 0.65 V cutoff. The Sn-based electrode delivered ∼700 mAh g^–1 reversible capacity over 100 cycles

“Natto” Binder of Poly-γ-glutamate Enabling to Enhance Silicon/Graphite Composite Electrode Performance for Lithium-Ion Batteries

Poly-γ-glutamate, a slimy constituent in a Japanese food, <i>natto</i>, consisting of fermented soybeans, is studied as the binder for silicon and graphite (Si/graphite) powder composite electrodes of lithium-ion batteries. All of the tested water-soluble natural polymers provide a better mechanical property of Si/graphite composite electrodes formed on Cu foil compared to conventional binder, poly­(vinylidene fluoride) (PVdF), leading to much improved battery performance. When lithium poly-γ-glutamate (Li-PGlu) is used as a binder, the Si/graphite electrode demonstrates a higher reversibility of electrochemical lithiation. Hard X-ray photoelectron spectroscopy results reveal that the surface of the silicon and graphite particles is uniformly covered with a thinner layer of Li-PGlu binder, and such uniform coverage enhances passivation for the Si/graphite electrode during charge–discharge cycles, dissimilar to that of PVdF. In Li-PGlu, not only the oxygen atoms but also the nitrogen atoms of carboxylate and peptide bonds can act as a Lewis base to coordinate lithium ions. The coordination at the electrode surface would show a synergy effect on desolvating the lithium ions to be inserted into Si and graphite across the interface more efficiently compared to that of polyacrylate and polysaccharides having no −NH– group. X-ray diffraction and laser microscope observations clearly confirm that a Li-PGlu cast film is amorphous and pore-free, whereas a PVdF film is crystalline and porous. The cycle performance of the Li-PGlu electrode is further improved by limiting the working voltage below 1.0 V vs Li and introducing FEC as the electrolyte additive because of improved passivation by the synergy effect of the binder coating, FEC addition, and potential limitation

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