Higher reversible capacity of 333 mA h g−1 in Na cell is demonstrated for hard carbon derived from HCl-treated argan.

Poly-γ-glutamate, a slimy constituent in a Japanese food, natto, 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.

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P2-Na2/3Ni1/3Mn2/3O2 (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na+/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly­(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.