“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.