sc7b01798_si_001.pdf (2.8 MB)
“Natto” Binder of Poly-γ-glutamate Enabling to Enhance Silicon/Graphite Composite Electrode Performance for Lithium-Ion Batteries
journal contribution
posted on 2017-06-16, 20:48 authored by Takahiro Mochizuki, Shoko Aoki, Tatsuo Horiba, Martin Schulz-Dobrick, Zhen-Ji Han, Sayuri Fukuyama, Hiroshi Oji, Satoshi Yasuno, Shinichi KomabaPoly-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.