Cross-Linked\nγ‑Polyglutamic Acid as an\nAqueous SiO_<i>x</i> Anode Binder for Long-Term\nLithium-Ion Batteries

Abstract

Silicon\noxide (SiO_<i>x</i>) has outstanding\ncapacity and stable lithium-ion uptake/removal electrochemistry as\na lithium-ion anode material; however, its practical massive commercialization\nis encumbered by unavoidable challenges, such as dynamic volume changes\nduring cycling and inherently inferior ionic conductivities. Recent\nliterature has offered a consensus that binders play a critical role\nin affecting the electrochemical performance of Si-based electrodes.\nHerein, we report an aqueous binder, γ-polyglutamic acid cross-linked\nby epichlorohydrin (PGA–ECH), that guarantees enhanced properties\nfor SiO_<i>x</i> anodes to implement long-term\ncycling stability. The abundant amide, carboxyl, and hydroxyl groups\nin the binder structure form strong interactions with the SiO_<i>x</i> surface, which contribute strong interfacial\nadhesion. The robust covalent interactions and strong supramolecular\ninteractions in the binder ensure mechanical strength and elasticity.\nAdditionally, the interactions between lithium ions and oxygen (nitrogen)\natoms of carboxylate (peptide) bonds, which serve as a Lewis base,\nfacilitate the diffusion of lithium ions. A SiO_<i>x</i> anode using this PGA–ECH binder exhibits an impressive\ninitial discharge capacity of 1962 mA h g^–1 and\nmaintains a high capacity of 900 mA h g^–1 after\n500 cycles at 500 mA g^–1. Meanwhile, the assembled\nSiO_<i>x</i>||LiNi_0.6Co_0.2MnO_0.2 full cell shows a reversible capacity of 155 mA\ng^–1 and displays 73% capacity retention after 100\ncycles.