Cross-Linked γ-Polyglutamic Acid as an Aqueous SiO_x Anode Binder for Long-Term Lithium-Ion Batteries

Abstract

Silicon oxide (SiO_x) has outstanding capacity and stable lithium-ion uptake/removal electrochemistry as a lithium-ion anode material; however, its practical massive commercialization is encumbered by unavoidable challenges, such as dynamic volume changes during cycling and inherently inferior ionic conductivities. Recent literature has offered a consensus that binders play a critical role in affecting the electrochemical performance of Si-based electrodes. Herein, we report an aqueous binder, γ-polyglutamic acid cross-linked by epichlorohydrin (PGA-ECH), that guarantees enhanced properties for SiO_x anodes to implement long-term cycling stability. The abundant amide, carboxyl, and hydroxyl groups in the binder structure form strong interactions with the SiO_x surface, which contribute strong interfacial adhesion. The robust covalent interactions and strong supramolecular interactions in the binder ensure mechanical strength and elasticity. Additionally, the interactions between lithium ions and oxygen (nitrogen) atoms of carboxylate (peptide) bonds, which serve as a Lewis base, facilitate the diffusion of lithium ions. A SiO_x anode using this PGA-ECH binder exhibits an impressive initial discharge capacity of 1962 mA h g^-1 and maintains a high capacity of 900 mA h g^-1 after 500 cycles at 500 mA g^-1. Meanwhile, the assembled SiO_x||LiNi_0.6Co_0.2MnO_0.2 full cell shows a reversible capacity of 155 mA g^-1 and displays 73% capacity retention after 100 cycles.