A dual force cross-linked γ-PGA-PAA binder enhancing the cycle stability of silicon-based anodes for lithium-ion batteries

Silicon (Si) is considered to be one of the most suitable anode materials for high energy density lithium-ion batteries due to its high theoretical specific capacity. However, the practical application of Si anode is limited by its huge volume change, leading to the rupture of electrode during cycling. In this study, a natural polymer of poly-glutamic acid (γ-PGA) is derived from nattomycin gum and poly-acrylic acid (PAA) through thermal cross-linking method and served as a dual force cross-linked binder. The two polymers are firmly bonded by dehydration to form the cross-linked γ-PGA-PAA binder with stable ester bonds, which readily creates hydrogen bonding forces with the hydroxyl groups on Si surface. The γ-PGA-PAA exhibits robust mechanical properties to achieve excellent electrochemical performance of Si-based electrode. When the γ-PGA-PAA is used as binder of Si nanoparticle anodes, the capacity retention of Si/γ-PGA-PAA electrode can reach 63.1% after 300 cycles at charge/discharge current density of 420 mA g−1 with a mass loading of about 0.7 mg cm−2. As a comparison, the capacity retentions of the Si/PAA and Si/γ-PGA electrodes are respectively 8.7 and 17.9% under the same conditions. At higher current density of 2940 mA g−1, the capacity retention of Si/PAA electrode has dropped dramatically to 23.3%, and the Si/γ-PGA-PAA electrode keeps still high-capacity retention of 72.5% after 400 cycles, while the Si/γ-PGA electrode can only last 40 cycles. When the three binders are applied to SiO electrode with 1 mg cm−2 loading, the γ-PGA-PAA binder yields also the highest capacity retention of 87.3% after 100 cycles at 160 mA g−1, while those corresponding to PAA and γ-PGA binders are 81.3% and 74.9%, respectively. XPS characterization results indicate that the γ-PGA-PAA binder has a strong adhesion to Si particles. This cross-linked binder network of γ-PGA-PAA effectively enhances the cycling performance and maintains the structural integrity of Si and SiO electrodes.