Slime-inspired polyacrylic acid-borax crosslinked binder for high-capacity bulk silicon anodes in lithium-ion batteries

Binder plays a key role in maintaining the integrity of battery electrodes. In this work, we investigate the relationship between binder properties and performances of Si electrodes in a lithium-ion battery (LIB). We learn from nature and use borax as a crosslinker between strands of polyacrylic acid (PAA) to form a network in the binder. By adding different amounts of borax, we can control the degree of crosslinking and thus the mechanical and chemical properties of the binder. We find that increasing the amount of borax crosslinker increases the modulus of the polymer network and the stability of the corresponding Si electrode up to about 5 wt% borax. Even with Si microparticles with an electrode density of 1.25 g cm⁻³, we obtain a capacity of about 2470 mA h g⁻¹ (3.1 mA h cm⁻²) and a capacity retention of 91.2% after 50 cycles. Cycle performance is extended beyond 500 cycles with a capacity limitation of 1000 mA h g⁻¹. Though, excessive amount of borax consumes the functional groups of PAA and compromises electrode adhesion, resulting in a decrease in cycle stability. Striking a balance between mechanical strength and adhesion of the binder is important for further development of alloy-based anode materials for LIBs.