Designing a Polyurethane Ionomer for Micro-Si Anodes: Adhesion, Ion Transport, and Cycling Stability from a Waterborne Binder

Abstract Silicon is an attractive anode material due to its high theoretical capacity; however, microsilicon (μSi) powders undergo large volume changes, repeated SEI damage, and particle debonding, leading to capacity-degrading transport losses. Here, we develop a water-borne polyurethane (WPU) ionomer binder composed of PTMEG/PEG soft segments and a PEG–citric-acid polyol. Partial LiOH neutralization converts carboxylic acids to lithium carboxylates, providing multipoint anchoring to Si/SiOx, reversible ionic associations, and Li-ion conduction pathways. Two formulations differing in PTMEG:di-PEG-CA ratio were synthesized via a solvent-minimized water-inversion route. NMR, HSQC, and FTIR confirm the targeted segmented ionomer architecture. The ionomeric WPUs are elastomeric (>1500% strain) with tunable modulus across formulations. Relative to a poly(acrylic acid) (PAA) binder, the ionic conductivity increases by an order of magnitude (1.8–2.2 × 10–4 vs 2.4 × 10–5 S cm–1), and the peel strength approximately doubles (∼2.0–2.2 vs ∼0.9–1.0 N cm–1). In μSi half-cells at 0.2 C, the capacity stabilizes at ∼2.1–2.3 Ah g–1 after 500 cycles with a Coulombic efficiency ≥ 99.98%, whereas PAA shows rapid fading to ∼1.0 Ah g–1. Postcycling Warburg slopes (19.3, 28.9 vs 123.2 Ω·s1/2) and GITT-derived diffusion coefficients confirm substantially faster Li+ transport for both ionomeric binders. These results establish LiOH-neutralized WPU ionomers as a practical, NMP-free binder platform for μSi anodes, with the degree of neutralization and soft/hard-segment ratio serving as key tuning parameters.