An Interfacial Balance Rule Governs Binder-Electrolyte Coupling in Lead-Free Perovskite Energy Storage

Abstract

Electrode binders are conventionally regarded as inert structural components. Here, we show that in lead-free perovskite supercapacitors, the binder defines the optimal electrolyte composition. Across a factorial matrix of poly(vinylidene fluoride) (PVDF) loadings and LiTFSI concentrations in CsSnCl$_3$ electrodes, the capacitance optimum shifts systematically with binder content along a single linear relationship, described by the Interfacial Balance Rule ($\lambda+\theta=1$), where $\lambda$ and $\theta$ are the normalized lithium-supply and polymer contributions at the optimized interfacial state. The same relationship holds for hybrid MASnCl$_3$, showing that the optimum is governed by the polymer-electrolyte interface rather than the perovskite lattice chemistry. Simulations using a pre-trained MACE machine-learned interatomic potential show that PVDF adopts a planar configuration on CsSnCl$_3$ and simultaneously interacts with cationic and anionic sites. This configuration homogenizes lithium adsorption energetics, introduces fluorine-mediated coordination, and confines lithium to a two-dimensional interfacial region while preserving lateral mobility. Tuning polymer coverage through surface density and chain length reveals a finite interfacial lithium accommodation capacity that marks the onset of out-of-plane aggregation. The Interfacial Balance Rule provides a macroscopic descriptor of this finite interfacial resource, balancing polymer-mediated lithium stabilization against limited accommodation space. Binder loading is therefore an active design parameter for polymer-regulated energy-storage interfaces.