Synergistic β-Phase Enhancement and Amorphization in PVDF-Based Solid Polymer Electrolytes Enabled by Femtosecond Laser Irradiation

Polymer-based solid electrolytes are promising candidates for high-rate all-solid-state lithium batteries (ASSLBs), yet their ionic conductivity is fundamentally limited by the coupled effects of high crystallinity and the dominance of the nonpolar α-phase. In this study, we propose a femtosecond laser-enabled microstructure regulation strategy that synchronously enhances amorphous-phase formation and polar β-phase enrichment in polyvinylidene fluoride (PVDF)-based composite electrolytes (CPEs). Two-temperature modeling reveals that femtosecond pulses generate ultrafast, spatially confined thermal transients that lead to picosecond-scale melting followed by rapid cooling with local cooling rates approaching ∼10⁷ K s^-1. Such extreme nonequilibrium cooling is essential for freezing the amorphous phase and stabilizing the metastable β-phase formed during the transient thermal cycle. In addition, the intrinsic spatial energy distribution of the femtosecond laser produces radially and depth-dependent thermal gradients, which drive spatially selective phase reconstruction. The central high-temperature region undergoes partial melting and amorphization, whereas the surrounding intermediate-temperature zones preferentially stabilize the β-phase. Guided by these insights, we identify an optimal effective pulse count of 3.75, yielding a 3-fold increase in ionic conductivity (7.4 × 10^-4 S cm^-1) relative to pristine films. Solid-state cells employing the laser-processed electrolytes exhibit improved cycling stability, achieving ∼11% higher capacity retention after 200 cycles. This work establishes femtosecond laser-induced spatial phase regulation as a powerful, solvent-free route to synergistically modulate amorphous and polar crystalline phases in PVDF, offering a promising route toward high-performance solid-state energy storage technologies.