Microplastic residues originating from lithium-ion battery components, including hydrophobic binder-derived poly(vinylidene fluoride) (PVDF) and separator-derived polyethylene (PE), are increasingly encountered in hydrometallurgical recycling streams, yet their impact on downstream lithium recovery remains largely unexplored. This study systematically investigates their influence on the reactive crystallization of lithium carbonate (Li2CO3) from aqueous lithium sulfate (Li2SO4) solutions. Precipitation at 80 °C was conducted at controlled polymer loadings (0.01–1.0 wt%), and crystallization kinetics, particle size distribution, filtration behavior, yield, and crystal morphology were evaluated using in situ monitoring, laser diffraction, and microscopy.PVDF markedly modified crystallization behavior, slightly reducing the turbidity onset times and promoting the formation of significantly finer particles, which led to a pronounced deterioration in filtration performance at higher loadings. Microscopy revealed that Li2CO3 crystallites preferentially nucleated and grew on PVDF surfaces, forming dense agglomerated structures rather than the larger, well-defined individual crystals observed in polymer-free systems. In contrast, PE exhibited similar but substantially weaker effects, with no clear evidence of surface-mediated nucleation. Despite these pronounced changes in crystal size, morphology, and separation behavior, the final Li2CO3 yield remained within 68–80% across all systems, with only a modest non-monotonic dependence on polymer loading.These findings demonstrate that hydrophobic polymer microplastics can act as effective heterogeneous nucleation sites, fundamentally altering crystallization pathways and downstream solid–liquid separation without significantly affecting equilibrium yield. The results highlight an overlooked process-level impact of microplastic contaminants and underscore the necessity of accounting for such impurities in the design and optimization of lithium-ion battery recycling flowsheets.