When fluorination becomes inactive: solvation exclusion and interfacial kinetics in Ni-rich lithium-ion batteries

Fluorinated carbonate additives are widely employed to stabilize electrolytes for high-energy lithium-ion batteries (LIBs), yet the interplay between fluorination degree, molecular geometry, and lithium-ion (Li⁺) solvation remains poorly understood. It is commonly assumed that increasing fluorination uniformly weakens Li⁺ solvation and improves interfacial stability. Here, we demonstrate that excessive fluorination-particularly in linear carbonate additives-can instead render the additives solvation-inactive through a previously unrecognized mechanism termed solvation exclusion. In this regime, highly fluorinated linear additives are expelled from the Li⁺ first solvation shell, leading to kinetically unfavorable interfacial processes. Using a multiscale approach integrating density functional theory, molecular dynamics simulations, multinuclear DOSY NMR, temperature-dependent electrochemical impedance spectroscopy, and long-term NMC90‖graphite full-cell testing, we systematically compare cyclic and linear fluorinated carbonates with graded fluorination. We reveal that cyclic carbonates with moderate fluorination remain active in Li⁺ solvation, promote controlled contact-ion-pair formation, lower charge-transfer activation energies, and form LiF-rich, mechanically robust interphases. Conversely, highly fluorinated linear additives trigger solvation exclusion, resulting in elevated activation barriers, organic-rich interphases, severe cathode cracking, and rapid capacity fading. Furthermore, galvanostatic intermittent titration measurements confirm that these performance disparities stem strictly from interfacial kinetics rather than bulk lithium diffusion. These findings establish that optimal electrolyte design requires balancing the fluorination degree and molecular geometry to preserve active solvation participation. Ultimately, this work provides fundamental insights and practical design principles for developing robust fluorinated electrolytes for Ni-rich LIBs, particularly under high-rate and low-temperature operations.