Surface Fluorination of LiNi0.5Mn1.5O4 by Fluoropolymer Thermal Decomposition: From Reactive Gas Speciation to Electrochemical Stabilization

Abstract Surface fluorination is an effective strategy to stabilize high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes against interfacial degradation, yet the role of the fluorinating agent and its decomposition pathway in governing surface speciation remains poorly understood. Here, PTFE and CYTOP are investigated as solid fluoropolymer-derived fluorinating agents for LNMO surface modification via thermal decomposition under inert atmosphere, yielding comparable surface fluorine contents of approximately 0.08 mmol g–1 for both treatments. X-ray diffraction and Raman spectroscopy confirm that both treatments preserve the cubic spinel framework, indicating that fluorination is confined to the particle surface. XPS reveals the formation of LiF-like species, M–F bonds and manganese oxyfluoride-like environments, while EPR reveals modifications of the manganese electronic environment after treatment. Galvanostatic cycling over 100 cycles at C/10 demonstrates improved capacity retention for both fluorinated samples: all three electrodes deliver an initial discharge capacity close to 143 mAh g–1. After 100 cycles, LNMO–PTFE retains 95% of its initial discharge capacity (136 mAh g–1), compared to 91% for LNMO–CYTOP and 90% for pristine LNMO (∼128 mAh g–1). TGA-MS and in situ gas-phase FTIR identify the reactive species generated during decomposition: PTFE mainly releases :CF2, •CF, •C2F3 and •CF3 fragments, whereas CYTOP produces a broader distribution of fluorinated species over a wider temperature range. These differences in decomposition chemistry directly control the nature of the fluorinated interphase and the resulting electrochemical stabilization of LNMO. This work establishes mechanistic guidelines for designing scalable fluoropolymer-based surface treatments for high-voltage cathode materials, as an alternative to reactive fluorine sources such as F2 or XeF2.