Room-Temperature Hydrogen-Enriched Water Pretreatment of LiFePO4 for Near-Surface Modification and Reduced Interfacial Resistance

Hydrogen-enriched water (HEW), an aqueous medium containing dissolved H2 and exhibiting a negative oxidation–reduction potential, was investigated as an additive-free, room-temperature pretreatment strategy for near-surface modification of LiFePO4 powders. The main effect observed under the HEW pretreatment protocol is not bulk structural reconstruction, but treatment-time-dependent modification of the near-surface chemical and textural state. X-ray diffraction (XRD) shows that the olivine lattice is retained without discernible peak-position shifts, while Raman spectroscopy indicates preservation of the main LiFePO₄ vibrational features with treatment-time-dependent spectral-shape changes. Together with progressive XRD peak broadening, these Raman features suggest increasing local structural/bonding heterogeneity without bulk lattice reconstruction. Consistently, X-ray photoelectron spectroscopy (XPS) reveals increased contributions from oxygen-containing surface species and oxygenated carbon-related components, suggesting modification of the local surface/adsorbate environment of LiFePO4 and the carbon-coated surface region. Nitrogen adsorption–desorption measurements, analyzed using the Brunauer–Emmett–Teller and Barrett–Joyner–Halenda methods, further indicate treatment-time-dependent textural redistribution, with the adsorption-derived contribution shifting from smaller-pore features to larger textural features and becoming most pronounced under the 6 h condition. Electron microscopy indicates that localized disorder remains confined to nanoscale surface regions. These near-surface chemical and textural changes correlate with electrochemical behavior: the 6 h pretreatment condition shows the highest discharge capacity and rate capability, together with the lowest combined interphase-related and charge-transfer resistance. Post-cycling XPS suggests a more oxygen-rich cathode–electrolyte interphase, and density functional theory calculations provide a model-based indication of reduced Li+ migration barriers in pore/void-containing local environments. Overall, the results identify a practical pretreatment window in which near-surface/textural modification is associated with improved interfacial kinetics without excessive disorder accumulation.