Surface defect‐enhanced conductivity of calcium fluoride for electrochemical applications

Abstract

Calcium fluoride is widely investigate known to be a promising solid material for optics, electronics, and electrochemistry. In this work, we report the successful preparation of calcium fluoride with enhanced defect structure obtained by the application of vapor pressure followed by high-energy ball milling, creating CaF 2 nano-powder, achieving increased ionic conductivities in the order of 1.9 · 10 −5 S·cm −1 at room temperature relying on fluoride surface interstitial defect with an activation energy of 0.35 eV. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR). It is revealed that the calcium fluoride electrolyte keeps in its basic cubic structure, becomes nano-powdered material with crystallite/ particle size of 12/30 nm. Surface defect structures are enhanced, which is visible with XPS, NMR, and EPR spectroscopy. The synthesized material provided considerable ionic conductivity enhancement introducing attraction for electrochemical testing for fluoride-ion batteries (FIB). KEYWORDSball milling, calcium fluoride, fluoride-ion battery, fluoride shuttle sensors, ionic conductivity, NMR, solid-state electrolyte

CaF 2 solid‐state electrolytes prepared by vapor pressure exposure and solid synthesis for defect and ionic conductivity tuning

“…19 F‐MAS‐NMR reveals that there is no detectable evidence of a surface defect peak promoting elevated conductivity enhancement as previously reported for higher vapor pressure exposure (RH 81 %) and liquid synthesis …”

“…The improvement of the conductivity of CaF 2 has been demonstrated by activating these grain boundaries using Lewis acids . Such and recent advances in nano‐technology have opened new perspectives to improve the FIB‐technology applying CaF 2 as an electrolyte for FIBs . From our findings, we conclude that there is considerable influence of humidity exposure on H 2 O, OH − and H 3 O + concentration on the surface defect structures of the nanoparticles.…”

“…X‐ray diffraction and NMR data are reported in Figure . The XRD patterns are related to the space group Fm‐3m accordingly the fluorite‐type structures . No direct influence of the humidity treatment onto the crystal structures was found.…”

“…From NMR it becomes evident that the F − ‐mobility can only partially be transformed to the macroscopic level due to insufficient ‐ not detectable ‐ surface defects. Still, since vapor pressure induced H 2 O molecules are adsorbed at CaF 2 (111) and introduce negative surface hydroxyl groups, positively charged hydrogen sites, and high‐energy milling under remaining oxygen contaminations introduce O 2− impurities, a promotion of defects, lattice distortions and geometry frustration especially at grain boundaries of different crystallites and particle surfaces can be considered. The ionic conductivities are attained with an improvement up to 4.7·10 −6 S·cm −1 at room temperature.…”

“…All‐solid‐state batteries are considered to be the safest batteries due to the almost complete absence of leakage, volatilization, and flammability . To realize such evolving solid‐state technology that operates at moderate temperatures improved solid electrolytes that have high F‐ion conductivities and favorable interaction with active electrode materials are indispensable for FIB . Pure solid‐state and cost‐efficient intrinsic doping mechanism (high‐energy ball‐milling), as for tysonite‐type compounds with fluorite‐type or vice versa introduce vacancies or interstitials for increased F‐ion mobility .…”