The Fluorination of Boron‐Doped Graphene for CF_x Cathode with Ultrahigh Energy Density

Abstract

The enhancement of the fluorination degree of carbon fluorides (CFx) compounds is the most effective method to improve the energy densities of Li/CFx batteries because the specific capacity of CFx is proportional to the molar ratio of F to C atoms (F/C). In this study, B‐doped graphene (BG) is prepared by using boric acid as the doping source and then the prepared BG is utilized as the starting material for the preparation of CFx. The B‐doping enhances the F/C ratio of CFx without hindering the electrochemical activity of the C–F bond. During the fluorination process, B‐containing functional groups are removed from the graphene lattice. This facilitates the formation of a defect‐rich graphene matrix, which not only enhances the F/C ratio due to abundant perfluorinated groups at the defective edges but also serves as the active site for extra Li+ storage. The prepared CFx exhibits the maximum specific capacity of 1204 mAh g−1, which is 39.2% higher than that of CFx obtained directly from graphene oxide (without B‐doping). An unprecedented energy density of 2974 Wh kg−1 is achieved for the as‐prepared CFx samples, which is significantly higher than the theoretically calculated energy density of commercially available fluorinated graphite (2180 Wh kg−1). Therefore, this study demonstrates a great potential of B‐doping to realize the ultrahigh energy density of CFx cathodes for practical applications.

Electroplated Silver-Modified CFx Cathode for Lithium Primary Batteries with High Rate Capability

“…There is also a weak absorption peak at 1311 cm –1 ascribed to the asymmetric stretching vibration of the −CF 2 group at the vacancy defect or at the edge of fluorinated graphene . The wide absorption band near 1611 cm –1 is attributed to the stretching vibration of the aromatic CC bond, which is consistent with the TEM results. Raman spectroscopy was performed to study the changes in the chemical structure of the CF x cathode before and after the electroplating treatment.…”

“…Raman spectroscopy was performed to study the changes in the chemical structure of the CF x cathode before and after the electroplating treatment. The two resonant peaks at ∼1350 (D band) and ∼1590 (G band) cm –1 in the Raman spectra (Figure S3) represent the lattice defects and the in-plane stretching vibration in the sp 2 -hybridized carbonaceous structure, respectively. ,− The intensity ratios of these two bands ( I D / I G ) of the prepared FA cathodes ranged from 1.13 to 1.16, further verifying the almost intact structure of the CF x particles after the electroplating treatment.…”

“…In the high-resolution C 1s spectrum of the CF x pristine powder (Figure S5a), the five consecutive peaks located at 291.3, 289.3, 287.9, 286.3, and 284.8 eV represent the characteristic peaks of −CF 2 bonds, covalent C–F bonds, semi-ionic C–F bonds, sp 3 C–C bonds and sp 2 CC bonds, respectively. , The presence of sp 2 CC bonds is consistent with the TEM images and FTIR spectra. In addition to the covalent C–F bonds in different forms, semi-ionic C–F bonds are usually formed in CF x compounds with relatively low degrees of fluorination; these bonds are prone to dissociation and are affected by Li + during the discharge process owing to their lower binding energy than covalent C–F bonds. ,, The corresponding high-resolution F 1s spectrum of the CF x pristine powder (Figure S5b) shows three peaks at 689.8, 688.8, and 687.7 eV, which are assigned to the fluorinated species in the perfluorinated, covalent (CF) n , and semi-ionic (C x F) n configurations, respectively. , After preparing the pristine powder into the electrode, the high-resolution C 1s spectrum of the CF x cathode (Figure S6a) showed five peaks similar to the pristine powder, but the peak strength and width varied. The changes in the sp 3 C–C bonds and sp 2 CC bonds peaks are caused by the addition of Super P conductive agent, while the changes in the −CF 2 bonds and covalent C–F bonds peaks are caused by the addition of PVDF binder.…”

“…At the beginning of discharge, the D Li + of both cathodes was between ∼10 –12 and ∼10 –11 cm 2 s –1 . The D Li + of these cathodes gradually increased to the range of ∼10 –9 to ∼10 –8 cm 2 s –1 as the discharge progressed because the conductive carbon generated in the initial stage of the electrode reaction lowered the diffusion barrier of Li + . Finally, it drops to approximately 10 –9 due to the formation of electrical and ionic insulating LiF crystals, hindering the contact between the active materials and electrolyte, and hence increasing the diffusion path of Li + .…”

In situ construction of oxygen vacancy-rich and fluorine-doped carbon-coated Ca2Fe2O5 for improved lithium storage performance

“…Besides, the high polarization of the C-F bond can produce strong electrostatic adsorption, promote the interaction with metal cations, enhance the connection between carbon coating and the chief material, and change the morphology of the transition metal material combined with the carbon base. 17,18 Herein, Ca 2 Fe 2 O 5 with a fluorine-doped and carbon-coated composite structure (denoted as CFO@FC) was in situ constructed by one-step calcination using polyvinylidene fluoride (PVDF) as the fluorine source and carbon source. Benefiting from the unique structure, increased active sites and accelerated reaction kinetics, unusual reversible capacities and admirable rate capability were achieved.…”