FeOF/TiO₂ Hetero-Nanostructures for High-Areal-Capacity Fluoride Cathodes

Abstract

Iron fluoride compounds offer an exciting pathway toward low-cost and high-capacity conversion-type lithium-ion battery (LIB) cathodes. However, due to the sluggishness of the electronic and ionic transport in iron fluorides, mass loadings of active materials in previous studies are typically less than 2.5 mg cm–2, which is too low for practical applications. Herein, we improve the charge transport in fluoride electrodes at both nano- and mesoscales to enable high-mass-loading fluoride electrodes. At the nanoscale, we prepare electronically conducting Li x TiO2 composites with FeOF nanoparticles to reduce electron transport distance to 5–10 nm, which is one of the shortest among reports. At the mesoscale, we design a percolating three-dimensional porous carbon nanotube (CNT) network to enable fast pathways for both electrons and ions. The resulting spongelike material, FeOF/TiO2@CNT, substantially enhances the kinetics of the conversion reaction in FeOF, boosts extra lithium storage capacity, and reduces the voltage hysteresis. Steady cycling over 300 cycles is achieved at a high mass loading of 8.7 mg cm–2 (FeOF/TiO2) (1.74 mAh cm–2). Such areal capacity of lithium storage is significantly higher than previously reported iron fluorides-based structures, a significant step forward toward the development of low-cost metal fluoride electrodes.

Phosphorus-Doped FeOF Nanoparticle-Based Cathodes for Lithium Storage

“…The peak position of F 1s (Figure b) in FeOF at 685.1 eV is assigned to Fe–F bonding . The O 1s spectra (Figure c) confirmed the appearance of the Fe–O peak at 530.3 eV in FeOF . The P 2p spectrum of P-FeOF exhibited a characteristic peak at 134.3 eV that can be ascribed to the bond of Fe–P (Figure d), which further demonstrates the introduction of P .…”

“…36 The O 1s spectra (Figure 2c) confirmed the appearance of the Fe−O peak at 530.3 eV in FeOF. 37 The P 2p spectrum of P-FeOF exhibited a characteristic peak at 134.3 eV that can be ascribed to the bond of Fe−P (Figure 2d), which further demonstrates the introduction of P. 38 The peaks at 132.1, 132.6, and 133.4 eV can be assigned to the P−C and P−O bonds. 38 As the introduction of P alters the electronic environment of FeOF, the XPS peaks of Fe, F, O, and P all shift, indicating that the electronic structure is optimized and the Li + transport kinetics are improved.…”

“…45 Notably, the R 1 peak disappears in subsequent cycles, which is caused by the irreversible reaction, the structure change of P-F6, and the formation of the CEI layer. 37 Because of the composition changes of P-F6 after the first cycle, the reduction peak position (R 2 ) moves (1.39 V → 1.51 V) in the subsequent cycle. 46 The CV profiles overlap well in the subsequent cycles indicating the high reversible performance of the P-F6 electrode.…”

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

“…A hierarchical structure composite made of FeOF and TiO 2 heteronanostructures distributed in a 3D porous CNT network was done for a high-mass-loading fluoride cathode. This “sponge” structuration gives a high areal capacity of 1.74 mAh cm –2 at a mass loading up to 8.7 mg cm –2 , far more than classic fluorides materials studied (<2.5 mg cm –2 ), and with TiO 2 helping with the electronically conduction upon lithiation, this synergy gave remarkable results (170 mAh g –1 after 300 cycles) . Fluorine-rich homogeneous passivation layer (HPL) was also proved to be effective on nickel–iron oxyfluorides (called NiFeOF) to avoid the metal dissolution into the electrolyte going on with the conversion mechanism, hence improving the cyclability .…”

“…This "sponge" structuration gives a high areal capacity of 1.74 mAh cm −2 at a mass loading up to 8.7 mg cm −2 , far more than classic fluorides materials studied (<2.5 mg cm −2 ), and with TiO 2 helping with the electronically conduction upon lithiation, this synergy gave remarkable results (170 mAh g −1 after 300 cycles). 145 Fluorine-rich homogeneous passivation layer (HPL) was also proved to be effective on nickel−iron oxyfluorides (called NiFeOF) to avoid the metal dissolution into the electrolyte going on with the conversion mechanism, hence improving the cyclability. 146 In the same idea, FeOF@ MnO 2 composite electrode demonstrated improved performances with respect to MnO 2 nanosheets and FeOF nanorods separately.…”