Fluorinated Carbons as Rechargeable Li-Ion Battery Cathodes in the Voltage Window of 0.5–4.8 V

Abstract

Fluorinated carbon (CF x ) cathodes have the highest theoretical energy density among lithium primary batteries. However, it is still a huge challenge to be reversible. Here, CF x is proposed as a high-performance cathode material for rechargeable lithium-ion batteries in the extended voltage window of 0.5–4.8 V. Specifically, the fluorinated graphite CF0.88 exhibits an initial specific discharge capacity of 1382 mAh g–1 (2362 Wh kg–1) and a specific discharge capacity of 782 mAh g–1 at the 2nd cycle and maintains a specific discharge capacity of 543 mAh g–1 (508 Wh kg–1) after the 20th cycle. This rechargeable behavior is associated with the conversion of CF x to LiF + C (>1.5 V) and then to Li1+x FC (0.5–1.5 V) during the initial discharge process; Li1+x FC is reversible to LiF + C in the following charge–discharge process (0.5–4.8 V). By extending the voltage window, CF x cathodes can show new electrochemical behaviors. Our research has provided new Li-free cathode materials for rechargeable batteries and insights for improving the performance of a Li/CF x secondary battery.

Energetics of Phase Transformation Mechanisms in Li-CFx Batteries

“…Previously, we reported obtaining good agreement between the experimentally determined average open-circuit voltage (OCV) (∼4.05 V) and theoretically calculated average OCV (4.27 ± 0.14 V) for the formation of LiCF as the discharge product for a CF cathode . The proposed structure of LiCF is shown in Figure c, consistent with previous reports of the structure of LiCF described as a lithium fluoride slab intercalated between graphene sheets. ,, The structure has space group number 7 ( P 1 a 1) with Li, F, and C atoms occupying the 2a positions and with lattice parameters 5.08 Å, 2.54 Å, 11.08 Å, 90°, 90°, and 120°.…”

“…6 The proposed structure of LiCF is shown in Figure 1c, consistent with previous reports of the structure of LiCF described as a lithium fluoride slab intercalated between graphene sheets. 13,48,55 The structure has space group number 7 (P1a1) with Li, F, and C atoms occupying the 2a positions and with lattice parameters 5.08 Å, 2.54 Å, 11.08 Å, 90°, 90°, and 120°.…”

Boosting the Energy Density of Li||CFx Primary Batteries Using a 1,3-Dimethyl-2-imidazolidinone-Based Electrolyte

“…The assembled Li||CF x primary battery has been widely applied in medical, military, and spatial fields. However, the discharge voltage plateau of the practical Li||CF x battery is much lower than the theoretical value, due to the low electronic conductivity and high overpotential of CF x electrodes, as well as robust C–F covalent bonds, which inherently restricts the practical energy density. , …”

“…However, the discharge voltage plateau of the practical Li||CF x battery is much lower than the theoretical value, due to the low electronic conductivity and high overpotential of CF x electrodes, as well as robust C−F covalent bonds, which inherently restricts the practical energy density. 9,10 Extensive efforts have been focused on improving the electrochemical performance of CF x materials, including optimization of the fluorination process, 11,12 construction of nanostructures, 13−15 and surface engineering of bulk materials. 16−18 The effects of fluorination conditions (fluorination temperature, especially) on discharge capacity and voltage plateau were studied by Feng et al 11 It was found that the F/C ratios of fluorinated hard carbon increased, while the discharge plateau inversely decreased as a function of fluorination temperature.…”

Fluorinated Carbon Materials and the Applications in Energy Storage Systems

“…Acting as cathode for primary batteries is CF x ’s main application in energy storage systems before the awareness about the important role of LiF, which is a key ingredient of the solid electrolyte interphase (SEI) in rechargeable LIBs. − The discharge product of CF x /Li batteries exactly bridges the primary battery and the secondary battery, which extends the application of CF x . Some attempts have been made to modify the electrode materials with CF x in situ or ex situ, including graphite anode and intercalation compounds cathode, as well as the popular Li metal anode.…”