FeF3·0.33H2O Cathode Enhanced by CNTs and Graphene: A High‑Performance Solution for Lithium‑Ion Batteries

Abstract

FeF₃·0.33H₂O offers a high theoretical capacity (712 mAh g^-1) through multi‑electron conversion chemistry, yet its intrinsic low electronic conductivity limits practical use. By incorporating a dual conductive network of carbon nanotubes (CNTs) and graphene, we synthesized a FeF₃·0.33H₂O/CNT+G nanocomposite via a liquid‑phase route followed by sintering and paste‑casting. Electrochemical tests in the 1.8–4.5 V window versus Li⁺/Li show a discharge capacity of 234.2 mAh g^-1 at 0.1 C, 193.1 mAh g^-1 after 50 cycles at 0.2 C, and 140.4 mAh g^-1 at 5 C. The synergistic CNT–graphene framework markedly reduces charge‑transfer resistance and enhances Li⁺ diffusion, achieving superior rate capability and cycling stability compared with pristine or single‑additive composites.

Introduction

Rechargeable lithium‑ion batteries (LIBs) dominate portable electronics and electric‑vehicle (EV) markets due to their high energy density and environmental friendliness. Conventional cathodes such as LiCoO₂, LiMn₂O₄, and LiFePO₄ are limited by single‑electron intercalation reactions, yielding theoretical capacities below 250 mAh g^-1. Metal fluorides, notably FeF₃, stand out for their high theoretical capacity (up to 712 mAh g^-1 via three‑electron transfer), high plateau voltage (~2.7 V), and excellent thermal stability. However, FeF₃ suffers from a wide bandgap that renders it electronically insulating, suppressing capacity, rate performance, and cycle life.

Several strategies have been employed to mitigate FeF₃’s conductivity deficit: element doping, surface coating, and composite formation with conductive additives. While doping and coating can modestly improve performance, forming a conductive network with carbon materials—especially CNTs and graphene—provides a robust, scalable solution. CNTs supply high surface area and flexibility, whereas graphene offers exceptional electronic conductivity and mechanical strength, together forming a three‑dimensional (3D) conductive scaffold that facilitates electron transport and buffers volume changes during cycling.

Results and Discussion

Structural and Morphological Characterization

TGA–DSC analysis confirmed the dehydration sequence of the FeF₃·3H₂O precursor, enabling calcination at 240 °C to yield the hexagonal tungsten‑bronze FeF₃·0.33H₂O. XRD patterns matched the standard PDF 76‑1262, with peaks at 13.79°, 23.62°, and 27.80° corresponding to the (110), (002), and (220) planes. SEM images reveal ~100 nm particles for the pristine material, while the CNT‑coated and CNT+graphene composites show uniform coverage of CNTs and graphene sheets on the particle surfaces, forming a continuous conductive network. TEM and HRTEM confirm intimate contact between FeF₃·0.33H₂O crystallites and the carbon framework; SAED patterns corroborate the hexagonal structure.

Electrochemical Performance

Galvanostatic tests (1.8–4.5 V) demonstrate that the FeF₃·0.33H₂O/CNT+G electrode delivers an initial discharge capacity of 234.2 mAh g^-1 at 0.1 C, outperforming the pristine (217.5 mAh g^-1) and CNT‑only (225.1 mAh g^-1) counterparts. After 50 cycles at 0.2 C, the composite retains 193.1 mAh g^-1 (85.5 % retention), while the pristine and CNT‑only electrodes fall to 146.2 and 170.3 mAh g^-1, respectively. Rate capability tests show discharge capacities of 228, 210.7, 194.4, 170.5, and 140.4 mAh g^-1 at 0.1, 0.5, 1, 3, and 5 C, underscoring the composite’s superior kinetic response.

Electrochemical impedance spectroscopy (EIS) reveals a markedly lower charge‑transfer resistance (R_ct = 50.9 Ω after the 3rd cycle) for the CNT+graphene composite compared with 115.7 Ω (pristine) and 68.2 Ω (CNT only). Lithium‑ion diffusion coefficients (D_Li+) derived from the Warburg region reach 1.67 × 10^-12 cm² s^-1 at the 3rd cycle and 1.21 × 10^-12 cm² s^-1 after 50 cycles, surpassing the other samples by 30–50 %. These findings confirm that the 3D CNT–graphene scaffold enhances electronic conductivity, reduces polarization, and accelerates Li⁺ diffusion.

Conclusions

We have successfully fabricated a FeF₃·0.33H₂O cathode integrated with a dual CNT–graphene conductive network via a straightforward liquid‑phase synthesis and paste‑casting process. The resulting nanocomposite delivers high specific capacity, excellent rate capability, and robust cycling stability without the need for binders. The synergistic effect of CNTs and graphene creates a continuous, flexible, and highly conductive scaffold that mitigates the intrinsic electronic bottleneck of FeF₃, positioning FeF₃·0.33H₂O/CNT+G as a promising high‑performance cathode for next‑generation lithium‑ion batteries.

Methods

Synthesis of FeF₃·0.33H₂O Powder

Fe(NO₃)₃·9H₂O (99.99 %) and NH₄F (98 %) were dissolved in ethanol and deionized water, respectively, and mixed under sonication. The resulting precipitate was washed, dried at 80 °C, ground, and calcined at 240 °C under Ar for 3 h to obtain FeF₃·0.33H₂O.

Preparation of FeF₃·0.33H₂O/CNT+Graphene Electrode

FeF₃·0.33H₂O was blended with 5 wt % CNTs, milled, and sintered at 240 °C for 3 h. The resulting powder was mixed with a 1–1.5 wt % graphene N‑methyl‑pyrrolidinone paste (0.5 g powder in 1.5 mL paste), stirred 4 h, and coated onto Al foil. The composite film was dried at 85 °C overnight to yield the binder‑free electrode.

Electrochemical Measurement

Coin cells (CR2032) were assembled under Ar with Li foil anode, Celgard 2400 separator, and 1 M LiPF₆ in EC/PC/DEC (1:1:1) electrolyte. Galvanostatic charge–discharge tests (1.8–4.5 V) were performed at room temperature on a Land CT‑2001A system. CV and EIS were conducted on a CorrTest CS310 workstation, with CV at 1 mV s^-1 and EIS from 100 kHz to 0.01 Hz (5 mV amplitude).