Lithium metal

Iron nitrate nonahydrate (Fe(NO₃)₃.9H₂O), Sodium hydroxymethyl cellulose (Na-CMC) and commercial α-Fe₂O₃ powder were purchased from Merck. Conducting graphite, Lithium metal are obtained from Alfa-aesar. Paracetamol (500 mg, acetaminophen) manufactured by Glaxo SmithKline Pharmaceuticals Ltd. was used in this study after its expiry. All chemicals were used as received without further purification.

The electrochemical behaviors of the LiFePO₄/graphene and LiFePO₄/C composites were evaluated with 2,025 coin-type batteries. The cathode electrodes were prepared by mixing 80 wt% active materials (LiFePO₄/graphene or LiFePO₄/C) and 10 wt% carbon black (TIMCAL) with 10 wt% polytetrafluoroethylene (PTFE, Aldrich) in isopropyl alcohol solution (99.5%, Aldrich). A uniform slurry was formed and pasted onto Al foils, dried at 120°C for 12 h, and then cut into circular electrodes with a diameter of 10 mm. Lithium metal (99.9%, Alfa-Aesar) was used as the anode, Celgard polypropylene (Celgard 2400) as the separator, and 1M LiPF₆ dissolved in ethylene carbonate and dimethyl carbonate (with a 1:1 volume ratio) as the electrolyte (MERCK KGaA, Germany). The cells were assembled in an argon-filled glove box (Etelux Lab2000, China). Cells were charged and discharged at room temperature using a LAND-CT2001A battery cycler (Wuhan, China) within the voltage range of 2.7–4.2 V (vs. Li⁺/Li). Cyclic voltammetry (CV) was performed with an Auto Potentiostat 30 system at a scan rate of 0.1 mVs⁻¹ between 2.5 and 4.2 V. Electrochemical impedance spectroscopy (EIS) profiles were obtained at the same open-circuit voltage by applying a 5-mV amplitude of the AC voltage with the frequency ranging from 100 kHz to 0.01 Hz.

Commercially available Polypropylene separator (Celgard 2325, 25 µm thickness), niobium (Nb) metal target (99.995% purity and 2 ˝diameter), Ar gas (99.99% purity), and LiNi_0.33Mn_0.33Co_0.33O₂ (LNMC, Sigma Aldrich), sulfur powder (99.99%, Alfa Aesar), lithium metal (99.99%, Alfa Aesar), LiPF₆ in DMC (Alfa Aesar), LiTFSI (99.9%, Sigma Aldrich), DME (99.5%, Sigma Aldrich), DOL (99%, Sigma Aldrich), LiNO₃ (99.99%, Sigma Aldrich) were purchased and used without further purification unless specified.