In recent years, it’s not too much to say that lithium-ion batteries (LIBs) are key components in electronic devices, which are often required to be mobile, or to be operated independently from system power supplies, such as smartphones, sensors, and electronic vehicles. Here, we propose a new Fe-based pyrophosphate cathode material Li_5.33Fe_5.33(P₂O₇)₄ for LIBs. It has been found that the new Fe-based cathode has a potential for Fe^2+/3+ redox couple approaching to 3.8 V, which is the highest among those of all Fe-based phosphate materials and pyrophosphate materials reported so far^[1], including LiFePO₄, Li₃Fe₂(PO₄)₃, LiFeP₂O₇, Li₂FeP₂O₇ and LiFe_1.5P₂O₇. In this report, we will discuss on its structural characterization and charge and discharge behaviors.

Methods

The cathode material was synthesized via the solid-state method. Stoichiometric amounts of precursors, Li₂CO₃, FeC₂O₄ and (NH₄)₂HPO₄, were wet-blended in acetone. After evaporation of the solvent, the resulting mixture was sintered at 500-650°C for 12 hours in Ar atmosphere to obtain the cathode material. Synchrotron X-ray powder diffraction patterns of the cathode material were obtained using a wavelength of 0.9996 Å. Rietveld refinement was carried out with RIETAN-FP program^[2].

Half-cell assembling was conducted in a dry room (dew point: <-70°C). The mixture of the cathode materials, Ketjen Black and polyvinylidene difluoride in a ratio of 85:10:5 wt% was dispersed in N-methylpyrrolidone. The resulting paste was coated on Al sheet, and then dried in vacuum at 40°C to evaporate the solvent. The Al sheet was cut out in disks (φ =16 mm), pressed under 9.5 MPa, and then dried in vacuum at 120°C overnight. 2032-type cells were assembled using the cathode sheets mentioned above as a positive electrode, Li metal disks (φ =16 mm) as a negative electrode, 1M LiPF₆solution in a 3:7 v/v mixture of ethylene carbonate/dimethyl carbonate as the electrolyte, and polypropylene separator (φ =18 mm). Galvanostatic charge and discharge measurements were carried out in CC mode (6.95 mA per 1 g of the cathode material). The voltage range was set to 2.0-4.5 V.

Results & Discussions

Figure 1a shows a synchrotron X-ray diffraction pattern of the cathode material, Li_5.33Fe_5.33(P₂O₇)₄. The rietveld analysis revealed that the cathode material belongs to triclinic system P-1, and that the lattice parameters were calculated as the following: a = 6.3813 Å, b = 8.5635 Å, c = 10.0275 Å, α = 107.937°, β = 89.863°, γ = 93.0035°. In the crystal structure, FeO₆ octahedrons form edge-sharing zigzag chains along the baxis, as displayed in Figure 1b.

Figure 2a and 2b show the first charge and discharge curves of a Li_5.33Fe_5.33(P₂O₇)₄/Li half cell and their derivative dQ/dV curves, respectively. Note that there is a plateau at 3.7-3.9 V in each of charge and discharge curves, near the charged state. In the derivative dQ/dV curves, there are four voltage peaks on charging, and three voltage peaks on discharging as displayed in Figure 2b. It is reasonable to say that the discharge voltage peak at 3.77 V corresponds to the charge voltage peak at 3.83 V, and, namely, that the observed Fe^2+/3+redox potential near the charged state is 3.80 V. This is the highest voltage among the cathode materials composed of Li-Fe-P-O reported so far. Both of the first charge and discharge capacities were approximately 105 mAh/g, which is 76% of the theoretical capacity (139 mAh/g).

The high potential of 3.80 V is probably related to the crystal structure with the FeO₆ edge-sharing chains (Figure 1b), in which the distance between neighboring Fe atoms is relatively small (3.22 Å at the smallest). This feature makes large Fe-Fe repulsion energy in the charged state with Fe³⁺, resulting in large difference in free energy between the charged and the initial states, which determines the redox potential.

References

[1] Masquelier, C.; Croguennec, L. Chemical Reviews 2013, 113(8), 6552-6591.

[2] Izumi, F.; Momma, K. Solid State Phenomena 2007, 130, 15-20.