P2-Type Na2/3Fe1/3Mn1/3Co1/3O2 As a New Positive Electrode for Na-Ion Batteries
A number of P2-type layered cathode materials have been synthesized as possible candidates for Na-ion battery application. These include Na2/3CoO2 , Na2/3Fe1/2Mn1/2O2 , Na2/3Co2/3Mn1/3O2  among others. Each of these systems each have a unique set of promising characteristics. We have found another promising candidate for a practical Na-ion cathode material. Na2/3Mn1/3Fe1/3Co1/3O2was synthesized and was found to have high energy density, good cycling stability and low polarization.
Phase pure samples were prepared by conventional solid state reaction at 950 °C in O2 from stoichiometric amounts of Na2O2, MnO2, Fe3O4 and Co3O4. X-ray diffraction (XRD) patterns were measured under Ar gas in a gas-tight sample holder. Electrodes were prepared from a slurry with 80% by weight active material, 12% Super-P carbon black and 8% PVDF binder dissolved in NMP. The slurry was then coated on Al foil using a coating bar with a 0.006" gap and dried in air for several hours at 120 °C. 2325 coin-type cells were constructed using 1M NaClO4(98% purity) dissolved in PC electrolyte with 2% by volume FEC. Two Celgard 3501 separators, one blown microfiber separator (3M Company) and a sodium foil counter electrode (Aldrich) were used. Cells were cycled with constant 10 mA/g current in indicated voltage ranges.
Results and Discussion
Figure 1 shows an XRD pattern of Na2/3Mn1/3Co1/3Fe1/3O2. The pattern shows no noticeable impurities. The pattern was fit using a Rietveld refinement with the P63/mmc space group. Lattice parameters of a = 2.89 Å and c = 11.1 Å were obtained, corresponding to a bulk density of 4.30 g/mL at 2/3 sodiation.
Figure 2 shows the first cycle voltage profile for Na2/3Mn1/3Co1/3Fe1/3O2 when charged to different voltages. It was found that substitution of Co into NaxFe1/2Mn1/2O2causes an increase in average voltage [2, 4]. Upon sodiation from 4.5 V, an average voltage of 3.00 V is achieved with 173 Ah/kg / 745 Ah/L of reversible capacity, yielding an estimated energy density of ~ 520 Wh/kg / 2240 Wh/L. When cycled to 4.0 V, average voltage is 2.77 V but the energy density is reduced to ~ 350 Wh/kg (1510 Wh/L), resulting from less access of the high voltage plateau.
Figure 2 also shows some of the consequences of accessing voltages above 4.1V using the current electrolyte formulation. Increasing use of the plateau increases voltage hysteresis and reduces reversibility, as evidenced by the decreasing negative capacity of the subsequent sodiation half-cycle. Ex-situ Mössbauer studies and the electrochemical performance of P2-type Na2/3Fe1/3Mn1/3Co1/3O2will be presented.
Figure 1 Rietveld refinement of the P2-Na2/3Fe1/3Mn1/3Co1/3O2XRD pattern.
Figure 2 First cycle voltage profile of P2-Na2/3Fe1/3Mn1/3Co1/3O2at different cutoff voltages.
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