P2-Type Na2/3Fe1/3Mn1/3Co1/3O2 As a New Positive Electrode for Na-Ion Batteries

Introduction

A number of P2-type layered cathode materials have been synthesized as possible candidates for Na-ion battery application. These include Na_2/3CoO₂ [1], Na_2/3Fe_1/2Mn_1/2O₂ [2], Na_2/3Co_2/3Mn_1/3O₂ [3] 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. Na_2/3Mn_1/3Fe_1/3Co_1/3O₂was synthesized and was found to have high energy density, good cycling stability and low polarization.

Experimental

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 Na_2/3Mn_1/3Co_1/3Fe_1/3O₂. 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 Na_2/3Mn_1/3Co_1/3Fe_1/3O₂ when charged to different voltages. It was found that substitution of Co into Na_xFe_1/2Mn_1/2O₂causes 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 Na_2/3Fe_1/3Mn_1/3Co_1/3O₂will be presented.

Figure captions

Figure 1  Rietveld refinement of the P2-Na_2/3Fe_1/3Mn_1/3Co_1/3O₂XRD pattern.

Figure 2  First cycle voltage profile of P2-Na_2/3Fe_1/3Mn_1/3Co_1/3O₂at different cutoff voltages.

References

[1] R.Berthelot, D. Carlier & C. Delmas, Nature Mat., 10(2011) 74-80.

[2] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, r. Okuyama, R. Usui, Y. Yamada and S. Komaba, Nature Mat., 11(2012) 512-517.

[3] D. Carlier, J. H. Cheng, R. Berthelot, M. Guignard, M. Yoncheva, R.Stoyanova, B. J. Hwang and C. Delmas, Dalton Trans., 40 (2011) 9306-9312.

[4] J.S. Thorne, R.A. Dunlap and M.N. Obrovac, J. Electrochem. Soc., 160 (2012) A361-A367.