Mechanically Milled Transition Metal Phosphides (M-P) As Anode Materials for Sodium Ion Batteries (M=Ti, Mn, Co, Cu, Zn)

Introduction

Sodium ion batteries are potentially low cost and high energy density battery technology. Phosphorus-based materials are attractive anode materials for sodium ion batteries as they offer high theoretical capacity at low voltages,^1,2however they have not been fully explored. In this study, several transition metal phosphides were synthesized and characterized in Na cells.

Experimental

Stoichiometric amounts of transition metal powders and red phosphorus powders were Spex-milled under Ar atmosphere for different time spans. Electrodes were made with active material, binder (PVDF or PI), carbon black at a ratio of 8:1:1 and dried under vacuum at 120 ℃ overnight. 1M NaPF₆in a solution of EC, DEC, FEC (volume ratio 3:6:1) was used as electrolyte and Na foil was used as counter/reference electrode. Cells were cycled between 0.005 V and 1.5 V at a C/40 rate under at 30 °C and 60 °C. X-ray diffraction (XRD) patterns were measured with a Rigaku Ultima IV X-ray diffractometer equipped with a Cu anode X-ray tube and a diffracted beam monochromator.

Results and discussion

Fig. 1 shows the XRD pattern of synthesized phosphides. All synthesized materials show pure desired phases with little or no impurities by XRD. Fig. 2 shows the voltage curves of ZnP₂ and Cu₃P electrodes at 30°C. The first sodiation capacities of most materials are much less than their theoretical capacities (listed in Table 1), which might be due to poor kinetics of active materials. Exceptions are ZnP₂ and Cu₃P, which show first sodiation capacities close to the theoretical values at 30°C. It was observed that increasing the cycling temperature increases the first discharge capacity, but has little effect on reversible capacity.

The reversible volumetric capacity of these materials at full sodiation state were calculated based on the molar volume of the M and Na₃P phases, and the results are summerized in Table 1. The reversible volumetic capacities of ZnP₂ and Cu₃P are 744 Ah/L and 922 Ah/L, respectively, which are considerably higher than that of hard carbon (~450 Ah/L). The irreversible capacities and polarization, however, are high for all the synthesized materials, which is typical of conversion reactions.

In a conversion reaction, M-P undergoes conversion reaction and forms Na-P phases and M metal upon sodiation. This was confirmed by ex-situ XRD measurements.  The ex-situ XRD patterns of fully sodiated and desodiated (discharged to 0.005V then charged to 1.5 V) Cu₃P are shown in Fig. 3. Drastic amorphization is observed upon sodiation and the peak intensity of Cu₃P decreases, while the peak intensity of Cu increases. After fully chargi, peaks attributed to Cu₃P appear again, confirming the reversible conversion reaction.

Conclusion

Transition metal phosphides were made from mechanical ball-milling and characterized. Conversion reactions occur during the sodiation of the materials. Most of these materials have much lower capacity than the theoretical capacities. ZnP₂ and Cu₃P have volumetric capacities that exceed that of hard carbon.