Na3V2(PO4)3-Graphene Nanocomposite As Stable Cathodes for Na-Ion Batteries

Introduction

 

Na-ion batteries, because of the high abundance and uniform geographic distribution of Na sources, have been regarded as low cost and high efficiency energy storage devices for stationary applications.^1-2 However, it is difficult to find high energy and stable cathode materials for reversible Na ion insertion and extraction because of the large radius of Na ions (~40% larger than Li ions). Recently, layer structured metal oxides and NASICON structured Na₃V₂(PO₄)₃ have been demonstrated to be good cathode candidates.^1-7 Na_0.44MnO₂ is known to have excellent cycling stability and can have a theoretical capacity of ~120 mAh/g when it cycles between Na_0.22MnO₂ and Na_0.66MnO₂. However, it has to be pre-cycled to 2V in order to fully use the capacity. Na₃V₂(PO₄)₃ is at discharge state and has a theoretical capacity of ~117 mAh/g. Yet it suffers from a low conductivity and usually needs large amount of carbon in electrode preparation. In this work, we synthesized Na₃V₂(PO₄)₃-graphene nanocomposite and demonstrated its excellent cycling stability and rate performance as cathodes for Na-ion batteries. A capacity of ~102 mAh/g was obtained at 0.5C current density and the capacity retention was ~99% after 600 cycles.

Results and Discussion

 

The Na₃V₂(PO₄)₃-graphene nanocomposite has 10% graphene. It was mixed with conductive carbon (10%) and binder (10%) and casted on an aluminum substrate. Long cycle stability was demonstrated in coin cells with Na metal as the counter electrode. A capacity of ~102 mAh/g was obtained at 0.5C and the capacity retention was ~99% after 600 cycles. We also studied the rate performance. The capacity at 2C rate is ~70 mAh/g.

 Fig. 1.  Long term cycling stability of the Na₃V₂(PO₄)₃-graphene nanocomposite.

 

 

Acknowledgement.

The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery & Energy Reliability (OE) (under Contract No. 57558). We also are grateful for enlightening discussions with Dr. Imre Gyuk of the DOE-OE Grid Storage Program. (A portion of) The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

 

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