Performance of CuP2 Negative Electrode for Na‐Ion Batteries with CNTs As Stabilizer

Transition metal phosphides (TMPs) are promising anode materials for sodium ion battery, thanks to their high theoretical specific capacities. Nevertheless, they suffer from large volume change and from poor conductivity during prolonged cycling. Here we systematically investigate the role of different kinds of single/multi-wall carbon nanotubes (SWCNTs/MWCNTs) as additives in order to stabilize copper phosphide particles (CuP2) as anode materials in sodium ion batteries (SIBs). All composites show enhancement in the overall capacity and cycling stability compared to the pristine CuP2 due to the well-connected CNTs on and between the CuP2 particles. At a high currency density of 1 A g-1, CuP2@SWCNTs composite with 13 wt.% SWCNTs can deliver a specific capacity over 400 mAh g-1 for more than 60 cycles, much better than conventional hard carbon materials. The CNTs enhance the conductivity and reduce capacity loss caused by the volume variation of CuP2 based electrode materials. Post-mortem analysis by scanning electron microscopy depicts a possible battery failure mechanism, whereby the solid electrolyte interface exists in the form of nanospheres distributing at the surface of the electrode. During the repeated cycling (volume change), more fractures and cracks occur resulting in continuous interfacial reactions and consumption of sodium and electrolyte.